EPA-R2-73-139
JANUARY 1973 Environmental Protection Technology Series
The Beneficial Use
of Storm Water
Office of Research and Monitoring
U.S. Environmental Protection Agency
Washington, O.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
1. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
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EPA-R2-73-139
January 1973
THE BENEFICIAL USE
OF STORM WATER
By
C. W. Mallory
Contract No. 68-01-0173
Project 11030 DNK
Project Officer
Sidney Beeman
Municipal Technology Branch
Environmental Protection Agency
Washington, B.C. 20460
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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EPA Review Notice
This report has been reviewed by the Environmental
Protection Agency and approved for publication.
Approval does not signify that the contents necessarily
reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or
recommendation for use.
11
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ABSTRACT
This report covers work originally performed by Hittman Associates in
1967 and 1968 under Contract No. 14-12-20 for the then Federal Water
Pollution Control Administration. Only a limited number of copies of
the report covering this work were produced at that time. The purpose
of this report is to make this information available for general distribution.
A system study was conducted to determine the technical and economic
feasibility of using small storage reservoirs throughout an urban com-
munity as-a means of storm water pollution control. Facilities were pro-
vided to treat the water prior to release or to provide sub-potable or
potable water for use in the community. A conventional approach to
controlling storm water pollution was defined for comparative purposes.
The study considered an 1140-acre watershed located in the new city of
Columbia, Maryland. Regression analysis techniques were used to
develop hydrologic models for predicting storm runoff following develop-
ment. Special water quality classifications were defined for the use of
treated storm water and estimates were made of the water demands as
a function of quality. Design and cost- data were developed parametri-
cally for storage, treatment facilities, distribution systems, andopera-
tion and maintenance as inputs to the system analysis model.
Computerized system analysis was used to select the optimal combina-
tions Of reservoir locations, type of treatment, and type of reuse on a
least cost per day basis. Alternatives were ranked and the optimal
practical solution determined considering the constraints on land use
imposed by existing development plans.
As a result of this work, it was determined that the use of local storage
and treatment does represent a feasible and economical method for storm
water pollution control. Further, the use of the treated water can sup-
ply a large portion of the fresh water demands of a typical urban resi-
dential community.
A demonstration program was planned and subsequently implemented by
the State of Maryland and the Environmental Protection Agency to eval-
uate erosion and sediment control practices on a 200-acre watershed in
Columbia, Maryland. The project includes a three-and-one-half-acre
lake, evaluation of cleaning and sediment handling methods, and sampling
and gaging stations to monitor changes in water quality and hydrology
during development.
111
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CONTENTS
Section
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
Abstract
List of Figures
List of Tables
Conclusions
Recommendations
Introduction
Storm Water Hydrology
Water Quality and Demands
Storm Water Quality
Storm Water Storage
Treatment Methods
Reuse Distribution Systems
System Model
System Model Outputs and Optimization
Development of Conceptual Designs
Local Collection, Storage, and Treatment of
Storm Water for Potable Reuse
Local Collection, Storage, and Treatment of
Storm Water for Sub-Potable Reuse
Local Collection, Storage, and Treatment of
Storm Water for Pollution Control
Conventional Storm Water Treatment System
Economic Comparison
Secondary Benefits
Demonstration Program
Acknowledgements
References
Appendices
Page
iii
vi
ix
1
5
7
15
41
57
65
97
103
111
131
143
147
159
171
177
185
193
205
213
215
219
v
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FIGURES
No. Pa§e
1 Complete Water System Incorporating Local
Storage, Treatment, and Reuse of Stormwater 8
2 Location Map of Columbia, Maryland 11
3 Wilde Lake Watershed - Sub-Water shed Map 14
4 Increased Yield vs. Fraction Impervious 20
5 Increased Yield vs. Rainfall Volume 21
6 Routing Method - Linear Storage Reservoir Model 25
7 Derivation of Flow-Duration-Frequency Curve
for Five-Year Storm Runoff 27
8 Effect of Impervious Fraction (I) on Five-Year
Storm Runoff 28
9 Effect of Lag Time (m) on Five-Year Storm Runoff 29
10 Average Runoff Rate vs. Storm Duration 30
11 Maximum Storm Volumes 31
12 Actual Daily Volume vs. Return Period
Wilde Lake 33
13 Calculated Daily Volume vs. Return Period -
Wilde Lake Watershed 34
14 Work Sheet - Typical Sub-Watershed Land Use
Tabulation and Imperviousness Factor Calculation 35
15 One-Year Daily Hydrograph for Sub-Watershed
No. 6 38
16 Wilde Lake Watershed - Service Areas 52
17 Work Sheet - Typical Sub-Watershed Land Use
Tabulation and Water Demand Calculation 53
18 Summary of Water Demands 55
VI
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FIGURES (Continued)
No. Page
19 Storage vs. Yield Example of Residual Mass
Tabulation Analysis Sub-Watershed No. 8 70
20 Storage vs. Yield - Comparison of Rippl Method
and Residual Mass Tabulation, Sub-Watershed No. 4 71
21 Storage Yield Characteristic vs. Proportion of
Demand Used for Sprinkling Lawns 75
22 Storage/Sprinkling Use Characteristic vs. Yield,
Sub-Watershed No. 16 76
23 Storage/Yield Characteristic vs. Reliability 79
24 Storage/Reliability Relationship vs. Average
Daily Demand 80
25 Tube Settler - Principles of Operation 85
26 Required Volume vs. Overflow Rate for Removal
of 10-Micron Silt Particle Under Ideal Conditions 87
27 Rainfall Intensity-Frequency-Duration Curves,
Howard County, Maryland 89
28 Storage/Pretreatment Rate Combinations vs.
Spill Percentages 91
29 Pretreatment Rate/Spill Percentage Relationships
vs. Storage 92
30 Storage/Reliability Characteristics vs. Yield,
Sub-Watershed No. 16 94
31 Class "AA" Treatment Systems 98
32 Distribution System Map - Wilde Lake Watershed 104
33 Wilde Lake Drainage Area Sub-Watershed Flow
Pattern 112
34 Table of Net Benefits Computer Run A 132
35 Table of Capital Cost Computer Run A 134
vu
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FIGURES (Continued)
No. Page
36 Table of Daily Cost Computer Run A 135
37 Table of Benefits - Computer Run A 137
38 Table of Net Benefits of Combinations of
Watersheds, Reuse Computer Run A 139
39 Table of Net Benefits of Combinations of
Watersheds, No Reuse - Computer Run A 140
40 Potable Reuse System Location Plan 148
41 Potable Reuse System Flow Diagram 151
42 Sub-Potable Reuse System - Location Plan 160
43 Sub-Potable Reuse System Flow Diagram -
Class "B" Treatment 163
44 Sub-Potable Reuse System Flow Diagram
Class "C" Treatment 164
45 Local Pollution Control System Location Plan 172
46 Plan of Conventional Treatment System 178
47 Conventional System Treatment Basin 180
48 Net Benefits vs. Value of Treated Water 189
49 Basins Used for Hydrograph Damping Investigation 195
50 Runoff at Point A 196
51 Runoff at Point B 197
52 Local Storage Pond in Natural Setting 201
53 Local Storage Pond in Wooded Park Setting 202
54 Local Storage Pond in Recreation Area Setting 203
Vlll
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TABLES
No. Page
1 The Wilde Lake Watershed, Columbia, Maryland 13
2 Final Regression Equation Coefficients 17
3 Typical Monthly Rainfall Totals, Baltimore
Customs House, 1900-1950 36
4 Calculated One-Year Storm Volumes for Wilde
Lake Sub-Watersheds 39
5 Distribution of Residential Water Use, After Reid 48
6 Distribution of Residential Water Use - Revised 48
7 Maximum Concentration of Selected Pollutants
by Reuse Category 50
8 Reported Storm Water Pollutant Concentrations 5f>
9 Principal Indices of Storm Water Pollution 58
10 Expected Maximum Pollutant Concentrations For
Storm Water in Wilde Lake Watershed 61
11 Effluent Standards for Water Discharged into
Wilde Lake 63
12 Storm Water Storage Reservoir Construction Types 66
13 Example of Residual Mass Tabulation - Sub-
Watershed No. 8 68
14 Ratio of Sprinkling Use During Selected Period to
Average Sprinkling Use 72
15 Example of Modified Residual Mass Tabulation
Storage vs. Proportion of Demand Used for
Sprinkling Lawns 73
16 Example of Modified Residual Mass Tabulation
Storage vs. Reliability 77
17 Settling Velocities of Selected Particles, after
Hazen 82
IX
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TABLES (Continued)
No.
Page
18 Minimum Sedimentation Basin Area Requirements
for Selected Particles, Sub-Watershed No. 14 83
19 Expected Quantity of Runoff in Excess of Selected
Storm Volumes 90
20 Proposed Treatment Processes 99
21 Construction Costs for Service Connections and
Internal Plumbing for Sub-Potable Reuse 106
22 Construction Cost vs. Service Area Distribution
System 108
23 Construction Costs vs. Service Area-Transmission
Lines 109
24 Expected Maximum Volume of One-Year Storms 114
25 Expected Available Supply from Storage 115
26 Total Annual Runoff vs. Sub-Watershed 116
27 Collection Areas - Computer Run A 123
28 Collection Areas Computer Run B 124
29 Collection Areas - Computer Run C 125
30 Combination of Areas - Computer Run A 126
31 Combination of Areas - Computer Run B 127
32 Combination of Areas - Computer Run C 128
33 Potable Reuse System Storage Reservoir
Collection Areas 147
34 Local, Collection, Storage, and Treatment of
Storm Water for Potable Reuse - Conceptual
Design 149
35 Potable Water Requirements, Total Storm Water
Runoff, and Amount Supplied 153
36 Local Collection, Storage, and Treatment of Storm
Water for Potable Reuse - Design and Construction
Costs 156
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TABLES (Continued)
No. Page
37 Local Collection, Storage, and Treatment of Storm
Water for Potable Reuse - Annual Operating and
Maintenance Costs 157
38 Sub-Potable Reuse System Storage Reservoir
Collection Areas 159
39 Local Collection, Storage, and Treatment of
Storm Water for Sub-Potable Reuse - Conceptual
Design 162
40 Sub-Potable Water Requirements, Total Storm
Water Runoff, and Amount Supplied 166
41 Local Collection, Storage, and Treatment of Storm
Water for Sub-Potable Reuse Design and
Construction Costs 167
42 Effect of Distribution System on Capital Costs 168
43 Local Collection, Storage, and Treatment of Storm
Water for Sub-Potable Reuse - Annual Operation
and Maintenance Costs 169
44 Local Collection, Storage, and Treatment of Storm
Water for Pollution Control Runoff by Sub-Water shed
and Pretreatment Unit with Pretreatment and
Reservoir Capacities 173
45 Local Collection, Storage, and Treatment of Storm
Water for Water Pollution Control Design and
Construction Costs 175
46 Local Collection, Storage, and Treatment of Storm
Water for Water Pollution Control - Annual Operation
and Maintenance Costs 176
47 Design Requirements for Conventional Treatment
Plant 177
48 Particle Removal with Conventional Treatment Basin 182
49 Design and Construction Costs - Conventional
Treatment System 183
XI
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TABLES (Continued)
e
No. Page
50 Economic Comparison of Pollution Control
Systems 186
51 Benefit/Cost Analysis of Water Supply and
Pollution Control Alternatives 187
XII
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SECTION I
CONCLUSIONS
Based on this system study, it was determined that the use of small
storage basins dispersed throughout an urban community for the control
of storm water pollution was technically feasible and economically attrac-
tive compared to other alternatives. Further, the storm water collected
in these basins could be treated to provide approximately half the water
demands of a typical urban residential community. In addition, a number
of secondary benefits could be derived through the use of this approach.
Four types of systems for the control of storm water pollution and for
supplying water were defined for evaluation purposes. These were:
1. Use of local storage basins with treatment to body contact
recreation standards for release downstream.
2. Use of local storage, basins with treatment to sub-potable
quality for reuse with separate distribution systems.
3. Use of local storage basins with treatment to potable
quality for distribution through existing water distribution
systems.
4. Use of a large basin to collect storm water from interceptors
and pumping stations with conventional treatment systems.
The comparative costs and performance characteristics of the four
systems are summarized as follows:
(1) (2) (3) (4)
Initial Costs ($) $830,000 2,598,000* 1,445,000 1,315,000
Fixed Costs ($/day) 119 373 207 189
Operation and
Maintenance ($/day) 55 151 194 68
Daily Costs ($/day) 174 524 401 257
Amount of Water
Supplied (gal/day) 0 556,000 460,000 0
Value of Water ($/day) 0 207 250 0
Net Operating Costs
($/day) 174 317 151 257
Cost per Year ($/yr) 63,400 115,800 55,200 93,000
Cost per Acre/Year
($/acre/yr) 55.25 104.30 48.05 81.71
Cost per Dwelling Unit
($/D.U.) 16.88 30.82 14.68 24.97
-'-'Includes water distribution system at$.l, 562, 000
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The other major« conclusions from this study are as follows:
1. A system of 10 small reservoirs dispersed throughout
an 1140-acre watershed was found to provide storage
capacity to receive in excess of 90 percent of the runoff
of storms with a one year return interval without encroach-
ment upon the existing land development plan.
2. The same effectiveness could be obtained with a lesser
number of larger reservoirs at a reduced cost, if areas
for the collection and storage of storm water were provided
in the initial land development planning.
3. The amount of storm water that can be collected and treated
could provide up to 52.5 percent of the water demands of a
typical urban residential development. The most economical
method of distributing the treated storm water would be to
treat the water to potable quality and inject the water into
existing water distribution systems.
4. Water treated to sub-potable water quality could provide
up to 46. 5 percent of the water demands of a typical urban
residential area; however, the cost of providing a second
distribution system for the sub-potable water would be
much greater than the cost of treating to potable water
quality for distribution through existing systems.
5. Treatment to sub-potable quality for reuse could be eco-
nomically attractive in those cases where a large demand
for sub-potable water exists for a few industrial or com-
mercial establishments, and the cost of separate distribution
systems can be reduced.
6. The use of small earthen dams for the construction of storm
retention ponds is by far the least-cost alternative. The cost
of constructed storage facilities of other types is generally
prohibitively expensive.
7. The small reservoirs used to collect storm water will also be
effective in controlling the excess runoff resulting from urban-
ization. With proper design, the runoff hydrographs can be
maintained near those of a natural area.
8. The ideal time to introduce storm retention ponds is prior to
development of the area and to use the ponds to control the
erosion and sediment generated by construction.
9. Using retention ponds for storm water management and for
sediment and pollution control will materially assist in the
preservation of ecology in urban areas.
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10. The use of collected storm water in sewered communities
could reduce nutrient releases since the water could be
effectively diverted to treatment facilities having nutrient
removal capabilities.
11. The use of storm retention ponds could be used for the
separation of combined sewers in selected situations.
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SECTION II
RECOMMENDATIONS
It is recommended that storm retention ponds be considered in the plan-
ning of the storm drainage facilities for new community development and
in modifications of the storm drainage systems in existing communities.
The ponds will provide an effective means of removing sediment and
dampen the peak storm water flows resulting from urbanization. The
availability of storage will reduce the capacity and costs of other drain-
ange and treatment systems.
It is recommended that storm retention ponds be introduced prior to
initial grading of the development site. This will provide a means of
trapping the large quantities of sediment generated by construction.
Following construction, the ponds should be cleaned and used as part
of the permanent system for storm water management and pollution
control.
It is recommended that demonstration projects be conducted to provide
information on the design, application, operation, and maintenance of
storm retention structures. Emphasis should be placed on methods of
cleaning and maintaining the ponds since the acceptability of this approach
will depend upon maintaining the ponds in an aesthetically acceptable con-
dition at reasonable costs. These projects should also consider methods
for the handling, processing, and disposal of sediment removed from the
ponds.
It is recommended that demonstration projects be conducted to develop
design and operating criteria for the application of storm retention ponds
as a method of storm water management. Design and performance infor-
mation should be obtained on relationships among pond levels, surface
area, intensity of storm events, and the quantity and quality of water
released. This should also include demonstration of retention structures
operating in a series-parallel arrangement to define interactions and
design and performance criteria for systems incorporating a number of
retention structures.
It is recommended that a demonstration project be undertaken to deter-
mine the feasibility of treating storm water for various reuse applications.
In this project, the emphasis should be placed on the development and
demonstration of a water treatment system capable of reliably treating
water to a quality suitable for distribution in existing water supply dis-
tribution systems. The treatment system should be automated for un-
attended operation and should have features for monitoring and auto-
matic shutdown if water quality is not suitable for supply.
