EPA/625/1-91/024
October 1991
Manual
Alternative Wastewater
Collection Systems
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Risk Reduction Engineering Laboratory
Cincinnati, Ohio
Office Of Water
Office of Wastewater Enforcement and Compliance
Washington, DC
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Notice
This document has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
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Contents
Chapter Page
I. OVERVIEW OF ALTERNATIVE CONVEYANCE SYSTEMS
1.1 Introduction 1
1.2 Pressure Systems ..3
1.3 Vacuum Systems 7
1,4 Small Diameter Gravity Sewers 22
1.5 Comparison with Conventional Collection 24
1.6 References 25
2 PRESSURE SEWER SYSTEMS
2.1 Introduction ....27
2.2 Detailed System Plan and Elevation Views 28
2.3 Detailed Description of On-Lot System Components 30
2.4 System Design Considerations 40
2.5 Construction Considerations 76
2,6 O&M Considerations 79
2.7 System Costs 84
2.8 System Management Considerations 88
2.9 References 90
3 VACUUM SEWER SYSTEMS
3.1 Introduction 93
3.2 System Plan and Elevation Views 95
3.3 Description of System Components 95
3.4 System Design Considerations 102
3.5 Construction Considerations..... 131
3.6 O&M Considerations 136
3.7 Evaluation of Operating Systems 141
3.8 System Costs 147
3.9 System Management Considerations 153
3.10 References 155
4 SMALL DIAMETER GRAVITY SEWERS
4.1 Introduction 157
4.2 Description of System Components 157
4.3 System Design Considerations 159
4.4 Construction Considerations..... 172
4.5 O&M Considerations 175
4.6 Review of Operating Systems 181
4.7 System Costs 181
4.8 System management Considerations 191
Hi
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Contents (continued)
Chapter Page
4.9 References 191
5 DESIGN EXAMPLES
5.1 Pressure Sewer System 193
5.2 Vacuum Sewer System 195
5.3 Small Diameter Gravity Sewers 204
IV
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Figures
Number Page
1-1 Installation of pressure sewer main 5
1-2 Grinder Pump (GO) system 6
1-3 Septic Tank Effluent Pump (STEP) system 6
1-4 Liljendahl-Electrolux vacuum system , ..8
1-5 Vacuum toilet 8
1-6 Colt-Envirovac vacuum system , 10
1-7 AIRVAC vacuum system 10
1-8 Major components of a vacuum sewer 13
1-9 AIRVAC valve pit/sump arrangement „ , 14
1-10 Upgrade/downgrade/level transport 15
1-11 Diagram of a typical vacuum station 16
1-12 Early design concept - reformer pockets , 18
1-13 Current design concept - pipe bore not sealed 18
1-14 Gravity sewer system example 21
1-15 Vacuum-assisted gravity sewer system example...... 21
1-16 Schematic of a SDGS system 23
2-1 Piping system appurtenances 29
2-2 Typical simplex GP package using slide away coupling and guide rails 32
2-3 Typical centrifugal GP package with pump suspended from basin cover 33
2-4 Duplex GP station 34
2-5 Typical progressing cavity-type GP package 35
2-6 Basic components of a progressing cavity grinder pump 36
2-7 STEP pump in external vault 37
2-8 Typical STEP package with internal pump vault 38
2-9 Head-discharge curves for typical GP and STEP systems 39
2-10 Wastewater flows for one home „ 42
2-11 Required pumping rates using flows from Reference 11 42
2-12 Design flows , 44
2-13 Zoning of GP or solids handling pump vault , 52
2-14 Zoning of a STEP system interceptor tank showing scum and sludge accumulation 55
2-15 Zoning of a STEP system interceptor tank showing liquid levels
at pump off, on, and high-level alarm 55
2-16 Two-compartment interceptor tank with hole in baffle wall where clear space expected 57
2-17 Two-compartment interceptor tank using combination tee and 1/4 bend 57
2-18 Multiple-unit interceptor tank and pump assembly 58
2-19 Head-discharge curves for one and multiple centrifugal pumps in parallel 61
2-20 Effective pump curve 62
2-21 Rotor and cutaway stator of progressing cavity-type pump 63
2-22 Typical progressing-cavity pump H-Q curve 64
2-23 Circuit diagram of a basic 120-volt control panel 66
2-24 Wastewater-type air release valve , 68
2-25 Basket strainer used with external pump vault 72
2-26 Multi-tray filter, used with external pump vault 72
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Figures (continued)
Number Page
2-27 Outlet tee fitted with well screen , -..,....., 73
2-28 Mesh placed over inlet ports of internal pump vault ;...; .......j. 73
2-29 Fully-screened internal pump vault ,...".. 74
2-30 Slotted pump vault ,..,..„...„.... 74
2-31 Example lot facility plan 77
3-1 Typical layout - vacuum sewer system 94
3-2 Water sewer/vacuum system similarities , 95
3-3 Profile view of typical vacuum sewer line 96
3-4 Plan and profile view - typical valve pit 97
3-5 Auxiliary vent location 99
3-6 Lift detail 99
3-7 Line diagram of a typical vacuum station 100
3-8 Vacuum lift capability 104
3-9 Static toss determination 104
3-10 Top view of crossover connection 108
3-11 Typical configurations for gravity connections 110
3-12 Typical fiberglass valve pit setting.. 111
3-13 Shallow fiberglass valve pit setting 113
3-14 Plan and elevation views of typical concrete buffer tank 114
3-15 Typical concrete dual buffer tank 115
3-16 Model D arrangement with external breather .-. 117
3-17 Model S arrangement - sump vented 118
3-18 Early system external breather dial . 119
3-19 Early system external breather dial 120
3-20 Auxiliary vent detail 121
3-21 AIRVAG cycle counter - two methods of connection 122
3-22 Division valve with gauge tap detail 124
3-23 Terminal access point detail 124
3-24 NPSHa calculation diagram with typical values 127
3-25 Typical elevations of level control probes 130
4-1 Components of a small diameter gravity sewer (SDGS) system 158
4-2 Typical pre-cast concrete interceptor tank 158
4-3 Service lateral installation using a trenching machine 160
4-4 Typical combination cleanout and air release valve detail 160
4-5 Typical STEP lift station detail 161
4-6 Alternative locations for interceptor tanks 165
4-7 Typical interceptor tank outlet baffles 165
4-8 Typical surge chamber detail , 166
4-9 Interceptor outlet flow control device 166
4-10a Typical cleanout detail 168
4-10b Typical cleanout detail 169
4-11 Ventilated cleanout detail „ 170
4-12 Australian boundary trap detail 171
4-13 Examples of drop inlets, external and internal 173
4-14 Soil odor filter detail 174
4-15 Example of general easement , 176
4-16 Mainline lift station with emergency storage 179
4-17 Emergency pumping manhole 180
VI
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Figures (continued)
Number Page
5-1 Example pressure sewer design ; 194
5-2 Design example layout 196
5-3 Design example profiles 197
5-4 Design example profiles 198
5-5 Design example profiles 199
5-6 SDGS design example system profile 205
VII
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Tables
Number Page
1-1 Vacuum Collection System Parameters 11
1-2 Vacuum Station Parameters 11
1-3 Summary of Vacuum System Types 11
1-4 Operating Vacuum Systems in the United States „ 19
2-1 Approximate Main Sizes Required to Serve Number of Homes Shown 46
2-2 Typical Requirements for Separation of Pressure Sewer Lines from Water Lines 47
2-3 Abbreviated Listing of PVC Pipe Dimensions 49
2-4 Sludge and Scum Accumulation at Glide, Oregon ..54
2-5 Typical Zoning Design For a 1,000-gai Interceptor Tank Serving a Single-Family Residence 56
2-6 Distribution of Causes for Call-Out Maintenance On Selected Grinder Pump
Pressure Sewer Projects ....81
2-7 Distribution of Causes for Call-Out Maintenance On Selected STEP
Pressure Sewer Projects 82
2-8 Average Installed Unit Costs (mid-1991) for Pressure Sewer Mains and Appurtenances ,. 85
2-9 Average Installed Unit Costs (mid-1991) for Grinder Pump Services and Appurtenances , 85
2-10 Average Unit Costs (mid-1991) for STEP Services and Appurtenances 87
2-11 O&M Cost Accounting Records forthe Glide, Oregon Pressure Sewer System 88
3-1 Recommended Lift Height , 104
3-2 Main Line Design Parameters : 105
3-6 Guidelines for Determining Line Slopes 105
3-4 Governing Distances for Slopes Between Lifts 105
3-5 Maximum Flow for Various Pipe Sizes 105
3-6 Maximum Number of Homes Served for Various Pipe Sizes 105
3-7 Service Line Design Parameters 108
3-8 "A" Factor for Use in Vacuum Pump Sizing 126
3-9 Discharge Pump NPSH Calculation Nomenclature 126
3-10 Values of V0for a 15-Minute Cycle @ Qmln for Different Peaking Factors 127
3-11 Spare Parts List Per Every 50 Valves 132
3-12 Specialty Tools and Equipment for Collection Systems 132
3-13 Specialty Equipment for Vacuum Station 132
3-14 Normal Operating Tasks and Frequencies 138
3-15 Preventive Maintenance Tasks and Frequencies 139
3-16 Operating Systems Visited in 1989 142
3-17 General Information on Operating Systems 143
3-18 Design/Construction Data - Collection System 143
3-19 Design/Construction Data - Vacuum Stations 143
3-20 O&M Data-General Information 144
3-21 O&M Data - Person-Hours/Year 144
3-2 O&M Data - Power Consumption/Year 144
3-23 O&M Data - Mean Time BetweenService Calls 144
3-24 Problem Classification 145
3-25 Average Installed Unit Costs (mid-1990) for Vacuum Sewer Mains and Appurtenances 148
VIII
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Tables (continued)
Number Page
3-26 Average Installed Unit Costs (mid-1 MO) for Vacuum Pits and Appurtenances 148
3-27 Average Installed Cost for Vacuum Station 149
3-28 Typical O&M Cost Components 150
3-29 Person-Hour Estimating Factors 150
3-30 Vacuum Station Power Consumption Estimating Factors, ,. 151
3-31 Typical Renewal and Replacement Factors for major Equipment 152
3-32 Annual Budget Example 153
4-1 Summary of SDGS Projects Reviewed 182
4-2 Summary of Interceptor Tank Characteristics Used in Selected projects 184
4-3 Summary of Collector Main Design Criteria Used in Selected projects 185
4-4 Comparison of System Component Use as a Function of Number of Conmnections
or Feet of Collection Main Installed in Selected Projects 186
4-5 Comparison of SDGS Construction Costs from Selected Projects 187
4-7 Comparison of Unit Costs of Components from Selected Projects....... 188
4-7 Summary of Component Costs from Selected Projects 189
4-8 Summary of Component Costs (by percentage) from Selected projects 190
5-1 Design Example Line Loss Calculations 200
5-2 Design Example Line Loss Calculations 201
5-3 Design Example Piping Calculations 202
5-4 Design Example Vacuum Station Calculations 203
5-5 Computations for SDGS Design Example 206
IX
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Acknowledgments
Authors:
William C. Bowne, Eugene, OR
Richard C. Naret - Cerrone & Assoc., Wheeling, WV (since 1991 - AIRVAC, Tampa, FL)
Richard J. Otis - Owen Ayres & Associates Inc., Madison, Wl 53704
Peer Reviewers:
Paul Farrell - Environment/One, Schenectady, NY
Donald Gray - West Virginia University, Morgantown, WV 26506
Margaret Klepic - Ohio EPA, Columbus, OH
Robert Langford - Airvac, Rochester, IN
Charles Pycha - U.S. EPA Region 5, Chicago, IL
James Wheeler- U.S. EPA-OW, Washington, DC
Technical Direction/Coordination:
James F. Kreissl - U.S. EPA-CERI, Cincinnati, OH
Denis J. Lussier- U.S. EPA-CERI, Cincinnati, OH
Charles P. Vanderlyn - U.S. EPA-OW, Washington, DC
Contract Management:
Arthur J Condren - James M. Montgomery Consulting Engineers, Pasadena CA
Heidi Schultz - Eastern Research Group, Arlington, MA
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Extensive review comments were also provided by the Inter-Agency Workgroup on Small Wastewater Systems,
whose active membership is:
Denis Lussier
U.S. EPA
Cincinnati, OH
Chuck Pycha
U.S. EPA
Chicago, !L
Brian Yim
U. S. EPA
Seattle, WA
Rao Surampalli
U.S. EPA
Kansas City, KS
Gary Morgan
Farmers Home Administration (FmHA)
Washington, DC 20250
David Kirkman
Department Of Housing and Urban Development (HUD)
Washington, DC
Curtis Townsend
National Park Service (NPS)
Lakewood, CO
Sam Gaddipati
Tennessee Dept. Of Health and Environment
Nashville, TN
Gordon Innes
California State Water Resources Control Board
Sacramento, CA
Randy Orr
NY State Dept. of Environmental Conservation
Albany, NY
George Keller
Maryland Department of The Environment
Baltimore, MD
Fred Reiff
Pan American Health Organization
Washington, DC
Rick Barror
US Public Health Service (IHS)
Rockville, MD
Albert Wright
The World Bank
Washington, DC
Alex Campbell
Ontario Ministry of the Environment
Toronto, Ontario, Canada
Randy Clarkson
Missouri Department of Natural Resources
Jefferson City, MO
Bhupendra Vora
Florida Department of Environmental Regulation
Tallahassee, FL
Margaret Klepic
Ohio EPA
Columbus, OH
XI
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CHAPTER 1
Overview of Alternative Conveyance Systems
1.1 Introduction
1.1.1 History
In the late 1960's, the cost of conventional gravity
collection systems in rural communities was found to
dwarf the cost of treatment and disposal. In response to
this condition efforts were initiated throughout the United
States to develop low-cost sewerage which could serve
the needs of the rural communities which constituted
over 80 percent of demand for centralized collection and
treatment.
In developing alternative collection systems for these
small communities, engineers turned to concepts which
had theretofore been either forgotten or ignored by the
profession.
Pressure sewers had only recently been conceived of as
a means of separating combined sewers in large cities
by Professor Gordon Maskew Fair of Harvard University
and installed by Mortimer Clift in a Irttletown in Kentucky.1
Vacuumsewers had been around since the 19th Century,
but had not been seriously considered for widespread
use until then. Small-diameter gravity sewers (SDGS)
also found 19th Century roots in the United States, but
the principles had been all but forgotten in the rush to
codify urban civil engineering technology. These systems
returned to the U.S. from Australia where they had been
employed successfully for several years.2
After initial demonstration projects had been underwritten
by the U.S. Environmental Protection Agency (USEPA)
(and its predecessor agencies) and the Farmers Home
Administration (FMHA), these technologies were given
special status underthe innovative and alternative (I&A)
technology provisions of the Clean Water Act of 1977.
Thus stimulated, these technologies flourished in small
communities which were able to secure grantsunderthis
program. More than 500 alternative sewer systems
were installed underthe I&A provisions, and a significant
number were also constructed with state, local and
private funding during the 1970's and 1980's.
1.1.2 Approach
In developing this design manual several approaches
were possible. Large committees of "experts" could have
been assembled, and a consensus document developed.
This approach had been used forthe 1986 Water Pollution
Control Federation MOP Number FD-123, but such efforts
tend to yield results of a conservative nature, reflecting
only certain issues to which even the least knowledgeable
can agree.
The course chosen was to utilize only the individuals with
the best practical experience in each of the three major
categories to prepare the sections on pressure, vacuum
and small-diameter gravity systems. The result is a
document which contains the most advanced state-of-
the-art for each of these systems. These individual authors
are:
Pressure
William C. Bowne
Consulting Engineer
Eugene, Oregon
Vacuum
Richard Naret
Cerrone & Associates
Wheeling, West Virginia
(since 1991 - AIRVAC Inc.; Tampa, Rorida)
Small Diameter Gravity
Richard J. Otis
Ayres & Associates
Madison, Wisconsin
The materials developed by these experts have been
extensively reviewed and edited for clarification and
pertinence to a wide sector of the international user
community.
Although the above approach was conceived to provide
the best source of information, there are some negative
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facets to ft. The most obvious is the dearth of references
cfted. Since the knowledge of the authors is gained
through a variety of fugitive and personal experience
sources, and not to any great degree from literature
presentations of varying quality and accuracy, the use of
references has been minimal. For those who seek
additional information on certain topical areas, lists of
such references can be obtained through Reference 3
and the National Small Flows Clearinghouse in
Morgantown, West Virginia.
As with any document certain issues are not discussed
herein in any significant detail because of the need to
concentrate on U.S. practice. Examples of these topics
include flat-grade sewers which are used almost
exclusively in Nebraska and "simplified sewer systems"
which are used in Brazil, Ghana and other countries.
Both have some degree of similarity with small-diameter
gravity sewers and are described elsewhere.
1.1,3 Commonalities
Although each of the alternative sewer technologies use
very different motive forces, there are many
commonalities in choosing, designing, constructing and
maintaining all of them. Clearly, all use lightweight plastic
pipe buried at shallow depths, with fewer joints due to
increased pipe lengths, when compared to conventional
gravity sewers. Each has the ability to save significantly
on capital investment if properly designed and installed in
rural areas where their inherent advantages can be
exploited. All have suffered from some misuse and
misapplication in early installations, as have all new
technologies. The purpose of this manual is to provide
the Information which will minimize future problems of
this type.
A common need of all alternative collection systems
(ACS) is proper administration and management. Since
the needs of these technologies are different from
conventbnal sewers and treatment facilities, operating
and maintenance (O&M) staff members must be properly
trained In the particular needs of the type of system
employed,
A common concern with all ACS types is the shallow
burial depth, which increases potential for damage from
the ground surface, e.g., excavation projects. Good
management and design can minimize this problem by
inclusion of marking tape and toning wires in the trench
and surface markers which direct excavators to the O&M
staff for assistance in locating facilities. Quality as-built
drawings and, possibly, geographic information systems
(GIS) on software will prove invaluable for all of these
systems.
The other concern is for a larger on-lot activity than
normally experienced with conventional sewers.
Homeowner involvement in the planning process is a
requirement for success with any ACS project to minimize
the potential for subsequent damage to public relations
and to maximizethe potential support of the homeowners
forthe project. Similarly, the system staff representative(s)
will be considered the embodiment of management, and
must be able to relate positively to the public.
1.1.4 Evaluation Issues
Each section of the manual is concerned with a specific
ACS technology. Each cites a series of site conditions
which favors that technology over conventional gravity
sewers. Unfortunately, that list is very similar for all three
types. Considering the commonalities discussed above,
only a few site conditions clearly favor a given ACS over
the others. The reasons why each technology has been
chosen forthe installed systems discussed in the manual
is rarely, if ever, due to careful and comprehensive
evaluation of each technology and subsequent
comparison. Usually the engineer is familiar with one
type of ACS and attempts to do a comparison of it to
conventional sewerage. Depending on how well that is
performed, the availability and rules of financial assistance
programs and the municipality's desires, a system is
chosen
In reality, all alternative sewer systems should be
considered for municipalities of 10,000 people or less.
Those communities of 3,500-10,000 population can likely
handle all ACS technologies with proper training. Small
communities under 1,000 population are probably the
most restrictive in terms of available O&M capability.
Anything more mechanical than a small-diameter gravity
sewer (SDGS) with no lift station should be given another
level of scrutiny for most of these locations. Arrangements
with county government, private management entities or
other larger utilities may eliminate this O&M barrier for
even the smallest communities, permitting an
unconstrained choice of the optimum technology for
each community.
In such cases all ACS systems should be fairly evaluated,
and this manual allows that, since each expert author has
presented the data for their ACS system. There are very
few instances where one ACS would be eliminated from
consideration since each can be combined with another
form of collection to overcome site limitations.
The most common combination is that of SDGS with
septic tank effluent pumping (STEP) sewers, This
combination is sometimes called effluent sewers, since
both employ septic tank pretreatment. A conventional
gravity sewer and vacuum sewer combination isdescribed
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in the text, which saved a small community considerable
capital expenditure. Grinder-pump (GP) pressure systems
are commonly used with conventional gravity sewers to
reduce total costs. Combinations of ACS, other than
"effluent sewers," are more rare. Theoretically, both
SDGS and STEP could feed into a vacuum or a GP
sewer, but the reverse could have repercussions since
the former two are designed to carry a wastewater which
does not contain heavy solids and grease. Although
unsubstantiated, any pressure or SDGS termination in
avacuumsewershou Id probably be carefully investigated
to determine the need for some form of gaseous emission
odor control at the vacuum station. All ACS types are
compatible with conventional sewers if interfacing
precautions described in the manual are followed.
Cold-weather conditions have not been dealt with to a
great degree in the manual, but all three ACS types have
been successfully employed in Canada. The primary
difference lies in insulation of piping and mechanical
components from severe winter temperatures.
The most seriousomissionisthatoftreatment subsequent
to conveyance by an ACS. Both SDGS and STEP
systems yield an effluent which is for all intents and
purposes the same as septic tank effluent. It is quite
biodegradable, weaker in terms of suspended and
settleable solids and all matter associated with this
fraction, e.g., organics and grease, than conventional
raw wastewater. It is generally anaerobic, contains
reduced sulfur species and readily emits the H2S form
upon stripping.1-3 GP systems contain a highly
concentrated form of raw wastewater since infiltration
and inflow (I/I) are generally negligible.1'3 Vacuum sewers
are quite violent in their internal action yielding a
wastewater similar in strength to conventional raw
wastewater, but highly aerated in terms of dissolved
oxygen content.
Conservative engineers have designed ACS treatment
systems identicallyto those for conventional wastewater.
Given the rural nature of these systems, most treatment
facilities have been stabilization ponds which are
somewhat insensitive to the wastewater characteristic
variance between ACStypes. Some mechanical treatment
systems (extended aeration and oxidation ditches) have
been used without significant difficulty for larger ACS
installations. Also, some subsurface soil absorption
systems have been successfully employed for some
smaller ACS sites.
Readers are referred to numerous other textbooks for
further information on wastewater characteristics vs.
treatment plant design. Given the variable nature of
available ACS wastewater data, use of pilot studies may
be prudent if treatment more sophisticated than
subsurface soil absorption or stabilization ponds with
effluent polishing is contemplated.
1.1.5 Perspective
The timing of this manual is such that experience with the
first generation of ACS has been documented. Problem
areas have been identified, and solutions generally have
been attempted and documented. This manual conveys
these experiences in a practical manner which permits
the engineering profession to design reliable systems
which small communities can manage.
Indeed there are some unknown or unresolved issues in
ACS technology, but these should disappear with time.
None are considered serious enough to retard continued
application of these systems. This manual is intended to
stimulate consideration of ACS technology and minimize
its misuse where ft is inappropriate to the problem
solution.
1.2 Pressure Sewers
1.2.1 Description
Pressure sewers are an outgrowth of the Congressional
directive in Section 104(q) of the Clean Water Act of
1972, to develop wastewater systems where
implementation of conventional practices is impractical,
uneconomical, or otherwise infeasible. Pressure sewers
have emerged asoneof the most popular and successful
of the collection system alternatives.
A pressure sewer is a small diameter pipeline, shaltowly
buried, and following the profile of the ground. Typical
main diameters are 5 cm (2 in) and 15 cm (6 in). Polyvinyl
chloride (PVC) is the usual piping material. Burial depths
usually are below the frost line, or a 75-cm (30-in)
minimum, whichever isgreater. In northern areas insulated
and heat-traced piping offer relief from these criteria.
Each home uses a small pump to discharge to the main.
This may be a grinder pump (GP), which grinds the solids
present in wastewater to a slurry in the manner of a
kitchen sinkgarbage grinder, or a septic tank and effluent
pump (STEP) system may be used. The septic tank of a
STEP system captures the solids, grit, grease, and
stringy material that could cause problems in pumping
and conveyance through the small diameter piping.
Grinder pumps to serve individual homes are usually 2 hp
in size, but 1-hp units are also used. Some installations
use 3-5 hp motors, but these are usually used when
serving several homes with one pumping unit. STEP
pumps are usually fractional horsepower.
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Theservlce line leading from the pumping unfttothe main
is usually 25-38 mm (1-1.5 in) diameter PVC. A check
valve on the service line prevents backflow, which is
insured with a redundant checkvalve at the pumping unit.
If a malfunction occurs, a high liquid level alarm is
activated. This may be a light mounted on the outside
wall of the home, or it may be an audible alarm which can
be silenced by the resident. The residentthen notifies the
sewer service district which responds to make the
necessary repair.
Installation of a pressure sewer main is shown in Figure
1 -1. General sketches showing GP and STEP installations
are shown in Rgures 1-2 and 1-3.
1.23 Potential Applications
The primary reason for the use of pressure sewers is
economic, but in some cases the decisions are
environmentally motivated.
In areas where rock is encountered when excavating to
install mainline sewers, pressure sewers can be cost
effective. The deep, wide trenches required to install
conventional sewers are expensivetoconstruct. Pressure
sewers require only shallow, narrow trenches.
Where groundwater is high, the deep excavations for
conventional sewers may enter that groundwater. In
some cases dewatering is not achievable. When
conventional sewers are installed undertheseconditions
the cost is high and the quality of installation is
questionable. Shoring can also add considerably to the
cost of conventional sewers.
Some topography does not favor gravity collection. One
typical example is around lakes where the homes are
built fronting the lake. The road serving these homes, and
often the only practical location for the sewer, may be
upstope from the homes. The profile of that route may go
up and down as it circles the lake; numerous and costly
pumping stations would be necessary if conventional
sewers were used.
Conventional sewers have a high cost per foot of sewer
installed. Where homes are sparse, the resulting cost
can be exorbitant. Pressure sewers can be installed less
expensively on a perfect basis.
Extremely flat terrain poses a problem to gravity sewer
installations since the gravity sewer must continually
stope downward. This causes the sewer to become
increasingly deep u ntii a lift station is necessary. Both the
deep excavations and the lift stations are expensive, and
the latter represents a considerable operation and
maintenance (O&M) expense.
Damage consequential to the installation of deep sewers
is a factor. In some cases blasting is required to install
sewers. This may cause upheaval of the road, damage
to nearby buried utilities and homes, and disruption to the
community. Deeply buried conventional sewers may
intercept and drain groundwater. In many cases the
groundwater will enter the gravity sewer as unwanted
infiltration.
Developments experiencing slow growth find pressure
sewers economically attractive. The front-end
infrastructure (mainline) is inexpensively provided. The
cost of the pumping units is deferred until the homes are
built and occupied. The cost for the pumping units may
also be financed with the home. Time value of money
considerations make this feature particularly attractive.
Pressure sewer equipment is also used in conjunction
with conventional systems. Where a low lying home or
basement is too low to allow gravity flow into a fronting
conventional sewer, a grinder pump or pressure sewer
type solids handling pump may be used at that home to
discharge to the sewer. Similarly, STEP units are used to
discharge to high lying drainfields, sand filters, mounds,
and other forms of on-s'rte wastewater disposal.
1.2.3 Extent of Use in the United States
The pressure sewer market size is large and growing. No
comprehensive lists have been kept to document pressure
sewer projects, but hundreds of systems are known to
exist throughout the United States. Locations of major
projects range from Florida to Alaska, and from Texas to
New York.
Many of these systems serve 50-200 homes. A few
systems serve over 1,000 homes each, and some systems
are now being designed that will serve over 10,000
homes. A few examples of larger projects are:
Horseshoe Bay, TX GP
Kingsland, TX GP
Saw Creek, PA GP
Anne Arundel Co., MD GP
Port St. Lucie, FL STEP
Buckeye Lake, OH STEP
Palm Coast, FL STEP
1,700 homes served
1,600 homes served
1,800 homes served
1,500 homes served
3,000 homes served
1,500 homes served
650 homes served
(planned for 20,000 homes to be served ultimately)
The use of pressure sewer components to serve low lying
homes fronting gravity sewers is substantial, but no
records havebeen kept to document the extent. Pressure
sewer components used with on-site disposal practices
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Figure 1-1.
Installation of pressure sewer main.
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*-'p.j«sfe_--ejj » «• 'M- * ••«• ,«4-
^SK^VI^'-'-l!*'**.-.
CVl?
^i^iSlir.'-. ^ai- ^ ,
fe^V^^gSgbfe^.J
-------
Figure 1-2.
Grinder Pump (GP) system.
1 CONTROL PANEL
2 BURIED ELECTRICAL CABLE
3 ELECTRICAL JUNCTION BOX
4 SEWAGE FLOW FROM HOME
5 PLUMBING DISCONNECT
6 SHUTOFF VALVE
7 SERVICE LINE TO MAIN
8 LEVEL SENSORS
9 CHECK VALVE
10 GRINDER PUMP
FIfluro 1-3.
Soptlc Tank Effluent Pump (STEP) system.
OTHER COMPONENTS
' SIMILAR TO
QR1NDERPUMP
-------
has become commonplace. Canada also uses pressure
sewers, as do several European and Asian countries.
Considering that pressure sewer technology emerged in
the late 1960s, the practice has indeed grown quickly.
1.2.4 Myths vs. Reality
Pressure sewers should not beglamorized as a panacea.
The endeavor should be to usethe technology appropriate
for the setting. If the appropriate technology is the use of
conventional sewers, orthe use of septic tank - drainfield
systems, that should be used. Conventional practices
are mature, well understood, and well accepted. Because
of particular grant funding conditions there have been
cases where the inappropriate use of alternative
technologies has occurred.
It has been a common error for people to learn of
excellent operating performance from especially well
designed and well built pressure sewer systems, and,
oddly, to expect the same performance from a shoddy
installation. Too often engineers inexperienced with the
technology have been employed. Components have
often been chosen without their having demonstrated
competence. Inspection has frequently been inadequate.
The result is a poor system, likely to be replaced in the
near future, and poor reputation gained undeservedly for
the pressure sewer concept.
Experience specific to pressure sewers is vital to provide
a good installation. A small system should be built first,
preferably with guidance from experienced people. Then,
performance of the operating system should be closely
observed to close the loop between planning, design,
construction, and long-term O&M.
The attitude and talent of the district owning and operating
the system are major factors. If the maintenance forces
orthe management reluctantly accept pressure sewers,
or do not have the ability to work with new concepts, the
project will probably be a failure.
A frequently held misunderstanding is that pressure
sewers are inherently maintenance intense. Experience
has not supported that opinion. Well designed pressure
sewers, made easy to maintain by design and attended
by qualified personnel, have been relatively easy to
maintain. However, they are not tolerant of withheld
maintenance, and incorrect operation and maintenance
can be worse.
The engineer and the district must be willing to interface
closely with the homeowners, and personnel assigned to
the task must be knowledgeable and skillfully diplomatic.
Each installation causes disruption to the homeowners
yard and inconvenience to them personally. The time
required for public relations is usually poorly conceived
and underestimated.
1.3 Vacuum Systems
1.3.1 History of Vacuum Sewer Technology
1.3.1.1 System Types
Vacuum sewer collection systems were patented in the
United States in 1888, when Adrian LeMarquand invented
a system of wastewater collection by barometric
depression.4 The first commercial applications of such
systems were by the Liljendahl Corporation (now known
as Electrolux) of Sweden in 1959.5 Since that time, three
other companies have been active in this market: Colt-
Envirovac, Vac-Q-Tec, and AIRVAC. There are significant
differences among these in terms of design concepts.
The major differences lie in the extent to which the
systems use separate black (toilet) and gray (the balance)
water collection mains. Electrolux uses a separate system
for these sources; Envirovac uses vacuum toilets and
one main; and AIRVAC and Vac-Q-Tec take the normal
household combined wastes. Other differences relate to
the location of the gravity/vacuum interface and to the
design of pumps, valves, lines, etc.
The Liljendahl-Electrolux system (Figure 1-4) was first
used in the Bahamas in the 1960s. In this concept,
separate black and gray water collection mains are used.
The black water is discharged to one of the vacuum
mains through a vacuum toilet (Figure 1 -5) whilethegray
water enters the other through the use of a specially
designed vacuum valve. The separate vacuum mains
are connected to the vacuum station. For critically water-
short areas, such as the Bahamas, the reduction in toilet
wastewater volume was a definite factor in the selection
of vacuum transport.1 The Bahama system was removed
from service in 1990.
A Vac-Q-Tec system, serving the Lake of the Woods
development near Fredericksburg, Virginia, was the first
residential vacuum collection system inthe United States.
This system uses concepts of the Liljendahl system but
has many important differences.6The Vac-Q-Tec system
requires no inside vacuum toilets or vacuum plumbing.
This system employs a single combined black and gray
water collection main. Large (2,840-L [750-gal]) storage
tanks are required at each residence. Finally, an external
power source is required for each valve since they are
electrically operated. In addition to the Lake ofthe Woods
system, several other Vac-Q-Tec residential systems
have been used by private developers.
7
-------
Figure 1-4. UIJondahl-EloctroIux vacuum sewer system.
i rvMNorvn i ruorvc
VACUUM PUMP—
r— BLACK WATER
COLLECTION TANK
AIR m
I
EWAGE
±
.1
I
r
J
I
Cl
\
.EANOUT
GRAY WATER
i
)
5
f
- VACUUM TOILET •
r BLACK WATER
VACUUM MAIN
CLEANOUT
mm
1
V
-Ji
J
mtam
J
mmm
•s
•
, BUFFER
^^ VOLUME
I— GRAY WATER VALVE — *
PUMPS
I""* i^ TO TREATMENT FACILITIES
Figure 1-5. Vacuum toilet.
VACUUM TO! LET
FLUSHING
MECHANISM
DISCHARGE VALVE
8
-------
The Cort-Envirovac system is the direct descendent of
the Liljendahl-Electrolux system (Figure 1-6). The Colt
system at South Seas Plantation near Fort Meyers,
Florida, served 33 residences. The houses had separate
black and gray water plumbing. The black water piping
from the vacuum toilet joined the gray water piping
immediately downstream of the gray water valve. A
single pipe with the combined contents transported the
wastewater to the vacuum station. The South Seas
Plantation system was removed from service a few
years after installation.
AIRVAC markets a pneumatically controlled and
operated vacuum valve which is used for combined grey
and black water systems (Figure 1-7). The AIRVAC
system allows for use of conventional plumbing in the
house, with the wastewater flowing by gravity to a
combined sump/valve pit. The valve starts its cycle
when it senses that approximately 38 L (10 gal) has
accumulated in the sump. It opens for a few seconds,
which is enough to evacuate the contents of the sump
as well as to allow atmospheric air to enter the system.
The wastewater/air mixture then travels to the vacuum
station.
AIRVAC's first system was installed in Mathews
Courthouse, Virginia, in 1970. Since then AIRVAC has
more than 35 additional systems operating in the United
States with many morecurrently being planned, designed,
or in construction. AIRVAC has also been very active in
the foreign market with operating systems in Australia,
Canada, Japan, Holland, and some other European
countries.
1.3.1.2 System Comparison
Each of the four systems has unique design features.
The major differences between these systems are
shown in Table 1-1. The water-saving feature of the
Electrolux and Colt systems is reported to be as much
as 27 percent of the total in a domestic application with
the use of vacuum toilets.7J9 AIRVAC and Vac-Q-Tec
systems can be altered to accommodate these and
other water-saving devices.
a. Services
Vacuum valves operate automatically, based on the
volume of wastewater behind the valve. Provided that
sufficient vacuu m is available in the main, the valves will
open after a predetermined volume of wastewater has
accumulated. Wastewater enters the mains through
these valves, followed by a volume of atmospheric air.
The valve is actuated by a pneumatic controller in all
systems except the Vac-Q-Tec system.1
TheVac-Q-Tec'sgrav'rty-vacuuminterfacevalveassembly
is unique in that it requires an external power source.1 The
valve can be monitored and operated from the vacuum
station through an extra set of contacts in the controller. A
separate cycling mode, called AutoScan, can be added,
which offers flexibility to the Vac-Q-Tec system, this
mode locks out the accumulated volume-cycle command
from each valve, and subsequently operates each valve
during low-flow periods. This flexibility allows the system
to store flows during peak periods and release them later
during low flow periods. All of the other systems must be
designed to handle peak flows. This f eatu re does, however,
add costs to the base system. Also, additional operating
and skilled electronicstechnicians are required to maintain
these complex systems.1
Depending on the manufacturer, the amount of water
entering the system with each valve operation varies. The
vacuum toilet admits approximately 1.1-1.5 L (0.3-0.4
gal)/flush, whereas the pneumatically controlled vacuum
valves admit 38-57 L (10-15 gal)/cycle.1
U.S. Navy research10 has reported that good transport
characteristics are found with sufficient inlet air and small
enough slug loadingsforthe available pressuredifferential
to overcome the liquid's inertia. This results in rapid slug
breakdown, re-establishing vacuum quickly at upstream
valves.
b. Collection Piping
Piping profiles differ, depending on uphill, downhill, or
level terrain. The pipe profiles recommended by each
manufacturer also differ. Only AIRVAC offers a complete
piping design program at this time.11
There have been two different concepts used in vacuum
design. In the first concept, the bore of the pipe is
purposely sealed during static conditions. This is
accomplished through the use of reformer pockets. In the
other concept, the bore of the pipe is not sealed. The
reformer pocket concept has been used by all four of the
manufacturers, although AIRVAC has since changed,
with all of their recent systems being designed using the
latter principle.11
All systems use PVC pipe. Both solvent-weld and gasketed
O-Ring pipe have successfully been used.
c. Vacuum Station
Vacuum stations, sometimes referred to as collection
stations, vary from manufacturer to manufacturer. Table
1-2 shows the varying design parameters of each type.
Electrolux and Colt vary their use of vacuum reserve tanks
with each installation, while Vac-Q-Tec and AIRVAC
-------
Figure 1-6. Colt-Envlrovac vacuum sewer system.
GRAY
WATER
VALVE
(
1
I
/
^
s
\
|s
j"
*^ «x
M\/Ar"H H IM T/"*ll crT"«
CLEANOUT
1
i.
. JJ
^f
J
k
1
X VACUUM PUMP-
COLLECTION TANK— i : !
AIR .
CLEANOUT
JJ
r*
*
""™t
SEWAGE PUMP-
TO TREATM
•r-
f
ENT-
TRANSPORT POCKET
VACUUM MAIN
FACILITIES
Figure 1-7. A1RVAC vacuum sewer system.
ft
1
k
*r
\
}
iA>
\,
fr
f
*"» ^
^
1
V
X*
^f
J
bA,
^,
s?
**»
RESEF
COLLECTION 1
J*
VACUUM
VALVE/SUMP '
LIFT
• VACUUM MAIN
VACUUM PUMP
SEWAGE PUMP.
TO TREATMENT FACILITIES •
10
-------
Table 1-1.
System Type
Electrolux
Colt-Envirovac
Vac-Q-Tec
AlRVAC
Table 1-2.
System Type
Electrolux
Colt-Envirovac
Vac-Q-Tec
AlRVAC
Table 1-3.
System
Type
Electrolux
Colt-Envirovac
Vac-Q-Tec
AlRVAC
Vacuum Collection System Parameters
House Piping Valve Type Piping Profile
Black and gray Black: vacuum Set configuration
separate toilets; gray: with traps
pneumatic valves
Black and gray Black: vacuum Set configuration
separate toilets; gray: with traps
pneumatic valves
Conventional Electrically actuated, Parallels terrain
plumbing pneumatic valve with traps
Conventional Pneumatic Set configuration
plumbing valve with profile
changes
Vacuum Station Parameters
i
Receiving Tank
Receiving Tank Evacuation Device
Separate black and Sewage pumps
gray water vessels.
Reserve tank use
varies by installation.
Common receiving Sewage pumps
vessel. Reserve
tank use varies.
One receiving Pneumatic ejectors
vessel plus
reserve tank.
One receiving Sewage pumps
vessel plus
reserve tank.
Summary of Vacuum System Types
Target No. U.S.
Market Current Status Systems
Residential Sold license 0
to Colt in 1970s
Shipbuilder Now a 5
Industrial subsidiary of Evak
Residential Ceased operation 5
in recent years
Residential Active 30
Collection Line
Black: 1-1/2"
& 2"; gray: 2" & 3;
PVC solvent weld
1 Single main, 3",4",
& 6"; PVC, special
"O" ring
Single main, 4*;
PVC Solvent Weld
Single main, 4", 6",
& 8; PVC, solvent
weld or "O" ring
Valve Monitoring and
Control Capability
No
No
Yes
! No
Design Approach
In-house
No design manual
In-house
No design manual
In-house
No design manual
Published design
manual
11
-------
always use reservoir tanks between the collection tank
and the vacuum pumps.1
1.3.1.3 Summary
Four manufacturers have played a major role in the
development of vacuum sewer systems. There are
significantdifferences in overall system philosophy, design
concepts, system components, and marketing
approaches (Table 1-3). While all four were active 20
years ago in the United States, only AIRVAC has continued
to place residential systems into operation on a regular
basis. Some of the early systems of Colt-Envirovac and
Vac-Q-Tec are currently being retrofitted with AIRVAC
valves.
1.33 Simplified System Description
1.3.2.1 Basic System Sketch
Figure 1 -8 shows the basic vacuum sewer system layout,
including the major components. This layout is based on
an AIRVAC type of system since it is the most common.
1.3.2.2 Components
A vacuum sewer system consists of three major
components: the vacuum station, the collection piping,
and the services. Each is described below;
a. Services
Wastewater flows by gravity from one or more homes
into a 114-L (30-gal) holding tank. As the wastewater
level rises In the sump, air is compressed in a sensor tube
which is connected to the valve controller. At a preset
point, the sensor signals for the vacuum valve to open.
The valve stays open for an adjustable period of time and
then closes. During the open cycle, the holding tank
contents are evacuated. The timing cycle is field adjusted
between 3 and 30 seconds. This time is usually set to
hold the valve open for a total time equal to twice the time
required to admit the wastewater. In this manner, air at
atmospheric pressure is allowed to enter the system
behind the wastewater. The time setting is dependent on
the valve location since the vacuum available will vary
throughout the system, thereby governing the rate of
wastewater flow.
The valve pit typically is located along a property line.
AIRVAC'S valve pit/holding tank arrangement (Figure 1-
9} isusually madeof fiberglass, although modified concrete
manhole sections have been used for special situations
(deep basements, large user, pressure/vacuum interface,
etc.). A non-traffic lightweight aluminum or cast iron lid is
available for yard installations. Where the installation will
be subjected to vehicular loading, a flush-mounted cast
iron lid is used. An anti-flotation collar may be required in
some cases.
b. Collection Piping
The vacuum collection piping usually consists of 15-cm
and 10-cm (6- and 4-in) mains, although more recent
installations also include 25-cm (10-in) mains in some
cases. Smaller (7.5-cm [3-in]) mains used in early vacuum
systems are no longer recommended, as the cost savings
of 7.5-cm vs. 10-cm (3-4 in) mains are considered to be
insignificant.
Both solvent welded PVC pipe and rubber gasketed pipe
have been used, although past experience indicates that
solvent welding should be avoided when possible. Where
rubber gaskets are used, they must be certified by the
manufacturer as being suitable for vacuum service. The
mains are generally laid to the same slope as the ground
with a minimum slope of 0.2 percent. For uphill transport,
lifts are placed to minimize excavation depth (Figure 1-
10). There are no manholes in the system; however,
access can be gained at each valve pit or at the end of a
line where an access pit may be installed. Installation of
the pipe and fittings follows water distribution system
practices. Division valves are installed on branches and
periodically on the mains to allow for isolation when
troubleshooting or when making repairs. Plug valve and
resilient wedge gate valves have been used.
c. Vacuum Station
The vacuum station is the heart of the vacuum sewer
system.11 It is similar to a conventional wastewater
pumping station. These stations are typically two-story
concrete and block buildings approximately 7.5 m x 9 m
(25 x 30 ft) in floor plan. Equipment in the station includes
a collection tank, a vacuum reservoir tank, vacuum
pumps, wastewater pumps, and pump controls (Figure
1-11). In addition, an emergency generator is standard
equipment, whether it is located within the station or
outside the station in an enclosure or is of the portable,
truck-mounted variety.
The collection tank, made of either steel or fiberglass, is
the equivalent of a wet well in a conventional pumping
station. The vacuum reservoir tank is connected directly
to the collection tank to prevent droplet carryover and to
reduce the frequency of vacuum pump starts andthereby
extend their life. The vacuum pumps can be either liquid
ring or sliding vane type. These pumps are usually sized
for 3-5 hr/d run-time. The wastewater discharge pumps
are non-clog pumps with sufficient net positive suction
head to overcome tank vacuu m. Level control probes are
installed in the collection tank to regulate the wastewater
pumps. Vacuum switches on the reservoir tank regulate
the vacuum pumps. A fault monitoring system alerts the
system operatorshould a low vacuum or high wastewater
level condition occur.
12
-------
Figure 1-8. Major components of a vacuum sewer system.
VACUUM MAIN #2
VACUUM MAIN 03
vmmazzx-mm
o o
9999,
6 3
9 9 9
.9 9 <* 9
6 6 6
H
VACUUM STATION
DIVISION VALVE
VALVE PIT
BUILDING SEWER
HOUSE
. •
VACUUM MAIN #1
3° VACUUM SERVICE LINE
13
-------
Figure 1-9. AIR VAC valve pll/sump arrangement
///=///=/.
VALVE CONTROLLER
ANTI-FLOTATION COLLAR
GRAVITY SEWERS
FROM 1-4 HOMES
,TRAFFIC OR NON-TRAFFIC COVER AVAILABLE
,MASS CONCRETE
INTERFACE VALVE |l /FIBERGLASS VALVE PIT
3" NO-HUB COUPLING (2)
TO VACUUM MAIN
CAP BONDED
TO STUB OUT
FIBERGLASS SUMP
14
-------
Figure 1-10. Upgrade/level/downgrade transport. (Reprinted courtesy of AIRVAC)
VACUUM MAIN
45° ELBOWS
UPGRADE TRANSPORT
///=///=///=///=///=///=/{/=///.=///;=
500 FT. TYPICAL
FLOW
LEVEL GRADE TRANSPORT
FLOW
DOWNGRADE TRANSPORT
15
-------
Rgura 1-11.
Diagram of a typical vacuum station. (Reprinted courtesy of AIRVAC)
VACUUM
PUMP
EXHAUST
*
POWER
VENTILATOR
FOR TOP
FLOOR
///=///=
FORCE MAIN
TO TREATMENT
PLANT
CONTROL
PANEL
o
0 S D
D g fl
o
o
o
o
VACUUM
PUMPS (2)
VACUUM
RESERVOIR
TANK
EQUALIZING
LINES (2)
SEWAGE
COLLECTION
TANK
VACUUM
GAUGE
ISOLATION
VALVE
=•/'/=//
VACUUM
SEWERS
PROBESi
16
-------
1.3.2.3 Operation
Vacuumor negative-pressure sewer systemsusevacuum
pumps at central collection stations to evacuate air from
the lines, thus creating a pressure differential.11 in negative
pressuresystems, a pneumatically operated valve serves
as the interface between the gravity system from the
individual user and the vacuum pipelines. Pressure
sensors in a wastewater holding tank open and close the
interface valve to control the flow of wastewater and air
into the vacuum system.
The normal sequence of operation is as follows:
• Wastewaterfromtheindividualserviceflowsbygravity
to a holding tank.
• As the level in the holding tank continues to rise, air is
compressed in a small diameter sensor tube. This air
pressure is transmitted through a tube to the controller/
sensor unit mounted on top of the valve. The air
pressure actuates the unit and its integral 3-way valve
which allows vacuum from the sewer main to be
applied to the valve operator. This opens the interface
valve and activates a field adjustable timer in the
controller/sensor. After a set time period has expired,
the interface valve closes.11 This happens as a result of
the vacuum being shut off, allowing the piston to close
by spring pressure.
* Thewastewaterwithinthevacuumsewerapproximates
the form of a spiral rotating hollow cylinder traveling at
38-45 cm/s (15-18 fps). Eventually, the cylinder
disintegrates from pipe friction, and the liquid flows to
low points (bottom of lifts) in the pipeline.
* The next liquid cylinder and the air behind it will carry
the liquid from the previously disintegrated cylinders up
overthe sawtooth lifts designed into the system. In this
manner, the wastewater is transported over a series of
lifts to the vacuum station.
The principles of operation of a vacuum sewer system
are not completely understood. An early concept was
that of liquid plug flow. In this concept, ft was assumed
that a wastewater plug completely sealed the pipe bore
during static conditions. The movement of the plug
through the pipe bore was attributed to the pressure
differential behind and in front of the plug. Pipe friction
would cause the plug to disintegrate, thus breaking the
vacuum. With this being the situation, reformer pockets
were located in the vacuum sewer to allow the plug to
reform and thus restore the pressure differential (Figure
1-12). In this concept, the re-establishment of the pressure
differential for each disintegrated plug was a major
design consideration.
In the current design concept, the reformer pockets are
eliminated so that the wastewater does not completely fill
or "seal" the pipe bore. Air flows above the liquid, thus
maintaining a high vacuum condition throughout the
length of the pipeline (Figure 1-13). In this concept, the
liquid is assumed to take the form of a spiral, rotating,
hollow cylinder. The momentum of the wastewater and
the aircarries the previously disintegrated cylinders over
the downstream sawtooth lifts. The momentum of each
subsequent air/liquid slug and its contribution to the
progressive movement of the liquid component of the
previous slugs are the major design considerations.
Both of the above design concepts are approximations
and oversimplifications of a complex, two-phase flow
system. The character of the flow within the vacuum
sewer varies considerably. The plug flow concept is
probably a reasonable approximation of the flow as it
enters the system, whereas the progressive movement
concept is probably a better approximation of the flow
throughout the vacuum main.
The significance of the air as a driving force cannot be
overemphasized. The atmospheric air expands within
the vacuum sewer, thusdriving the liquid forward. The air
affects not only the liquid in the associated air/liquid slug,
but also the liquid downstream.
1.3.3 Potential Applications
Below are the general conditions that are conducive to
the selection of vacuum sewers.
• Unstable soils
• Rat terrain
• Rolling terrain with small elevation changes
• High water table
• Restricted construction conditions
• Rock
• Urban development in rural areas
Experience has shown that for vacuum systems to be
cost effective, a minimum of 75-100 customers is needed
per custom vacuum station. Package vacuum stations
have proven to be cost-effective for service areas of 25-
150 customers. The average number of customers per
station in systems presently in operation is about 200-
300. There are a few systems with fewer than 50 and
some with as many as 2,000/station. There are
communities which have multiple vacuum stations, each
serving hundreds of customers.
Hydraulically speaking, vacuum systems are limited
somewhat by topography. The vacuum produced by a
vacuum station is capable of lifting wastewater 4.5-6 m
(15-20 ft), depending on the operating levelof the system.
17
-------
Figure 1-12. Early design concept- reformer pockets.
FLOW
-REFORMER POCKET
Rgure 1-13, Current design concept - pipe bore not sealed.
AiR SPACE
• SEWAGE AT REST
18
-------
This amount of lift many times is sufficient to allow the
designer to avoid the lift station(s) that would be required
in a conventional gravity system.
1.3.4 Extent of Use in the United States
Table 1 -4 shows the operating residential vacuum sewer
systems in the United States as of January 1990. There
are another dozen or so presently in the construction
phase, with more being planned and designed.
In addition to the above residential systems, several
industrial facilities use vacuum systems to collect
wastewater.3 These companies include the Scott Paper
Company pulp and paper mill in Mobile, Alabama, with 25
AIRVAC valves; Stauffer Chemical Company in Baton
Rouge, Louisiana, with 7 AIRVAC valves; and Keystone
Steel and Wire Company in Peoria, Illinois, with 29
AIRVAC valves. ENVIROVAC type systems using
vacuum toilets are used in remote construction camps
and park restroom facilities, and along with another
vacuum system manufacturer, Jered Industries, in many
shipboard installations. These types of installations are
beyond the scope of this report and will not be addressed.
1.3.5 Myths vs. Reality
Many myths exist concerning vacuum sewer systems. In
reality, a vacuum system is not unlike a conventional
gravity system. Wastewater flows from the individual
homes and utilizesgravfty to reach the point of connection
to the sewer main. The equipment used in the vacuum
station is similar in mechanical complexity to that used in
a conventional lift station. The most common myths
concerning vacuum sewer technology are discussed
below.
MYTH: Vacuum sewers are only to be considered where
flat terrain exists.
REALITY: Vacuum sewers should be considered in
level, downhill, and uphill terrain. The practical limit of
Table 1-4. Operating Vacuum Systems in the United Slates
Project Name
Martingham
Foxcliff Estates
Country Squire Lakes
Mathews Courthouse
Plainville
Eastpoint
Westmoreland
Fallen Leaf Lake
Fairmont
Quean Anne's County
LaFargeville
Charlotte
Ohio Co. - Cedar Rocks
Ohio Co. - Peters Run
Ohio Co. - Short Creek
Friendly PSD
Central Boaz PSD
Red Jacket PSD
Washington Lands PSD
Cedar Grove
Lake Chautauqua
Lag Marina
Emmonak
Swan Point
Alton
White House
Morrlstown
Lake Manitou
Thflfa%fi
1 1 lol CTdct
Sanford
Claywood Park
New Cumberland
Big Sandy
Lanark Village
Pattersontown
Beallsville
Salmon Beach
Noorvik
Big Bear Lake
Centertown
Stafford Township
Ocean Pines
Lake of the Woods
Shipyard Plantations
Palmetto Dunes
Captain's Cove
Project Location
St Michaels, MD
Marti nsville, IN
North Vemon, IN
Mathews, VA
Plainville, IN
Eastpoint, FL
Westmoreland, TN
South Lake Tahoe, CA
Somerset County, MD
Queen Anne's Co., MD
LaFargeville, NY
Charlotte, TN
Wheeling, WV
Wheeling, WV
Wheeling, WV
Friendly, WV
Parkersburg, WV
Red Jacket, WV
Washington Lands, WV
Lexington Park, MD
Celeron, NY
Norfolk, VA
Emmonak, AK
Swan Point, MD
Alton, KY
White House, TN
Morristown, NY
Rochester, IN
Thoroea NY
1 1 ITO oa<3, IV I
Sanford, FL
Parkersburg, WV
New Cumberland, WV
Charleston, WV
Lanark Village, FL
Pattersontown, FL
Beallsville, PA
Puget Sound, WA
Noorvik, AK
Big Bear Lake, CA
Centertown, KY
Manahawkin, NJ
Berlin, MD
Locust Grove, VA
Hilton Head Island, SC
Hilton Head Island, SC
Greenback, VA
System Type
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRV&f^
/sin v/^w
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
ENVIROVAC
ENVIROVAC
ENVIROVAC
ENVIROVAC
VAC-Q-TEC
VAC-Q-TEC
VAC-Q-TEC
VAC-Q-TEC
VAC-Q-TEC
vertical lift, although experimental systems are being
tested which may increase the feasible vertical lift limit.
MYTH: Vacuum sewers should not be considered when
the potential for gravity flow exists.
REALITY: Many times a broad view of an area's terrain
automatically rules out vacuum sewers as an alternative
to be considered. However, a closer look may reveal
many small advantages, that, when considered
collectively, add up to a significant savings.
19
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An example of this occurred In the Ohio County PSD-
Peters Run project in Wheeling, West Virginia. In that
project, it only seemed logical to the designer to use
conventional gravity sewers. The area was rural with
residential development following a creek. However,
upon closer inspection, it was evident that the gravity
main would be required to cross the creek in various
places, since the development was on both sides. With
the creek bank being 3-m (10-ft) deep and the creek
crossing requiring 1 m (3 ft) of cover, the gravity sewer
would have been 4-m (13-ft) deep for most of its length
{Rgure 1-14). At the terminus of the system, a lift station
was needed to pump the wastewater to a plant, which
was located above 100-yr flood elevation.
By utilizing vacuum, the designer used "lifts" to raise the
main above the bedrock level to a depth of 1.2-1.5 m (4-
5 ft) (Rgure 1-15). The vacuum station that was required
was nothing more than the lift station that was required
In the gravity layout, with the exception of the addition of
vacuum pumps. This additional expense was more than
offset by the savings of the line installation. The
"inexpensive" conventional gravity system would have
required deep, difficult excavations with much rock. The
vacuum alternative had much shallower excavations
w'rth little rock. In essence, the vacuum system was
installed as a "vacuum assisted-gravfty sewer" with
significant cost savings.
wear are very inexpensive. A vacuum valve and controller
can be rebuilt for about $30. Rebuild frequency is5-1 Oyr.
MYTH: The vacuum pumps must run 24 hr/d to keep
vacuum on the system.
REALITY: The typical vacuum station is designed so that
the vacuum pumps operate about 3-5 hr/d.
MYTH: It takes a tremendous amount of energy to keep
constant vacuum on the systems.
REALITY: The average sized vacuum station contains
20-hp vacuum pumps. Considering a run-time of 5 hr/d
and the cost of electricity at $0.08/kWh, the cost of power
for the vacuum pumps is about $185/month, A system
this size can and typically does serve 200-300 customers.
MYTH: The operation of a vacuum system requires a
person with a college degree.
REALITY: Any person that is mechanically inclined can
operate a vacuum system. Most of the systems in
operation in the U.S. have operators with no more than
a high school education.
MYTH: Since vacuum sewers are mechanized, they
undoubtedly are unreliable.
REALITY: Early vacuum systems were not without their
problems. However, component improvements, design
advancements, and experience with thetechnology have
resulted In systems that are very reliable.
MYTH: Vacuum sewers are operation and maintenance
intensive.
REALITY: In general, vacuum sewers maybe less costly
to construct than conventional sewers, but may be more
expensive to operate and maintain. However, the
magnitude of the O&M effort has been greatly overstated.
This ts due largely to the little historical data that exist
coupled with the conservative nature of most engineers.
MYTH: Replacement parts are expensive.
REALITY: The components of the vacuum station are
not unlike those of a conventional pumping station. The
small parts of the vacuum valve that are subjected to
MYTH: If the vacuum valve fails, wastewater will back up
into my house.
REALITY: Vacuum valves can fail in either the open or
closed position. One failing in the closed position will
result in backups. This would be analogous to a blockage
or surcharging of a gravity sewer. Fortunately, failure in
this mode is rare. Almost all valve failures happen in the
open position. This means that the vacuum continues to
try to evacuate the contents of the pit. The vacuum
pumps usually run continuously to keep up, as this failure
simulates a line break. In these cases, a telephone dialer
feature available in vacuum stations notifies the operator
of this condition. Correction of the problem can generally
be made in less than an hour after the operators arrive at
the station.
In short, many of the major objections to the use of
vacuum systems are not well founded. These systems
have been acceptable in a variety of applications and
locations. Any hypothetical or abstract difficulty that can
be applied to the vacuum system can also be applied to
the moreconventional systems. In any event, the vacuum
system offers the same convenience as any othertype of
20
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Figure 1-14. Gravity sewer system example.
MANHOLE'
GRAVITY SEWER
t
. BEDROCK
Figure 1-15. Vacuum-assisted gravity sewer system example.
BEDROCK
21
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public sewer system with reference to the actual discharge
from the home and meeting the needs of the particular
locality.
1.4 Small Diameter Gravity Sewers
1.4.1 Description
Smatldiametergrav'rty sewers (SDGS) are rapidly gaining
popularity in unsewered areas because of their low
construction costs. Unlike conventional sewers, primary
treatment is provided at each connection and only the
settled wastewater is collected. Grit, grease and other
troublesome solids which might cause obstructions in the
collector mains are separated from the waste flow and
retained in interceptor or septic tanks installed upstream
of each connection (Rgure 1-16). Wfththe solids removed,
the collector mains need not be designed to carry solids
as conventional sewers must be.
Large diameter pipes designed with straight alignment
and uniform gradients to maintain self-cleansing velocities
are not necessary. Instead, the collector mains may be
smaller in diameter, laid with variable or inflective
gradients. Fewer manholes are used and most are
replaced by cleanouts except at major junctions to limit
Infiltration/inflow (I/I) and entry of grit. The required size
and shape of the mains is dictated primarily by hydraulics
ratherthansolidscarryingcapabilrtiesaswithconventional
gravity sewers.
Designers must still, however, be cognizant of I/I and
ultimate growth in sizing these systems. Construction
costs are reduced because SDGS may be laid to follow
the topography more closely than conventional sewers
and routed around most obstacles within their path
without installing manholes. The interceptor tanks are an
Integral part of the system. They are typically located on
private property, but usually owned or maintained by the
utility districts so that regular pumping to remove the
accumulated solids for safe disposal is ensured.
SDGS were first constructed in Australia in the 1960's.
They were used to provide a more cost effective solution
than conventional gravity sewers to correct problems
with failing septic tank systems in densely developed
urban fringe areas. The SDGS were designed to collect
the effluent from existing septic tanks. Since the tanks
would remove the suspended solids that might settle or
otherwise cause obstructions in the mains, smaller
collector mains 10 cm (4 in) in diameter, laid on a uniform
gradient sufficient to maintain only a 45 cm/s (1.5 fps)
flow velocity were permitted. This alternative has been
estimated to reduce const ructioncostsby30-65 percent.
