United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/051 June 1985
Project Summary
Investigations of Existing
Pressure Sewer Systems
Joseph W. Rezek and Ivan A. Cooper
Fifteen pressure sewer systems were
visited to evaluate operation and main-
tenance history and wastewater treat-
ability. This report details design, con-
struction, and operation and mainte-
nance (O&M) characteristics at these
sites, and it highlights common prob-
lem areas. Reliability of equipment, fre-
quency of repair, treatment considera-
tions, costs, and organization models
are also discussed.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH. to an-
nounce key findings of the research pro-
ject that is fully documented in a separ-
ate report of the same title (see Project
Report ordering information at back).
Introduction
The number of pressure sewage collec-
tion systems has increased because
smaller communities need low cost solu-
tions for collection and treatment of
wastewater. The many advantages of
pressure systems include: lower construc-
tion costs associated with less expensive
piping, excavation, dewatering, and shor-
ing; lower construction costs in areas
with rock formations close to the surface,
hilly terrain, or high groundwater; in-
creased development opportunities be-
cause homesite units need be added only
when a homeowner decides to build; and
greatly reduced infiltration and inflow
(I/I) as a result of sealed sewer lines.
However, pressure systems are more
mechanized than gravity systems and
potentially have more maintenance re-
quirements.
Treatment considerations for pressure
sewer systems may differ from conven-
tional gravity systems. One type of pres-
sure system sends a reduced organic load
to the treatment plant because of pre-
treatment by a septic tank. These reduced
loads may result in higher treatment
plant capacity when conventional design
criteria are used. However, another type
of pressure system usually has a higher
organic loading than does a gravity sys-
tem because the flow volume is not
increased by I/I.
To gather data on existing pressure
sewer systems, 15 systems at 9 sites
were visited. The sites were chosen
because their variations in terrain, equip-
ment, age, and treatment methods repre-
sented a good cross-section of the pres-
ent use of this technology. Interviews
Were held with system operators, con-
tractors, pump repairers, state and local
officials, and major pump manufacturers.
Design
There were two types of pressure
sewersseptic tank effluent pump(STEP)
systems and grinder pump (GP) systems.
The STEP units consisted of an intercep-
tor (septic) tank, pumping chamber with
an effluent pump, and several appurte-
nances, including piping, valves, level
control system, alarm, and electrical ser-
vice lines. GP facilities used similar
appurtenances but had a pumping cham-
ber/storage tank with a grinder pump
instead of a septic tank. Most manufac-
turers offered packaged units of both
types, but savings were sometimes pos-
sible with individually designed packages.
The full report discusses in detail the
materials of construction, equipment
specifications, and site-specific installa-
tion features of onsite units. Most notable
were certain common design problems:
corrosion of coated steel, cast iron, and
brass appurtenances, and failure of pres-
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sure switches used for level control.
Proper ventilation and use of plastic,
bronze, and stainless steel were the best
preventive steps for corrosion control.
Mercury float switches were used in
newer installations and were more reli-
able than previously used pressure switch-
es. Gas and odor usually were not a prob-
lem when proper ventilation was used.
Several methods were used for sizing
the pumps and piping, based on one or
more of the following criteria:
elevation of each pumping unit;
estimated number of pumps running
at any given time;
peak water demand curves;
minimum scouring velocity of 3 to 5
fps;
friction headloss;
head delivered by pumps; and
hydraulic grade lines.
Mainline sewer system designs were
straightforward, with short branches and
few changes in direction. A positive head
was maintained on all pumps to prevent
grease and solids buildup, air lock forma-
tion, and pump siphoning. Precise topo-
graphical surveys were not critical be-
cause pressure sewers allowed for more
flexibility during construction than did
gravity sewers. Pipe locator systems and
color coding were recommended to avoid
mistaken cross connections and to aid in
repair and maintenance.
Air release valves were required at all
high points in the system to vent gas
accumulation and avoid pressure build-
ups and air locks. Several operators sug-
gested using more air release valves
because changing sewage velocities
changed the points at which air accumu-
lated. Manual valves are less expensive
than automatic valves, but they are time-
consuming to operate and maintain;
automatic valves were therefore recom-
mended to reduce O&M costs.
Cleanouts and in-line shutoff valves
were included in the mainline sewers to
aid in cleaning and maintenance. The use
of cast iron valves in the mainline sewers
did not lead to any corrosion problems.
Some problems occurred in systems
that were started up at only 5% to 10% of
design capacity. Problems were more
severe in GP systems because slow veloc-
ities allowed grease and detergent depo-
sition. In STEP systems, slow velocities
meant longer residence time in sewers
and greater chances for gas and odor
formation. Possible solutions included
combination gravity and pressure sewer
systems and flushing stations to supple-
ment low flows.