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SECTION III
INTRODUCTION
GENERAL
Simply stated, the local storage, treatment, and reuse of storm water
is a concept for the control of storm water pollution in which the storm
water runoff is collected in small storage basins dispersed throughout
an urban area, treated to remove pollutants, and further treated for use.
With this concept, the benefits derived from the use of storm water
are used to offset the cost of effecting pollution control of storm water.
This final report on "The Beneficial Use of Storm Water" contains a
description of the system study, design, and evaluation of the local
storage, treatment, and reuse of storm water and the analysis of the
economic and technical feasibility of this concept.
The system study is based on the new city of Columbia, Maryland, and
specifically Wilde Lake and its associated watershed which is located
in the first village of Columbia. Wilde Lake is an artificial lake con-
structed in 1966, surrounded by apartments, townhouses, and individual
residences, now approaching completion. This study is further based
on the land development plans for this area, established lot boundaries,
and other physical, aesthetic, and sociologic requirements existing
within the area. The objective of this study has been to develop systems
that could control the storm water pollution of Wilde Lake and to perform
a comparative evaluation of the performance and economic aspects of
these systems.
USE OF SYSTEM ANALYSIS
In order to subject the local storage, treatment, and reuse of storm
water to comprehensive examination, system analysis techniques were
used. This required the development of models to describe the various
subsystems and compilation of generalized input data to define the design,
economic, and performance parameters involved. The subsystems that
comprise the overall system andthe interrelationship to the characteristics
of the study area and other utility systems are shown diagramatically on
Figure 1. Rainfall and water from a public water supply are inputs to
the system and evapotranspiration, soil percolation, surface drainage
out of the watershed, consumptive uses, and sanitary sewer flow are
outputs. The principal parts of the overall system considered in the
analysis are:
Storm Water Storage
Storm Water Pretreatment
Final Treatment
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EVAPO- TRANSPIRATION
RAINFALL (P) .
CO
STORM WATER
COLLECTING
SYSTEM
EFFECTIVE
RUNOFF
(QE)
STORM WATER
STORAGE
STORM v
YIELD (Q )_
STORM WATER
PRETREATMENT
NET c
YIELD (CT)
OVERFLOW TO „
SURFACE DRAINAGE (L )
OVERFLOW TO ,
SURFACE DRAINAGE (L )
E-XCESS WATER TO
SURFACE DRAINAGE (LY)
PUBLIC SUPPLY
MAKEUP WATER (QM)
TREATED WATER
USES
WASTE TREATED
WATER
CONSUMPTION (CN)
PUBLIC SUPPLY
DEMAND (QP) -
PUBLIC SUPPLY USES
(WP)
WASTE PUBLIC
WATER
FINAL
TREATMENT
TREATED
WATER (Q1)
LOSSES TO SANITARY
SEWER (WY)
(WY)
SANITARY SEWER
COLLECTING
SYSTEM
SEWER
FLOW (WT)
CONSUMPTION (C)
Figure 1. Complete Water System Incorporating Local Storage, Treatment,
and Reuse of Storm Water
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Treated Water Storage
Treated Water Uses
Following development, the system models were then applied using the
data and characteristics of the Wilde Lake watershed in Columbia,
Maryland. The formulation of the models and the .results from1 applying
these models to a specific watershed are contained in this report. It
is noted that the methodology and the models used in this study are gen-
eralized and can be adapted to a variety of situations.
The objective function used in the formulation of the system model is
described as follows:
Water released to surface drainage from the Local Storage,
Treatment, and Reuse of Storm Water System shall meet or
exceed stated effluent standards, and;
This condition will be achieved at the lowest net system cost.
This function is subject to all applicable physical, technological, legal,
and institutional constraints, as described in the following sections.
The term "lowest net system cost" is understood to be interchangeable
with the term "highest net system benefit" and is defined as:
min(SC + aSCf -SB ) (1)
\ a i a/
or, stated differently
- aSC-) (2)
a a
where:
C = marginal annual operating and maintenance costs
cL
Cf = marginal fixed construction and project costs
B = marginal annual benefits
3.
a - capital recovery factor
Equation 2 is described as the "net benefit function" and subsequent
discussion will be in terms of maximizing this function. The capital
recovery factor used in Equation 2 is defined as:
a = - L_ - (3)
1 (1 + i)m
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where:
i - annual rate of interest or cost of capital
m = period of amortization
SITE SELECTION
Early in the formulation of the Storm. Water Reuse program, it became
apparent that the new city of Columbia, Maryland, offered several unique
advantages as a study site for the systems analysis. The Storm Water
Reuse concept is specifically directed to urbanized areas where water
runoff can be clearly identified as a source of water pollution. In order
to perform a systems analysis in such an area, the physical character-
istics of the area must be known in some detail, including hydrologic
factors, land uses, population densities, etc. Since the method of anal-
ysis was to be kept generalized wherever feasible, a study area was
deliberately selected where gage data were not available for rainfall and
runoff. The requirements that led to the close examination of Columbia,
however, were those related to the planned demonstration of the concept
following analysis.
Columbia is a completely planned new town being developed by Howard
Research and Development Corporation of Baltimore, Maryland, an affil-
iate of the Rouse Company, on a 27 square mile site in Howard County,
Maryland (Figure 2). Designed for a 1980 population of 110, 000 people,
Columbia is midway between Baltimore and Washington astride the busy
Northeast Corridor. Construction started in 1966 and the first residents
moved into the city in June 1967. By the first anniversary of the arrival
of these residents, more than 5000 people were living in the community
and construction was proceeding on schedule. The complete town will
consist of nine villages with each village made up of a number of neigh-
borhoods. A principal feature of the Columbia plan is the extensive use
of water as a focal point for the activities of the community. The town
site includes portions of the Little Patuxent, Middle Patuxent, and
Patuxent river basins, and five man-made lakes totaling over 500 acres
of water surface are scheduled. Two of these, Lake Kittamaqundi and
Wilde Lake, are already in existence.
The availability of the complete Columbia plan permits the conduct of
a systems study of the Storm Water Reuse concept in an area which is
not yet fully developed. If this study is based on the planned charac-
teristics of the area after development, a demonstration program can
be conducted which will fully evaluate the results of the systems analysis
while taking advantage of the reduced costs of concurrent construction
Since the entire area is being developed by one organization, coordi-
nation and cooperation are greatly enhanced. Another fortunate feature
of Columbia is the integration of many different types of land use
including low to high density residential development, numerous type's
10
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Figure 2. Location Map of Columbia, Maryland
11
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of commercial uses, and various industrial uses. In some areas of
Columbia, a wide variety of these uses can be found in a single water-
shed. As a result, the influence of various land uses on storm water
quality and quantity as well as the accompanying possibilities for reuse
can be evaluated in the systems study and subsequent demonstration.
WILDE LAKE WATERSHED
In order to simplify hydrologic aspects of the systems study as well as
to create an easily definable water quality objective, it was decided to
perform the study on the drainage area of one of the artificial lakes. Of
the two lakes which would be available for the demonstration program,
only one, Wilde Lake, had a drainage area substantially within the
Columbia project. Furthermore, a great diversity of land use exists in
the Wilde Lake watershed. Construction in the watershed is scheduled
for completion before 1970, permitting concurrent construction of dem-
onstration facilities, followed almost immediately by operation of these
facilities in a fully developed area.
The Wilde Lake watershed consists of 1140 acres, or about 1.8 square
miles. Other information concerning the lake and its watershed is listed
in Table 1. Figure 3 is a map of the area, showing a number of sub-
watersheds which were used in the hydrologic analysis reported in Sec-
tion IV.
With the cooperation and assistance of Howard Research and Development
Corporation, the Wilde Lake watershed was chosen as the site for the sys-
tems analysis effort of the Storm Water Reuse program, and the south-
west quadrant of the watershed was identified as the tentative site of a
first-phase demonstration facility following satisfactory completion of
the systems analysis. To permit evaluation of various storm water re-
use concepts in as many configurations as possible, the study watershed
was divided into 22 sub-watersheds. These were delineated by natural
ridge lines wherever possible, permitting the runoff from each area to
be collected at a single point.
12
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TABLE 1. THE WILDE LAKE WATERSHED, COLUMBIA, MARYLAND
Watershed Area
Planned Population
No. Dwelling Units
Planned Land Use - Columbia
Open Space
Single Family Res. - Low Density
Single Family Res. - Medium Density
Town houses
Garden Apartments
Mid-rise Apartments
Employment Centers
School Sites
Public Rights-of-way
Total - Columbia
Expected Land Use - Outside Columbia
Open Space
Single Family Res. - Low Density
Public Rights-of-way
Total - Outside Columbia
Wilde Lake Water Surface Area
Estimated Maximum Capacity
Fraction Impervious Construction
in Watershed
1, 140 Acres
12, 254 Persons
3, 757
147. 6
13. 3
185. 8
114. 8
66. 6
4. 9
24. 8
37. 7
115. 8
711. 3
Acres
Acres
Acres
Acres
Acres
Acres
Acres
Acres
Acres
Acres
194. 5 Acres
191. 0 Acres
22. 1 Acres
407. 6 Acres
21.1 Acres
48, 200, 000 Gallons
0. 22
13
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\
LEGEND
WILDE LAKE
WATERSHED
7 SUB-WATERSHEDS -
FWPCA Contract No.l4-]2-20
WltDE LAKE WATERSHED
Columbia, Md.
Figure 3. Wilde Lake Watershed - Sub-Water shed Map
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SECTION IV
STORM WATER HYDROLOGY
As one of the initial phases of the Storm Water Reuse project, it was
necessary to develop methods of adequately describing the storm water
flow characteristics of the Wilde Lake drainage area. Two different
types of hydrologic data were required:
A continuous daily runoff hydrograph from each of the
sub-water sheds in the basin
Peak flows in each sub-watershed as a function of
rainfall recurrence interval
The first of these was needed to estimate the total volume of water
available for reservoir storage and reuse for any time interval, and
the second to estimate sedimentation efficiency and system operation
during large storms. Methods were required for obtaining both types
of data without benefit of gaged stream flow records at Wilde Lake.
•Fortunately, many methods exist for calculating peak flows from
drainage areas lacking stream flow data. There are, however, few
methods which can be applied to the development of a daily runoff
hydrograph in an area where basic hydrologic data are unavailable.
Since this problem was the first to be approached and required consid-
erable effort for solution, it will be discussed in detail in the following
sections.
AVAILABLE METHODS FOR CALCULATING CONTINUOUS RUNOFF
HYDROGRAPHS
The method to be used in the derivation of a runoff hydrograph for the
sub-watersheds of Wilde Lake had to satisfy the following requirements:
1. It had to provide a continuous hydrograph of daily runoff
volumes for each sub-watershed and be applicable to the
variety of impervious conditions of the watersheds.
2. It had to be a fairly simple method requiring a minimum of
data input and a minimum amount of time spent in its
development.
3. It had to be derivable without any basic hydrologic data
except land use and rainfall.
For small rural watersheds such as Wilde Lake, there exists no
standard simple method of providing a runoff model that would satisfy
these requirements. To develop such a method would have required
15
-------
more effort than could have been justified under this project since the
selection of runoff values satisfied only a; small part of the project goals.
Therefore several existing approaches to the solution of similar prob-
lems were examined with the purpose of selectingthebest suited approach
and modifying.it as necessary.
Each of the methods that were examined has been used in the past to
estimate runoff volumes and uses as one of the input parameters a
rainfall record for the drainage area. The following approaches were
considered.
Unit Hydrograph
This involves calculating for a drainage area the hydrograph that would
be produced by one unit of "effective precipitation' (actual rainfall minus
infiltration and other losses) occurring during a specific period of
time (1). This unit hydrograph is then used to calculate the runoff
hydrograph from any distribution of effective rainfall. On small drain-
age basins, the time intervals which would be required for rainfall
input would be measured in minutes. The calculation of effective pre-
cipitation from actual precipitation would require a knowledge of infil-
tration losses during each rainfall at any time of the year.
Linear Storage Reservoir Models
This computer method, which has been employed by the U. S. Department
of Agriculture (2), utilizes routings of effective precipitation through
linear storage reservoirs with a built-in lag time. This method gives
excellent results for individual storms, but also requires short-time
interval inputs as well as infiltration knowledge to calculate effective
precipitation.
Digital Simulation Models
Several models of this type exist with many variations. The basic prob-
lem with them, however, is that they require a great deal of detailed
input information to describe the watershed.
Regression Models
This method takes a dependent variable and one or more independent
variables and derives the coefficients relating the variables so that the
variation between the observed dependent variable and the calculated
dependent variable is at a minimum. Regression equations involving
rainfall and runoff have been used in past hydrologic studies in areas
where gaged data exist. However, it is possible to derive a regression
equation for a gaged watershed and apply it to another ungaged area so
long as the areas are physically and hydrologically similar.
16
-------
It was decided that the adoption of some form of regression model
would probably be the most fruitful approach. The procedure that was
chosen was to select from the Maryland area another watershed that
had the same physical and hydrologic conditions as the Wilde Lake area
and had at least five years of rainfall and runoff data. A regression
analysis could then be performed on this watershed and the results
applied to Wilde Lake.
The advantage of this approach is that a detailed knowledge of infiltration
and soil conditions is not required since the overall effects of such
parameters are indicated by the regression coefficients. The regression
model does not have to present an exact cause-effect relationship between
variables as long as its predictive ability is good and the variable rela-
tionship is reasonable.
DEVELOPMENT OF REGRESSION MODELS
After examining the available runoff data in the Maryland area, only
one watershed was located having physical characteristics similar to
those of Wilde Lake. This watershed, on the West Branch of Herring
Run in Baltimore County, is of the same approximate size and shape,
has the same topographic features, and is located in the same geologic
zone as the Wilde Lake drainage area. For these reasons it is ideally
suited for the derivation of a rainfall-runoff model which could then be
applied to Wilde Lake with modifications to account for varying percent-
ages of imperviousness in the sub-watersheds.
A number of regression models were tested including models in which
the effect of antecedent precipitation on runoff and the seasonal variations
of base flow were considered. The model found to provide the best
correlations with actual data consisted of a third order polynomial
shown in Table 2.
TABLE 2. FINAL REGRESSION EQUATION COEFFICIENTS
Q.. = A + BRt + CRt2 + DR..3 (4)
Regression Coefficients Correlation
A B C D R R2
Yearly 0.5247 3.016 1.187 -0.168 0.858 0.736
Seasonal
Winter 0.885 3.449 0.808 0.099 0.756 0.572
Spring 0.625 2.044 1.917 -0.419 0.874 0.764
Summer 0.269 1.849 1.969 -0.260 0.974 0.949
Fall 0.353 1.297 4.591 -1.389 0.927 0.859
R. = rainfall in inches/day
Q. = runoff in cfs-days/mile2-day
17
-------
APPLICATION OF THE REGRESSION MODEL TO WILDE LAKE
Although the Herring Run and Wilde Lake areas are similar in most
respects, the sub-watersheds of Wilde Lake will have a variety of
impervious percentages after development. Past studies of the effects
of urbanization on runoff (3) have shown that an increase in impervious
cover will decrease the lag time between peak rainfall and runoff,
increase the peak runoff rate, and increase the runoff yield per unit
area. It is only this last effect which will affect the derived runoff
model in this case, since the areas are too small for lag time changes
to be important on a daily flow basis, and the peak flows will only affect
the pipe sizings.
It was therefore necessary to develop a procedure for modifying the
model to take into account a full range of impervious conditions. This
involves several assumptions:
1. For the Herring Run area, which is 11.9 percent impervious,
equations can be derived representing unit flow from both the
completely pervious and completely impervious portions of
the basin.
2. The yield from the completely impervious portions will be less
than 100 percent of the rain that falls on them, since some
water will flow from impervious surfaces to pervious surfaces
where it will be lost to infiltration.
3. While this percentage will actually vary, an average value
can be calculated and used in the equations.
These assumptions were used in the derivation of the following equation,
which is fully described in Appendix A:
A + KXI'R. +(^i-')[(B-KXI)R. + CR.2 + DR.3] (5)
where:
Qi - runoff/unit area from any Wilde Lake watershed on
the ithday
K = conversion factor between in. /day and cfs/mi2 = 26. 88
X - fraction of rainfall actually running off impervious
areas
I = fraction imperviousness (Herring Run = 0. 119)
:' = fraction imperviousness of any Wilde Lake watershed
R^ = rainfall on the fth day
A, B, C, D - regression coefficients derived previously
-------
The only unknown in this equation is X, the percentage of runoff from
impervious areas. To obtain a value, the results of a study of effects
of urbanization on runoff from selected Texas watersheds were used (3).