Routine maintenance also proved to be low in cost. As a
result, by 1986 over 80 systems had been constructed
with up to 4,000 connections per system.12
In the United States, small diameter gravity sewers were
not introduced until the mid-1970's.z The first systems,
located in Mt. Andrew, Alabama and Westboro,
Wisconsin, were small demonstration systems with 13
and 90 connections respectively. The Mt. Andrew system
was constructed as a variable grade system with sections
of sewer depressed below the static hydraulic grade
line.13 The Westboro system was designed with uniform
gradients using the more conservative Australian
guidelines.14 The Westboro system was estimated to be
30 percent less costly than conventional sewers.
As knowledge of the success of these systems spread,
SDGS began to gain acceptance and by the mid-1980's,
over 100 systems had been constructed. The designs of
most of the systems constructed prior to 1990 followed
the Australian guidelines, but as experience has been
gained, engineers are finding that the guidelines can be
relaxed without sacrificing performance or increasing
maintenance costs. Variable grade systems in which the
sewers are allowed to operate in a surcharged condition
are becoming more common. Minimum flow velocities
are no longer considered as a design criterion. Instead,
the design is based on the system's capacity to carry the
expected peak flows without raising the hydraulic grade
line above the interceptor tank outlet inverts for extended
periods of time. Inflective gradients are allowed such that
sections of the mains are depressed below the static
hydraulic grade line. Despite these significant changes
f romthe Australian guidelines, operation and maintenance
costs have not increased.
Small diameter gravity sewer systems consist of:
* House Connections are made at the inlet to the
interceptor tank. All household wastewaters enter the
system at this point.
* Interceptor Tanks are buried, watertight tanks with
baffled inlets and outlets. They are designed to remove
both floating and settleable solids from the waste
stream through quiescent settling over a period of 12-
24 hr. Ample volume is also provided for storage of the
solids which must be periodically removed through an
access port. Typically, a single-chamber septic tank,
vented through the house plumbing stack vent, is used
as an interceptor tank.
• Service Laterals connect the interceptor tank with the
collector main. Typically, they are 7.5-10 cm (3-4 in) in
diameter, but should be no larger than the collector
main to which they are connected, They may include a
check valve or other backflow prevention device near
the connection to the main.
22
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Figure 1-16.
Schematic of a SDGS system .
INTERCEPTOR TANK-2^
INFLECTIVE
GRADIENT
BUILDING
SEWER
EFFLUENT
Collector Mains are small diameter plastic pipes with
typical minimum diameters of 7.5-10 cm (3-4 in),
although 3-em (1.25-in) pipe has been used
successfully. The mains are trenched into the ground
at a depth sufficient to collect the settled wastewater
from most connections by gravity. Unlike conventional
gravity sewers, small diameter gravity sewers are not
necessarily laid on an uniform gradient with straight
alignment between cleanouts or manholes. In places,
the mains may be depressed below the hydrau lie grade
line. AJso, the alignment may be curvilinear between
manholes and cleanouts to avoid obstacles in the path
of the sewers.
Cleanouts, Manholes and Vents provide access to
the collector mains for inspection and maintenance. In
most circumstances, cleanouts are preferable to
manholes because they are less costly and can be
more tightly sealed to eliminate most infiltration and grit
which commonly enter through manholes. Vents are
necessary to maintain free-flowing conditions in the
mains. Vents in the household plumbing are sufficient
except where depressed sewer sections exist. In such
cases, air release valves or ventilated cleanouts may
be necessary at the high points of the main.
* Lift Stations are necessary where elevation differences
do not permit gravity flow. Either STEP units (See
Pressure Sewer Systems) or mainline lift stations may
be used. STEP units are small lift stations installed to
pump wastewater from one or a small cluster of
connections to the collector main, while a mainline lift
station is used to service all connections in a larger
drainage basin.
Although the term "small diameter gravity sewers" has
become commonly accepted, it is not an accurate
description of the system, since the mains need not be
small in diameter (the size is determined by hydraulic
considerations) nor are they "sewers" in the sense that
they carry wastewater solids. The most significant feature
of small diameter sewers is that primary pretreatment is
provided in interceptortanks upstream of each connection.
With the settleable solids removed, it is not necessary to
design the collector mains to maintain minimum self-
cleansing velocities. Wtthoutthe requirement for minimum
velocities, the pipe gradients may be reduced and, as a
result, the depths of excavation. The need for manholes
at all junctions, changes in grade and alignment, and at
regular intervals is eliminated. The interceptor tank also
23
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attenuatesthewastewaterflow rate from each connection
which reduces the peakto average flow ratio, below what
is typically used for establishing design flows for
conventional gravity sewers. Yet, except for the need to
evacuate the accumulated solids in the interceptortanks
periodically, SDGS operate similarly to conventional
sewers.
14.2 Application
Small diameter gravity sewers have potential for wide
application. They are a viable alternative to conventional
sewers in many situations, but are particularly well-suited
for tow-density residential and commercial developments
such as small communities and residential fringe
developments of larger urban areas. Because of their
smaller size, reduced gradients and fewer manholes,
theycanhaveadisttnct cost advantageover conventional
gravity sewers where adverse soil or rock conditions
createmainlineexcavationproblemsorwhere restoration
costs in developed areas can be excessive. In new
developments, construction of the sewers can be deferred
until the number of homes built warrant their installation.
In the interim, septic tank systems or holding tanks can
be used. When the sewers are constructed, the tanks
can be converted for use as interceptortanks. However,
SDGS usually are not well suited in high density
developments because of the cost of installing and
maintaining the interceptortanks.
1.4.3 Extent of Use In the United States
The use of small diameter gravity sewers has been
rapidly increasing in the United States. They have been
referredto by different names including Australian sewers,
variable grade sewers (VGS), small bore sewers (SBS),
septic tank effluent drains (STED) and common effluent
drains (CED). They are all similar in design except that
the Australian sewers or CED typically are designed to
have uniform gradients with a minimum flow velocity of
0.30 cm/s (1 fps). The others do not require uniform
gradients, but will allow inflectivegradients where sections
of the sewer are depressed below the hydraulic grade
line. Minimum flow velocities may not be required.
The use of small diameter gravity sewers has been
largely limited to existing rural communities. The first
SDGS system was installed in 1977 and by the early
1990's over 200 systems were operating. Increasingly,
they have been used for residential fringe developments
and new subdivision and resort developments where the
topography is favorable. Frequently, the systems built
are hybrid gravity and pressure systems, which have
sometimes been called "effluent sewers" or "solids-free
sewers."
Experience with the sewers has been excellent. The
sewers have proven to be trouble-free with low
maintenance requirements. As a result, confidence with
the systems has grown and the designs have become
less conservative.
1.4.4 Myths Versus Reality
Deterrents to the use of small diameter gravity sewers
have come from both the engineering/regulatory
community andthe potential users themselves. Engineers
and regulatory agencies have been reluctant to promote
SDGS because of the concern over long-term
performance. The concern has been over whether the
sewers could handle the flows without backups or
obstructions occurring. This concern is fading as
experience shows the sewers to be relatively trouble-
free.
Potential users have discouraged their use because of
the conception that SDGS are "second-rate" systems.
Typical concerns are for odors and whether the system
can be expanded to accommodate growth. However,
where SDGS have been installed, users have found
them to perform no differently than conventional sewers.
Several early systems did have odor problems due to
ignorance of the odor potential of free-falling interceptor
tank effluent and problems due to improper house and
tank sealing. By minimizing turbulence in the mains and
at lift stations and providing proper venting, odor problems
have been easily overcome. With proper planning,
expansion can be accommodated and with properdesign,
odors problems are avoided.
Construction of SDGS may not be the lower in cost than
conventional sewers in all unsewered developments.
The cost of installing interceptor tanks is a significant
cost. Usually existing septic tanks cannot be used as
interceptor tanks because they are not watertight and
cannot be inspected and repaired cost effectively. As in
any project, all reasonable alternatives should be
evaluated before design commences.
1.5 Comparison with Conventional
Collection
The following features of the various sewerage alternatives
are considered in planning a project.
1.5.1 Population Density
Conventional sewers are typically costly on a lineal foot
basis. Where housing is sparse, resulting in long reaches
between services, the cost of providing conventional
sewers is often prohibitive.
24
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Pressure sewers, small diameter gravity sewers, and
vacuum sewers are typically less costly on a lineal foot
basis, so often prove to be more cost-effective when
serving sparse populations.
Conversely, where the required length of sewer between
service connections is comparatively short, the cost of
providing conventional sewers is usually affordable unless
some other obstacle is present, such as adverse slopes
or rock excavation. Even when conventional sewers are
slightly more expensive than alternative sewers, their
use maybe preferred as conventional sewerage is an old
and mature practice.
1.5.2 Growth Inducement
The minimum allowed size of conventional sewers is
generally 20-cm (8-in) diameter, to accommodate sewer
cleaning equipment. Being comparatively large in diameter
(and capacity), conventional sewers are often seen as
being growth inducing. This is especially true if
assessment costs to fronting properties are high, which
prompts property owners to develop the property for
housing. This is often the most profitable alternative for
the property owner and as such provides the greatest
financial relief from the assessment.
Pressure sewer, small diameter gravity sewer, and
vacuum sewer mains may be intentionally downsized to
limit growth. Innovation in assessment rates can reduce
the need for prope rty development as a meansof escaping
imposed charges. However, designer/owners may wish
to allow for some level of growth. This can be incorporated
with alternative sewers as well as conventional ones.
1.5.3 Ground Slopes
Where the ground profile over the main slopes
continuously downward in the direction of flow,
conventional or small diametergravrty sewers are normally
preferred. If intermittent rises in the profile occur,
conventional sewers may become cost-prohibitively deep.
The variable grade gravity sewer variation of small
diameter gravity sewers, by use of inflective gradients
and inconjunctionwith STEP pressuresewer connections,
can be economically applied. Vacuum sewers may be
particularly adaptable to this topographic condition, so
long as head requirements are within the limits of available
vacuum (see vacuum sewer section).
In flat terrain conventional sewers become deep due to
the continuous downward slope of the main, requiring
frequent use of lift stations. Both the deep excavation and
the lift stations are expensive. SDGS are buried less
deep, owing to the flatter gradients permitted. Pressure
sewers and vacuum sewers are often found to be practical
in flat areas, as ground slope is of little concern. In areas
where the treatment facility or interceptor sewer are
higher than the service population, pressure sewers and
vacuum sewers are generally preferred, but should be
evaluated against SDGS systems with lift stations.
1.5.4 Subsurface Obstacles
Where rock excavation is encountered, the shallow
burial depth of alternative sewer mains reduces the
amount of rock to be excavated.
Deep excavations required of conventional sewers
sometimes encounter groundwater. Depending on
severity, dewatering can be expensive and difficult to
accomplish.
1.5.5 Discharge to Gravity Sewers
Where homes are in proximity to a conventional gravity
sewer, but where conventional service is impractical,
alternatives may often be used. Grinder pumps or solids
handling pumps are used at individual homes to discharge
from low-lying homes to the conventional sewer. Vacuum
sewers are used to serve large enough groups of homes
to justify the cost of the vacuum station.
STEP pressure sewers are commonly used in conjunction
with SDGS. Such hybrid installations are more common
than strictly STEP or strictly SDGS, even though these
sewers are usually classified as one of these types. Their
discharge into some conventional sewers may be feasible,
but the discharge of sulffdes to the sewer must be
evaluated when such discharges are large enough to
constitute a significant portion of the total flow.
1,5.6 Discharge to Subsurface Disposal Fields
STEP pressure sewer equipment is commonly used to
discharge septic tank effluent to subsurface disposal
fields that are distant or located at higher elevations than
the homes served. SDGS may also be used for
conveyance of effluent to subsurface disposal facilities
so long as ground slopes are favorable for gravity flow.
1.6 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Alternatives for Small Wastewater Treatment
Systems. EPA/625/4-77/011, NTIS No. PB-299608.
U.S. Environmental Protection Agency, Cincinnati,
Ohio, 1977.
25
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2. Otis, R.J. Small Diameter Gravity Sewers: An
Alternative for Unsewered Communities. EPA/600/
2-86/022, NTIS No. PB86-167335. U.S.
Environmental Protection Agency, Cincinnati, Ohio.
1986.
3. Alternative SewerSystems. WPG F Manual of Practice
(MOP) No. FD-12, 1986.
4. A. LeMarquand. Sewerage or Drainage of Houses,
Towns, or Districts, and Apparatus Therefore. Patent
No, 377681, Feb. 7,1988.
5. Water, Our Most Precious Possession. Liljendahl
Vacuum Company, Ltd., Stockholm, Sweden,
undated.
6. B.C. Bumsetal. Method and Apparatus forConveying
Sewage. Patent No. 3,730,884,1973.
7, Envirovac Technical Information, Colt Industries,
Beloft, Wis., undated.
8. D.W. Averil and G.W. Heinke. Vacuum Sewer
Systems. Report prepared for the Northern Science
Group of the Canadian Department of Indian Affairs
and Northern Development, 1973.
9. I.A. Cooper and J. W. Rezek. Vacuum SewerSystem
Overview, Presented at the 49th Annual Water
Pollution Control Federation Conference,
Minneapolis, Minn., Oct. 3-8,1976.
10. Skillman, E.P. Characteristics of Vacuum Wastewater
Transfer Systems. ASME Intersociety Conference
on Environmental Systems, 1976.
11. Design Manual, AIRVAC, Vacuum Sewerage
Systems, Rochester, IN., 1989.
12. South Australian Health Commission, Public Health
Inspection Guide No. 6: Common Effluent Drainage
Schemes. Adelaide, South Australia. 1986.
13. Otis, R.J. An Alternative Public Wastewater Facility
for a Small Rural Community. Small Scale Waste
Management Project. University of Wisconsin.
Madison, Wisconsin. 1978.
14. Simmons, J.D..J.O, Newman and C.W. Rose. Small
Diameter, Variable-Grade Gravity Sewers for Septic
Tank Effluent. In: On-Site Sewage Treatment.
Proceedings of the Third National Symposium On
Individual and Small Community Sewage Treatment.
American Society of Agricultural Engineers, ASAE
publication 1-82. pp 130-138.1982.
26
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CHAPTERS
Pressure Sewer Systems
2.1 introduction
2.1.1 Background
Historically there have been only two choices for the
disposal of domestic wastewater: either conventional
sewers were used, usually having lift stations as needed
within the system, and dischargingto municipal treatment
works, or septic tanks and drainfields were used.
Conventional sewers were generally used in cities and
largertowns, and septic tanks and drainfields were more
common in rural areas.
These technologies are mature and time honored, but
there are cases where neither method is well suited.
Where rock excavation is encountered the deep
excavations for conventional sewers can result in
excessive costs. Similar obstaclesto conventional sewer
construction include high groundwater, and terrain that
does not slope favorably for gravity collection. Where
homes are spaced distantly the resulting long length of
sewer between homes results in unacceptably high
costs.
Septictankdrainfleldsencounterdifficulties when located
in tight clay soils that have low absorption rates, or in
areas of high groundwater. In these cases effluent often
surfaces above the drainfields. Where soils are too
porous the effluent may flow too freely and enter the
groundwater without sufficient treatment.
In instances such as described above, pressure sewers
and other alternate technologies should be considered.
2.1.2 History
Lift stations have long been used with conventional
sewers, and occasionally household pumps having solids
handling capabilities have been used to lift wastewater
from low lying homes to conventional sewer mains.
Pumps have long been used to discharge effluent from
septic tanks to distant or elevated drainfields, but until
the last 15 years there has been no widespread use of
pressure sewer systems.
Mortimer Clift was among the first to report on pressure
sewer technology regarding a system serving 42 homes
in Radcliff, Kentucky.1 Solids handling pneumatic ejectors
were used which discharged via 7.5-cm (3-in) diameter
service lines to a 10-cm (4-in) main, following the concepts
of a patent issued to him in 1965. The system was
eventually abandoned due to equipment problems, but no
uncorrectable obstacles were apparent regarding the
pressure sewer concept.
With concern for the limited capacities of the numerous
combined storm - sanitary sewers existing in the United
States., Gordon M. Fair proposed a solution announced
publicly in 1965. Fair suggested conveying domestic
wastewater in a pressure sewer main separate from
storm water, with the main being hung from the crown of
the existing combined sewer. His patented "converted
sewer system", issued in 1968, was assigned to the
public.
Fair's proposal prompted a study of the concept by the
American Society of Civil Engineers (ASCE), for the
Federal Water Pollution Control Administration. During
this time General Electric subcontracted with the ASCE to
develop a grinder pump package following the directives
of the ASCE engineers. The final report, published in
1969, concluded unfavorably toward the concept due to
costs associated with placing a sewer within a sewer, but
the pressure sewer concept using grinder pumps was
otherwise endorsed.2
Paul Farrell was a project engineer with General Electric
and was highly involved in the ASCE study and in
development of the grinder pump package. He continued
his efforts with a pressure sewer demonstration project in
1970 at Albany, New York, which served 12 townhouses.
This was probably the most extensively monitored system
ever built, and was the subject of detailed and thorough
analyses3.
27
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General Electric withdrew from further involvement while
Environment-One Corporation continued the grinder
pump development and went on to other demonstration
projects, such as Grandview Lake, Indiana, which served
93 homes. Mr. Farrell !s still with Environment-One
Corporation. Other pioneers in pressure sewers included
Harold Schmidt, who proposed the use of septic tank-
effluent pump (STEP) pressure sewers. His first STEP
installation was made serving subdivision homes in Port
Charlotte, Florida, in 1970. That system now serves over
1,000 homes. Ken Durtschi also proposed a STEP
pressure sewer system to serve over 500 homes at
Priest Lake, Idaho. With his design and under his direction,
the system was built in the early 1970s. Gary Klaus also
proposed a system that involved the use of a solids
handling pump which discharged via a flexible hose to a
pipeline mounted on the dock. The wastewater was
conveyed from there to a floating septic tank which
discharged chlorinated effluent to the river. Beginning in
the early 1970s, Cecil Rose, then chief engineer for the
Farmers Home Administration, became a proponent of
pressure sewers, especially those conveying septic tank
effluent.
USEPA's Office of Research and Development provided
support for several early demonstrations of pressure
sewers, including Grandview Lake; Indiana, Phoenixville,
Pennsylvania; Albany, New York; and Bend, Oregon.**
Subsequently, the Agency produced a Small Community
Technology Transfer Seminar series which included a
report on pressure and vacuum sewer systems in 19777
When the Clean Water Act of 1977 gave increased
Federal funding to all projects that incorporated these
alternative collection systems, the numberof documented
installations proliferated in the United States from
approximately 100 to more than 600 by the end of the
Construction Grants Program in 1990. This number may
be significantly largertoday if all the systems not funded
by the Federal Government are also included. Of the
documented total, pressure sewers constitute the most
numerous class, followed closely by small diameter
gravity sewers;vacuum sewers systems are well behind.
2.2 Detailed System Plan and Elevation
Views Showing Locations(s) of Ail
Components
A simplified sketch showing a section of pressure sewer
main is shown as Figure 2-1. In the figure, discharge is
presumed to a treatment and disposal facility. If discharge
is made to a conventional sewer, pretreatment facilities
may be required.
Pretreatment facilitiesare located andsized in accordance
with the contact time required forthe particular method of
pretreatment. For example, chlorine reacts almost
immediately whereas aeration may require several hours.
Another parameter dictating pretreatment station location
is the flow regime in the main. For example, air injection
into a main could only be feasible if the main were steeply
rising, and if there were substantial pressure in the main
to cause the dissolution of oxygen into the waste stream.
isolation valves (IVs) are located about the same as in
water line practice. At the intersection of mains two IVs
are usually placed as shown in the figure, although some
engineers prefer using a valve cluster of three IVs.
Isolation valves are sometimes placed at the upstream
end of mains, to facilitate subsequent main extensions.
IVs are also placed on each side of areas where
subsequent disruption of the main can be anticipated,
such as at bridge or stream crossings, where future road
construction is foreseen, or in areas of unstable soils.
Where reaches between IVs become long, intervening
IVs are sometimes used to divide the long reach into
shorter lengths.
On long, steep grades, IVs are located to accommodate
pressure testing requirements. Other IVs may be used
as a part of the design of other facilities, such as with
cleanouts or with flow meters.
Cleanouts (COs) are sometimes provided and when
provided, in-line cleanouts are most typically placed
where pipe sizeschange. This is in anticipation of cleaning
the main using a pipe cleaning pig. Terminal cleanouts
(TCOs) maybe located at the ends of mains. If thedesign
of the cleanout is such that it rises to ground surface, a
manual or automatic air release valve is often fitted to the
high point.
Air release valve stations may be manual (MARV) or
automatic (AARV). When the main is submerged under
a static head the air release valves are normally located
at summits. If the upstream end of a main terminates on
a rising grade, an air release valve is used there. In two-
phase flow regimes the air releases are placed as
described elsewhere. A soil bed or other facility for odor
absorption is sometimes provided at AARV stations,
especially if located where odors would be a nuisance or
if the particular station is expected to release much gas.
Service saddles, tees, or tapped couplings are used to
join the service line to the main. A corporation stop is
often provided there, and a check valve if the main is
accessible. An isolation valve and check valve may be
provided on the service line, sometimes located at the
28
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Figure 2-1.
Piping system appurtenances.
IV
3"PVC
4"PVC
TCO
IV
AARV
CO
FLOW
PMS
3"PVC
A) PUN VIEW
IV
TCO
B) PROFILE OF MAIN
GS
IV CV
PLAN VIEW OF SERVICE LINE
AND CONNECTION TO MAIN
FM-FLOW METER
IV - ISOLATION VALVE
CO - CLEANOUT
TCO - TERMINAL CLEANOUT
PMS - PRESSURE MONITORING STATION
AARV - AUTOMATIC AIR RELEASE VALVE
CS - CORPORATION STOP
CV - CHECK VALVE
29
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road shoulder or at the road right-of-way line. The
location is chosen based on anticipated damage caused
by such activities as road grading or snow plowing, and
based on subsequent accessibility. For example, service
line IVs and CVs are usually located outside of paved
areas.
Row meters are valuable, particularly when assessing
capacity of the system and when quantifying the extent
of I/I the system receives. They are also used to flow-
pace pretreatment facilities using chemical injection. For
economic reasons, however, they are often not provided.
They are generally of greater benefit on larger systems
serving several hundred homes but should be considered
for small systems as well. Most large systems use
permanent installations. In smaller systems portable
meters may be used to determine flows at particular
points in question.
Pressures within the system can be measured in the GP
or STEP pump vault if the discharge piping will
accommodate the meter, and providing no check valve is
used between the gauge and the main. However, most
systems use service line check valves, which precludes
this practice and makes necessary the provision of
pressure monitoring stations (PMS).
Pressure monitoring stations are simple, inexpensive,
and should be provided on systems where there may be
questions as to operating conditions. The recording
pressure gauge is usually moved from station to station
as needed. PMSs can help reveal air-caused headlosses,
and unexpectedly high or low flows. This is done by
comparison of the theoretical hydraulic grade line versus
that measured.
2.3 Detailed Description of On-Lot
System Components
2.3.1 Available Systems
Thepressuresewerpumping unit periodically discharges
flow from the home or homes served to a pressure sewer
main where the flow is conveyed to a point of treatment
and disposal.
With a grinder pump (GP) system, the purpose of the
grinding action is to reduce the size of troublesome solids
present in wastewater making a pumpable slurry to be
conveyed in the small diameter service lines and mains.
Solids handling (SH) systems rely on user cooperation to
ensure that particularly troublesome solids are not
discharged to the pumping unit. Such matter includes
hard solids, plastics, rags, sanitary napkins, and stringy
material.
Effluent pumps (STEP) are merely open-impeller pu mps
which are capable of pumping wastewater from which
troublesome solids have been removed by the septic
tank,
2.3.2 Detailed Descriptions
The septic tank of a STEP system captures most grit,
grease, and other troublesome solids, and retains the
sludge and floatable matter. The effluent is also reduced
in strength in terms of BOD, SS and certain other
parameters. This is of major benefit when drainfield or
sand filter treatment is used, and may be a secondary
benefit when mechanical forms of treatment are used.
The pump vault provides storage for a working volume
between the "pump on" and "pump off' liquid levels, so
the pump does not cycle on and off too frequently. The
storage volume between "pump on" and "high level
alarm" allows the inflow rate from the home to temporarily
exceed the discharge rate of the pump without triggering
an alarm.
The "reserve space" between the high level alarm and
the point where the pump vault is full and overflow is
impending is particularly important. A small reserve
space volume will not store sufficient flow to avoid
overflow or backup in the home, causing inconvenience
to the user prior to the time that service personnel can
arrive and correct the malfunction.
When STEP systems are used the reserve space provided
by the septic tank is usu ally quite large, in the order of one
day's average flow, which allowsthe service personnel to
attend to maintenance calls less urgently.
Emergency overflows are sometimes provided when
grinder pumps are used owing to limited storage capacity
of the pump vault. When using STEP systems the septic
tank encompasses a large storage volume above the
normal water level and such overflows are rarely required.
However, overflows can be easily disposed of to the
drainfield. The raw wastewater overflow of GP or SH
systems should go to a drainfield via a septic tank or be
discharged to a holding tank. Any overflow management
should be limited to short-term emergencies and should
not permit inflow back into the system.
Pump manufacturers provide preassembled packages
of pressure sewer components, including the pump,
pump vault, in-vault piping and valves, liquid level sensors,
electrical control panel, electrical junction box, and
associated equipment. The availability of such packages
greatly simplifies the duties of the application engineer,
and often has the distinct advantage that the assembly
has been refined as dictated by prior experience. There
30
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is also a single source of responsibility in the event of
malfunction.
installation within the home, often in a basement. A
duplex grinder pump station is shown in Rgure 2-4.
In some cases, component systems have been designed
by the application engineer and built by the owner or the
contractor's supplier. There havebeentwodistinct motives
for the site-specific component approach. One is
economic: the assembly can often be produced at less
expense if the components are purchased separately
and assembled by the owner or contractor. However,
inferior components are often used, the components
selected may not work well together, and quality control
has been known to suffer.
The second reason site-specific component systems
have been used has been the view that a better system
can be built, custom made for the particular project
needs. This approach has been successful on only a few
projects where prototypes were fully developed, tested
and refined over a period of time, or where the design
engineer had considerable experience with pressure
sewer technology.
Atypical simplex GP package is shown in Figure 2-2. The
pump vault is typically fiberglass and usually 60-90 cm
(2-3 ft) in diameter. The depth varies with the dictates of
ground topography and the volume of reserve space to
be provided. Typically specified depths of basins are
usually 1.5-2.4 m (5-8 ft).
The pump is shown to be suspended, and aligned with a
slide-away coupling by guide rails. This design has
historically been favored by most centrifugal pump
manufacturers. A long-handled operating wrench is used
to reach the shutoff valve from ground surface. A lifting
chain is provided to aid in removing the pump.
Three mercury-float-switch liquid level sensors are within
the pump vault: "Pump off", "pump on", and "high level
alarm". In some cases a redundant off float is added,
which may not only stop the pu mp but sometimes activates
a low level alarm. If a duplex installation is made, an
additional float switch is used for the lag pump-duplex
cycle.
Wiring is extended from the electrical junction box in the
pump basin to an electrical control panel. The control
panel is usually mounted on the outside wall of the home,
but in some cases it is pedestal mounted adjacent to the
basin.
Figure 2-3 shows a centrifugal grinder pump suspended
from the basin cover, ratherthan using the guide rail-slide
away coupling arrangement. This system is intended for
One popular semi-positive displacement (SPD) grinder
pump package differs considerably from the centrifugal
pump GP packages shown previously. A typical design
is shown in Figure 2-5.
The progressing cavity typeofsemi-positivedisplacement
pump is suspended into the basin. The pumping core is
comprised of the pump, motor, grinder, piping, valving,
and electrical controls. Liquid levels are sensed using a
trapped air type of pressure sensor, somewhat like a
bubbler system, butwithoutthecompressor. Thissystem
has no moving parts in contact with the wastewater. The
components are shown in Figure 2-6.
An external control panel is not needed with this type of
pump, but an external branch circuit disconnect is
sometimes used, and an external high level alarm
annunciator (horn or light).
Figure 2-7 depicts a STEP pump in a pump vault external
from the septic tank. Some GP installations are the same
and would only vary in that the pump has a slightly
different appearance. When a GP is used as shown in
this figure, stainless steel legs are screwed into the
bottom of the pump since most GPs are made to be
suspended.
This installation does not employ guide rails. Instead of
connecting the discharge via a slide away coupling, a
discharge hose is used which extends to within a few
inches of the top of the basin where a quick-connect
coupling and the isolation valve can be reached from
ground surface. This is the type of installation usually
favored by makers of GP component systems. STEP
systems are usually of a different type, shown below.
SH systems are typically as shown in Figure 2-7.
A STEP system is shown in Rgure 2-8. This concept
employs a pump vault internal in the septic tank, as
contrasted against the external pump vault shown in
Figure 2-7. Both approaches are widely used for STEP
systems.
The effluent pump rests on the floor of the pump vault,
and discharges via a flexible discharge hose that connects
to the service line piping with a quick-connect coupling
near ground surface. Three mercury float switches are
used, and an external electrical control panel (not shown)
is employed, as in GP practice.
31
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FIgurt 2-2. Typical simplex GP package using slide away coupling and guide rails. (Courtesy F.E. Myers Pump Co.)
COVER
JUNCTION BOX
DISCHARGE
PIPE
HIGH WATER ALARM
MERCURY
LEVEL CONTROL
INLET
ON a OFF
MERCURY
LEVEL CONTROLS
GRINDER
PUMP
FIBERGLASS
BASIN
CONCRETE
32
-------
Figure 2-3. Typical centrifugal GP package with pump suspended from basin cover. (Courtesy Barnes Pump Co.)
33
-------
Figure 2-4, Duplax GP slaHon. (Courtesy Barnes Pump. Co.)
34
-------
Figure 2-5.
Typical progressing cavity-type GP package. (Courtesy Environment/One Corp.)
35
-------
Flgura 2-6. Basic components of a progressing cavity grinder pump. (Courtesy Environment/One Corp.)
INTERNAL PRESSURE
SWITCHING
1J8rMALENPT
HIGH LEVEL ALARM
SENSING TUBE
ANTI-SIPHON
8 CHECK VALVE
36
-------
Figure 2-7.
STEP pump in external vault. (Courtesy Barnes Pump Co.)
COVER ELECTRICAL
JUNCTION BOX GATE VALVE
QUICK COUPLING
4" PVC INLET
ADAPTER
PVC DISCHARGE
HOSE ASSEMBLY
PUMP VAULT
PVC DISCHARGE
PIPING
LIFTING ROPE
DISCHARGE
MERCURY
FLOAT SWITCHES
ALARM
PUMP ON
FLOAT POLE
ASSEMBLY
CHECK VALVE
PUMP OFF
ANTI-FLOATATION
COLLAR
37
-------
Figure 2-8.
Typical STEP package with Internal pump vault. (CourtesyBarnes Pump Co.)
COVER
COVER
GATE VALVE
INLET
MERCURY FLOAT SWITCHES
QUICK COUPLING
FLEXIBLE
DISCHARGE HOSE
ALARM
PUMP ON
PUMP OFF
CHECK VALVE
SEPTIC TANK
A riser extends to ground surface, providing access to
the pump vault. In some designs the pump vault is
removable through the riser, for access to the tank. In
other designs the pump vault and riser are integral in
which case access to the tank is made through the cover
on the inlet end of the tank which may be buried or may
also have a riser extending to ground surface.
Inlet holes (ports) are provided in the pump vault as
shown, which are located below the lowest predicted
elevation of the scum in the tank during the lowest water
level condition, but above the maximum height of the
sludge layer.
The liquid level in the entire septic tank rises and falls in
response to flows from the home and the pumping cycle.
When the liquid level rises for example 7.5 cm (3 in)
(about 50 gal), the pump turns on and pumps down the
whole tank 7.5 cm (3 in). If a malfunction occurs and the
liquid level rises sufficiently above the pump-on level, a
high level alarm is activated. The reserve space is
located between the top of the floating scum and the soffit
of the tank.
The GP, SH, and STEP systems shown in the figures are
available as simplex units, intended to serve one home,
or perhaps up to 3 homes. Larger systems are also used,
to serve many homes. The designs vary from duplex
versions of the designs shown to full-scale wastewater lift
stations. When large STEP installations are made, either
large septic tanks are used, or several tanks are placed
in series, with the final tank being the pump tank.
Pneumatic ejectors have been used to a limited extent on
STEP systems, but none are now known to be marketed.
Some STEP installations use submersible water well
pumps, which must be used in conjunction with an inlet
screen since well pumps have no solids handling capability.
Well pumps also must be placed in a tube to simulate a
well casing, to cause water entering the pump to flow past
the motor, providing cooling. Progressive cavity pumps
38
-------
Figure 2-8.
Head-discharge curves for typical GP and STEP systems.
1 - 2 HP CENTRIFUGAL STEP PUMP
2 - 2 HP CENTRIFUGAL GP
3 -1/2 HP CENTRIFUGAL STEP
4 - MZ HP PROGRESSIVE CAVITY STEP
5 -1 HP PROGRESSIVE CAVITY GP OR STEP
6 -1/2 HP 7-5TAGE SUBMERSIBLE WATER WELL STEP
10
DISCHARGE (GPM)
have been used some on STEP systems as well as on
GP systems.
The pump characteristics vary with the manufacturers,
but a general overview of the head - discharge curves is
shown in Figure 2-9.
2.3.3 Materials of Construction
The use of proper tankage is particularly important. If a
tank fails, the contractor must re-enterthe homeowner's
property with heavy equipment, excavate to remove the
failed tank, and place the new tank. It is a most costly
and visible mistake.
Pu mp vaults are most commonly made of FR P (fiberglass
reinforced polyester). Those provided by the pump
manufacturers are usually quality products, having a
minimum wall thickness of about 6 mm (1/4 in). Some
are gel coated which provides a smooth protective
surface, and those that are not gel coated have resin
rich surfaces intended to prevent glass fibers from being
exposed that could cause wicking.
Some FRP vaults may be as thin as 1.5 mm (1/16 in) in
places. The product may not be produced from quality
materials or with quality workmanship, and fibers may be
exposed. These are usually produced by other than the
original pump manufacturer (aftermarket items).
Other pump vaults have been made of both high density
and low density polyethylene (PE). PE is more flexible
than FRP, so where the vaults are intended as a structural
member, they must be thicker than FRP. PVC has also
been used, usually made from sewer pipe.
Septic tanks on STEP systems have been made of
reinforced concrete, FRP, and PE. Depending on the
quality of the product, all of these materials have been
successful, and in other cases all have been unsuccessful.
In certain instances reinforced concretetanks have cracked
badly, admitting groundwater. FRP tanks have cracked or
39
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split open, in some cases have collapsed, and in other
cases water seeped through the walls. Some PE tanks
have deformed so badly as to not be functional and to
demand replacement. In other instances they have
collapsed totally.
Where the liquid level in the septic tank is lower than the
groundwater, infiltration can occur if the tank is not water
tight. There are numerous projects where I/I into the
upstream sewer and septfctank has morethan quadrupled
flows, pushing the pressure sewer system beyond its
capacity and rendering it a failure.
Judging structural integrity by observation of the tank
being used as a part of an existing septic tank - soil
absorption system has proven misleading. First, "rt is
often impossible to know if a septic tank used with an
absorption system leaks or not. Secondly, tanks which
apparently do not leak in septic tank-soil absorption
service may leak under pressure sewer service owing to
potentially lower groundwater levels.
To evaluate the septic tank structurally, it is necessary to
prepare a loading diagram depicting the loads the tank
will be subjected to, commensurate with burial depth,
groundwater depth, soil types, foundation, bedding and
backfill to be used, and other parameters. Following this
task the tank isdesigned by usual engineering analyses.
Generally, concrete tank designs follow American
Concrete Institute standards, assuming one-way hinged
slabs spanning the shorter dimension. Non-traffic load
designs usually result in concrete tanks having an
equivalent of 10-cm (4-in) walls and top and bottom
slabs, with #5 bar reinforcement at about 20-cm (8-in)
centers. A thickness of sacrificial concrete above the
water line may be provided in anticipation of corrosion by
H2S04,
The thickness required of FRP tanks varies considerably
with tank shape andthe quality of the FRP product. When
this has been evaluated, the usual conclusion has been
to require an average wall thickness of 6 mm (1/4 in), with
a minimum thickness at any point of 5 mm (3/16 in).
Because polyethylene is so flexible the shape of PE
septic tanks is crucially important. Flexible tanks can
deform to a shape of structural weakness if not properly
designed. PE tank designs generally rely considerably
on empirical refinement, taken from monitored experience
on numerous installations under varying conditions.
Quality control of the tank manufacturing process must
be assured. It has been common for tank construction to
be poorly executed.
Septic tank effluent and the septic tank atmosphere are
corrosive due to the hydrogen sulfide present above the
water line and the potential for sulfuric acid formation.
The wastewater in a grinder pump vault may also become
septic duetothewastewaterbeing sometimes held in the
pump vaultfor extended periods. Exposed appurtenances
must be suitably corrosion resistant.
In most cases where the pumping package has been
supplied by a manufacturer with considerable pressure
sewer experience, the engineer can be reasonably
assured that acceptable materials have been used.
When component systems are built the engineer must
pay strict attention to materials choices.
The materials chosen for corrosion resistance vary
according to the material properties needed for structural
and other reasons.
Austen'rtic stainless steel, particularly Type 316 and in
some cases Type 304 have proven to give excellent
service. Fasteners are produced from this material, also
such items as hose clamps. Martensitic stainless steel,
such as Type 416, has generally proved unacceptable.
Some plastics are virtually unaffected by exposure to
H2S while others are not. PVC, ABS, and PE, all materials
that have long been used in sewerage service, appear
acceptable. Nylon, however, is affected by H2S and
H2SO4, and is not acceptable.
Copper products, e.g., alloys of brass or bronze, provide
limited success. Besides corrosion considerations brass
is subject to dealloying, while some bronze, such as 85-
5-5-5, will give better performance. The terms brass and
bronze are used loosely despite having different meanings;
the engineer is advised to evaluate these materials with
caution.
2.4 System Design Considerations
2.4.1 Hydraulics
2.4.1.1 Design Flows and Their Variability
a. Average Daily Flows
Fundamental to the design of a sewer system is the
determination of design flows. Where actual flow
characterization data are available they should be used.
An allowance of 380 Ucap/d (100 gpcd) has been used
as a general rule in the design of conventional sewer
systems.8 However, that general rule may allow for more
infiltration than may occur when pressure sewers are
40
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used, and it allows for some amount of commercial and
industrial use that may not be present in pressure sewer
design. Experience with pressure sewerage has shown
a lower allowance to be more in order.
During the early stages of pressure sewer development
extensive investigations were made into domestic water
consumption during periods of low outside water use,
with the correlation that water consumption would closely
parallel sewerf lows. These studies showed flows of 150-
230 L/cap/d (40-60 gpcd). Flow measurements were
made on conventional sewers serving residential
communities during periods when I/I was not occurring,
with the same conclusions.
At this time, thousands of flow measurements have been
made on pressure sewer systems with wide demographic
spread.9 The result of these measurements has
corroborated findings of the earlier studies; that flows are
typically 150-230 Ucap/d (40-60 gpcd), with little weekly
or seasonal variation.
The availability and quality of water affects water use and
consequently sewer flows, as does water pressure,
community affluence, nature of occupancy, and attitudes
of the users regarding water conservation. Because of
these variables and to provide a safety factor, the flow
rate normally assumed for design is 190-265 L/cap/d (50-
70 gpcd).
While pressure sewers are sometimes thought to be free
of I/I, it can occur in the non-pressurized portions of the
system, e.g., the building sewer and the tank. In some
cases I/I has been extreme, due to leaking building
sewers or house roof drains being connected to the
building sewer, due to pump vault risers being set below
ground level which allows surface waterto enter, or in the
case of STEP systems due to leaky septic tanks. It is
prudent to make an allowance for I/I when adopting a
design flow, based on the extent of I/I control given to the
project.
Daily peak flows may exceed design values by several
times and occur several times per day but these are of
little importance due to their short duration. There are
also periods of zero flow.
Flow variations are related to mainline sizing and pump
selection. That is, an oversized system will tend to have
more peaky flows than a system with smaller diameter
mains, where the pumps run longer per cycle at lower
discharge rates.
On the Glide, Oregon STEP system, peak hour flows
were found to occur about twice/day, at flow rates of 40-
65 percent of design peak flows. The Glide system at that
time served 560 equivalent dwelling units (EDUs) and
was sized to serve an ultimate population of 2,380 EDUs.
The 32-km (20-mi) piping system is 7.5-30 cm (3-12 in)
in diameter.
b. Peak Flows From Homes and Required Pumping
Rates
Besides average daily flow rates and their variabilities ft
is important to consider other factors, such as the rate of
flow from the individual home to the septic tank or GP
vault. This flow rate can be quite high at times.
The American Society of Civil Engineers2 reported peak
flows that may occur about twice per year as being 98 L
(26 gal) in a 4-min period, or 408 L (108 gal)/hr. They go
on to describe the simultaneous discharge from a bathtub
and clothes washer resulting in a 174-L (46-gal) discharge
over a 2-min period, and having a high probability of
occurrence.
Bennett10 reported surge flows of 230 L (60 gal) in a 7-min
period. Jones11 reported findings similar to those of
ASCE and Bennett and applied the data to regression
analyses. The results of the various stud ies are shown on
Figure 2-10.
If the purpose of the pressure sewer pump was to
discharge flows as fast as they enter the tank, required
pumping rates would be quite high to accommodate
these instantaneous peak flows. However, the purpose
of the pressure sewer pump is to discharge flows at a rate
such that the level in the tank will not reach the high water
alarm level, and with a high degree of confidence, will not
overflow the basin. The reserve volume within the tank
between pump on and high level alarm is used to
attenuate peaks and is a factor in establishing required
pumping rates.
Required pumping flow rates should not be confused with
design flow rates used for sizing mains, as the latter does
not consider attenuation of peak flows from the home
provided by the volume held between pump on and alarm
levels in the pump vault.
Rgure 2-11 adopts the 1 -percent regression of Reference
11 and Rgure 2-10 and presents pumping rates required
given four different volumes of reserve space. The
curveson Figure 2-11 have been calculated based on the
following equation:
Q = (V-S)/t
Equation 2-1
Where,
41
-------
Figure 2-10.
Rgur»2-11.
Wastowaler flows for one home
300
250
200
ISO
100
,50
10
20
30
TIME (MINUTES)
Required pumping rates using flows from Reference 11, <1-percent regression.
STORAGE
VOLUME.
GAL.
(PUMP ON TO
ALARM LEVEL)
10
TIME (MINUTES)
42
-------
Q = Minimum required pump discharge rate (gpm)
V = Volume of peak wastewater flow from the
home (gal)
S = Storage volume between pump on and alarm
(gal)
t = Time (minutes)
Figure 2-11 shows, for example, a minimum required
pumping rate of 10 L/min (2.6 gpm) if 132 L (50 gal) of
reserve is provided. However, a higher pumping rate is
not detrimental in most cases. When using a grinder
pump system where pipeline cleansing velocities are
required, a higher pumping rate may be needed for that
purpose.
c. Design Flows
Design flows are maximum flow rates expected to occur
once or twice per day, and are used to size the pressure
sewer mains. Flow rates in excess of design flows can
occur under certain situations to be described later, so
design flows should not be taken as the maximum flow
rate possible to occur.
Two design approaches have been used; the probability
method and the rational method.
The probability method proposes the maximum number
of pumps theoretically expected to be running at any
time. Then, with the discharge rate of the pumps being
known or assumed, the design flow is the product of the
number of pumps running times the pump discharge
rate.
Many pressure sewer pumps are centrifugal, having
gradually sloping head-discharge curves, so the discharge
rate varies considerably depending on the discharge
pressure. Consequently the pumping rate in the probability
method is only loosely assumed when centrifugal pumps
are used. The probability method would best apply to
pumps having vertical or near vertical head-discharge
curves, such as semi-positive displacement pumps, e.g.,
progressing cavity types.
The rational method can logically be applied when either
centrifugal pumps or semi-positive displacement pumps
are used. The rational design has almost exclusively
become the accepted method of practice.
The rational method proposes a design f lowcorresponding
to the number of homes served by the pressure sewer,
which is used to size the mains and to construct the
design hydraulic grade line. Pumps are then selected
that can discharge into the main at an acceptable flow
rate given the design discharge pressure.
Environment QfJfi. The design handbook of Environment
One Corporation, manufacturers of progressing cavity
type grinder pumps and effluent pumps, tabulates the
number of pumps expected to be running simultaneously
versus the number of pump cores connected to the
system.12 Design flow rates as shown in Figure 2-12 are
then determined by the product of 11 gpm (the discharge
rate of their pump) times the number of pumps running.
Information from a study by ASCE2 was used to derive
the E-1 design flow rate curve which was then refined by
operating experience with projectsusing their equipment.3
The ability of their pumps to operate at least 25 percent
above design pressure accommodates occasional peak
flow needs in excess of design.
ASCE. A study by the ASC E2 was accomplished early in
the history of pressure sewer development, based on
water supply demand rates in northern latitudes and
during periods when outside water use was minimal.
Based on Johns Hopkins University data and referencing
work by McPherson, tables of flow were prepared.
Measurements of 15 systems in the northeastern United
States were shown, serving 44-410 EDUs. In northern
California, measurements were made of 13 systems
serving 63-295 EDUs. Curves were drawn from these
data expressing a ratio of maximum peak hour of any day
to average annual use.
Hydromatic. Hydromatic pump company sponsored a
study and report by Battelle Institute which explored the
use of centrifugalgrinder pumps.13 Anumber of information
sources were cited, including that from the Grandview
Lake,4 Indiana pressure sewer project, the Albany, New
York study,3 and work by Watson, Farrell, and Anderson.14
A specific equation for peaking factor was taken from Fair
and Geyer's text,15 citing Harmon's measurements of
conventional wastewater flow:
(18+P)08/(4+P)os
Equation 2-2
Where,
Qp = Peak flow
Qa = Average flow
P = Population in thousands
Eight tables were presented ranging from flows of 660-
1,515 Us (175-400 gpd)/EDU. Although not stated within
the report, Equation 2-2 fits the data in Battelle's tables
exactly, excepting for low flows where departure was
apparently regarded as necessary owing to the differing
natureof pressure sewer collection as contrasted against
43
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Figure 2-12. Design flows.
300
200
100
ASCE
KKDRTHEASTERN
U.S.
u 100 200
EQUIVALENT DWELLING UNITS
gravity sewer collection. A design flow of 1 L7s (15 gpm)
was suggested for one home, and 1.6 L7s (25 gpm) for 5
homes.
Rows shown in the tables are reported to be recurring
peak flows, to occur once or twice per day.
The table corresponding to average flows of 740 L7s (195
gpd), and occupancy of 3.5 people/EDU has been used
most frequently in pressure sewer design, and was used
for the curve shown in Figure 2-12.
Battelle's tables have been widely used for both GP and
STEP designs.
Barnes. Thedesign manual provided bvthis manufacturer
of centrifugal grinder pumps and effluent pumps proposes
a peaking factor of 3> and assumes that flows occur over
an 18-hr period, which results in a peaking factor of 4 over
a 24-hr period.16 Their equation is re-expressed as
Equation 2-3. A minimum flow of 1 L7s (15 gpm) is
suggested. The Barnes recommended design curve as
presented in Figure 2-12 assumed an average flow of
760 Us (200 gpd)/EDU.
Q = 4 Q Equation 2-3
400
500
F.E. Myers. This pump manufacturer's design handbook
provides a plot of peaking factor versus the number of
dwellings served, with the peaking factor varying from
4.8 at zero EDU to 3.4 at 700 EDUs.17 They suggest an
average daily flow expected when serving 3-bedroom
homes of 950-1,510 L7s (250-400 gpd).
Simplified Equation. By examination of the curves shown
in Rgure 2-12, a simplified equation has been fitted. The
reasoning in proposing a simplified equation is that a
precise determination of flows is not possible to achieve
anyway, especially with regard to infiltration allowance.
The simplified equation is easy to use and easy to modify
to suit project needs.
Q = AN + B
Equation 2-4
Where,
Q =
A
N
B
Design flow (gpm)
Acoefficient selected by the engineer, typically
0.5
Number of EDUs
A factor selected by the engineer, typically 20
In the usual form, the equation is Q = 0.5N + 20, but may
be varied to account for anticipated high water use (and
44
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correspondingly high wastewater flows), to allow for a
greater safety factor, and especially to allow for I/I,
Varying the coefficient A steepens or flattens the curve
while varying the factor B raises or lowers the curve.
The design curves proposed by the various manufacturers
have been widely used. In the vast majority of cases the
systems have performed well, indicating that the design
curves are adequate. However, many systems have
been sized for growth that has not yet occurred, so these
systems have not yet been fully tested. Most systems are
not equipped with flow meters that would measure peak
flows, nor are pressure readings routinely taken, so
acceptable performance is only judged by the lack of
nuisance high water alarms during peak periods.
Systems that have proven inadequate typically received
I/I far in excess of that anticipated.
2.4.1.2 Minimum Flow Velocities in Pipes
The term "self cleaning velocity" refers to the flow velocity
required to convey solids along with the water carrier. To
maintain an unobstructed pipeline, that velocity should
be sufficient to transport grit that may be present in the
wastewater, to prevent grease plating on the crown of the
pipe, and to scour and resuspend previously settled
matter.
When force mains are used to convey conventional
wastewater or when a grinder pump pressure sewer is
used, the typically ascribed self cleaning velocity is
usually taken as about 60-90 cm/s (2-3 fps). That velocity
should occur once ortwice daily. The higher velocity of 90
cm/s (3 fps) is preferred with regard to scouring concerns,
but the higher flow rates correspond with higher
headlosses and the need for higher head pumps.
The septic tank of a STEP system is effective at capture
of grit and grease. It is logical that the required self
cleaning velocity would be much reduced when septic
tank effluent is pumped. Assuming values of s «1.1 and
Dg=0.2 mm for septic tank effluent, and applying Camp's
equation for sediment transport,18 results in a self-cleaning
velocity of less than 30 cm/s (1 fps).
Experience has shown that if GP flow velocities are too
low grease collects at the crown of the pipe, restricting
the cross sectional area and interfering with the transfer
of air to air release valves, and increasing headlosses
against which the GPs must pump.3 Grit can also collect
at the invert of the pipe.
There have been occasional reported instances where
velocities on small portions of GP systems have been so
low that pipeline clogging has occurred, necessitating
pipeline cleaning.
Experience with STEP systems has shown that solids
are not deposited even when velocities are less than 30
cm/s (1 fps). However, a self-cleaning velocity of 30 cm/
s (1 fps) may be conservatively used.
With either GP or STEP systems, sand and other debris
can enter the pipeline during construction which can
become cemented due to contact with the septic
wastewater, and difficult to remove. The pipeline should
be kept capped at all times during construction except
when pipe laying is being actively accomplished, and
other measures taken to insure that the pipe is kept
clean.
Mains should be designed to withstand the forces of
pipeline cleaning by pigging, should that become
necessary. The more common need for pigging is to
remove debris that entered the pipe during construction.
Pig launching stations have been provided on a few
projects, but usually they are regarded as an unnecessary
expense and encumbrance.
Pressure monitoring stations have occasionally been
used on large projects. The methodical and routine
taking of pressure readings can help reveal progressive
pipeline clogging, but more often it shows effects of air
binding.
2.4.1.3 Applicable Equations
The Hazen-Williams Equation is most often used to
forecast headloss, but the Manning Equation is
acceptable. When the Hazen-Williams Equation is used
the C factor is selected by the engineer, typically being
130-140 for plastic pipe. A corresponding Manning n
value is 0.010.
Hazen-Williams:
V=1.318CR063Sa54
Manning:
V=1.486/nRzaS1'2
Where,
Equation 2-5
Equation 2-6
V = Velocity of flow (fps)
C = Hazen-Williams flow coefficient
R = Hydraulic radius
S = Slope of energy gradient
n = Manning flow coefficient
45
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Where pipelines flow partly full, as in some gravity
reaches wrthinacombinedpressuresewer-smalldiameter
gravity sewer, the velocity can be easily calculated by the
Pomeroy Equation:
Table 2-1. Approximate Main Sizes Required to Serve
Number of Homes Shown (Using Q = 0.5 N+20,
and V = 2 fps)
V = 6.8 S041Q024
Equation 2-7
Where,
V -
S
Q =
Velocity of flow (fps)
Slope of energy gradient
Row rate (gpm)
Pipelines
2.4.2.1 Mainlines
a. Geometry
The geometry of a pressure sewer system is similar to
that of a water distribution system, but normally in
dendriform pattern, as opposed to a network in which the
pipelines are looped. The purpose of the branched layout
is to have a predictable minimum self-cleaning velocity in
the mains, but a disadvantage is that redundancy is not
provided as it is in a looped system. With a network, a
section of the piping system can be shut down for repairs
without interrupting flow from all upstream inputs, as flow
from them is naturally redirected.
In some pressure sewer designs a network pattern is
used with mainline isolation valves in the normally closed
(NC) position. These isolation valves are located such
that the system operates the same as a dendriform
layout would, excepting during a period when a portion of
the main is shut down for repairs, in which case the
normally closed valve is temporarily opened and flows
are redirected. The practice of network layout using NC
valves is more common with STEP system design than
with G P systems owing to their reduced need for cleansing
velocities.
Pressure sewer geometry also differs from most water
supply systems in that some reaches of the pressure
sewer may flow part full (by gravity). In profile, pressure
sewer systems are sometimes arranged to pump only
upslope, or to confine downslope piping to steep and
distinct reaches where hydraulic conditions are more
predictable.
b. Pipe Sizing
For rough planning purposes the Equation 2-8 may be
used, and a velocity of flow assumed at 60 cm/s (2 fps).
Table 2-1 shows the resulting number of homes that
could be served by various mainline sizes.
Pipe diameter
(in)
2
3
4
6
8
No. Homes Served
6
60
120
240
560
Q = 0.5N + 20
Equation 2-8
Where,
Q =
N
Design flow, gpm
Number of homes served
There is little economy in using 5-cm (2-in) mains. 7.5-cm
(3-in) pipes can be installed in the same trench, with the
same backfill, labor, and engineering, yet the 7.5-cm (3-
in) main has considerably more capacity. Also, saddle-
type, "wet-tap" 32-mm (1-1/4 in) service line connections
can be made to the 7.5-cm (3-in) main, but 32-mm (1.25-
in) wet taps cannot be made to the 5-cm (2-in) main. 7.5-
cm (3-in) pipe is readily available in gasketed joint, but 5-
cm (2-in) is commonly available only in solvent weld joint.
Forthese reasons 7.5-cm (3-in) is becoming the smallest
preferred main size. An exception to this practice is when
5-cm (2-in) pipe is needed to maintain cleansing velocities.
c. Routing
In most cases pressure sewer mains are located outside
of and adjacent to the edge of pavement and
approximately parallel to the road or street, which reduces
the expense of pavement repair and traffic control. In
areas subject to unusual erosion, the preferred location
is often within the paved area. This location is also
favored by some municipalities as being an area where
subsequent excavation is less likely and more controlled,
and therefore being a location more protected from
damage.
An advantage to the use of pressure sewers is that the
small diameter plastic pipe used is somewhat flexible and
can be routed around obstacles. This feature allows
pressure sewers to follow a winding path as necessary.
The pipe should be bent in a long radius if possible, not
in a radius less than that recommended by the pipe
manufacturer. The minimum radius recommended by
the Uni-Bell Handbook of PVC Pipe19 for classes of pipe
most used as pressure sewer main is given by Equation
2-9. In diameters larger than about 10 cm (4 in), the pipe
46
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is stiff and the practicality of achieving the allowed radius
must be considered.
Rb/OD = 200
Equation 2-9
Where,
Rb = Minimum radius of the bending circle.
OD = Outside diameter of the pipe.
Pressure sewer mains are normally buried with a cover
of about 75 cm (30 in). In a few cases where economy is
paramount and subsequent damage is unlikely, they are
buried more shallowly. In colder climates the depth of
bury may be dictated by frost penetration depths. In the
northern U.S. they are often placed at 1.2-1.5 m (4-5 ft),
and in arctic conditions as deep as 2.4 m (8 ft) or deeper.
In cases where these depths result in excessive capital/
installation costs, alternative pipe materials which
incorporate insulation and even heat tracing have been
successfully employed.20
When using large diameter mains, the height of the
isolation valves may dictate a minimum burial depth so
the valve operator is sufficiently below ground surface.
The height of air release valves can also dictate burial
depth if the valves are to be fully underground.
The separation of pressure sewers from water supply
mains and laterals often requires that the pressu re sewer
be buried deeper than would be required for other
reasons. In most instances the separation requirements
between the sewer and private wells and streams are
dictated by state health departments, and the
requirementsdifferthroughoutthe United States. Typical
requirements are shown in Table 2-2.
Profiles of mains are recommended and usually, but not
always, shown on the plans. They may be omitted if the
mains are only a few hundred feet long, if air release
stations are not needed, and if there are no obstacles to
be crossed.
Culvert and utility crossings often dictate numerous
variations in the burial depth of pressure sewer mains,
with many resulting sags and summits in the pipeline
profile. In some cases these variations in the profile are
hydraulically detrimental regarding the accumulation of
air at the summits. When the variations are regarded as
detrimental, reaches of the pressure sewer main may be
placed at a particular depth to allow for the crossings, or
otherwise profiled to minimize summits.
To minimize damage to the pressure sewer main caused
by subsequent excavation, ground surface route markers
Table 2-2. Typical Requirements for Separation of
Pressure Sewer Lines From Water Lines21
Parameter Requirement
Parallel Installations
Crossings
Locate sewer as far as practical from water
main. Minimum separation 3m (10ft). If sewer
Is closer than 3 m (10 ft) from water main,
sewer Is to be located 30 cm (12 in) lower than
the water main. In some jurisdictions, when
doser than 3 m (10 ft), sewer is to be of water
main materials or encased. Other jurisdictions
allow water and sewer in the same trench if the
sewer is 30 cm (12 in) lower.
Crossing is to be as nearly perpendicular as
practical. Sewer tobe30cm(12in) lower than
water main. Some Jurisdictions require that no
joints be used in the sewer main within 3 m (10
ft) of the crossing.
are sometimes placed adjacent to or above the main,
warning excavators of its presence. Good as-constructed
plans are helpful in identifying the pipeline location, and
a cable buried with the main can be induced with a tone
so the main can be field located using common utility
locating equipment.
A warning tape marked "pressure sewer" is sometimes
placed shallowly in the pipeline trench to further notify
excavators. When the tape is placed lower in the trench,
e.g. adjacent to the pipe, it is called an "identification"
tape. The tape can be metalized so it can be detected
with utility locating devices, but most tape cannot be
induced with a tone, so metalized tape should be placed
shallowly to be detected.
d. Trench Section
Trenching may be accomplished using a backhoe, wheel
trencher, or chain-type trencher. The choice of equipment
is usually dictated by the contractor based on equipment
availability and the material to be excavated.
Imported material termed "pipe zone backfill" is often
placed to surround the main several inches if material
excavated from the trench is regarded as unsuitable for
use as that material. Pipezonebackfillisusuallygranular,
as pea gravel or coarse sand. Fine sand or soil is
generally not as desirable as it bulks rather than flowing
into place densely under the pipe haunches.
The remaining backfill material required is often specified
by the agency controlling the road or street, especially if
the mains are located within the pavement.
47
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In some cases a lean cement-sand slurry is used for
backfill. This option is particularly attractive when a
narrow trench is used, the mains are located within the
pavement, and prompt restoration fortraffic is important.
e. Pipe Materials
PVC is most widely used. Polyethylene has also been
used, especially when the number of joints must be
minimized, or when ft is selected for visual identification
in contrast to PVC water mains. Polyethylene has also
been used for lake crossings, and in insulated and heat-
traced form for arctic installations.
The commonest PVC mains are iron pipe size (IPS)
1,400 kPa {200 psi) working pressure rated, standard
dimension ratio (SDR) 21, or 1,100 kPa (160 psi), SDR
26. Even though the operating pressures in the mains
may be far tower than the working pressure rating of the
pipe, tower rated pipe is not normally recommended.
Thinner wall pipe is more likely to be seriously damaged
during installation. Also, the mains should be built to
withstand the pressure of hydraulic cleaning using a pipe
cleaning pig. SDR 26 pipe has been reported to suffer
damage when used with high-head pumping units.4 In
some cases SDR 26 is used only in sizes 10 cm (4 in) and
larger, to avoid the thin wall characteristic of smaller
pipes.
On small diameter gravity sewer projects, PVC sewer
pipe, ASTM D-3033 or D-3034 is often used. This pipe
has a different outside diameter than IPS, and in some
sizes, the outside diameter of D-3033 is different from
that of D-3034, so the availability of fittings should be
investigated.
When PVC mains iargerthan about 20 cm (8 in) are used,
AWWA C-900 pipe is sometimes specified, often for
reasons of the availability of proper fittings. This pipe is
available in two types, iron pipe size, and cast iron pipe
size, the latter being a different outside diameter than
either IPS or sewer pipe.
Pressure irrigation pipe (PIP) is often used to fabricate
pump vaults and otherappurtenances, and has a different
outside diameter than any of the other pipes mentioned.