Construction
Proper construction of onsite facilities
was extremely important to ensure over-
all system success. Improper installation
allowed I/I and debris to enter the system
and caused numerous onsite problems.
Ease of maintenance, proximity to power
supplies, freezing conditions, and aes-
thetics were all considered at good instal-
lations.
Mainline pressure sewer construction
was similar to water main installation
and allowed more flexibility than conven-
tional sewer construction. However, good
jointing and careful bedding were essen-
tial to system success. Common causes
of later problems were poor pipe bedding,
installation at improper depths, and allow-
ing dirt or debris to enter the lines during
installation.
Operation and Maintenance
The full report details site-specific O&M
practices and problems for several sys-
tems. Although most O&M is related to
onsite units, maintenance of these units
was minor, and all of the types of pump-
ing units showed good reliability. Improp-
er installation, homeowner misuse, mal-
functioning level control and alarm sys-
tems, air-binding of pumps, plugging of
pumps and piping, and motor burnout
were the most common problems. Reli-
ability was defined as the mean time
between service calls (MTBSC), which
was the mean time interval, in years, that
system components lasted without serv-
ice calls. The MTBSC for pumps ranged
from 1.2 to 19.6 years, based on systems
with 1 to 8 years of operational exper-
ience.
A problem common to all GP units at all
of the sites was failure of the pump's sta-
tororboot. Failure was usually caused by
dry-running, excessive solids, or abnor-
mally high discharge pressure caused by
grease buildup.
Most common problems with onsite
appurtenances related to valves. Al-
though several sites reported no valve dif-
ficulties, larger and older systems had
problems such as grease plugs in check
valves, frozen check valves, leaking check
valves, and corrosion. Every system using
cast iron valves experienced corrosion
problems, and several systems had trou-
ble with corrosion of brass valves. Poly-
vinyl chloride valves were tried in some
systems, but they were not widely used
because of a high incidence of sues:
cracking. Heavy-duty bronze valves wen
the most resistant to corrosion.
Operation and maintenance of main-
line sewers was minimal, and other than
grease accumulation, most problems
were related to construction and installa-
tion. Most designs provided the capability
of flushing the pressure sewer lines, but
operators only did so when a blockage
occurred. Providing this maintenance
only on an as-needed basis is not recom-
mended. Allowing a system to run until
failure from blockage could lead to more
expensive solutions than flushing.
Odor control was a problem at several
of the locations visited, but usually at lit
stations and not at onsite units or treat-
ment plants. In addition to good ventila
tion, several methods (including hydro
gen peroxide addition) were used tc
overcome odor problems.
Routine preventive maintenance of b<
onsite units and mainline facilities re-
duced the frequency of emergency break-
down maintenance. Annual or semi-
annual inspections of onsite facilities, lif
stations, and air-release valves was rec-
ommended. Routine flushing of all line;
was also recommended.
.A final O&M concern that put a grea
demand on an operator's time was super
vision and coordination of on-lot installa-
tions. Several operators reported that 1C
to 33 percent of their time was spent or
this activity, which was usually no
included in the original O&M budget.
Treatment
Compared with gravity-collected sew
age, GP wastewater was typically 25 t<
50 percent more concentrated because o
the exclusion of I/I; it also containei
smaller particles as a result of the grind
ing action of the pumps. GP wastewate
was noted to be anaerobic when enterin
the treatment plant. No problems wen
reported in treating these wastes, but i
was suspected that higher mixed liquo
suspended solids would have to be main
tained because of greater primary efflu
ent organic concentration.
STEP wastewater was generally les
concentrated than gravity-collected sew
age because of the pretreatment by th
septic tank. Only one plant was visite
that treated 100 percent STEP wastes
and that plant produced a high qualit
effluent. Treatment and disposal of sep
tage also had to be considered for STE
systems.
Both types of wastwater contained li*
tie or no I/I, and neither required a co
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minutor at the treatment plant. There
were no reports of problems with flow
variations at the treatment plants.
Costs
The use of pressure sewers has in-
creased because of significant cost sav-
ings in certain areas such as low-density
developments, lakefront communities,
slow-growth developments, and areas
with hilly terrain, shallow bedrock, and
high groundwater.
The report cited actual costs or engi-
neering estimates for many systems that
showed capital cost savings of 25 to 90
percent over gravity systems. Total on-lot
capital costs during the 1969-77 con-
struction period ranged from $700 to
$2100 per dwelling. The pumping unit
made up to 40 to 70 percent of the on-lot
costs. Mainline capital costs varied widely
because of local construction conditions.
Treatment plant costs were not signifi-
cantly different from the costs of gravity
sewer treatment plants, though some
savings should result from the lack of I/I
volume.