By relating increased runoff to increased imperviousness on the same
watershed, the following equation was derived:
• - QI 1 1-1
"R
n 1(1
where;
Qj = runoff volume (as calculated by the equation given in
Appendix A) for an area with an impervious fraction of I
R = rainfall volume
By taking individual storm data and calculating Qj, a value of X can be
calculated for each storm, and these values can be averaged over all
storms. The value of X thus obtained is:
X = 0.8586
Using this value in the equation for Q-, the final equation for runoff on
the Wilde Lake watersheds becomes:
Q. = A+Ri [23. 08I'+g^)(B-2. 77)] +R.2 [C^jj+Ri3 [D(^)] (7)
Separate equations for each sub-watershed and for each season can be
derived by using the appropriate values of I1 and the regression coef-
ficients A, B, C, and D, and from these equations the continuous runoff
hydrograph can be calculated using a daily rainfall input.
To illustrate the actual effects of imperviousness on the runoff yield
per unit area, an equation can be developed giving the percent increase
in unit yield of an area with a fraction imperviousness of I' over the
natural area (0 impervious fraction).
lOOT
P = TTTTT - - - LJ1T (Appendix A) (8)
\: i; - KXI+B+CR.+DR.
K. i i
where P = percent increase in yield over the natural state. Figures 4
and 5 show, respectively, P versus I' for different values of R^ and P
versus Ri for different values of I'. The results of the Texas study on
effects of urbanization are also indicated in Figure 4. It can be seen
that for every value of I', the precipitation values between about 0. 2 inch
and 1. 0 inch have the greatest effects on P. This would be expected
19
-------
700
R=0.50"
= 1.00"
R = 0.25'
= 2.00"
= 3.00"
0.10
0.20 0.30 0.40
Fraction Impervious
0.50 0.60
Figure 4. Increased Yield vs. Fraction Impervious
20
-------
700
600
s
D
-*-
D
z
-------
since during small rainstorms the pervious areas would contribute very
little runoff, and during heavy rains, the pervious areas would eventually
contribute large volumes because the infiltration capacity would be
exceeded.
SENSITIVITY ANALYSIS
As one method of testing the usefulness of the final modified regression
equation, a sensitivity analysis was made of the equation coefficients.
Such an analysis indicates how sensitive the calculated values of runoff
are to minor variations in these coefficients and also provides a measure
of the reliability of the equation over the range of rainfall values. To
perform the type of sensitivity analysis required, the regression equa-
tion was partially differentiated in the terms of each coefficient, and
the resulting differential equations were used to calculate the change in
runoff as a function of the change in coefficients. This was done for
both the upper and lower limits of the imperviousness parameter.
It was determined that the final regression equation is not abnormally
affected by coefficient changes, and throughout the rainfall range the
equation remains stable.
STORM HYDROGRAPH AND PEAK FLOW MODEL DEVELOPMENT
In order to calculate the expected peak flows for a storm of any recur-
rence interval, a method was desired which could use the rainfall
intensity-duration-frequency curves for Howard County as a basic
input. Although the standard procedure for this type of calculation is
to use the Rational Method with an appropriate runoff coefficient for
each watershed, a more sophisticated approach which would provide
more detailed information was selected. This approach, mentioned
previously, involves the routing of a rainfall hyetograph through a
linear storage reservoir model to obtain a complete storm hydrograph.
It has been used with great success by the U. S. Department of Agricul-
ture in analyzing storm runoff from small agricultural watersheds (2),
and it required only minor changes to be adapted to this study.
As the first step in this analysis, the Howard County rainfall curves
were fitted to an equation relating the intensity of rainfall, the duration
of rainfall, and the return period of the storm. This equation, in a
standard form, is:
2QOT0.176
~ i—1~ (9)
(t + 25)1' *-
22
-------
where:
i = intensity (inches/hour)
T = return period (years)
t = duration of rainfall (minutes)
Using this equation, rainfall hyetographs for a 60-minute storm with
return periods on one, two, five, and ten years were synthesized.
These hyetographs were developed with the following conditions:
1. For each storm, the intensity of rainfall for any duration
from five minutes to 60 minutes is identical to the intensity
calculated by the rainfall equation.
2. The hyetograph is approximately symmetric about the peak
intensity.
3. The minimum time interval for calculating intensities
is five minutes.
In addition to these conditions, the 60-minute storm volume is only
slightly less than the volume of a 24-hour storm of the same return
period, and therefore is analogous to the daily rainfall and runoff
volumes used in the regression analysis.
The linear storage reservoir model, which calculates the runoff hydro-
graph, requires effective precipitation as an input. Since effective
precipitation is simply the rainfall hyetograph minus any losses due
to infiltration or surface storage, a method was needed to alter the
hyetograph to include these losses. This was done by using an infil-
tration equation for the losses on the pervious portion of a watershed
and by assuming the rainfall on the impervious portion is entirely
available for runoff. The chosen infiltration equation was one originally
derived by Horton (4) which had been used in a Johns Hopkins University
study of the rainfall in runoff processes (5) and for which values of infil-
tration coefficients had been calculated for the Baltimore area. The
steps involved in changing the original hyetograph are summarized in
the following equations, which were applied to the hyetograph for each
interval of At:
PPer
PT -f = PT - ff + It - f Yl-e~at]il (10)
Imp Imp L o \ c o/\ /J v '
where:
f = infiltration loss = f + (f - f Vl-e~at\
o ( c o)( )
f = initial infiltration rate (inches/hour) = 2.5
23
-------
f = final infiltration rate (inches/hour) = 1.5
c
a = exponential decay constant (hour~ ) = 0. 7
p = effective rainfall on pervious areas
Per
P = rainfall on impervious areas = original hyetograph
Imp
0whenf>Plmp
PEff
where:
?_,.,, - final effective precipitation hyetograph
-hit!
I = fraction imperviousness of the watershed
The resulting effective precipitation hyetograph has been applied as _
input to a linear storage reservoir model of the runoff process. This
model uses the following continuity equation as its basis:
q + q S9 ~ S
I. = * 2 + 2A, 1 (13)
i 2 At
where:
I. = average inflow rate for time period At(cfs)
q = outflow rate (cfs)
S = storage volume (feet^)
At - time increment between times 1 and 2, seconds
The effect of storage on the outflow depends on the physical properties
of the entire drainage area and on the volume of storage. Normally
the storage will impart a time delay to the outflow with respect to the
inflow into the basin. This concept is expressed by the equation:
where M is the basin storage coefficient, which may be considered to
be equal to the lag time (2).
In order to obtain the best estimates of peak flow, timing, and hydro-
graph shape, investigations have shown that two routings of the rainfall
24
-------
through half of the indicated storage are required, where the output
from the first reservoir becomes the input to the second.
h
s - s
bl 2
Q! = h
q - S
S2 ~2
«2,
Figure 6. Routing Method - Linear Storage Reservoir Model
From the two preceding equations and the double-routing technique,
a single equation can be derived to take effective precipitation as the
input to the first reservoir and calculate the output from the second
reservoir.
This final equation for routing the effective precipitation is:
Q;
VP + 9P
Eff. Eff.
J
(15)
where:
m
At
runoff in cfs/acre during the i^ time interval
lag time (minutes)
time interval (minutes)
Using a computer, the effective precipitation hyetographs for each storm
return period, and for a range of imperviousness from 0.1 to 0.5, were
used as input to the routing equation. Various lag times from 4 minutes
to 20 minutes were assumed and a total of 48 hydrographs representing
each combination of return period, imperviousness, and lag time was
obtained as output.
These data were then reduced by calculating for each hydrograph the
maximum average flow for durations from five minutes to 60 minutes,
or in equation form:
Q, = Max [ZyQ/d| , where d = duration in minutes (16)
The resulting values of QJ, plotted against duration (for every combina-
tion of lag time, imperviousness, and return period), provided a set
of curves that are analogous to the rainfall intensity-duration-frequency
25
-------
curves. The evolution of these runoff curves is summarized in graph-
ical form in Figure 7, and the effects of all the parameters on Q can be
seen in Figures 8 through 10.
In order to more easily use the data that these curves represent, an
equation was graphically fitted to all of the points for durations up to
30 minutes. This equation related all of the parameters, eliminating
the need for interpolation between the original curves and allowing a
rapid determination of peak flows for any sub -watershed. The equation
which follows has a multiple correlation coefficient of 0. 98 when
compared to the original data points.
Qd = 8T
0. 955T0' 033I(2. 482-0. 129m+0. 010m2-0. 00033m3)
where:
m = lag time (minutes) 4<_ m < 20
d - duration (minutes) 5< d <_ 30
T = return period (years) 1< T < 10
I = impervious fraction 0. !<_ I < 0. 5
By applying this equation to the sub -water sheds, storm flow determina-
tions could be made for any storm up to 10 years in magnitude, and
the results of these determinations and their application to sedimenta-
tion requirements are described in a subsequent section.
As a final application of this storm hydrograph analysis, the volumes of
the individual hydrographs were calculated and plotted against impervi-
ousness and return period. These runoff volumes represent approxi-
mately the maximum runoff volumes of a 24-hour storm for each return
period since the original 60-minute rainfall hyetograph volume was
almost identical to the 24-hour rainfall volume. A graph of storm vol-
umes is shown in Figure 11.
COMPARISON OF REGRESSION MODEL AND STORM HYDROGRAPH
MODEL
Since two entirely different, techniques were used to analyze storm water
runoff, a comparison was made of the results of the two. Because both
methods ultimately calculated the volumes of runoff, storm volumes
were chosen as the basis for comparison.
26
-------
(5)
= 5, I =0.1, m = 12
(1) Intensity - duration-frequency
curve for a five-year storm
rainfall
(2) Synthesized hyetograph
(3) Effective precipitation hyetograph
(4) Runoff hydrograph for area with
12 minute lag time
Flow-duration-frequency curve
for five-year storm runoff
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Time (minutes)
Figure 7. Derivation of Flow-Duration-Frequency Curve
for Five-Year Storm Runoff
-------
O
cti
03
<-n
O
K
O
a
txO
cfl
d
s
'x
a!
Lag Time = 8 minutes
10
20
30 40
Time (minutes)
50
Figure 8. Effect of Impervious Fraction (I) on Five-Year Storm Runoff
28
-------
o>
SH
O
Cti
K
-------
CO
o
CD
S-i
O
cfl
n!
K
O
§ 2
K
0)
bo
CO
Fraction Impervious 1-0.3
Lag Time m = 4
10
15
20
25 30 35 40
Duration (minutes)
45
50
55
60
Figure 10. Average Runoff Rate vs. Storm Duration
-------
0.04
o
cti
ao
a
s
3
.— <
O
a
JH
O
x
cti
0.03 —
0.02
0.01 —
0. 1
0.2 0.3 0,4
Fraction Impervious
0. 5
Figure 11. Maximum Storm Volumes
31
-------
A five-year computer calculation of daily flows for the Wilde Lake water-
shed, as determined by the regression model, was examined and a fre-
quency distribution of the runoff was plotted. This was then converted
to a plot of daily flow versus return period as shown in Figure 12. To
calculate the volumes of storms using the hydrograph approach, curves
similar to those in Figure 11 were employed, using a fraction impervi-
ousness of 0. 22. This provided storm volumes for storms with return
periods of one, two, five, and ten years, which could be compared to
the corresponding volumes in Figure 12.
A plot of storm volumes calculated by both methods is given in Figure 13.
It can be seen that the different techniques gave approximately the same
results with a maximum difference of only eight percent for a two-year
storm.
INPUT TO THE SYSTEM ANALYSIS
In order to use the final regression equation (Equation 7) to calculate con-
tinuous runoff hydrographs for each of the Wilde Lake sub-watersheds,
it was necessary to first determine the fraction imperviousness of each
watershed. Since land use data were readily available for the Columbia
project, a tabulation was made of the acreage that was to be devoted to
each type of land use in every sub-watershed. A factor was applied to
each use to represent the percent imperviousness estimated for that type
of use. By multiplying the impervious factor by the number of acres and
summing for all land uses, the total number of pervious and impervious
acres could be obtained.
For the portions of watersheds that were situated outside the Columbia
boundaries, a similar procedure was followed. In this case, either ex-
isting land uses or zoning maps were used to tabulate the land uses. The
same impervious factors were applied to the non-Columbia acreage as
were used for similar uses in the Columbia project.
Figure 14 is an actual tabulation of land uses, residence units, popula-
tion, and imperviousnesses for sub-watershed number 17. The same
tabulation was prepared for all other watersheds and the overall imper-
viousness (shown in the last line of Figure 14) was used as I in Equation 7.
The only additional data required to calculate runoff hydrographs were
daily precipitation for the Wilde Lake area. After examining monthly
precipitation data from four rain gages in the vicinity of Columbia, it
could be seen that the Friendship Airport weather station data agreed
most closely with the monthly rainfall calculated for Columbia by the
Thiessen method. Therefore, five years of daily rainfall data were
obtained from Friendship Airport to be used as input to the regression
model.
-------
cd
P
30
Estimated
CD
G
O
i—i
i—i
03
bo
S
o
25
20
0)
a
i — i
o
15
CO
CO
10
,
,
,
4 567
Return Period (years)
10
Figure 12. Calculated Daily Volume vs. Return Period - Wilde Lake
-------
30
TO 25
G
O
i—<
•—i
cfl
ttD
c
O
3 20
a
3
15
O
-}->
co
10
Calculated by Regression Analysis
Calculated by Hydrograph Analysis
4567
Return Period (years)
10
Figure 13. Calculated Daily Volume vs. Return Period
Wilde Lake Watershed
-------
Project 3519
SUB-DRAINAGE AREA
DATA SHEET
Coll. Area No. j f
COLUMBIA PROJECT
NON-COLUMBIA
LAND USE
OPEN SPACE
Single Family -
Low Density
Single Family -
Medium Density
Town- Houses
Garden Apartments
Mid-Rise
Apartments
Employment
Centers
School Sites
Public
Rights-of-way
OPEN SPACE
Single Family -
Low Density
Public
Rights-of-way
COLUMBIA TOTAL
NON-COLUMBIA
TOTAL
AREA TOTAL
AREA
(Ac.)
10.7
12.5
4.9
6.8
3.6
7.4
2.5
95
O.I
579
O.I
58.0
UNITS/
Ac.
RESID.
UNITS
POPUL./
UNIT
POPUL.
1.2
4.0
11,0
17.0
28.O
50
54
116
101
4.0
4.0
3.5
2:5
2.5
200
189
290
253
1.2
321
321
4.0
932
t|V
932
%
IMPERV.
o.o
7.0
I6.O
33.O
2O.O ;
22.Q
IOQO
18.0
85,0
o;o
,7.0
8.5
37.7
37.6
IMPERV.
AREA
(Ac.) •
,
/
E.O
1.6
'\A
O.8
i
7.4
0.5
8.1
21.8
21.8
Figure 14. Work Sheet - Typical Sub-Watershed Land
Use Tabulation and Imperviousness Factor Calculation
35
-------
These data were first altered to more closely represent the long-term
precipitation record at Friendship Airport. To perform the alteration,
a simple normalization technique was used which preserved the histor-
ical time distribution of rainfall, but which changed the daily volume of
rainfall on a random basis. This alteration eliminated the extremely dry
or wet months, which on the average would be unlikely to occur in a given
five-year period, without eliminating most daily extremes or greatly
changing yearly totals. The procedure used involved the following data
input:
1. A set of 30 monthly rainfall totals to represent typical monthly
rainfalls expected. These were obtained from a 50-year dis-
tribution of monthly precipitation at the Baltimore Customs
House. These values are shown in Table 3.
2. The continuous daily rainfall record for Friendship Airport
expressed in inches for the five-year period 1960 through 1964.
TABLE 3. TYPICAL MONTHLY RAINFALL TOTALS
BALTIMORE CUSTOMS HOUSE, 1900-1950
Month
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Total Monthly
Precipitation
(inches)
0.40
0. 69
0.92
1. 18
1.40
1. 52
1.71
1. 85
2. 00
2. 15
2. 29
2.41
2. 59
2. 71
2.90
Month
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Total Monthly
Precipitation
(inches)
3. 05
3.21
3.41
4.
5.
5.
60
80
4. 01
4. 28
4. 50
80
10
50
6. 00
6. 65
7. 45
10. 70
A computer program was used to take each month's daily rainfall data
and sum these data to give the total monthly rainfall. This monthly value
(Mi) was compared to the set of 30 typical monthly values and the month
that was closest to Mi numerically was chosen. (This can be called Ti. )
The difference between Mt and T-- was calculated and M; was increased
or decreased by this amount so that it equaled Ti. The adjustment was
done in the following manner:
1. If Mi was greater than Ti, one day during the month was chosen
at random and 0. 01 inch of rainfall was subtracted. (If that day
36
-------
had no rainfall, then another day was chosen. ) This was done
repeatedly until M. = T..