SeeTable2-3 foran abbreviated listingof pipe dimensions.
Refer to the Unibell Handbook of PVC Pipe19 for a more
thorough listing. Checking with manufacturers for
availability is required as many companies do not produce
all the pipes said to be available.
PVC pipe has a high coefficient of thermal expansion;
about 3/8-in of length variation/100 ft of pipe/10°F
temperature change.
Coefficient = 3.0 x 10* in/in/°F
Equation 2-10
Considerable temperature changes will be experienced
during pipeline installation, and some degree of
temperature change will occur during operation, with
climate changes and effluent temperature changes. To
reduce expansion and contraction induced stresses,
flexible elastomeric ("rubber ring") joint pipe is preferred
for use as mains.
If solvent weld joint pipe is used, the pipe manufacturer's
recommendations for installation regarding temperature
considerations should be followed. The Uni-Bell Handbook
of PVC Pipe19 also provides guidance as to proper
practices.
Fittings most often used are of the solvent-weld joint
type. They are more commonly available than gasketed
joint fittings, and expansion and contraction are allowed
for in the remaining pipe joints. Care must be taken for
proper solvent welding, especially when using largerpipe
sizes that are difficult to handle.
f. Appurtenances
Isolation valves (IVs) are used on pressure sewer mains
much as they are on water mains. Gate valves may be
used, or resilient-seated gate valves, and occasionally
ball valves are used. Typical locations for IVs are at
intersections, both sides of bridge crossings, both sides
of areas of unstable soil, and at periodic intervals on long
routes. The intervals vary with the judgment of the
engineer, but are typically about 0.8-1.6 km (0.5-1 mi).
Cleanouts are occasionally provided. The most common
type consists of a valved wye extending to ground
surface that can launch a pipe cleaning pig. When
cleanouts are provided, they are typically placed at the
ends of mains, and where main diameter sizes change.
Thrust anchors should be used as they are in water main
practice. Even though the operating pressure in the main
may be low enough that thrust anchors may not seem to
be required, the main should be hydrostatically tested
following installation at a pressure of about 1,400 kPa
(200 psi). A properly installed PVC pipeline will easily
pass that pressure test, but a poor installation will be
revealed. Thrust anchors and a quality installation may
also be required if pipeline cleaning by pigging is
anticipated.
48
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Table 2-3.
Abbreviated Listing of PVC Pipe Dimensions
IRON PIPE SIZE (IPS)
Ham. O.D.
(in)
1/2 0,840
3/4 1.050
1 1,315
1-1/4 1.660
1-1/2 1.900
2 2.375
2-1/2 2.875
3 3.500
4 4.500
6 6.625
8 8.625
10 10.750
12 12.750
Min. wall
SDR-26 SDR-21
CL. 160 CL 200
0.090
0.113
0.137
0.167
0.173 0.214
0.255 0.316
0.332 0.410
0.413 0.511
0.490 0.606
Sen. 40 Sen. 80
0.109 0.147
0.113 0.154
0.133 0.179
0.140 0.191
0.145 0.200
0.154 0.218
Note: Other thicknesses
include SDR-13.5,
SDR-32.5, and SDR-41.
SEWER PIPE PSP ASTM D-3033
Norn. O.D.
On)
4" 4.215
6 6.275
8 8.160
10 10.200
12 12.240
15 15.300
Min. wall
SDR-41 SDR-35
0.125 0.125
0.153 0.180
0.199 0.233
0.249 0.291
0.299 0.350
0.375 0.437
* 4" Is SDR 33.5
SEWER PIPE PSM ASTM D-3034
Mom. O.D.
(in)
4" 4.215
6 6.275
8 8.400
10 10.500
12 12.500
18 18.700
Min. wall
DR-42 DR-35
0.125
0.180
0.200 0.240
0.250 0.300
0.300 0.360
0.536
AWWA Q-900 CAST IRON PIPE SIZE fCIPC}
Nom. O.D.
(in)
4 4.800
6 6.900
8 9.050
10 11.100
12 13..200
Min. wall
DR-25 DR-18
PC.100 PC.150
0.192 0.267
0.276 0.383
0.362 0.503
0.444 0.617
0.528 0.733
DR-14
PC.200
0.343
0.493
0.646
0.793
0.943
Table 2-3. Abbreviated Listing of PVC Pipe Dimensions
(continued)
AWWA C-900 IRON PIPE SIZE
Nom.
(in)
4
6
8
10
12
O.D.
4.500
6.625
8.625
10.750
12.750
PIP IRRIGATION
Nom.
(in)
4
6
8
10
12
15
18
20
O.D.
4.13
6.14
8.16
10.20
12.24
15.30
18.36
20.40
DR-25
PC.100
0.180
0.265
0.345
0.430
0.510
PIPE
Min.
SDR- 100
50' Hd.
0.065
0.070
0.080
0.100
0.120
0.150
0.180
0.200
Min. wall
DR-18
PC.150
0.250
0.368
0.479
0.597
0.708
wall
SDR-32.5
125psi
0.127
0.189
0.251
0.314
0.377
0.471
-
-
DR-14
PC.200
0.321
0.473
0.616
0.768
0.911
Note : Other thicknesses
include SDR-41,
SDR-51,andSDR-93.
Air release valves are required on major systems and
require hydraulic analysis for placement. Water-system-
type air release valves have been tried, mostly without
success due to corrosion or clogging with sludge.
Wastewater-type air release valves are recommended.
On pressure sewer systems serving more than about
500 homes, the provision of pressu re monitoring stations
(PMS) is advised. These consist of a small diameter
service line connected to the side of the main, and
extending to a terminus in a valve box or vault. An
isolation valve is provided at the terminus, and a fitting
necessary for connection to a mobile pressure sensor -
recorder that may be moved from station to station. PMS
are used to occasionally record pipeline pressures (the
hydraulic gradient), to measure how the piping system is
performing. This is particularly of interest over time, or ft
may be of interest if the placement or performance of air
release assemblies are in question.
Flow meters are of considerable value, especially on
large systems. Magnetic-type flow meters are the most
common.
2.4.2.2 Service Lines
a. Geometry
Pressure sewer service lines are typically arranged
similar to water services. A typical location is near and
49
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parallel to property lines, but where property line locations
are not well known, ft is advisable to maintain some
distance from them.
It is good practice to field mark the location of the service
I'mewilh boldly identified lath afew days priorto installation.
This serves as a reminder to the property owner about
the intended location and may cause the owner to
recognize some reason that the location should be
changed. It also serves as an advance noticeto neighbors
if property boundaries are in dispute.
Most municipalities prefer locating the service line where
ft will not be driven over, but other jurisdictions prefer
locating the service line within the paved driveway. The
reasoning is that subsequent excavation and associated
damage to the service line may be less likely within the
pavement.
Service lines should be located distant from potable
water lines to reduce possibility of cross contamination.
They should also be distant from other buried utilities if
possible, due to the possibility of damage caused by the
subsequent excavations for maintenance or repair of
those utilities.
Service line profiles may normally undulate without much
concern. The velocity and du ration of flow and the typical
diameter and length of service lines are such that air
rarely collects in the summits to such an extent as to
cause hydraulic problems.
b. Pipe Sizing
Typical service lines serving individual homes are 32 mm
(1,25 in) in diameter, but have varied from 19 to 38 mm
(0.75-1.5 In). A primary reason for what might seem to be
over sizing is to limit headloss, so head-limited pumps
can discharge at an adequate rate. A second reason for
the seemingly large 32-mm (1.25-in) diameter is that
check valves in that size will easily pass any solids that
the pump can discharge.
Multiple EDU (MEDU) service lines are often 5-cm (2-in)
diameter or larger and should be separately evaluated for
hydraulic capacity.
To evaluate service line sizing, ordinates are subtracted
from the pump curve for various discharge rates
corresponding to headlosses in the service line. The
resulting plot is the effective pump curve, that is, the
characteristics of the pump at the main,
c. Trench Section
Where soil types allow the use of chain-type trenchers,
use of trenchers is sometimes specified for service line
installations as they cause less disruption to the property
owners yard than backhoes. Rocky soils and some
clayey soils that will not self-clean from the trencher teeth
may be impractical to excavate using a trencher.
Street crossings are often accomplished by pushing a
steel conduit under the street to act as a sleeve for the
service line that is installed inside. Other street crossings
are bored, or use a "hog." Open cutting of the street is
done where other means are impractical.
Service lines are buried below the frost penetration
depth, and usually at a minimum of 45-60 cm (18-24 in),
as a measure of protection from subsequent excavations.
In rocky settings in moderate climates, service lines are
sometimes buried only 30 cm (12 in).
Bedding and backfill materials for service lines are usually
the native material taken from the trench excavation,
especially when a trencher is used and the material is
well broken up. When the service lines are installed under
travelled ways or when rock excavation is encountered,
surrounding the service line with imported pipe zone
backfill is advised.
d. Pipe Materials
Schedule 40 PVC is the most commonly used service
line material. In small sizes, such as the normal 32-mm
(1.25-in), conventional 1,100-kPa (160-psi), PVC pipe
has a thin wall that is subject to damage during
construction. It is for this reason that the heavier walled
schedule 40 pipe is normally used, while lower-pressure-
rated pipe is used for the larger diameter mains.
PVC service lines usually use solvent-weld-type fittings.
Rubber ring fittings are not commonly available in small
diameters. Also, service lines are short as compared to
mains so thermal expansion and contraction is a lesser
concern. The manufacturers instructions should be
followed regarding pipe laying where substantial
temperatu re changes are expected. Expansion joints are
available as a separate fitting, intended for use on
solvent-weld-joint pipe.
Polybutylene and polyethylene have also been used.
Either of these materials can be installed without joints
and are favored to place within conduits at street crossings.
Compression fittings are used in preference to insert
fittings which reduce the size of the opening in the pipe.
The service line should be pressure-tested. A common
practice is to hydrostatically test the line while visually
examining all joints prior to backfilling.
50
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e. Appurtenances
Connections to the main can be made by tee or by service
saddle. Tees can only be used when they are installed
while the main is being placed. Service saddles can be
used to make wet taps to the main in service. To place
tees, the service line location must be accurately and
reliably known at the time the main is installed.
Intuitively, wyes are sometimes thought to be preferable
to tees, for connection of the service line to the main.
However, wyes are not hydraulically superior. Neither
are wyes available in pressure rated PVC. There has
been limited use of non-pressure rated drain, waste, and
vent (DWV) wye fittings, but they are available only in
limited sizes, and their use is seldom seen.
Connections to the main should be of high quality,
considering the large number of them, considering that
they are buried, and that a small errorcan be compounded
to be a large problem.
A corporation stop is typically used at the service saddle,
and a gate valve or ball valve also used for isolation,
sometimes placed at the street right of way line. A buried,
redundant check valve is also often used on service lines.
So the check valve can be later found, it is placed
adjacent to the isolation valve which has a valve box riser
to ground surface.
Alternately, the check valve and isolation valve are
placed in a valve box. The valve box is usually too small
to allow field personnel to remove and reinstall a valve,
so the box has to be dug up to provide access. The box
allows operation of the isolation valve handle, and marks
the location of the facilities. When a valve box is used,
consideration should be given to frost protection.
A toning wire or metalized marker tape buried with the
service line facilitates future location.
2.4.2.3 Building Sewers
The term building sewer refers to the gravity flow pipe
extending from the home to the interceptor tank or pump
vault. In many cases state or local authorities regulate
installations of building sewers. The Uniform Plumbing
Code is often referenced.21
The building sewer should slope continuously downward
as specified by state code, usually at a slope of not less
than 0.25in/ft, or 2 percent grade. Desirably, the pumping
unit should be located near the home so the building
sewer is short, with less need for maintenance and less
opportunity for I/I.
If an existing building sewer does not have a cleanout,
one should be placed outside and close to the home.
Some agencies prefer having a cleanout at the dividing
line where agency maintenance begins. For example, if
a sewer district had placed 6 m (20 ft) of new building
sewer to join an existing building sewer and if that point
marked where district maintenance begins, a cleanout
may be used there.
When G P or SH systems are used and the bu ilding sewer
enters the pump vault without a tee or ell, maintenance
of the building sewer may be accessed via the pipe end.
However, some agencies prefer discouraging the pipe
cleaning contractors from entering the pump vault.
Bends in building sewers should be avoided where
possible, and a cleanout used for each aggregate change
in direction exceeding 57°.*
Infiltration via leaking building sewers has been common,
as has the connection of roof or yard drains. A quality
installation is advised, which determines the existence of
these and eliminates them during construction. Smoke
testing has been effectively used to reveal sources of
extraneous water, but care must be taken to keep the
smoke from entering homes.
PVC or ABS piping materials are most widely used.
Some regulatory agencies require ABS in certain
locations, such as in proximity to the home, or under
driveways where external loads may be high.
Direct burial (UF) or conduited wiring from the home to
the pumping unit is often placed in the same trench with
the building sewer, but this practice may require approval
of the regulating authorities.
2.4.3.1 Grinder Pump and Solids Handling Pump
Vaults
The various manufacturers of pressure sewer equipment
provide somewhatdifferent packages, but many generally
resemble that shown in Figure 2-13. These vaults are
typically made of FRP and vary in diameter from 60 cm
to 120 cm (24-48 in), with the larger sizes usually being
applied to duplex installations involving two pumps to
serve a group of homes. The height may vary from 1.2 m
to 2.4 m (4-8 ft).
The pump vault may be divided into zones, with each
zone describing a particular purpose. Each manufacturer
may have recommended dimensions to fit their own
equipment, but to gain an understanding of the functions,
an example is given. For ease in mental calculation of
volumes held in circular basins, Equation 2-11 is used:
51
-------
FIflUi* 2*13.
Zoning of GP or solids handling pump vault.
WORKING
VOLUME
(C)
BUILDING
SEWER
SLIDE AWAY
COUPLING
FLOOR
LIQUID
LEVEL
SENSORS
52
-------
V = 6 D2 (approximate)
Equation 2-11
Where,
V = Volume per foot of depth (gal)
D = Inside diameter of basin (ft)
Referring to Figure 2-13, the pump inlet is seen to be
suspended some distance (a) above the floor of the vault.
This dimension may typically be about 10 cm (4 in).
The pump inlet is submerged some distance (b) below
the lowest operating liquid level in the vault, the pump
"off" level. This dimension may be about 2.5 cm (3 in), or
a greater distance may be preferred to prevent vortexing.
When using grinder pumps this dimension b is kept small
so floating grease will not accumulate excessively.
The pump "on" level may be 30 cm (12 in) or so (c) above
pump "off." If a 60-cm (2-ft) diameter basin is used, the
working volume would be 91 L (24 gal) pumped/cycle
(ignoring the volume displaced by the pump, and the
wastewater entering the vault during the pumping cycle).
The alarm level is above the pump on level by some
amount (d). In this example assume 15 cm (6 in) so the
volume held in the basin between on and alarm is 45 L (12
gal), a small allowance.
If a duplex installation is used, the lag pump would turn
on at the level shown as alarm in Figure 2-13, and the
alarm level would be correspondingly higher.
Once the liquid level rises above the crown of the
incoming building sewer, ventilation via the roof vent of
the home is interrupted. Some users of GP or SH
systems install a small P-trap vent through the cover or
upperwall of the vault for continued ventilation under this
condition.
Between the alarm level and the top of the basin is the
reserve volume (e), if ventilation has been provided.
Alternatively, an overflow may be provided to a holding
tank or other structure (not shown).
The vau ft cover may be bolted on or, with some designs,
the cover will be lifted up to allow a spillage on the lawn
in preference to having backflow into the home in the
unlikely event both the pump and check valves fail. If
sized large enough, the P-trap vent accomplishes the
same thing.
2.4.3.2 Septic Tanks (Interceptor Tanks)
In pressure sewer and small diameter gravity systems
the septic tank has often been called an interceptor tank
owing to the differences between septic tanks used in
conjunction with drainfields versustanks used on pressure
sewer systems. The interceptor tank is generally an
engineered product of higher quality, stronger, and more
water tight. Except for the possible incorporation of a
pump and its containing vault, an interceptor tank is
functionally the same as a septic tank.
A comprehensive study on septic tank sludge and scum
accumulation was accomplished in the 1940s by the U.S.
Public Health Service.23 Over 600 references were made
to develop information from previous research, and
practices were reviewed in 12 countries plus the United
States. Over 200 operating tanks also were studied, as
well as many full-scale laboratory systems. A regression
analysis of their observations resulted in equations relating
sludge and scum accumulation,,with time and are
presented here as Equations 2-12 and 2-13:
Sludge accumulation:
Scum accumulation:
= 0.45T-0.12
Equation 2-12
Equation 2-13
Where,
V = Volume per capita (cu ft)
T = Time in years
Of the total scum accumulation, about 1/3 was reported
to lie above the liquid level, and 2/3 below.
Sludge clear space was described as that distance
between the top surface of the sludge and the outlet tee.
To avoid scour and carryover of solids, a minimum
sludge clear space of 15 cm (6 in) was suggested.
Scum clear space, the distance between the bottom of
the scum layer and the outlet baffle or tee (or inlet ports
in the pump vautt) was recommended to be a minimum
7.5 cm (3 in), but pressure sewer experience suggests 1 5
cm (6 in) to be a better allowance since scum is a
particularly problematic material if allowed to enter the
system. .
When Equations 2-12 and 2-13 are solved for typical
single family occupancies and 3,780-L (1,000-gal)
interceptor tanks of usual shape, and using 1 5-cm (6-in)
clear spaces for scum and sludge, the pump vault inlet
port level appears best placed at about one-third of the
lowest liquid level in the tank.
53
-------
An audit of septage accumulation was made on the
Glide, Oregon STEP system by the Douglas County
Department of Public Works. Sludge and scum levels
were measured in 400 tanks which had been in service
for8-years. In one analysis of that study, measurements
in 186, 3,780-L (1,000-gal) STEP tanks serving single
family residences were selected for evaluation. Results
are shown in Table 2-4.
Fromthe Glide study and otherobservations, the following
guidelines were proposed for estimating average sludge
and scum accumulation at single family residences, with
the caveat that accumulations vary greatly from home to
home:
1. Annual combined sludge plus scum accumulation:
33 gal/home.
2. Scum comprises about 1/3 of the combined volume
of sludge plus scum.
3. About 1/3 of the scum lies above the water (effluent)
level.
4. Pump vault inlet ports should normally be located at
about 1/3 of the depth below the 'pump off' level.
Uhasbeendescribedthat sludge and scum accumulations
vary so much as to make accurate forecasts futile.24
Observations on hundreds of interceptor tanks reinforce
his position. However, no better general placement of the
outlet ports has been dictated than that given above.
Zoning of the interceptor tank for scum and sludge
accumulation and location of the ports in the pump vault
are shown in Rgure 2-14.
When the pump vault is an integral part of the tank, the
liquid level in the entire interceptor tank rises and falls in
response to incoming flows and in response to pumping.
Although liquid level control settings may vary, typical
settings for single home application in 3,780-L (1,000-
gal) tanks are 7.5 cm (3 in) between pump off and on, and
7.5 cm (3 in) between on and high water alarm. Most
3,780-L (1,000-gal) interceptor tanks contain about 30 L7
cm (20 gal/in) depth, so the 7.5-cm (3-in) settings
correspond to about 230 L (60 gal). Figure 2-15 depicts
these liquid levels.
The various dimensions may differ as dictated by the
individual design engineer. One typical design for a
single family residence, using a 3,780-L (1,000-gal) tank
and the guidelines for sludge and scum accumulation
noted above is given in Table 2-5. (Note that septic tanks
normally hold 10 percent more than their rated volume,
so a 3,780-L (1,000-gal) tank would contain 4,160 L
(1,100 gal) filled).
Table 2-4. Sludge and Scum Accumulation at Glide, Oregon
(186 1,000-gal tanks)
Mean
S. Dev.
Min.
Max.
Time
(yr)
8.2
0.7
7.2
9.1
Occupants
(No.)
2.75
1.18
1
6
Sludqe
(gai)
195
98
20
530
Scum
(gai)
92
60
0
300
Total
(gai)
289
114
60
650
This type of table is used to locate the positions of liquid
level sensors and pump vault ports, but is not a reliable
indicator of the amount of sludge and scum to be
accumulated at the time septage removal is necessary.
This is because accumulations vary so greatly from
home to home. Scum and sludge accumulations at
facilities otherthan single family homes vary considerably.
For example, restaurants produce considerable grease;
while laundromats produce considerable heavy sludge,
but little scum.
2.4.3.3 Multiple-Compartment Tanks
The use of baffled, or multiple compartment septic tanks
has often been considered. Objectives have been to
reduce the concentration of suspended solids in the
effluent, and to limit the consequence of digestion upset.
Some have speculated that if septage is not removed as,
scheduled, flow into the first cell of a two compartment
tank may be plugged with sludge or scum before low
quality effluent is discharged from the second cell, but
this speculation has proven unreliable.
Baffling is sometimes thought to achieve improvement in
performance by providing longer detention time, better
dispersion, reduced short circuiting, keeping sludge and
scum away from the outlet, and reduction of turbulence.
But opinion differs as to whether the performance of two
smallertanks (the product of providing baffles) is superior
to that of one larger tank.
Studies have been made on the performance of single-
and multiple-compartment tanks by the U.S. PublicHeaKh
Service,23 Winneberger,24 Baumann and Babbitt,25 and
Jones.11 These studies have generally concluded that
single compartment tanks are economical, practical, and
perform well, but that baffling is advantageous to
performance. The degree of improved effluent quality
has been described as ranging from "microscopic" to
"statistically significant."
Performance varies considerably depending on how
compartmentation is accomplished. Two types of
compartmented tanks are shown in Figures 2-16 and 2-
17. In the former figure, a hole or window is provided in
54
-------
Figure 2-14.
Zoning of a STEP system Interceptor tank showing scum and sludge accumulation.
RISER & COVER
LEVEL SENSOR ASSEMBLY
JUNCTION BOX
•4- SERVICE LINE
INLET
PORTS
PUMP VAULT
PUMP
Figure 2-1S. Zoning of a STEP system Interceptor tank showing liquid levels at pump off, on, and high level alarm.
RISER & COVER
LEVEL SENSOR ASSEMBLY
JUNCTION BOX
-4-SERVIGE LINE
INLET
PORTS
PUMP VAULT
PUMP
55
-------
FIgura 2-16. Two-compartment interceptor tank with hole in baffle wall whore clear space expected.
__/*-
Flgur* 2-17. Two-compartment interceptor tank using combination tee and 1/4 bend.
—JL
ra
56
-------
Table 2-5. Typical Zoning Design For a 1,000-gal
Interceptor Tank Serving a Single-Family
Residence
Parameter
Sludge
Sludge clear space (6-in)
Scum dear space (6-in)
Submerged scum
Floating scum
On-off pump cycle (3-in)
On-alarm (3-in)
Reserve space*
Volume
(gai)
370
120
120
120
60
60
60
190
Total
1,100
Reserve is 720 L (190 gal) above the alarm level
when there is 230 L (60 gal) of floating scum, or 950
L (250 gal) when there is no floating scum.
the baffle wall separating the two cells, placed in the
center of where the first compartment clear space is
expected to be. The latter f igu re shows a combination tee
and 1/4 bend with the invert placed at the liquid level of
the first cell, and the inlet located in the clear space of the
first cell. Generally, the latterdesign has been credited as
providing the better performance.
If an internal pump vault is placed in the second cell of the
tank, as shown in Figure 2-16, the liquid level will rise and
fall throughout the full length of the tank in response to
pumping, as shown in Figure 2-15.
However, if an internal pump vault is placed in the second
cell of the tank shown as Figure 2-17, the liquid level
fluctuates only in the second cell. If the second cell is
small and if the usual zoning volumes are used between
pump off, pump on, and high level alarm (normally about
230 L [60 gal] each), the liquid level fluctuations may be
considerable. This causes the liquid level within the
second cell to be quite low at pump off level, and subjects
the tank to a more unbalanced structural loading due to
the soil backfill. This has been of most concern when
flexible, plastic septic tanks, or concrete tanks of marginal
strength are used.
Baffle walls must be made strong enough to withstand
the liquid level being at ope rating height on one side of the
wall, but with the other cell being empty. This has been
difficult to accomplish with some plastic tank designs.
Experience with thousands of single-cell septic tanks
used on STEP systems has shown a single riser to be
sufficient. If multiple cells are used, each cell must be
fitted with a riser to provide access for septage removal.
To further evaluate multiple-compartment tanks, the
purpose of the tank as used in a STEP system should be
reviewed. If discharge is to a municipal treatment works,
the purpose is generally to capture the grit, grease, and
stringy material that would present a problem to pumping,
foul the liquid level sensors, and possibly cause
obstructions in the piping system.
Single-celltanks have been used extensively and perform
satisfactorily for this purpose. Pump clogging occurs in
only about one percent of the installations annually.
Clogging of mains is nonexistent. It seems little benefit
would be derived by the use of multiple-compartment
septic tanks.
If, however, discharge is made to a soil absorption field
or other similar facility where the maximum reduction of
suspended solids may be critical, properly designed
multiple compartmenttanks will perform better functionally.
The need for quality, structurally adequate, infiltration-
free tankage is usually regarded as more apparent than
the need for compartmentation. However, research on
tank shape, inlet and outlet fittings, gas baffle deflectors,
and other design factors is encouraged.
2.4.3.4 Tanks Serving Multiple Homes
Most projects standardize on 3,780-L (1,000-gaI) tanks
to serve any size home, and are used to serve up to 3
homes. The benefits of standardization outweigh the
benefits of variable tank sizing. However, the frequency
of septage pumping increases, but not proportionately,
with the number of people served.
When MEDU installations are made, such as to serve a
restaurant, apartment complex, mobile home court, or
other facility having flows greater than that expected from
about 3 EDUs, larger tankage is needed. The use of
precast tanks placed in series has been successful.
Multiple tanks in parallel have experienced problems in
dividing flows evenly. In some cases single large precast
tanks have been used. In a few cases, large tanks have
been cast in place, but this practice is often the least
desirable option due to site disruption.
Figure 2-18 shows a MEDU installation using precast
tanks placed in series. Two pumps are placed in the final
tank. To make use of reserve space within the tanks
without breaching the lowest tank riser, all risers should
extend at least to the elevation of the soffit of the highest
tank. It is often impractical to provide as much reserve
space in the tanks as would be desired, in which case
emergency overflow to an existing or new drainfield is
sometimes provided.
57
-------
Figure 2-18. Multlpte-unlt Interceptor tank and pump assembly.
TOPS OF ALL RISERS SHALL BE AT ELEVATION EQUAL OR ABOVE SOFFIT OF HIGHEST TANK SERVICE LINE
DUPLEX PUMPS
(ONE PUMP SHOWN FOR CLARITY)
*(2ND PUMP ON AT ALARM
FLOAT FOR LEAD PUMP)
-------
To insure ventilation between tanks, even if the liquid
level in any tank is higher than the crown of the inlet or
outlet fittings, a small pipe "jumper" may be placed to join
risers.
Sizing relies in part on the judgment of the engineer, with
regard to flows and to character of the wastewater.
Expectations of sludge and scum accumulation are
guided by experience with the types of facilities being
served. Sizing of MEDU grinder pump vaults follows
conventional sewerage pumping practice.
One method used for septic tank sizing is provided by
Equation 2-14:*
V = 0.750 + 1,125
Equation 2-14
Where,
V = Volume of septic tank (gal)
Q = Daily flow (gpd)
Daily flow can be estimated by making comparisons with
similar facilities in the area having metered water. Other
methods are described in the Design Manual for Onsite
Wastewater Treatment and Disposal Systems.16
As for septage removal frequency, experience with
hundreds of interceptor tanks on pressure sewer systems
has shown that tanks which meet the above criteria
serving typical single-family residences require pumping
at about 10-yr intervals. However, septage accumulation
varies widely from home to home, and an interval of
about 8 yr is often adopted.
The best method is to remove septage when it begins to
encroach on the clear space minimums, which requires
measurements using devices described best in several
publications.23-24'27 Septage removal from tanks
discharging to soil absorption fields should be more
frequent due to the variability of septage accumulation
and due to the severe damage caused if solids are
discharged to them.
MEDU establishments such as restaurants require
septage removal much more frequently. Restaurants
discharge large and troublesome volumes of grease and
the extremely hot dishwasher water adds to the problem.
Excess grease should be removed prior to discharge to
the interceptor tank. Methods of accomplishing this are
outside the scope of this document, but are presented
elsewhere.11'23-28
2.4.4 Pumps/Electrical Service
2.4.4.1 Pumps
a. General
Centrifugal pumps are most commonly used, followed by
progressing cavity pumps. Submersible water well pumps
having 5 or more stages (impellers) are sometimes used
on STEP systems, and pneumatic ejectors have seen
limited use in the past, but none are presently being
marketed.
For approximate estimations of horsepower required for
particular head and flow conditions, the following equation
can be used. Equation 2-15 has been derived from the
definition of horsepower being 33,000 ft-lb/min, and the
weight of water being 8.34 Ib/gal:
Hp = QH/(3,960E)
Equation 2-15
Where,
Hp = Horsepower
Q = Discharge rate (gpm)
H = Head (ft of water)
E = Pump and motor efficiency (%)
Equation 2-15 is used for only the most general purposes
since pump selection depends considerably on specific
pump characteristics. It is used, however, in conjunction
with pump characteristic curves or testing data to
determine efficiency.
The ability of a pressure sewer pump to run at shutoff
head is a consideration. Shutoff head conditions occur
when an isolation valve has been closed, or following a
long power outage when many pu mps run simultaneously,
dominating some of the pumps to shutoff head.
Some pumps have the ability to run dry for extended
periods. A dry running condition occurs when the pump
has become air bound or when the "off" liquid level sensor
malfunctions.
If the liquid level in the pump tank is higher than the
hydraulic grade line of the main, siphoning to the main
can occur after the pump motor has turned off. This can
lower the liquid level in the pump vault to the elevation of
the pump intake, allowing air to be drawn into the pump
and causing some pumps to become air bound.
In areas where the power supply undergoes extremes of
voltage variation or brownouts, motors tolerant of these
conditions may be preferred, such as permanently split
capacitor (PSC) motors.
59
-------
b. Centrifugal Pumps
The most common GP or STEP pressure sewer pump is
the submersible centrifugal.
The head-discharge curve is typically shaped as shown
in Figure 2-19a. When two or more pumps discharge
simultaneously into a common header the abscissas are
additive, and the resulting curve is as shown in Rgure 2-
19b. Because in pressure sewer use the pumps are
located at different stations and different elevations
along the main, the analysis becomes more detailed.
This procedure is given by Ranigan and Cudnik.13
Since the pump discharges via a service line to a
pressure sewer main, headlosses in the service line can
be subtracted from the ordinates of the pump H-Q curve
to form the effective pump curve at the main, as shown
in Figure 2-20.
Centrifugal pumps draw most power at maximum
discharge, at the far right of the H-Q curve, and draw the
least power at shutoff head. Efficiencies are shown on
some published curves, with greatest efficiency usually
being about mid point on the curve.
STEP system centrifugal pumps can discharge light
solidsthat may be present in septic tank effluent. Typically
the pumps can handle solids ranging from 6-mm to 18-
mm (0.25-0.75 in) diameter. Small solids handling pumps,
intended for raw wastewater use, can pump solids of
about 5-7.5 cm (2-3 in) in diameter. Grinder pump
impellers can handle only small solids since the pumping
of large solids is not necessary due to the grinding action.
STEP impellers are usually made of bronze or plastic. If
a pump having an iron impeller sets in septic wastewater
without operating for a while, e.g. when the homeowner
is on vacation, the impeller can become bonded to the
volute by iron sulfide. Often the motor will not have
enough starting torque to break the impeller loose from
the iron sulfide, occasioning a service call. Bronze and
plastic impellers are generally more resistant to this
bonding than iron impellers.
Some grinder pumps use iron impellers, but have fewer
occasions of iron sulfide binding than STEP pumps
because the GP uses a higher horsepower, higher
starting torque motor and the wastewater is less likely to
contain sulfide species at the pump vault than a STEP
system. .
Iron sulfide plates .onto the iron casing, especially in
STEP systems. In most casesthe effect is only cosmetic,
but in some cases corrosion and metal loss occurs. The
corrosion depends on the sulfide generating potential of
the wastewater (e.g. suit ate in water supply, BOD, and
temperature). If the pump is constantly submerged it will
normally corrode less than if it is sometimes exposed to
the atmosphere.
Most centrifugal, submersible pumps used on pressure
sewer systems can operate at shutoff head without
damage for extended periods. Some engineers specify
that a 3-mm (1/8-in) diameter hole be provided in the
pump casing, to leak slightly during shutoff conditions,
causing circulation and cooling. Since the holes tend to
clog, some engineers favor using orifice bleeder valves
forthis purpose. These are simple, neoprene valves that
have a f lapperto partially close the orifice during discharge.
Most submersible pumps can run dry for extended
periods without damage. Dry running is often caused by
an air bound condition following siphoning. To prevent
siphoning, a hole or orifice bleeder valve is sometimes
provided to admit air and break the siphon. The hole or
valve can also allow the air to escape, preventing air
binding even if siphoning occurs. With STEP systems
using pump vaults inserted in the interceptor tank,
uncontrolled siphoning can causethe pump vault contents
to be lowered such that the pump vault can float out of
position.
Most high-head centrifugal pressure sewer pumps use
3,450-rpm, 2-pole motors rather than slower running
1,725 rpm, 4-pole motors.
In general, for centrifugal pumps steeper H-Q curves and
higher heads are obtained by the impeller being shallow
(having less solids handling capacity), having fewer
vanes, and having more wrap to the shape of the vanes.
Larger diameter impellers are associated with higher
heads. There are various impeller designs, such as
enclosed, open, and vortex. Each type and design has its
own characteristics.
Submersible, multiple stage water well pumps are small
and light-weight which make them easier for field
personnel to handle. Due to the multiple stages, high
running speed, and shallowness of the impellers, high
heads are achievable using fractional horsepower motors.
Well pumps are damaged if run at shutoff head, so
employ the drilled hole or bleeder mentioned previously
to leak sufficient flow to lubricate the pump and to keep
the motor from excessively overheating.
If run at excessively high heads, a well pump develops a
downthrust condition, and if run at excessively high
discharge rates, an upthrust condition develops. To limit
the latter condition an orifice restriction is sometimes
60
-------
Figure 2-19. Head-discharge curves for one and multiple centrifugal pumps In parallel.
100
50
P
til
til
50
DISCHARGE (GPM)
100
150
150
100
50
§
a Single pump H-Q curve
ONE
PUMP
RUNNING
I
ABSCISSAS ARE ADDITIVE.
0 50
DISCHARGE (GPM)
100
150
200
250
300
b. H-Q curves for multiple pumps
61
-------
Figure 2-20. Effsctlve pump curva.
100
80
60
40
20
I
PUBLISHED
PUMP
CURVE
HEADLOSSES IN THE SERVICE LINE ARE SUBTRACTED
FROM THE PUBLISHED PUMP CURVE. (HEADLOSSES FOR 100
FEET OF 1-1/4" SERVICE LINE WERE USED IN
THIS EXAMPLE.)
EFFECTIVE
PUMP
CURVE
_L
20
DISCHARGE (GPM)
40
60
80
100
and consequently preventing the pump from running at
too high a discharge rate.
Well pumps must be used in conjunction with screens or
fitters, since they have no solids handling capability. They
must also be installed in a sleeve resembling a well
casing, so water flows past the motor to provide cooling.
c. Progressing Cavity Pumps
A progressing cavity pump is a semi-positive
displacement, screw type-pump consisting of a single
helical rotor turning eccentrically in a double helical
elastomeric stator having twice the pitch length of the
rotor. This forms a series of spiral shaped sealed cavities
that progress from suction to discharge. Figure 2-21
shows the rotor and cutaway stator of a progressing-
cavrty pump.
Most progressing-cavity pumps used on pressure sewer
systems have the pumping elements submerged, but dry
well installations are sometimes used. The progressing-
cavity pump self-primes reliably.
Atypical head-discharge curve is shown as Figure 2-22.
The H-Q curve is steep, discharging a predictable flow
rate. The mid range discharge rate is generally adopted
as the pump flow rate, irrespective of the slight change
caused by head variation. When numerous progressing-
cavity pumps discharge simultaneously into a common
header (pumping in parallel), the resulting flow is taken as
the adopted discharge rate for one pump times the
number of pumps running. This isin contrast to centrifugal
pump analysis where the composite pump curve is
plotted, and the discharge rate is taken to be the
intersection of the composite curve and the system
curve.
The power draw of a progressing-cavity pump increases
with increasing discharge pressure.
Progressing-cavity pumps do not have a natural shutoff
head but instead continue increasing pressure within the
capacity of the motor and the ability of the components
to withstand the pressure. If run against a closed valve,
amperage draw increases and the thermal overload
eventually trips out, shutting off the motor. After cooling,
the thermal overload automatically resets and the pump
62
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Figure 2-21. Rotor and cutaway stator of progressing cavity-type pump. (Courtesy of Environment/One Corp.).
63
-------
Figure 2-22.
160
Typical progresslng-cavity pump H-Q curve.
140
120
100
80
60
40
20
§
IU
u..
D
I
J_
0 10
DISCHARGE (GPM)
20
tries again to discharge. If the valve is still closed, the
thermal overload again heats and trips out, and the
process repeats. The thermal overloads used are capable
of withstanding this duty.2
If the closed valve was located right at the pump discharge,
the stator would likely be destroyed by running at
excessively high pressure. However, in pressure sewer
applications some air is normally present in the piping
system to cushion the effect, so a closed mainline
isolation valve may present a less severe condition.12
Some installations use a pump-mounted pressure relief
valve to avoid overpressure conditions, some rely on the
protection provided by the motor thermal overload, and
others sense the discharge pressure and turn off power
to the pump motor until the high pressure condition has
been corrected.
The stator will be destroyed if a progressing-cavity pump
is run dry, and if the dry running continues, the rotor and
other components will be damaged. However, if the rotor
and stator are wet enough for lubrication they can be run
without damage.
Progressing-cavity pumps do not become air bound, but
anti-siphoning devices are recommended to be used with
them to prevent air release into the pressure sewer and
to keep the rotor and stator lubricated.
Four-pole motors, running at 1,725 rpm are used.
2.4.4.2 Electrical service
Most grinder pumps and solids handling pumps, which
require more starting torque than effluent pumps, use
capacitor start-capacitor run (CSCR) motors, and some
use capacitor start-induction run (CSIR) motors.
Centrifugal effluent pumps use CSIR or permanently split
capacitor (PSC) motors, and some fractional horsepower
effluent pumps use shaded pole (SP) motors. Nearly all
motors have automatic reset thermal overload protection
built in.
Power consumption is normally low; often less than
$1.00/month. Power use can be estimated by Equation
2-16, or more correctly by Equation 2-17:
P = 745 Hp T (approximate)
P=TIEF
Equation 2-16
Equation 2-17
Where,
P = Power consumption (watt-hours)
H = Horsepower
64
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T = Running time (hours)
1 = Amperage
E . Voltage
F = Power factor (percent)
As points of reference, typical power factors are about 50
percent for CSIR motors, and higher for PSC motors.
CSIR motors will generally draw more amperage than
PSC motors.
Malfunctions of the liquid level sensors are a major cause
of service calls. Accordingly, special attention should be
given to specifying good sensors and applying them
properly.
Mercury float switches have been the most commonly
used liquid level sensors. One float is required for pump
off, one for pump on, and one for high level alarm. If
redundant off is desired, an additional switch is used.
Duplex installations add one more switch for lag pump
on.
Most mercury float switches are pilot devices that control
the motor start contactor (relay) in a control panel, but
some are motor rated. Some variations of mercury float
^switches provide a differential so that one float can
control both pump on and off. Motor rated, differential
switches are sometimes used on economically motivated
installations.
Grinder pump and solids handling pump systems
encounter rags and grease in the raw wastewater that
interfere with the movement of mercury switches. Larger
size floats have generally been more reliable.
Some STEP pump vaults are so small that the mercury
switches have little room to operate. Also, when the
pump vault is internal in the septic tank the differential
between floats is small (typically about 7.5 cm [3 in]). The
small space and small differential require that the mercury
switches be given a short tether. The short tether and the
likelihood of the switches coming in contact with pumping
equipment in the vault often cause these installations to
be unreliable.
Displacer-type liquid level sensors have seen limited
use. Displacers are floats that move vertically in the
pump vault, thus conserving space and presenting fewer
opportunities for being obstructed. Most displacers are
built to provide a differential between pump on and pump
off. Some are pilot devices, while others are motor-rated.
Bubbler type liquid level sensors have also been used.
These consist of a small compressor which purges air
down a sensing tube in the wet well. The pressure in the
sensing tube increases as the liquid level in the wet well
rises. Pressure switches in the control panel sense the
pressu re to turn the pump on or off, or to activate alarms.
Bubblers are more common than mercury switches on
pressure sewer installations in Europe. In the U.S. their
use is usually limited to more expensive installations, or
they are used where the liquid level sensors must be
"explosion proof-rated. To achievethat standard, mercury
switches use intrinsically safe relays which are expensive,
complicate the control panel, and have had a history of
being troublesome.
Environment-One uses a trapped air system, similar to
the bubbler but without the compressor.
Numerous other level sensing methods have seen limited
use, such as diaphragm switches, reed switches, probes,
transducers, and ultrasonics.
The electrical control panel is usually mounted on the
outside wall of the home, but is sometimes placed on a
pedestal near the pumping unit.
Features of the control panel vary greatly depending on
the choices of the designer. Figure 2-23 shows typical,
simplecontrol panel circuitry using mercury float switches.
This controller depicts a 120-voIt panel not requiring a
panel mounted capacitor, and works as follows:
Assume the hand-auto switch is in the auto position, in
which case that circuit is open. In the hand mode, the
switch powers the motor contactor coil. Off is often
provided by the fuse or circuit breaker being openable.
As the liquid level in the wet well rises, the, off float is
closed and eventually the rising water closes the on float.
Power then passes through the closed on float switch to
the coil of the motor contactor, which electrically closes
and holds contacts M1 and M2. M1 being closed, power
is provided to the motor.
The pump motor is now running, so the liquid level in the
wet well is receding. As the liquid level in the wet well
recedes, the on float opens but the motor contactor coil
remains energized via the still closed off float and the
closed contact M2. As the liquid level continues to fall, the
off float switch opens, discontinuing power to the motor
contactor coil and opening contact M1 which stops the
motor and opening contact M2 so the motor contactor
coil no longer receives power via the off float.
If the liquid level in the wet well rises above the on float
and continues rising, it eventually causes the alarm float
switch to close. This provides power to the alarm lamp.
65
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Figure 2-23.
Circuit diagram of a basic 120-volt control panel.
G U N
MI-
MOTOR WITH BUILT-IN THERMAL OVERLOAD PROTECTION
Power is also applied to the audible alarm via a normally
closed contact A2. The alarm may be silenced by pushing
a button mounted on the control panel door. This energizes
the alarm relay coil, opening contact A2 and closing
contact A1. The alarm relay coil remains energized via
the now closed contact A1. When the high liquid level
condition has been corrected, the alarm float switch
opens and the alarm circuitry is automatically restored to
the condition shown in the figure.
A simple panel might have the following features:
1. Motor start relay
2. Audible high water alarm
3. High water alarm light
4. Control circuit fuse
5. Audible alarm push button silence
6. Audible alarm automatic reset relay and circuitry
7. Power leads accessible fortaking amperage readings
8. Hand-off-auto (HOA) toggle switch
9. Terminal strip
If the pymp uses a capacitor start motor, the capacitor
may be in the control panel or it may be mounted on the
motor. With a capacitor start - capacitor run motor both
capacitors are in the control panel.
Numerous additional features can be added to control
panels.
In some cases they have proven to be detrimental owing
to the complexity produced. This is particularly the case
if the service personnel that maintain them are not
accomplished in electrical matters.
Care should be exercised in providing a proper balance
of control panel features versus the benefits of simplicity.
Additional features sometimes used in control panels:
1. Elapsed running time meter
2. Event counter
3. Power failure alarm
4. Overload protection in addition to that built in the
motor
5. Various switches and status lights
6. Low voltage transformers
7. Seal failure lights
66
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On STEP systems having a mixture of 115- and 230-volt
motors, it has been common to provide 230-volt power
with neutral to each control panel. In that way, either
voltage pump can be run.
A junction box is used at the pumping unit to join the
mercury switch wiring and pump wiring to the branch
wiring extending from the pumping unit to the control
panel. This wiring is either rated for direct burial or is
placed in a conduit. Each mercury switch uses two
conductors and the pump uses 3 conductors. Accordingly,
a typical installation involving a pump and 3 mercury
switches requires 9 wires. One leg of each mercury
switch can be commoned, so 7 wires extend from the
pumping unit to the panel.
Junction boxes have often been a maintenance problem
due to leakage and corrosion. Splices made inside them
should be made watertight. Corrosion-resistant materials
should be used on the junction box. Abundant room for
access to the junction box is required for ease of service.
On some installations the pumping unit is placed so near
the home that wiring can be run directly from the pumping
unit to the control panel without need for a junction box.
The conduit must be perfectly sealed to prevent the
migration of corrosive and flammable gases to the panel.
The use of simple and easily understood electricals is
generally advised. Thought must be given to electrical
safety and to the certified electrical qualifications of the
workmen. Familiarity with the National Electrical Code is
required. Some electricals are listed by testing
laboratories, e.g. Underwriters Laboratories, Canadian
Standards Association, or Factory Mutual. This provides
a measure of protection from electrical shock and from
concerns over liability.
Power can be taken from the homeowner's load center
(the circuit breaker panel). If the panel has no remaining
space, sometimes two wafer breakers can be used in
place of one standard sized breaker. Circuit breakers
should be clearly rfiarked for identification.
A problem in using the homeowner's load center is that
access into the home is usually required which is a
practice generally avoided by contractors and engineers.
When the load center is used, the homeowner is typically
required to provide the branch circuit to a specified
location, instead of the work being included inthecontract.
Sometimes the work is improperly accomplished or not
done in the time required.
An alternative to the use of the homeowner's load center
is to take power from the meter base. In this case the
power is run hot from the meter base to a fused safety
switch placed as close to the meterbase as possible. Use
of this option requires specific approval of the regulatory
authorities, such as the state electrical inspector and/or
the state fire marshal, and also the approval of the
electric utility.
2.4.5 Valves/Cleanouts
2.4.5.1 Mainline Isolation Valves
Isolation valves are used on pressure sewer mains to
close a reach of the main for repairs if necessary, and to
accommodate pressure testing of limited lengths of
main. In some cases plastic valves have been used,
generally ball valves, but more commonly bronze gate
valves are used in sizes up through 7.5-cm (3-in) diameter,
and epoxy coated cast iron valves are used in larger
sizes.
Bronze and cast iron are subject to plating with sulfide,
but generally have performed well. This assumes the
pipeline flows full as opposed to being exposed to air at
the crown. If air (gases) were present there, the hydrogen
sulfide gas present could be converted to sulfuric acid. If
the pipeline flows full, the corrosive acid cannot form.
Standard AWWA gate valves have been used but those
using a resilient wedge and closing on a smooth bore
have been found to close more tightly.
2.4.5.2 Air Release Valves
A common air release valve is shown as Figure 2-24. The
valve inlet is connected to the top of the main. If the valve
itself is placed to one side of the main, a service line can
be used for the connection. The service line connects to
the top of the main (where air is accumulated), and
slopes continually upward to the valve inlet.
Normally the body of the air release valve is partly full of
water (effluent). If it is full enough to buoy up the float, the
rubber needle closes the orifice that passes through the
topof the valve body. As air (or gas) enters the valve from
the main, the trapped air accumulates and the liquid level
in the valve body is correspondingly lowered. Finally
enough air is trapped that the liquid level is lowered
enough that the float is no longer buoyed up. When this
happens the needle falls away from the orifice seat and
the previously trapped air is expelled out the orifice.
The air release valve can also admit air under vacuum
conditions. If the main undergoes a vacuum, as happens
when siphoning occurs, the effluent in the valve body is
drawn out of the valve and into the main. Since there is
67
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Flgura 2-24.
Waattwator-type air ralaaaa valve. (Courtesy APCO Valve and Primer Co.).
ORIFICE
LEVERAGE FRAME
COVER BOLT
COVER GASKET
CONNECTING LINK
LEVER PIN
BODY
FLOAT
COVER
NEEDLE LEVER
NEEDLE
RETAINING RING
FLOAT LEVER
CLOSE NIPPLE
ISOLATION VALVE
DRAIN VALVE
\
CLOSE NIPPLE
INLET
68
-------
no effluent in the valve body to buoy up the float, the
rubber needle falls away from the orifice seat. Air can
then be drawn into the valve and into the main by the
vacuum, through the orifice.
The amount of airthat an air release valve can release or
admit depends on the orifice size and the pressure
differential across the orifice. For perspective, a8-mm (5/
16-in) diameter orifice will pass about 7 Us (15 cfm) of
free air at 6.9 kPa (1 psi), and about 5 Us (20 cfm) at 69
kPa(10psi).
Automatic air release valves should be wastewatertype.
Water system valves have occasionally been used but
they usually become inoperative due to a gel-like
accumulation. Also, water type valves may be produced
from materials not sufficiently corrosion resistant, and
they may have unsuitable opening sizes.
A soft durometer needle should be used where pressure
is low, which is typical of pressure sewer application.
Also, a large orifice size needs to be specified, for
example 8-mm (5/16-in) diameter, to accommodate the
comparatively large volumes of air in and out of the valve
under the low pressures involved. The valve
manufacturers should be consulted for guidance.
The valve bodies are cast iron, thoroughly epoxy coated.
Working parts should be Type 316 stainless steel or a
plastic proven by experience to be suitable.
Air release valves, not vacuum valves or combination air
release and vacuum valves, are typically sufficient, but
standard hydraulic evaluations would apply.
Because of the possible odor, air release valves are often
vented to soil beds for odor absorption. Activated carbon
canisters have also been used for treatment of off-gases,
but with only limited success. Further data are presented
elsewhere.29 When the valves are located distant from
dwellings and/or where they are expected to expel little
gas they are normally vented to atmosphere, in which
case the discharge needs to be vented outside of the
valve box or the valve, box and appurtenances must be
made of corrosion resistant materials.
Soil beds used for odor absorption generally consist of a
perforated pipe bedded in gravel, underlying at least 45
cm (18 in) of loam soil backfill. Drainage should be
provided if the soil bed is located in an area of high
groundwater. Sizing depends on the volume of gas
expected, but usually soil beds are greatly oversized as
the extra sizing is low in cost and provides a generous
assurance that the bed will perform despite possible
adverse conditions.
Where the location of air release facilities be precisely
determined, manifolded connections to the main may be
made. These manifolds should include a connection
several pipe diameters below the point where a hydraulic
jump would form.
The geometry and valving should be such that each
connection can be flushed clean by maintenance staff.
2.4.5.3 Cleanouts
Where cleanouts are used on the mains they usually are
pipe cleaning pig launching stations. These are placed at
pipeline terminations and where pipe diameters change.
Mostly they are used to insure thatthe pipelines are clean
of construction debris following pipeline installation. This
removes the speculation of debris-caused blockages in
the event that hydraulic anomalies occur.
The cleanouts typically extend to ground surface and
may constitute summits that accumulate air(gas). Manual
air releases are usually fitted to them.
2.4.5.4 Service Line Valves
Most of the valves and appurtenances available were
conceived and designed for purposesotherthan pressure
sewer use. They may work well on this application, or
they may not. Advice is to proceed cautiously, and if at all
possible, to see that the product has been used
successfully on previous installations with the same
application.
Corporation stops are often used at the main, fastened to
the service saddle. Being difficult to replace, a quality
product is desired. They are bronze, and should be full
port opening.
Bronze gate valves have been used as isolation valves.
These tend to perform well but do not seal drip tight.
Unless high quality valves are used the handles corrode
when exposed to soils and moisture, and they corrode
badly if exposed to the wastewater atmosphere.
Plastic ball valves have been successfully used. These
close tightly and because of the 1 /4 turn of the handle, it
is easy to see at a glance if the ball valve is open or
closed. Union type valves are preferred, but if buried
there is no access for adjusting the union. Perhaps
su rprisingly, some plastics become brittle due to exposu re
to hydrogen sulfide. Other plastics are not strong enough
to withstand external forces, such as differential
settlement. The proper material should be specified,
such as PVC. Most manufacturers offer tables showing
the corrosion resistance of their materials.
69
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Two check valves are often used on services, with one
being located at the pumping unit and the other being
buried at some designated point along the service line,
such as at the street right of way line or near the main.
Service lines are quite subject to damage by subsequent
excavations. The purpose of having two check valves is
to prevent a spill in event of damage to the service line,
and as redundant protection against backtlow.
Bronze swing check valves have been successfully
used, but do not close drip tight. Resilient seats improve
the closing characteristics, but such valves are equipped
with rotating disks that have a bolt protruding through the
disk. The bolt tends to gather stringy material. A clear
waterway is preferred.
Wye-pattern swing checks are preferred overtee-pattem
valves when installed horizontally (due to the gravity
component provided) but can be used in either the
horizontal or vertical mounting position.
PVC swing checks using neoprene flappers have
performed well. Thesetendto have large, clear waterways
and open and close easily without becoming stuck by
sullides. However, some brands are cheaply made and
quality control varies.
Many ball check valves, especially spring operated ones,
have protruding parts that catch stringy material. Other
designs have clear openings and are suitable.
Proven experience with the valve is desired. This can
usually be provided when the valve is part of a package
provided by an experienced pressure sewer products
manufacturer.
2.4.5.5 Building Sewer Appurtenances
Cleanouts are used on building sewers, located near the
home or at the point of demarcation between homeowner
mainten ance and district maintenance. Typical cleanouts
are shown in Chapter 3.
Check valves termed "backwater valves" are made to be
installed on building sewers, but are rarely used. These
are swing check valves with the working parts being
removable from the top of the valve, and are available in
7.5-, 10-, and 15-cm (3-, 4-, 6-in) diameter.
2.4.6 Miscellaneous Appurtenances
2.4.6.1 Septic tank effluent screens
While septio tanks are effective at the removal of
troublesome solids, they are not 100 percent effective.
Some solids are discharged in the effluent. Some effluent
pumps have limited ability to handle these occasional
solids, making necessary the use of effluent screens or
filters, especially at particular installations where
maintenance history shows that solids carryover is
common. Water well pumps have no solids handling
ability so must always be u sed with a screen. If discharge
is made to an on-sfte disposal system having distribution
laterals with small orifices, effluent screening is sometimes
suggested to reduce the possibility of orifice clogging.
Basket strainers were fitted to the discharge of a number
of septic tanks used on STEP systems in the early 1970s
in Florida,24 and later in Oregon. These were observed
over a period of time and showed that solids carryover
varied greatly from home to home. Strainers at most
installations were found to be nearly free of solids after
months of use while those at certain other homes were
full and overflowing.
The principal reason forthedifferences in solidscarryover
seems to be the practices of the user with respect to
disposal of troublesome items. Some solids are prone to
carryover as they attain a neutral buoyancy, and are not
removed by sedimentation or flotation. Other solids are
susceptible to being gas-lifted from the sludge layer and
entrained in the tank effluent.
The proximity of the scum or sludge layer to the septic
tank outlet is a major factor relating to solids carryover.
Scum and sludge accumulation can vary considerably.
While the clear space between the top of the sludge and
the bottom of the scum layer may be within an expected
range, either scum or sludge may constitute a large
fraction of the total volume such that one of the two can
encroach unpredictably on the outlet.
Another common reason for solids carryover has been
that the "pump off' liquid level sensor has been set too
nearthe level of the inlet ports, allowing submerged scum
to be drawn intothe pump vault. Also common is siphoning
that can occur through the pump and discharge piping if
the hydraulic grade line of the main is below the liquid
level in the septic tank. This lowers the scum layer
sufficiently that scu m discharges from the tank as effluent.
In cases where scum is carried over, pumps and screens
(if used) can be clogged.
To describe the volume and composition of carried over
solids, guidance can be taken from the Manila, California
STEP system. All flow from this system serving 350
homes is discharged to a main pumping station, fitted
with basket strainers having about 6-mm (1 /4-in) diameter
holes. Annually, about 1,640 cm3 (100 cu in) of solids/
home are captured by the strainers. The solids are
comprised of cigarette filters, hair, prophylactics, tiny
70
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grease balls, clumps of detergent, common earthworms,
plastic sandwich wrap, and lint. Some carryover of scum
is suspected at this project for the reasons outlined
above.
The recognition that some solids pass through septic
tanks and the use of effluent screens is not new.
Winneberger showed that 6-mm (1/4-in) mesh screens
were recommended by the U.S. Public Health Service in
1920.*
Screened pump intakes have been tried, mostly without
success. The poor performance has been due to the
small screen area, the resulting high velocity through the
screens which tends to hold solids on the screens. Their
horizontal position permits trapping of floatable material.
Also, since the floor of the pump vault is only a few inches
below the screened pump intake, solids that fall away
from the screen come to rest only a short distance from
the screen and are resuspended each time the pump
runs, and captured again by the screen. Vertical screens
have been more self cleaning.
Effluent screens fall into two basic categories: 1) those
that are a part of the septic tank outlet device, and 2)
screened pump vaults, located within the septic tank.
Basket strainers are simple devices of the formercategory,
hung on the septic tank outlet, and shown in Figure 2-25.
These capture the solids so they can be easily seen for
identification and to quantify the volume of carryover.
Basket strainers have mostly been used for research
rather than for pump protection, since they are not self-
cleaning. Manual emptying of basket strainers is
periodically required.
A multi-tray filter shown in Figure 2-26 is a patented
device that replaces the septic tank outlet tee. It uses
stacked plates, separated by a gap of about 1.5 mm (1/
16 in) between each plate, and housed within an open-
bottomed plastic case. Some solids slough off the filter
and fall to the bottom of the septic tank. The plates may
be lifted out of the case as a u nit and rinsed off to remove
solids that may accumulate between the plates.
Slotted PVC well screens have been fined to septic tank
outlet tees as shown in Figure 2-27. The screen can be
cleaned with a round swab or brush placed inside the well
screen.
The simplest type of screened pump vault is one where
mesh is placed over the vault inlet ports, as shown in
Figure 2-28. About 6-mm (1/4-in) plastic mesh is usually
used. Because of the small screen area, blinding of this
type of screen is usually experienced after a year or so,
requiring routine cleaning of the mesh which is attempted
by hosing the screen from the inside of the vault. Often
hair, lint, and other fibrous or stringy material becomes
tangled in the mesh and the vault has to be removed from
the septic tank for manual cleaning.
The screened vault, a patented method, is shown in
Figure 2-29. A large, basket shaped plastic screen of 1.5
mm (1/16in) mesh fits inside the pump vault but is slightly
smaller in diameter, providing an annular space between
the vault and the basket. After removing the pump and
liquid level controls, the basket can be lifted out for
cleaning.
A slotted pump vault is shown in Figure 2-30. A variation
of this design not shown is for the vault inlet to be mesh
instead of slotted. The slotted vault is made of PVC well
screen, usually 30-40cm (12-16 in) indiameter{depending
on the size of the pump and appurtenances). If discharge
is to a municipal treatment works, the location of the
screen or slots is less critical. Some digested sludge may
be pumped in this case, which is to be avoided if
discharge is to a d rainf ield or other facility more sensitive
to effluent quality.
In overview of the various designs, proponents of screens
that are part of the septic tank outlet tee usually favor the
external pump vault arrangement, which better allows
factory packaging of pu mp components. They also prefer
that flow through the screen be governed by the natural
flow rate through the septic tank rather than coinciding
with pumping rates, as with internal vault designs.
Proponents of screened pump vaults internal to the
septic tank favorthe economy of the internal vault and the
integrated packaging of the septic tank and pump vault.
With most screen designs, if the liquid level in the septic
tank rises sufficiently, as can happen during a long power
outage, the screen can be bypassed from above, carrying
scum inside the screen. If the screened vault becomes
blinded, the pump vault can be floated out of position due
to the higher liquid level outside the vault.
Slotted screens are believed to be more resistant to
clogging than mesh screens due to the one-way bridging
action provided by the long slot as opposed to the two-
way bridging action of the mesh. However, the mesh
better insures that long, stringy solids are captured.
The use of screens is vitally important when non-solids
handling pumpsareused. When effluent pumpsare used
capable of passing 13-mm (1/2-in) or larger solids, only
about one percent of the pumps experience clogging per
71
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Rgure 2-25. Basket strainer used with external pump vault.
RISER & COVER —
SEPTIC TANK
Flflur* 2-26. Multi-tray filter, used wih external pump vault.
BASKET STRAINER
HUNG ON SEPTIC
TANK OUTLET PIPE
PUMP VAULT
RISER & COVER
MULTI-TRAY FILTER
PUMP VAULT
SEPTIC TANK
72
-------
Figure 2-27.
Outlet tee fined with well screen.
RISER & COVER
WELL SCREEN-
ra
SEPTIC TANK
Figure 2-28. Mesh placed over inlet ports of Internal pump vault
PUMP VAULT
RISER & COVER
• MESH PLACED
OVER INLET
PORTS
SEPTIC TANK
PUMP VAULT
73
-------
Figure 2-28. Fully-*cro«ned Internal pump vault. (Courtesy Orenco Systems, Inc.)