The majority of system O&M costs
related to onsite O&M. These costs varied
depending on the level of maintenance
required for the on-lot units, and they
ranged from $48 to $180 per dwelling per
year. The level of maintenance required
was related to the forethought given to
O&M during design and to the extent of a
preventive maintenance program. O&M
costs for effluent pumps were estimated
to be less than those for grinder pumps
because of fewer moving parts. Mainline
sewer O&M costs were estimated to be
about one-fourth those of gravity sewer
O&M costs.
Individual home electrical costs were
very low and were comparable to the cost
of using a coffee maker. Treatment plant
energy costs were similar to gravity sys-
tem plants, but some savings should be
realized from lack of I/I volume.
System Organization Models
Mainline pressure sewers and treat-
ment facilities were owned and operated
by one of three organizational units: a
government authority (such as a sewer
district, municipal utility district, city, or
county), a private utility company, or a
cooperative or homeowner's association.
Under government or private utility organ-
izations, onsite facilities could be owned
by either the homeowner or the central
authority. Generally, maintenance of on-
site units was the responsibility of the
party who owned the unit. Some central
authorities contracted out for O&M serv-
ices, while others performed all work
in-house.
Under cooperative arrangements, the
individuals owned and maintained the
pump units. An elected homeowner's
association board oversaw the system
and dealt with calls from the home-
owners. For mainline repairs, the home-
owners called the association, who in
turn called a local contractor. For on-lot
repairs, the homeowner was free to call
either a local contractor or to enter into
some type of maintenance contract.
All system operators interviewed dur-
ing the course of the study recommended
an overall comprehensive management
system that offered perpetual mainte-
nance on the complete system with
emergency service charges built into the
monthly user fee.
Systems with formal maintenance organ-
izations had fewer complaints and fewer
system problems. Moreover, customers
showed more interest and were kept bet-
ter informed about system operation.
Where homeowners lacked formal as-
sistance, many expressed an interest in
securing help from a formal, centralized
entity.
Conclusions and
Recommendations
Design and Construction
Pressure systems should be used only
if there is a clear and significant cost-
effectiveness over conventional grav-
ity systems. Slight capital cost advan-
tages will probably not outweigh high-
er O&M costs.
Grinder-pump pressure sewer sys-
tems designed for future use that is
significantly greater than present use
have problems resulting from insuffi-
cient scouring velocities.
Because corrosion is a major problem,
especially in STEP systems, plastic,
bronze, and stainless steel should be
used for valve materials.
Mercury float switches should be used
instead of pressure switches for level
control.
Automatic air release valves should be
used instead of manual air release
valves to reduce O&M requirements.
Alarm systems should be provided to
alert the homeowner to malfunctions.
Systems should use pipe locaters and
color-coded pipe to aid maintenance
and to avoid accidental breaks and
cross-connections.
Operation and Maintenance
Both GP and STEP pump units showed
acceptable reliability.
Onsite pumping units that are light
enough for one person to lift are easier
to service.
Annual or semiannual inspection of
pumps and sewer system components
results in less emergency breakdown
maintenance.
All sewer systems shutoff valves and
air-release valves should be inspected
and manually operated at least twice a
year.
Odors can be a problem, but these
usually occur at lift stations and not at
onsite units or treatment plants.
Significant operator time is spent on
supervising and coordinating installa-
tion of new onsite units, and O&M
budgets should reflect this time.
Treatment. Costs, and
Organization
GP sewage has higher organic loads
than gravity-collected sewage because
of the absence of I/I. STEP sewage is
less concentrated than gravity sewage
because of pretreatment in the septic
tank.
Both types of pressure sewage are
amenable to conventional wastewater
treatment.
Installed costs of onsite units ranged
from $1000 to $2000 for STEP units
and from $ 1300 to $2500 for GP units.
Systems with a formal, central main-
tenance organization had fewer cus-
tomer complaints and fewer system
problems. Customers showed more
interest and were kept better informed
about system operations. Where such
organizations did not exist, home-
owners showed an interest in obtain-
ing help from a formal, centralized
entity.
The full report was submitted in fulfil-
lment of Contract No. 68-03-2600 by
Rezek, Henry, Meisenheimer and Gende,
Inc., under the sponsorship of the U.S.
Environmental Protection Agency.
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Joseph W. Rezek and Ivan A. Cooper are with Rezek. Henry, Meisenheimer and
Gende. Inc.. Libertyville, IL 60048.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Investigations of Existing Pressure Sewer
Systems, "(Order No. PB 85-197 044/AS; Cost: $14.50. subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
i, U.S. GOVERNMENT PRINTING OFFICE: 1985-55M16/27088
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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