2. If Mi was less than Tj_, one day during the month was chosen
at random and 0. 01 inch of rainfall was added. (Once again,
only days on which rainfall actually occurred were changed.)
This was done repeatedly until M. = T..
As a numerical example of this, assume that for January 1962 a total of
5. 02 inches of rain fell according to the sum of the first 31 daily volumes.
It is found by comparison to the 30 typical monthly values that the typical
month which is numerically the closest to 5. 02 inches is one with 5. 10
inches of rain. This value therefore represents T..
T. - M. = 0.08 inch (18)
Since MI is less than TI, the 0. 08 inch must be added to the daily rain-
fall during the month of January 1955. A day on which rainfall occurred
would be picked at random and 0.01 inch would be added to the rainfall
data for this day. This procedure would have to be done 8 times until
the original 5. 02 inches of January rainfall would equal 5.10 inches.
This procedure was followed for each of the 60 months in the five-year
record and the adjusted daily precipitation was then used as the input to
the regression model. The computer was used to output daily runoff
values (neglecting base flow) for each sub-watershed and combination of
watersheds. A typical year's output for one of the sub-watersheds, in-
cluding coefficients, is shown in Figure 15.
It should be noted that the five years of rainfall data, when compared to
the long-term record, consist of one very wet year, three dry years,
and one average year. The occurrence of below average precipitation
during three years should provide a conservative input to an economic
study of storm water reuse.
STORM VOLUMES
To provide the estimates of one-year storm volumes for different water-
sheds, the results of the hydrograph analysis were employed. For every
sub-watershed, the values of imperviousness and size were used to ob-
tain from Figure 11 the total volumes of a one-year storm. These values
were then tabulated and are presented in Table 4. The application of
these volumes to the sizing of reservoir ponds and sedimentation facili-
ties will be discussed in a subsequent section.
GAGING AND SAMPLING
As a part of this program, a gaging and sampling station was established
in the Wilde Lake watershed. The results of this work are contained in
Appendix B.
37
-------
OJ
co
RUN— OFF
DATA RUN NO.
iGRESSION COEFFICIENTS
A a
JAN
FEB
MAR
APR
MAY
JUNE
JULY
AUG
SEPT
UCT
NOV
DEC
I
2
3
4
5
6
7
a
?
10
IT
12
13
14
IS
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
O.O
o.~o
0,0
0.0
0.0
0.0
0 .0
O.O
0.0
O.T>
0.0
0.0
JAN.
0-V59
O. 53
0.05
0.62
0.05
O.O3
0.27
6.6134 0.
6 . 61 3 4 0".
6.6134 0.
5.4347 I".
5.4347 1.
5.4347 1.
5.2710 1 .
5.2T10 1.
5.2710 1.
4.S080 3.
4.8080 3.
4.8080 3.
FEB. MAR.
O.O5 O.09
0.95
0.10 0.06
0.04
0.70 0 .66
0.17
0.02 0.12
0 .09
0.32 0 .39
0.18
0.12
0.46 0.54
0 .29 0.12
0.34
0.02
0 .09 0.04
0 .64
= 6
C
6777
6777
6777
60"78
6078
6078
6515
6515
6515
8510
8510
8510
APR.
0.01
0.23
0.67
0.46
I .03
0.16
0.09
0.02
0 . 14
0.21
0.03
AREH
O
0.0827
0.0827
0.0827
-OV35T9
-0.3519
-0.3519
-0.2183
- 0 . 21 83
-0.2183
-f. 1654
-1. 1654
-1 .165*
MAY
0.25
0.02
0.12
0.32
0.34
0.44
0.04
0.22
O.C4
- 0.1078 SCT.7*
A
JUNE JULY
0.09
OV07 OV26
~
0.36
O'i02
0.10
XT. 77 0.01
0.02
0.13 0.21
1 .29 0.08
0.02
0.90
0.07
-•
0.51
0 .05
I .39
0.46
0.02
0.22
0.01
0.02
AREA =
0 . 1078
AUG.
0.42
avis
IT. 06""
0.04
0.04
0.23
0.06
1.15
0.49
0.36
SEPT.
0.03
0.98
0.01
0 .03
0.05
0 .02
OCT.
0.15
0 ,06
0 . 80
1 .89
0.02
NOV.
0 .27
0 .07
O.,07
~.P .04
0.06
0.15
0 .57
o-. t o
,-o.o;6
0 jt>6
0.45
.0.08
0^42
0.44
0.07
<0. 13
"0.07
0.01
Figure 15. One-Year Daily Hydarograph for Sub-Water shed No. 6
-------
TABLE 4. CALCULATED ONE YEAR STORM VOLUMES
FOR WILDE LAKE SUB-WATERSHEDS
Storm Volume
Watershed (Million Gallons)
1 0.461
2 0.586
3 0.530
4 0.342
5 0.530
6 1.104
7 0.803
8 0.626
9 1.460
10 1.258
11 0.854
Watershed
12
13
14
15
16
17
18
19
20
21
22
Storm Volume
(Million Gallons)
0.594
0.697
1.030
0.236
0. 782
1.130
0. 290
0. 924
1.203
0.690
0. 358
39
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SECTION V
WATER QUALITY AND DEMANDS
Storm runoff has been stored and reused locally within the catchment
area for centuries. Many island communities, such as Gibralter, Guam,
and Bermuda, have been entirely dependent on local efforts to maximize
this type of water resource to supply their needs. The Department of
Agriculture published handbooks to assist farmers in constructing farm
cisterns as supplementary water supplies. In contrast to water falling
on clean and well-maintained, frequently impervious catchments, the
storm runoff from a typical urban area has acquired heavy pollutant
loads before it enters a storm water collection system. In addition to
discharging large quantities of pollutant into natural watercourses,
storm runoff is sometimes permitted to enter combined sewer systems
or may accidentally enter sanitary sewers through leaks, improper con-
nections, etc. , where it causes overflows and further increases stream
pollution.
The reuse of urban runoff as a supplementary water resource presents
a number of problems, detailed elsewhere in this report. It is clear that
the treatment of storm water to drinking water quality is potentially a
difficult and expensive task. Section VIII discusses the treatment prob-
lem in some detail. Since many possible water uses do not require
drinking water quality, an investigation was made of applications of sub-
potable water within an urban community. A literature search was made
to discover other applications of storm water to sub-potable reuse. No
local reuse of storm water was reported in the literature surveyed, but
numerous applications of sub-potable water systems, including reclaimed
wastewater, were reported (6, 1, 8, 10, 13, 20, 21, 22). Next, the kinds of
reuse available in the Columbia area were investigated and listed. The
various types of reuse were categorized into four water quality classes
and criteria developed for estimating the total demand in each. Finally,
demands were calculated for the Wilde Lake watershed and each of its
component sub-watersheds.
A literature survey has been made of previously reported efforts to re-
cover a wide variety of waste and sub-potable waters for distribution to
uses that would otherwise be supplied by public potable water systems.
This search was made to determine previous technical success with such
projects but, more particularly, to determine levels of demand for vari-
ous types of reuse and to discover whether public acceptance of such
systems was achieved. The largest portion of reported applications was
devoted to use of reclaimed wastewater, normally from the municipal
wastewater treatment plant. Although this involves a water resource
somewhat more difficult to treat and probably much more unlikely to win
widespread acceptance, published experience has been summarized to
permit an examination of a more severe case than the one under study.
41
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-WATER RECLAMATION'
TKe expression "wastewater reclamation" is used to overcome ptfblie
aesthetic- or, more properly, semantic - objections to the term "sewage
.treatment" (6)., In a broad sense, "wastewater reclamation is defined
as'.ih'e purposeful upgrading of the quality of sewage, rendering it reus-
able by agriculture, industry, or the public (11). Planned wastewater
-reclamation has been practiced for over 30 years. All the basic use
categories except drinking water supply are being directly served by
reclaimed waters. Even though the present quantity of reclaimed waste-
water being used is still rather small, planned reclamation appears to
be on the threshold of becoming a major consideration in the augmenta-
tion of water resources (9).
Wastewater reclamation falls into two categories:- incidental and planned.
Waste treatment plants discharging into fresh waters provide incidental
reclamation in that the effluent may be used again. On the other hand,
.planned reclamation involves the production of water suitable for direct
application to a beneficial use. Treatment facilities are financed and
constructed specifically for this purpose (9).
Reclamation of wastewater may be directed toward any of the following
applications:
Specialized industrial uses
Crop and domestic'irrigation
Recreational lakes;and ponds
Ground water recharge
Non-potable domestic uses, e.g., toilet flushing (6)
A brief summary of reported experience in each of these areas of use
follows.
INDUSTRIAL USES
-Direct use of treated municipal wastewaters by industry is presently
small, but the increasing number of undertakings attest to the enormous
potential that exists (13). .Most of the recirculation by industry has been
limited to using industrial wastewater for cooling purposes and a few have
virtually closed systems requiring only small amounts of makeup water
(24) Mining, metallurgical, and allied industries located in arid regions
of the United States have pioneered the reclamation of wastewater and
many large industrial plants find it desirable to treat and reuse waste-
water because of significant financial savings (10). The literature reports
that more than 150 industries located in 38 states are reclaiming indus-
trial wastewater (10). Two well-known examples are Bethlehem Steel
42
-------
Company's plant at Sparrow's Point, Maryland, which uses effluent from
Baltimore's domestic sewage treatment plant (13), and the Kaiser Steel
Company at Fontana, California, which reclaims industrial and domes-
tic wastes from'its own plant (6).
The quantities of water used by industry may often be radically reduced
by proper reuse. The steel industry offers the best example of this. At
the Kaiser Steel Plant at Fontana, the net use of water has been reduced
to 1000 gallons per ton of steel. This is a substantial saying when com-
pared to the national average of 65, 000 gallons per ton of steel. Not only
will the reuse of water affect the total quantity of water used, but reused
water can often be purchased at a considerably lower cost (23; see also
15-22).
AGRICULTURAL USES
Irrigation requires more water than any other use (24). Methods of irri-
gation with reclaimed water include: flooding, spraying or sprinkling,
ridge and furrow, and subsurface irrigation. Although most state health
standards do not permit the use of sewage for truck farming regardless
of the degree of treatment, treated effluent can be used on cotton, beets,
vegetables grown for seed production, pasture crops, and woodlands (13).
Crop irrigation has been used as an aid to disposal of community wastes,
with irrigation being merely incidental to the disposal operation. At
Fresno, California, the city sewage farm takes the place of a secondary
treatment plant that would be needed to sufficiently clean the wastewater
for disposal to a canal or stream (10).
RECREATIONAL USES
One of the outstanding examples of recreational use of reclaimed waste-
water is the installation at Golden Gate Park in San Francisco. This
activated sludge plant is providing an effluent which is chlorinated and
used for maintaining the level of decorative lakes and the irrigation of
pastures, grass, and gardens. Irrigation of golf courses using reclaimed
wastewater has increased in recent years. The Marine bases at El Toro
and Pendleton, California, are examples. At El Cajon, near Palo Alto,
California, there are semipublic courses now under construction which
will use sewage effluent for irrigation. It has been shown that golf
courses employing sewage effluent are able to maintain themselves with-
out additional fertilization (10).
A reclamation project has been undertaken by the Santee County Water
District in San Diego County, California, which provides for the develop-
ment of a series of artificial lakes fed by the effluent of a standard acti-
vated sludge sewage treatment plant. There are four lakes, two of which
are open to the public for recreational uses. Swimming is currently not
permitted, and catching fish for human consumption is not allowed. How-
ever, a recreational area is provided, via use of reclaimed wastewater,
in an area which affords little attraction of this type (13).
43
-------
GROUND WATER. RECHARGE
Replenishment of ground waters through artificial recharge has been
given considerable attention. In many areas, ground water levels have
been rapidly falling and are not being restored through natural means.
Annual water replacement may lag behind withdrawals due to the slow
movement rate of ground water and limited opportunities for surface
waters to penetrate the earth's surface. Waters suitable for artificial
recharge may be classified generally as flood waters, industrial wastes,
and municipal sewage effluents (13).
Numerous methods are used in artificial recharge operations. The old-
est and still most common technique is the utilization of holding basins.
A second method is the use of a modified stream bed. A stream chan-
nel is widened and treated by a combination of methods to increase the
percolation rate (13). Ditches and furrows may also be used. The basic
arrangements are contour, lateral, and branching types. Where slopes
are relatively uniform, flooding provides an economical means of re-
charge. Another recharge method is the pumping of water through in-
jection wells directly into porous strata (13, 16).
The Whittier Narrows water reclamation project in Los Angeles County,
California, is an example of ground water recharge potential (12). The
object of this project is to conserve and assist in restoring the Water
resources of the agricultural area of Southern California. Treated sew-
age is distributed into spreading ponds. The reclaimed waters then per-
colate through to the ground water aquifers and are eventually used in
the irrigation of agricultural lands. Operation of the Whittier Narrows
plant is being closely watched by various agencies interested in public
health, education, pollution control, and industry (13).
DOMESTIC USES
Domestic uses of reclaimed wastewater are generally limited to lawn
watering and toilet flushing with a few notable exceptions. These excep-
tions are relatively recent developments. Some of the domestic uses
reported are:
1. Sites of Grand Canyon National Park, where treated domestic
wastewater was first used in the park in 1926 for toilet flushing,
lawn sprinkling, cooling water, and boiler feedwater at the
power plant.
2. Pomona, California, where municipal sewage plant effluent
has been used for domestic irrigation of lawns and gardens
in a suburban home development since 1929.
3.
1931.
San Diego State Teachers College, California, where sewage
effluent was first used for lawn and shrubbery irrigation in
44
-------
4. Golden Gate Park, San Francisco, California, which was men-
tioned earlier, first used activated sludge plant effluent in 1932
for lawn watering and maintaining water levels in some of the
park lakes (6).
A pilot plant in Tucson, Arizona, has demonstrated that potable water may
be made from municipal waste treatment effluent. The process is auto-
matic and uses effluent from an activated sludge process, followed by
softening, flocculation, filtration, demineralization, and carbon absorp-
tion (14).
A more 'striking example is the experience of Chanute, Kansas. In 1956,
when drought dried up the local water supply, effluent from the city's
secondary treatment plant was diverted to the water supply reservoir.
Treated wastewater was recycled through the city 8 to 15 times during
a five-month period under the supervision of public health authorities
(8,14).
DUAL SYSTEMS
For situations where domestic water requirements can be partially met
by a nearby source of reasonably clean water, a dual water system may
be used. Dual water systems can be defined as those involving distribu-
tion of two grades of water to consumers through independent pipe net-
works (7).
The idea of dual water systems for domestic supply received consider-
ation in the United States as early as 1894. At first, universal adoption
of the concept was not advocated. It was urged, however:
"Where there is a supply of naturally pure water sufficient to
meet culinary and drinking requirements, it (should) be sup-
plied through a separate system, leaving the larger require-
ments to be furnished from the nearest, cheapest source of
reasonably clean water" (7)
The water of Coalinga, a city of 6300 located in South Central California,
is highly mineralized and unsatisfactory for drinking purposes and most
home uses. Because of the poor quality of this supply, softer water is
shipped to Coalinga from Armona, California, by railroad car. Since
1931, this water has been distributed to the homes through a system
constructed in parallel with the hard water system. The hard water
supply is disinfected for safety reasons. Thus, Coalinga has been
operating a dual water system for-over 35 years (7).
Avalon, a town on Catalina Island offshore from Los Angeles, operates
a dual water system. This system has been in operation since 1914. A
fresh water distribution system supplies water for drinking, cooking,
and bathing. A salt water system supplies ocean water for fire protec-
tion and sanitary purposes. It appears that the operation of this system
will continue due to the shortage of fresh water on the island (7).
45
-------
Another example of a dual water system is the one in operation at the
University of Wisconsin since sometime prior to 1913. The University
draws water from Lake Mendota which is treated and pumped to the
buildings and used for showers, laboratories, air conditioning, fire pro-
tection, boilers,and other uses not involving human consumption. Drinking
water is purchased from the city of Madison and supplied to the same
buildings through a separate distribution system (7).
For public health reasons, community-wide distribution of biologically
safe and unsafe waters through a dual system cannot be seriously con-
sidered. If the question of dual water systems is examined on the con-
dition that both supplies are safe for drinking purposes, then the concept
appears worthy of study. The two supplies are usually termed "potable'
and "sub-potable" (7).
The sub-potable supply should be maintained at a quality level such that
occasional inadvertent use for drinking purposes will not result in harm
to the consumer. The supply must be free from harmful biologic forms
and toxic chemicals. Elements of the sub-potable system that furnishes
water for normal and fire fighting demands must provide hydraulic capac-
ity for fire flows plus the coincident draft for toilet flushing, irrigation,
and other domestic and industrial uses.