RISER & COVER
Figure 2-30. Slotted pump vault
SEPTIC TANK
PUMP VAULT
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PUMP VAULT
SEPTIC TANK
74
-------
year. Accordingly, most installations using effluent pumps
do not use screens.
Type 316 and Type 304 stainless steet, PVC, polyethylene,
ABS, and FRP.
2.4.7 Odors and Corrosion
Grinder pump and solids handling pump basins are
odorous only to the extent the fresh, raw wastewater is
odorous. When the wastewater is retained in the basin for
some time it becomes septic and begins producing
hydrogen sulfide gas.
On occasions when odor control is required at grinder
pump installations, enzymes have been added, either by
the homeownerorthe maintenance district. The enzymes
can also be beneficial in reducing grease accumulation in
the pump vault, but are expensive when required for any
length of time.
With STEP systems the tank effluent is always septic,
and potentially odorous and corrosive. Some of the H2S
can escape from the septic tank, however, and some
may be captured by the floating scum layer. The BOD of
the effluent is lowerthan that of raw orground wastewater.
Since sulfide production is generally proportional to
BOD, the STEP system effluent has a reduced potential
for total H2S production.
A septic wastewater atmosphere is characterized as
being odorous, corrosive, and toxic. The rotten egg odor
of H2S is repulsive and detectable in concentrations as
low as 3 ppb H2S is itself corrosive but is also reactive with
thiobacillus bacteria present on the appurtenances and
walls of the pressure sewer vault above the water line to
form sulfuric acid, H2SO4. In sufficient concentrations
H2S causes acute poisoning, paralyzing the respiratory
center. Methane is also produced by septic wastewater,
which is asphyxiating, as is the carbon dioxide and
nitrogen present in wastewater gas. When sufficiently
ventilated the atmosphere in the pump vault is usually at
a safe level for brief exposure.
For odor control, basin covers are typically gasketed or
made such that escaping gases are vented into the soil,
or ventilation is provided by the roof vent of the home.
While H2S is heavier than air, it is presumed drafted away
with the greater volume of air and lighter gases present.
So long as turbulence is minimized in the basin to limit the
amount of H2S liberated, odors are rarely reported via
roof ventilation. In most cases where problems have
been investigated improper house venting has been a
major contributor.
Proper materials must be selected for resistance to
corrosion. Most packages assembled by manufacturers
of pressure sewer components comply reasonably with
this requirement. Particularly well-suited materials include
Because of the toxic and asphyxiating atmospheric
conditions possible in the vaults, designs should be
made where exposure of the service personnel to these
gases is unnecessary and difficult. More than brief
exposure is to be avoided.
Wheregrinderpumpsor solids handling pumpsdischarge
a short distance, as in a typical service line to a receiving
gravity sewer, the residence time in the pipeline is usually
short enough for the wastewater to be relatively fresh or
even stale, but not so septic as to present a problem of
odors or corrosion at the receiving sewer. When they
pump to a pressu re sewer system where the wastewater
is retained in the pipeline for more than about 30 minutes,
hydrogen sulfide begins to be produced. With increased
retention time the effluent becomes totally septic, with all
the attendant concerns.
The septic aspect of the wastewater in the mains presents
no particularproblem as the PVC pipelinesare unaffected.
Isolation valves should be made corrosion resistent, as
should air release valves. Air releases should vent to an
odorcontrol facility, such as a soil absorption bed if much
gas is expected to be expelled or if the air release is near
habitation.
There have been cases where pressure sewer-collected
septic effluent has been discharged to gravity sewer
mains without causing nuisance or corrosion. In these
cases the pressure sewer flows were small, the receiving
sewer flows were large and aerobic, and flows were
introduced as quiescently as possible to avoid driving off
H2S. More commonly, however, the discharge is odorous,
corrosive, and, in extreme cases, can cause the
atmosphere within the receiving sewer to become
hazardous.
Concrete sewers are attacked by H2S. PVC and vitrified
clay pipes are inert. The usual situation is that the
receiving concrete manhole corrodes more rapidly than
any other sewer component due to turbulence during
transition.
When discharging a pressure sewer to a receiving
conventional sewer, a reach of the latter should be
chosen which has a substantial quantity of flow. Some
jurisdictions have used a 5:1 flow
ratio, however such generic rules are not equally
applicable for all situations. In cases where pretreatment
for sulfide must be provided, aeration is a favored method
for converting the sulfide to thiosulfate, but requires a
substantial reaction time. Chemicals have been used,
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such as chlorine, a strong oxidizing agent and bactericide,
hydrogen peroxide which is a source of oxygen, and
ferrous sulfate which can either act as a catalyst for
oxidation of sulfides or precipitate them. AH of these
chemicals require mixing, and some require substantial
reaction times while others react instantly.
Much information is available on the characteristics and
prelreatment of septic wastewaters.29 Unfortunately,
information has historically been ignored or poorly
understood by the parties involved in design.
Where septic pressure sewer-collected effluent is
discharged directly to a municipal treatment facility,
odors have not been a problem if the discharge is
submerged and diffused, and if the receiving basin is
large, well mixed, and aerobic.
2.5 Construction Considerations
2.5,1 Line Changes
Because of the flexibility of small diameter PVC piping
and because the pressurized flow regime is not very
sensitive to horizontal alignment, minor field changes in
alignment usually pose no particular problem and are
often made by the construction manager. Major changes
should be evaluated and approved by thedesign engineer
prior to implementation. Changes in alignment should be
well documented and shown on as-constructed plans.
2.5.2 Grade Control
Profiles are shown on the plans, but in most cases the
pressure sewer main is shown to be at a constant depth
below ground surface, such as 75 cm (30 in). The profile
is necessary for the proper evaluation of air release
station use and placement. At culvert crossings, or for
separation from watermainsorotherutilities, the pressure
sewer profile may vary from the standard depth. The
configuration at these locations must be specified by the
designer for air release valve considerations, and by
contractors so that the project is properly bidable.
As with horizontal alignment, minor changes in vertical
alignment may be made in the field, but major changes
should be evaluated in advance by the design engineer.
The as-constructed plans should reflect the installed
profile.
2.5.3 Service Connections
Grinder pump vaults and septic tanks should be located
in an area of stable soil, accessible for construction, but
not subject to vehicular traffic. Some GP and SH
Installations are made in basements.
Some designs locate the vault within a few feet of the
home, so wiring can pass directly from the vault to the
control panel without need for an electrical junction box
in the pump vault. The building sewer is short, reducing
maintenance needs and reducing the possibility of
receiving infiltration from poorly made pipe joints.
Other designs use long building sewers and place the
pumping unit next to the side property line and near the
street. This design is uncommon but is used in certain
areas, especially where ground-slopes are flat and
building lots are mounded, thereby providing sufficient
downward slope away from the home. The pumping unit
is most easily accessed with this design. Where adjacent
homes have the pumping units adjoining the same
property line, the construction easement from each
homeowner is narrowerthan would otherwise be required.
When serving existing homes the pumping unit is usually
located near to the existing septic tank, which terminates
the existing building sewer. Being close to the existing
septic tank, excavation material for the new tank can be
conveniently placed as backfill inside the hole left by the
old tank, crushed in place. Usually, old septic tanks are
abandoned by being crushed in place or backfilled with
sandorgravel. Occasionally the old tank is removed. The
new tank may occupy the same location as the old tank.
On the rare occasions when existing tanks are considered
useable, they must be extensively tested for leaks and
illegal inflow sources. These procedures are usually
considered too expensive, and contribute to the usual
100 percent replacement approach.
When serving existing homes, planning tank and service
line locations must be done with the homeowner's
involvement. It is common for the appropriate site for the
tank to conflict with the homeowner's wishes to build a
driveway, deck, garage, or additional bedroom. The
homeowner may know the locations of such features as
water wells or buried utilities;, which are to be avoided.
The family pet burial ground can be a sacred area to be
avoided, at all costs.
Whenever possible, lot facility plans should be drawn
when serving existing homes. These larger scale plans
show sensitive areas, and show the locations of the
planned improvements. They also list such information
asthe homeowner's name, phone nu mber, contact person
(if the owner is non-resident), whetherthe resident works
nights (day sleeper), if the dog bites, and numerous other
points of valuable information, A simple lot facility plan is
shown as Figure 2-31. The additional information, usually
on the reverse side of the lot facility plan, is:
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Flgur* 2-31.
Example lot facility plan.
Owner: A.J. Condren
Address: 132 Jones Street
Station: 130+60 JS
JONES ST.
= 20'
EP
FUTURE
DECK
PL&-
FENCE I
EP - EDGE OF PAVEMENT
S - SEWER
RW- RIGHT OF WAY
W - WATER LINE
DW - DRIVEWAY
DF - DRAINFIELD
ST - SEPTIC TANK
SL - SERVICE LINE
GP - GRINDER PUMP
E - ELECTRICAL PANEL
PL - PROPERTY LINE
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Owner: A.J, Condren
Address: PO Box 1190, Fairbanks, AK 9i543
Station:! 30*60 JS
Phone: (503)345-3001
Contact person: Joe Baily (renter - resident)
Phone: home 543-4640, work 543-5569
Locations of GP & SL approved by owner? Yes
Easement signed and recorded? Yes
Special notes:
1. Renter (Joe Baily) has been designated to make
local decisions on behalf of the owner.
2. Baily is not available Saturdays.
3. Do not trench near roots of 4" Oak tree.
4. Mount electrical control panel 1' higher than normal
to account for access from future deck.
5. Replace building sewer to the house foundation.
(Existing building sewer has root intrusions and is
known to be broken.)
Lot facility plans should be prepared in advance of
construction, so the time consuming contacts can be
completed. Also, having lot facility plans at the time of
construction bidding enables the contractor to better
estimate costs and, later, to schedule work.
As locations for the tanks, service lines and electrical
service are decided in the field, and agreed to by the
involved parties, conspicuous lath may be placed. These
mark the agreed location for file photos and remind the
homeowner of their locations. Owing to long delays prior
to construction the markings may require replacement
before construction to jog homeowners regarding these
locations. It is common for the homeowner to phone the
districtafewdayslatertosaythey have realized (reminded
by the lath) that the routes are unacceptable for whatever
reason. While the changes may be costly annoyances,
they are preferable to making errors and subsequent
changes after the installation has begun, and provide an
opportunity for better public relations.
The contractor, sometimes together with the construction
manager, should contact the homeowner or resident
again a few days in advance of construction. This helps
minimize inconvenience, better coordinates the work,
and is generally required public relations. The authority
mustdesignate acoordinatortoensure minimal disruptions
to the homeowners.
Videotapes of the property before and after construction
eliminate many disagreements about property damage
during construction.
2.5.4 Equipment Substitutions
It sometimes happens that the contractor awarded the
job will place orders for equipment that does not comply
with the specifications, and not notify the engineer or
owner of this. The substitution may not be discovered
until the equipment arrives, usually after a considerable
time has passed and the construction season is growing
short. Claims are made that the equipment is equal to
that specified. The client and engineer are sometimes
pressured into accepting what they believe is substandard
equipment.
To avoid this situation some system owners have opted
to purchase key equipment separately, and to furnish the
equipment to the contractor. This not only insures that
the specified equipment will be used but may have an
added benefit of lower cost. However, since the contractor
is not in control of that equipment supply there may be
concerns for delay if the equipment does not arrive on
schedule. The contractor may also take the position that
proper operation is beyond the contractor's control. This
can be partially avoided if contractors are aware of the
owner's plans prior to bidding.
In some cases where the construction contractor provides
the equipment, a list of the particular equipment is
required to be furnished as a part of the bid, or formal
submittals are required. Inothercases, only preapproved
equipment is allowed, with the list being acknowledged in
writing by the contractor. The construction manager is
furnished with copies of equipment orders. ;
Where substitutions are allowed, the securing of samples
is advised. If the equipment substituted is a component
part of a larger assembly, a prototype of the assembly
should be required for examination and testing. This
provision should be a part of the contract specifications.
Not all substitutions are detrimental. In some cases the
suppliers offer substitutions based on substantial
experience, and their suggestions should be heeded. ,
2.5.5 Testing n
Mains should be hydrostatically pressure-tested following
established procedures, such as those of the American
Waterworks Association. Test pressures should be at or
near the working stress rating for the pipe .materials.
Property accomplished installations can easily pass these
tests. If the installation is flawed, that should be known
during construction rather than later during operation
when the mains are filled with wastewater. The mains
may be subject to pressures considerably higher than
design during pipeline cleaning operations. u .=,.
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When custom (non-commercial) pumping units are used,
prototypes should be built for examination and testing.
Experience has shown that many septic tanks used on
STEP systems leak, allowing infiltration. Some plastic
tanks deform badly due to backfill loads. Septic tanks
should be thoroughly tested and/or be otherwise proven.
Concrete tanks are often tested by filling with water to the
soffit of the tank and observing seeping and water levels
over a period of time. Fiberglass and polyethylene septic
tanks may also be hydrostatically tested, and may be
further tested by imposing a vacuum in the tank to
simulate backfill loading pressures.
2,5.6 Scheduling/Warrantees
Depending on the method and timing of construction,
some components can be installed very early during
construction, and some very late. There have been some
cases where the 1 -year equipment warranty period had
expired for some of the installations before the f acil ity had
initiated operation and the pump had ever been used.
The contract documents need to be explicit in stating
precisely when the warranty period starts.
2.6 O&M Considerations
2.6.1 O&M Manual
Many pressure sewer systems operate without an O&M
manual. Other systems have O&M manuals that resemble
design manuals. Neither practice has proven satisfactory.
The O&M manual should primarily be a reference
document, intended for the regular use of the service
personnel.
Major manufacturers of pressure sewer equipment have
manuals or catalogs available for the components or
packages supplied by them. Some of the information is
generally suitable for insertion into the overall manual.
For example, air release valve manufacturerscan provide
cutaway drawings that are beneficial to understanding
how the valve works, and details listing and identifying
the various parts used. Some septic tank manufacturers
have complete O&M manuals describing, for example,
how and when septage should be removed.
A "system analysis plan" is particularly useful. This plan
capsulates the project on one or two sheets, showing
infrastructure pipe routes and sizes, valve locations,
profiles, nodes of the accumulated number of ED Us
contributing to each reach, and static and dynamic
hydraulic grade lines.
Septic sewer gases are expected at STEP septic tanks,
at relatively inactive GP vaults, and points where pressure
sewers discharge. The proper practices of pressure
sewer maintenance are usually such that the gases are
not hazardousdueto dilution with ambient air, but service
personnel need to be aware of the potential hazards of
these gases. For example, hydrogen sulfide is known to
deaden a persons ability to smell it after brief exposure.
This may cause the person to wrongly think the gas has
drifted away. Many deaths have been attributed to
exposure to septic sewer gases.
Service personnel should be advised of procedures and
precautions regarding exposure. Designs should be such
that no exposure is required, or at the most, only brief and
limited exposure to a diluted atmosphere.
Extensive electricals are used at each pumping unit,
typically including an electrical control panel, mercury
float switches, and electrical junction boxes. Service
personnel are exposed to the possibility of electrical burn
or shock in their maintenance duties. They may be
standing on wet ground while servicing electricals, and
working under adverse conditions such as darkness, rain
or snow. Electrical safety should be included in the O&M
manual. Designs should limit exposureof O&M personnel
to live electrical systems.
Sanitation practices should also be covered in the design
manual as with any project involving contact with
wastewater.
Simple, straightforward designs are more easily
understood and better maintained. Maintenance functions
and management programs should be fully considered
during the design phase.
2.6.2 Staffing Requirements
Staffing requirements are dependent on the type of
pressure sewer involved, the features provided, and the
quality of the system. For example, some systems use
heavy pumps that require two people to lift them from the
vault, while others use light-weight pumps easily lifted by
one person.
Some designs have a large reserve volume available in
the tank for storage after the high level alarm sounds, or
an overflow to a standby drainfield. These are "fail soft"
provisions that allow the service personnel to better
schedule their activities, while not significantly
inconveniencing the user. In contrast, some systems use
small basins that hold only a few gallons beyond the
alarm level. These systems need to be attended as soon
as an alarm condition is known, sometimes late at night
or on holidays or weekends.
Quality designed and constructed systems, and those
systems receiving regular preventive maintenance
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generally experience fewer service calls in response to
alarms.
Service calls likely to require exposure to dangerous
conditions, e.g., live electrical or sewer gas exposure,
should be attended by at least two people for safety
purposes.
Nearly all systems have defects that are not discovered
during construction. If the correction of these defects is
assigned to the maintenance forces, excessive
maintenance can result. A preferable alternative is to
identify the defects, and to correct them as a capital
improvement.
Recognizing all of the variables involved that cause some
systems to be more maintenance intensive than others,
no general rule can be made regarding staffing needs.
However, systems that are well designed, well built, and
properly maintained may experience a mean time between
service calls (MTBSC) of about 4-years, loosely meaning
that 1/4 of the pumping units will be serviced in a year.
The personnel-hours spent per service call depends on
whether two personnel are required or one, and the
nature of service typical of the particular system. The
mean times range from 20 minutes per service call to
about 3 hours.
A minimum of two personnel are required to be trained
and available. On some systems one person can attend
to the service calls, but a fully trained and experienced
backup is needed for times when the lead person is
unavailable. The two people do not necessarily have to
be full time employees, but at least one person has to be
available on call.
In the most general sense, two full time employees have
usually been found sufficient to maintain a well-designed
system of about 1,000 pumping units.
2.6.3 Operator Training
Typically, the malfunctions of most service calls are
electrically related. The service personnel should be
trained accordingly and should have the appropriate
credentials.
Familiarity and a basic knowledge of plumbing practices,
particularly as applied to pumping, is required. Personnel
should be advised on practices regarding exposure to
septicsewergases. Public relations are important. Service
personnel should be able to represent the sewer district
diplomatically. Training in this subject area may be
advised.
2.6.4 Spare Parts Inventory
The number of spare parts required is related to the
particular system being operated and maintained.
In general, sufficient pipeline fittings and special tools
should be on hand to quickly repair any rupture that may
occur in the piping system. Anticipation of need during
non-working hours should be made, when parts would
not be available from outside sources.
All working parts of the pumping units, for example the
pump itself, liquid level sensors, check valves, and
control panels should be on hand in sufficient quantity to
quickly restore operation in event of malfunction. The
need for inventory can be refined by experience with the
particular system, with consideration given for particular
parts that cannot be obtained quickly. In general, an
inventory of 5 percent of the number of pu mps in operation
is sufficient.
Many pressure sewer systems are intended for growth
within the system, and for piping extensions to be made.
These systems usually have enough parts and equipment
on hand to satisfy maintenance needs.
2.6.5 As-Built Drawings/Maps
The prevention of damage to pressure sewer mains and
service laterals caused by subsequent excavations should
be a high priority.
Warning signs have been posted on some projects,
periodically placed along the route to notify or remind
excavators of the presence of the pipeline. Toning cable
is installed adjacent to the pipeline on some systems,
which can be induced with a tone and located using
standard utility-locator devices. Identification tape buried
with the pipelines further warn excavators. And reliable
as-built drawings are beneficial, especially to those
agencies planning future excavations.
As-constructed lot facility plans, showing locations of the
pumping unit, electrical service, service line and building
sewer are equally helpful. These are of particular benefit
when new service personnel are engaged who are
unfamiliar with the locations of key site features.
2.6.6 Maintenance
2.6.6.1 Normal
Service calls made in response to phone calls by the
home residents constitute most maintenance activities.
The amount and type of maintenance required varies
widely between projects, depending on the equipment
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Table 2-6. DistrlbutfonofCausesforCall-OutMalntenance
On Selected Grinder Pump Pressure Sewer
Projects
Category Percent of Occurrences
Electrically related
Pump related
Miscellaneous
Pump vault related
Piping related
25-40
20-25
20-30
5-15
5-15
used, quality of installation, and other natural and human
factors.
a. Grinder Pump Maintenance
To describe maintenance functions generally, Table 2-6
was prepared based on the experiences of two major
projects, one using centrifugal pumps and the other using
progressing cavity pumps. Each project kept detailed
daily records of service calls. A tedious examination of
the maintenance records was made, augmented by
detailed personal examinations of the projects and visits
with the maintenance staff.
Causes for maintenance were divided as follows:
Electrically related: Where mercury float switches were
used, about 50 percent of the service calls related to their
malfunction. Grease accumulation was the principal
cause of the float switches not operating properly. The
typical correction was removal of the grease, adding
enzymes to the pump vault, and providing extra enzymes
to the home resident along with the request that they
dispose of cooking grease elsewhere. Other descriptions
of malfunctions included the float switches being
obstructed, loose, cracked, stiff cords restricting
movement of the float, and simply "not working".
Where pressure switches were used in lieu of float
switches, about 20 percent of the electrically caused
service calls related to them. Adjustment or replacement
of the switch was usually accomplished.
Problems with electrical control panels were highly diverse,
ranging from inoperative components to loose wiring,
and problems with the intrusion of ants or other insects.
Control panel problems constituted 25-55 percent of the
electrically related causes for service calls.
Fuses were blown or circuit breakers were tripped or
turned off in 7-18 percent of the electrically related
service calls. The reporting was not clear as to whether
the breakers were tripped out due to an overload or
ground fault, or turned off by the homeowner.
Problems with splices contained in electrical junction
boxes can be a major cause of maintenance, especially
if the splices in the junction boxes are not water tight.
Junction boxes typically leak, despite claims to the
contrary.
In about 3 percent of the electrical problem cases, the
homeowner's power supply was at fault.
Other electrical problems too diverse to categorize
represent perhaps 5 percent of these service calls.
Pump related: Pumps were often removed and replaced
on one project, and seldom removed on the other. On the
project where replacement was more common, it was
usually due to the pump drawing excessive amperage,
indicating that something was jammed in the grinder
mechanism. Troublesome materials found in grinders
were mentioned in the reports as being washrags, sanitary
napkins, underwear, kitty litter, handkerchiefs, and similar
items.
Miscellaneous: One system reported overflowing
wastewater onto the yard or backups in the home on 60
percent of the miscellaneous service calls. This was to be
expected as the pump vaults were small. The other
system investigated used emergency overflowsto holding
tanks, so experienced rare cases where surfacing or
piping backups occurred.
Odor problems were reported on one of the systems.
Usually the field personnel attributed this to lack of use,
causing the vault contents to be septic. Where vents
were used in the vault covers, they were checked to see
if water was in them preventing the transmission of gases
to atmosphere.
Pumps or service lines were sometimes found to be
airbound. Check valves were reported to be leaking
slightly in only 2 percent of the miscellaneous service
calls.
A large fraction of the miscellaneous service calls were
too diverse in nature to categorize.
Pump vault related: One project used pump vaults and
piping within the vaults that had been designed and
produced by a local supplier, rather than that available
from the pump manufacturer. Over half of their pump
vault related service calls were related to changing this
inadequate piping system.
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The intrusion of water into the pump vaufts was noted on
both projects. In some cases the water entrance was due
to roof drain discharging in proximity to the pump vault,
or due to careless irrigation practices. In many instances
thepump vault was placedtoo low, so water could puddle
around and over the vault cover.
Other pump vault related service calls were diverse,
ranging from damage inadvertently caused by the home
resident to faulty installation during construction.
Piping related: Piping related causes for service calls
were similar for both projects. About 50 percent of the
calls were for clogged or broken building sewers (usually
a maintenance responsibility of the homeowner). Forty
percent of the calls related to damaged service lines, and
the remainder were diverse.
b. STEP System Maintenance
Two major STEP projects were selected for detailed
evaluation, both having kept extensive daily records of
service calls. Both systems used submersible centrifugal
effluent pumps. Table 2-7 indicates causes for call-out
maintenance.
Causes for maintenance were divided as follows:
Electrically related: Both systems used mercury float
switches, and both systems also used displacer switches
on early installations. The mercury float switches have
been the cause of numerous maintenance calls, but less
than the displacers. Some of the displacers remain in
service on one project and have been particularly
maintenance intense due to imperfect design, but not
dueto problems inherent with the concept. Well over half
of all electrically related service calls are attributed to
adjustment or failures of the liquid level sensors. The
remaining reasons for electrical malfunctions are spread
among problems with junction boxes (used on one of the
two projects), electrical control panels, and tripped or
thrown breakers.
Pump related: One project used low cost pumps, relying
on the philosophy that they are relatively easy and
inexpensive to replace, while the otherproject used more
expensive pumps. Both approaches have been
reasonably successful. The low cost pu mps are removed
whenever there appears to be a pump related problem.
They are taken to the shop for evaluation, and in the
majority of cases they are found to be clogged with a
cigarette filter or prophylactic. Some of these clogging
problems are attributed to an unannounced change
made in the design of the pump which made it less able
to pass solids than the original design which was evaluated
arid selected. Theclogging object is removed in the shop,
Tabl e 2-7. Distribution of Causes for Call-Out Maintenance
on Selected STEP Pressure Sewer Projects
Category Percent of Occurrences
Electrically related
Pump related
Miscellaneous
Tank related
Piping related
40-60
10-30
20-40
1-5
5-10
and the pump is placed back in service during a
subsequent service call.
The project using more expensive pumps encounters
clogging on only a trivial number of pumps; about 1
percent of the installations. The need to remove these
pumps from the basins is rare.
Miscellaneous: Air bound service lines are a common
occurrence on one system, and air bound mains are
reported on the other. This is one of the most common
sources of maintenance calls.
One system's staff attributes the problem to reaches of
the main having so few homes presently served that peak
flows are of such short duration that air accumulations
are not purged from the main. The other system has
reaches experiencing the two-phase flow of air and
water. Improved venting practices have successfully
corrected problem areas, but have not been applied to
the project as a whole.
Tank related: Few tank related service calls occur, but
this is because high quality tankage was used on both
projects. Tankage problems have been so extreme on
some other projects using poor quality tanks that little
attempt was made to deal with the problems by
maintenance forces. Instead, replacement was
undertaken as a capital improvement.
Overflows infrequently occurbecause of the large reserve
volume provided in the septic tank.
Piping related: Clogged building sewers represent half of
these service calls, even though maintenance of the
building sewer is the responsibility of the home owner.
Damaged service lines represent the other half of the
service calls.
2.6.6.2 Preventive
Pipeline locating by sewer staff may be frequently needed
by utility companies or others planning excavations in the
area. Mainline isolation valves should be exercised
annually. Air release stations should be checked for
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proper operation, the frequency being best established
by experience with the particular system. If pretreatment
devices are used, they need to be attended regularly.
There is little preventive maintenance practical to
accomplish at most pumping units. The pumps and
ancillary components are not routinely removed from the
vaults.
On routine maintenance visits the pu mps and controllers
are run through their cycles to see that all aspects are in
working order. Voltage and amperage readings are taken.
High amperage indicates that something is restricting the
movement of the rotor, usually a clogged impeller or
grinder. Motor starter contacts are sometimes cleaned,
especially if the area experiences problems with insect
intrusion.
The first yearortwo after installation, earthwork settlement
is common around the pump vaults and may need to be
corrected. Often the tank settles, causing the top of the
vault to be below ground surface where inflow could
enter. The tanks then require a water-tight extension to
be retrofitted.
Mostly, routine maintenanceconsists of visual inspection.
Grinder pump vaults (particularly with float switches) are
washed down to reduce grease buildup. Where enzymes
are popular, they may be added. STEP pump vaults
accumulate less grease, so there is less need to clean
them. The sludge and scum accumulation in the STEP
tank should be monitored during these visits.
2.6.6.3 Emergency
Even though they may be infrequent, mainline ruptures
are possible. Repair materials and equipment should be
reasonably accessible for such needs.
When pump vaults are used that have small reserve
volumes between the high level alarm setpoint and the
top of the basin, response time must be prompt or an
overflow or backup in the home will occur. Either way, the
user is inconvenienced. In many cases, maintenance
forces respondtosuchcallswheneverthecall is received,
even during late night or early morning hours.
During extended power outages pump basins may be
filled and overflowing or backing up in the home. However,
water use and corresponding wastewater flows are greatly
reduced during power outages. An emergency overflow
or the provision of adequate reserve space is a "fail soft"
provision. Portable standby generators and gasoline-
powered pumps have been provided in a few cases, but
rarely if ever used.
2.6.7 Record Keeping
The O&M manual should contain forms for this purpose.
A record should be kept of routine maintenance, along
with a summary list of corrective action to be taken. A
data base should be prepared to document call-out
maintenance. With a data base, a printout of previous
maintenance for any particular address can be prepared
for the service person attending a call. This is especially
beneficial to new employees, to alert them as to probable
causes of the malfunction. Repeat calls to particular
addresses for repetitive problems are common. These
are clearly shown by data base reporting.
2.6.8 Troubleshooting
About all the field personnel usually know about the
performance of a pressure sewer main is that ft is
adequate.
As growth occurs an increased frequency of high level
alarms at the individual pumping units is usually due to
the mains having reached capacity or having developed
a problem with air-caused headless.
A preferable method to measure the performance of the
mains is to take readings at pressure monitoring stations
located at key locations along the route of the main.
These are taken periodically, and compared with previous
readings that correspond to times when lower populations
were being served, periods of especially high or low
infiltration, periods when air accumulations in the mains
may be expected, and other critical times. In that way the
hydraulic performance of the system is measured and
documented. Air binding conditions can be identified,
located, and corrected. Knowledge of the capacity and
other characteristics of the system is continually refined.
During service calls for the pumping units, the service
personnel usually attempt to troubleshoot the problem
and correct it in the field. A typical scenario is as follows:
First, they confirm that power is being supplied to the
control panel and observe the liquid level in the tank to
confirm that a high water condition exists. If no reserve
volume remains available to receive flow, they may pump
the basin down some by running the pump manually (if it
will run), or by using a mobile pump.
then, they may turn the power off so they can work more
safely with the malfunctioning installation. Guided by
their experience with pumping units on the project, or a
history of performance of the particular pumping unit
being serviced, they first address the most likely causes
of the problem. A common first check is of the mercury
float switches, often obstructed by grease or otherwise
being out of position.
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They will run the pump on the manual switch setting in the
electrical control panel, and take amperage readings. A
high-amperage reading usually indicates a jammed
impeller. A low amperage reading usually indicates an
air-bound pump.
By this process the cause of the malfunction is identified
and corrected in the field. Not infrequently, a repair is
made that is believed to have been the cause of the
malfunction, only to receive another call for service the
next day. In some cases components are taken to the
shop for further evaluation.
2.7 System Costs
2.7.1 Construction
2.7.1.1 General
Awidevariety of factors cause construction costs to vary
considerably from project to project.
Topographical matters, such as the steepness orf latness
of an area can impact costs. The proximity and number
of culvert or buried utility crossings can greatly influence
the cost of pipeline installations. Geological issues such
as rock excavation, fragile soils, or high groundwater
conditions are major considerations in preparing cost
estimates.
Prices also vary with geographical setting, typically being
higher in northern metropolitan areas, for example, than
In the rural south.
The attitude of the public to be served is important.
Consider, for example, the reluctance a contractor would
have to enter private property for installation of pressure
seweron-tot facilities if the property owners wereopposed
to the project. Unit bid prices may reflect the attitudes of
the public, particularly with area contractors.
The burial depth of pipelines, compaction requirements,
restoration required, degree of field inspection, and
traffic control are other examples of causes for cost
variance. Dueto economy of scale, large projects usually
experience tower unit prices, but this may be tied to the
bidding climate and the availability of contracting
companies large enough to bid the job.
Familiarity with pressure sewer installation plays an
important role. It has been common for the first projects
built in an area to experience higher unit prices than
subsequentprojectsduetouncertaintiesofthecontractor.
For this reason some engineers have arranged for
interested contractors to make a few installations on a
time and materials compensation basis prior to the time
of bidding. This allows the contractors to sharpen their
understanding of the project requirements, and to refine
estimates. It also better acquaints the contractors with
the engineers, demonstrates installations to the client
and the public, and frequently results in final design
revisions.
Funding and regulatory requirements play a part in
project cost estimating to the extent that the regulations
may be a help or hindrance to the contractor, the client,
and the engineer.
Because of the many variables, accurate cost estimating
guidelines are beyond the scope of this manual, but
some generalities are given.
2.7.1.2 Piping systems
Piping systems are best estimated using guidance from
water system projects built in the same area, if similar
materials and specifications are used. The best situation
is one where the water line project was designed and
construction managed by the same engineering firm
producing the sewer cost estimate. In this way, project
similarities and differences can be factored into the
comparison from intimate familiarity. If the estimating
engineers are not familiar with the project they are
obtaining guidance from, they should become familiar
with the specifications for that project to rationalize
differences between the projects. In some cases, the
Associated General Contractors (AGC) can provide helpful
insight.
The prices for piping materials can usually be obtained
through local suppliers. PVC is generally priced by the
pound. At present (mid 1991), in large quantities, a cost
of $1,30/kg ($0.60/lb) can be used, but this may fluctuate
considerably. PVC weighs about 1,425 kg/m3 (89 Ib/ou
ft)-
In the absence of better information, Table 2-8 provides
estimating data for planning purposes only, This table
was prepared by reviewing bid tabulations from numerous
projects throughout the United States. Some projects
were known to have been built so cheaply that long term
performance was questionable. Other projects had
unusually stringent specifications or other features that
resulted in particularly high costs. Both the high cost and
low cost bid tabulations were discarded, leaving mid
range averages to produce this table.
'••<:..','
Pipe prices include furnishing and installing the pipe,
excavation, bedding, backfilling, compaction, pressure
testing, cleanup, and related requirements. Not included
84
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Table 2-8. Average Installed Unit Costs (mid-1991) for
Pressure Sewer Mains and Appurtenances
Table 2-8. AverageUnitCo3ts(mld-1991)forGrinderPump
Services and Appurtenances
Item
Unit Cost
Item
Unit Cost
2-in Mains
3-in Mains
4-in Mains
6-in Mains
8-in Mains
Extra for mains in A.C. pavement
2-in Isolation valves
3-in Isolation valves
4-in Isolation valves
6-in Isolation valves
8-in Isolation valves
Automatic air release stations
7.50/LF
8.00/LF
9.00/LF
11.00/LF
14.00/LF
5.00/LF
250/each
275/each
350/each
400/each
575/each
1,500/eaeh
2-hp centrifugal grinder pump
- List price
- Quantity price
Simplex GP package
- List price with 30-ln vault
- Quantity price with 30-in vault
- Installation
4-in building sewer
1.25-in service line
Abandon septic tank
1,200/each
600/each
4,100/each
1,800/each
500 - 1,500/each
16/LF
6/LF
400/each
are allowances for such items as rock excavation,
engineering, and administration.
2.7.1.3 Grinder Pump Services
As shown in Table 2-9, a typical list price for a 2-hp pump
is presently about $1,200. The list price for a simplex
grinder pump package is about $4,100. This example
package includes the following. (The package would
differ if a progressing cavity type pump were supplied,
e.g. the pump would likely be rated at 1 horsepower, and
liquid levels would be sensed using a trapped air system.
However, costs should be similar.)
• Pump-2hp
« Pump vault - 30-in diameter x 8-ft deep
• Pump vault cover
• Slide-away rail assembly
• Piping within vault, with gate valve and check valve
* Electrical junction box with cord grips
* Mercury float liquid level sensors
* Electrical control panel, including:
« NEMA 3R enclosure
- Circuit breaker
- Capacitors
- Motor start contactor
- Hand-off - auto switch
- Audible and visual alarm
- Audible alarm silence with auto reset
When provided in quantity, prices are considerably
reduced. Typical volume prices are about $600 for the
pump alone, and $1,400 for the pump package using a
60-cm (24-in) diameter pump vault, or $1,800 fora pump
package using a 75-cm (30-in) vault
Some dealers specialize in providing GP packages at low
prices, a typical price being about $1,300. However,
many of the components are not provided by the pump
manufacturer, but are instead purchased by the dealer
from a variety of sources. The dealer often assembles
some of the parts. The assembly may or may not have
been field-tested and refined.
A low-cost, dealer-provided package usually differs from
the factory package in several ways. The pump vault is
typically smaller, usually 60-cm (24-in) diameterx 1.5-m
(5 ft) deep, and is made of thin wall FRP. Hose isused for
the discharge piping instead of hard piping, or galvanized
metal piping may be used. No slide away, quick-disconnect
coupling is provided. The pump rests on feet screwed
into the bottom of the pump instead of being suspended
as most factory packages are made. No electrical junction
box is provided, i.e., wiring is routed directly from the
pump vault to the control panel without splicing. The
electrical control panel is often made simpler and with
lower cost components. A visual alarm is provided instead
of audible and visual, which also eliminates the need for
audible alarm automatic reset circuitry.
Some dealer-provided packages may be quite good, but
the least expensive packages usually compromise quality,
pump vault size, and some other features.
In some cases, sewer districts acquire the various parts
and pieces, and assemble the packages themselves,
including manufacture of the electrical control panel. A
savings of about $200/package has been reported by
several districts following that practice, with variable
results.
Installation costs of GP services vary considerably
depending on the standards being met, the degree of
restoration required, whether existing or new homes are
being served, and a host of other factors.
85
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The lowest cost installations are made by sewer district
personnel serving new homes priorto landscaping being
done. Inthese Instances, costs as low as $-400 have been
reported. Contractors' bid prices are usually higher and
typically are $500-1,500.
From the above, the total furnished and installed cost of
simplex GP services ranges from $1,800 to over $4,000,
not including building sewer replacement, abandonment
of the old septic tank (if applicable}, and service line
installation.
When serving existing homes, the building sewer is often
replaced to limit I/I. For economic reasons GP vaults are
sometimes placed only a few feet from the home which
limits the amount of building sewer required and the
length of wiring needed.
"Abandonment" of the old septic tank is usually required
by the authorities, which means removing and properly
disposing of the septage, then either filling the septic tank
with sand or crushing it in place and backfilling the hole.
2.7.1.4 STEP Services
Prices of effluent pumps vary according to the quality
provided. For examples of the varying qualities, some
pumps use ball bearings while others use sleeve bearings.
Some use Type 316 stainless steel fasteners while
others may use a type of stainless steel that is not
corrosion-resistant to the STEP effluent. Motor types
vary, as do impellers and seals. Some STEP pumps are
builtforthespecificpurposewhileothers are inexpensive
sump pumps, used as pressure sewer pumps whether
suited or not.
Pump prices also vary accordingtothe head theyproduce,
and consequently vary with the horsepower of the motor
required.
The list price for a typical low-head effluent pump of good
quality is about $300, while a higher head, high quality
pump has a list price of about $800. A factory-provided
package, using a pump vault external to the septic tank,
has a list price of about $2,500-3,000 depending on the
pump supplied. This example package includes the
following:
Pump -1/3 to 2 hp
Pump vault - 24-in diameter x 6-ft deep
Pump vault cover
Slide-away rail assembly
Piping within vault, with gate valve and check valve
Electrical Junction box with cord grips
Mercury float liquid level sensors
Electrical control panel, including:
- NEMA 3R enclosure
- Circuit breaker
- Motor start contactor
- Hand-off - auto switch
- Audible and visual alarm
- Audible alarm silence with auto reset
The STEP pump vault is smallerthan a G P vault because
reserve volume is provided in the septic tank.
When provided in quantity, prices are considerably
reduced.
STEP packages use a discharge hose assembly more
commonly than the slide-away rail assembly and hard
piping listed above. Also, they are often provided by a
pump dealer rather than the factory. The pump dealer
may provide the package as a specialty sideline, or may
custom assemble them according to the design and
specifications provided by the engineer. The latter service
can also be provided by the factory, especially if a
quantity order is involved.
Atypical price for a dealer-provided STEP package using
a pump vault external to the septic tank may vary from
$700 to$1,500 depending on the particularpump supplied,
the quantity ordered, and other variables. Packages
made to be inserted into the septic tank, as contrasted
against the external vault design, are generally about
$200 less expensive.
A new water-tight septic tank is often required due to
infiltration expected to enter the existing septic tank.
Water-tight septic tanks vary in cost according to the
quality provided, and according to the materials used.
Concrete tanks are usually less expensive than well
constructed and engineered FRP or polyethylene tanks,
but are heavy and more difficult to install in confined
spaces. Prices for quality tanks of either material generally
are $600-1,000.
Installation costs vary depending on whether new or
existing homes are served, the degree of restoration
required, if new or existing septic tanks are used, the size
of the project, and a host of other factors. Mid range
installation costs for retrofitting existing septic tanks have
been about $600-1,200, and when installation of a new
tank is required, costs are about $1,000-1,500. These
costs do not include replacement of building sewers,
service line installation, connection to the main,
abandonment of the old septic tank, or restoration.
Average, generalized prices for STEP equipment and
installations are shown in Table 2-10. Price extremes
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Table 2-10. Average Unit Costs (mid-1991) for STEP
Services and Appurtenances
Item Unit Cost
Effluent pump list price
Effluent pump quantity price
Simplex factory package list price
Quantity package price w/externa] vault
Quantity package price w/intemaJ vault
New septic tank
Installation (retrofit of existing tank)
Installation (with new septic tank)
4-in building sewer
1.25-in service line
Abandon septic tank
($)
300 - 800/each
200 - 500/each
2,500 - 3,000/each
700 -1,500/each
600 -1,200/each
600 - 1,000/each
600 - 1,200/each
1,000 -1,500/each
14 - 18/LF
4 -8/LF
300 -500/each
extend far beyond the ranges given here, but are not
indicative of genera! prices for typical well-built projects.
2,7.2 Operation and Maintenance
The operation and maintenance cost for pressure sewers
varies greatly, depending on system size and quality. In
some instances pipelines have been laid with lax
specifications and little inspection. The subsequent
maintenance required and lowerlevel of reliability probably
offsets the initial cost savings.
The maintainability of the on-tot facilities plays an important
part in determining the cost of long term maintenance.
For example, some systems are difficult to maintain in
that removal of the pump, liquid level controls, and
electricals are difficult and unpleasant tasks. In such
cases, when a service call is occasioned the service
personnel tend to only patch the problem with the result
that the number of service calls received increases over
time. Preferred for long-term economy are designs where
the working parts are easily tested, repaired, and if
possible, shop or factory reconditioned.
Many systems have inadequatecost accounting systems,
making O&M cost analyses difficult to accomplish or
misleading. Also, systems vary widely as to type. Some
aregrinderpump systems whileothers are STEP systems.
Some systems have their own treatment facilities to
operate and maintain, while others discharge to nearby
facilities in other jurisdictions. I n some cases maintenance
is provided by contractors, and in other instances
maintenance is provided by the controlling agency's staff
who may have part time duties other than maintenance
of the pressure sewer.
As a result, a detailed analysis of particular project
circumstances is required to forecast O&M cost.30 With
that caveat, O&M costs are reported here for one system.
The Glide, Oregon pressure sewer involves about 20
miles of main. A total of 680 EDUs are served by 420
STEP installations, 12 GP units, and 10 variable grade
gravity sewer connections. The setting is primarily
residential but several schools are served, several
restaurants, mobile home parks, and similar installations.
The system has been in service since the late 1970s
(over 10-years).
Glide is a quality system that was built to demanding
standards, and closely inspected. All main installations
were specially bedded and backfilled, and pressure-
tested. Heavy-duty pumps were used.
Two full-time field personnel maintain the system. While
both attend the collection system, one of the personnel
focuses primarily on O&M of the treatment facilities
{oxidation ditches and mixed media filtration), while the
other persons primary obligation is with the collection
system.
A detailed cost accounting system is in place. Typical
O&M costs incurred are shown in Table 2-11.
Labor costs reported in Table 2-11 include approximately
50 percent fringe benefits, including PICA, retirement,
sick leave, and vacation.
No facility amortization or equipment rental is included,
nor is customer billing and accounting.
Given the 680 EDUs served, and the total O&M annual
costof$142,200,or$17.43/month/EDU.ExcludingO&M
for the wastewater treatment plant, and the WWTP
prorated share of the overhead, remaining overhead and
O&M of the pressure sewer is $4.77/month/EDU.
2.7.3 User Charges and Assessments
When conventional sewers are used, the front-end
construction cost is often high. In comparison, pressure
sewers generally have a lower initial cost. When the
pressure sewer main has been installed and treatment
facilities are on line, service to the nearby properties is
available. The higher cost item of the on-lot facilities
(pump, etc.) is deferred until service is needed or until the
property is built upon. Subsequent users finance the
capital cost of theirown on-lot facilities as opposed to that
being a district obligation.
This encourages innovation and sometimes departure
from traditional methods of assessment. The high initial
cost of conventional sewers may financially require
assessment to be applied to all fronting properties,
whereas pressure sewer mains may affordably bypass
properties not intended to be served.
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T*bl» 2-11. O&M Coat Accounting Records for the Glide,
Oregon Pressure Sewer System ($1,000)
Item Overhead WWTP Collection Services Total
Labor
Materials
Total
%
28.6
2.1
30.7
22
56.7
24.3
81.0
57
2.3
0.6
2.9
2
16.3
11.3
27.6
19
103.9
38.3
142.2
100
Average costs for 1988 and 1989.
An "Association" approach to financing is possible, as
opposed to the "District" approach. An, association, as
described here, obtains revenues from user and
connection charges but has no taxation authority.
Pipelines may be sized to serve those properties desiring
service (plus some allowance), as opposed to the
conventional sewer - district practice of sizing sewers to
serve an entire area at 100 percent anticipated
development.
As a guideline for future projects, probably little can be
learned from the service and connection charges made
in communities now served by pressure sewers. This is
because the economics vary so widely. Some projects
have received substantial grant funding while others
received none. Some projects can be constructed
economically while others will be expensive. Reasons for
the cost variances include the availability of existing
treatment facilities, rock excavation, size of project, and
other parameters described throughout this manual.
In a review of 5 projects, monthly service charge rates
were set to compensate O&M expenses for the services,
collection system, treatment, and for management.
Monthly rates were $12-20. Connection charges pay for
on-Iot facility materials. Sometimes installation charges
are included, and in other cases they are paid separately
by the homeowner.
Also included in the connection charge fee is a prorata
share of the piping system infrastructure and treatment
facilities. Connection charges were about $2,000-4,000.
Additional information regarding the time value of money
associated w'rth the use of pressure sewers is contained
in a book by Thrasher.9 Deferred costs of subsequent
pump Installations and O&M costs equate to a lower
present worth.
2.8 System Management Considerations
2.8.1 Homeowner Responsibilities
During the planning and design phase property owners
should become informed about the project and once
properly informed, become involved. This is true of all
sewer projects, and not unique to alternative collection
systems. However, with these systems it has been
particularly common for communities to react prior to
becoming accurately informed. Eithertheuse of pressure
sewers is falsely glamorized or unduly criticized. Early
public dissemination of accurate information is a critical
role of the utility and the engineer.
Prior to construction, the property owners may be asked
for any knowledge they may have of the location of the
existing septictank, drainfield, buried utilities, or properly
lines. The property owner may also have some role in the
placement/location of the on-Iot facilities. In locating
these, thought should be given to property alteration
plans, such as the widening of a driveway or the addition
of a patio or deck.
Disruption to the community will be less for installation of
pressure sewer mains than for conventional sewers, but
disruption for installation of the services can be greater.
Throughout the planning-design-construction process,
cooperation of all parties including homeowners results
in a more easily accomplished project and lower costs.
Use of the system should comply with requirements of
the user ordinance. Typical requirements includethat the
homeowner should not drive or build over the tank, and
should protect the facilities from damage. Discharge of
flammables, acids, excessive amourttsof grease, sanitary
napkins, or other non-wastewater items is discouraged.
This requirement differs little from user ordinance
requirements for conventional sewers. Proper use of the
system results in lower user charges and improved
reliability.
2.8.2 Sewer Utility Responsibilities
The utility should take seriously the need for early and
accurate public address. On some larger community
projects, opinion leaders of the community representing
diverse geographical areas and varied cultural
backgrounds have been sought. These people are invited
to be special advisors to the utility, and in turn receive
advance, more detailed information than is possible to
disseminate at large public meetings. The personal
contact they have with their neighbors can transfer
information more meaningfu lly than is possible otherwise.
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While maintenance and operation of a pressure sewer
may not be more difficult than that of a conventional
sewer, the technology is less well known. The utility
should have the flexibility and talent required to adapt to
unfamiliar practices. A degree of long-term commitment
is required. The regulatory agency may want assurance
of the utility's capabilities, to encourage their use of
alternative sewers.
Tactful public relations are important, as they are with
any utility function. Personnel in contact with the public
become the utility's ambassadors. Selection of personnel
should be made accordingly, and special training and
ongoing support may be advised.
Detailed daily records should be kept of maintenance
functions, and a summary report should be made annually.
This report should quantify the maintenance requirements
and make recommendations for project improvements.
Involvement of the maintenance forces is required.
Continuing involvement of the design engineer will close
the loop between planning, design, construction, operation
and maintenance, and will result in improved future
designs.
The utility should be capable of responding to whatever
routine or emergency needs may be presented.
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2.9 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Clift.MA Experience with Pressure Sewage. ASCE-
SED, 94(5):865,1968.
2. Combined Sewer Separation Using Pressure Sewers.
ASCE. FWPCA Report ORD-4, 1969.
3. Carcich, I.G. Hetling, L.J., and R.P. Farrell. A Pressure
Sewer Demonstration. EPA Number R2-72-091,
1972.
4. Hendricks, G.F. and S.M. Rees. Economical
Residential Pressure Sewage System with No
Effluent. EPA/600/2-75/072, 1975.
5. Mekosh.G and D. Ramos. Pressure Sewer
Demonstration at the Borough of Phoenixville,
Pennsylvania. EPA Number R2-73-270,1973.
6. Eblen, J.E. and L.K. Clark. Pressure and Vacuum
Sewer demonstration Project-Bend, Oregon. EPA/
600/2-78/166, NTIS No. PB287-146, 1978.
7. Alternatives for Small Wastewater Treatment
Systems. EPA/625/4-77/011, NTIS No. PB299608.
U.S. Environmental Protection Agency, Cincinnati,
OH, 1977.
8. Great Lakes - Upper Mississippi River Board of State
Sanitary Engineers, Recommended Standards for
Sewage Works, Health Education Service, Albany,
NY. 1978.
9. Thrasher, David. Design and Use of Pressure Sewer
Systems. Lewis Publishers. 1988.
10. Bennett, E., K. Linstedt and J. Felton. Rural Home
Wastewater Characteristics. Proc. of the National
Home Sewage Symposium, American Society of
Agricultural Engineers, 1974.
11. Jones, E. Domestic Water Use in Individual Homes
and Hydraulic Loading of and Discharge from Septic
Tanks. Proc. of the National Home Sewage Disposal
Symposium, American Society of Agricultural
Engineers, 1974.
12. Environment-One Corporation. Design Handbook,
Low Pressure Sewer Systems Using Environment-
One Grinder Pumps. 1989.
13. Flanigan, L.J. and R.A. Cudnik. State of the Art
Review and Considerations forthe Design of Pressure
Sewer Systems. Weil-McLain Company, Ashland,
Ohio, 1974.
14. Watson, K.S., Farrell, R.P. and J.S. Anderson. The
Contribution from the Individual Home to the Sewer
System. JWPCF 39(12):2039-2054,1967.
15. Fair, G.M. and J.C. Geyer. Water Supply and
Wastewater Disposal. J.Wiley & Sons, New York,
NY, 1954.
16. Brinley, R.K., Olmstead, R.D. and S.M. Wilkinson.
Design Manual for Pressure Sewer Systems.
Peabody Barnes Inc., Mansfield, Ohio, 1982.
17. Conery, W.J. Pressure Sewer Piping System Design.
F.E. Myers Co., Ashland, Ohio, 1981.
18. Minimum Velocities for Sewers. Journal Boston
Society of Civil Engineers, 29(4):286,1942.
19. Uni-Bell Plastic Pipe Association. Handbook of PVC
Pipe. 1982.
21. International Association of Plumbing and Mechanical
Officials, "Uniform Plumbing Code", 1985.
20. Cooper, B.J. Buried Pipes in Frozen Soil. In:
Appropriate Wastewater Management Technologies
for Rural Areas Under Adverse Conditions, D.H.
Waller and A.R. Townshend, eds., Tech. Press,
TUNS, Halifax, N.S., CANADA 1987.
22. Kim, S.W. Indiana State Board of Health I/A
Newsletter, No. 5, Feb. 1981.
23. Weibel, S.R., et al. Studies on Household Sewage
Disposal Systems, Parts 1, 2, and 3. U.S. Public
Health Service Publications, 1949 -1954.
24. Winneberger, J.H. Septic-Tank Systems, A
Consultant's Toolkit. Butterworth Publishers, 1984.
25. Baumann, E.R. and H.E. Babbitt, H.E. An
Investigation of the Performance of Six Small Septic
Tanks. University of Illinois Engineering Experiment
Station, Bulletin Series No. 409, Vol. 50, No. 47,
February 1953.
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26. Design Manual: Onsite Wastewater Treatment and
Disposal Systems. EPA/625/1-80/012, NTIS No.
PB83-219907, U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1980.
27. Handbook: Septage Treatment and Disposal. EPA/
625/6-84/009, NTIS No. PB88-184015. U.S.
Environmental Protection Agency, Cincinnati, Ohio,
1984.
28. Siegrist, R,L, Anderson, D.L. and J.C. Converse.
Commercial Wastewater On-Site Treatment and
Disposal, Proceedings of the 4th National Home
Sewage Symposium, ASAE, Proc. 07-85,1985.
29. Design Manual: Odor and Corrosion Control in
Sanitary Sewerage Systems and Treatment Plants.
EPA/625/1-85/018, U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1985.
30. Farrell, R.P. Operating and Maintenance Experience
with Some Pressure Sewer Systems. In: Proc. of the
Fourth National Conference, Individual Onsite
Wastewater Systems, National Sanitation
Foundation, 1977.
91
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Page Intentionally Blank
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CHAPTERS
Vacuum Sewer Systems
3.1 Introduction
The use and acceptance of alternative wastewater
collection systems have expanded greatly in the last 20
years. One of these alternatives, vacuum sewers, has
been used in Europe for over 100 years. However, it has
been only in the last 25 years or so that vacuum transport
has been utilized in the United States.
In this period of time, significant improvements have
been made in system components. In addition,
experience with operating systems has led to
advancements in design, construction, and operational
techniques. These factors have all contributed to vacuum
sewer systems being a reliable, cost-effective alternative
for wastewater conveyance.1
Vacuu m sewerage is a mechanized system of wastewater
transport. Unlike gravity flow, it uses differential air
pressure to move the wastewater. It requires a central
source of power to run vacuum pumps which maintain
vacuum on the collection system (Figure 3-1). The
system requires a normally closed vacuum/gravity
interface valve at each entry point to seal the lines so that
vacuum is maintained. These valves, located in a pit,
open when a predetermined amount of wastewater
accumulates in the collecting sump. The resulting
differential pressure between atmosphere and vacuum
becomes the driving force that propels the wastewater
towards the vacuum station.
A vacuum system is very similar to a water distribution
system, only the flow is in reverse (Figure 3-2).2 This
relationship would be complete if the vacuum valve was
manually opened, like a waterfaucet. With proper design,
construction, and operation a vacuum system can be
made to approach a water system in terms of reliability.
The choice of collection system type is usually made by
the consulting engineer during the planning stages of a
wastewater facilities project. This choice is the result of
a cost-effectiveness analysis. Where the terrain is
applicable to a gravity system, the vacuum system many
times is not even considered. While gravity may be cost
effective inthese situations, many small factorsconsidered
collectively may result in a vacuum system being the
proper choice. Vacuum sewers should be considered
where one or more of the following conditions exist:
» Unstable soil
• Flat terrain
• Rolling land with many small elevation changes
« High water table
• Restricted construction conditions
• Rock
• Urban development in rural areas
The advantage of such systems may include substantial
reductions in water use, material costs, excavation costs,
and treatment expenses. In short, there is a potential for
overall cost effectiveness. Specifically, the following
advantages are evident:
• Small pipe sizes, usually 7.5-, 10-, 15-, and 20-cm (3,
4,6,8-in) are used.
• No manholes are necessary.
* Field changes can easily be made as unforeseen
underground obstacles can be avoided by going over,
under, or around them.
• Installation at shallow depths eliminates the need for
wide, deep trenches reducing excavation costs and
environmental impact.
* High scouring velocities are attained, reducing the risk
of blockages and keeping wastewater aerated and
mixed.
• Unique features of the system eliminate exposing
maintenance personnel to the risk of H2S gas.
• The system will not allow major leaks to go unnoticed,
resulting in a very environmentally sound situation.
• Only one source of power, at the vacuum station, is
required.
• The elimination of infiltration permits a reduction of size
and cost of the treatment plant.
93
-------
Figure 3-1. Typical layout - vacuum sewer system. (Courtesy AIRVAC)
94
-------
Figure 3-2.
Water sewer/vacuum system similarities. (Courtesy AIRVAC)
WATER SUPPLY SYSTEM
WATER
VALVES
r VMLVCO -\
VA
WATER TANK
(+) PRESSURE
WATER PUMP
SEWAGE PUMP
VACUUM
VALVES
SEWAGE COLLECTION
TANK (-) PRESSURE
VACUUM COLLECTION SYSTEM
The history of vacuum sewer technology is discussed in
Chapter 1. The differences in overall system philosophy,
design concepts, system components, and marketing
approaches of four manufacturers are discussed. Each
of these companies have made significant contributions
to the vacuum sewer industry. Presently, almost all
systems in operation in the United States are AIRVAC
systems. For this reason, the remainder of this Chapter
focuses on that approach. This in no way represents any
endorsement of that system, but merely reflects the
present state of the art in vacuum sewerage. In fact, at
least one competitor has recently entered the U.S market.
That manufacturer, Iseki, uses an approach similar to
AIRVAC.
3.2 System Plan and Elevation View
Figure 3-3 shows a plan and profile view of a typical
vacuum sewer line. Figure 3-4 shows a plan and profile
view of a typical valve pit.
3.3 Description of System Components
A vacuum sewer system consists of three major
components: the service, the collection mains, and the
vacuum station.
3.3.1 Services
The services in a vacuum system consist of the following
components:
• Vacuum valve
• Auxiliary vents
• Valve pit/sump, or
• Buffer tanks
The vacuum valve provides the interface between the
vacuum in the collection piping and the atmospheric air
in the building sewer. System vacuum in the collection
piping is maintained when the valve is closed. With the
valve opened, system vacuu m evacuates the contents of
the sump. The valve is entirely pneumatic by design, and
has a 7.5-cm (3-in) opening size. Some states have
made this a minimum size requirement, as this matches
the throat diameter of the standard toilet.
Valve pits and sumps are needed to accept the wastes
from the house. These may consist of one setting with
two separate chambers. In these cases, the upper
chamber houses the vacuum valve and the bottom
chamber is the sump into which the building sewer is
connected. These two chambers are sealed from each
other. The combination valve pit/sump is made of
95
-------
Rgui* 3-3. Profits v!«w of typical vacuum s«wer lln«.
to
c»
-------
Figure 3-4.
Plan and profile view - typical valve pit
GRADE-
POSITION FOR ANTI-FLOATATION RING 13" DIA.
(IF REQUIRED)
6" WIN. LENGTH GRAVITY
STUB WITH GLUED ON CAP,
CAST IRON FRAME AND LID
FIBERGLASS VALVE PIT
2" OR O.2% SLOPE
WHICHEVER GREATER
VACUUM SEWER MAIN
(DIRECTION OF FLOW
'B' INTO PAPER)
4" GRAVITY SEWER WITH SLIP COUPLING
Z" SENSOR LINE
i" SUCTION LINE
SHOWING UP TO 4 GRAVITY
CONNECTIONS TO SUMP
_ FIBERGLASS SUMP 30" OR 54"
DEEP. SUMP 30" I.D. AT TOP,
16"I.D. AT BOTTOM
-------
fiberglass, and is able to withstand traffic loads. For
deeper settings, the fiberglass pit/sump arrangement
may be replaced by a concrete manhole section in which
the vacuum valve is mounted. In this arrangement, only
one chamber exists. Buffer tanks are used for large
customersorwhenapressure/vacuumorgravity/vacuum
interface is desired, as would be the case with a hybrid
system.
A 10-cm (4-in) auxiliary vent is installed on the
homeowner's service lateral, downstream of all of the
house traps (Figure 3-5). This vent is necessary to
provide the volume of air that will follow the wastewater
into the main. This also circumvents the problem of
inadequate house venting which had resulted in trap
evacuation in early installations.1 Some operating entities
require the vent to be located near a permanent structu re
for aesthetic and protection reasons. In climates where
temperatures fall below freezing, this vent must be
located a minimum of 6 m (20 ft) from the valve pit. In this
manner, the heat from the wastewater acts to warm the
freezing atmospheric air thus reducing the possibility of
freezing of some of the valve components.
3.3J2 Collection Mains
The collection piping network consists of the following
components:
• Pipe
• Fitting
• Lifts
• Division valves
The piping network is connected to the individual valve
pits and the collection tank. Schedule 40, SDR 21 and
SDR 26 PVC pipe have been used, with SDR 21 being
the most appropriate and most common. Both solvent-
welded and gasketed have been used. Experience has
shown that there are fewer problems with the gasketed
type pipe. Where gasketed pipe is used, the gaskets
must be certified for use under vacuum conditions.