PROBLEMS AFFECTING PUBLIC ACCEPTANCE
The literature is essentially silent on public reaction to the aesthetics of
wastewater reclamation and dual water systems. It can be observed,
however, that the public has accepted existing projects. Even though
this acceptance has occurred, the aesthetics of reclamation are still con-
sidered an important factor (9).
Hundreds of U. S. cities draw their water from inland streams. They are
in fact, using the diluted sewage effluent of upstream cities. Although the
presence of such pollution is generally known, consumer acceptance of the
water does not appear to be appreciably reduced. Dilution and time appear
to dissipate aesthetic objections. An assumed public aversion to the con-
cept of direct wastewater reclamation has been a major deterrent to ex-
pansion of this practice (9).
When dual water systems are mentioned, objections regarding health
hazards are raised. These objections arise from associating the word
dual with the hazardous combination of potable and contaminated water
(7). Early in the development of dual water systems, it was established
that the most serious difficulty in operating such systems "will be found
in the proper regulation of the use of purified water" (7) It was felt
that some customers would use the pure water for other than intended
uses, while others, either carelessly or to avoid cost, would not use
it at all.
46
-------
Conversely, the two water supplies in existing dual water systems are
both safe for drinking purposes. The "potable" supply is maintained at
an excellent quality, suitable in all respects for drinking, cooking, and
bathing. The "sub-potable" supply, inferior in chemical quality but
equal in safety, is suitable for, waste transport, lawn and garden irrigation,
street flushing, fire protection, and other uses not requiring water of
high quality. Each dwelling served by a dual water system requires two
service lines and two meters (7).
The difference between treatment systems for water reuse and wastewater
treatment for disposal to streams seems to be one of degree rather than
of kind. At least no sharp distinction is evident in the literature. Streams
and lakes are natural systems for bio-treatment. Their water is often
reused several times before discharge to the sea. Present public accep-
tance of "natural" processes is considered greater than acceptance of
engineering processes (4).
REUSE CLASSIFICATIONS
Storm water was considered as a potential water source for every level
of water use existing within the Wilde Lake area. Although each discrete
use has its own unique water quality requirement or standard, an estimate
will be made to draw the various uses into a limited number of categories,
simplifying further analysis. Whenever a group of use types is defined
as one category, the water quality standard selected for that category
must meet the requirements of each use considered.
Water use can be broadly described as being either residential, commer-
cial, industrial, or public. Table 5 lists various types of residential
water uses, with approximate percentages of total consumption. Linaweaver
et al. collected data from more than 3000 residences in public metered
water and public sewer areas of the eastern United States and analyzed
that data in terms of residential consumption not including sprinkling use
and sprinkling use alone .(26). All areas studied in the eastern U.S.
averaged 56. 7 gallons per capita per day for nonsprinkling residential
use. The thirteen local areas studied ranged from 42. 9 to 71. 1 gallons
per capita per day (27). Examination of similarities between areas
studied by Linaweaver and the Wilde Lake region of Columbia led to the
selection of 61.5 gallons per capita per day as an estimate of water con-
sumption within the, dwelling unit.
Sprinkling use was conclusively correlated, in the Linaweaver work, to
the irrigable area associated with the dwelling unit and the local deficiency
between potential evapotranspiration and summer precipitation (28). Three
areas were located within the immediate vicinity of Columbia for which
data were available, known as Glenmont, English Manor, and Northwest
Branch Estates. All are served by the Washington Suburban Sanitary
Commission and are comprised of homes similar in value and lot size
to those in Columbia. Average annual sprinkling use, as computed by
Linaweaver, divided by average irrigable area for each of these locations
47
-------
resulted in sprinkling usages ranging from 253 gallons per day per acre
to 921 gallons per day per acre (27). In order to arrive at a value for
estimating demand in Columbia, these three areas were averaged, and
the mean was found to be 518 gallons per day per acre. Table 5 was
revised with slight regrouping of uses, due to certain uses sharing
common plumbing fixtures, making separation unfeasible. The revised
table is shown as Table 6.
TABLE 5. DISTRIBUTION OF RESIDENTIAL
WATER USE. AFTER REID (25)
Toilet flushing
Bathing
Dishwashing
Drinking and other kitchen
Laundering
Lawn sprinkling
Auto washing
% of Total
28
23
4
3
10
29
3
Gal/Capita/Day
24.0
20. 0
3. 75
2. 75
8.5
25. 0
2.5
TABLE 6. DISTRIBUTION OF RESIDENTIAL
WATER USE - REVISED
Quality
AA
Use
Human consumption, food prepa-
ration, general kitchen use
Gal/ Gal/
Capita/Day Day/Ac re
6. 5
A Bathing, laundering, auto washing
B Lawn sprinkling
C Toilet flushing
31. 0
24. 0
518.0
The water quality considerations shown in Table 6 have been assigned in
decreasing order to quality level. Water quality Class "AA" is defined
as meeting the U.S. Public Health Service Drinking Water Standards and
is intended for all potable water uses (29). Class "A" is virtually identi-
cal to Class "AA" except for taste and odor considerations not important
in water which will not actually be consumed or used in food preparation.
Class B has somewhat more lenient requirements, particularly with
respect to suspended solids, except that it must be disinfected to the
48
-------
same standard as Class "AA, " thus providing safety for accidental inges-
tion. Class "C, " also disinfected, has little more than a suspended solids
limit and minimal requirements for corrosivity. Specific limits on var-
ious quality parameters are shown in Table 7.
Commercial water use might contain all of the use categories discussed
above in addition to air conditioning loads and other miscellaneous com-
mercial uses. In all cases, the water demands of each commercial user
must be evaluated with respect to the four water quality levels described
above and defined in Table 7. Particular types of commercial use vary
widely not only in the specific uses involved, but in the relative magnitude
of each of them.
Industrial uses fall, to a great degree, into the same type of breakdown
as commercial uses. Additionally, industry frequently has large demands
for process water, whose required quality varies from very low to sub-
stantially higher than potable water quality, depending on the industry
and the process employed. In order to compute demands for each water
quality, a particular industry must be analyzed in detail to determine its
requirements by quality level. In the study area, the Wilde Lake water-
shed of Columbia, no process-water-using industry is contemplated,
making this type of analysis unnecessary in the present study.
Public water uses include applications such as street cleaning, sewer
flushing, fire protection, and snow melting. Street cleaning and sewer
flushing account for very little water use on a long-term basis, compared
to the available supply. Providing water for fire protection implies that
high flows will be available from a system of very high reliability. Fire
insurance rating bureaus normally insist that the required fire flows for
a community like Columbia be available entirely from elevated storage,
independent of pumping capacity. The lowest requirement in the Wilde
Lake watershed applies to the detached single family residential areas,
where 120, 000 gallons of storage must be continuously available. In
order to consider a number of separate systems, each of them must have
the full fire flow capability. Furthermore, the area under study is al-
ready served by the Howard County Metropolitan Commission with a con-
ventional public water supply system designed to provide necessary fire
flows. For these reasons, fire flows were not considered as possible uses
of the storm water within the local reuse concept. Another public use
considered was the flushing of streets following snowfalls to melt and
remove accumulated snow. This is nonconsumptive use from the stand-
point of the storm water system, since the runoff returns to the storage
facility. This type of snow removal is confined to those occasions when
pavement temperature is above 40°F and air temperature is at least 30°F,
to prevent refreezing of the water as ice. The flushing must also be
done at a time when the pavement will have the opportunity of drying
prior to the next freezing cycle, effectively preventing use of the tech-
nique at any time except under bright sunlight. Unfortunately, the circum-
stances which permit the use of water flushing of snow usually result in
rapid melting of the snow without flushing. The water required on the
infrequent occasions when this procedure is indicated is considered to be
a negligible amount.
49
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TABLE 7. MAXIMUM CONCENTRATION OF
SELECTED POLLUTANTS BY REUSE CATEGORY
Required Water Quality*
Maximum Concentration
Constituent
(mg/i unless indicated)
Alkyl benzene sulfonate
Ammonia (NH^)
Arsenic
Barium
Cadmium
Calcium
Carbon chloroform extract
Chloride
Chromium (hexavalent)
Copper
Cyanide
Fluoride
Iron
Lead
Magnesium
Magnesium+Sodium sulfate
Manganese
Nitrate (as
Oxygen, dissolved (minimum)
Phenolic compounds (as phenols)
Selenium
Silver
Sulfate
Total solids .
Zinc
BOD (5-day) mg/4
Coliform (MPN per 100 m 4)
pH (units)
Color (units)
Turbidity (units)
Suspended solids
Phosphates
Volatile suspended solids
*Based upon maximum concentrations.allowed by the USPHS, the World
Health Organization, and the Water Quality Standards of the State of
Maryland.
--Higher suspended solids are permitted by various water quality stan-
dards. Limit based on sediment control and water contact recreation.
AA
0.5
0.5
0.01
1.0
0.01
75
0.2
250
0.05
1.0
0.01
1.5
0. 3
0.05
50
500
0. 05
45
5.0
0.001
0.01
0.05
250
500
5.0
1
7.0
15
0-3
1.0
A
0. 5
0. 5
0. 05
1.0
0.01
75
0.2
250
0.05
1.0
0. 2
3.0
0. 3
0. 1
150
500
0. 1
50
5. 0
0.002
0.05
0.05
250
500
15
70
6.0
20
3-8
1.0
B
0. 5
0.5
0. 05
1.0
0.01
75
0. 2
250
0.05
1.5
0.2
3.0
0.3
0.1
150
1000
0. 5
50
4.0
0.002
0. 05
0. 05
400
500
15
240
6.0
30
8-15
1 r\ •-'•' "•''
j_ \j '\-- 'I-*
1.0
c
0. 5
0. 5
Of 05
1.0
0.01
75
0.2
250
0.05
1.5
0.2
3.0
0. 3
0.1
150
1000
0.5
50
4.0
0.002
0.05
0.05
400
500
15
240
6.0
30
15-20
30**
1.0
50
-------
DISTRIBUTION OF DEMANDS
In order to permit evaluation of various levels of treatment for each of
the sub-watersheds and combinations of sub-watersheds, the water de-
mands were tabulated, by water quality class, for each sub-watershed.
The boundaries of the sub-watersheds were revised slightly for this
tabulation in order that the area of computation be similar to that which
would actually be serviced by a discrete distribution system within the
sub-watershed. Figure 16 shows the distribution areas, divided by
broken lines, except for the sub-watersheds lying wholly outside the
Columbia project, where the natural drainage lines will continue to de-
fine the areas. A second tabulation was made of land uses within the
revised sub-watersheds, analogous to that shown previously as Figure 14.
This tabulation, shown as Figure 17, also lists irrigable area, defined
as the difference between total area and impervious area, excepting open
space. This is the approximate area of all lawns, gardens, and school
grounds which are likely to be irrigated by lawn sprinklers during the
summer.
The usage coefficients described earlier for each of the water quality
classes are applied to the appropriate parameters and residential water
use is computed for each quality level. Other demands, including, com-
mercial and institutional uses, are estimated individually for each instal-
lation and listed on the sheet. Residential and other demands can be
totalled to yield water demand by water quality class for the sub-watershed.
This operation was performed on each sub-watershed in the entire drainage
area to evaluate total demands. Prior to assembling this information for
input to the systems analysis computer program, however, the demands
listed for Class "B" uses, lawn sprinkling, were derated. The storage
requirements associated with several patterns of water demand were
evaluated. It was found that the nonuniform behavior of sprinkling de-
mands would place a significant requirement on reservoir size. Since
other sources of supply would be required during certain periods, an
analysis was performed to determine the extent to which sprinkling de-
mands could be met with reservoirs of reasonable size.
It was determined that when sprinkling demands were satisfied with an
87. 5 percent reliability and the balance of the demand with a 100 percent
reliability, the reservoir requirement was essentially independent of the
fraction of total demand which was composed of sprinkling uses. In other
words, sprinkling demands are interchangeable with other demands, pro-
vided no more than 87. 5 percent of the sprinkling use is satisfied by the
system. The balance of the demands would be supplied from the public
water system through a makeup water connection. As a result of this
analysis, sprinkling demands in the watershed were reduced to 87. 5 per-
cent of their original value, then treated on the same basis as other,
uniform demands with respect to storage requirements. The methods
used in making this analysis are described in Section VII.
Examination of the water use categories indicates that eight possible com-
binations of water use exist. This includes all possible combinations of
the lower three quality classes, as well as all four classes together.
51
-------
CJ1
LEGEND
WILDE LAKE
WATERSHED
/ SERVICE AREA
BOUNDARIES
Figure 16. Wilde Lake Watershed - Service Areas
-------
Project 3519
SUB-DRAINAGE AREA
DATA SHEET
Distr. Area No.
u
LU
•— >
o
Q£
Q-
co
— \
0
U
, <
Zffl
i=
0
u
LAND USE
OPEN SPACE
Single Family -
Low Density
Single Family -
Medium Density
Town- Houses
Garden Apartments
Mid-Rise
Apartments
Employment
Centers
School Sites
Public
Rights-of-way
OPEN SPACE
Single Family -
Low Density
Public
Rights-of-way
COLUMBIA TOTAL
NON-COLUMBIA
TOTAL
AREA TOTAL
AREA
(Ac.}
IO.7
12.5
4.9
6.8
3.6
14.
9.5
62O
62O
UNITS/
Ac.
1.2
4.0
II.O
I7.O
28.0
1.2
RESID.
UNITS
50
54
116
IOI
321
321
POPUL./
UNIT
4.0
4.0
3.5
2.5
2.5
4.0
POPUL.
2OO
189
29O
253
932
932
%
IMPERV.
o.o
7.0
I6.O
33.0
2OD
22.0
IOO.O
I8.O
85.O
0.0
7,O
85.O
45.O
45. 0
IMPERV.
AREA
(Ac.)
2.0
1.6
1.4
Q8
I4.O
8.1
27.9
27.9
OTHER
DEMANDS
ESTIMATED WATER DEMANDS (AVG. DAY)
POPULATION X 6.5 G/D/Person (AA) =
POPULATION X 31.0 G/D/Person (A)
= 23.4 Ac. X 518.0 G/D/Acre (B)
POPULATION X 24.0 G/D/Person (C)
ESTIMATED DEMAND OF WILDE (AA)
LAKE VILLAGE GREEN SHOP-
PING CENTERx LIBRARYxAND
COMMUNITY CENTER.
DEMAND
(G/D)
12X120
22x370
3x800
400
8x750
Figure 17. Work Sheet - Typical Sub-Watershed Land Use
Tabulation and Water Demand Calculation
53
-------
It was assumed that, should potable water be produced, there would be
no incentive for separating plumbing systems, therefore all demands
would be connected to a single system. In all the other cases, where
sub-potable water is being produced, the plumbing systems constructed
for sub-potable water distribution might be connected to any combina-
tion of fixtures.
The water demands computed for each sub-watershed, after derating of
the Class "B" demand as discussed above, were combined into each of
the possible use combinations and displayed on a table. This table was
input to the computer program to serve as the basis for evaluating vari-
ous alternate reuse systems. A printout of this table appears as Figure
18.
54
-------
**** OUTPUT ****
***«• WATER DEMANDS _ BY. SFRVIf.E _LEVF1 ****
WATERSHED
AA
1
2
3
4
_5
6
7
8
9
10
11
12
_L3
14
±5
16
JJ
18
19
20
2J
22
550.0
550.0
210.0
1960.0
2550.0
17310.0
. -9080.0
4050.0
2730.0
45460.0
3150.0
1480.0
_234Q.Q_..
5280.0
810.0
2310.0
6060.0
1200.0
1140.0
3460.0
_. 5430.0.
1460.0
2600.0
2600.0
990.0
9180.0
12150.0
82550.0
43310.0
18230.0
13000.0
33670.0
15000.0
7070.0
11160.Q .
23500.0
3840.0
11040.0
28890. 0-
5700.0
5460.0
17800.0
25890.0
6940.0
7210.0
7298.0
3120.0
6799.0
4078.0
20983.0
11830.0
7980.0
36930.0
9520.0
12100.0
5800.0
10330.0
15910.0
3130.0
21480.0
10610.0
5710.0
12190..Q-
17590.0
10560.0
6210.0
2020.0
2020.0
770.0
7480.0
9410.0
63910.0
33530.0
15060.0
10100.0
34760.0
11620.0
5470.0
8640.0
21690.0
2980.0
8540.0
22370.0
4420.0
16280.0
20040.0
5380.0
A,B
9810.0_
9898.0
4110.0
15979.0
16228.0
103533.0
55140.0_
26210.0
49930.0
43190.0
27100.0
12870.0
21490.Q_
39410.0
6970.0
32520.0
39500.0
11410.0
_.17650..0__
35390.0
36450.0
13150.0
A,C
4620.0
4620.0
1760.0
16660.0
21560.0
146460.0
76840^0.
33290.0
23100.0
68430.0
26620.0
12540.0
i9aoa.a
45190.0
6820.0
19580.0
51260.0
10120.0
96BO.O
34080.0
_ 45930.0
12320.0
A.B.C
1 1A3O.O
11918.0
4880.0
23459.0
25638.0
167443.0
88670,0
41270.0
60030*0
77950.0
38720.0-
18340.0
^Dl^fl-O
61100.0
995O.O
41060.0
61870.0
15830.0
?lfl70T0
51670.0
564<»0»0
18530.0
B,C
<)?~*n.n
9318.0
ifl^An,n
23040.0
4703fi,o
44280.0
2^72f».0
11270.0
1BQ70.0
37600.0
6HO.O
30020.0
•*2
-------
SECTION VI
STORM WATER QUALITY
In order to specify the various treatment alternatives which might be con-
sidered in the development of the optimum system to achieve the desired
water quality requirement, it is necessary that the pollutional content of
the storm water be known to some reasonable degree. At the least, the
nature and approximate maximum levels of concentration of the principal
pollutants should be known. The first attempt towards determining these
parameters was to search published reports for analysis of urban runoff
from similar areas. Numerous references to storm water quality were
located, but only a few went further than isolated, unrelated pieces of
data (30-38). Among the several extensive studies of storm water pollu-
tion, most failed to relate specific pollutants to conditions in the watershed
such as land use, industry, industrial and commercial wastes, etc. Pol-
lutant concentrations quoted seldom included any reference to the intensity
of runoff at the time of sampling or the hydrograph previous to the sam-
pling time. One of the more complete studies of storm water pollution
was performed in Cincinnati, Ohio, by Weibel et al. , yet it did little more
than set up some broad ranges of pollutant concentrations for watersheds
similar to the one analyzed (31). Table 8 summarizes the observed range
of a number of pollutants found in storm water at a variety of locations
throughout the world. Each of the studies used to develop this table were
sufficiently comprehensive to exclude the possibility of a chance or "freak"
occurrence of an unrepresentative concentration of some specific pollutant.
It can be seen that many of the items vary by two or more orders of magni-
tude throughout reported experience. It was concluded that the literature
survey would be of little value in establishing a pollutant baseline for the
Columbia location, due to the very wide variance of reported data. The
sections below describe the tentative listing of maximum concentrations
for expected pollutants, followed by the results of the water quality sam-
pling station, intended to verify the tentative data.
TENTATIVE DESCRIPTION OF RUNOFF QUALITY
A review of the specific pollutants mentioned in various studies led to a
classification of the parameters as shown in Table 9.
Solids
None of the studies reported above contained any quantitative data on
floating solids. Inlet structures are normally expected to exclude large
floating objects (boards, tree branches, toys, etc. ) but the existence of
open drainage channels downstream from the closed storm water collection
system permits such objects to enter the flow. In addition, leaves, small
branches, paper or cardboard objects, etc. , are commonly found in all
storm water collection systems. Much of the debris will eventually ab-
sorb water and sink, joining suspended solids, but some will remain
bouyant until it is manually or mechanically removed from the stream.
57
-------
TABLE 8. REPORTED STORM WATER
POLLUTANT CONCENTRATIONS*
Observed Range
Dissolved Solids
Suspended Solids
Volatile
Nonvolatile
BOD
COD
DO
Total Nitrogen (NO2, NO3,
Organic)
Total Phosphate
Coliform Total/100 ml
Fecal Strep/100 ml
Chlorides
30 -
154 -
26 -
38 -
119 •-
6. 9 -
18 -
6.4 -
2.3 -
0.47 -
40
median-
11 •
8, 000
228
36,250
98
292
625
3, 100
8.0
11. 8
1, 400
240, 000
20, 500
160
Number
Location^
. ' .1,. /
1 ."
6
1
.. 1
9:
2
' 1
, 3
2
4
' ' 2
*Refs. 30, 31, 34, 35, 36, 37, 38
Oxygen Balance
Nutrients
Bacteria
Miscellaneous
Physical and
Chemical
Parameters
TABLE 9. PRINCIPAL INDICES OF
STORM WATER POLLUTION
Pollutant
Floating Solids
Suspended Solids
Dissolved Solids
Biochemical Oxygen Demand (BOD)
Chemical Oxygen Demand (COD)
Dissolved Oxygen (DO)
Total Nitrogen (N,NO2, NOs, etc.)
Total Phosphorous (P, PO4, etc.)
Coliform Group
Fecal Streptococci
PH
Temperature >
Oil and Grease
Chlorides
Pesticides
Surfactants
Unit of Measure
Ib/day
mg/ JL
mg/ i (5-day)
mg/ H
mg/ji
mg/ !L
MPN/100 ml
MPN/100 ml
pH units
°C
mg/4
mg/A
mg/ml
mg/ IL
58
-------
To a degree, the nature and size of these objects will be determined by
the physical layout of the collection system. No estimate was made for
the incidence of floating solids.
Suspended solids include all those particles so fine or so bouyant that
they are suspended in the stream. It may include both particles which
will settle following loss of stream velocity and those which are either
colloidal or at neutral bouyancy. These solids may or may not be volatile
in nature. The volatile solids are generally composed of various organic
substances,, both animal and vegetable in origin, which are characterized
by putrescibility and eventual decomposition into simpler, stable materials.
Volatile solids of vegetable origin may include cut and fallen vegetation
(leaves, grass, etc.), humus, and discarded food. Animals contribute
offal and carcasses of various sizes and stages of decomposition. Non-
volatile suspended solids are primarily soils and general surface debris
and are inherently stable and inorganic in nature. Analysis of runoff from
similar watersheds in the Cincinnati study cited earlier led to the tenta-
tive assumption that, after construction is completed, runoff from Columbia
will probably not exceed 300 mg/4 suspended solids, 60 mg/4 of which
might be volatile.
Dissolved solids may include various soluble minerals and salts, as well
as small quantities of other organic and inorganic materials which might
be slightly soluble. Decomposition of volatile suspended solids often pro-
duces soluble organic products. Some dissolved materials have been pre-
cipitated from the atmosphere by the rainfall and were dissolved before
striking the ground, but most have entered the water as suspended solids
and have subsequently dissolved. Many dissolved substances are of inter-
est themselves as specific pollutants, such as chlorides, pesticides, sur-
factants, etc. Since these separately discussed pollutants are the only
dissolved solids expected to be of any interest in the Columbia runoff,
no limit was created for dissolved solids.
Oxygen Balance
Biochemical Oxygen Demand (BOD) is an index determined by a labora-
tory procedure for measuring the amount of oxygen consumed, in five
days, by natural agencies in stabilizing the volatile material in the water.
It is a reasonable estimate of the oxygen that will be consumed in nature
as the various organic materials decompose. Untreated domestic sewage
has a BOD on the order of 200 rag/A, while Maryland Department of Water
Resources water quality criteria classify as "good" a stream having less
than 3. 5 rag/ & BOD. It was estimated that the storm runoff might contain
up to 25. 0 mg/4 BOD.
Chemical Oxygen Demand (COD) measures the amount of oxygen consumed
under specified conditions in the oxidation of organic and oxidizable inor-
ganic matter, corrected for the influence of chlorides. COD and BOD tend
to complement one another, although there is no inherent relation between
the two. The difference is primarily one of the mechanism of oxidation.
In the present analysis, the BOD test is considered more appropriate
than COD, so no estimate was made of storm water COD.
59
-------
Dissolved Oxygen (DO) measures the quantity of oxygen available in ,the
water in molecular form. It may range from 0.0 up to saturation, which
is a function of water temperature and, to a lesser extent, atmospheric
pressure. Saturation levels may range, at sea level, from 7. 1 mg/£ at
35°C to 14. 4 mg/£ at 1°C. It can be assumed that falling raindrops are
100 percent saturated and that any loss of oxygen content is a result of -
rising temperature or of physical contact with oxidizable contaminants,
or both. It was assumed that the DO of storm water might be as low as
0.0 mg/0.
Nutrients
Both nitrogen and phosphorous are effective nutrients and are commonly
found in water as nitrates and phosphates. Nitrogen might also be present
as nitrites and as dissolved or organic nitrogen, due to the decomposition
of organic material. The presence of either of these materials is usually
indicative of decomposing organic materials in the stream or in the water-
shed, or of the direct application of chemical fertilizers containing these
materials to land in the watershed, resulting in a high nutrient level of
runoff. The presence of these substances in excessive concentrations
contributes to algal blooms and to distortion of the ecological balance of
the receiving body of water. It was assumed that the runoff under study
would not exceed 3.0 mg/2. Nitrogen (measured as NOg) or 1. 0 mg/0
phosphorous (measured as
Bacteria
Due to the great difficulty of isolating and identifying all of the possible
pathogenic (disease-producing) bacteria which might be present in storm
water, it is common practice to test for presence of one of the fecal
bacteria and use that strain as an indicator of fecal contamination. If
such contamination is found to be present, it is considered possible that
pathogens might also be present. If no fecal bacteria are identified, the
presence of pathogens is considered unlikely. The fecal bacteria used
most widely as an indicator is the coliform group of bacteria, particu-
larly the Escherichia Coli, a relatively large rod-like bacteria. It is
readily detected by means of any of several standard laboratory methods.
Since coliform bacteria may be present in soil as well as humans and
animals, more specific indicators, such as the fecal streptococci group,
are sometimes used. In all cases, test results are reported as most
probable number (MPN) per 100 ml. Since public health considerations
dictate that chlorination should be practiced regardless of the bacterial
concentrations, no limit was suggested for the probable bacterial pollution.
Miscellaneous Physical and Chemical Parameters
From the many parameters that could be considered under this heading
the only ones which are expected to be of any significance in the Wilde
Lake area are chlorides, oil, and grease. No quantitative estimate
was made for oil and grease, but it is virtually certain that runoff from
streets, driveways, and parking lots will include noticeable quantities
It is estimated that chlorides, primarily from salt used to melt snow in the
60
-------
winter; may range as high as 160 mg/jfc. Table 10 lists the tentative
storm water quality description for the Wilde Lake watershed.
TABLE 10. EXPECTED MAXIMUM POLLUTANT CONCENTRATIONS
. rFOR STORM WATER IN WILDE LAKE WATERSHED
Type
Solids
Pollutant
Floating Solids
Suspended Solids
Volatile Suspended Solids
Oxygen Balance BOD
Nutrients Nitrogen (N, NO2,
Miscellaneous
Physical and
Chemical
Parameters
Phosphorous (P, PO.^)
Chlorides
Oil and Grease
(as NO3)
(as PO4)
Expected Max.
Concentration
Present
300 mg/ji
60 mg/ H
25 mg/1
3. 0 mg/ JL
1. 0 mg/A
160 mg/ i
Present
The results of the water quality sampling program conducted in the
Wilde Lake watershed are contained in Appendix B.
WATER QUALITY STANDARDS FOR WILDE LAKE
The Maryland Department of Water Resources has recently adopted a
comprehensive set of water quality standards regulating the state's
interstate and principal intrastate waters, incompliance with federal
requirements (40). Although Wilde Lake is not specifically assigned a
minimum water quality in these standards, its intended use is clearly
consistent with the Class "C" uses outlined in the regulations — water
contact recreation, fish propagation, agricultural and industrial water
supply. These standards make the following requirements of bodies
of water intended for Class "C" uses:
1. Fecal coliform bacteria less than 240 MPN/100 ml
2. Dissolved oxygen greater than or equal to 4. 0 mg/£
at any time, 5. 0 mg/1 monthly average
3. pH greater than or equal to 6. 0, less than or equal
to 8. 5
4. Temperature less than or equal to 93 F (35 C)
61
-------
In addition, general requirements are made, prohibiting harmful concen-
trations of any other pollutants which might be present. Harmful concen-
trations are to be determined in terms of the intended uses of the body of
water which is to be protected.
A conservative interpretation of these water quality standards and the
ability of Wilde Lake to neutralize pollutants entering it from storm
water runoff led to the definition of effluent standards for all water
deliberately discharged into Wilde Lake or its tributary watercourses.
These standards take into account the possibility of spills during severe
storms and have been further adjusted to coincide with the water quality
criteria adopted for Class "C" water uses in the previous section.
Table 11 lists the parameters being used to define effluent water quality
together with the maximum concentration of each.
62
-------
TABLE ,11. EFFLUENT STANDARDS FOR
WATER DISCHARG'ED INTO WILDE LAKE*
Solids ,', ,,,.,
Oxygen Balance
Nutrients
Bacteria
Miscellaneous
Physical and
Chemical
Parameters
Floating Solids
Suspended Solids
Volatile Suspended Solids
BOD
Nitrogen (as
Phosphorous (as
Fecal Coliform
pH
Oil and Grease
Chlorides
Surfactants (ABS)
Ammonia (NHs)
Arsenic
Barium
Cadmium
Calcium
Carbon Chloroform Extract
Chromium (hexavalent)
Copper
Cyanide
Fluoride
Iron
Lead
Magnesium
Manganese
Phenolic Compounds
(as phenols)
Selenium
Silver
Sulfate
Zinc
Color
None
30.0 mg/4
None
3.5
50.0 mg/4
.1.0 mg/4
240 MPN/100
6. 0 or greater
None
nig/4
5 mg/4
5 mg/4
05 mg/4
0 mg/4
01 mg/4
rag/A
2 mg/4
05 mg/4
2 mg/4
0 mg/Ji
3 mg/4
1 me/^-
250
0
0
0
1
0
200
0
0
1
0
3
0
0
150
0. 5 mg/4
0
0
0
400
15
30
.002
. 05
05 mg/^
mg/Ji
mg/Ji
color units
= Refs. 40 and 41
63
-------
SECTION VII
STORM WATER STORAGE
The Wilde Lake watershed of Columbia is similar to other recent sub-
urban developments in its approach to the storm drainage problem. The
natural drainage channels have been retained with their original topog-
raphy and the floodplains are used, in part, to satisfy the open space
requirement of the community planning concept. The floodplains are
normally surrounded with building lots and these, in turn, are circum-
scribed by roads. The storm drain system consists of many relatively
small inlet and pipe systems, each draining independently into a flood-
plain. After discharge, the storm water follows natural drainage lines
to Wilde Lake. Roads are generally located at or near the ridge lines
of the various sub-drainage areas and only a few cases exist within the
Wilde Lake watershed where an appreciable amount of runoff is collected
by a single closed collection system before discharge to a floodplain.
CONSTRUCTION TYPES
The early planning of the Local Reuse System study provided for consid-
eration of unconventional storage facilities, such as natural or artificial
underground cavities, slip-formed vaults beneath streets, etc., as well
as more conventional approaches to storage within the floodplain. Exam-
ination of the drainage area indicated that significant quantities of storm
water were not available at any place in the watershed, except at loca-
tions within the natural floodplains. Of the 22 sub-watersheds consid-
ered, only number 10 enters Wilde Lake as a wholly closed system with-
out having been substantially collected within a natural floodplain. When
storage is to be provided in the floodplain, two types were considered -
open ponds and constructed storage. In sub-watersheds where floodplain
space is not available, such as number 10, conventional constructed
storage was considered, along with unconventional approaches, such as
special vaults under roads and parking lots. Construction costs for such
vaults were found to be on the order of $0. 20/gallon of storage capacity
and maintenance problems were considered to be severe. Since conven-
tional constructed storage is competitive with other methods from sizes
of 50, 000 gallons upward and maintenance is facilitated by the improved
access, parametric costs for unconventional approaches were not devel-
oped and no further consideration of them was judged necessary.
In the case of constructed storage, costs are dependent on location with
respect to existing grade. In most cases, the storage facility must be
placed below grade to permit gra.vity inflow under all conditions. Occa-
sionally, local topography may permit a tank to be constructed partially
or completely above grade. To maintain maximum flexibility, the below
grade and above grade cases were separately analyzed and costed. Tanks
of steel and of prestressed concrete construction were also investigated.
Open ponds were studied and the optimum combination of excavated and
65
-------
natural storage determined for ponds in topography similar to Columbia.
Table 12 summarizes the types of reservoir construction which were
studied and cost estimated.