HDPE pipe has also been used in Europe. Typical sizes
include 7.5-, 10-, 15-, and 20-cm (3, 4, 6, 8-in) pipe.
PVC pressure fittings are needed for directional change
as well as for the crossover connections from the service
line to the main line. These fittings may be solvent-
welded or gasketed. Again, the recent trend is to avoid
solvent-welded fittings where possible.
Lifts or vertical profile changes are used for uphill liquid
transport. These lifts are generally made in a sawtooth
fashion. A single lift consists of two 45-degree fittings
connected with a short length of pipe (Rgure 3-6).
Since vacuum sewers are exposed to repeated energy
inputs, pipe movement is possible if proper installation
practices are not followed. Early systems used concrete
thrust blocks at each fitting. More recent systems have
been installed without concrete thrust blocking. The
theory behind this is that the pressure is inward rather
than outward as would be the case in a positive pressure
situation. However, a more important concern is that
each fitting is a point of possible joint failure. Failure of the
fitting may occur because of trench settlement rather
than thrust. Forthis reason, care must be exercised in the
backfill and compaction operations. Granular backfill
material covering the fitting, coupled with mechanical
compaction, is a must if thrust blocking is to be eliminated.
If thrust blocking is used, athin plastic membrane should
cover the pipe prior to the concrete pour.
Division valves are used for isolation purposes during
troubleshooting. Both plug and resilient-wedge gate valves
have been used. Recent systems have included gauge
taps installed just downstream of the division valve. This
tap makes it possible for one person to troubleshoot
without having to check vacuum at the station. This
greatly reduces emergency maintenance expenses, both
from a time and manpower standpoint. Therefore, this
tap is recommended.
Different pipe location identification methods have been
used. These include magnetic trace tape in the top of the
trench, metal toning wires above the pipe, and color
coding of the pipe itself.
3.3.3 Vacuum Station
Vacuum stations function as transfer facilities between a
central collection point for all vacuum sewer lines and a
pressurized line leading directly or indirectly to a treatment
facility.1 The following components are included in the
vacuum station (Figure 3-7).
• Vacuum pumps
• Wastewater pumps
• Generator
• Collection tank
• Reservoir tank
• Controls
• Motor control center
• Chart recorder
• Fault monitoring system
Vacuum pumps are needed to produce the vacuum
necessary for liquid transport. The operational history of
vacuum sewers indicates that the optimum operating
range is 40-50 cm (16-20 in) Hg. The pumps, however,
should have the capability of providing up to 63 cm (25 in)
Hg as this level is sometimes needed inthetroubleshooting
98
-------
Figure 3-5. Auxiliary vent location.
Figure 3-6. Lift detail.
FLOW-*'
VACUUM MAIN
45° PVC SOLVENT-WELD
SCHEDULE 40 DWV FITTING
SCHEDULE 40 OR SDR 21 PVC PIPE
99
-------
Rgura 3-7.
Une diagram of a typical vacuum station (Courtesy AIRVAC)
CONTROLS AND
ALARMS ;
TELEPHONE
ALARM
VACUUM RESERVOIR TANK
. 2" VACUUM VALVE
SEWAGE COLLECTION
TANK
SUMP IN
BASEMENT OF
VACUUM STATION
VACUUM SEWERS \
TO TREATMENT -
PLANT
y VACUUM GAUGE
VACUUM SWITCH
COMPOUND GAUGE
VACUUM
RECORDER
S.G. SIGHT GLASS
100
-------
process. Redundancy is a minimum requirement with
each pump capable of providing 100 percent of the
required air flow (cfm).
Vacuum pumps may be either the liquid-ring or sliding-
vane type. A liquid-ring vacuum pump utilizes a service
liquid as a sealing medium between an offset impeller
and the pump casing. As the impeller spins, the service
liquid is forcedagainstthepumpoutercasing by centrifugal
force, and air is compressed and forced out of the
discharge pipe by the eccentric liqu id action. The vacuu m
is created as more air is drawn in to be compressed.
When liquid-ring pumps are used, oil is recommended as
the seal liquid. Sincethe service liquid continually circulates
when the pump is in operation, a service liquid tank must
be provided. The tanks are vented with an outlet to the
outside. Since the service liquid carries a significant
quantity of heat away from the pump, a heat exchanger
is required.
Sliding-vane vacuum pumps may also be used. For
these types of pumps an air filter is required. This filter is
located on the inlet line between the reservoir tank and
the vacuum pump so as to remove paniculate material
which might cause excessive impeller wear if it were to
enter the pump volute. The use of sliding-vane vacuum
pumps has increased recently. The reason for this is the
lower power consumption required for a given pump
capacity. On the negative side, there have been problems
reported with the vulnerability of these pumps should
liquid be carried into them. In this situation, the pumps
must be taken out of service to remove the liquid; the
result is a shortened service life. By contrast the liquid-
ring pump can usually withstand an accident of this type
with very little damage. Design precautions, such as an
electrically controlled plug valve between the collection
tank and the reservoir tank, can be added to the piping
system in order to protect sliding-vane pumps.
Wastewater pumps are required to transfer the liquid that
is pulled into the collection tank by the vacuum pumps to
its ultimate point of disposal. Dry pit pumps have been
used extensively, although submersible wastewater
pumps located on guide rails within the collection tank
may be used as an alternative. The most frequently used
pump hasbeen the non-clog type. Redundancy is required,
with each pump capable of providing 100 percent of the
design capacity. The level controls are set for a minimum
of 2 minutes pump running time to prevent excessive
pump starting and related, increased wear. The pumps
should have shutoff valves on both the suction and
discharge piping to allow for removal during maintenance
without affecting the vacuum level.
Check valves are used on each pump discharge line and
on a common manifold after the discharge lines are
joined to it. Equalizing lines, consisting of small diameter
clear PVC pipe connecting the pump discharge to the
collection tank, are usually required.2 The purpose of
these lines is to remove airfromthe pump andto equalize
the vacuum across the impeller. In addition, they will
prevent the loss of prime should a check valve leak.
Since this setup will result in a small part of the discharge
flow being recirculated to the collection tank, a decreased
net pump capacity results.
In some recent designs, equalizing lines have been
eliminated by using horizontal wastewater pumps with a
continuously flooded suction. This is accomplished
through the use of a ball check valve on the pump suction
piping between the collection tank and the wastewater
pumps. Prior to using this concept, the designer should
carefully weigh whether the pumping cost savings are
significant enough to risk the possibility of failure of the
ball-check and the problems that could result.
Wastewater pumps are typically located at an elevation
significantly below the collection tank to minimize the net
positive suction heat (N PSH) requirement. In conjunction
with NPSH requirements, pump heads are increased by
7 m (23 ft) to account for tank vacuum. Both vertical and
horizontal pumps can be used.
Materials of construction for pumps include cast iron with
stainless steel shafts, while aluminum, bronze, and brass
should be avoided. Fiber packing is not recommended.
Double mechanical seals which are adaptableto vacuum
service should be used.2
A standby generator is a must. It ensures the continuing
operation of the system in the event of a power outage.
Standard generators that have been used in other
wastewater applications are available from a variety of
manufacturers.
The wastewater is stored in the collection tank until a
sufficient volume accumulates, at which point the tank is
evacuated. It is a sealed, vacuum-tight vessel made of
either fiberglass or steel. Fiberglass tanks are generally
more expensive, but do not require the periodic
maintenance (painting) of a steel tank. Painting may be
required every 5-6 years. Vacuum, produced by the
vacuum pumps, is transferred to the collection system
through the top part of this tank. The part of the tank
below the invert of the incoming collection lines acts as
the wet well. A bolted hatch provides access to the tank
should it be necessary.
101
-------
Most collection tanks are located at a low elevation
relative to most of the compon ents of the vacuu m station.
This minimizes the lift required for the wastewater to
enter the collection tank, since wastewater must enter at
or near the top of the tank to ensure that vacuum can be
restored upstream. Many times this results in a deep
basement required in the vacuum station.
A vacuum reservoir tank is located between the vacuum
pumps and the collection tank. It has three functions: 1)
to reduce carryover of moisture into the vacuum pumps;
2) to act as an emergency reservoir; and 3) to reduce the
frequency of vacuum pump starts.3 Like the collection
tank, H can be made of either fiberglass or steel.
The vacuum pumps are controlled by vacuum switches
located on the reservoir tank. Usual operating level is 40-
50 cm (16-20 in) Hg with a low level alarm of 35 cm (14-
in) Hg, The wastewater pumps are controlled by a
probe{s) located insideof thecollectiontank. One method
Includes using seven probes, one for each of the six set
points of the pu mping cycle and one as a grou nd. Another
method relies on a single probe that is capable of
monitoring all of the set points. These probes are the
capacitance-inductive type. They require a transmitter/
transducer to send a 4-20 mA signal to the control panel.
The motor control center houses all of the motor starters,
overloads, control circuitry, and the hours run meter for
each vacuum and wastewater pump. The vacuum chart
recorder, collection tank level control relays, and a
telephone dialer are also normally located within the
motor control center.
Vacuum gauges are used on all incoming lines as well as
on both the collection tank and the reservoir tank. Their
purpose is to allow the operator to monitor the system.
These gauges are very important in the troubleshooting
procedures. Chart recorders for both the vacuum and
sewer pumps are needed so that system characteristics
can be established and monitored. Like gauges, these
recorders are vital in the troubleshooting process.
A fault monitoring system is needed to alert the operator
of any irregularities, such as a low vacuum level. These
systems actuate an automatic telephone dialer. There
are a number of manufacturers that have this type of
equipment.
For small systems a skid-mounted station is available.3
This u n'rt contains all the compo nents of a typical vacu u m
station and must be installed inside a protective structure.
3.4 System Design Considerations
3.4.1 Hydraulics
The discussion of Section 2.4.1 is equally appropriate for
vacuum sewers. AIRVAC 7.5-cm (3-in) valves have a
capacity of 2 L/s (30 gpm) when connected to a 15-cm (6-
in) or larger main, assuming the necessary vacuum, 13
cm {5 in Hg), is available at the valve site.2 To achieve this
capacity with the normal 38-L (10-gal) discharge cycle
with an average valve-open time setting of 6 seconds, a
vacuum valve would be required to open and close 3
times/minute, with a "rest" of only about 15 seconds
between cycles. This may not present a problem to a
valve that is relatively close to the vacuum station
connected to a 15-cm (6-in) main. A different situation
exists for a valve at the far end of the system that is
connected to a 10-cm (4-in) main. Vacuum response (the
ability of the vacuu m main to quickly recover to the same
level of vacuum that existed prior to the cycle) is of
absolute importance in vacuum sewer design. Vacuum
response is a function of line length, pipe diameter,
number of connections, and the amount of lift in the
system.
Should a situation exist that is cause for concern, the
designer should consider using a buff er tank to attenu ate
peak flows. In this case, the idea is not for the valve to
keep up with incoming flow, but ratherto use tank reserve
volume to prevent backups until the valve is capable of
emptying the tank.
Design flows are maximum flow rates expected to occur
once or twice a day, and are used to size the vacuum
sewer mains as well as the various vacuum station
components. Instantaneous flow rates in excessofdesign
flows can occur under certain situations. Chapter 2
describes various studies of flow versus equivalent
dwelling units (EDUs). The equation for estimating design
flow based on the number of homes to be served is as
follows:
where:
Q
A
N
B
Q = AN + B
Design flow (gpm)
Acoefficient selected bytheengineer, typically
0.5
Number of EDUs
A factor selected by the engineer, typically 20
In the usual form, the equation is Q = 0.5N + 20, but may
be varied to account for anticipated high water use, to
102
-------
allow for a greater safety factor, and to allow for some
infiltration and inflow (I/I).
Theterm "self cleaning velocity" refers to the flow velocity
required to convey solids along with the water carrier. To
maintain an unobstructed pipeline, the velocity should be
sufficient to transport grit that may be present in the
wastewater, prevent grease plating on the crown of the
pipe, and scour (resuspend) previously settled matter.
This velocity is generally 60-90 cm/s (2-3 fps).
Tangential liquid velocities in the typical vacuum sewer
are 4.6-5.5 m/s (15-18 fps), obviously well above the
minimum required for self cleaning. A common claim
from the manufacturers of vacuum sewer components is
that there has never been a blockage reported in a
vacuum system. Whether true or not, the high transport
velocities suggest that the probability of blockages
occurring are remote.
An understanding of the vacuum transport process is
needed by the system designer. With the saw-tooth
profile, and as long as no vacuum valves are operating,
no wastewater transport takes place. All wastewater
remaining in the sewers will lie in the low spots. Since the
wastewater will not seal the bore of the pipe in these static
conditions, little vacuum loss is experienced throughout
the system when low or no flow is occurring.
When a sufficient volume of wastewater accumulates in
the sump, the vacuum valve cycles. The differential
pressure that exists between the sewer main and
.atmosphere forces the sump contents into the main.
While accelerating, the wastewateris rapidly transformed
into foam and soon occupies only part of the pipe cross
section so that the momentum transfer from the air to
water takes place largely through the action of shear
stresses. The magnitude of the propulsive forces starts
to decline noticeably when the vacuum valve closes, but
remains important as the air continues to expand within
the pipe. Eventually friction and gravity bring the
wastewater to rest at a low spot. Another valve cycle, at
any location upstream of the low spot, will cause this
wastewaterto continue its movement toward the vacuum
station.1
Vacuum systems are designed to operate on two phase
(air/liquid) flows with the air being admitted for a time
period twice that of the liquid. Open time of the AIRVAC
valve is adjustable; hence, various air to liquid ratios are
attainable.
Normally, the vacuum pumps are set to operate at 40-50
cm (16-20) in Hg of vacuum. The minimum vacuum of 40
cm (16 in) Hg results in a total available head loss of 5.5
m (18 ft); 1.5 m (5 ft) of this head loss is required to
operate the vacuum valve, leaving 4 m (13 ft) available
for wastewater transport (Rgure 3-8), The summation of
friction loss and static lift from any one point on the sewer
network to the vacuum station must not exceed 4 m (13
ft). This relationship is expressed by the following
equations:
where:
V
= Vacuum available in the collection system for
transport, ft
= Minimum vacuum available at the vacuum
station, typically 16 in Hg (which roughly equals
18 ft of water)
= Vacuum requiredtooperatethevacuumvalve,
5 in Hg (which roughly equals 5 ft of water)
With:
V, = Static Loss + Friction Loss
and the normal values substituted in the above equation,
the equation is simplified:
Static Loss + Friction Loss = 13 ft of H2O
Static losses are those incurred by using lifts, or vertical
profile changes. Profile changes are accomplished by
using two 45-degree fittings joined by a section of pipe.
For efficient use of the energy available, profile changes
should be as small as possible. Numerous lifts are
recommended over one large lift.2 Table 3-1 shows the
recommended lift height for various pipe sizes.
Static losses are calculated by subtracting the pipe
diameter from the lift height (Figure 3-9):
Static Loss = Lift Height - Pipe Diameter
Friction loss charts for SDR 21 PVC pipe and a 2:1 air/
liquid ratio have been developed by AIRVAC, and are
contained in their design manual.2 Friction losses are
only calculated for sewers that are laid between 0.2 and
2.0 percent fall. Friction losses in falls greater than 2.0
percent are ignored. The hydraulic calculations should
be performed including friction loss. A separate calculation
can then be made to see how much of the total loss is
static versus friction.
103
-------
Figure 3-8.
Vacuum lift capability.
VACUUM
PUMP
NORMAL OPERATING RANGE I
30" HG = 34'
20" HG = 23' H20
• 16"HG = 18'H20
18' TOTAL AVAILABLE LIFT
:£ REQ'D FOR VALVE OPERATION
13' AVAIL. FOR SEWAGE TRANSPORT
:£ TYPICAL FRICTION LOSS
10' AVAIL. FOR STATIC LIFT
Table 3-1. Recommended Lift Height
Pipe Diameter Lift Height
(in)
3
4
6
8
10
(«)
1.0
1.0
1.5
1.5
2.3
Figure 3-9.
Static loss determination.
ii
t
t
LI FT HEIGHT
STATIC LI FT
our-1 I
i I
STATIC LIFT = LIFT HEIGHT - PIPE DIAMETER
104
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3.4.2 Mains, Service Lines, and Building Sewers
3.4.2.1 Mains
a. Geometry and Sizing
The geometry of a vacuum sewer system is similarto that
of a water distribution system. Rather than looped,
however, it is normally in a dendriform pattern.
It is desirable to have the vacuum station located as
centrally as possible. This lends itself to a system with
mufti-branches. This is very important, as multiple main
branches to the vacuum station give added operating
flexibility. For example, with a system having 3 branches
serving 300 customers, the worst case scenario is that
100, or one-third, of the customers may be without
service while a problem is corrected. By contrast, the
worst case scenario assuming a similarly sized system
with one branch would have all 300 customers out of
service during the same period.
When laying out a vacuum system, the designer should
select pipe runs that:
* Minimize lift
• Minimize length
* Equalize flows on each main branch
The length of collection lines are governed by two factors.
These are static lift and friction losses. As previously
discussed, the su mmation of these two amountsgenerally
cannot exceed 4 m (13 ft). Due to restraints placed upon
each design by topography and wastewater flows, it is
impossible to give a definite maximum line length. One
operating system has a single main line branch exceeding
4,300 m (14,000 ft) in length.
Vacuum sewerdesign rules have been developed largely
as a result of studying operating systems. Important
design parameters are shown in Table 3-2 and 3-3.
Table 3-4 shows at what length the 0.2 percent slope will
govern versus the percentage of pipe diameter for the
slope between lifts.
AIRVAC hasdeveloped atable recommending maximum
design flows for each pipe size. Table 3-5 shows these
recommendations.1
Substituting the values of maximum flow (Q) from Table
3-5 into the previously cited simplified equation (Q=0.5N
+20) and solving for N, will give the maximum number of
homes served for each line size. Table 3-6 shows these
results.
Tabl* 3-2.
Main Line Design Parameters
Minimum distance between lifts, ft
Minimum distance of 0.2-peroant
slope prior to a series of lifts, ft
Minimum distance between top of
lift and any service lateral, ft
Minimum slop, percent
20
50
6
0,2
Table 3-3.
Guidelines for Determining Une Slopes*
Line Size
4" Mains
6" Mains
Use Largest of:
- 0.2%
- Ground slope
- 80% of pipe dia. (between lite only)
-0.2%
- Ground slope
- 40% of pipe dia. (between lite only)
* Assuming minimum cover at top of slope
Table 3-4.
Pipe Diameter
(in)
4
4
6 & larger
6 & larger
Governing Distances for Slopes Between Lifts
Distance Governing Factor
<135ft
»135ft
<100ft
>100ft
80% of pipe diameter *
O.2% slope
40% of pipe diameter
0.2% slope
Table 3-5. Maximum Flow for Various Pipe Sizes
Pipe Diameter
Maximum
Maximum Row
(in)
4
6
8
10
(gpm)
55
150
305
545
Table 3-6. Maximum Number of Homes Served for Various
Pipe Sizes
Pipe Diameter
(in)
4
6
8
10
Number of
Homes Served
70*
260
570
1,050
* The recommended maximum length of any 4-in run is 2,000 ft, which
may limit the amount of homes served to a value less than 70.
105
-------
The values in Table 3-6 should be used for planning
purposes or as a starting point for the detailed design. In
the latter case, estimated site-specific flow inputs along
with the friction tables should be used in the hydraulic
calculations. A correctly sized line will yield a relatively
small friction loss. If the next larger pipe size significantly
reduces friction loss, the line was originally undersized.
Experience has shown that there is little economy in
using 7,5-cm (3-in) pipe for mains. Forthis reason, 10 cm
(4 in) is the minimum recommended main size.
b. Routing
In most cases, vacuum sewer mains are located outside
of and adjacent to the edge of pavement and
approximately parallel to the road or street, which reduces
the expenses of pavement repair and traffic control. In
areas subject to unusual erosion, the preferred location
is often within the paved area. This location is also
favored by some municipalities as being an area where
subsequent excavation is less likely and more controlled,
and therefore a location more protected from damage.
One of the major cost components of a vacuum system
is the valve pit. With two or more homes sharing one
valve, overall system construction costs can be
significantly reduced, resulting in major cost advantage.
To do this, however, may require the main line to be
located in private property, typically in the back yard.
There are two disadvantages to this type of routing. First,
ft requirespermanent easements from oneof the property
owners, which may be difficult to obtain. Second,
experience has shown that multiple house hookups are
a source of neighborhood friction unless ft is located on
public property. The designer should carefully consider
the tradeoff of reduced costs to the social issues prior to
making the final routing decision.
An advantage to the use of vacuum sewers is that the
small diameter PVC pipe used is flexible and can be
easily routed around obstacles. Thefeature allows vacuum
sewers to follow a winding path as necessary. The pipe
should be bent in as long of a radius as possible, never
less than that recommended by the pipe manufacturer.
The equation from the Uni-Bell Handbook of PVC Pipe
forthe minimum radius of bending is presented in Chapter
2. This equation is:
where;
OD
FyOD a 200
Minimum radius of the bending circle
Outside diameter of the pipe
Vacuum sewers are normally buried with a cover of 90
cm (3 ft). In a few cases, where economy is paramount
and subsequent damage is unlikely, they are buried with
less cover. The depth of burial in colder climates is
usually dictated byfrost penetration depths. Inthe northern
United States, they are often placed at 1.2-1.5 m (4-5 ft).
Even though linefreezingisaconcem with most engineers,
it usually is not a problem with vacuum mains since
retention time in small-diameter lines is relatively short
and turbulence is inherent. Long periods of disuse could
still lead to freezing conditions however, unless the
system were adjusted to ensure relatively complete
evacuation of the mains (atechnique used for "winterizing"
some resort systems).
The separation of vacuum sewers from water supply
mains and laterals often requires the vacuum sewer be
buried deeper than would be required for other reasons.
Horizontal and vertical separation requirements are
dictated by State Agencies. These requirements vary
from state to state.
Profiles of the mains should always be shown on the
plans. Slopes, line sizes and lengths, culvert and utility
crossings, inverts, and surface replacements are typically
shown on the profiles.
Culvert and utility crossings often dictate numerous
variations in the depth of burial of vacuum sewer mains,
with many resulting sags and summits in the pipeline
profile. Unlike pressure mains, where air accumulates at
a summit requiring an air release valve, vacuum sewers
are not affected by high points in the profile. The sags,
however, may present a problem, as they typically will
add lift to the system. In addition, if not designed and
constructed properly, a sag may trap wastewater at low
flow periods blocking off the low part of the sewer.
To minimize damage to the vacuum sewer main caused
by subsequent excavation, route markers are sometimes
placed adjacent to the main, warning excavators of its
presence. Accurate as-constructed plans are helpful in
identifying the pipeline location, and a cable buried with
the main can be induced with a tone so the main can be
field located using common utility locating equipment.
A warning tape marked 'Vacuum sewer" is sometimes
placed shallowly in the pipeline trench to further notify
excavators. When the tape is placed lower in the trench,
e.g., adjacent to the pipe, H iscalled an identification tape.
The tape can be metalized so it can be detected with
utility locating devices. Most tapes cannot be induced
with a tone at a significant depth, so metalized tape
should be placed shallowly to be detected. It is also very
106
-------
or twisted during its placement or the detectable surface
area will be reduced.
c. Trench Section
Trenching may be accomplished by using a backhoe,
wheel trencher, or chain-type trencher. The choice of
equipment is usually dictated by the contractor based on
the material to be excavated, depth requirements,
topography and available working space.
Imported material termed "pipe zone backfill" is often
placed to surround the main if material excavated from
the trench is regarded as unsuitable for this purpose.
Pipe zone backfill is usually granular, such as pea gravel
or coarse sand. Fine sand or soil is generally not as
desirable as it bulks, rather than flows, into place under
the pipe haunching.
The remaining backfill material required is often specified
by the agency controlling the road or street, especially if
the mains are located within the pavement.
In some cases a lean cement-sand slurry is used for
backfill. This option is particularly attractive when a
trencher is used, the mains are located within the
pavement, and prompt restoration for traffic control is
important.
d. Pipe Materials
PVC thermoplastic pipe is normally used for vacuum
sewers in the Unitsd States. In some European
installations, HOPE and ABS have been successfully
used. In certain cases, ductile cast iron pipe (DCIP) has
also been used, assuming the Joints have been tested
and found suitable for vacuum service.
The most common PVC mains are iron pipe size (IPS),
200 psi working pressure rated, standard dimension ratio
21 {Class 200 SDR 21 PVC). Class 160, SDR 26, and
Schedule 40 PVC have also been used. From a pressure
standpoint, the lower class pipe is acceptable. However,
thinner wall pipe is more likely to be damaged during
installation. Further, there is little cost difference between
SDR 21 and SDR 26 when the excavation, backfill, and
surface restoration are considered. For these reasons,
Class 200, SDR 21 is recommended.
PVC pipe has a high (3.0 x 10* in/in/°F) coefficient of
thermal expansion; about 3/8-in length variation/100 ft of
pipe/10°F change in temperature. Considerable
temperature changes will be experienced during pipeline
installation, and some degree of temperature change will
occur during operation, with climate and shallow soil
temperature changes. To reduce expansion and
contraction induced stresses, flexible elastomeric joint
("rubber ring" joint) pipe is preferred. If solvent-welded
joint pipe is used, the pipe manufacturers
recommendations for installation regarding temperature
consideratfonsshouldbefollowed.The Uni-Bell Handbook
of PVC Pipe also providesguidance as to properpractices.
In the past, solvent-weld fittings were most often used
with Drain, Waste, and Vent (DVW) pipe. These fittings
are more commonly available than gasketed joint fittings
(one major line component, the wye required for each
connection to the main, is not made in gasketed PVC).
Expansion and contraction, while a concern if the entire
system were solvent-welded, are allowed for in the
gasketed main-line pipe joints.
Presently, there is a move toward eliminating all solvent-
welding. At least one of the major fitting manufacturers is
making gasketed wye fittings. Spigot adapters (an adapter
fitting that can be solvent welded in a controlled
environment into each of the three legs of the wye
resulting in three gasketed joints) can also be used.
3.4.2.2 Service Lines
Vacuum sewer service lines run from the vacuum main
to the valve pit. Typically these lines are near and parallel
to property lines.
It is good practice to boldly field mark the location of the
service line with properly identified lath a few days prior
to installation. This serves as a reminder to the property
owners about the intended location and may cause them
to recognize some reason why the location should be
changed. It also serves as an advance not ice to neighbors
if property lines are in doubt.
Most municipalities prefer locating the service line where
it will not be driven over. Others, however, prefer locating
the service line within the paved driveway. The reasoning
is that subsequent excavation and associated damageto
the service line is less likely within the paved section.
Service lines should be located distant from potable
water lines to reduce the possibility of cross contamination.
They should also be distant from other buried utilities, if
possible, due to the possibility of damage caused by
subsequent excavations for maintenance or repair of
those utilities.
All connections to the main, e.g..crossover connections,
aremade "bverthetop" (Figure 3-10). This isaccomplished
using a vertical wye and a long radius elbow. Due to the
restraints placed upon the depth of sewers by the
connecting sewers entering "over the top," engineers
should consider the ground cover required on these
107
-------
Figure 3-10. Top vl»w of crossover connection.
MAIN LINE •
90" ST. ELL TURNED 45°
.40WYEFTG
FLOW
p|T
Table 3-7.
Service Line Design Parameters
Minimum distance from lift to valve pit, ft
Minimum distance from lift to crossover, ft
Minimum slope between lifts
5
5
2 in or 0.2%
(whichever is larger)
108
-------
connections at the design stage. Table 3-7 gives design
parameters when lifts are required in the service line.
Service lines are typically 7.6 cm (3 in) in diameter. An
exception to this occurs when a buffer tank is used.
Buffer tanks are used for large flows which otherwise
result in frequent valve cycles. To maintain good vacuum
response at the buffer tank, a 15-cm (6-in) service line is
recommended.
As with the vacuum sewer mains, Class 200, SDR 21
PVC pipe typically is used for the service lines. Solvent-
welded DWV fittings are used, although rubber ring
fittings are becoming more common.
3.4.2.3 Building Sewers
The term building sewer refers to the gravity flow pipe
extending from the home to the valve pit setting. In many
cases, state or local authorities regulate installations of
building sewers. The Uniform Plumbing Code is often
referenced.
For residential service, the building sewer should be 10
cm (4 in) and slope continuously downward at a rate of
not less than 0.25 in/ft (2-percent grade). Desirably, the
valve pit setting should be located near the home so the
building sewer is short, with less need for maintenance
and less opportunity for infiltration and inflow (I/I). Line
size for commercial users will depend on the amount of
flow and local code requirements.
Bends should be avoided in building sewers, and a
cleanout used for each aggregate change in direction
exceeding 57°.
Infiltration via leaking building sewers has been common,
as has the connection of roof and yard drains. A quality
inspection during homeowner connection is advised to
determine if these situations exist. If so, steps should be
taken to require their elimination priorto final homeowner
connection.
To minimize the risk of damage to the fiberglass valve pit
during homeowner connection to the system, a stub-out
pipe of sufficient length, typically 1.8 m (6 ft), from the
valve pit is recommended. The orientation of the valve pit
as it relates to the house and the wye connection is
dependent on the number of connections to the pit
(Rgure3-11).
If the home piping network does not have a cleanout
within it, one should be placed outside and close to the
home. Some agencies prefer having a cleanout at the
dividing line where agency maintenance begins (i.e., the
end of the 1.8-m [6-ft] stub-out pipe).
3.4.3 Valve Pit Settings
3.4.3.1 General
a. Fiberglass Settings
The premanufactu red, fiberglass type of valve pit setting
is by far the most common. This type of setting is
composed of fourmainparts;thebottomchamber(sump),
the top chamber (valve pit), the plate that separates the
two chambers (pit bottom), and the lid (Figure 3-12).
Wastes from the home are transmitted to the sump via
the building sewer, the inlet entering the sump 45 cm (18
in) above the bottom. Up to four separate building sewers
can be connected to one sump, each at 90 degrees to
one another. A tapered shape helps facilitate thebackf illing
procedure.
The sump has a wall thickness of 5 mm (3/16 in) and is
designed for appropriate traffic loading with 60 cm (2 ft)
of cover.1 Elastomer connections are used for the entry
of the building sewer. Holes for the building sewers are
field cut at the position directed by the engineer.
The sumps employed thus far have two different heights:
76 and 135 cm (30 and 54 in). Both are 45 cm (18 in) in
diameter at the bottom and 90 cm (36 in) in diameter at
the top, with the smaller size having a capacity of 208 L
(55 gal) and the larger one 380 L (100 gal).
The valve pits house the vacuum valve and controller.
They are usually fabricated with filament-wound fiberglass
with a wall thickness of 5 mm (3/16 in) in order to be
suitable for appropriate traffic loading.1 The valve pit is
normally 90 cm (36 in) in diameter at the bottom and is
conically shaped to allow the fitting of a 60-cm (23.5-in)
diameter clear opening cast iron frame and cover at the
top. Depths are normally 1 m (42 in). One 7.5-cm (3-in)
diameter opening, with an elastomer seal, is pre-cut to
accept the 7.5-cm (3-in) vacuum service line.
The pit bottom is made of reinforced fiberglass that is 6-
mm (1/4-in) thick at the edges and 8-mm (5/16-in) thick
in the center. These bottoms have been molded by the
resin injection process.1 Valve pit bottoms are provided
with holes pre-cut for the 7,5-mm (3-in) suction line, 10-
cm (4-in) cleanout/sensor line and the sump securing
bolts. Sealing between the valve pit bottom and the sump
is done in the field using a silicone, butyl tape rubber
sealant or neoprene O-ring. The pit bottom should have
a lip which allows the valve pit to rest on top of it.
Cast iron covers and frames, designed for heavy traffic
loading, are typically used. The frame weight is generally
40 kg (90 Ib) and the lid weight about 45 kg (100 Ib). When
109
-------
FJgur» 3-11. Typical configurations for gravity connections.
4' GRAVITY
LINE
4" GRAVITY
LINE
VACUUM
SEWER MAIN-
S'1 VACUUM
SERV. LINE
7
VACUUM
VALVE
PIT
'
bJ
2-HOUSE CONNECTION
VACUUM
SEWER MAIN-
VACUUM VALVE PIT
4" GRAVITY
LINE
SINGLE HOUSE CONNECTION
110
-------
Figure 3-12. Typical fiberglass valve pit setting.
ANTI-FLOTATION
COLLAR IF REQUIRED
GRAVITY SEWERS
FROM 1-4 HOMES
• TRAFFIC OR
NON-TRAFFIC LID
\\ s FIBERGLASS VALVE PIT
WATERTIGHT BOND
PIT BOTTOM
FIBERGLASS SUMP
111
-------
a lighter lid is desired, such as in non-traffic situations, a
lightweight aluminum or cast iron lid may be used. These
types of lids do not have frames, but rather are fitted to
the valve pit through the use of two J-boHs. These lids
should clearly be marked "Non-Traffic."
Ashaltowerarrangement is possible, if sodesired (Rgure
3-13). This arrangement would be used in areas where
high ground water or poor soils exist and depth of the
building sewers is very shallow.
b. Concrete Settings
Certain situations call for the use of concrete valve pit
settings:
• When the deepest fiberglass setting is not sufficient to
accept the building sewer.
• When a large flow is anticipated requiring flow
attenuation.
• When an interface between two system types (e.g.,
pressure and vacuum) is needed.
The deepest possible fiberglass setting is 2.4 m (8 ft).
The building sewer depth at the valve pit setting is
therefore limited to 2 m (6.5 ft), since the building sewer
enters 45 cm (18 in) above the bottom. Should a deeper
setting be required, a concrete valve pit setting may be
used. The maximum recommended depth for a concrete
pit is 3m (10 ft).
These types of settings are typically constructed of 1.2-
m (4-ft) diameter manhole sections, with the bottom
section having a pre-poured 45-cm (18-in) diameter
sump. It is very important that all joints and connections
be watertight to eliminate ground-water infiltration. Equally
important is the need for a well-designed pipe support
system, since these tanks are open from top to bottom.
The support hardware should be of stainless steel and/
or plastic.
Earlier in this section hydraulics were discussed, with the
general conclusion that 5.5 m (18 ft) of system loss was
available; 4 m (13 ft) for the collection piping and 1.5 m
(5 ft) for the valve operation. The 1.5 m (5 ft) generally
corresponds to the amount of lift required to evacuate the
sump contents, assuming the deepest fiberglass valve
pit setting is used. Increasing this depth by use of a
concrete setting will result in a decreased amount of lift
available for the collection system. Depending on the
specific location of the deep setting required, the engineer
may opt to serve the home by other methods. Should a
deep concrete setting still be the choice, the engineer
should seek written agreement of the manufacturer
regarding any warrantees.
c. Buffer Tanks
For large flows that require attenuation, a buffer tank
should be used. Buffer tanks are typically used for
schools, apartments, nursing homes, and other large
users. They are designed with a small operating sump in
the lower portion, with additional emergency storage
available in the tank.
Like the deep concrete valve pit settings, the buffer tank
is typically constructed of 1.2-m (4-ft) diameter manhole
sections, with the bottom section having a prepoured 45-
cm (18-in) diameter sump (Figure 3-14). The same water
tightness and pipe support concerns apply to the buffer
tank.
When an interface between two system types is needed,
a dual buffer tank should be used. For example, a hybrid
system may be used, with vacuum sewers serving the
majority of the service area and pressure sewers serving
the low-lying fringes. At some point, a transition will be
needed between the pressure flow and vacuum flow.
A dual buffer tank is similar to a buffer tank, with the
exception that it is larger to accommodate two vacuum
valves (Rgure 3-15). These tanks typically utilize 1.5-m
(5-ft) diameter manhole sections. Dual buffer tanks may
also be used if the single buffer tank does not have the
capacity for the large flows. A single buffer tank has a 2-
L/s (30-gpm) capacity while a dual buffer tank has a 4-L/
s (60-gpm) capacity.
3.4.3.2 Appurtenances
Anti-flotation collars are sometimes used on the fiberglass
valve pit settings (See Figure 3-12). Buoyancy calculations
should be performed by the designer to see if they are
necessary. Past experience has shown that these collars
are usually not needed. Should they be used, care must
be taken during the valve pit installation as poor bedding
and backfill may lead to settlement problems. Settlement
of the concrete ring most likely will coincide with damage
to the building sewer and/or the pit itself.
3.4.4 Vacuum Valves
3.4.4.1 Valve Arrangements
All vacu u m valves except Vac-Q-Tec operate without the
use of electricity. The valve is vacuum operated on
opening and spring assisted on closing. System vacuum
ensures positive valve seating.
The AIRVAC valves have a 7.5-cm (3-in opening), are
made of schedule 80 ABS and have stainless steel
shafts, delrin bearings and elastomer seals.2 The valve is
equipped with a rolling diaphragm-type vacuum operator
and is capable of overcoming all sealing forces and of
112
-------
Figure 3-13. Shallow fiberglass valve pit selling. (Courtesy AIRVAC).
BREATHER
MASS CONCRETE
FOUNDATION BLOCKS (4)
FIBERGLASS VALVE PIT
CRUSHED ROCK
CD
3" SUCTION LINE
FIBERGLASS SUMP
113
-------
Figure 3-14. Plan and elevation views of typical concrete buffer tamk. (Courtesy AIRVAC)
GRAVITY INLET
3" VACUUM MAIN
4' CONCRETE MANHOLE
BREATHER
PIPE
'.r •'.'• .••*'.:•-'.*:-v-;'-r' "'••\-
3" VACUUM
MAIN
GRAVITY
INLET
114
-------
Figure 3-15. Typical concrete dual buffer tank, (Courtesy AIRVAC)
6" GRAVITY
INLET
BREATHER
PIPES
5' CONCRETE
MANHOLE
3" VACUUM MAIN
8' MAXIMUM
115
-------
opening using vacuum from the downstream side of the
valve, AH materials of the valve are chemically resistant
to normal domestic wastewater constituents and gases.
The controller/sensor is the key component of the valve.
The device relies on three forces for its operation:
pressure, vacuum, and atmosphere. As the wastewater
level rises in the sump, it compresses air in the sensor
tube. This pressure initiates the opening of the valve by
overcoming spring tension in the controller and activating
a three-way valve. Once opened, the three-way valve
allows the controller/sensor to take vacuum from the
downstream side of the valve and apply ft to the actuator
chamber to fully open the valve. The controller/sensor is
capable of maintaining the valve fully open for a fixed
period of time, which is adjustable over a range of 3-10
seconds. After the time period has elapsed, atmospheric
air Is admitted to the actuator chamber permitting spring
assisted closing of the valve. All materials of thecontroller/
sensor are fabricated from a plastic or elastomer that is
chemically resistant to normal domestic wastewater
constituents and gases.
Two type of vacuum valve are available: the Model D and
the Model S. The valves in both arrangements are
identical, but rely on a different piping/plumbing
arrangement for their source of atmospheric air needed
for proper controller operation.
a. Model D
The Model D arrangement gets atmospheric airthrough
a tube which is connected to an external breather pipe
(Rgure3-16).
The Model D arrangement is the most reliable since there
is little chance of water entering the controller. However,
some dislike this type of arrangement because of
aesthetics and/or fear of vandalism to the external
breather. Experience has shown that these are problems
are relatively minor.
b. Models •' . :,
The Model S is a "sump-vented" arrangement. One of the
three controller tubes is connected to the cleanout/
sensor piping. This piping extends into the lower sump,
which Is connected to the building sewer. The building
sewer is open to atmospheric air through the 10-cm (4-
Sn) auxiliary vent (Figure 3-17).
While eliminating some of the concerns associated with
the external breather, the Model S valve has some
potential problems.
First, it is necessary that the sump be air and watertight.
Should system vacuum at the valve be Jess than 13 cm
(5 in) Hg, the valve will not operate. Wastewater will
continue to fill the sump. If the sump is watertight, it will
become pressurized, with a "bubble" of airtrapped at the
top of the sump. When system vacuum is restored, the
bubble of air will be used by the controller in the valve
closing process. However, should water completely fill
the bottom sump in the same scenario, the valve will
open and stay open since it will lack the atmospheric air
needed for closure. This open valve will cause a loss of
system vacuum, which may affect other valves at different
locations in a similar fashion.
Second, the installation of the homeowner's building
sewer becomes more critical. A sag in the building sewer
alignment will trap water and not allow the free flow of
atmospheric air.
Some engineers have experimented with a blend of the
two concepts, by utilizing a breather that gets its air from
the top chamber. This has been successful in areas
where there is no chance for surface water to enter the
top chamber. The danger is that water may enter the top
sump filling it to a level above the breather. This water will
directly enter the controller and cause problems with
valve closure. In either the Model D or the Model S valve,
water in the top chamber would be of no concern, if the
controller itself was watertight. However, this is not
always the case if a problem has occurred. Since
preventing water from entering the top chamber has
proven to be a difficult task, venting from the top chamber
is discouraged.
Because the Model D arrangement is less susceptible to
problems than the Model S, it is the recommended type.
Since the two valve are physically identical, it is possible
to convert from a Model S to a Model D, and vice versa.
3.4.4.2 Appurtenances
a. External Breather
The external breather has been discussed in the previous
section. An early version of this included a 38-mm (1.5-
in) galvanized pipe extending 60-90 cm (2-3 ft) above
ground nearthevalve pit setting (Figure3-18). Abreather
dome is needed to prevent clogging from small insects.
This size of pipe was necessary for self support. Smaller
pipe has been used in concert with imbedded post
support as well. Both arrangements have been
successfully used in many operating vacuum systems,
although some perceived problems with aesthetics and
vandalism still exist, AIRVAC also offers an alternative
external breather arrangement. This consists of 32-mm
(1.25-in) polyurethane pipe that is anchored in the ground
by concrete (Figure 3-19). This material is very flexible,
making it virtually vandal proof. No matter what
,116
-------
Figure 3-16. Model 0 arrangement with external breather. (Courtesy AIRVAC)
BREATHER DOME
LOCATED ABOVE
FLOOD LEVEL
GRAVITY SEWERS FROM
1-4 HOMES
3" SUCTION LINE
FIBERGLASS VALVE PIT
TO VACUUM MAIN
CLEANOUT/
SENSOR PIPE
FIBERGLASS SUMP
117
-------
FIgur* 3*17. Modal S arrangement - sump vented. (Courtesy AIRVAC)
FIBERGLASS VALVE PIT
GRAVITY SEWERS
FROM 1-4 HOMES
0=3
TO VACUUM MAIN
3" SUCTION LINE
FIBERGLASS SUMP
118
-------
Figure 3-18.
Early system external breather dial, (Courtesy AIRVAC)
GALVANIZED
BREATHER DOME—--"
SOCKET X%" MPT
PVC ADAPTER
'/ft" MPT X Vi" SOCKET
PVC ADAPTER
Vfe" BREATHER T
ECCENTRIC GALVANIZED-
90* REDUCING ELBOW
. 3/i" SCHEDULE 80 PVC PIPE FOR BREATHERS
UP TO 35' LONG, THEN INCREASE TO
1" SCHEDULE 80 PVC PIPE FOR BALANCE
119
-------
Figure 3-19.
Early system external breather dial. (Courtesy AIRVAC)
BDP—GREEN PLASTIC -
BREATHER DOME
2' STANDARD LENGTH
1WI.D.X %" WALL
GREEN POLYURETHANE
TUBING
GRADE-
BREATHER
,—LINEGROMMET
1'/4" COMPRESSION X 1
FPTPVC ADAPTER
18"
1V4" X 1V4" MPT-
NYLON ELBOW
11/4" PIPE SIZE POLYETHYLENE
PIPE, SDR7, 200 PSI
120
-------
Figure 3-20.
Auxiliary vent detail. (Courtesy AIRVAC)
STRUCTURE OR POST
4"—180* BEND
W/SCREEN
FLEXIBLE
COUPLING-
FROM CUSTOMER
RESIDENCE
•A
• 4' RISER PIPE
. 4"—45 BEND
4" WYE
\
I
2
^
TO VACUUM
!i VALVE PIT
arrangement is used, two items require attention. First,
the entire breather piping system from the dome to the
connection at the controller must be watertight. Second,
the piping must slope toward the valve pit setting.
b. Auxiliary Vent
A 10-cm (4-in) PVC vent is required on the building
sewer. Its purpose is to provide a sufficient amount of air
to act as the driving force behind the liquid that is
evacuated from the lower sump (see Section 1.3 for a
discussion on system operation). With a Model S
arrangement, a secondary function is to provide the
necessary atmospheric airforpropercontrolleroperation.
The auxiliary vent is made of 10-cm (4-in) PVC pipe and
fittings (Figure 3-20). Most entities require it to be located
against a permanent structure, such as the house or a
wall. To prevent valve freezing in cold climates, the vent
should be a minimum of 6 m (20 ft) from the valve pit
setting, thereby allowing the warmth from the wastewater
time to warm the air.2
c. Cycle Counter
To mon'ttorthe number of valve cycles, a cycle counter is
available.2 This device is designed for mounting directly
on the vacuum valve or the valve pit wall (Figure 3-21).
The unit is enclosed in a watertight housing with a clear
nylon top.
With this device, it is possible to monitor the number of
cycles of a particular valve. Cycle counters typically are
utilized where a large water use is expected in order to
determine if the valve is reasonably capable of keeping
up with the flow.
Some entities use the cycle counter as a metering
device.5 Knowing the number of cycles and the volume
approximate per cycle, one can estimate the amount of
wastewater through the vacuum valve over a given
period.
Others use the device as method of determining illegal
stormconnectionstothe vacuum sewer. Theflowthrough
121
-------
Figure 3-21. AIRVAC cycle counter - two methods of connection.
ATTACH CYCLE COUNTER
TO PIT WALL WITH
SELF TAPPING SCREWS
ON VALVE PIT WALL
ATTACH CYCLE COUNTER
TO VALVE BONNET BOLTS
ON VALVE
122
-------
the valve can be estimated and compared to metered
water use. From this, it is possible to determine if
extraneous water is entering the vacuum sewer and
generally in what amounts.
It is not necessary to have a cycle counter for each valve
(unless they are being used as a metering device for
billing purposes), as they are small and can easily be
moved from location to location. It is recommended that
the spare parts list include several cycle counters.
3.4.5 Division Valves and Cleanouts
3.4.5.1 Division Valves
Division valves are used on vacuum sewer mains much
as they are on water mains. Plug valves and resilient-
seated gate valves have both been successfully used,
although care must be exercised in the selection process
to insure reliability. Typical locations for division valves
are at branch/main intersections, at both sides of a bridge
crossing, at both sides of areas of unstable soil, and at
periodic intervals on long routes. The intervals vary with
the judgment of the engineers, but is typically 450-600 m
(1,500-2,000 ft).
The valves should be capable of sustaining a vacuum of
60 cm (24 in) Hg. Contract specifications should call for
a certified test from an independent laboratory to verify
this.
AIRVAC recommends that the body, bonnet, closure
element, and trunions be fabricated of cast iron equal to
ASTM A126, Class B, with the closure element being
covered with a precision molded Buna-N facing to act as
the resilient seating surface; with a mating surface of 90
percent pure nickel polished to a 14 RMS finish.2
Valves 10 cm (4 in) and smaller may be directly actuated
while all 15-cm (6-in) and larger valves should be provided
with gear actuators.
The operating nuts should be of cast iron equal to ANSI
A126 Class B. The connecting pin or key should be
stainless steel. Aluminum nuts are not acceptable.
The valves should be installed in a valve box conforming
to local codes, with the operating nut extended to a
position where it is accessible with a standard valve
wrench.
Recent designs have included a gauge tap, located on
the downstream side of the division valve (Figure 3-22).
Its purpose is to allow vacuum monitoring by one person
in the field, rather than requiring two people (one to
operate the valve in the field and one to read the vacuum
gauge at the vacuum station).
3.4.5.2 Cleanouts
Cleanouts, called access points in vacuum sewer
terminology, have been used in the past. Their use is no
longer recommended in systems with high valve density
since access to the vacuum main can be gained at any
valve pit. However, some state codes still require
Cleanouts to be installed at specified intervals. In these
cases and in stretches where valves are non-existent,
access points should be constructed similar to Figure 3-
23.
. 3.4.6 Odors and Corrosion
There are few odor problems reported with vacuum
sewers. There are three contributing factors responsible
for this: 1) the system is sealed, 2) air is introduced in
great volumes at each flow input, and 3) detention times
are short.
The entire system, from the valve pit setting to the
vacuum station, is sealed. The valve pit sump containing
the wastewater should be tested for tightness both at the
factory and in the field after installation. The piping
system contains no air releases. The collection tank at
the vacuum station, into which all of the sewer mains
empty, is a vacuum-tight vessel.
There is a large amount of air introduced into the system
at each valve pit setting. The airto liquid ratio, by volume,
typically is 2:1 or higher. This amount of air aids in the
prevention of septic wastewater.
The typical valve cycle volume is about 38 L (10 gal). This
small volume results in frequent valve cycles. Once in the
main, the wastewater travels at velocities in excess of 4.5
m/s (15 fps). Also, the relative liquid to air volume in the
main is quite low. These factors result in a short detention
time, which also aids in the prevention of septic
wastewater.
The one exception to the above discussion on odors
occurs when concrete buffer tanks are used. Unlike the
fiberglass settings, there tanks are open from the sump
to the top of the pit. Operating personnel must be careful
of sewer gas buildup in these tanks when performing
maintenance, although the volume of wastewater present
in the tank usually is not large enough to produce
dangerous levels of hydrogen sulflde. Also, these types
of tanks typically are used to attenuate large flows,
allowing the wastewater more time to turn septic. This
does not present a problem in the mains, since the sealed
PVC piping is unaffected by septic wastewater.
123
-------
Rflur* 3-22.
Division valve with gauge tap detail. CAST IRON
VALVE BOX
& COVER
CIRCULAR CONCRET
COLLAR (TYPICAL)
DIVISION VALVE
IS "/I
vi i i y
J
h
8"X8"X8"
CONC. BLOCK 1
H
/£
^
•T—
,
j
5£l
1 I
E
3/6" BARB
ADAPTOR W/CAP
COMP FITTING
POLYETHELENE
TUBING
• 3/4« MPT
() FLOW*"
f
GAUGE TAP
Figure 3-23.
Terminal access point detail.
CIRCULAR CONC. COLLAR
(ROAD & BERM ONLY)
C.I. FRAME
& COVER
RISER
CAP & SPIGOT
(TO BE CLEAR
OF LID SWING)
18"DIA. X30"
PVC PIPE
45° BENDS
(2 REQ'D)
VACUUM
SEWER MAIN
FLEXIBLE
COUPLING
CONC. BLOCK (3 REQ'D)
GRANULAR FILL
124
-------
All of the system parts in contact with wastewater are
either PVC, ABS, FRP, rubber, or stainless steel, which
are corrosion resistant.2 As such, corrosion has not been
a problem in vacuum sewers.
The accumulation of grease is a cause for concern in
conventional lift stations as well as in vacuum stations.
Grease builds up on level controls and on the sides of the
vacuumcollection tank. Greasetraps are typically required
in applications such as restaurants to minimize these
problems.
Grease has not presented problems in vacuum sewer
mains. When the wastewater is evacuated from the
sump, the suction generally pulls floatable grease into
the vacuum mains. Since the wastewater moves through
the mains at high velocities, there is little opportunity for
grease in the sewer to build up in the system to any level
which could cause a blockage.
3.4.7 Vacuum Station Design
Nomenclature used in the station design is given below:
Term
max
Q!
CL
."<>
V.
TDH
H.
Definition
Station peak flow (gpm)
Station average flow (gpm)
Station minimum flow (gpm)
Discharge pump capacity (gpm)
Vacuum pump capacity (cfm)
Collection tank operating volume (gal)
Collection tank volume (gal)
Reservoir tank volume (gal)
System pump-down time (min)
Piping system volume (gal)
Total system volume (gal)
Total dynamic head (ft)
Static head (ft)
Friction head (ft)
Vacuum head (ft)
3.4.7.1 Vacuum Pumps
Vacuum pumps may be either the sliding-vane or the
liquid-ring type. In either case, the pumps should be air-
cooled and have a minimum (ultimate) vacuum of 74.4
cm (29.3 in) Hg at sea level. The pumps should be
capable of continuous operation. Duplicate pumps, each
capable of delivering 100 percent of the required airflow,
should be provided.
To size the vacuum pumps, the following empirical
equation has been successful:"
"A" varies empirically with mainline length as shown in
Table 3-8.1 The minimum recommended vacuum pump
size is 70 L/s (150 cfm).
Lubrication should be provided by an integral, fully
recirculating oil supply. The oil separation system should
also be integral. The entire pump, motor, and exhaust
should be factory assembled and tested with the unit
mounted on vibration isolators, and should not require
special mounting or foundation considerations.
3.4.7.2 Discharge Pumps
Duplicate pumps, each capable of delivering the design
capacity at the specified TDH should be used.
Each pump should be equipped with an enclosed, non-
clog type, two port, grey iron impeller, statically and
dynamically balanced, capable of passing a 7.5-cm (3-in)
sphere. The impeller should be keyed and fastened to a
stress proof steel shaft by a stainless steel lockscrew or
locknut. Pumps should have an inspection opening in the
discharge casing.
A certification from the pump manufacturer that the
pumps are suitable for use in a vacuum sewerage
installation is strongly recommended.
Equalizing lines are to be installed on each pump. Their
purpose is to remove air from the pump and equalize the
vacuum across the impeller.1 Clear PVC pipe is
recommended for use as small air leaks and blockages
will be clearly visible to the system operator. On small
discharge pumps (generally less than 100 gpm), the
equalizing lines should be fitted with motorized full port
valves which close when the pumps are in operation.
To size the discharge pumps, use the following equation:
(Typical peak factors are 3-4.)
The TDH is calculated using the following equation:
TDH is calculated using standard procedures for force
mains, However, head attributed to overcoming the
vacuum in the collection tank (Hv) must also be considered.
This value is usually 7 m (23 ft), which is roughly
equivalent to 50 cm (20 in) Hg (typical upper operating
value). Since Hv will vary depending on the tank vacuum
level (40-50 cm [16-20 in] Hg, with possible operation at
much lower and higher levels during problem periods), it
125
-------
Tabt* 3-8. "A" Factor for Use In Vacuum Pump Sizing
Longest Una Length A
(tt)
0-3.000
3,001-5,000
5,001-7,000
7,001-10.000
>1 2,000
5
6
7
8
11
is prudent to avoid a pump with a flat capacity/head
curve.
Where possible, horizontal wastewater pumps should be
used, as they have smaller suction losses compared to
vertical pumps. To reduce the risk of vortexing in the
collection tank, the pump suction line should be 5 cm (2
in) larger than the discharge line. Wastewater pump
shafts should be fitted with double mechanical shaft
seals with the seal chamber pressurized with light oil.
Net positive suction head (NPSH) calculations are
important in the discharge pump selection process. In
Nomenclature and typical values used in the NPSH
calculations are given in Table 3-9.
To calculate NPSH,, use the following equations:
Table 3-9. Discharge 'Pump NPSH Calculation
Nomenclature
Term Definition Typical Value
NPSH. Net positive suction head available, tt
h. Head available due to atmospheric
pressure, ft
Head available due to atmospheric
pressure at liquid level less
vacuum in collection tank, ft
Maximum collection tank vacuum, ft
ht Depth of wastewater above pump
centerline, ft
h^ Absolute vapor pressure of wastewater
at its pumping temperature, ft
h, Friction loss in suction pipes, ft
h^ Vacuum equalizing head provided by
1-in equalizing lines, ft
NPSH, Net positive suction head required by
the pump selected, ft
33,9 @ Sea level
33,2@500ft
32.8 @ 1,000 ft
29.4 @ 4,000 ft
18,1 @ 16-lnHg
22.6 @ 20-inHg
1.0 (min.)
0.8
2.0 (vert pump)
0.0 (nor. pump)
3.0 (min.)
the plans. The tanks should be sand-blasted and painted
as follows:
NPSH, must be greater than NPSHf. NPSH. and TDH
should be calculated for both the high and tow vacuum
operating levels and compared to the NPSH, at the
corresponding point on the head/capacity curve. Figure
3-24 is a diagram for calculation of NPSHa in a vacuum
system.
3.4.7.3 Vacuum Tanks
Two vacuum tanks are required by AIRVAC for each
custom station: the collection tank and the reservoirtank.
Other manufacturers may not require the reservoirtank.
Basic construction of the tanks is similar, differing only in
size, shape, and type and location of the openings. Both
steel and fiberglass tanks are acceptable. Steel tanks
should be of a welded construction and fabricated from
not less than 6-mm (1/4-in) thick steel plates. The tanks
should be designed for a working pressure of 50 cm (20
in) Hg vacuum and tested to 71 cm (28 in) Hg vacuum.
Each tank should be furnished with the required number
and size of openings, manways, and taps, as shown on
• Internally: One coat of epoxy primer and two coats of
coal tar epoxy.
• Externally: One coat of epoxy primer and one coat of
epoxy finish.
Each tank should be supplied complete with sight glass
and its associated valves. Fiberglass tanks may be
substituted using the same specifications. Fiberglass
tanks are to have 1,000-kPa (150-psi) rated flanges.
The operating volume of the collection tank is the
wastewater accumulation requiredto restart the discharge
pump. It usually is sized so that at minimum design flow
the pump will operate once every 15 minutes. This is
represented by the following equation:
where:
Qr
Q
•<*>
Qa/2
Qmiai = Qa x Peak Factor
126
-------
Figure 3-24. NPSHa calculation diagram with typical values. (Courtesy AIRVAC)
HEQ (31)
HAVT=HA-VMAX(10,8-)
HA (33.41)
VACUUM TANK
HIGH 20" HG (22.6')
VMAX (22,61
HVPA (0.78')
HF(2-)
NPSHA (12' WIN.)
1" EQUALIZING LINE
/— OPEN AT ALL TIMES
i
SEWAGE PUMP
127
-------
Table 3-10. Values of V, for a 15-Minute Cycle at Qmln for
Different Peaking Factors
Peaking Factor
Vo
3.0
3.5
4.0
2.08xQmtx
1.84xQmtt
1.64 xQmt,
Table 3-10 gives the value of Vo for a 15-minute cycle at
Of* for different peaking factors. The total volume of the
collection tank should be 3 times the operating volume (V,
» 3VJ, with a minimum recommended size of 1,500 L
(400 gal).
After sizing the operating volume, the designer should
check to ensure an excessive number of pump starts per
hour will not occur. This check should be performed for
a wastewater inflow equal to one-half the pump capacity.
When designing the collection tank, the wastewater
pump suction lines should be placed at the lowest point
on the tank and as faraway as possible from the main line
inlets. The main line inlet elbows inside the tank should
be turned at an angle away from the pump suction
openings.
The recommended size of the reservoir tank for most
applications is 1,500 L (400 gal). In special cases, it may
be larger.
After the vacuum pump, collection tank, and reservoir
tank are sized, system pump-down time for an operating
range of 40-50 cm (16-20 in) Hg should be checked.1 This
calculation will show the amount of time it will take the
selected vacuum pumps to evacuate (pump-down) the
collection piping from 40-50 cm (16-20 in) Hg. This
equation is shown below:
where:
I
t = 0.045 [(2/3)^ + (V^-V,,) + VJ
System pump-down time (min)
Volume of collection system piping (gal)
Volume of collection tank (gal)
Operating volume of collection tank (gal)
Volume of reservoir tank (gal)
Vacuum pump capacity (cfm)
In no case should T be greater than 3 minutes or less
than 1 minute. If greater than 3 minutes, the capacity of
the vacuum pumps should be increased and if less than
1 minute, thesizeof the reservoirtankshould be increased.
3.4.7.4 Standby Generator
The standby generator should be capable of providing
100 percent of standby power required for the station
operation. It typically is located inside the station, although
generators located outside the station in an enclosure
and portable generators are also acceptable.
3.4.7.5 Station Piping
This item includes piping, valves, fittings, pipe supports,
fixtures, drains, and other work involved in providing a
complete installation.
Station piping includes all piping within the station,
connecting piping to the vacuum reservoirtank, collection
tank, vacuum sewer lines, and force mains.
Wastewater, vacuum, and drain lines larger than 10 cm
(4 in) should be cast iron, ANSI B16.1,860 kPa (125 psi)
for exposed installations. For buried installations,
mechanical joint, ANSI A21.11, AWWA C111, cast iron
should be used. Fittings should be flanged and mechanical
joint as appropriate (ANSI AS1.10, AWWA C110). Red
rubber gaskets 3-mm (1/8-in) in thickness should be
used on all flanges. Vacuum lines as well as other lines
under 10 cm (4 in) should be Schedule 80 PVC. Building
sanitary drains are to be PVC DWV pipe and fittings.
The piping should be adequately supported to prevent
sagging and vibration. It also should be installed in a
manner to permit expansion, venting, and drainage.
For fiberglass tanks, all piping must be supported so that
no weight is supported by the tank flanges. Flange bolts
should only be tightened to the manufacturer's
recommendations. Provisions must be allowed for
inaccurate opening alignment.
All shut-off valves fitted within the collection station
should be identical to those used in the collection system
piping, with the exception that they be flanged.