TABLE 12. STORM WATER STORAGE . :
RESERVOIR CONSTRUCTION TYPES , ; ,-
Natural Storage Open Ponds
Constructed Storage Steel Tanks - Above Grade
Steel Tanks - Below Grade
Concrete Tanks - Above Grade
Concrete Tanks Below Grade
SIZING CRITERIA
The storm water storage facilities included in the Local Storage, Treat-
ment, and Reuse of Storm Water concept function as multiple purpose
reservoirs and,as such, are subject to several potentially conflicting
requirements. Their basic purposes, in the order of the priority implied
by the orientation of this study, are water quality control, water supply,
and the creation of a pleasing and harmonious asset to the local environ-
ment. As water quality control reservoirs, they are expected to be
designed and operated so as to prevent degradation of their contents,
maintain sufficient release of the natural watercourse to protect down-
stream uses, and possibly effect sedimentation. The water supply use
dictates certain relationships of storage capacity, supply, and yield,
dependent on, among other things, the reliability with which the yield
must be maintained. Finally, the appearance of the facility must be at
least consistent with its surroundings and might well be designed to be
an asset to the area. The presence of the facilities must be acceptable
to the community and pose no particular safety hazard or nuisance.
In studying size requirements for the local storage units, the individual
reservoir locations were first analyzed to determine the yield versus
capacity relationships, assuming 100 percent reliability over the five-
year period of the synthetic hydrograph. Similar analyses were performed
to determine relationships with 99, 98, 95, 90, and 80 percent reliability for
the five-year period. These analyses were performed for constant yields
and for yields approximating the patterns of several different levels of
lawn sprinkling use. Next, water quality considerations were analyzed
in terms of operating and design requirements, and the storage units
studied as sedimentation basins. Finally, sizes and design criteria were
developed which met requirements imposed by all three purposes. The
following sections describe in detail the development of these criteria.
66
-------
STORAGE/YIELD CHARACTERISTICS FOR WATER SUPPLY USE
A common approach to water supply reservoir sizing problems is the Mass
Diagram or Rippl Method (42). When performed graphically, this method
plots cumulative supply against cumulative yield so that cumulative surplus
or deficiency can be examined. A simple graphical manipulation is suffi-
cient to measure maximum required storage directly for the time period
plotted. The identical result may be obtained with somewhat better reso-
lution by applying the following continuity equations to tabular data:
Qi + Si = D.+Si + 1 (19)
where:
Q. = inflow on the i day
S. = storage volume at the start of the i day
D. = yield on the i day
This equation may be applied to each successive time period throughout
the available -record, and the maximum value found for Si is the maximum
required storage, comparable to that found by the graphical technique.
In order to achieve meaningful results in small watersheds subject to
large, short-term fluctuations in runoff rates, a five-year daily synthetic
hydrograph was developed (Section IV), containing 1825 data points. These
data were developedf or each of the 22 sub-watersheds. Each of these must be
analyzed over a wide range of demand levels, or yields. This last require-
ment is made to permit an economic tradeoff between the revenue of addi-
tional water uses and the incremental cost of additional storage. A prelim-
inary appraisal of the difficulty of a conventional mass balance continuity
analysis of 1825 data points for each of the watershed yield combinations
led to investigation of alternate approaches to the reservoir sizing
problem.
The selected method was that of Residual Mass Tabulation, a technique
for approximating the result of the method described above with many
fewer calculations and in a manner much better suited to the high speed
processing characteristics of the digital computer (42). Table 13 is a
portion of a residual mass tabulation for one of the sub-watersheds in
the Wilde Lake area. The two left-hand columns list total accumulated
runoff (in cfs-days) for various periods (in days) in the synthetic record.
The first line indicates runoff for the driest day of record; the second
line indicates the total runoff for the driest five consecutive days of
record; the third line indicates the total for the driest 10 consecutive
days of record; etc. For example, the eleventh line indicates that
during the driest 60 consecutive days of record, the total accumulated
runoff was 1. 0 cfs-day. This listing continues until the period listed
equals the total record, 1825 days, and the accumulated runoff equals
the total runoff for the period, 83. 09 cfs-day in this example. The second
67
-------
TABLE 13. EXAMPLE OF RESIDUAL MASS TABULATION -
SUB-WATERSHED NO.
CO
Period
(days)
1
5
10
15
20
25
30
35
40
50
60
80
100
150
200
250
300
350
400
450
500
600
700
800
900
1000
1200
1400
1600
1825
Runoff
(cfs-days)
0.0
0.0
0. 0
0.0
0.0
0. 0
0.0
0. 12
0.22
0. 37
1.00
1.54
1.70
3. 32
4, 65
7. 53
9. 97
12. 33
13.85
15. 58
10.00
23.05
29.52
33.71
39.03
43.37
52.26
61. 56
72. 27
83.09
Demand(cfs)=0. 0125
Outflow Storage
(cfs-days)
0. 0125
0.0625
0. 1250
0. 1875
0.2500
0. 3125
0. 3750
0.4375
0.5000
0. 6250
0.7500
1.0000
1.2500
1.8750
2.5000
3. 1250
3.7500
4. 3750
5.0000
5.6250
6. 2500
7. 5000
8.7500
10.0000
11.2500
12. 5000
15.0000
17 5000
20.0000
22.8120
0.0125
0.0625
0. 1250
0. 1875
0.2500
0. 3125
0. 3750
0.3175
0.2800
0.2550
-0.2500
-0. 5400
-0.4500
-1.4450
-2. 1500
-4.4050
-6.2200
-7. 9550
-8.8500
-9.9550
-11.7500
-15. 5500
-20.7700
-23.7100
-27.7800
-30.8700
-37.2600
-44.0600
-52.2700
-60.2780
Demand(cfs) = 0. 0200
Outflow Storage
(cfs-days)
0.0200
0. 1000
0.2000
0.3000
0.4000
0.5000
0. 6000
0.7000
0.8000
1.0000
1.2000
1.6000
2.0000
3.0000
4.0000
5.0000
6.0000
7.0000
8.0000
9.0000
10.0000
12.0000
14.0000
16.0000
18.0000
20.0000
24.0000
28.0000
32.0000
36. 5000"
0.0200
0. 1000
0.2000
0. 3000
0.4000
0.5000
0.6000
0. 5800
0. 5800
0. 6300
0.2000
0.0600
0.3000
-0.3200
-0.6500
-2. 5300
-3. 9700
-5.3300
-5.8500
-6. 5800
-8.0000
-11.0500
-15.5200
-17.7100
-21.0300
-23.3700
-28.2600
-33. 5600
-40.2700
-46.5900
Demand(cfs)=0. 0400
Outflow Storage
(cfs-days)
0.0400
0.2000
0.4000
0.6000
0.8000
1.0000
1.2000
1.4000
1.6000
2.0000
2.4000
3.2000
4.0000
6.0000
8.0000
10.0000
12.0000
14.0000
16.0000
18.0000
20.0000
24.0000
28.0000
32.0000
36.0000
40.0000
48.0000
56.0000
64.0000
73.0000
0.0400
0. 2000
0.4000
0. 6000
0.8000
1.0000
1.2000
1. 2800
1.3800
1. 6300
1.4000
1.6600
2. 3000
2. 6800
3. 3500
2.4700
2.0300
1.6700
2. 1500
2.4200
2.0000
0.9500
-1. 5200
-1.7100
-3. 0300
-3.3700
-4.2600
-5. 5600
-8.2700
-10.0900
Demand(cfs)=0. 0600
Outflow Storage
(cfs-days)
0.0600
0.3000
0.6000
0.9000
1.2000
1.5000
1.8000
2. 1000
2.4000
3.0000
3.6000
4.8000
6.0000
9.0000
12.0000
15.0000
18.0000
21.0000
24.0000
27.0000
30.0000
36.0000
42.0000
48.0000
54.0000
60.0000
72.0000
84. 0000
96.0000
109.5000
0.0600
0. 30,00
0.6000
0. 9000
1.2000
1.5000
1.8000
1.9800
2. 1800
2.6300
2.6000
3.2600
4.3000
5.6800
7.3500
7.4700
8.0300
8.6700
10.1500
11.4200
12.0000
12.9500
12.4800
14.2900
14.9700
16.6300
19.7400
22.4400
23.7300
26.4100
Maximum Storage
0.3750
0.6300
3.3500
(supply deficit)
-------
pair of columns, headed "DEMAND (CFS) = 0.0125, " show the method
of analysis for a constant yield of 0. 0125 cfs. The column headed "OUT-
FLOW" lists the total yield for each of the periods listed in the first
column (number of days times daily yield). The column headed "STORAGE"
lists the volume drawn from storage found by subtracting the "RUNOFF"
column from the "OUTFLOW" column. The largest positive number in
this column is the approximate size, in cfs-day, of the required reservoir,
given this demand. This process is repeated successively with various
demands until the entire possible operating range has been explored. In
each case, the largest positive number in the storage column is taken as
the required reservoir size. When the "STORAGE" column does not show
a negative entry in the last row, a supply deficit is indicated. That is,
the total available supply is less than the total required yield and no finite
reservoir can be constructed which would provide this yield during the
period analyzed.
After the resudual mass tabulation has been performed for a sufficient
number of yields to describe the range of possible operation, a plot is
made of yield versus storage requirement. A typical plot is shown on
Figure 19. In this case, the units of storage have been converted from
cfs-day to gallons and those of yield from cfs to gallons/day. Since the
method used samples of various time periods and computes storage for
the sampled periods only, the storage requirement found will be only an
approximation of the actual requirement. It may be equal to the actual
requirement or, more probably, slightly smaller. For this reason, the
plot shows a smooth curve intersecting or passing above the plotted points.
This curve can be taken as a continuous relationship between storage re-
quirements and yield for the period of the synthetic hydrograph used to
generate the points. Figure 20 shows the storage/yield characteristic of
another sub-watershed as well as the results of a graphical RipplMethod
solution for the same area. The correspondence between the two methods
was considered satisfactory for the purposes of this study, and all subse-
quent storage analyses were made by the method of residual mass tabu-
lation, using digital computers.
STORAGE/YIELD CHARACTERISTICS FOR SPRINKLING USES
The determinations of storage requirements described above assumed
that the demand on the storage facility, or yield, would be constant in
terms of volume/day. This assumption is relatively valid for domestic
uses such as toilet flushing, bathing, laundering, food preparation, etc.
The only significant use which cannot be treated as a constant demand is
lawn sprinkling. In addition to its totally seasonal characteristic in many
areas of the country, the pattern of use is highly irregular from day to
day. In their major study on residential water use, Linaweaver and
others found the relationships between the most severe and the average
periods of sprinkling use in a residential area near the Columbia site
(27). These are shown in Table 14.
69
-------
Required Storage Capacity (MG)
X
P
3
' CD rt>
SH CO
^-i >_.. i—•
P a CD
CT ^
CD £
w P
J I
CO ^ CO
^ ^ S"
(T) i-=l O
S-SS
3
cr
0)
03
P Kj
h^-
CD
i—'
s a
>
3
P
^
CO
(-"
CO
-------
O
bo
ctS
s
IT
4)
1.8
1.6
1.4
1.2
in
' U
0.8
0.6
0.4
0.2
Mean Runoff =
13, 121 gallons/day
— Rippl Method
Residual Mass Tabulation
5000 10,000
Yield (gallons/day)
15,000
Figure 20. Storage vs. Yield -
Comparison of Rippl Method and Residual Mass Tabulation
Sub-Watershed Number 4
71
-------
TABLE 14. RATIO OF SPRINKLING USE DURING SELECTED
_ PERIOD TO AVERAGE SPRINKLING USE _
Glenmont and Northwest Branch Estates,
Washington Suburban Sanitary Commission,
October 1963 to September 1965
Ratio of Sprinkling Use
Time Period to Average Use (27, p. 6)
Peak Hour 21.20
Maximum Day 7.17
Maximum 92 Days 2.16
365 Days 1. 00
A continuous curve was fitted to the four data points listed by a parabolic
regression technique, yielding the following equation:
(20)
RI = 7.26d Standard error of estimate =
0. 0061; o = 0.993
where:
R = ratio between average sprinkling use during period,
d, and annual average sprinkling use
d = period of sprinkling use being studied in days
This equation can now be used to estimate the ratio of sprinkling use
during the most severe period of a selected length to the average sprin-
kling use. When average sprinkling use levels are developed te Section V
.
as an input to this calculation. irrigaDie area is
tohee sprinkling demand patterns can be applied
"u d is
te -- -- d--e -^rr sr
72
-------
TABLE 15. EXAMPLE OF MODIFIED RESIDUAL MASS TABULATION -
STORAGE VS. PROPORTION OF DEMAND USED FOR SPRINKLING LAWNS
Sub-watershed #16, Demand (cfs) = 0.0232
(g/d)= 15,000
Period Runoff
(days) (cfs-days)
0% Sprinkling
Outflow Storage
25% Sprinkling
Ratio Outflow Storage
100% Sprinkling
Ratio Outflow Storage
1
5
10
15
20
25
30
35
40
50
60
80
100
150
200
250
300
350
0.0
0. 0
0.0
0.0
0.0
0.0
0.0
0. 10
0. 18
0. 30
0. 86
1. 30
1.46
2.89
4.05
6.70
9. 12
11.45
0.
0.
0.
0.
0.
0.
0.
0.
0.
1.
1.
1.
2.
3.
4.
5.
6.
8.
0232
1160
2320
3480
4640
5800
6960
8120
9280
1600
3920
8560
3200
4800
6400
8000
9600
1200
0. 0232
0. 1160
0. 2320
0.3480
0.4640
0.5800
0.6960
0.7120
0.7480
0.8600
0. 5320
0. 5560
0.8600
0. 5900
0. 5900
-0.9000
-2. 1600
-3. 3300
2. 565
1.807
1. 588
1.481
1.414
1. 366
1.329
1. 300
1. 276
1.238
1.209
1. 167
1. 137
1.087
1.056
1.034
1.017
1.004
0.0595
0.2096
0.3684
0. 5154
0.6561
0.7923
0.9250
1.0556
1. 1841
1.4361
1. 6829
2. 1660
2.6378
3. 7828
4.8998
5.9972
7.0783
8. 1525
0.0595
0.2096
0. 3684
0. 5154
0.6561
0.7923
0. 9250
0. 9556
1.0041
1. 1361
0. 8229
0.8660
1. 1778
0.8928
0.8498
-0.7028
-2. 0417
-3.2975
7.
4.
3.
2.
2.
2.
2.
2.
2.
1.
1.
1.
1.
1.
1.
1.
1.
1.
260
229
351
924
654
463
316
200
103
951
835
666
546
349
224
136
069
015
0. 1684
0.4906
0.7774
1.0176
1.2315
1.4285
1. 6119
1.7864
1. 9516
2.2632
2. 5543
3.0921
3. 5867
4. 6945
5. 6794
6. 5888
7.4402
8.2418
0. 1684
0.4906
0. 7774
1. 0176
1.2315
1.4285
1. 6119
1. 6864
1.7716
1. 9632
1. 6943
1. 7921
2. 1267
1.8045
1. 6294
-0. 1112
-1. 6798
-3. 2082
Maximum Storage
0.8600
1.1778
2.1267
-------
sprinkling 25 percent sprinkling, and 100 percent sprinkling. The
"RATIO" columns are determined by evaluating Equation 20 for each
time period and adjusting this ratio as follows:
R., = pR. +d-p) (21)
where:
R. , = • adjusted ratio for i period
R. = ratio from Equation 20 for i period
p = fraction of use assumed as sprinkling
These adjusted ratios are then entered in the "RATIO" columns in the
rows corresponding to the correct time period. The tabulation proceeds
as before, except that the "DEMAND" column is the product of the average
daily demand, the number of days in the period, and the corresponding
adjusted ratio. The results of this tabulation indicates significant increase
in required storage as a direct result of an increasing proportion of the
water being used to irrigate lawns. Figure 21 presents the results of a
complete tabulation for one sub-watershed showing a family of curves for
various proportions of sprinkling use. Figure 22 is another presentation
of the same data, comparing sprinkling use fraction to required storage
for a range of total average daily use levels. As discussed in Section V.
this type of analysis was used to derive the derating factors for sprinkling
uses used in the study.
STORAGE /YIELD CHARACTERISTICS AS A FUNCTION OF RELIABILITY
Both the constant use and the sprinkling use storage characteristics invest-
gated above have been oriented toward a requirement of 100 percent reliable
operation of the storage system based on a five-year synthetic hydrograph.