Check valves fitted to the vacuum piping are to be of the
57-kg (125-lb) bolted bonnet, rubber flapper, horizontal
swing variety. Check valves are to be fitted with Buna-N
soft seats.
Check valves fitted to the wastewater discharge piping
are to be supplied with an external lever and weight to
ensure positive closing. They also should be fitted with
soft rubber seats.
On the upstream side of each side of each vacuum sewer
isolation valve, a vacuum gauge of not less than 11 cm
(4.5 in) in diameter shouId be installed. Gauges should be
positioned so that they are easily viewed when the
128
-------
isolation valves are operated. Diaphragm seals should
not be used with compound gauges.
3.4.7.6 Motor Control Center
The Motor Control Center (MCC) is to be manufactured,
assembled, wired, and tested by the factory in accordance
with the latest issue of NEMA Publication ISC2-322, for
Industrial Controls and Systems. The vertical section
and the individual units shall bear a UL label, where
applicable, as evidence of compliance with UL Standard
845.
Wiring inside the MCC is to be NEMA Class I!, Type B.
Where Type B wiring is indicated, the terminal blocks
should be located in each section of the MCC.
The enclosure should be NEMA Type 12-wrth-Gasketed
Doors. Vertical sections shall be constructed with steel
divider side sheet assemblies formed or otherwise
fabricated to eliminate open framework between adjacent
sections or full-length bohed-on side sheet assemblies at
ends of the MCC.
The MCC should be assembled in such a manner that it
is not necessary to have rear accessibility to remove any
internal devices or components. All future spaces and
wireways are to be covered by blank doors.
3.4.7.7 Level Controls
The discharge pumps and alarms are controlled by
seven probes inside the collection tank. These probes
are 6-mm (1 /4-in) stainless steel with a PVC coating. The
seven positions are:
1. Ground probe
2. Both discharge pumps stop
3. Lead discharge pump start
4. Lag discharge pump start
5. High level alarm
6. Reset for probe #7
7. High level cut-off: stops all discharge pumps (auto
position only) and vacuum pumps (auto and manual
positions)
Figure 3-25 presents approximate elevations of these
probes in the collection tank relative to the discharge
pumps and incoming vacuum mains.
An acceptable alternative to the seven probes is a single
capacitance-inductive type probe capable of monitoring
all seven set points. This type of probe requires a
transmitter/transducer to send a 4-20 mA signal to the
MCC.
3.4.7.8 Telephone Dialer
A voice communication-type automatictelephonedialing
alarm system should be mounted on a wall adjacent to
the MCC. The system should be self-contained and
capable of automatically monitoring uptofourindependent
alarm conditions.
The monitoring system shall, upon the opening of any
one alarm point, access the telephone lines, wart for the
dial tone, and begintodialthefirstoffourfield programmed
telephone numbers. The system will then deliver a voice
message indicating a two digit station number and the
fault status at that station. The message will be repeated
a preset amount of times with sufficient space between
messages to allow the called individual to acknowledge
receipt of the call. Acknowledgment of the message is
accomplished by pressing a touch tone star (*) key on the
telephone between messages. Following the
acknowledgment, the system will vocalize a sign-off and
hang up. The system then enters a 30-minute delay to
allow adequate time for follow-up measures to be taken.
If, during the delay, another fault occurs, the system will
begin recalling. Additionally, the system can be called at
any time from a standard telephone, whereupon it will
answerthe call and deliver a vocalized message indicating
the station number and fault status at the location.
If the delay elapses and faults still exist, the system will
begin dialing In 1-minute intervals attempting to deliver
the f au It message. If no acknowledgment is received, the
system will hang up, wait 60 seconds, and call the next
priority number. After dialing the last priority number, the
system will, if necessary, return tothe first priority number
and repeat the sequence indefinitely.
If the monitoring system is to be housed in the MCC,
provisions must be made to isolate the system from
interference.
The monitoring system should be provided with
continuously charged batteries for 24 hours standby
operation in the event of a power outage.
3.4.7.9 Vacuum Gauges
All vacuum gauges should be specified to have a stainless
steel bou rdon tube and socket to be provided with 13-mm
(1/2-in) bottom outlets. Polypropylene or stainless steel
ball valves should be used as gauge cocks.
Vacuum gauges should be provided at the following
locations1:
• On the side of the vacuum reservoir tank in a position
that is easily viewed from the entrance door.
129
-------
Figure 3-25. Typical elavaUona of tevel control probes. (Courtesy Al RVAC)
VACUUM COLLECTION TANK
INCOMING VACUUM
SEWER
ATLEVEL OF BOTTOM
OF INLET ELBOWS
ON VACUUM SEWERS
4" MIN. BELOW PROBE #7
4" MIN. ABOVE PROBE #4,
4" MIN. BELOW PROBE #6
4"-6" ABOVE PROBE #3
33" MIN. FROM
BOTTOM OF INLET
ELBOW TO TOP OF
DISCHARGE PUMP
VOLUTE
15"-24" ABOVE PROBE #2
TO GIVE RUN TIME OF PUMP
SEWAGE PUMP
GROUND 1"ABOVE
TANK BOTTOM
6" ABOVE DISCHARGE
PUMP VOLUTE
-------
» On the collection tank in a position that is easily viewed
from the stairway leading to the basement.
• On each incoming main line to the collection tank,
immediately upstream of the isolation valve on the line.
These gauges should be in a position above the
incoming main lines that is easily viewed from the
operating position of the isolation valves.
The connection from the incoming main lines to the
vacuum gauges should be made of PVC or CPVC pipe.
Copper pipe is not to be used for this purpose.
3.4.7.10 Vacuum Recorder
The MCC should contain a 7-day circular chart recorder
with a minimum chart diameter of 30 cm (12 in). The
recording range should be 0-75 cm (0-30 in) Hg vacuum,
with the 0 position at the center of the chart. The chart
recorder should have stainless steel bellows.
3.4.7.11 Sump Valve
The basement of the vacuum station should be provided
with a 38cm x 38cmx 30 cm (15 inx 15 in x 12 in) deep
sump to collect washdown water. This sump will be
emptied by a vacuum valve that is connected by piping to
the collection tank. A check valve and eccentric plug
valve should be fitted between the sump valve and the
collection tank.
3.4.7.12 Spare Parts Inventory
For optimum operating efficiency, it is necessary that a
sufficient inventory of spare parts be kept. Some of the
spare parts, such as fittings and pipe, can be purchased
through local builder's supply companies. However, there
are parts that are unique to vacuum systems that cannot
be purchased locally. Typically, these spare parts are
included as part of the construction contract. Table 3-11
is a recommended list of spare parts that should be
supplied to the owner during the construction phase.
In addition to spare parts, certain specialty maintenance
tools and equipment are needed and are listed in Table
3-12.
The vacuum station also requires spare parts. These
range from spare pump seals to fuses. Specialty Hems
that should be considered are given in Table 3-13.
Especially vital for the vacuum station are spare
microprocessor based electronic components. This type
of equipment is used for the level controls and the fault
monitoring systems and is very sensitive to power spikes.
These system components are vital to the station, as
they essentially operate and monitor the system. Losing
level controls to some type of failure will quickly cause
severe operational problems, such as loss of vacuum or
discharge pump malfunctions. Not having spare
equipment amplifies the problem since this would require
the system to be operated manually. This would require
an operator on a continual basis (until the spare part
arrived) to cycle both the vacuum pumps and the
wastewaterpumps manually. Loss of thefauK monitoring
system also results in significant O&M increases, as
witnessed by early system problems before such systems
were included in the vacuum station designs. Similarly,
response time to system malfunctions will be extended.
3.5 Construction Considerations
Construction of a vacuum sewer system is similar to
conventional systems. Utilizing small diameter pipes in
shallow trenches and having the ability to avoid
underground obstacles virtually at will makes this type of
const ruction attractiveto contractors. There are, however,
certain inherent construction problems associated with
vacuum sewers.
It is imperative that inspection be performed by those
with a thorough knowledge of vacuum sewer technology.
The design of the system and its hydraulic limits must be
understood by at least one member of the construction
team.
3.5.1 Pipeline Construction
The use of chain-type trenchers is sometimes specified
for service line installation where soil types allow, as they
cause less disruption to the property owner's yard than
does a backhoe. Rocky soils and some clayey soils that
will not self clean from the trencher teeth may be
impractical to excavate using a trencher.
Street crossings are often accomplished by the bore
method in which an auger is used and a steel casing is
pushed in the resulting opening under the street. The
casing acts as a sleeve forthe service line that is installed
inside. Other street crossings are "free bored" by the use
of a "hog". Open cutting of the street is done where boring
is impractical.
Vacuum service lines are buried a minimum of 75 cm (30
in), since the vacuum line that exits the valve pit does so
at a depth of 68 cm (27 in).
Many contractors use a backhoe for the service line
excavation, since this same equipment is also required
for the excavation of the valve pit, which typically is
located close to the main sewer. Many times this results
in over-excavation of the service line trench. Over-
excavation, coupled with the use of fittings which are
typically required between the valve pit and main, can
131
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T«b!o3-11.
Spare Parts List Per Every 50 Valves
2 each Vacuum valves
4 each Controllers/sensors
2 each Sensor/cleanout tubes
2 each Controller/sensor rebuilding kit
5 each Valve cycle counters
4 each Three-Inch "no hub" couplings
2 each Valve pits
2 each Valve pit bottom plates
1 each Standard collection sump
1 each Deep collection sump
2 each Valve pit covers
5ft Clear valve tubing: 3/8-in.
10ft Breather tubing: 5/8-in.
Table 3-12. Specialty Tools and Equipment for Collection
Systems
1 each Portable vacuum pump
2 each Portable vacuum chart recorders
100 each Vacuum charts
3 each Chart pens
2 each 0-20 in W.G. magnehelic gauges
2 each 0-50 In W.G. magnahelic gauges
1 each 12 VOLT DC submersible pump
15 ft Pump discharge hose
2 each No-hub torque wrenches
2 each Vacuum gauges
1 each Flexible mercury manometer
1 each Controller test box
1 each Pipe locator
T«bt» 3-13.
Specialty Equipment for Vacuum Station
1 each Inductance probe
1 each Probe transmitter
1 each Probe microprocessor card
2 each Vacuum switch
2 each Vacuum gauge
1 each Auto dialer microprocessor card
lead to future problems if proper bedding and backfill
materials are not used.
Since the native material and intended contractor
equipment are not always known, it is recommended that
the contract documents specify surrounding the service
line with imported pipe zone backfill.
3.5.1.1 Line Changes
Unforeseen underground obstacles are a reality in sewer
line construction. Water and gas lines, storm sewers, and
culverts at unanticipated locations all may present
difficulties during construction. Natural underground
conditions, such as rock, water, or sand also may present
more problems than anticipated.
With the "straight line, constant grade" nature of gravity
sewer construction, these obstacles usually result in field
changes. These field changes might include installing an
additional manhole and/or removing and relaying part of
the pipe at a different grade. The grade change could
affect the depth and grade of the entire gravity sewer
system. Another lift station may have to be installed.
Alterations at the treatment plant could be required.
Unfortunately, this scenario is all too common. The end
result is an increase in contract price through change
orders.
One key advantage of vacuum sewers is the flexibility
they allow for line changes during construction.
Unforeseen underground obstacles usually can be
avoided simply by going under, over, or around them.
There may be cases where line changes will be necessary,
due to hydraulic limitations. However, the likelihood of
this is greatly reduced when compared to conventional
gravity sewers.
One must be very careful not to make a change for the
sole sake of making construction easier. Every line
change should be carefully evaluated for its effect on the
performance of the overall system. Will the change
increase the amount of lift in the system (and ultimately
result in increased power costs)? Will the change result
in an undesirable hydraulic condition at a key location in
the system? How will the operation and maintenance be
affected? Will the change put the pipeline in a location
that the O&M equipment cannot reach? Will it result in
operational problems in the future? All of these concerns
must be weighed against the potential construction cost
savings prior to a change being authorized.
Line changes are made through the use of fittings. No 90-
degree bends should be used in vertical or horizontal line
changes.2 Concrete thrust blocking generally is not
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necessary, however, backfill compaction in this zone of
abrupt change in direction is vital.
3.5.1.2 Grade Control
Like line changes, grade changes should not be made
without a thorough evaluation of how that change will
affect overall system performance. The ability to make
grade changes to avoid obstacles is an advantage but
the abuse of this freedom can result in major problems.
This issue has been the cause of conflicts between the
contractor and the engineer in past projects. The
engineer's inspector instinctively desires to eliminate lifts
to improve the system hydraulics. This results in a
deeper installation. The contractor, on the other hand, is
constantly wanting to add lifts to result in a shallower
installation. As long as neither party loses sight of the
system's hydraulic limits and the effect on operational
costs, a conflict does not have to take place.
Vacuum sewers must be laid with a slope toward the
vacuum station. The only exception to this is where
vertical profile changes (lifts) are located. The pipe must
slope toward the vacuum station between lifts.
A minimum of 0.2-percent slope must be maintained at
all times.2 To ensure this, a lasertypically is required. The
use of automatic levels also is acceptable when handled
by an experienced instrument operator. In areas where
an obvious (>0.2 percent) downhill slope exists, the
pipeline may follow the contour of the ground. Grade
should routinely be checked by the engineer's inspector.
3.5.1.3 Service Connections
Most contractors have a crew installing main lines and a
second crew installing the services (valve pit settings). It
is common for the line crew to install a wye fitting on the
main to accept later piping from the pit crew. Typically,
the pit crew installs the pit and then connects the pit to the
main. Connecting these two fixed points, which have
different elevations, with rigid piping sometimes can be
difficult. Many times the result is the excessive use of
fittings. This situation can be avoided by proper planning
and coordination between crews.
The depth of each service setting typically is custom
designed. The relationship between the ground elevation
where the pit setting will be situated and the elevation of
the customer's basement/building sewer dictates the
depth of service required. In addition, the length of
connecting lateral required must be considered to allow
for sufficient slope of the building sewer. Prefabricated
valve pits with fixed dimensions can sometimes make pit
location critical. Moving the pit to a lower elevation, while
allowing additional fall for the building sewer, may result
in lift being necessary to connect to the main (elevation
of main is generally independent of elevation of the pit).
Moving the pit to a higher elevation may result in insufficient
fall available for the building sewer. Each valve pit
location should be evaluated for adequacy and verified
by the engineerto the contractor prior to shipment of the
valve pits.
Forservice lines, the construction is almost always within
private property. Forthis reason, easements are required
from property owners for the installation and future
maintenance of the system. Easement acquisition is the
responsibility of the Sewer Authority, either directly or
through contract with a right-of-way agent. In either case,
the hydraulic limits of the system must be understood
prior to making any changes that may be requested by
the property owner. For this reason, ft is recommended
that Authorities employ the services of the design engineer
for all or part of this task.
The user's responsibility begins at the vacuum pit stub-
out. The length of the stub-out typically is 1.8 m (6 ft).
Shorter stub-out lengths are undesirable. They would
require the homeownerto excavate cbsertothe fiberglass
valve pit to connect to ft, possibly causing damage or
disturbing the backfill/compaction around the pit in the
process.
Most operating entities require the homeownerto replace
or be responsible for the replacement of the building
sewer from the house foundation to the stub-out
connection, since vacuum sewers are not designed to
handle extraneous water. By accepting old, possibly
defective building sewers, the operating entity is taking a
riskon operation and maintenance problems, particularly
"waterlogging".
The building sewer is temporarily under vacuum during
the valve's open cycle. Forthis reason, the pipe material
must be able to withstand those forces without collapsing.
It is standard practice for 10-cm (4-in), Schedule 40, SDR
21, or SDR 26 PVC pipe to be used for this purpose; 680
kg (1,500 Ib) crush-type pipe is not acceptable, nor is
SDR 35 PVC.
The homeowner is usually responsible forthe installation
of the 10-cm (4-in) auxiliary vent also. This vent is
necessary for the proper operation of the valve. It should
be located no closer than 6 m (20 ft) from the valve pit.
It is desirable for this vent to be located against a
permanent structure, such as the house itself, a fence, or
a wall.
There have been attempts by some engineers in the past
to include this work as part of the construction project.
This was found to be unworkable, as it required the
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contractor to have complete knowledge of the
homeowner's plumbing system prior to the project bid.
Many times, the homeowner was unsure of the exact
location of his building sewer. Contractor liability was
increased since excavation near the foundation was
required. This led to high unit bid prices. Finally, installing
a vent on the building sewer beforethe system was ready
for operation often did nothing more than vent the septic
tank, which created odor problems.
All of the work required by the homeowner must be
inspected by the operating entity prior to final connection.
This ensures the proper and efficient operation of the
system. Compliance with the Sewer Use Ordinance is
the only remaining user responsibility.
The vacuum valve is not installed until the customer is
ready to connect to the valve pit setting. It is common for
the contractorto install the valve pit/sump, including all of
the necessary piping, during collection system
construction. The valve is supplied to the Authority for
their installation at a later date. In this manner, the
Authority can systematically install the valves as each
customer requests connection. Each valve is "timed-out"
with the controller time setting being recorded for future
reference. The time setting on the first few valves is
typically changed once or twice after startup as system
hydraulics continually change until all customers are
connected. All valves are then fine tuned to operate as a
complete system.
3.5.2 Equipment Substitutions
Equipment subm'rttatsshould beapproved by the engineer
prior to the contractor ordering the specific equipment.
Virtually all of the components in a vacuum system are
available from more than one manufacturer. Even though
much research and development currently is taking
place.thevacuumsewerindustry hasgrownand improved
largely by the trial and error method. For this reason, the
engineer should be extremely careful of substitutions of
specified equipment, especially in a piece-meal fashion.
This is not to say that new or alternative brands should
automatically be ignored; it simply means that care and
judgment should be exercised priorto any majordeviation.
Should a substitution be desired, the contractor should
submit the following information to the engineer to allow
for a complete evaluation of the situation:
• Identify the product by stating the manufacturer's
name and address, trade name of product, and the
catalog or model number.
» Include product data such as shop drawings, samples,
etc.
» Give itemized comparison of substitution with specified
equipment, listing variations.
• Givequality and performancecomparisonof substitution
with specified equipment
« Give cost data comparing substitution with specified
equipment.
* List availability of maintenance services as well as
replacement parts.
• Show the effect of substitution on the project schedule.
• Show the effect of substitution on other related
equipment.
The engineer may require a bond if there is no precedent
information on the proposed substitution.
3.5.3 Testing
3.5.3.1 Sewers
Prior to the installation/connection of any valve, the
complete system of sewers should be vacuum tested.
The engineer should be present during the entire testing
period. Any leak not discovered due to flawed testing
procedures, either intentional or u nintentional, will quickly
become evident once the system is operational, as the
operatorcan simply check the vacuumchartsdailyto see
if any leaks are present. Testing generally involves the
following:1
• At the end of each day's work, the mains and crossovers
laid that day should be plugged and subjected to a
vacuum of 60 cm (24 in) Hg, allowed to stabilize for 15
minutes and then not lose more than 1 percent vacuum
pressure/hr test period. During the daily testing, all
joints should be exposed. If any section of the sewer
fails the test, it should be reworked prior to laying new
sections of sewer.
« When the system of sewers and crossovers has been
completed, the complete network should be subjected
to 60 cm (24 in) Hg vacuum pressure, allowed to
stabilize for 15 minutes then not lose more than 1
percent vacuum pressure/hr for a 4-hr period.
During testing, temperature and/or climatic conditions
may vary. The following conditions may affectthe vacuum
readings in the pipe being tested:
» A drop in temperature may occur. The effect will be
cooling of the pipe and the air in tt thus causing
contraction of both. The contracting of the airwfthinthe
pipe will cause an increase in vacuum.
• A change in the barometric pressure may also occur.
Before rainfall, the barometric pressure may drop by
approximately 12 mm (0.5 in) Hg. A vacuum gauge
measures the difference in pressure between the
volume being tested and the atmospheric pressure.
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Therefore, any change in barometric pressure will
cause an equivalent change in pressure on the gauge
being used in the vacuum test.
Where climatic changes may occur during a vacuum test,
it is recommended that pipe temperature and atmospheric
pressure be recorded at the beginning and end of the
test, and the test results adjusted to correct for these
changes.
3.5,3.2 Vacuum Station
All vacuum piping and connected appurtenances in the
vacuum station should be tested for tightness. A final test
of the station piping should be done. In this test the
station piping is subjected to 60 cm (24 in) Hg. There
should be no loss of vacuum in the 4-hr test period.2
All of the controls should be tested at startup to see that
the system is functioning as designed. This includes the
vacuum pump controls, the wastewater pump controls,
and the telephone dialer.
3.5.4 Historical Construction Problems
The above sections provide insight into some of the
inherent problems associated with vacuum sewer
construction. Problems can be avoided by sound design,
proper inspection, and preconstruction planning/
coordination between the engineer and the contractor.
As part of the development of this manual, six operating
systems were visited in 1989. Based on these visits, four
problems were prevalent during construction. Two of
these were short-term and two resulted in later, long-
term problems.
3.5.4.1 Short Term
a. Solvent Welding in Cold Weather
Solvent welding PVC pipe in temperatures approaching
or below freezing led to vacuum leaks. Some contractors
made the mistake of keeping the glue warm, only to apply
ft to a pipe that was much colder. This led to leaking joints
and difficulty in passing the final leakage test.
This problem can be avoided by not solvent-welding
joints during temperatures of 4°C (40°F) and colder.
However, since this may not be practical for some jobs,
a better solution would be to minimize the number of
solvent-welded joints in the system.
Along these lines, there has been a significant move in
the vacuum industry toward theuse of gasketed pipe and
fittings. In all cases, installation of piping should be in
accordance with the proper specification of the Plastic
Pipe Institute, ASTM F-645-80 or ASTM D2774-72.
b. Pit Alignment
The prefabricated valve pit setting allows for gravity stub-
out connexions only at predetermined legations. Likewise
the location of the 7.5-cm (3-in) vacuum service line is
predetermined. Collectively, thetop and bottom chambers
may only be assembled in a few different arrangements.
Some contractors, concerned with facing the stub-out
direction toward the house, failed to consider the already
existing location of the wye connection to which the 7.5-
cm (3-in) service line would connect. Some corrected the
situation by re-installing the pit at a location that could be
accepted by both connection points. Others used fittings
to make the necessary adjustment.
Pre-installation planning, rather than last minute decision
making will eliminate the alignment problems.
3.5.4.2 Long Term
a. Valve Pit Settlement
Poor workmanship by the contractor led to valve pit
settlement. This resulted in alignment problems for the
owner at the time of the valve installation (vacuum line
entering the pit moved from its level position). Improper
valve alignment can lead to future valve malfunctions.
Valve pit settlement problems can be avoided by better
quality control on both the contractor's and inspector's
part during construction. Taking time to ensure proper
alignment and proper compaction around the pit will
greatly reduce the likelihood of this problem occurring.
b. Excessive Use of Fittings
Two fixed points, at varying inverts and locations, but
requiring rigid connection piping, resulted in the contractor
using an amount excessive use of fittings. These fittings
many times were located within the pit excavation. This
overexcavated zone is one where lack of compaction
could easily lead to future settlement, which can lead to
fitting failures.
The use of fittings in the service lines can be minimized
by proper planning and coordination between line and pit
crews. To minimize the difficulties, some contractors
install the valve pets first.6 The use of gasketed fittings,
which adds a certain degree of flexibility, will also alleviate
some of these problems.
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3.6 Operation and Maintenance
Considerations
3.6.1 Operation and Maintenance Manual
To operate a vacuum sewer system requires proper
training. O&M Manuals are a vital part of this training
process. Problems arose in some of the early vacuum
systems due to the lack of such aids. Manufacturers and
engineers are now recognizing this fact and are reacting
accordingly with improvedtechnical assistance and O&M
Manuals.
A well written O&M M anual shou Id containthe information
necessary to achieve the following goals:3
• To provide an accessible reference for the wastewater
collection system operators in developing standard
operating and maintenance procedures and schedules.
• To provide a readily available source of data, including
permits, design data, and equipment shop drawings
which are pertinent to the particular system.
• To provide the system operators assistance and
guidance in analyzing and predicting the system
efficiency.
• To provide the system operators assistance and
guidance in troubleshooting the system.
While an O&M Manual is a valuable tool, it should not be
viewed as the source of the ultimate solution to every
problem. The degree of efficiency of the system depends
on the initiative, ingenuity, and sense of responsibility of
the system operator. Also, the manual should be
constantly updated to reflect actual operational
experience, equipment data, problems, and implemented
solutions.
The O&M Manual should contain the following information
as a minimum.
3.6.1.1 Design Data
All design data should be given. Included would be
information relating to the system make-up, such as the
number of valves, line footage, and line sizes. Also
included would be component sizing information,
anticipated operating ranges, and other important design
considerations. As-built drawings showing all system
components should be included.
3.6.1.2 Equipment Manuals
Installation and maintenance manuals from the
manufacturers of the majorequipment should be included.
A list showing the manufacturer and supplier as well as
contact persons, addresses, and phone numbers should
be compiled.
3.6.1.3 Warranty Information
All warranties, including effective dates, should be listed.
3.6.1.4 Shop Drawings
A list of all approval drawings should be made which
identifies the manufacturer, model number, and ageneral
description of the equipment. A copy of each approval
drawing should be included with the O&M Manual.
3.6.1.5 Permits and Standards
All applicable permits, such as the National Pollutant
Discharge Elimination System (NPDES) permit, should
be included in the manual. Applicable water quality
standards should also be included.
3.6.1.6 Operation and Control Information
This section should include a description of the overall
system. The major components should be identified. The
following information should be given for each major
component:
Relationship to adjacent units
Operation
Controls
Problems and troubleshooting guides
Maintenance
Preventive maintenance schedule
Equipment data sheet
3.6.1.7 Personnel Information
A description of the manpower requirements, including
qualifications and responsibilities should be listed.
3.6.1.8 Records
A list of the type of records, as well as a list of reference
materials that are important, should be included.
3.6.1.9 Preventive Maintenance
All equipment should be listed and cross-referenced to
equipment catalogs. Maintenance schedules should be
established.
3.6.1.10 Emergency Operating and Response
Program
This section should include a description of actions and
responses to be followed during emergency situations.
Included should be a list of contact persons, including
addresses and phone numbers for those responsible for
various community services.
3.6.1.11 Safety Information
A safety plan should be developed which includes
practices, precautions, and reference materials.
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3.6.1.12 Utility Listing
A list of all utilities in the system area should be given,
including contact persons, addresses, and phone
numbers.
Much of the information contained in the O&M Manual
can easily be recorded on a database-type computer
program. Computersoftwareisbecomingmoreaffordable
to small communities and can provide powerful aids to
the O&M staff.
3.6.2 Staffing Requirements
3.6.2.1 General Information
Information gathered from operating systems suggests
that the effort to operate and maintain modern vacuum
systems has been overstated. Vacuum systems are
mechanized and will, therefore, normally require more
O&M than a conventional gravity sewer system. This
must be allowed for in system planning.
3.6.2.2 Operator Responsibilities
The system operator is responsible for the following
activities:
* Analyzes and evaluates operation and maintenance
functions and initiates new procedures to insure
continued system efficiency
* Gathers and reviews all data and records same forthe
preparation of reports and purchase requests
• Recommends all major equipment purchases and
system improvements
* Maintains effective communication with other
employees, municipal and government officials, and
the general public
* Conducts daily operation and maintenance of the
system
• Inspects the system daily to determine the efficiency of
operation, cleanliness, and maintenance needs
• Prepares work schedules
« Prepares operational reports and maintenance reports
• Determines remedial action necessary during
emergencies
3.6.3 Operator Training
It is desirable forthe operating entity to hire the system
operator while the system is under construction. This
allows the operator to become familiar with the system,
including the locations of ail lines, valve pits, division
valves, and otherkey components. Toaddfurthertraining,
manufacturers may offer lengthy (e.g., 2 weeks) training
program at their facility.
For example, AIRVAC provides a small-scale vacuum
system at their manufacturing center. This setup includes
clear PVC pipe with various lift arrangements where
trainees can watch the flow inside a clear pipe during a
wide variety of vacuum conditions. Faults are simulated
so that the trainee can gain troubleshooting experience.
The operator is taught valve operation and its overhaul.
Finally, vacuum station maintenance is taught.
The best training isgained by actual operating experience.
Many times, however, the knowledge is gained at the
expense of costly mistakes. This is especially true at
startup time. During this time, the engineer, who provided
day-to-day inspection services during construction, is
gradually spending less time onthe system. The operator
is busy setting vacuum valves and inspecting customer
hookups. Complicating the situation is the fact that the
operating characteristics of the system continually change
until all of the customers are connected and all of the
valves are fine-tuned. However, with the operator(s)
being preoccupied with other tasks, this fine tuning
sometimes is not done; problems develop; and system
credibility can be lost.
This 'training gap" is present at the startup of virtually
every vacuum system. This is an area of the technology
that needs improvement. One solution is forthe engineer
to budget a 3-6 month, on-site training service to aid the
system operator in the fine tuning and troubleshooting of
the early problems. The operator will benefit from the
engineer's systematic approach to problem solving. This
most likely will instill a certain degree of confidence in the
operators) concerning the system. Operator attitude is
vital to the efficient operation of a vacuum system.
3.6.4 As-built Drawings and Mapping
It is common in the industry for changes to be made
during construction. The changes should be reflected on
the as-built drawings. Asthe name implies, these drawings
depict exactly how the system was bu ilt. This is a vital too!
to the operating entity for maintenance, troubleshooting,
and future improvements or extensions to the system.
An index map showing the entire system should be
included in the as-built drawings. Shown on this map will
be all key components, line sizes, line identifications,
valve pit numbering and locations, and division valve
locations. Detailed plan sheets of each line of the collection
system should be included, with dimensions necessary
to allow the operatorto locate the line as well as all related
appurtenances.
Unique to a vacuum system is the need for an as-built
hydraulic map. This is similar to an index map but also
includes special hydraulic information:
• The locations of every lift.
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• The amount of vacuum toss at key locations, such as
the end of a line orthe intersection of a main and branch
line.
• Number of main branches, number of valves in each
branch, and total footage (or volume) of pipe in each
branch.
This simple but vital information allows the operator to
make intelligent decisions when fine tuning or
troubleshooting the system.
Another tool that is helpful to the operator is an as-built
drawing of each valve pit setting. These drawings will
show the location of the setting relative to some permanent
markers (house, power pole, etc.), the orientation of the
gravity stub-outs, the depth of the stub-outs, and any
other pertinent site-specific information. These records
are used by the operator as new customers connect to
the system.
The vacuum station drawings should be altered to reflect
changes made during construction. Especially important
in these drawings are dimensions, since any future
modification will depend on available space.
3.5.5 Maintenance
3.6.5.1 Normal Maintenance
A properly designed vacuum station will be equipped with
a fault monitoring system, such as a telephone dialer.
This system monitors the operation of both the vacuum
station and the collection system, and automatically
notifies the operator of tow vacuum, high levels of
wastewater in the collection tank, and power outages.
Normal operation includes visiting each vacuum station
daily. Some daily maintenance procedures include the
recording of pump running hours, and oil and block
temperature checks. Once an operator is familiar with the
operating characteristics of the system, a simple visual
check of the gauges and the charts in the station will
provide an adequate alert of any problems. This visual
check along with recording operating data generally
takes about 30 minutes. Weekly procedures include
checking battery terminals, battery conditions, and
operational testing of the standby generator. Monthly
procedures include cleaning of the collection tank sight
glass, a check of the mechanical seal pressurizers of the
wastewater discharge pumps, and a test of the telephone
alarm system.
On a normal day, the operator will not be required to visit
the collection system. Normal station gauge and chart
readings are an indication that the collection system is
fine. Depending on a system's history of breakdown
Table 3-14.
Normal Operating Tasks and Frequencies
Pally
Visually Check Gauges/Charts
Record Pump Run-Times
Check Oil and Block Temperatures
Weekly
Check Battery Conditions
Exercice Generator
Change Charts
Monthly
Clean Sight Glass
Check Mechanical Seal Pressurizers
Test Alarm System
maintenance, some periodic inspection may be required.
This would include the inspection and manual operation
of each valve at some regular interval. The breather lines
should be inspected forthe accumulation of moisture. An
experienced operator will quickly learn the sounds of a
properly functioning valve.
Table 3-14 summarizes the daily, weekly and monthly
tasks.
3.6.5.2 Preventive Maintenance
Wastewater collection systems operate and must be
maintained 365 days a year. Variations in operation and
maintenance workloads occur, making it imperative that
preventive maintenance be planned and scheduled.3
This will ensure that there is no idle time during non-peak
workload periods. Inspection and maintenance planning
and scheduling involves time, personnel, equipment,
costs, work orders, and priorities.
A preventive maintenance schedule for all major
equipment should bedeveloped. To initiate the preventive
maintenance tasks, a work order system must be
established. This system identifies the required work,
priority of task, and any special information, such as the
tools or parts required for the Job. These work orders
provide a record of work completed.
Scheduled maintenance on the collection piping should
be minimal. Areas where difficult or unusual conditions
were encountered during construction should be visited
periodically. Other areas to be visited include steep
slopes and potential slippage areas.
At least twice a year, the division valves should be
checked. This is done by moving the valve through the
entire opening and closing cycle at least once. This
procedure will keep valves in operating condition. In
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Table 3-15,
Preventive Maintenance Tasks and Frequencies
Yearly
Exercise Division Valves
Inspect Vacuum Valves
Check Controller Timing
Check Plug and Check Valves at Station
Inspect Vacuum and Station Pumps For Wear
5-6 Years
Remove Valve and Replace Worn Parts
Rebuild Controller
addition, it will familiarize the operating personnel with
the location of all valves.
All vacuum valves should be inspected once a year.2
They should be manually cycled to see that they are
operating property. The controller timing cycle should be
recorded and compared to the original setting. If
necessary, the timing should be reset. This entire
procedure can be done by one person, requiring about
10-15 minutes/valve.
About every 10 years, each vacuum valve should be
removed, a spare put in its place, and the old valve
returned to the workshop.3 The valve should be taken
apart and inspected for wear. If needed, the seat should
be replaced. When the valve is reassembled, a new shaft
seal and bearing should be fitted. The seals and
diaphragms of the controller/sensor should be replaced
every 5 years. These procedure can be done by one
person, requiring 45 minutes for the valve and 1 hour for
the controller.
Preventive maintenance for the major equipment at the
vacuum station should be done in accordance with
manufacturers' recommendations. Yearly maintenance
might include removal from service and comprehensive
inspection of check valves, plug valves, vacuum pumps,
wastewater pumps, generator, and the telephone dialer.
Table 3-15 summarizes the preventive maintenance
tasks and their frequencies.
3.6.5.3 Emergency Maintenance
Although very little effort is required on a day-to-day
basis, there will be times that emergency maintenance is
necessary. This effort usually requires more than one
person, particularly when it involves searching for a
malfunctioning valve. Many times problems develop
after normal working hours, requiring personnel to be
called out on an overtime basis. Emergency orbreakdown
maintenance can occur in the piping system, at the
vacuum station, or at the vacuum valve.
Assuming proper design and construction, there is very
little that can go wrong in the piping system. Occasionally,
a line break will occur, due to excavation forother utilities
or landslides, causing a loss of system vacuum. Using
the division valves, the operator can easily isolate the
defective section.
Malfunctions at the vacuum station are generally caused
by pump, motor, or electrical control breakdowns.
Redundancy of most components allows forthe continued
operation of the system when this occurs.
Most emergency maintenance is related to malfunctioning
vacuum valves caused by either low system vacuum or
extraneous water. Failure of the valve is possible in either
the closed or open position. A valve failing in the closed
position will give the same symptoms as a blocked
gravity line, that is, thecustomer will experience problems
with toilet flushing or backup of wastewater on the
property. A phone call from the affected party makes
identification of this problem easily rectif iable. This rarely
happens since virtually all valve failures occur in the open
position. When this happens, a loss of system vacuum
occurs as the system is temporarily open to atmosphere.
The fault monitoring system will recognize this low vacuum
condition and alert the operator of the problem. Acommon
causeof failure in this position istheentrance of extraneous
water into the controller.
A procedure for locating the source of a vacuum failure
has been developed by AIRVAC as follows:4
• When a low vacuum condition occurs in the system,
isolate each incoming line to the collection tank to
identify the problem line.
• Close off the problem line. Open the remaining lines to
clear the wastewater from them,
* Allow vacuum in the operational lines to reach the
maximum vacuum level possible; then close these
lines off.
• Open the line with the problem.
• Starting at the collection tank, go to the first division
valve on the problem line. Connect a vacuum gauge to
a nearby vacuum valve (orto a gauge tap, if one exists)
downstream of the division valve. Close the division
valve and observe if vacuum builds up. If it does not,
the problem is between the vacuum station and the
division valve. If vacuum rises, repeat the process on
the next division valve. Before reopening each division
valve, allow vacuum to build up in the nonproblem
sections of the sewer to clear those sections'
wastewater.
139
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• After isolating the problem section, check each valve
pit to locate the malfunction. Often this can be
accomplished by driving to each pit and listening for the
sound of rushing air in the auxiliary vent.
• After locating the malfunctioning valve, follow the
manufacturer's valve troubleshooting procedures.
• If no valves are malfunctioning, check for underground
construction that could have caused a break in the
transport piping. Also, walk the route of the problem
sewer and look for evidence of a break, such as a
sunken area.
The above procedure is a systematic approach to locating
the source of vacuum loss. The time for the first 4 steps
is generally about 2-3 minutes, while the entire procedu re
generally can be completed within30minutes.4Sometimes
a shortcut can be taken. In the recent study of operating
systems, H was found that many times the same valve(s)
fail. This is usually due to some particular hydraulic
condition at that specific locale. In these situations, the
operators check these valves before any other isolation
is done. In another situation, a skilled operator can
usually tell how far from the vacuum station the problem
is by analyzing the vacuum charts as the degree of
vacuum loss is inversely proportional to distance from
the station. This allows for the simplification of the
isolating procedure.
Valve failures, if not located and corrected within a couple
of hours, may cause failures in other parts of the system.
A valve that is hung open or that continuously cycles will
cause system vacuum to drop. If the vacuum pumps
cannot keep up with this vacuum loss, the result is
insufficient vacuum to open other valves. This may lead
to backups. When vacuum is finally restored, a large
amount of wastewater, in relation to the amount of air, will
be introduced into the system possibly resulting in
"waterlogging." When this occurs, the system must be
manually operated, allowing the vacuum pumps to run
longer than usual. The manual repetitive cycling of the
vacuum and wastewater pumps in effect increases the
capacity of the vacuum station. This repetitive cycling is
continued until the system "catches up." At that point, the
system Is returned to its automatic mode.
There have been attempts made to determine the time
necessary to locate a failed valve.4 At the Plainville,
Indiana system, a valve was caused to fail at a location
unknown to the maintenance personnel. This valve was
located and the problem corrected in 21 minutes. System
operators report that the typical valve failure is located
within 30-45 minutes. Most also cite driving time from
their housetothe system as being thecrfticalfactor in this
response time. A key component in continual operation
is an effective alarm system, along with available
maintenance personnel.
3.6.6 Record Keeping
Good records are important for the efficient, orderly
operation of the system. Pertinent and complete records
provide a necessary aid to control procedures asthey are
used as a basis of the system operation. The very first
step of any troubleshooting procedure is an analysis of
the records. This is especially true of the collection
system. A wealth of information is contained in the basic
records kept on a daily basis. The following types of
records should be kept:
• Normal maintenance records
• Preventive maintenance records
• Emergency maintenance records
• Operating costs records
These records should be carefully preserved and filed
where they are readily available to operating personnel.
All records should be neat and accurate and made at the
timethedataareobtained.lt is good practice to summarize
this data in a brief monthly report and a more complete
annual report. These reports should be submitted to the
local authorities such as Town Council and the Mayor.
Thiskeepsthoseofffcialsappraisedofprogress.problems
and long term needs of the facility.
3.6.6.1 Normal Maintenance Records
The following information should be recorded on a daily
basis:
• Date
• Personnel on duty
• Weather conditions
• Routine duties performed
« Operating range of vacuum pumps
* Run-times of vacuum pumps
• Run-times of wastewater discharge pumps
» Run-time of standby generator
* Flow data ,
• Complaints received and the remedies
• Facilities visitors
• Accidents or injuries • , ,
• Unusual conditions ;
• Alterations to the system
; - * ' -„;..• t ,,
3.6.6.2 Preventive Maintenance Records
Adequate records provide informationthattellsoperational
personnel when service was last performed on each
system component and indicates approaching service or
preventive maintenance requirements. Efficient
scheduling of these maintenance tasks can then be
;• 140
-------
made which avoid interference with other important
aspects of system operation.
Results of periodic inspections should be kept. This
would include a list of all potential problems, the likely
cause of these problems, the repairs necessary to solve
the problem, and recommendations for future
improvements to minimize recurrence.
3.6.6.3 Emergency Maintenance Records
Records should be kept concerning all emergency
maintenance. This includes the following:
« Date and time of occurrence
• Person(s) responding to problem
• Description of problem
• Remedy of problem
• Parts and equipment used
* Total time to correct problem
• Recommendations for future improvements
3.6.6.4 Operating Cost Records
To insure budget adequacy, it is very important to keep
accurate information concerning the costs of all operation
and maintenance hems. Costs include:
Wages and fringe benefits
Power and fuel consumption
Utility charges
Equipment purchases
Repair and replacement expenses
Chemicals
Miscellaneous costs
3.7 Evaluation of Operating Systems
3.7.1 Early Vacuum Systems
Early vacuum systems wereoften plagued with consistent
operational problems. These systems were installed
without sufficient field experience with system
components. In addition, operation and maintenance
guidelines were not yet available. As a result, several
operational problems were encountered.
Early vacuum systems experienced problems because
of the lack of knowledge of two-phase transport. Small
vacuum mains, improperly planned vacuum main profiles,
too large liquid slug volumes, and insufficient air all
resulted in transport problems.5
Vae-Q-Tee systems located sensitive electronic valve
control equipment in 208-L (55-gal) drums near the
wastewater holding tanks. Corrosion of these drums and
the control boxes caused numerous problems and the
complexity necessitated skilled techniciansto successfully
operate the system.4
Early Coft and AIRVAC systems lacked components that
are now generally accepted as minimum design
standards.2 The lack of standby power indirectly caused
some vacuum valve boots to rupture. During power
outages, liquid built up behind these valves. When power
was restored and the valves cycled, the large volume of
water could not be completely drained before the timed
valve operation sequence closed the boot on the fast-
moving water instead of on air. This resulted in unusually
high forces which ruptured the valve boots. These
problems were alleviated to a large degree when standby
power was added to the system.
Al RVAC's early valve pits were made of a tar-impregnated
paper which deformed when placed in unstable soil or in
areas subjected to vehicle traffic. This deformation
eventually led to damaged valves. Additional problems
resulted from the use of valve pits without bottoms in
areas of high ground water.4 In this case, water entered
the sensor-controller causing the valves to continually
cycle and eventually deplete system vacuum. Corrective
measures included replacement of the early valve pits
with fiberglass pits capable of withstanding traffic loadings.
Breather tube extensions aboveground and controller
modifications have minimized these past problems.
A better understanding of vacuum sewer hydraulics,
improved system components, and establishedoperation
and maintenance guidelines have led to significant
operational improvements.
3.7.2 Recent Vacuum Systems
Although operational reliability has improved with each
successive generation of systems, some problems still
exist. Six operating systems were visited in 1989 so that
meaningful operation and maintenance data could be
generated. An attempt was made to visit systems that
would give a good cross section of the technology.
Topography, geographical location, size, and varying
design concepts were considered in the selection process.
One early system was visited to see if improvements
through the years have resulted in increased reliability.
Table 3-16 presents information on the operating systems
visited.
3.7.2.1 General Information
Table 3-17 presents general information on each of the
systems visited.
141
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Table 3-16.
Operating Systems Visited In 1989
Project Nome
Ocean Pines
Westmoreland
Ohio Co-Phase 1
Lake Chsutnuqua
Control Boaz PSD
Whita House
Date
Location Operational
Berlin, MD
Westmoreland, TN
Wheeling, WV
Celeron, NY
Parkersburg, WV
White House, TN
1970
1979
1984
1986
1988
1988
System
Type
VAC-Q-TEC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
AIRVAC
3.7.2.2 Design/Construction Data
Table 3-18 shows design and construction data as they
relate to the collection systems of each of the systems
visited.
Table 3-19 shows design and construction data as they
relate to the vacuum stations of each of the systems
visited.
3.7.2.3 Operation and Maintenance Data
Significant improvements have been made in the
components in the last 10 years, particularly with the
valve controller unit. This has been attributed to a
combination of research and testing and an increased
quality control effort. Continuingtoeducatethedesigners,
builders, and operators of propertechniques will result in
a further reduction of the problems. Evidence of this is
starting to surface in some of the more recently
constructed systems.
Operation and maintenance data were gathered on each
of the systems visited. These data are presented in
Tables 3-20 through 3-23.
3.7.3 Trend
Tnefailure rateof some of theearly vacuum systems has
been documented: in general, the mean time between
service calls (MTBSC) of some of these systems ranged
from less than 1 to more than 8 year; all but one of these
systems had a MTBSC of less than 4 years.4 By contrast,
all of the systems studied in 1989, with the exception of
the early system, had a MTBSC of greater than 6 years,
with the average being almost 10 years. This indicates
that compon ent improvements and design advancements
have had a positive impact on the reliability of vacuum
systems.
3.7.4 Historical Operating Problems
Each of the systems visited experienced some type of
problem which predominated as a demand on O&M staff
time. However, most were short lived. Table 3-24
describes the types of problems found.
A sophisticated statistical analysis was not performed to
develop the above percentages. The percentages were
determined by a combination of frequency of occurrence
and demand on staff time as reported by the system
operators that were interviewed. They are presented
only for the purposes of putting the different problems
into perspective.
Assuming the percentages roughly approximate reality,
80 percent of the problems to date can be categorized as
avoidable. This does not mean that they were not or are
not problems. Withgood design, construction, inspection,
training, and quality control, however, these problems
can be avoided in future systems.
3.7.4.1 Component Defects
a. Broken Spring-Valve Failure in Open Position
Shortly after startup of one system, an unusual amount
of valve failures were occurring. It was determined that
these failures were caused by a broken spring in the
valve controller. An investigation revealed that the springs
were made of defective material. The springs were
systematically replaced with new ones, an effort that two
people accomplished on a part-time basis over the span
of a month.
This effort did not require the removal of the valve itself.
Using spare parts, the District put new springs in the
controller component of the valve at their workshop. After
a sufficient amount of controllers were rebuilt, they were
taken to the field and replaced the controllers with the
defective springs, a procedure that took 10 minutes per
valve. These defective controllers were then brought
back to the shop where the procedure was repeated u ntil
all controllers were rebuilt with the proper spring. This
problem has not since returned.
b. Unreliable Valve Controllers
The original valves in one system had a controller that
was different from the ones used presently. They were
found to be very unreliable. The result was more valve
failures, and, hence, more O&M expense than originally
anticipated. Inthe 1970s, Al RVAC designed and patented
their own controller. After the successful testing of these
controllers at their Indiana facility, they began mass
production.
Every valve in the system has since been retrofitted with
the AIRVAC controller. The failure rate has been greatly
reduced since the changeover.
142
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Tab!* 3-17.
General Information on Operating Systems
Ocean
Pines
Feet of Pipe 285,000
No, Vacuum Stations 12
No. Valves 1,500
No. Homes Served 3,500
Topography Flat
Soils Sandy
Community Age New
Seasonal Population No
Mean income High
Table 3-18.
Valve Type
Pipe Material
Diameters, in
Min. Cover, ft
Min, Slope, percent
Div. Valve
Thrust Blocking
Multi-branches
Longest Line, ft
Design Concept
Pit/House Ratio
Type Sumps
Design/Construction
Ocean
Pines
VAC-Q-TEC
Solvent
Welded
4
3
None
Plug
No
Yes
Unknown
None
0.43
Concrete
700 gal.
Westmoreland
83,000
4
490
540
Rolling
Rock
Old/New
No
Middle
Ohio
County
43,000
1
200
250
Hilly
Clay
Old
No
Middle
Lake
Chautauqua
121,000
4
900
2,500
Rat
Sandy
Old
Yes
Middle
Central
Boaz
39,000
1
180
350
Flat
Sandy
Old/New
No
Middle
White
House
65,000
2
260
360
Rolling
Rock
Old/New
No
Middle
Data - Collection Systems
Westmoreland
AIRVAC
Model-D
Solvent
Welded
3,4,6
4
0.2
Plug
Yes
Yes
Unknown
Reformer
Pockef
0.91
2 pit
Fiberglass
Ohio
County
AIRVAC
Model-S
Solvent
Welded
3,4,6
3
0.2
Plug
No
Yes
8,000
Early
AIRVAC
0.80
Fiberglass
Lake
Chautauqua
AIRVAC
Model-S
Rubber
Gasket
3,4,6.8
4
0.2
Plug
Yes
Yes
8,500
Early
AIRVAC
0.36
Fiberglass
Central
Boaz
AIRVAC
Model-D
Rubber
Gasket
3,4,6
3
0.2
Gate
No
Yes
6,500
Early
AIRVAC
0.51
Fiberglass/
Concrete
White
House
AIRVAC
Model-D
Solvent
Welded
3,4,6,8,10
4
0.2
Plug
No
Yes
14,000
Current
AIRVAC
0.72
Fiberglass
* Converted over the years to meet "early AIRVAC* design standards.
Table 3-19.
No. Probes
Equalizing Lines
Odor Filters
Design Construction
' Ocean
Pines
Multi
No
Yes
Data - Vacuum Stations
Westmoreland
Multi
Yes
No
Ohio
County
Single
Yes
No
Lake
Chautauqua
Single
Yes
No
Central
Boaz
Single
No
Yes
White
House
Multi
Yes
No
143
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Table 3-20.
O&M Data - General Information
No. Operating Personnel
Other duBes?
No. Persons Needed for
Vacuum Part of System
Table 3-21, O&M
Normal MaJnt, hr/yr
Prev. Malnt, hrfyr
Emerg. Malnl, hrtyr
Total Person-Hourefyr
No. (X Valves
Pereon-Hours/yr/Valve
No. of Customers
Person-Hours/yr/Cust.
Ocean
Pines
8
No
8
West
Moreland
6
Yes
1
Ohio
County
5
Yes
1
Lake
Chautauqua
7
Yes
2
Central
Boaz
1
Yes
1
White
House
3
Yes
1
Data - Pereon-Houre/Year
Ocean
Pines
16,640
2,500
1,660
20,800
1,500
13.9
3.500
5.9
West
Moreland
1,915
245
192
2,352
490
4.8
540
4.4
Ohio
County
260
100
240
600
200
3.0
250
2.4
Lake
Chautauqua
365
1,013
48
1,426
900
1.6
2,500
0.6
Central
Boaz
315
90
384
789
180
4.4
350
2.3
White
House
260
200
60
520
260
2.0
360
1.5
Table 3-22, O&M Dais - Power Consumption/Year
Power Costtyr, $
No. Valves
Cost/yr/Vafve, $
No, Customers
Cosl/yr/Cuslomer, $
kWWyr/Customer
TaWi 3-23. O&M
No. Valves
No. Service Calls/yr
MTBSC.yr
Ocean
Pines
120,000
1,500
80.00
3,500
34.29
570
Data • Mean Time
Ocean
Pines
1,500
1,500
1.0
West
Moreland
15,000
490
30.60
540
27.77
460
Ohio
County
2,400
200
12.00
250
9.60
160
Lake
Chautauqua
27,540
900
30.60
2,500
11.30
190
Centra)
Boaz
4.800
180
26.51
350
13.71
230
White
House
3,900
260
15.00
360
10.83
180
Bewtoen Service Calls (MTBSC)
West
Moreland
490
48
10.2
Ohio
County
200
24
8.3
Lake
Chautauqua
900
40
22.5
Central
Boaz
180
30
6.0
White
House
260
24
10.8
144
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Table 3-24.
Problem Classification
Problem
Component Defect
Design Shortcomings
Operator Error
Construction Related
Equipment Malfunction
Extraneous Water Related
Responsible Percent of Total
Party Problems
Manufacturer
Engineer
Operator
Contractor
Manufacturer
Customer
40
10
5
25
5
1S
c. Controller Shaft/Seal Problems Multi-Firing Valves
Some valves were found to cycle more than once after
the sump was emptied. A redesigned controller shaft and
seal has solved this problem. Controllers with the old
shaft and seal have been retrofitted with the latest
version.
d. Plug Valve Problem
The type of division valve used in most operating systems
is the plug valve. Some systems have experienced major
problems with plug valves while others have not.
Problems reported in one system included times when
system vacuum pulled the division valve closed thereby
cutting off system vacuum beyond that point. As workers
attempted to re-open these valves, frequently the valve
shaft was twisted off due to the heavy load applied by
system vacuum. When this happened, the division valve
could not be opened, resulting in part of system being out
of service until corrective measures were taken.
Some system operators reported problems with plug
valves not holding vacuum when closed. This renders
them useless as a troubleshooting tool, which is their
primary purpose.
Problems with plug valves have been attributed to quality
differences between manufacturers, in some cases, and
the defective materials in others. Properly manufactured
plug valves can and have been used successfully.
Some recent systems have used resilient-wedge gate
valves instead of plug valves with success. With proper
care in the selection process, both types of valves can
successfully be used.
e. Defective Tubing-Valve Failure in Open Position
Some valve failures that occurred after startup of one
particular system were caused by defective tubing in the
controller. These tubes were systematically replaced
with new ones and this problem disappeared.
3.7.4.2 Design Shortcomings
a. Discharge Pump Cavitation
Some systems have reported problems with wastewater
discharge pump cavitation. This was due to the
characteristics of the pump itself. A vortex plate installed
in the bottom of the collection tank corrected this problem.
The real solution lies in proper design. Net positive
suction head (NPSH) calculations should be performed.
Section 3.4 discusses these calculations. A pump having
sufficient NPSH available should be selected.
b. Leaking Check Valves
Check valves between the vacuum pumps and the
reservoir tank installed in a vertical pipe run resulted in
vacuum loss problems in the station piping. Locating the
check valves in a horizontal run of station piping corrected
this problem.
c. Oversized Vacuum Pumps Keep up with Open
Valve
One of the newest systems in operation was designed
with the more recent and more conservative design
guidelines. This resulted in larger vacuum pumps than
would be selected using the old design standards. While
this additional capacity has helped in terms of system
pump down-time, it also has caused a small problem.
The vacuum pumps appearto have sufficient capacity to
keep up with an open valve. This being the case, low
vacuum is not recorded at the station, even though it is
occurring at the location of the open valve. In addition, the
automatic telephone dialer is not activated. This results
in increased run-time of the vacuum pump and increased
power costs. The operator must be very observant when
analyzing the daily vacuum charts to notice that the
pumps are cycling more frequently than normal.
Otherwise, a valve that is hung open may go unnoticed
for days resulting in a waste of power.
A simple solution to this problem involves installing
another relay in the control wiring that causes the dialer
to call the operator if both pumps are running together for
a predetermined amount of time{say 10 minutes), despite
the vacuum level.
3.7.4.3 Operator Error
a. Wastewater Pulled Into Vacuum Pumps
A high wastewater level in the collection tank can be
caused by pump failure (usually control related rather
than a pump malfunction) or by more flow coming in than
the wastewater pumps are capable of pumping out. Most
145
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system designs call for an automatic shutdown of both
vacuum and discharge pumps in the event of this
happening. Some systems, however, do not have
provisions to prevent the automatic mode from being
overridden by manual operation.
The natural inclination, when faced with a zero-vacuum
situation, is for the operator to manually operate the
vacuum pumps in an attempt to restore system vacuum.
DoingthiswKhafull collection tank can result in wastewater
being pulled into the vacuum pumps. This result in
damage to the vacuum pumps, especially with the sliding-
vane type.
The proper procedure, assuming the discharge pumps
work, is to valve off the incoming lines and turn the
vacuum pumps off. The discharge pumps are then used
to pu mp the collection tank down to the normal operating
level. The line valves can then be opened and the
vacuum pumps returned to their automatic mode. Good
design will include electrical provisions to prevent
overriding the automatic mode without these steps first
being taken.
3.7.4.4 Construction Related
a. Line Break
Line breaks, caused by a trench settlement, were most
likely the result of inadequate bedding materials or poor
compaction during construction.
b. Broken Finings
There have been cases of loss of system vacuum due to
broken fittings. In all but a few cases, the failures have
occurred at fittings at or near the valve pit. This is most
likely due to insufficient compaction around the valve pit
coupled with system rigidity due to solvent welded fittings.
c. Construction Debris-Valve Failure in Open Position
These systems experienced more problems than others
on startup. Construction debris (stones, small pieces of
pipe, etc.) inthe homeowner's building sewer causes the
vacuum valve to hang open until manually cleared.
These problems usually disappear within a few weeks of
startup. Problems of this nature are easily discovered by
the operator simply by listening to the auxiliary vent.
Hearing the constant rushing of air is a good indication
that the valve is hung open. Opening the valve pit,
unscrewing the valve body, and clearing the obstruction
is a 10-minute procedure.
d Extreme Heat in the Station
Temperature approaching 70°C (175°F) were reported
in one system shortly after startup. The hot air from the
vacuum pumps was blown directly at the motor control
center (MCC). The extreme heat caused relay and
starter problems in the MCC, which resulted in pump and
telephone dialer malfunctions.
An analysis of the situation revealed that the vacuum
pumps were wired incorrectly. Ratherthan the pump fans
coming on when the temperature reached 65°C (150°F)
and remaining on until the pumps turned off, the fans
came on at the same time the pumps did and ran for a
timed 10 minutes. This has been corrected and the result
is a much cooler vacuum station. Future plans call for the
hot air to be directed away from the MCC through duct
work.
e. Broken Cleanouts
A few cases of broken cleanouts have occurred. Breaks
in fittings adjacent to the cleanouts were believed to be
caused by pit settlement due to poor compaction during
construction.
3.7.4.5 Equipment Malfunction
a. Faulty Level Control Transmitter
At one system, a problem with one of the level probes
occurred. The probe failed to send the proper signal to
the motor control center to turn the vacuum pumps off
during periods when high wastewater levels existed in
the collection tank. This resulted in wastewater being
pulled into the vacuum pumps causing damage. Another
time the probe failed to send the proper signal to the
motor control center to turn off a discharge pump. This
resulted in the pump continuing to run until the tank was
dry. The problem was traced to a faulty transmitter which
was replaced.
Since failure of the probe can lead to the ruining of a
vacuum and/or a wastewater pump, the operator
developed a simple inexpensive backup system, involving
the use of magnets strapped to the site glass above and
below the highest and lowest set point of the probe. A
float-mounted magnet located inside the site glass and,
held inside a plastic tube, moves with the level of the tank.
Should the probes fail, the floating magnets and one of
the fixed magnets will meet, causing a circuit to close and
the appropriate pumps to either start or stop.
b. Telephone Dialer Malfunctions-Interference
At one system, the automatic telephone dialer
malfunctioned by calling the operator only to report that
the system "was all clear." Investigation of this problem
revealed that interference from the motor control center,
caused by electrical spikes, was sending false signals to
the dialer. Shielded cable added to the dialer wiring did
not help. The dialer was taken out of the MCC and
mounted on a nearby wall. This corrected the problem.
146
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It is now known that the microprocessor-based equipment
is very sensitive to power spikes. Provisions should be
made during design to filter the power supply or to
provide a separate smooth power supply to this kind of
equipment.
c. Telephone Dialer Malfunction-Undetermined
A big problem in one of the earlier systems has been the
inability of the telephone dialers to function properly.
Without a dialer, which provides 24-hr/d coverage,
problems go unnoticed until the next work day. While this
has reduced the amount of overtime significantly, it has
also resulted in increased pump run-times. The operating
personnel apparently feel that the system is reliable
enough (i.e., oversized vacuum pumps can keep up with
a valve failure) and valve failures are so infrequent that
replacing the dialers cannot be economically justified.
The above rationale may be a case of false economy.
Allowing a low vacuum situation to go undetected for
hours will result in unnecessary additional run-time of the
vacuum pumps. This will result in increased power costs
as well as increased wear on major equipment. This
increased wear leads to decreased equipment life. A
high-wastewater level in the collection tank typically
results in total system shutdown until the operator corrects
whatever is wrong. With no dialer to notify the operator of
this situation, the system may be down for hours at a
time. This will lead to waterlogging, which will require a
significant labor effort to correct.
Even more serious is the liability potential that exists as
a result of damage done by system backups into the
customer's homes. Forthese reasons, telephone dialers
are considered to be a necessity in the vacuum station.
3.7.4.6 Extraneous Water Related
a. System Waterlogging
One of the most severe problems that can occur with the
collection system is lossof vacuum dueto"waterlogging."
This occurs when wastewater is admitted to the system
with insufficient atmospheric air behind it. This results in
ever-decreasing vacuum levels beyond the waterlogged
section. This may lead to insufficient vacuum to open the
vacuum valves and ultimately to wastewater backup in
the home. In most cases, waterlogging is caused by too
much liquid (extraneous water) entering the system at
one time. This is a very difficult problem to address since
it typically is related to illegal, and difficult to find, storm
water connections by the customer. An aggressive I/I
program during the hookup stage will keep problems
such as these to a minimum.
b. Water in Controller-Valve Failure in Open Position
Component defects have been responsible for many of
the valve failures in the past. With these failures being
drastically reduced by component improvements, one of
the remaining ways for a valve to fail is through water in
the controller. Water in the controller will prevent ft from
completing its cycle and the valve will remain open. This
problem is more likely to occur with the Model "S" valve
than with the model "D" valve.
In either the Model "8" or Model "D" valve, water can
enter the controller in two general ways. In the first way,
water that is present in the upper valve chamber enters
directly into the controller as a result of the controller not
being air and/or vacuum tight. A tightness test is normally
performed at installation, during the annual preventive
maintenance visft, and any time emergency maintenance
is performed on the valve. The other possibility isthrough
condensation in the breather tubing. A properly installed
breather line, with adrainleading to the valve, will prevent
this second case from occurring.
With the Model "S" valve, a third possibility exists. In this
case, water comes directly from the lower sump to the
controller. The Model "S" valve requires a watertight seal
between the upper and lower sections of the valve pit so
that a bubble of air can be trapped for use by the valve
controller during its cycle. Should the seal be broken and
water take the place of air, it will be drawn into the
controller by vacuum when the valve cycles.
3.8 System Costs
3.8.1 Construction Costs
3.8.1.1 General
Certain site conditions contribute to the high cost of
conventional sewer installation. These include unstable
soils, rock, and a high water table. In addition, restricted
construction zones or areas that are flat may also result
in high construction costs. By using vacuum sewers, the
construction costs for these difficult conditions can be
reduced. Smaller pipe sizes installed at shallower depths
are the prime reasons for this. The uphill transport
capability, even when used only to a very small degree,
may save many dollars in installation costs. One other
major advantage is the extent to which unforeseen
subsurface obstacles can be avoided. Each of these
considered individually results in lower costs. Considered
collectively, they may result in substantial cost savings.
It is not uncommon for a wide range of bids to be received
on an engineer's first vacuum project. Contractors
unfamiliar with vacuum sewers may bid high simply
because of the fear of the unknown (e.g., bid as if the
147
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project were a gravity sewer). Equally possible is a
contractor who bidstcxs low becauseof an underestimation
of the effort required (e.g., bid as If ft were a water line).
Usually the cost is somewhere between these two
extremes. Once the contractors have experience
constructing vacuum sewers, the spread between low
and high bid narrows. It then becomes much easier for
the engineer to estimate the construction costs of future
projects.
Many factors affect construction cost bids. Material
surpluses or shortages, prevailing wage rates, the local
bidding climate, geographic area, time of year, soundness
of the design documents, and the design engineer's
reputation areexamplesof these. Funding and regulatory
requirements also play a part in project cost estimating to
the extent that the regulations may be a help or hindrance
to the contractor, the client, and the engineer.
Because ofthe many variables, accurate cost-estimating
guidelines are beyond the scope of this manual. The
following general guidelines, however, will be of help to
the estimator.
3.8.1.2 Piping System
Pipe installation prices are a function of pipe diameter,
pipe material, trench depth, and soil conditions. Other
factors, such as shoring, dewatering, and restricted
construction area conditions will have a large effect on
the price.
Piping systems are best estimated using guidance from
water system projects built in the same area, if similar
materials and specifications are used. The best situation
is one where the water line project was designed and
construction managed by the same engineering firm as
is producing the sewer cost estimate. In this way, projects
similarities and differences can be factored into the
comparison. If the estimating engineers are not familiar
with the project they are obtaining guidance from, they
should become familiar with the specifications for that
projectto rationalize differences between theprojects. In
some cases, the Associated General Contractors (AGC)
can provide helpful insight.
In the absence of better information, Table 3-25 provides
estimating data for planning purposes only. This table
was prepared by reviewing bid tabulations from numerous
projects throughout the United States. Some projects
were known to have been built so cheaply that long term
performance was questionable. Other projects had
unusually stringent specifications or other features that
resuHed in particularly high costs. Both the high cost and
low cost bid tabulations were discarded, leaving mid
range averages to produce the table.
Table 3-25. Average Installed Unit Costs (Mid-1990) for
Vacuum Sewer Mains and Appurtenances
Item Unit Cost
4-in Mains
6-ln Mains
8-in Mains
Extra for Mains in A.C. Pavement
4-in Division Valve
6-in Division Valve
8-|n Division Valve
Gauge Tap
Lifts
($)
11.00/LF
14.00/LF
17.QO/LF;
5.00/LF'
350/ea,
400/ea.
550/ea.
50/ea.
50/83.
Table 3-26. AvwagelnstalledUnltCosls(M!cJ-1990)forValve
Pits and Appurtenances
Item Unit Cost
($)
Standard Setting 2,300/ea.
Deep Setting 2,500/ea.
Single Buffer Tank 3,000/ea.
Dual Buffer Tank 4,000/ea.
Extra for Anti-Flotation Collar 100/ea.
4-in Auxiliary Vent 50/ea.
External Breather 50/ea.
Optional Cycle Counter 125/ea.
Pipe prices include furnishing and installing the pipe,
excavation, bedding, backfilling, compaction, vacuum
testing, cleanup, and similar requirements. Not included
are allowances for such items as rock excavation,
engineering, and administration.
3.8.1.3 Valve Pits
Valve pit prices will vary depending on the type of valve,
type of pit, and depth of the pit.
IThe mbsjt common valve pit setting (standard setting)
consists of the following:
» Model D-3-in vacuum valve w/controller
• 16 in x 30 in fiberglass tapered sump - 30-in deep
• 27 in x 36 in fiberglass valve pit - 42-in deep
• Pit bottom _ ;,
• Cast iron frame and cover
• 3-in suction line
• 2-in sensor line
• All piping/tubing hardware
• Grpmmets for openings
• 4-in gravity .stub-out - 6-ft long , .
148
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Table 3-27,
Average Installed Cost (mid-1990} for Vacuum Station
item
Package Station
Package Station
Package Station
Custom Station
Custom Station
Custom Station
No.
Customers
10-25
25-50
50-150
100-300
300-500
>500
Equipment
Cost*
($)
50,000
75,000
90,000
120,000
140,000
170,000
Building
Cost
($)
25,000
30,000
40,000
50,000
60,000
75,000
Installed
Cost
($)
20,000
25,000
30,000
40,000
50,000
75,000
Total
Cost
($)
95,000
130,000
160,000
210,000
250,000
320,000
' Includes Generator
Deeper settings contain the same items, except the 30-
in deep sump is replaced by a 54-in deep sump.
Single buffer tanks consist of the following:
• Model D-3-in vacuum valve w/controller
• 4-in diameter concrete manhole - variable depth
* Cast iron frame and cover
• 3-in suction line
• 2-in sensor line
* All piping/tubing hardware
« All pipe support hardware
• 4-in gravity stub-out - 6-ft long
Table 3-26 gives average installed unit prices for the
various valve pit settings and appurtenances. The prices
include furnishing and installing the valve pit, excavation,
bedding, backfill, compaction, vacuum testing, and surface
restoration.
3.8.1.4 Vacuum Stations
The price of a custom-designed vacuum station depends
very much on the equipment selected, the type of structure
it is housed in, and the amount of excavation required.
Equipment cost varies with the capacity required for each
component. Equipment for a typical vacuum station
includes:
Vacuum pumps (2)
Wastewater Pumps (2)
Generator
Collection tank
Reservoir Tank
Level controls
Motor control center
A package vacuum station, used for smaller (<250 gpm)
projects, includes the above equipment, with the exception
of the generator and reservoir tank. This equipment is
assembled, painted and tested at the factory and then
shipped on a skid for placement in the building.
Table 3-27 gives average installed prices for both custom-
designed stations as well as for package stations. The
prices include the equipment (including the generator for
all stations), station piping, electrical, excavation, site
restoration, and labor.
3.8.2 Estimating Techniques
Most engineers are capable of estimating capital costs in
their geographic area of practice once major material
component costs are known. Installation cost can be
developed by analyzing the effort required to install
conventional technology and applying the appropriate
modifications to the unit prices to correct forthe local site
conditions. Using sound engineering practices the
engineershould be ableto make a fairly accurate estimate
of the construction costs. The following steps may be of
some value for engineers considering vacuum sewers
for the first time:
* Analyze bid tabulations of other vacuum projects. It is
desirable to use a project that is in the same geographical
area and has similar site conditions.
* Request a set of plans and specifications in order to
gain afu II understanding of what each bid item includes.
Ask about any peculiar or unusual bidding conditions.
* Estimate the cost of the system by applying modified
unit prices that take into consideration the locale and
site conditions.
• Correct for inflation by applying the appropriate cost
index. , ...
Because of the many factors that influence a bid, ft is
difficult to say if the above procedure will yield information
of quantitative value, although qualitative assistance is
likely. ' : . '
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T*b!«3*20.
Typical O&M Cost Components
Labor
Clerical
Power
Utilities
Transportation
Supplies/Maintenance
Misc. Expenses
Equipment
Future Connections
Normal, preventive, emergency maintenance
Billing
Vacuum and sewage pumps
Water, phone, fuel
Vehicle amortization, fuel, Ins.
Maintenance contracts
Insurance, professional fees
Renewal and replacement (sinking fund)
Set-aside amount (If required)
Table 3-29. Person-Hour Estimating Factors
Low High Average
Normal
Preventive
Emergency
Total
0,4
0.5
0.2
1.6
{ Person- Hour/Yr/Valve)
3.9 1.7
1.1 0.7
1.2 0.8
4.8 3.2
3.8.3 Operation and Maintenance Costs
The obvious advantage to utilizing vacuum sewers is a
savings of capital costs. This savings, however, may
come at the expense of higher O&M costs. The question
then becomes, "Is the savings of capital cost enough to
offset the increase in O&M expenses?" A present worth
analysis, where life cycle costs are converted to present
values, will answerthisquestion. This analysis, however,
is only valid if reasonable capital and O&M cost figures
are used.
Because of the relatively new nature of the vacuum
sewers, very little historical cost data exist. This is
especially true for operation and maintenance costs.
This lack of data has led many to the conclusion that
vacuum sewers must be O&M intensive. A review of
operating records of systems discussed in this chapter
suggests that previously published O&M figures may no
longer apply. Reasons for this are twofold. First, the
previous figures were based on very limited data on a few
early systems. Second, component improvements have
resulted in significantly fewer service calls and lower
O&M costs. These factors require that the previously
published data be used only as a knowingly conservative
estimate of the state of the art.
Information gathered from the systems that were visited
in1989wasusedinthe formation ofthe estimating tables
that follow. Data from the Ocean Pines system was not
included since this system is not representative of the
current state of vacuum technology.
Table 3-28 shows typical O&M cost components.
3.8.3.1 Labor
Labor costs are estimated by multiplying the number of
person-hours required by the hourly rate, and adding
fringe benefits. The annual person-hour requirements
are made up of normal, preventive, and emergency
maintenance.
For normal maintenance, an operator is not required 24
hours a day to monitor the system, as the telephone
dialer does this for him. However, someone must be
available around the clock in case the telephone dialer
calls with a problem. In this respect, vacuum systems are
unique. Very few problems in a vacuum system can go
uncorrected for any length of time without causing a
cumulative effect.
The operating entity's overall responsibilities should be
considered when estimatingthe labor costs. For example,
the entity may also be responsible for other sewer
facilities, orpossibly even water facilities. In these cases,
operating personnel are usually shared. At the end ofthe
year, the time charged to the operation of the vacuum
system most likely will relate exactly to the effort required
(e.g., 1 hr/d plus some hours charged to emergency
maintenance). If the overall facilities are large enough to
warrant different shifts, emergency work most likely will
be done without overtime being required.
An entirely different situation exists forthe entity operating
nothing but a similarly sized vacuum system. Typically,
a full-time operator is hired. This person charges 8 hr/d
to the maintenance ofthe system although most days he
will spend much less than this. Should a problem develop
after normal working hours, he most likely will be paid
overtime. Even though both operators will spend the
same amount of actual maintenance time, the amount of
billed time will be entirely different. The engineer should
carefully analyze the client's overall operation, taking into
consideration the possibility of shared duties, prior to
making an estimate of the labor costs.
Preventive maintenance is usually scheduled to be done
by the normal workforce during off-peak working hours.
Because of this, preventive maintenance is usually
reported as normal maintenance.
Emergency maintenance many times requires personnel
after normal working hours. The result is overtime pay.
Emergency maintenance typically requires two operators
or one plus an assistant.
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Table 3-29 presents a range of person-hours required
per year as a function of the number of vacuum valves.
As such, the values shown should be considered as
realistic values for new systems with proper design,
construction, and operation.
When a full-time operator is to be hired, regardless of
anticipated workload, the values in Table 3-20 should not
be used. In this case, the estimated annual person-hour
requirements should include the full-time hours of
employment plus an estimateof the overtime (emergency
maintenance) hours, taking into consideration overtime
work generally requires two people. No allowance is
needed for normal or preventive maintenance since
these can be performed during normal working hours.
The following equation can be used to estimate annual
labor hours for the case of a full-time operator. This
equation assumes a 40-hr workweek and that 50 percent
of the emergency maintenance will occur during normal
working hours.
Table 3-30.
Vacuum Station Power Consumption
Estimating Factors
2080 hr/yr + (0.5 x EMF x # valves x 2)
Where,
EMF
Emergency Maintenance Factor from Table 3-
29 .
As an example, a system with 200 valves and an average
EMF (0.8 hr/yr/valve) would yield an annual labor
requirement of 2,080 +160, or 2,240 hr. Given the range
of EM Ffrom Table 3-29, this requirement could vary from
2,120hrto2,320hr.
3.8.3.2 Clerical
This item includes wages for the clerical staff. Also
included would be the billing costs such as envelopes
and stamps. Like labor costs, the value of this item most
likely will depend on whether the operating entity has an
existing, ongoing operation which requires office staff. If
so, the costs would be limitedtothedirectbilling expenses
only,
3,8.3.3 Power
Power is required forthe vacuum pumps, the wastewater
pumps, and the heating and lighting of the vacuum
station. Once the design is completed, ft is possible to
accurately estimate the annual power consumption.
Power costs for both the vacuum and wastewater pu mps
are estimated by using the following general equation:
P = 0.746 T H C
Low
High
Average
160
(kWh/Yr/Customer)
460
250
where:
P = Annual power cost ($)
T = Time of operation (hr/yr)
H = Horsepower
C = Cost of electricity ($/kWh)
This equation does not include any surcharge the power
company may assess on peak demands. The local rate
structure of the power company should be analyzed and
an appropriate surcharge estimated.
The capacity of the vacuum pumps, and the resulting
horsepower requirements, can be determined by using
the equations in Section 3.4. Likewise, the run-time of the
vacuum pumps can also be determined. An allowance,
typically an additional 10-20 percent should be made in
the run-time to account for leaks, breaks, and valve
failures.
The capacity of the wastewater pumps and the resulting
horsepower requirements were discussed previously.
Daily run-time can be estimated by dividing the average
flow (gpd) by the normal discharge rate (gpm) and again
by 60 to convert the units. Annual run time is then
computed by multiplying the result by 365.
There are also power costs for heating, ventilation^ and
lighting at the vacuum station, which generally amount to
less than $50/month.
For planning purposes, values shown in Table 3-30 can
be used to estimate the annual power consumption for
the entire vacuum station.
It should be noted that the high factor shown in Table 3-
30 was for a early vacuum system. The average figure is
a more accurate representation of the power requirements
of recent vacuum systems.
3.8.3.4 Utilities
Utilities at the vacuum station generally include water,
telephone, and fuel. Water may be required for sinks,
hose bibs, or toilets. A telephone is required forthe fault
monitoring system. Some systems make use of cellular
phones, while others use radio communications rather
than a telephone. Fuel may be required for the standby
151
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generator. The cost of these utilities generally is less than
$50/month.
3.8.3.5 Transportation
Vehicle expensesto maintain the system will be incurred.
For estimating purposes, a mileage rate multiplied by the
estimated annual miles will suffice. This rate should
include vehicle amortization, depreciation, taxes, and
similar expenses. Mileage willbe required forthe following
tasks:
« Daily visits to the vacuum station
• Annual visits to valve pits for preventive maintenance
• Periodic emergency maintenance
3.8.3.6 Supplies/Maintenance
As with a conventional system, certain supplies will be
required. Restocking of spare parts and inventory is
included in this item, as are oil, fuses, charts, and chart
pens. Initial purchase of items on quantity discount
should be maximized to take advantage of the lower unit
costs when compared to subsequent prices for
replacement.
Service contracts for emergency generators, as well as
fuel forthe generators, may also be included in this item.
3.8.3.7 Miscellaneous Expenses
Miscellaneous expenses include insurance and
maintenance on the system structures as well as
professional services (engineering, accounting, legal)
that may be required during the year.
3.8.3.8 Equipment Renewal and Replacement
An annual set-aside account should be established to
generate sufficient funds for major equipment renewal
and replacement. The annual cost is estimated by dividing
the replacement cost by the useful life. This amount is
generally set aside in an interest bearing account until
needed. Present dollars can be used in the estimate
since the interest earnings most likely will offset inflation.
Alternative methods dictated by regulatory agencies also
can be employed.
Table 3-31 lists the major equipment items and their
useful life.
3.8.3.9 Future Service Connections
The costs of future service connections may have to be
Included in the O&M budget. Unlikeconventional systems,
where a future connection may be relatively inexpensive,
a future vacuum connection can be quite costly. The
costs of a valve, valve pit/sump, fittings, pipe, and labor
may exceed $2,000.
Table 3-31. Typical Renewal and Replacement Factors for
major Equipment
Item
Vacuum Pumps
Sewage Pumps
Vacuum Valve rebuild
Misc. Station Equipment
Useful Life
(yr)
10
10
6
10
Most systems that were visited simply charge the future
customer the cost of the connection. This does not
appear to be a problem, since most new houses would
have a similar expense for an on-lot system. In this case,
the annual operating budget does not have to include a
line item for future service connections.
In West Virginia, however, this is not the case.6 Sewer
utilities are regulated by a Public Service Commission.
This agency hesitates to approve a tap fee greater than
$250, regardless of the type of system. For this reason,
the operating entity must include the cost of future
connections in their rate package as an annual set-aside.
The total installation cost is discounted by the $250 tap
fee and multiplied by the number of estimated annual
future connections. Since some future customers may
be able to connect to an existing valve pit, a sharing factor
maybe applied. Thismethod requires present customers
to subsidize the cost of future connections.
A modification of this concept includes prefinancing a set
amount of future connections in the construction capital
budget, and adjusting the annual set-aside amount
accordingly. An example calculation of this set-aside
amount follows:
No. vacuum customers
No. vacuum valves
Sharing factor, percent
20-year growth rate, percent
Cost of new service, $
Tap fee, $
No. prefinanced connections
Net cost of connection, $
Total set-aside required, $
Annual set-aside required, $
140
110
78
15
2,000/ea
250/ea
8
2,000-250.1,750
1,750x8 = 14,000
14,000/20= 700
3.8.4 User Charge Assessments
To generate sufficient revenues to offset expenses, the
operating entity must develop a user charge system.
Components of the annual budget are:
• O&M expenses
Labor
Clerical
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Table 3-32. Annual Budget Example
Vacuum
Pressure/Gravity
Treatment Plant
Total
O&M
Labor
Clerical
Power
Utilities
Transportation
Supplies/Maintenance
Miscellaneous Expanses
Chemicals
Equipment Replacement/Repair
Future Service Connection
Total O&M
Debt Service
Total Annuai Budget
3,500
460
2,200
750
1,400
1,500
1,000
0
1,600
700
13,110
16,655
$29,765
2,000
540
2,000
500
200
200
500
0
3,100
65
9,690
16,655
$26,345
14,300
0
6,900
750
400
1,630
2,000
1,200
2,200
0
29.380
11,710
$41,090
19,800
1,000
11,100
2,000
2,000
3,330
3,500
1,200
6,900
1,350
52,180
45,020
$97,200
Power
Utilities
Transportation
Supplies/Maintenance
Miscellaneous Expenses
Equipment Renewal and Replacement
Future Service Connection
• Debt Service
There are different methods of developing rates. A
common method includes a charge based on metered
water consumption, wfthaflat rate for nonmetered users.
The rate structure may be the same for all levels of
usage, or may decline as water use increases. Others
simply charge a flat rate for all users. No matter what
system is used, the rates must be sufficient to generate
the required revenue.
Mosttariffs include provisions fortap fees. Thesegenerally
cover initial users of the system as well as future users.
The initial tap fees are generally used to cover startup
expenses. Tap fees for future connections are used to
offset all or part of the actual cost to make the connection.
As an example, the projected annual operating budget of
a project in West Virginia1 is presented in Table 3-32. This
project is scheduled for construction in 1991, with
operation to begin in 1992. The project consists of a
hybrid collection system (vacuum, pressure, and
gravity) and a treatment plant.
General Project information for Annual Budget
Example:
Pressure/ Treatment
Vacuum Gravity Plant Total
Const. Costs, $
No. Connections
1,000,000 1,000,000
140 160
750,000 2,750,000
300 300
3.9 System Management Considerations
3.9.1 Homeowner Responsibilities
Use of the system should comply with requirements of
the user ordinance. Typical requirements include that
the homeowner should not drive or build overthe tank,
and should protect thefacilftiesfromdamage. Discharge
of flammables, acids, excessive amounts of grease,
sanitary napkins, or other non-wastewater items is
discouraged. This requirement differs little from user
ordinance requirements for conventional sewers.
Proper use of the system results in lower user charges
and improved reliability.
3.9,2 Sewer Authority Responsibilities
Once all customers are connected, the Authority is
focused on providing reliable, efficient service to their
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customers. To achieve this, the operating personnel
must be capable, dependable, and knowledgeable. Of
utmost importance is attitude. An operator that does not
believe in the system will ultimately cause the system to
operate below its potential, in terms of reliability and
costs. Conversely, one with a good attitude usescreativity
to get more out of the system than wasoriginally planned.
To operate any system at a high level of efficiency
requires a Sewer Use Ordinance. This document sets
consistent rules for all users to follow. Included are
material specifications for the building sewer, minimum
slope requirements, and vent locations. Of extreme
Importance to the Authority is a limitation of use of the
vacuum sewer to sanitary wastes only, as extraneous
water will cause operational problems.
An active program for the identification of extraneous
water sources should be developed. This may include
smoke testing and dye testing. To identify and quantify
sources of extraneous water, the Authority can take
advantage of an integral feature of a vacuum system: the
cycle counter. This device, when connected to the valve,
will record the nu mber of times the valve opens in a given
period. Knowing that each cycle is approximately 38 L
(10 gal), the Authority can estimate, based on water
consumption records, the minimum number of cycles
expected overthat period. Acount much in excess of the
expected may be a sign of extraneous water. To quantify
the amount, the number of cycles is multiplied by 38 L(10
gal) and compared to the water consumption. Listening
to the auxiliary vent for sounds of running water when no
wastewater-generating activity is taking place may also
provide clues for sources of extraneous water.
The Authority also is responsible for future extensions of
the system. This includes proper planning, design, and
construction of such extensions. Finally, future
connections to the existing system are made by the
Authority in accordance with the provisions of the Sewer
Use Ordinance.
3.9.3 Other Entities
During the planning, design, and construction of sewer
systems, there are many different entities involved. Two
of these are regulatory agencies and the engineer. It is
during these times that many decisions are made and
details finalized. Often, these entities view the startup of
a system as their final involvement. While this attitude is
understandable, it is not acceptable. Continuing
involvement is needed to help a develop an experience
base with newer systems which permits intelligent
applications in the future.
The engineer should spend a significant amount of time
during the startup of the system. Tests should be run and
problems simulated to see if the system is operating as
designed. Periodically, the operating records should be
analyzed for budget sufficiency purposes. Problems and
their solutions should be recorded. In short, the engineer
should use the operating experience of the system to
help develop improvements in future designs.
Likewise, regulatory agencies should follow up on the
operation of a new system. Information on problems,
including causes and the remedies, should be gathered.
Cost data should be obtained. This type of information
can then be used for future projects.
It is this present lack of information that causes many
engineers and regulatory agencies to shy away from a
new technology. It has become easierto be conservative,
and hence unduly critical, rather than to learn the details
of a new technology, no matter how cost-effective.
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3.10 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. R. Naret, Vacuum Sewers:Construction and
Operation Experience. Presented at the 1988 ASCE
Water Resources Planning Management Conference
in Norfolk, VA.
2. AIRVAC Design Manual, Rochester, IN., 1989,
3. O. F. Robles. Operation and Maintenance Manual for
the Ohio County Public Service District, 1990.
4. Alternatives for Small Wastewater Treatment
Systems. EPA/625/4-77-011, NTIS No. PB-299608.
U.S. Environmental Protection Agency, Cincinnati,
Ohio, 1977,
5. J.W. Rezek and I. A. Cooper. Preliminary Report on
Vacuum Sewer System at Lake of the Woods, VA.,
Volume 1: Hydraulic Field Test Program. Report
prepared for Lake of the Woods Service Company
and Utilities, Inc., Henry, Meisenheimerand Gende,
Inc., Libertyville, Illinois, November 1974.
6. Cerrone & Associates, Inc. Union Williams Public
Service District, Final Engineering Report. January
1990.
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Page Intentionally Blank
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CHAPTER 4
Small Diameter Gravity Sewers
4.1 Introduction
Small diameter gravity sewers (SDGS) are a system of
interceptor tanks and small diameter collection mains.
The interceptor tanks, located upstream of each
connection and usually on the property served, remove
grease and settleable solids from the raw wastewater.
The settled wastewater is discharged from each tank via
gravity or by pump (septic tank effluent pumping [STEP]
unK) into thegravHycollectormains usually located in the
public right-of-way. The mains transport the tank effluent
to a treatment facility or connection with a conventional
gravity sewer interceptor.
Because the interceptor tanks remove the troublesome
solids from the waste stream, the collector mains need
not bedesigned to carry solids. This reduces thegradient
needed and, as a result, the depth of excavation. The
need for manholes at all junctions, changes in grade and
alignment, and at regular intervals is eliminated resulting
in further potential cost savings.
The sewer diameter can also be reduced because the
interceptortank attenuates the wastewaterf lowto reduce
the peak to average flow ratio. Yet, except for the need
to evacuate the accumulated solids from the interceptor
tanks periodically, SOGS operate similarly to conventional
sewers.
The compatibility of STEP systems with SDGS allows an
efficient low-cost hybrid collection alternative in many
unsewered developments. A hybrid design can often
eliminate or minimize the need for lift stations to reduce
both capital and operation and maintenance (O&M)
costs. It is cautioned that grinder pump (GP) systems
are not compatible with SOGS because the waste solids
and grease are not removed from the waste stream
before discharge to the collector main. Also, effluent
sewer termination into vacuum systems could have odor
potential at the central vacuum station.
4.2 Description of System Components
Typical small diameter gravity sewer systems consist of:
building sewers, interceptortanks, service laterals, collector
mains, cleanouts, manholes and vents, and lift stations
(see Figure 4-1). Other appurtenances may be added as
necessary
4.2.1 Building Sewers
All wastewaters enter the small diameter gravity sewer
system through the building sewer. It conveys the raw
wastewaters from the buildingtothe inlet of the interceptor
tank. Typically it is a 10-15 cm (4-6 in) diameter pipe laid
at a prescribed slope, usually no less than 1 percent,
made of cast iron, vitrified clay, acrylonitrile butadiene
styrene (ABS) or polyvinyi chloride (PVC).
4.2.2 Interceptor Tanks
Interceptortanks perform three important functions: 1)
removal of settleable and floatable solids from the raw
wastewater, 2) storage of the removed solids, and 3) flow
attenuation. Thetanks aredesigned for hydraulic retention
times of greater than 24 hours when two-thirds full of
solids to permit liquid-solid separation via sedimentation
and flotation. Outlet baffles on the tanks prevent floating
solids from leaving the tank. The tank has sufficient
volume to store the solids until which time they can
removed, typically on 1-10 year cycles for residential
connections and semi-annual^ or annually forcommerciai
connections with food service. Anaerobic digestion does
take place within the tank which reduces the volume of
accumulated sludge and prolongs the storage time. The
interceptor tanks also provide some su rge storage which
can attenuate peak flows entering the interceptortank by
more than 60 percent.1*
Septic tanks are typically used as interceptor tanks
(Figure 4-2). Pre-cast reinforced concrete and coated
steel tanks are usually available locally in a variety of
sizes. Rberglass (fiber reinforced plastic, FRP) and high
density polyethylene tanks (HOPE) are also available
regionally. Pre-cast concrete tanks are most commonly
157
-------
Figure 4-1.
Components of a small diameter gravity sewer (SDGS) system.
SOLUBLE BOD LATERAL
INFLECTIVE
GRADIENT
SETTLEABLE SOLIDS
BUILDING
SEWER
EFFLUENT
Figure 4-2.
Typical pre-cast concrete Interceptor tank.
INLET
OUTLET
158
-------
used in SDGS systems, but polyethylene and fiberglass
tanks are gaining in popularity because they are more
watertight and lighter in weight for easier installation.
However, FRP and HOPE tanks require more care in
proper bedding and anti-flotation devices where high
ground water occurs.
4.2.3 Service Laterals
Service laterals connect the interceptor tank to the
collection main. The laterals are usually plastic pipe no
larger in diameter than the collector main. They are not
necessarily laid on a uniform grade nor with a straight
alignment (Figure 4-3).
Optional lateral appurtenances include check valves,
"p"-traps or "running" traps and corporation stops. Most
existing SDGS systems do not include these options, but
check valves are being used more frequently to prevent
backups into low-lying connections during peak flows.
"P"-traps have been retrofitted on connections where
odors issuing from the house plumbing vent stack have
been a problem. Corporation stops are used primarily on
"stub outs" reserved for future connections.
4.2.4 Collector Mains
Collector mains convey the settled wastewater to either
a lift station, manhole of a conventional gravity sewer
system or directly to a treatment plant. Plastic pipe, with
solvent weld or rubber gasket joints is used almost
exclusively. However, flexible, high density polyethylene
pipe with heat fused joints has also been used successfully.
4.2.5 Manholes and Cleanouts
Manholes and cleanouts provide access to the collector
main for various maintenance tasks. Since hydraulic
flushing is sufficient to clean the mains, the use of
manholes is usually limited. Common practice is to use
manholes only at major junctions because they can be a
significant source of infiltration, inflow and sediment.
Cleanouts are typically used at upstream termini, minor
main junctions, changes in pipe size or alignment, high
points, and at intervals of 120-300 m (400-1,000 ft).
4.2.6 Air Release Valves and Vents
Vents are necessary to maintain free-flowing conditions
in the mains. In SDGS systems installed with continuously
negative gradients, the individual house connections will
provide adequate venting if the sewer lateral is not
trapped. In systems where inflective gradients are allowed,
the high points in the mains must be vented. Air release
or a combination of air release/vacuum valves are
commonly used in combination with a cleanout (Figure 4-
4). Individual connections located at a summit can also
serve as a vent if the service is neither trapped nor fitted
with a check valve.
4.2.7 Lift Stations
Lift stations may be used at individual connections which
are too low in elevation to drain by gravity into the
collector. They are also used on the collector mains to lift
the wastewater from one drainage basin to another.
Individual lift stations are essentially STEP systems and
are usually simple reinforced concrete or fiberglass wet
wells following the interceptor tank with low head, low
capacity submersible pumps operated by mercury float
switches (Figure 4-5). In a few systems where the static
lift is great, high head, high capacity turbine pumps have
been used successfully. This is only possible if the
wastewater effluent is screened prior to pumping to
eliminate any solids that might ciog the turbines. Mainline
lift stations were originally similar in design to the
"residential" lift stations, but becauseof corrosion problems
which commonly occur in the wet well, the use of dry wells
is becoming more common to reduce corrosion problems
and to facilitate maintenance.
4.3 System Design Considerations
4.3.1 Hydraulic Design
A small diameter gravity sewer system conveys settled
wastewater to its outlet by utilizing the difference in
elevation between its upstream connections and its
downstream terminus. It must be set deep enough to
receive flows by gravity from the majority of the service
connections and have sufficient capacity to carry the
expected peak flows. Therefore, design decisions
regarding its location, depth, size and gradient must be
carefully made to hold the hydraulic losses within the
limits of available hydraulic head energy. Where the
differences in elevation are insufficient to permit gravity
flow from an individual connection, energy must be
added to the system by a lift pump (see Chapter 2). The
number and location of individual lift stations or STEP
units are usually determined from comparisons of their
costs of construction, operation and maintenance with
the cost of construction and maintenance of deeper and/
or larger diameter (smaller headloss) sewers. The
hybridization of SDGS with STEP is common.
4.3.1.1 Design Flow Estimates
The hydraulic design of sewer mains is based on the
estimated flows which the sewer must carry. Since
wastewater flows vary throughout the day, the sewer
main must be designed to carry the expected prolonged
peak flows, typically the peak hour flow. Conventional
sewer design assumes 380 L//cap/d (100 gpcd) times a
typical peaking factor of 4 for collector mains. This
estimate includes allowances for commercial flows and
infiltration. However, experience with SDGS has shown
that these design flow estimates greatly exceed actual
159
-------
Figure 4-3.
Service lateral Installation using a trenching machine.
Figure 4-4.
Typical combination cleanout and air release valve detail.
SEALED LID
2- PVC BALL
VALVE
POLYRIBBED VAULT
2-TEE
160
-------
Figure 4-5. Typical STEP lift station detail.
QUICK COUPLING —
INLET
PVC DISCHARGE
HOSE ASSEMBLY
PUMPVAULT
COVER ELECTRICAL
I JUNCTION BOX GATE VALVE
v / /
PVC DISCHARGE
PIPING
LIFTING ROPE
DISCHARGE
MERCURY
FLOAT SWITCHES
PUMP ON
FLOAT POLE
ASSEMBLY
CHECK VALVE
PUMP OFF
ANTI-FLOATATION
COLLAR
161
-------
flows because most SDGS serve residential areas where
daily per capita flows are far less than 380 L/cap/d (100
gpcd); the peak to average flow ratio is also less than 4
because the interceptor tanks attenuate peak flows
markedly.2
Measured average daily wastewater flow per capita is
approximately 170 L/d (45 gpd).3"5 However, in small
communities and residential developments where little
commercial or industrial activity exists, average per
capita wastewaterf tows in sewers may be as much as 25
percent less.6 Household wastewater flow .can vary
considerably between homes but it is usually less than
227 L/d (60 gpd) and seldom exceeds 284 L/cap/d (75
gpcd).7 Typically, 190 L/cap/d (50 gpcd) is assumed for
wastewater flows in residential areas where actual water
use data are not available. Commercial flows are estimated
individually using established criteria.7
The collector mains are sized to carry the maximum daily
peak flows rather than the average flow. In residential
dwellings, the rate of wastewater discharged from the
building depends on its water use appliances and fixtures
used. Instantaneous peak flows are typically 0.3-0.6 L/s
(5-10 gpm).7 Maximum flows of 0.1 L/s (1.7 gpm) may
occur/-9 However, the interceptor tank in SDGS systems
attenuates these peaks dramatically. Monitoring of
individual interceptortanks shows that outlet flows seldom
exceeded 0.06 L/s (1 gpm) and most peaks ranged
between 0.03 and 0.06 L/s (0.5-0.9 gpm) over periods of
30-60 minutes. There were long periods of zero flow.2
The degree of attenuation depends on the design of the
Interceptor tank and/or its outlet.
In addition to wastewater flows, allowance must be made
for potential clear water infiltration/inflow (I/I). Common
sources of infiltration in SDGS systems are the building
sewer and interceptor tank. In SDGS systems in which
the existing septic tanks were used as interceptor tanks,
wet weather flows have been significantly higherthan dry
weather flows. Leaking building sewers, cracked tanks
and poorly fitting tank covers are the most common
sources of infiltration. Where all new tanks were installed
and the building sewers tested or replaced, the ratio of
wet weathertodry weatherflows have been much lower.2
In all systems, foundation drains and roof leaders may be
significant sources of inflow and SDGS projects should
attempt to eliminate them during construction. Despite
these attempts, the designer may wish to allow for some
degree of unavoidable I/I; but this allowance should be
significantly lower than typical conventional gravity
systems due to their higher elevation and smaller
diameter.
Experience with SDGS systems has shown that the
criteria used to estimate design flows have been
conservatively high. Design flows have generally been
190-380 L/capita/d (50-100 gpcd) with peaking factors of
1 to 4. More recent designs have been based on flows per
connection of 545-1,635 L/d (0.1 -0.3 gpm). These design
flow estimates have been successful because the
interceptortanks have storage available above the normal
water level to store household flows for short peak flow
periods.
4.3.1.2 Flow Velocities
Conventional sewer design is based on achieving "self-
cleansing"velocitiesduring normal daily peak flow periods
to transport any grit which may enter the sewer, scour
grease and resuspend solids that have settled in the
sewer during low flow periods. However, in SDGS
systems, the primary treatment provided in the interceptor
tanks upstream of each connection remove grit, and
most grease and settleable solids. Studies have shown
that the remaining solids which enter the collectors and
any slime growths which develop within the sewer are
easily carried when flow velocities of 15 cm/s (0.5 fps) are
achieved.2
Experience with SDGS has shown that the normal flows
which occur within the systems are able to keep the
mains free-flowing. Thus, SDGS need not be designed to
maintain minimum flow velocities during peak flows
although many state agencies require that minimum
velocities of 30-45 cm/s (1.0-1.5 fps) be maintained
during daily peak flow periods.
Maximum velocities should not exceed 4-5 m/s (13-16
fps). At flow velocities above this limit, air can be entrained
in the wastewaterthat may gather in airpocketsto reduce
the hydraulic capacity of the collector. Drop cleanouts or
manholes should be employed where the pipe gradient
results in excessive velocities.
4.3.1.3 Hydraulic Equations
Hydraulic equations used for design of the sewer mains
are the same as those used in conventional gravity
sewers. However, unlike conventional gravity sewers,
sections of SDGS systems are allowed to be depressed
below the hydraulic grade line such that flows may
alternate between open channel and pressure flow,
Therefore, separate analyses must be made for each
segment of the sewer in which the type of flow does not
change.
.,. . / - '. . -
Both Manning's and Hazen-Williams pipe flow formulas
are used. Roughness coefficients used range from 0.009
to 0.015 for Manning's "n".and 100 to 150 for Hazen-
Williams "C". Typical "n" and "C" values are 0.013 and
162
-------
140 respectively.2 Nomographs and hydraulic elements
graphs may be found elsewhere.10
Design depths of flow allowed in the sewer mains have
been either half-full or full. Most older systems designed
with uniform gradients have used half-full conditions to
dictate changes in pipe size. However, systems with
variable gradientsaltowthecollectormaintobe surcharged
at capacity. In these systems, pipe size changes are
dictated by the relative elevation of the hydraulic grade
line to any service connection elevation.
Design procedures follow conventional sewer design
except in sections where pressure flow occurs. In these
sections, the elevation of the hydraulic grade during daily
peak flow conditions must be determined to check that it
is fowerthan any interceptortankoutlet invert. If not, free-
flowing conditions will not be maintained at every
connection. Where the hydraulic grade line is above a
tank invert, the depth of the sewer can be increased to
tower the hydraulic grade line, or the diameter of the main
can be increased to reduce the frictional headless or a
STEP unit can be installed at the affected connection to
lift the wastewater into the collector. If short term
surcharging above any interceptortank outlet inverts is
expected, check valves on the individual service lateral
may suffice to prevent backflow.
4.3.2 Collector Mains
4.3.2.1 Layout
The layout of SDGS is a dendriform or branched system
similar to that of conventional sewers except that the
mains are usually not laid down the street center line so
that expensive pavement restoration is avoided. In most
cases, SDGS are located alongside of the pavement in
the street right-of-way. If there are numerous services on
both sides of the street, collectors may be provided on
both sides to eliminate pavement crossings. Another
alternative is to locate the collectors down the back
property lines to serve a whole block with one collector.
The backlot alternative may be the most accessible to
homeowners since most septic tank systems are located
in the backyard. Therefore, homeowners are not required
to reroute the building sewer to the front of the lot, but
access for interceptor maintenance may be limited.
Since new interceptor tanks are usually installed, SDGS
are installed most often in the front of the lots. If necessary,
the building drains are redirected to the front.
4.3.2.2 Alignment and Grade
The horizontal alignment of SDGS need not be straight.
Obvious obstacles such as utilities, large trees, rock
outcrops, etc. should be avoided with careful planning,
but unforeseen obstacles can often be avoided by bending
the pipe. The radius of the bend should not exceed that
recommended by the pipe manufacturer forthe conditions
under which ft is to be installed.
The gradient of SDGS must provide an overall fall
sufficient to carry the estimated daily hourly peak flows,
but the vertical alignment need not be uniform. Inflective
gradients, where sections of the main are depressed
below the static hydraulic grade line, are permissible if
the invert elevation is controlled where the flow in the pipe
changes from pressure to open channel flow. The
elevation of these summits must be established such that
the hydraulic grade line does not rise above any upstream
interceptortank outlet invert during peak flow conditions.
Adequate venting must also be provided at the summit.
Between these critical summits, the profile of the sewer
should be reasonably uniform so unvented air pocketsdo
not form which could create unanticipated headlosses in
the conduit and excessive upstream surcharging.
4.3.2.3 Pipe Diameter
The pipe diameter is determined through hydraulic
analysis. It varies according the to the number of
connections and the available slope. The minimum
diameter used is typically 10 cm (4 in), but 5-cm (2-in)
diameter pipe has been used successfully in recent
projects. Where the 5-cm (2-in) diameter pipe is used,
the interceptor outlets have used flow control devices to
limit peak flows, and check valves to prevent flooding of
service connections during peak flow periods. The costs
of the flow control devices and check valves generally
cancel savings realized from the smaller pipe; 10-cm (4-
in) diameter pipe, therefore, is most commonly used as
a minimum size.
4.3.2.4 Depth
The depth of bu rial for the collector mains is determined
by the elevation of the interceptor tank outlet invert
elevations, frost depth or anticipated trench loadings.
Any of these conditions may control. In most cases,
designers do not attempt to set the depth such that all
connections can drain by gravity. Where gravity drainage
from a residential connection is not possible, STEP lift
stations are used. An optimum depth is selected to
minimize the total construction costs due to mainline
excavation and the installation of STEP units. However,
the depth must not be less than that sufficient to prevent
damage from anticipated loadings. Where the pipe is not
buried below pavement or subject to traffic loadings, the
minimum depth is typically 75 cm (30 in); however, a
depth of 60 cm (24 in) is considered minimum for
conventional pipe. Pipe manufacturer should be consulted
to determine the minimum depth recommended. In cold
climate areas, the frost depth may determinethe minimum
depth of burial unless insulated pipe is used.
163
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4.3.2.5 Pipe Materials
PVC plastic pipe is the most commonly used pipe material
in SDGS systems. Standard dimension ratio (SDR) 35 is
used in most applications, but SDR 26 may be specified
for road crossings or where water lines are within 3 m (10
ft). For deep burial, SDR 21 may be necessary. Where
the use of STEP un'rts is anticipated, only SDR 26 or 21
should be used for the affected segment of collector
mains because of pressurizing requirements and the
compatibility of pipefrttings. Typically, elastomerfc (rubber
ring) Joints are used, however, for pipe smaller than 7.5
cm (3 in) in diameter, only solvent weld joints may be
available.
HOPE has been used infrequently, but successfully. Pipe
joining is by heat fusion.
4.3.3 Service Laterals
Typical service laterals between the tank and the sewer
line are 10-cm (4-in) diameter PVC pipe, although laterals
as small as 3 cm (1-1/4 in) in diameter have been used;
they are not recommended. The service lateral should be
no largerthan the diameter of the collector main to which
H is connected. The connection is typically made with a
wye or tee fitting. Where STEP units are used, wye
fittings are preferred.
Occasionally, check valves are used on the service
lateral upstream of the connection to the main to prevent
backfloodi ng of the service connection du ring peakf lows.
If used, it is important that the valve be located very close
to the collector main connection. Air binding of the
service lateral can occur if the valve is located near the
interceptor tank outlet.
4.3.4 Interceptor Tanks
4.3.4.1 Location
The interceptor tanks should be located where they are
easily accessible for periodic removal of accumulated
solids. Typically, they serve a single home and are
located near the house between the foundation and the
collector main adjacent to or in place of the existing septic
tank. Single tanks serving a group of homes have not
been accepted by users because of fear of backups;-
therefore, tanks at each connection are recommended.
If the collector main is located on the opposite side of the
building, reversal of the building drainage may be
desirable, but not necessary. Access for maintenance is
the critical factor in location. In some projects, the tank
has been located in the public right-of-way to eliminate
the need for the utility district to enter private property to
pump the tank (Figure 4-6).
4.3.4.2 Design
Prefabricated, single-compartment septic tanks are
typically used for interceptor tanks in SDGS systems.
Most projects standardize the use of 3,785-L (1,000-gal)
tanks for all residential connections. For commercial
establishments, local septic tank codes are commonly
used to determine the necessary volume. For a given
volume, several tank designs may be available locally.
Shallow tanks, or tanks with the greater water surface
area for a given volume are preferred designs because
of the greater flow attenuation that they provide.
Inlet and outlet baffles are provided in conventional
septic tanks to retain solids within the tank. These baffles
are adequate for SDQS applications. The inlet baffle
must be open at the top to allow venting of the interceptor
tank through the building plumbing stack. On the outlet,
various "gas deflection" baffles or outlet screens may be
used to capture low density or neutral buoyancy solids
that might otherwise pass through the tank (Figure 4-7).
These devices are not necessary, however, since these
solids have not been shown to cause problems in SDGS
systems.
Flow control devices have been used on interceptor
outlets to limit peak flow rates to a predetermined
maximum. Surge chambers were added to interceptor
tanks in early projects.11 The surge chamber contained a
standpipe with a small orifice drilled near the bottom
(Figure 4-8). During peak flow periods, the chamber
provides storage for the wastewater while the orifice
controls the rate of flow from the tank. These chambers
are no longer used because the orifices plug readily so
the chambers are not effective in flow attenuation. They
also require about 30-45 cm (1.0-1.5 ft) of headless
which may require deeper burial of the collectors and, as
a result, higher construction costs. Also, odor problems
have resulted due to the free fall of available interceptor
tank effluent. Flow control devices are now available that
areplaced within the interceptortank and usethefreeboard
provided in the tank for storage (Figure 4-9).
Water-tightness is a critical criterion in selection of an
interceptor tank. For that reason, existing septic tanks
are infrequently converted to interceptor tanks. Earlier
systems attempted to use the existing septictank at each
home to reduce construction costs. It was found that
septic tanks are difficult to inspect and repair properly.
SDGS systems reviewed which had significant numbers
of old tanks all had high ratios of wet weather to dry
weather flows.2 Common practice now is to replace all
tanks. Currently, there is no standard procedure for
existing tank leakage inspection. This practice has the
added advantage of requiring the property owner to
replace the building sewer to ensure greater
164
-------
Figure 4-6.
Alternative locations for interceptor tanks.
, PL.
PL
1 INTERCEPTOR
', TANK
INTERCEPTOR
TANK
PL
SERVICE
LATERAL
INTERCEPTOR
TANK
PL'
R/W
COLLECTOR MAIN
Figure 4-7.
Typical interceptor tank outlet baffles.
FLOW LINE
OUTLET SCUM
BAFFLE
FLOW LINE
GAS DEFLECTION
BAFFLE
TANK OUTLET
PIPE
165
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166
-------
watertightness. Some projects incorporate the
replacement of the building sewer to allow inspection of
the building plumbingto eliminate roof leaders, foundation
drains and other unwanted connections that contribute
clear water inflow.
Access to the tank for periodic inspections and solids
removal is required. A sufficiently large opening over the
tank inlet or outlet to allow inspection and effective sludge
removal should be provided. However, because of the
tank'sseptic conditions, unsupervisedorunaccompanied
personnel must not enter the tank. All applicable safety
codes must be followed in the design of these facilities.
The opening should be a minimum of 45-cm (18-in)
square or in diameter. A watertight riser terminating 15
cm (6 in) above grade with a bolted or locking air tight
cover is preferred to a buried access.
4.3.4.3 Material
Prefabricated septic tanks are typically used for interceptor
tanks. They are available in reinforced concrete, coated
steel, fiberglass and high density polyethylene.
Unfortunately, the quality of manufacture varies from
locality to locality. Therefore, it is necessary to carefully
inspect and test random tanks for structural soundness
forthe intended application andfor watertightness. Coated
steel tanks are not recommended because the coating is
easily damaged during storage and installation, leading
to severe corrosion and short tank life.
All tank joints must be designed to be watertight. The
joints include tankcovers, manhole risers and covers and
inlet and outlet connections. Rubber gasket joints for inlet
and outlet connections are preferred to provide some
flexibility in case of tank settlement.
4.3.5 Manholes and Cleanouts
In most SDGS systems, cleanouts are used instead of
manholes, except at major junctions at mains. Since
hydraulic flushing is all that is necessary to maintain the
mains in a free-flowing condition, cleanouts provide
sufficient access to the mains. Cleanouts are less costly
to install than manholes and are not a sourceof infiltration,
inflow or grit. Since the SDGS system is not designed to
carry grit, elimination of manholes is strongly
recommended. Manholes represent a potential sourceof
odors and of grit and other solids in SDGS systems.
Cleanouts are typically located at upstream termini of
mains, junctions of mains, changes in main diameter and
at intervals of 120-300 m (400-1,000 ft) (Figure 4-10).
Cleanouts may also be used in place of drop manholes.
The cleanouts are typically extended to ground surface
within valve boxes.
Manholes, if used, are located only at major junctions.
The interiors should be coated with epoxy or other
chemical resistant coating to prevent corrosion of the
concrete. The covers used are typically gas-tight covers
to limit the egress of odors and inflow of clear water.
Where depressed sections occur, the sewer must be well
vented. Cleanouts may be combined with air relief valves
at high points in the mains (Figure 4-4) or an open vent
cleanout installed (Figure 4-11).
4.3.6 Valves
Air release, combination air release/vacuum and check
valves may be used in SDGS systems. Air release and
combination air release/vacuum valves are used for air
venting at summits in mains that have inflective gradients
in lieu of other methods of venting. These valves must be
designed for wastewater applications with working
mechanisms made of type 316 stainless steel or of a
plastic proven to be suitable. The valves are installed
within meter or valve boxes set flush to grade and
covered with a water tight lid (Figure 4-4). If odors are
detected from the valve boxes, the boxes may be vented
into a small buried gravel trench beside the boxes (see
Section 2.4.8).
Check valves are sometimes used on the service
connections at the point of connection to the main to
prevent backflow during surcharged conditions. They
have been used primarily in systems with 5-cm (2-in)
diameter mains. Many types of check valves are
manufactured, but those with .large, unobstructed
passageways and resilient seats have performed best.
Wye pattern swing check valves are preferred over tee
pattern valves when installed horizontally. Although the
systems with 10-cm (4-in) diameter mains have operated
well without check valves, their use can provide an
inexpensive factor of safety for these applications as
well. An alternative method used to prevent pumping
backups in some projects has been to provide an
interceptor tank overflow pipe to the drain field of the
abandoned septic tank system. Care must be exercised
to prevent backflow through such connections in areas
with high groundwater. In Australia, a "boundary trap" is
included at every connection which provides an overflow
to the ground surface if backups occur (Figure 4-12).ia
4.3.7 Odors and Corrosion
Odors are a commonly reported problem with SDGS
systems. The settled wastewater collected by SDGS is
septic and therefore contains dissolved hydrogen sulfide
and other malodorous gases. These gases tend to be
released to the atmosphere in quantity where turbulent
conditions occur such as in lift stations, drop cleanouts or
hydraulic jumps which occur at rapid and large changes
167
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Fig ur» 4-1 On. Typical cleanout detail.
CONG. COLLAR
HEAVY DUTY
WATER LINE CASTING
Q.-'i
, .'..-
V - . J * .' .
--••' • '-. «-.• •.•'.""
2" + PIPE DIA.
GRANULAR MATERIAL
COMPACTED
4-, 6". OR —7
8" PIPE /
168
-------
Figure 4-1 Ob. Typical cleanout detail.
4^ BEND
(b) ELEVATION
GRADE SURFACE AWAY
FROM CLEANOUT
CLEANOUT
ADAPTER &
THREADED
PLUG
IOS
COMPACTED VDH & STONE
OR SAND TO SUPPORT RISER
169
-------
Figure 4*11. Ventilated cieanout detail.
24"
GRADE SURFACE AWAY
FROM CLEANOUT
RETURN BEND OR TWO 90°
BENDS WITH CLOSE NIPPLE
FIT END WITH 8 MESH 18 QA.
STAINLESS STEEL SCREEN
fiHUrn
18" SQUARE CONCRETE PAD
COMPACTED VDH & STONE
OR SAND TO SUPPORT RISER
~)D1RE
——$ OF Pi
DIRECTION
OF FLOW
(MIN)
170
-------
Figure 4-12, Australian boundary trap detail.
VENT (MOSQUITO PROOF)
REINFORCED COVER & BLOCK
jv * ' /
-------
of grade or direction in the collector main. The odors
escape primarily from the house plumbing stack vents,
manholes or wet well covers of lift stations.
The odors have been controlled by minimizing turbulence
and sealing uncontrolled air outlets. Drop inlets have
been effective in eliminating odors at lift stations (Figure
4-13). Gas-tight lift station covers should be installed if
odors are persistent and odor control provided for the
fresh air vent. An effective odor control measure is to
terminatethe vent in a buried gravel trench (Rgure 4-14).
Carbon filters have been used successfully, but require
regular maintenance. Manholes should be replaced with
cleanouts; however.if used, the manholes should have
gas-tight covers. Odors from improperly designed house
plumbing vents have been controlled most easily by
sealing the vent on top of the interceptor tank outlet tee
or by installing water traps in the service lateral.
The atmosphere created by the released gases is very
corrosive. Corrosion is a common problem in lift stations.
Corrosion resistant materials must be used (see Section
2.4.8). More recent SDGS systems have used wet well/
dry welt design for lift stations to reduce the exposure of
mechanical components to the corrosive atmosphere.
4.4 Construction Considerations
4.4.1 General
Construction of small diameter gravity sewers is similar
to construction of conventional gravity sewers except
that strict horizontal and vertical control of main alignment
is not required. As a result, construction can proceed
much more quickly and be less costly. However, SDGS
systems require that a significant portion of the work be
performed on private property to install the. interceptor
tank and service lateral. Because the property owner can
be very demanding in surface restoration, many general
contractors are uncomfortable with such work. As a
result, the bids received for construction may be few and
inflatedto account forthe workonthe service connections.
To maximize the potential for cost savings, consideration
should be given to letting two construction contracts, one
to install the mains and the other to perform all work on
private property. The contractor for the private work'
shouJdbeacontractorwho is experienced in working with
property owners such as a local septic tank system.
Installer.
4.43 Mainline Construction
.':.»
4.4.2.1 Une Changes ;
Setting the line of the collector main should be performed
with the objective of minimizing site restoration costs.
Detailed surveys during the design phase may not have
been performed because of cost'considerations. As a
result, all obstacles in the intended path of the main may
not have been identified on the plan sheets. Since
straight alignment is not required for SDGS, changes in
the alignment within maximum pipe deflection limits can
be made in the field to avoid large trees, fences, pavement,
etc. that could increase restoration costs. Most changes
can be made by the construction manager, but major
changes in alignment should be evaluated by the design
engineer. Any changes made should be documented
and shown on the as-built drawings.
4.4.2.2 Grade Control
Strictvertfcalcontrol of SDGS during mainline construction
is not necessary. In most cases, the pipe may be joined
above ground and laid in the trench. However, the pipe
should be laid as uniformly as is reasonable to minimize
headlosses and potential points where gas can collect, if
significant changes in the pipe profile are required to
avoid utilities or at various crossings, they must be
evaluated by the design engineer for air release valves.
All changes must be documented and recorded on the
as-built drawings.
4.4.2.3 Trench Construction
Trenching may be done by backhoe or trenching
equipment. Over-excavation is not a critical concern if
the change in the pipe invert elevation is not greaterthan
one pipe diameter nor so sudden that the integrity of the
pipe is threatened.
Select backfill for bedding and surrounding the pipe is
necessary if the native trench spoil contains cobble or
does not fill around the pipe snugly. Granular materials
such as medium or coarse sand or pea gravel is usually
used. Local requirements may control. To help locate the
pipe or warn excavators working in the area, color coded
warning tape should buried in the backfill 10-20 cm (4-8
in) below grade directly over the main. Pipeline markers
which relate the pipe to existing permanent above ground
structures should also be used.
4:4.3 Service Connections
Service connections include the building sewer,
interceptor tank and service lateral to the collector main.
Usually, the utility district is responsible for installation of
the interceptor tank and service lateral while the user is
responsible for installing the building sewer and its
connection to the interceptor tank. However, in some
cases, the utility district has also taken responsibility for
installing the building sewer to help ensure a watertight
connection and minimal inflow from illegal connections.
In laying out the service connection, the property owner
should be involved. The owner should be consulted
172
-------
Figure 4-13.
Examples of drop Inlets, external (a) and Internal (b).
INCOMING
SEWER
DROP
INLET
DISCHARGE
SUSPENDED LEVEL
CONTROL SENSORS
SUBMERSIBLE
SEWAGE PUMP
a) OUTSIDE DROP INLET
b) INSIDE DROP INLET
INLET SEWER
STRAP SET AS
ORDERED
173
-------
Flgur»4-14. Soil odor Biter detail
a) SIDE VIEW
,1
1
j
t
1
J
1
t
12- M!N
12-
MIN.
r
k
18"
MIN.
(
12"
MIN.
f
III
^
q
^- — -s
[ — '
V
1 VENTED CAP
- 4 ,$5_
||n= GEOTEXTILE FABRIC
III! ^
a-
""£=;' <^ <^* ^ ^ f~!.../rzi..e^ <=>. r-> <^» r^
rJ Ti=- 4" PERFORATED PVC PIPE
b-l It i v
1 \\ ^
I oO
~|_J c- s — **^ ''^ — ^—^ — '— * — *— -* — ^^ — i!— * — <=s — '^~^ 1 j jEDy
10' fc
SEASONALLY HIGH WATER TABLE
Ai
VENT FROM
LIFT STATION
b) FRONT VIEW
18-
'« -
VENTFROM
LIFT STATION
OL VENT TO
* ATMOSPHERE
24- MIN.
174
-------
concerning the preferred location of the tank and service
lateral, existing utilities and other areas of the property to
be avoided. The proposed route and any areas to be
affected by the construction activities should be video
taped to provide a reference during restoration work.
Early SDGS systems attempted to utilize existing septic
tanksat each connection as interceptortanks to minimize
construction costs. In such instances, the existing tanks
were pumped and inspected prior to being accepted by
the utility. However, where a significant number of existing
tanks were included, clear water infiltration and inflow
has been a problem. Current practice is to install all new
interceptor tanks to limit infiltration and inflow. To provide
the tightest system as possible, the user should be
required to install a new building sewer subject to a leak
test by the utility district. The existing tank must be
pumped and abandoned by removal or by destruction
and filling with inert solid material.
The interceptor tanks should be located where they can
be reached easily for routine pumping by vacuum trucks.
However, the tanks should be clear of any area subject
to vehicular traffic. To facilitate maintenance, the
interceptortanks have been located in the public right-of-
way in some projects. This approach avoids many of the
problems associated with construction on private property,
but does increase the hookup cost to the property owner
since the owner is usually responsible for the building
sewer to the interceptor tank.
Tank installation must follow the manufacturer's
specifications. Proper bedding and flexible, water-tight
inlet and outlet connections must be used. Flotation
collars maybe required to prevent flotation when the tank
is pumped in areas that experience high water conditions.
As-buiK site plans should be prepared after the service
connection is made. The plans should show the location
of all elementsof the service and referenced to permanent
structures on-srte. Where possible, photographs should
be included in the record for the utility district.
4.4.4 Testing
Water-tightness testing of the collection mains, interceptor
tanks, service connections and building sewers should
be performed during construction. Testing procedures
and criteria appropriate for conventional gravity sewers
are used for testing the mains, service connections and
building sewers. Both vacuum and hydrostatic tests are
used for interceptor tanks. Typical acceptance criteria
are less than 2.5 cm (1 in) toss of Hg vacuum after 5
minutes with an initial vacuum of 10 cm (4 in) of Hg or a
drop in water level of 2.5 cm (1 in) after 24 hours in an
overfilled tank. With the hydrostatic test, it is necessary
that the tank be filled to at least 60 cm (2 ft) above the top
ofthetanktocheckcovers and manhole riser connections.
Typically, the property owner is responsible for testing of
the building sewer to the satisfaction of the utility district
before connection.
4.5 Operation and
Considerations
Maintenance
4.5.7 Administration
Utility or special purpose districts are commonly formed
to administer, operate and maintain SDGS systems
located outside municipal boundaries. These districts
vary in structure and powers from state to state, but they
typically have most of the powersof municipal government
except for methods of generating revenues.
The sewer utility should be responsible for maintenance
of the entire system. This includes all interceptor tanks
and any appurtenances such as STEP units located on
private property. Typically, the utility district assumes
responsibility for all SDGS components downstream
from the interceptor tank inlet. In some projects, the
responsibility for maintenance of the components located
on private property have been left to individual property
owner. This avoids the need to enter private property.
However, since the interceptor tank is critical to the
proper performance of the SDGS system, responsibility
for maintenance should be retained by the district. It is
strongly recommended that the district assu me ownership
or equivalent responsibility for the interceptor tank and
the components downstream of the tank to ensure
access and timely appropriate maintenance.
To obtain access to the SDGS components located on
private property, perpetual general easements are
typically secured from the owner. The easements can
take several forms, but general easements or easements
by exhibit are recommended over metes and bounds
easements because of the time and expense of writing
metes and bounds. In some cases, the easements are
obtained without compensation to the owner, while, in
others, a nominal charge is provided. In ail cases,
property owners are entitled to some compensation
under the Uniform Relocation act; but waiver thereof
implies their support for the community's endeavor.
Where it is necessary to cross private property with the
collector mains, metes and bounds easements are usually
used. An example of a general easement is presented in
Figure 4-15
4.5.2 Operation and Maintenance Manual
An operation and maintenance manual is essential to
every project. Although most ^maintenance tasks are
relatively simple and usually do not involve mechanical
175
-------
Figure 4-15. Exampl B of general easement.
KNOW ALL MEN BY THESE PRESENTS:
That, In consideration of One Dollar and other good and
valuable consideration paid to the undersigned
respectively, hereinafter referred to as GRAN! ORS by
the utility district, hereinafter referred to as GRANTEE,
the receipt whereof is hereby acknowledged, the
GRANTORS each, f ortheir respective heirs, distributees,
personal representatives, successors and assigns, do
hereby grant, bargain, sell, transfer, convey, release,
quit claim and remise unto the GRANTEE, its successors
and assigns, a PERPETUAL EASEMENT to erect,
construct, install, lay, use, operate, maintain, inspect,
alter, clean, remove and replace sewer pipes, pumps,
interceptor tanks and all appurtenances necessary and
Incident to the purposes of the easement, and, in
connection with the same, temporarily to place machinery
and materials which may be necessary to effect the
purposes of the easement upon lands of the respective
GRANTORS situate in the name of county and state
TOGETHER WITH the right of ingress and egress over
adjacent lands of GRANTORS, their respective heirs,
distributees, personal representatives, successors and
assigns, as the same may be required in order to effect
the purposes of the easement. The location of the
easement on the landsof each GRANTOR is respectively
shown on Sheet No. of Contract No. forthecontract
drawings of the local entity, dated .
The GRANTEE expressly agrees that any and all
disturbance to the surface of the lands of the GRANTOR
will be promptly repaired and to the extent possible
restored to their pre-existing condition, whether such
disturbance takes place during the initial installation or at
any time thereafteras may be occasioned by subsequent
repairs or maintenance to the said sewer line and
interceptor tank with the easement area.
Executed at the local entity on the respective dates as
follows:
Date
Signature Street Address Tax Acct.
equipment, the manual does provide a valuable reference
for location of components and services and typical
drawings detailing the design and construction of each
component. In addition, the manual should contain a
comprehensive maintenance log to document all
maintenance performed and any performance problems
and the corrective actions taken. A good manual should
contain, at a minimum, the following:
1. Description of the system
Adescriptfonofthesystem and eachof its components
should be provided. The component descriptions
should include the function of each, their relation to
adjacent components and typical performance
characteristics. Specific design data, shop drawings,
as-built plans and profiles of the collector mains and
detailed plan drawings of each service connection
are essential.
2. Pescription of the system operation
Normal operation, emergency operation situations
and procedures and failsafe features should be
described.
3. System testing, inspection, apd monitoring
The purpose, methods, and schedule of all
recommended testing, inspections and monitoring
should be described. Sample recording forms should
also be included.
4. Preventive maintenance procedures and schedules
A clear description of all preventive maintenance
procedures is needed with specific schedules for
their performance.
5. Troubleshooting
A description of common operating problems, how
they may be diagnosed and procedures to correct
them is extremely helpful to O&M personnel.
6. Safety
Safety practicesand precautions should be described
to alert personnel to the potential hazards and
methods to avoid or mitigate them. The dangers of
working with septic wastes which generate dangerous
hydrogen sulfide and methane gases must be
emphasized.
7. Recordkeeping Logs and Forms
Sample recordkeeping forms and logs should be
provided.
8. Equipment Shop Drawings and Manuals
Shop drawings and installation and maintenance
manuals of all major equipment should be included.
176
-------
Manufacturers and their suppliers should be listed
with contact names, addresses and telephone
numbers.
9. Utilities List
A list of all utilities in the project area, location maps
and contact names, addresses and phone numbers
should be provided.
10. System Drawings
Complete as-built drawings of the system are
necessary. Detailed drawings of the service
connections showing the precise location of all
components with maintenance logs for each should
be included.
4.5.3 Staff and Equipment Requirements
Operation and maintenance requirements of SDGS
systems are generally simple in nature, requiring no
special qualifications for maintenance staff other than
familiarity with the system operation. The operator's
responsibilities will be limited largely to service calls, new
service connection inspections and administrative duties.
In most systems, interceptor tank pumping is usually
performed by an outside contractor under the direction of
the utility district.
Maintenance equipment is also limited. A truck mounted
centrifugal suction pump can be used to provide most
emergency operation equipment needs. Sufficient hose
should be purchased to reach between cleanouts. Other
equipment can be provided by outside contractors as
needed.
For STEP installations (including lift stations), O&M
requirements are described in Section 2.6.
4.5.4 Operator Training
Specialized training for SDGS maintenance personnel is
not necessary. Basic plumbing skills, however, are
desirable. If a significant number of service connections
include STEP units, an understanding of pumps and
electrical controls is also helpful. (For a small number of
such units, ft is common forthe utility district to retain local
plumbing and electrical contractors to be available for
any necessary repairs.)
The staff should be aware of the dangers of exposure to
sewer gases and to avoid entry into confined spaces
unless properly protected. Since a significant portion of
the system is located on private property, it is important
that the staff have good communication skills and a
willingness to work with residents.
4.5.5 Spare Parts Inventory
Because SDGS systems have few mechanical parts, the
need to maintain a spare parts inventory is limited.
However, if individual STEP units are included in the
system requiring that spare pumps and controls must be
available for emergency repairs. A minimum of two spare
pumps and the associated float switches and controls
should be maintained for small systems (see Section
2.6.4 for furtherdiscussion). Pipe and pipe fittings should
be kept on hand to repair any pipeline breaks that may
occur. Spare interceptor lids and riser rings should also
be kept.
4.5.5 As-Built Drawings
As-buitt drawings of the entire SDGS system including all
on-lot facilities are essential to maintenance of the system.
Curvilinear alignments and few manholes or cleanouts
make locating the collector main routes difficult unless
accurate drawings tying the location of the line to
permanent structures are developed. As-built drawings
of each individual service connection should also be
made. These drawings are necessary when repairs are
needed or when the components must be located to
avoid damage due to other construction activities.
4.5.7 Maintenance
4.5.7.1 Normal
Normal maintenance is generally limited to call-outs by
users. The call-outs are usually due to plumbing backups
or to odors. In nearly every case reported, the plumbing
backups were due to obstructions in the building sewer.
Although the building sewer is the property owner's
responsibility, most utilities have assisted the owner in
clearing the obstruction. Odor complaints are common.
As with the plumbing backups, faulty venting in the
building plumbing is usually the cause. If improved
venting fails to eliminate the odor complaints, the
interceptor inlet vent can be sealed or running traps
placed in the service lateral to prevent the sewer main
from venting through the service connection.
4.5.7.2 Preventive
Preventive maintenance includes inspection and pumping
of the interceptor tanks, inspection and cleaning of the
collection mains and inspection and servicing of any
STEP units or lift stations.
a. Interceptor Tanks
The interceptor tanks must be evacuated periodically to
prevent solids from entering the collector mains.
Prescribed pumping frequencies are typically 3-5 years,
but operating experience indicates that a longer time
between pumpings, of 7-10 years, is usually adequate
(see Section 2.4.4). Restaurants and other high use
177
-------
facilities, such astavems, require more frequent pumping.
Common practice is to require additional grease removal
andpumptanksservingthesefacilitiesevery6-12months.
Tank inspection is usually performed immediately after
the tank has been evacuated to check for cracks, leaks,
baffle Integrity and general condition of the tank. If
effluent screens are used on the tank outlet, they must be
pulled andcleaned by flushlngw'rth water. Annual flushing
of the screens is recommended if they are to be effective.
A preliminary septage handling and disposal plan must
be developed which complies with existing State and
Federal regulations. However, most utilities do not perform
the pumping themselves. Private pumpers are usually
hired through annual contracts to pump a designated
numberoftanks each year and to beon call foremergency
pumping. The utility is responsible for the conduct of its
contractors and must provide oversight to ensure
compliance. The septage removed is usually land spread
ordischarged into a regional treatment plant.13 During the
pumping operations, utility district personnel should be
present to record the depth of sludge and thickness of
any scum in each tank so that the schedule can be altered
according to actual accumulation rates.
b. Collector Mains
Periodic inspection and cleaning of the collector mains
are the usually recommended maintenance functions.
Hydraulic flushing is most often recommended for
cleaning. Pressure hoses to push "pigs" through the
mains have also been suggested as a cleaning method,
but are not recommended if the collector mains are SDR
35 pipe. Reported performance of systems has been
good and, therefore, inspection and flushing has not
been deemed necessary by most utilities and has seldom
been performed. In systems where the mains have been
inspected, no noticeable solids accumulations have been
noted. The experience with SDGS in Australia is similar.
Many large systems there have been operating over 30
yearswithoutmaincieaning.However.reguIarinspection
and flushing is still recommended for long flat sections in
which daily peak flow velocities are less than 15 cm/s (0.5
fps).
c. Lift Stations
Mainline lift stations should be inspected on a daily or
weekly basis. Pump operation, alarms and switching
functions should be checked and running times of the
pumps recorded. The discharge rate of each pump
should be calibrated annually.
4.5.7.3 Emergency Calls
Mainline or service lateral obstructions and lift station
failures require that emergency actions be taken to limit
the time the system is out of service to prevent
environmental or property damage that might occur. It
requires that the utilities have defined emergency
operation procedures.
a. Obstructions
If an obstruction occurs, the utility must be able to
respond quickly such that backups do not occur at
upstream service connections. Experience has shown
that most obstructions are caused by construction debris
which cannot be removed by simple flushing. It may
require that the main be excavated to remove the
obstruction. While the obstruction is cleared, the utility
must be prepared to pump from the cleanout, manhole or
interceptor tank immediately above the obstruction to a
cleanout or manhole below. A centrifugal suction pump
or truck-mounted pump works well for this.
Fortunately, obstructions have been rare. All reported
obstructions have occurred soon after construction or
after an improperly inspected service connection was
made. Construction debris has been the cause.
Obstructions from other causes have not been reported.
b. Lift Stations
Lift stations may fail due to loss of power or a mechanical
failure. Standby emergency generators can be provided
for power during prolonged outages, but the generators
can be costly and require regular maintenance. Because
of the costs, many small communities have provided
added storage atthe lift station (Figure 4-16) and/ortruck
mounted pumps that can pump from the wet well to a
downstream hose connection on the force main (Figure
4-17). This latter method also works well for mechanical
failures.
4.5.8 Record Keeping
Good record keeping of all operation and maintenance
duties performed is essential for preventive maintenance
and trouble shooting when problems occur. A daily log
should be kept and maintenance reports on all equipment
filed. Flows atthe mainline lift stations should be estimated
daily by recording the pump running times. This is helpful
in evaluating whether infiltration or inflow problems are
developing. A record of each service call and corrective
action taken should be filed by service connection
identification number. This record should include tank
inspection and pumping reports. These records are
particularly useful if reviewed just prior to responding to
a service call out.
4.5.9 Troubleshooting
4.5.9.1 Odors
Odors are the most frequently reported problem with
SDGS systems. Odors typically occur at lift stations and
178
-------
Figure 4-16.
Mainline lift station with emergency storage.
11" GALVANIZED VENT PIPE
' 10' MIN. HEIGHT ABOVE GRADE
-» TO CONTROL PANEL
PUMP CONTROL [•"
SWITCHES
PIPE GALLERY
FOR
OVERFLOW STORAGE
SL DE AWAY COUPLING
179
-------
FJgtw« 4-17.
Emergency pumping manhole.
STD. S'.DIA. MANHOLE
WITH TYPE 2 FRAME
AND TYPE A COVER
FORCEMAiN
FROM
LIFT
STATION
THREADED BRASS NIPPLE, QUICK
DISCONNECT COUPLING & CAP
PLUG VALVE WITH
OPERATING HANDLE
. ^
180
-------
from house plumbing stack vents, particularly at homes
located at higher elevations or ends of lines. Odors are
most pronounced where turbulence occurs. The
turbulence releases the obnoxious gases dissolved in
the wastewater.
Odors at lift stations have been successfully eliminated
by installing drop inlets that extend below the pump shut
off level. This eliminates most of the turbulence. Other
successful corrective measures include soil odor filters
(Rgure 4-14), air tight wet well covers and vents that
extend 3-5 m (10-15 ft) above grade.
Odors at individual connections often originate in the
collection main. If a sanitary tee or similar baffle device
is used at the interceptortank inlet or outlet, the top of the
tee can be sealed or capped to prevent the gases
escaping into the building sewer. P-traps or running traps
on the service lateral have also been used (Rgure 4-12).
In some cases, extension of the main further upslope to
where ft can be terminated in a vented subsu rface gravel
trench has been employed successfully. The trench
filters the odors before venting the gas to the atmosphere.
4.5.9.2 Corrosion
Corrosion is a problem that is usually most evident at lift
stations and manholes. Nonferrous hardware must be
used in lift station wet wells. Concrete manholes and wet
wells must be coated with corrosion resistant materials.
Alternatively, corrosion problems can be reduced in lift
stationsby using wet well/dry well const ruction with awell
vented wet well.
4.5.9.3 Infiltration/Inflow
Clear water I/I was a common problem with earlier SDGS
systems that used a high percentage of existing septic
tanks for interceptor tanks. Leaking tanks or building
sewers were the primary entry points of clear water.
Systems that have installed all new interceptortanks and
pressure test building sewers and tanks have few I/I
problems. , . _
4.6 Review of Operating Systems
Because knowledge of the performance of operating
small diameter sewer systems in the United States is
relatively sparse, twelve operating systems were selected
for review. The systems selected were based on size,
date on-line, type of design (uniform grade versus variable
grade), and local terrain. A summary of the projects
selected for detailed review is presented in Table 4-1.
Summaries of the interceptor tank and collector main
designs are presented in Tables 4-2 and 4-3. Comparisons
of system component use as a function of total feet of
collector main installed are presented in Table 4-4.
All the systems reviewed have performed well.
Obstructions have not occurred in any of the systems
despite the fact that mainline flushing has not been
performed. Odors have created nuisance problems in
several systems, but control measures have been
effective. Corrosion is a common problem in lift stations
because the corrosive atmosphere created by the septic
waste was not anticipated. In the systems where existing
septic tanks and building sewers were retained, infiltration
has been a significant problem. This has not been the
case where newtanks and building sewers were installed.
Routine maintenance is limited to weekly inspection of
mechanical equipment and pumping of the interceptor
tanks every 5-7 years. Call outs have been infrequent
and usually due to problems in the building sewer rather
than the systems themselves.
4.7 System Costs
4.7.1 Construction
Construction costs were obtained from the twelve SDGS
systems reviewed. Total construction costs adjusted to
January, 1991 are presented in Table 4-5. Unit costs of
components and their averages adjusted to January,
1991 are presented in Table 4-6. The costs of the
components per foot of pipe installed and their adjusted
averages for the various projects are presented in Table
4-7. Table 4-8 presents the component costs as a
percent, of the total construction costs.
Based on the costs from the twelve systems, the
components were ranked from most to least costly:2
1. Collector mains
2. Interceptor tanks (including service lateral)
3. Mainline lift stations
4. Pavement restoration
, 5. Crossings (road, stream, utility)
6. STEP lift stations
;7. Manholes
8. Site restoration
9. Force main
10. Miscellaneous
This ranking suggests in which areas efforts should be
made in system design and construction to reduce the
total costs.
Costs of installing the collector mains and the interceptor
tanks and service laterals typically account for over 50
percent of the total cost of construction. The exact order
of the other construction cost categories will depend on
the characteristics of the individual project. Therefore,
181
-------
Summwy of SDQS ProJ*cts Rtvbwtd
oo
ro
Community
Mt Andrew,
AL
Westboro,
Wl
Badger, SD
Avery, ID
Maplewood,
Wl
S. Corning,
NY
New Castle,
VA
Miranda,
CA
Gardiner,
NY
Lafayette,
TN
No.
Pop. Features Connections Total 3-ln 4-ta
(«)
100 Gently sloping. 31 2,500 50{3-In)
50 (2-ln)
200 Gently sloping. Deep 87 18,846 - 77
well-drained soils.
105 Flat to gently rolling. 53 6,616 - 77
90 Narrow, steep-sided 55 6,690 - 100
mountain valley bottom.
Moderately deep soils.
150 Flat. Very shallow 61 5,800
creviced bedrock.
2,000 Flat valley bottom. 606 45,525 - 77
Steep side slopes.
Poorly-drained soils.
190 Gently sloping. High 64 6,955 - 64
seasonal water table.
Bouldery soil.
300 High, moderately to 100 9,617 9 91
steeply sloping river
terrace. Deep alluvial
soils.
500 Gently sloping. 109 19,330 - 71
1,500 Top of plateau and 510 45,310 - 47
steep side slopes.
Length LemrfV
6-ln 8-fn Pressure Connection Comments
(%) (ft)
81 *lnfJective gradient with sections
depressed below HGL
• 2-ln minimum diameter drains.
• No manholes or daanouts.
• Some pressure Inlets.
5 18 217 » Uniform gradient
« Curvilinear alignment between
manholes.
« Hybrid gravity/pressure system.
23 - - 125 « Uniform gradient
122 * Uniform gradient
• No horizontal control maintained
during construction.
* Pipe gallery reserve storage in
lift stations.
100 95 • Uniform gradient
* Emergency pump manholes
below each lift station.
23 - - 70 • Uniform gradient
• "Sump manholes" Isolate
sections of network.
36 - 109 * Uniform gradient
96 • Uniform gradient.
19 10 - 177 • Uniform gradient
• Curvilinear alignment between
manholes and cleanouts.
53 - - 89 • Uniform gradient
• Curvilinear alignment between
manholes and cteanouts.
• Aerated lift stations for odor control.
Data
On-Une
July 1975
Sept 1977
Nov. 1980
Sept 1981
Nov. 1981
July 1983
May 1982
Nov. 1982
Dec. 1982
Sept 1983
-------
Table 4-1.
Summary of SDGS Projects Reviewed (continued)
Community
West Point,
CA
Zanesville,
OH
Muskingham
Co., OH
No.
POD. Features Connections Total
430 Very hilly with steep 155 18,000
slopes. Shallow bedrock.
1,880 Very hilly with steep 711 61,362
slopes. Shallow bedrock.
2,150 Very hilly with steep 767 89,748
slopes. Shallow bedrock.
Length
3-in 4-in 6-in
100 (2-in)
18 (2-in) 2
10 (3-in)
79 (2-in) 3 2
16 (3-in)
Length/
8-in Pressure Connection
(ft)
116
70 86
117
Comments
• Inflective gradient with sections
depressed below HGL.
• 2-in minimum diameter mains.
• Buried at constant depth, but no
sections depressed below HGL.
• 2-in minimum diameter mains.
• Inflective gradient with sections
• 2-in minimum diameter mains.
• Surge tanks installed following
interceptor tanks.
Date
On-Line
Nov. 1985
Oct. 1986
Nov. 1986
-------
TftbI»4-2.
Summwy of (nt«c»ptor Tank ChwectwlBlJcs Used In S«!«ct»d Pro]*ct»
Repiacom8ntRat8(%)
Community
Mt Andrew,
AL
Westboro,
W!
Badger, SO
Avery, ID
Maplewood,
Wl
S. Corning,
NY
New Castle,
VA
Miranda,
CA
Gardiner,
NY
Lafayette, TN
West Point,
CA
Zanesville
OH
Muskingham
Co., OH
Sizing Criteria
508-gal tank for all connections.
Wisconsin Administrative Code
1,000-gal tank for all connections
1,000- gal single residence
1,500-gal double residence
Wisconsin Administrative Code
1 ,000-gal tank for all connections
2.5-day detention time based on
metered flow
2-day detention time based on
metered flow
New York Administrative Code
Tennessee Administrative Code
1,000-gal tank for all residential
applications
1 ,000-gal tank for all applications
1 ,000-gal tank for all applications
Mn. Size (gal)
508
1,000
1,000
1,000
1,000
1,000
1,000
780
750
750
1,000
1,000
1,000
Access
• Burled manhole
* Buried manholes
* 4-in Inspection port to grade
• Burled manhole
• 6-in Inspection port to grade
• Buried manhole
• 4-in Inspection port to grade
• Buried manhole with
O-ring gaskets
• 4-in inspection port to grade
• Buried manhole
• 6-in inspection port to grade
* Manhole to grade
* Buried manhole
• 4-in Inspection port to grade
• Burled manhole
• Manhole to grade
• Manhole to grade
• Manhole to grade
Special Features Estimated
• Precast concrete 1 00
• Outiet consists of 6 2-In darifler tubes @ 60 degrees
• Precast concrete 90
• Precast concrete 25
* Precast concrete 1 00
* Double layer mastic on risers
• Precast concrete 100
• Located near road to minimize length of connection
• Precast concrete with waterproof coating 75
« Wall baffles with horizontal tea section on outlet
• 2 outlets: upper one to existing dry well to act as overflow
• Precast concrete 46
• Fiberglass tanks <1 ,500-gal capacity 75
• Precast concrete 2-chambers >1 ,500-gal capacity
• Coated and water tested
* Structural specification
• Precast concrete 65
•Structural specification
• Precast concrete 5
• High-density polyethylene 1 00
• Screened outlet
• Flow controlled outlet
• High-density polyethylene 100
• Spherical shape
• 2-in diameter outlet
• Gas deflection baffle
• High-density polyethylene or concrete
• 2-in diameter outlet
• Separate surge tank
Actual
100
95
50
100
100
83
58
90
20
23
100
100
60
-------
Tabl«4-3.
Summary of Collector Main Design Criteria Used In Saleetad projects
03
01
Community
Mt Andrew,
AL
Westboro,
WI
Badger.
SD
Avery,
ID
Maplewood,
WI
S. Corning,
NY
New Castle,
VA
Miranda,
CA
Gardiner,
NY
Lafayette,
TN
West Point,
CA
Zanesville,
OH
Muskingham
Co., OH
Design
Flow
0.4 gpm/connection
72 gpcd
Min. Pipe
Diameter
(in)
2
4
50 gpcd, min. pipe 4
size based on fixtures
TO gpcd
peaking factor - 3
75 gpcd
peaking factor =4
200 gpd/connection
peaking factor = 4
50 gpcd
peaking factor = 4
175 gpd/connection
peaking factor = 2
80 gpcd
peaking factor =4
350 gpd/connection
225 gpd/connection
peaking factor = 2
0.5 gpm/connection
0.5 gpm/connection
4
6
4
4
3
4
4
2
2
2
Min. Design
Velocity
(fps)
N/A
1.5 (0.5 full)
1.0 (full)
1.5 (0.5 full)
1.5 (0.5 full)
1.5 (full)
1.0 (0.5 full)
1.5 (0.5 Ml)
1.5 (0.5 full)
1.0 (0.5 full)
0
0
0
Minimum Slops
3-in 4-in
(9
6-in 8-in
=)
Inflective gradients
0.67
0.60
0.40
-
0.40
0.50
0.94 0.67
0.67
0.36
0.67
0.28 0.19
-
0.27
0.20 0.15
0.50
-
0.40 0.40
0.36
Inflective gradients
Inflective gradients
(constant buried depth)
Inflective gradients
Roughness*
Coefficient
C-150
n = 0.013
n= 0.011
n- 0.010
n = 0.013
n= 0.011
(Kutter's)
n=0.013
n- 0.013
n- 0.01 3
C=100
n=0.012
C = 150
C = 150
Pipe
Material
PVC
SDR 26
PVC
SCH40
PVC
SDR 35
PVC
SDR 35
PVC
SDR 35
ABS & PVC
SDR 35
PVC
SDR 35
PVC
SDR 35
PVC
SDR 26
PVC
SDR 35
PVC
SDR 26
PVC
SCH40 &
SDR 35
PVC
SDR 35 &
SDR 26
Depth (ft)
Alignment
Curvilinear
Curvilinear
Straight
Curvilinear in
horiz. plane
Straight
Curvilinear
Straight
Straight
Curvilinear
Curvilinear
Curvilinear
Curvilinear
Curvilinear
Min.
1
7
5
5
6
4.5
3
4
4
2.5
3
3.5
4.5
Avg.
1
7.5
7.5
5
6
7
5.5
8
7.5
4
3.5
3.5
4.5
* C = Hazen-Williams; n = Manning's (except as noted)
-------
CO
0>
T*W«4~4. Compad*on of System GOTpontntU»M«Furwtk>noir&mb«
Category
Community
Westboro,
Wl
Badger, SD
Awry, ID
Maptewood,
Wl
S. Coming,
NY#1
S. Coming,
NY #2
Newcastle,
VA
Miranda,
CA
Gardiner,
NY
Lafayette, TN
West Point,
CA
Zanesville,
OH
Average
Mainline
217
125
122
95
63
77
109
96
177
89
116
83
114
Service
Laterals
Feet of Category/Connection
217
125
122
95
63
77
109
96
98
89
94
123
109
Septic
Tante
050
0.38
1.02
0.80
0.88
0.81
0.30
0.72
0.94
0.05
1.03
0.43
0.67
Manboiss
S71
348
3,345
1,450
3,096
2,994
541
1,374
716
1,678
-
2,609
1,702
Cteanouts
-
-
394
112
310
342
104
163
805
781
857
272
414
Manhotes&
Cteanouts
Feet of Main/Category
571
348
352
104
281
307
84
146
379
533
857
246
351
Residential
Lift Stations
1,450
6,616
-
-
3,715
2,395
-
962
2,761
22,655
-
163
5,090
Mainline
Lift Stations
5,143
3,308
1,673
1,933
-
-
6,955
-
19,330
5,034
18,000
-
7,672
Force
Main, ft
82
10.3
2.4
1.7
-
-
3.1
46.7
6.6
4.6
3.6
-
9.3
-------
Table 4-5.
Comparison of SDGS Construction Costs ($} from Selected Projects
03
Community
(Bid Date)
Westboro, Wl
(Jan. 1977)
Badger, SD
(July 1980)
Avery, ID
(Aug 1980)
Maplewood, Wl
(Dec. 1980)
S. Corning, NY #1
(April 1981)
S. Corning, NY #2
(April 1981)
New Castle, VA
{April 1981)
Miranda, CA
July 1981
Gardiner, NY
(Aug 1981)
Lafayette, TN
(May 1982)
West Point, CA
(March 1985)
Zanesville, OH
(Feb. 1985)
Average
Construction Costs
245,635
103,281
290,280
265,903
810,345
1,218,301
212,696
666,703
596,246
737,844
695,432
2,798,913
Bid
2,959
1,949
5,278
4,359
2,738
3.930
3,623
6,667
5,470
1,447
4.487
3,887
CostfConnectten
Adjusted to January 1991
5,668
2,856
7,632
6,168
3,789
5,438
5,014
8,909
7,309
1,823
5,164
4,471
5,353
Bid
13.03
15.61
43.39
45.85
43.63
50.87
30.58
69.33
30.84
16.29
38.64
46.65
CosVft of Main
Adjusted to January 1991
24.96
22.87
62.74
64.88
60.38
70.40
42.32
92.64
41.22
20.52
48.67
53.66
50.44
-------
Table 44.
Comparison of Unit Costs ol Comporwnts from S*J«t»d Projects
Community
(Cost Index)
MaJn(Sm)
2-in 3-ln 4-in
6-in
8-ln
(Avg, Depth, ft)
Westboro.WI
(2494)
Badger, SD
(3260)
Aveiy, ID
(3304)
Maptewood, Wl
(3376)
S. Corning, NY #1
(3452)
S. Corning, NY #2-
(3452)
Newcastle, VA
(3452)
Miranda, CA
(3575)
Gardiner, NY
(3575)
Lafayette, TN
(3792)
West Point, CA
(4151)
Zanesville, OH
(4153)
•Adjusted Average
(4777 -Jan. 1991)
5,33
(7.6)
4.15
(7.9)
8.75
.
- . 5.70
(7.5)
11.84
(7.6)
7.00
(5.5)
19.59 20.15
(9.2) (16.7)
8.90
(6.9)
6.45
(3)
7.26
(3.5)
7.87 8.09 10.48
(3.5)
8.70 17.74 12.19
(3.5) (6.3) (7.4)
-
6.46
(10.8)
-
8.90
(5.7)
8.00
(8.0)
11.95
(6.9)
-
-
16.29
(8.6)
7.30
(3)
. -
-
13.44
(7.2)
7.50
(9)
-
-
-
-
15.20
(7.8)
15.00
(9.1)
-
17.79
(9.9)
-
-
-
19.98
(9.0)
Manholes
(Seach)
420
672
2,000
633
1,350
2,168
1,110
2,217
1,056
1,076
-
477*
1,660
Cleanouts
($each)
-
-
100
69
150.
110
79
261
300
107
300
771
290
MaMrtt
Lift Stations
(leach)
10,366
21,400
8,554
20,721
-
-
20,000
-
15,000
6,322
40,000
-
24,325
Force Main
($each)
0.55
0.39
1.64
2.92
-
-
2.60
0.17
0.32
0.37
5.60
-
2.33
Residential Septic Tante flnstatted)
Lift Stations (750-o.al) (1,000-gal)
($/ft) ($aaeh)
1,097
250
-
.
6,000
6,000
-
4.749 1,477
2,000 600
2,600
-
1,543
4,143 1,388
($each)
327*
450
1,500
650
800
1,250
750
1,990
650
500
1,600
1,335
1,315
Connections
($«)
-
4.28
4.61
8.10
5.36
9.70
7.00
10.00
4.51
3.45
9.73
8.71
9.08
_• Includes service connection. .
,*-.-Includes ball valves and release valves.
-------
Table 4-7.
Summary of Component Costs from Selected Projects
Community
(Cost Index)
Westbora.Wl
(2494)
Badger, SD
(3260)
Avery, ID
(3304)
Maplewood.WI
(3376)
S. Corning, NY #1
(3452)
S. Coming, NY #2
(3452)
New Castie, VA
(3452)
Miranda, CA
(3575)
Gardiner, NY
(3575)
Lafayette, TN
(3792)
West Point, CA
(4151)
Zanesville, OH
(4153)
Adjusted Average
(4777 -Jan. 1991)
In-Place
Pipe
557
2.67
8.57
17.30
13.36
15.11
9.89
24.36
15.07
6.90
7.26
8.09
15.10
Lift
Manholes Cleanouts Stations
0.60 - 1.65
1.93 - 3.23
0.60 0.25 5.11
0.44 0.62 10.72
0.44 0.48
0.72 0,32
2.40 0.78 2.88
1/S1 1.60
1,47 0,37 0.78
0.64 0.14 1.26
0,35 2.22
0.18 1.05
1.42 0.79 4.95
Force
Main
0.55
0.39
1.64
2.92
-
-
2.60
0.17
0.50
0.37
1.56
-
1.66
Bldg. Septic
Sewer Tanks
0.76
0,03
-
-
1:62
2.51
-
4.94
0,72
0,11
-
9.46
3.22
($/ft pipe installed)
1.68
1.36
12.71
8.02
11.59
13.80
9.76
18.24
3.62
1.78
16.13
6,86
11.70
Service
Conn.
,«
2.59
0.69
2.79
7.72
11.87
-b
7.44
2.50
4.19
6.00
8.71
7.13
Street Site
Repair Crossings Restoration
1,47 0.22 0.75
-* 0.23 -*
J> 12.49 -b
1.72 -" 1.29
3.57 1.12 3.08
2,37 1.64 2.11
J> -i> J»
9.48 -" 0.53
2.97 2.07 0.77
0.56 0.34 -"
1.47 0.36
5.72 - 1.12
4.34 3.45 2.12
Misc.
0.06
-
1.33
0.03
0.65
0,42
2.27
0.96
-
-
3.29
5.45
2.01
Total
13.03
15.61
43.39
45.85
43.63
50,87
30.58
69.33
30.84
16.29
38,64
46.65
57.89
* Included in septic tank costs,
b Inducted in pipe costs.
-------
T«W»4-8.
Summwy of Componwit Costa from Seloctod Projtota
tO
o
Communly
(Cosllndex)
WflStbOrO, Wl
Badger, SD
Aveiy, ID
Maplewood, Wl
S. Coming, NY#1
S. Coming, NY #2
New Caste, VA
Miranda, CA
Gardiner, NY
Lafayette, TN
West Point, CA
ZanesvHIe, OH
Average
IrvPlace
Pipe Manholes
40 5
30 12
20 1
38 1
31 1
30 1
32 8
35 2
49 5
42 4
19
17 1
26 2
Cleanouts
1
1
1
1
3
2
1
1
1
2
1
Uft
Stations
13
21
12
23
9
-
3
8
6
-
9
Fores
Main
Bdg.
Sewer
Septic
Tante
Service
Conn.
(percent of total construction cost)
4 6 13 -
2 0 9 23
4 - 29 2
6 - 17 6
4 27 18
5 27 23
9 - 32
0
2
2
4
-
3
7
2
1
-
20
6
26
12
11
42
15
20
11
8
26
15
19
15
Street
Repair
11
4
8
5
14
10
3
4
12
12
Site
Crossings Restoration Msc,
2
1
29
3
3
-
7
2
1
-
7
6 1
3
3 0
7 1
4 1
7
1 1
2
-
8
2 12
4 3
-------
cleanouts might be higher, crossings lower, etc. However,
the top two categories will almost always dominate. Pipe
installation costs are affected most by the depth of
excavation. Where frost does not control the depth at
which the sewers must be installed, shallow placement
can reduce the total costs significantly. Consideration
should be given to eliminating gravity drainage for
basement drains. Greater use of individual STEP units
can also reduce the required depth of the collectors.
Several projects have shown that hybrid systems using
pressure connections into gravity collectors can be cost
effective in areas of undulating topography. Reducing
the depth may also eliminate the need for some mainline
lift stations. Shallow placement will allow the use of
continuous trenching equipment as well.
The cost of installation of the interceptor tanks and
service laterals includes the cost of evacuation and
abandonment of the existing septic tank. Installation
costs should be reduced by combining more than one
connection on one tank. However, this is seldom done,
except in a few instances where tanks are placed on the
right-of-way, farther from the served dwellings. Many
contractors are reluctant to work on private property
because of the insistence of the property owner about
complete restoration of their property. Several methods
have been used to mitigate this problem to control the
cost. Video taping of each site prior to construction helps
to resolve complaints concerning appropriate restoration.
Letting a separate contract for the service connections to
allow a smaller contractor who is typically more
accustomed to working with property owners to perform
the work has been effective.
Placement of the interceptor tanks in the public right-of-
way eliminates the need to enter private property
altogether. This latter approach is seldom used because
of space restrictions and the additional cost to the
property owner for longer building sewer connections.
4.7.2 Operation and Maintenance
The most significant operation and maintenance costs of
projects reviewed are labor, interceptor tank pumping
and system depreciation. An operator must be on call at
all times, but the time required for preventive maintenance
is small. Most projects do not employ full time staff,
finding that 5-10 hr/wk is sufficient for service calls or
emergency maintenance.
Interceptor tank pu mping is u su ally performed by outside
contractors. Most projects are pumping each tank every
2-3 years which has been found to be more frequent than
necessary. Pumping of residential tanks every 7-10
years appears to be sufficient in most instances.
Commercial establishments, particularly those with food
service may require pumping every 6-12 months.
Other operating and maintenance costs include
administration, utilities, insurance and occasional repairs.
These costs account for 20-30 percent of the total
operation and maintenance costs.
4.7.3 User Charges/Assessments
User charges typically include administration, operation
and maintenance, depreciation and debt retirement costs.
In most projects, flat rates for residential connections are
charged because water meters are not provided.
Surcharges are usually placed on commercial connections
based on assumed water use. In the projects reviewed,
usercharges ranged were $10-20/month per connection.
Flat rates are also frequently used for assessments.
These may take the form of hookup charges. A two tiered
system is common. The first tier is for connections made
at the time of system construction. The second is for
future connections. Existing users at the time of
construction are usually provided the interceptor tank
and service lateral while future users must pay for the
tank and lateral in addition to the hookup fee.
4.8 System Management Considerations
4.8.1 User Responsibilities
Typically, the user is responsible for only the building
sewer from the building to the interceptor tank. If a STEP
unit is included as part of the service connection, the
owner is also responsible for providing power to the
control panel. Beyond these limited responsibilities, the
owner must also see that access to all components of the
system located on the property is unimpaired.
4.8.2 Sewer Utility Responsibilities
The utility is usually responsible for the installation,
operation and maintenance of the entire system
commencing at the inlet to the interceptor tank. Outside
contractors may be employed to perform some tasks
such as installing service connections or pumping of the
interceptor tanks.
4.9 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
191
-------
1. Baumann, E.R., E.E. Jones, W.M. Jakubowski and
M.C. Nottingham. 1978. Septic Tanks. In: Home
Sewage Treatment, Proceedings of the Second
National Home Sewage Sreatment Symposium.
American Society of Agricultural Engineers. St.
Joseph, Michigan.
2. Otis, R.J. 1986. Small Diameter Gravity Sewers: An
Alternative for Unsewered Communities. EPA/600/
2-86/022, NTIS No. PB86-167335. U.S.
Environmental Protection Agency. Cincinnati, Ohio,
1986
3. Anderson, J.S. and K.S. Watson. 1967. Patterns of
Household Usage. JAWWA 59:1228-1237.
4. Bennett, E.R. and E.K. Linstedt. 1975. Individual
Home Wastewater Characterization and Treatment.
Completion Report Series No. 66. Environmental
Resources Center. Colorado State University. Fort
Collins, Colorado.
5. Siegrist, R.L., M. Witt, and W.C. Boyle. 1976.
Characteristics of Rural Household Wastewater. J.
Env. Engr. Div., Amer. Soc. Civil Engr. 102:553-548.
6. Otis, R.J. 1978. An Alternative Public Wastewater
Facility for a Small Rural Community. Small Scale
Waste Management Project. University of Wisconsin.
Madison, Wisconsin.
7. Onsite WastewaterTreatmentand Disposal Systems.
U.S. Environmental Protection Agency. EPA/625/1-
80/012, NTIS No. PB83-219907. U.S. Environmental
Protection Agency, Cincinnati, Ohio. 1980.
8. Watson, K.S., R.P. Farrell, and J.S. Anderson. 1967.
The Contribution From the Individual Home to the
Sewer System. JWPCF (39):2039-2054.
9. Jones, E.E.,Jr. 1974. Domestic Water Use in
Individual Homes and Hydraulic Loading of and
Discharge from Septic Tanks. In: Proceedings of the
National Home Sewage Disposal Symposium. Amer.
Soc. Agricul. Engr. ASAE publication 1 -75. St. Joseph,
Michigan.
10. Gravity Sanitary Sewer Design and Construction.
WPCF Manual of Practice No. FD-5. Water Pollution
Control Federation. 1982.
11. Simmons, J.D., J.O. Newman, and C.W. Rose.
1982. Small Diameter, Variable-Grade Gravity
Sewers for Septic Tank Effluent. In: On-Site Sewage
Treatment. Proceedings of the Third National
Symposium on Individual and Small Community
Sewage Treatment. American Society of Agricultural
Engineers, ASAE publication 1-82. pp 130-138.
12. South Australian Health Commission. 1986. Public
Health Inspection Guide No. 6: Common Effluent
Drainage Schemes. South Australia.
13. Septage Treatment and Disposal. EPA/625/6-84/
009. NTIS No. PB88-184015, U.S. Environmental
Protection Agency, Cincinnati, Ohio. 1984.
192
-------
CHAPTERS
Design Examples
5.1 Pressure Sewers
in this example the rational method of design is used.
Variations of this method exist, but most variations differ
little in conclusion. The example is shown graphically but
may be accomplished mathematically or by computer.
No portions of the example collection system are shown
to be higher than the point of discharge, so two-phase
flow does not exist. If two-phase flow were present
calculations should be madeforthe additional headlosses
not accounted for in standard hydraulic equations. Thus,
the design example is simplified, explaining basically the
pipe size selection and pump selection processes.
For this example, flows are based on Equation 5-1,
which assumes that infiltration and inflow are well
controlled;
This example simplifies the procedure some, but intends
to convey the basic ideas without being unnecessarily
complicating. The procedure is as follows:
1. On a plan of the area to be served, group the homes
or dwelling units (DU) and total them into accumulated
nodes. On Rgure 5-1 they are shown as 270 DU, 80
DU, and 45 DU.
Q m 0.5N + 20
Where,
Q = Design flow (gpm)
N = Number of homes served
2. Determine design flows for each nodeusing Equation
5-1. On Figure 5-1 they are 155, 60, and 43 gpm,
corresponding to the DUs noted in Step 1.
3. For each separate reach between nodes, select a
trial pipe diameter and determine the slope of the
hydraulic grade line (S) and velocity of flow (V).
Velocity is determined by the equation V= Q/A. Slope
is determined by the Hazen Williams equation. Write
the trial pipe diameter (D) and corresponding S and
Equation 5-1 v by the node.
The minimum scouring velocity required if using
grinder pumps is 60 cm/s (2 fps) or preferably 90 cm/
s (3 fps). Using STEP systems, minimum self-cleaning
velocity is not usually considered.
Pipeline sizes are taken as nominal, even though most
commonly used sizes and types of PVC have inside
diameters slightly larger than nominal.
The Hazen-Williams equation is used for calculating
headloss:
1.318CR063S054
Equation 5-2
Where,
V = Velocity of flow {fps)
C = Flow coefficient (140 in this example)
R = Hydraulic radius
S = Slope of hydraulic grade line
In the example three-inch mains are used as the
smallest size for reasons mentioned elsewhere in this
manual, but 5-cm (2-in) mains could be used to serve
small numbers of DUs.
4. Beginning at the most downstream point (point of
discharge elevation 445.0 at station 0+00 in the
example), draw the hydraulic grade line.
5. On the profile, plot the low liquid level elevations of
each tank at the stations where the tank and pump
assemblies will be installed.
6. Scale the elevation difference between the liquid
level elevation in the tank and the hydraulic grade line
193
-------
Ftflur»5-1.
Example prassuro sewer design.
LINE A
EXAMPLE
PUMP
LOCATION
I'
•< FLOW
\
POINT OF
DISCHARGE m
LINE A 17+75=
LINE B 0 + 00
LU
A) PLAN VIEW
500
450
400
17+75))
Q - 60 gpm
D-3"
S-1.1%
V = 2.7fps
HYDRAULIC •
GRADE LINE
PIPELINE.
PROFILE
< DU = 45 (LINE B)
Q= 43 gpm
D = 3"
S = 0.6%
V = 2.0 fps
LINEB
20+00 25+00
A 17+75-
B 0 + 00
0+00 5+00
10+00
194
-------
at that station - 20 m (61 ft) in the example. This gives
the head that the pump must discharge against.
7, Given the design pumping head (Step 6), select a
pump having an effective pump curve that can
discharge a sufficient flow rate through its service
line without being so near to shutoff head that long
term performance would be questionable.
If required pumping heads exceed the capabilities of
available pumps, return to Step 3 and try using larger
diameter piping. This will result in flatter slopes of the
energy grade line, and lowerthe required pumping head.
5.2 Vacuum Sewer System
[This design example is a variation of one from the AIRVAC design
manual, which is protected under U.S. Patent Number 4179371. Changes
to the text were made to allow for terminology consistency with the rest
of thisdocument and for the sake of simplicity. These changes do not alter
in any way the context of the example.]
Consider the vacuum sewer layout in Figure 5-2. The
location of the vacuum station, sewers, and valves have
been selected in accordance with the requirements of
Section 3.4, which are restated below:
* Minimize lift
• Minimize length
• Where possible, equalize flows on each sewer
Each valve is assumed to serve two homes and peak flow
per home is 0.64 gpm, or 1.28 gpm/valve installation.
Three main sewers will be required to effectively serve
the area depicted in Figure 5-2. Each of these main
sewers is connected directly to the vacuum tank at the
vacuum station. Sewers are not joined together into a
manifold outside the station.
Division valves must be located to isolate areas of the
sewer network for troubleshooting purposes.
Profiles (Figures 5-3 through 5-5) have been prepared for
a portion of main #2. The profiles for mains #1 and f 3
would be similar.
Profiles for main #2 follow principles stated in Chapter 3:
• Maximum length of 10-cm (4-in) sewer - 2,000 ft
• 80-percent pipe diameter drop, or 0.2-percent fall
between lifts - whichever is greater on 4-in and smaller
mains
« 40-percent pipe diameter drop, or 0.2-percent fall
between lifts - whichever is greater on 15-cm (6-in) and
larger mains
• Where the ground profile falls greater than 0.2 percent
in the flow direction, the sewer profile follows the
ground
The location of vacuum valves, division valves, and
branch sewer connection points follows the principles
stated of Section 3.4.
The buffer tank valve installation shown between Points
C and D is representative of a high-flow user, such as a
laundromat or school; 10 gpm is used as the rate for this
location.
Main 3 is representative of a sewer main laid down an
alley way, which allows up to 4 homes to be connected
to each vacuum installation.
5.2.1 Procedure
Commence at Point F. Calculate losses to Point D.
Calculate losses and flows from Point E to Point D.
Determine the line with the greatest loss and carry
forward. In this example, the total line loss from Point F
to Point D is greater than the total line loss from Point E
to Point D. Therefore, only the total line loss from Point
F to Point D is carried forward, and line F-D-C becomes
the main sewer for total line loss calculation purposes.
The total line losses for all upstream flow from Point D to
Point C are calculated in Table 5-1 and continue towards
the vacuum station. Note that line losses were not
calculated forthe branch entering Point B;this is because
it is clearly evident that there are negligible losses
present. This piping and additional flows have been
entered at Point B forthe remaining mainlinecalculations.
See Table 5-2 for calculation of line losses in main #2,
from Point D through Point A. The friction losses for
slopes greater than 2.0 percent have been ignored, and
calculated static losses due to a profile change equal the
lift height minus the pipe inside diameter.
Using the same method, total line loss, flows and pipe
sizes can be calculated for main f 1 and #3. Flows, pipe
sizes, and lengths for these mains have also been
estimated to allow piping and vacuum station calculations
to be completed.
The last step involves the preparation of piping and
vacuum station calculation sheets from the sewer profiles
(see Tables 5-3 and 5-4).
The engineer must then select suitable standard size
pumps andtanks, usually in concert withthe manufacturer,
and recalculate the vacuum station requirements using
the selected equipment sizes. Vacuum and wastewater
pump sizes should be selected to allow for additional
195
-------
Flgura 5-2.
Design example layout.
196
-------
Figure 5-3. Design example profile*.
525'
525'
520'
515'
520'
515'
510'
507'
505'
502'
500'
510'
507'
505'
502'
500'
495'
GROUND SURFACE
PROPOSED VACUUM SEWER
495
00
2+00
4+00
6+00
8+00
10+00 12+00
14+00 16+00
LEGEND: K4 DIVISION VALVE
f CONNECTION FROM A BRANCH SEWER
SCALE: HORIZONTAL 1"=200'
VERTICAL r=S
SEWERS LAID TO 0.2% FALL IN FLOW DIRECTION
-------
Dttlgn oxwipto prof!J«*.
525'
525'
(0
03
520'
515'
495'
520'
515'
495'
16+00 18+00 20+00 22+00 24+00 26+00
MAIN «2
LEGEND: X DIVISION VALVE
f CONNECTION FROM A BRANCH SEWER
28+00
30+00
SCALE: HORIZONTAL
VERTICAL
32+00
1"=200'
1"=5'
34+00
SEWERS LAID TO 0.2% FALL IN FLOW DIRECTION
-------
Figure 5-5. Design example profiles.
525'
520'
515'
510'
507'
505'
502'
500'
495'
00
LEGEND:
=///.
525'
520'
515'
510'
507*
505'
502'
500'
495
2+00
4+00 6+00
MAIN #2
8+00
10+00
12+00
SCALE: HORIZONTAL 1"=200'
VERTICAL 1"=5'
DIVISION VALVE
CONNECTION FROM A BRANCH SEWER
SEWERS LAID TO 0.2% FALL IN FLOW DIRECTION
199
-------
Design Bcwnpfc Un* Low Catcutetkms («if eakntlttton* hi ft)
ro
o
o
Station
to Station
(0)8+80-8+60
8460-7-fOO
7+00-5+50
5+50-4+00
4400-1-tOO
1400- 0(0)
Branch sewer G-C
Ur»
Lsoqth
Mean
Q
Accum.
H./100
Head
Loss Una
Static
Loss
Toy
Loss Una
Start with Losses carrladfoiward from F-D:
20 25.6 25.6 0.0184 0.0037 0.0037
160 26.9 28.2 0.0198 0.0317 0.5 0.5317
150 29.4 30.7 0.0225 0.0338 1 1.0338
150 37.0 43,3 0.0352 0.0528 1 1.0538
300 45.9 48.4 0.0528 0.1584 1 1.1584
100 49.0 49.7 0.0594 0.0594 1 1.0594
loins at this ooint: calculate G-C before continulna with main.
Accum.
Head LOSS 5-in
1.5535
1.5572
2.0889
3.1227
4.1755
5.3339
6.3933
Roe Length
4-ln 6-in
20
160
150
150
300
100
No.
Wn Valves
2
2
3
4
1
Calculation for G-C yields an accumulated head loss of 5.0243, which Is less than 6.3933. Therefore, F-D-C head losses (6.3933) are carried over for the remaining mainline calculation, shown In
Table 5-2.
-------
Table 5-2.
Design Example Line Loss Calculations (all calculations In ft)
ro
o
Station
to Station
Line
Length
Mean
Q
Accum.
K/100
Head
Loss Line
Static Total
Loss Loss Line
Start with Losses carried forward from F-D-C:
(0)30+15-29+95
29+95-26+50
26+50-21+40
(6)21+40-8+13
8+13-5+00
5+00-2+00
2+00 - 0 (A)
Total
20
345
510
1,327
313
300
200
97.1
100.3
104.1
124.6
131.0
133.0
136.1
97.1
103.5
111.2
130.4
131.7
134.2
138.0
138.0
0.2097
0.2219
0.0718
0.1009
0.1101
0.1132
0.1180
0.0419
0.7656
0.3662
1.3389
0.3446
0.3396
0.2360
0.5 0.5419
0.7656
8.0670
1.3389
0.38 0.7246
0.38 0.7196
0.58 1.1160
Accum.
Head Loss 3-in
6.3933
6.9352
7.7008
8.0670
9.4059
10.1305
10.8501
11.9661
11.9661
Pipe Length No.
4-in 6-in 8-in Valves
20
345 5
510 6
750 1,327 9
(branch)
313 1
300 2
200 3
4,815 2,540 2,650 101
Accumulated head losses are under 13 ft, so design is acceptable.
-------
PipoLenglh
Urn 4-4n
1 2,400
2 4,815
3 3,700
Total 10,915
Avg. Crossover Length
Total 3* Pipe
6* 8-tn
1,400
2,718 2,650
2,200
6,315 2,650
Poek
Flow
(gpm)
78,1
1380
49.9
266.0
No.
Crossovers
31
42
10
83
40
3,320
No.
Valves
62
102
31
195
No.
Homes
124
200
78
402
Volume of Pipework (based on SDR 21 PVC plpa):
Vp - (0.0547 x Length 3-in + 0.0904 x Length 4-tn + 0.1959 x Length 6-in + 0.3321 x Length 8-in) cu ft
o Vp (182 + 987 •»• 1,237 + 880)- 3,286 cu ft
Vp » 7.5 x 3,286 CU ft-24,645 gal
2/3 Vp = 16,430gal
-------
Table 5-4.
Design Example Vacuum Station Calculations (all calculations In ft)
w
Peak Flow
Average Flow
Minimum Row
Vacuum Pump Capacity Required
Determine A from:
Longest Line Length
(ft)
0-5,000
5,001-7,000
7,001 - 10,000
10.001 - 12,000
12,001 - 15,000
6
7
8
9
11
Vacuum Pump Capacity Required
Discharge Pump Capacity
Collection Tank Operating Volume:
(for 15-min cycle at Q^J
Total Volume of Collection Tank =. Va
Vacuum Reservoir/Moisture Removal Tank - Vrt
System Pump Down Time for Operating Range of 16-20 in Hg Vacuum:
QmiB,Peak factor "Q^j/3.5
(Ax Qm J/7.5 gal/cu ft
(7x266y7.5gal/cuft
NOTE: Min. Q^ISOacfm
15 (38) (266-38)7266
3V0 3(489)
NOTE: Min Va= 400 gal
t - (0.045)[ (2/3KV,,) -.- (Va- V,) -.- VJ/Q,,
t should be less than 3 minutes.
If over, increase Q^ to give t under 3 minutes.
If t under 1 minute, increase Vn.
(0.045) [(16,430) + (1,500 - 500) + (400)1/300
NOTE: Use actual tank and pump values
266 gpm
76 gpm
38 gpm
248acfm
266 gpm
489 gal
1,468 gal
400 gal
2.67 minutes
Select next standard
pump size = 300 cfm
Round up to 100-gal interval
-------
house connections to be made without overloading. For
very large vacuum stations, 3 vacuum pumps may be
used to prevent use of extremely large pumps. Typically,
25-hp vacuum pumps are the largest used.
5.3 SDGS
An SDGS isto be constructed to serve a small unsewered
development The development is subdivided into lots
with 30 m (100 ft) frontage. Sixteen lots are currently
occupied with single family homes. The proposed sewer
will serve a total of 25 lots, but an additional 10 upstream
from the proposed terminus of the sewer may be
subdivided later. Therefore, the SDGS is designed for 35
residential connections.
The first step in design is to draw a system profile,
beginning with the ground surface profile. The location
and elevation of all interceptor tank outlet inverts should
be added. The profile of the sewer is drawn such that it
Is below all the tank outlet invert elevations and limits
depths of excavation (See Rgure 5-6). Subsequent
hydraulic analysis will show whetherthe proposed profile
Is satisfactory.
To perform the hydraulic analysis, the sewer is divided
Into convenient sections. Each section should have a
relatively u niform gradient or flow condition (open channel
or surcharged) to simplify the computations.
The computations for this example are presented in
Table 5-5:
Column 1:
The selected sections are numbered beginning from the
sewer outlet (Station 0+00).
Column 2:
Downstream station of the individual section is recorded.
Column 3:
Upstream station of the individual section is recorded.
Column 4:
The design flow is based on the total number of
connections contrfouting flowtothesection. The estimated
flow per connection used in this example is 0.04 Us (0.6
gpm).
Column 5:
The length of the sewer section is determined by,the
distance between thedownstream and upstream stations
(Columns 2 and 3)
Column 6:
The proposed elevation of the sewer invert atthe upstream
station of the section is recorded.
Column 7:
The proposed elevation of the sewer invert at the
downstream station of the section is recorded.
Column 8:
The difference in the upstream and downstream section
stations is determined by subtracting Column 6 from
Column 7.
Column 9;
The average slope of the section is calculated by dividing
Column 8 by Column 5. If Column 8 is zero, then
surcharged conditions must be assumed.
Column 10:
The proposed pipe diameter is recorded.
Column 11:
The capacity of the sewer section is calculated. In this
example, the Manning equation was used with a "n" of
0.013.
Column 12:
The ratio of the design flow to the calculated pipe
capacity (full pipe flow) is determined by dividing Column
4 by Column 11. If less than 1, the pipe flows partially full.
If greater than 1, the pipe is surcharged,
Column 13:
The velocity at full pipe flow is determined by dividing the
Column 11 by the cross sectional area of the pipe. This
computation is necessary for the following calculations.
Column 14:
The depth of flow at design flows is determined by using
a hydraulic elements chart from any sewer design or
hydraulic handbook.
Column 15:
The velocity at design flow is determined from the
hydraulic elements chart. It is not necessary for design,
but is often of interest to the designer.
Those sections that are continuously surcharged (Column
14) and long sections laid level are critical sections in
SDGS design. If wastewater backups into individual
connections isto be prevented, the slope of the hydraulic
grade line during peak flow conditions should not be
allowed to rise above any service connection inverts. If
this occurs, the pipe must be increased in diameter, the
" 204
-------
Figure S-6. SDGS design •xampl* •ystsm proflto.
ro
o
10
z
o
GROUND SURFACE
CRITICAL ELEVATION
MAX. HEAD PERMISSIBLE
ON FULL SECTION
0*00
2+00
3+00
4+00
5*00
LENGTH OF DRAIN (METERS)
-------
T«W«S-6.
Computttkmt for SDO3 D«i)gn ExampJt
Ratio of
Raboof Velocity Deptiof Velocity at
Section Station
No. (from)
(D (2)
1 0+00
2 0+60
0+60
3 1+43
4 2+20
2420
8
°> 4C 2+20
1J 1+43
5 4+00
6 4+33
Station
(to)
(3)
0+60
1+43
1+70
2+20
4+00
4+07
4+00
2+20
4+33
4+73
Design
Flow
(Us)
(4)
1.40
1.32
1.32
1.16
1.00
1.00
1.00
1.16
0.64
0.56
Length
(m)
(5)
60
83
110
77
180
187
180
77
33
40
Upstream Downstream
Station Station
(m)
(6)
2.4
2.4
2J«
5.2
5.2
5.5
5.2
2J
6.0
6.0
(m)
(7)
0.8
2.4
2.4
2.4
5.2
5.2
2.5"
2.4
5.2
6.0
Etev.
Difference
(m)
(8)
1.6
0
0.5
2.8
0
0.3
2.7*
0.1
0.8
0
Slope
(9)
0.027
0
0,005*
0.026"
0.001"
0.036
0
0,002*
0.015"
0.0004"
0.015
0.001
0.024
0
0.005"
Pipe
Diameter
(mm)
(10)
50
100
50
100
50
50
100
50
50
100
50
50
Flowat DeslgntoFufl atFuK Ftowto
Full Pipe Pipe Flow Pipe Flow PIpeDJa,
(Us)
(H)
1.32
0.46
1.32
1.32
1.55
1.00
1.00
1.00
0.29
1,85
1.27
0.56
(12)
1.06
0.17
1.00
0.75
1.00
0.50
3.90
0.63
0.51
1.00
(nvs)
(13)
1.10
0.17
0.78
0.13
1.00
0.23
0.63
0.28
(14)
0.32
Surcharge
0.73
Surcharge
1.00 .
0.38
0.57
Surcharge
Design
Flow
(nvs)
(15)
0.66
0.17
0.76
0.13
0.50
0.16
0.54
0.28
a Maximum rise or slope at the HGL, based on upstream condition.
b Slope of HGL necessary to carry the design flow.
c Recomputat'on of pipe hydraulics due to change In sewer profile.
d Necessary elevation and elevation difference to carry the design flow.
-------
downstream invert elevation lowered, or STEP units
installed at the affected connections.
,ln this example, Sections 2 and 4 are continuously
surcharged. To determine if the pipes in these sections
are adequately designed, the maximum slopes of the
hydraulic grade lines through the sections are sketched
on the profile beginning from the outlet from the surcharged
sections. The grade lines must remain below all con nectton
inverts if gravity connections are to be used. Using the
maximum slopes, the hydraulic capacities of the sections
are calculated. These capacities must be greater than
the design flows through the sections. Section 2 is
surcharged from Station O-t-60, the outlet where free
discharge occurs, to a point where the hydraulic grade
line intersects the pipe upstream.
Because all service connections must remain above the
hydraulic grade line to allow gravity drainage, the
maximum slope of the grade line is established by
connection 6 (Figure 5-6). The maximum slope is
determined to be 0.005 m/m. A 5-cm (2-in) diameter pipe
would require a slope of 0.026 m/m to carry the design
flow. This slope would cause the hydraulic gradelineto be
above connection 6, so a 10-cm (4-in) diameter pipe was
selected.
In Section 4, connections 15 and 17 establish the
maximum slope of the hydraulic grade line. The sewer
profile originally proposed sets the static water level of-
this surcharged section at the same elevation as
connection 15. Either a STEP unit must be used at this
connection or the sewer invert elevation lowered at
Station 2+20. If a STEP unit is used at connection 15,
connection 17 will establish the maximum slope of the.
grade line at approximately 0.002 m/m. A 5-cm (2-in)
diameter pipe is not large enough to carry the design flow.
Therefore, a 10-cm (4-in) diameter pipe is selected.
Other options would have been to provide STEP units at
both connections or lower the sewer invert at Station
0+10. An economic analysis would be necessary to
determine the most cost effective solution.
207
•U.S. GOTISIMBrr PRINTING OFFICE: 1994-550-001/80338
------- |