Since the Local Storage, Treatment, and Reuse of Storm Water concept
required that the system function only as a supplemental supply to a con-
ventional public water distribution system, the storage requirements asso-
ciated with various levels of system reliability were also analyzed. For
purposes of this analysis, 90 percent reliability is defined as the ability of
the system to supply at least 90 percent of the total water demand in any
given year. Once again, the residual mass tabulation was employed to deter-
mine storage/yield characteristics. In this case, an assumption was made
that the portion of the total demand which the storage system would fail to
supply would be that occurring during the driest period. Table 1-6 is an ex-
ample of a tabulation of a sub-watershed with a 15, 000 gallon/day total de-
mand which would be either 100 percent, 98 percent, 95 percent, or 90 per-
cent supplied by the reservoir under study. In each case, the "DEMAND11
column is reduced by the annual difference between reservoir yield and
demand for each year or fraction thereof. In the example given, the
74
-------
4. 0
ai
C
_0
—<
cd
O
c
O
nJ
Q,
oi
U
•a
0)
3
a"
ffi
.3. 0
1.0
10,000 20,000
Yield (Gal/day)
30, 000
Figure 21. Storage Yield Characteristic vs. Proportion
of Demand Used for Sprinkling Lawns
75
-------
4.0
3.0
O
o
oJ
a
ni
u
0)
M
aJ
t,
O
CO
"D
0)
Sn
3
O"
-------
TABLE 1.6. EXAMPLE OF MODIFIED RESIDUAL MASS TABULATION -
STORAGE VS. RELIABILITY
Sub-watershed #16, Demand
-------
98 percent column is reduced by 0. 1694 cfs-day* for all time periods of
one year or less, by 2 x 0. 1694 cfs-day for time periods of two years or
less but more than one year, etc. A complete analysis of the sub-water-
shed for various levels of reliability is shown in Figure 23. The same
information is displayed in Figure 24 as the required storage as. a func-
tion of supply reliability for various levels of demand.
The previous paragraphs have summarized an investigation of storage
requirements for the 22 sub-watersheds in the Wilde Lake area, derived
as a continuous function of such variables as total demand, proportion of
demand devoted to lawn sprinkling, and supply reliability. This analysis
has indicated that storage requirements are highly nonlinear with demand,
relatively sensitive to sprinkling uses, and highly sensitive to the avail- ;
ability of supplemental water from other sources. The method of analysis
employed, a modified residual mass tabulation technique, was well suited
to the application of digital computer methods and produced satisfactory
results within the scope of the initial assumptions. Since the same sim-
ulated rainfall record was applied to all sub-watersheds, little error will
be introduced by summing the cumulative runoff columns of the tabulations
for several sub-watersheds to analyze the characteristics of the combi-
nation. Unfortunately, it is necessary to perform the complete tabulation
for each possible combination of sub-watersheds in order to determine the
storage/yield characteristic for each possible reservoir application.
WATER QUALITY CRITERIA
In providing for detention of water for any purpose, reasonable care must
be taken to prevent degradation of the water in storage. In the application
under study, possibilities exist for degradation as a result of decomposition
of suspended or floating organic materials, flushing of previously settled
solids into downstream flows, growth of algae in the case o.f open storage
facilities, and the creation of nuisance conditions due to breeding of flies
and mosquitoes from aquatic larvae. Fortunately, most organic materials
found in storm water are not readily putrescible and may be retained for
a reasonable period before appreciable decomposition occurs. Periodic
removal of accumulated materials is normally sufficient to avoid difficulty
from this source, as well as minimizing the possibility of flushing settled
material downstream. Design criteria can be developed to minimize spills,
further reducing the occurrence of this problem. Algal blooms can be con-
trolled by chemical treatment and by the maintenance of a relatively high
throughput as a proportion of average contents, which will serve to mini-
mize the growth of insect larvae as well. Provision must be made for
sufficient attention to the condition of the reservoir, such as daily inspec-
tions, to preclude the formation of a quality degradation condition,
It would be desirable to exclude trash from the storage facility by the
installation of a mechanical trash screen at the storm water inlet. Such an
installation, however, would be prone to accumulate trash and clog during
periods of high flow, thus causing overflows and possible flood damage.'
* 0. 1694 cfs-day = '[(100 - 98)15, 000/646, 316 x 100] 365
78
-------
4. 0'
3.0
O
c
o
-------
3.0
Sub-Watershed #16
28, 140 Gal/Day
25,000 Gal/Day
ra
fi
O
i — i
i — i
nl
o
o
rt
a
aj
O
n!
S-i
O
T3
(U
CT*
0)
2.0
1.0
I I I I
I / I/
50
Reliability (%]
20,000 Gal/Day
15,000 Gal/Day
10,000 Gal/Day
5,000 Gal/Day
100
Figure 24. Storage/Reliability Relationship vs.
Average Daily Demand
80
-------
For purposes of the system study, therefore, it was assumed that trash
would be allowed to enter the storage facility for later manual removal.
The outlet from this facility, which would operate at flows much below
storm flow rates, could be screened to exclude trash from the treatment
works and downstream areas without creating appreciable maintenance
problems.
An important aspect of water quality control along a natural watercourse
is the protection of downstream uses through maintenance of minimum
flows. In the case of Wilde Lake, it was considered desirable to protect
existing stream bed ecology as well as maintain evaporation losses in
Wilde Lake along with the release requirement imposed on the Wilde
Lake dam by the Maryland Department of Water Resources. These fac-
tors will be taken into account in the development of operating criteria
in a later section.
SEDIMENTATION
The operation of the storage facility as a sedimentation basin is of inter-
est in sizing the facility. Large quantities of suspended solids must be
removed from storm water in order to meet the minimum water quality
standards described in Section V. In order to accomplish this objective,
a reservoir may be operated as a combination storage and sedimentation
facility, or a storage facility may be followed by a separate sedimenta-
tion device. Sedimentation will occur whenever the velocity of the storm
flow is reduced by directing it into a structure or basin of large cross
section, compared to the stream channel. Since the storage reservoir
will function as a sedimentation basin regardless of its design criteria,
the first approach was to analyze the effectiveness of this type of sedi-
mentation device.
The unhindered settling velocity of free and discrete particles suspended
in a liquid has been given (42) as:
Ps " P V
vs
where Cj^ is the Newtonian drag coefficient, a function of particle geom-
etry, fluid characteristics, and Reynolds number, and where:
V = particle volume
A = projected area of the particle
L*
g - acceleration of gravity
p = mass density of the particle
p = mass density of the fluid
81
-------
If a velocity, vo,is defined for a specific ideal sedimentation chamber in
the following terms:
Q (23)
vo = A
where:
Q = overflow rate
A = surface area of chamber
then it can be shown that:
when v >v , the particle settles; and
so
when v 0) solids in significant quantities. Hazen has calculated
the settling velocity of various particles representative of those found in
storm water as shown in Table 17.
TABLE 17. SETTLING VELOCITIES OF
SELECTED PARTICLES, AFTER HAZEN
(Ref. 48, p. 1474)
Particle Diameter Settling Rate
Kind of Material (uj (cm/sec)
Coarse sand 1000 10. 0
Coarse sand 200 2. 1
Fine sand 100 0. 8
Fine sand 60 0.38
82
-------
. , TABLE 17. SETTLING VELOCITIES'OF
SELECTED PARTICLES, AFTER HAZEN
(Ref. 48, p. 1474) (Continued)
Particle Diameter Settling Rate
Kind of Material (y_) (cm/sec)
Fine sand 40 0. 21
Silt 10 0.015
Coarse clay 1 0.00015
Fine clay 0.1, 0.0000015
Since the distribution of particle sizes is not known, a conservative
approach to sedimentation facility design is indicated. Hydrologic investi-
gations reported in Section IV estimated the mean runoff rate from sub-
watershed number 14 at approximately 29. 5 gallons per minute (gpm). A-
range of mean overflow rates might be considered in design of the sedi-
mentation facility, but obviously none can be less than the mean runoff
rate. Since reducing the overflow rate tends to reduce th'e area require-
ment for; a given settling velocity (Equation 22), Table 18 represents the
minimum areas where settlement can be achieved under ideal conditions
for the particles indicated.
TABLE 18. MINIMUM SEDIMENTATION BASIN AREA REQUIREMENTS
FOR SELECTED PARTICLES. SUB-WATERSHED NUMBER 14
Particle Diameter Minimum Area Requirement
Kind of Material (u) (square feet)
Coarse sand 1000 , 0.20
Fine sand 100 2. 50
Silt ', 10 133.56
Coarse clay 1 13, 356
Fine clay 0. 1 1, 335, 600
In practice, the area requirements would be considerably larger since
the overflow rate would have to be greater and flow and velocity distri-
butions would be greatly different than the ideal conditions.
As a result of, this evaluation, it was concluded that storage basins of the
sizes that would be feasible in most locations would .not alone produce
83
-------
the desired water quality. Although the basins would serve to remove
most of the larger size materials, the unfavorable velocity gradients that
would exist during storm events would disturb the sedimentation process.
This would cause the carryover of the smaller particles. For this reason,
it was determined that supplemental treatment would be required to re-
move the small particles. However, the storage pond would still provide
for the removal of a large portion of the solids and would reduce the
sediment handling requirements of the supplemental treatment devices.
PRETREATMENT UNIT
Alternatives to the use of conventional sedimentation basins include the
use of specialized sedimentation devices offering high effective-to-total
surface area ratios, the application of chemical coagulants and coagulant
aids to agglomerate small or light particles, and combinations of these
techniques. Examination of settling requirements for particles below
1. 0 micron in diameter led to selection of a combination approach - the
use of an advanced sedimentation technique with chemical coagulants
and/or coagulant aids. This facility, combined with chlorination of settled
water, is referred to in later sections as the "Pretreatment" process.
All runoff captured by the storage facility is subjected to this process,
whether for subsequent final treatment or discharge to the stream.
Costs for the pretreatment phase were based on the use of a tube settler
as marketed by Neptune-Microfloc, Inc. , and described by Hansen and
Gulp (46). This device employs a series of small diameter plastic tubes
inclined upward approximately 60 degrees from the horizontal. The
flocculated water is permitted to pass upward through the tubes at con-
trolled flow rates, typically between 3. 0 and 5. 0 gpm/square foot of cross
sectional area. Figure 25 illustrates the principle of operation of the tube
settler. Water moving up the tube at velocity Vf contains particles having
a settling velocity vg. The resultant velocity, vr, tends to move the parti-
cles toward the tube wall, where they become trapped in a layer of parti-
cles previously removed. A two-inch diameter tube four feet in length can
be operated at flow velocities as high as 2. 84 gpm/square foot tube area
and still permit a 10 micron silt particle to move completely across the
tube. The corresponding loading on a conventional sedimentation basin is
0. 015 cm/second or 0. 221 gpm overflow rate per square foot of surface
area (Equation 22) (see Table 17). This comparison is based on ideal condi-
tions and does not take into account hindered settling conditions caused by
the concentration of particles as they near the sludge collection zone. In
the case of plain sedimentation, basins are commonly maintained at a
minimum depth of five to eight feet to allow sufficient space above the
sludge storage zone for hindered settling to occur in a low velocity bound-
ary zone between the top layer, where flow is close to average overflow
rate, and the static bottom layer. In the case of the tube settler, the
steep inclination of the tubes causes the sludge to counterflow along the
side of the tube as it accumulates, falling into a sump below the tube
assembly. The tube settler configuration also requires influent and
84
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Vector Relationships for Particle in Tube, Settler
Typical Operation of the Tube Settler
Figure 25. Tube Settler - Principles of Operation
85
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effluent plenums to distribute the flow to the tubes and to collect it after
clarification Figure 26 shows a comparison of the volume of typical
sedimentation facilities designed to remove 10-micron silt particles at
various overflow rates. The tube settler volumes include provisions
for influent flume, influent plenum, sludge storage sump, and effluent
plenum, while the conventional sedimentation volumes are based on an
average depth of 5. 0 feet to provide for sludge collection and storage.
Inside partitions and supporting structures are included, but outside walls
are not.
Other sedimentation devices, such as tray settlers, were investigated.
Performance characteristics fell between conventional sedimentation
and the tube settler, but sediment removal and maintenance problems
were more severe than either of the two techniques compared above.
PHYSICAL AND AESTHETIC CONSIDERATIONS
Inherent in the concept of the Local Storage, Treatment, and Reuse of
Storm Water- system is the close proximity of the system elements to the
properties which benefit from the system. In addition to engineering
and economic considerations then, it is essential that the physical appear-
ance of the facilities be consistent with the immediate environment. In
the case of Columbia, the storage and pretreatment units must be,located
in the floodplains to permit gravity flow through the units. In general,
the floodplains of the Wilde Lake watershed have been dedicatedto public
use as open space. They are typically lightly forested with some shrub-
bery and underbrush and are bounded on all sides by residential land uses.
Any construction within these areas must be particularly unobtrusive in
order to achieve acceptability within the community. The pretreatment
units and constructed storage can be placedbelow grade and any necessary
superstructure, i.e., hatchways, vents, etc. , can be dis,guised with shrub-
bery, fences, etc. In the case of open pond storage, the design of the pond
must insure that its appearance under all conditions of operation will be one
of an attractive, natural pond. All construction must be confined to an area
which will permit at least a fringe of trees along the perimeter, thus avoid-
ing a conspicuous discontinuity in the open space. Subsequent cost esti-
mates of constructed storage and pretreatment units are based on their
installation entirely below grade, while open pond estimates provide for
design features to insure their acceptability.
SIZING STORAGE FOR MULTIPLE PURPOSES
Earlier paragraphs described methods for determining required storage
capacities for various conditions of water demand. It was found that for
a given demand with a known proportion of lawn sprinkling uses, the
storage requirement is a function of the reliability of the system or,
stated differently, a function of the quantity of water drawn from the
existing public water supply system in times of dry weather. Water
quality considerations were found to affect operation policy more than
86
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1000
4)
O
•r-l
•§
O
T3
0)
fn
•iH
3
cr
0)
100
10
Plain Sedimentation
Tube Settler
I I I I I I
I I I I I I
10
100
Overflow Rate (gallons/minute)
Figure 26. Required Volume vs. Overflow Rate for.
Removal of 10-Micron Silt Particle Under Ideal Conditions
87
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design size, since meeting the standards by sedimentation in the reser-
voir was found to be impractical. The application of the pretreatment
step following storage does, however, impose a different type of yield
requirement on the storage facility, in that it must be provided with a
constant flow for proper operation, although that flow need not be con-
tinuous. Physical and aesthetic considerations were examined and found
to apply primarily to design criteria other than size, except that avail-
able land may limit the maximum attainable storage in a particular
location.
Since the operation of the pretreatment unit appears to place a definable
size requirement on the reservoir, that requirement will be evaluated
first, then reviewed in the light of other requirements. Based upon pre-
vious discussions, the following assumptions are made with respect to
the operation of the storage facility as a supply to the pretreatment unit:
1. The reservoir will be operated on a fill-and-draw basis,
storage capacity calculated as the difference between
minimum and maximum pools.
2. The highest proportion of total runoff feasible must be
actually intercepted by the reservoir and subjected to
pretreatment.
3. When outflow to the pretreatment unit occurs, it must
be at a constant controlled rate.
4. Outflow need not occur at all times, e.g., intermittent
operation of the pretreatment unit is permissible.
The proportion of the total runoff which is captured by the storage facility
is a function of the volume of individual storms and the probability of
available storage coincident with the occurrence of each storm. Since
the reservoir is on a fill-and-draw cycle, the probability of available
storage is a function of both the maximum storage capacity and the draw
rate, or the rate of operation of the pretreatment unit. The analysis of
reservoir performance will be in terms of recurrence intervals of storms
and it will be initially assumed that storage is available to capture the
entire volume of some design storm. When storms of larger volume or
longer return interval occur, the excess quantity will be spilled, by-
passing the pretreatment unit. Figure 27 shows rainfall-intensity-
duration curves for storms of various recurrence intervals in Howard
County, Maryland. From the definition of recurrence interval, the fol-
lowing identity exists:
E(n) = (24)
where:
E(n) - expected number of storms meeting or exceeding
conditions for a return interval T (years) occurring
during period P (years)
88
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co
CO
25 year return interval
20 year return interval
10 year return interval
5 year return interval
1 year return interval
50 60 70 80
Rainfall Duration (minutes)
100
110
120
Figure 27. Rainfall Intensity-Frequency-Duration Curves,
Howard County, Maryland
-------
Rainfall-intensity-duration curves can be used to generate runoff-
intensity-duration curves for specific watersheds, using the methodology
outlined in Section IV. By application of probability and expectation the-
ory to these curves, the quantity of runoff occurring in excess of the
storm volume corresponding to any selected return interval may be cal-
culated. Table 19 lists the results of one such analysis!
TABLE 19. EXPECTED QUANTITY OF RUNOFF IN
EXCESS OF SELECTED STORM VOLUMES
(Imperviousness - 0. 30, m = (lag time) = 4 minutes)
(Total annual runoff = 220, 307 gallons/acre)
Base Storm Expected
Return Interval Excess Runoff Excess
(years) (gallons/acre) (%) '
1 16,160 7.34
5 5,050 2.29
10 3,066 1.39
20 1,857 0.84
25 1,584 0.74
50 962 0.44
100 579 0.26
Based on the most conservative calculation of storm volumes and on
intensity-frequency-duration curves calculated to be conservative with
respect to long-term records, a one-year storm volume storage capa-
bility can be expected to capture at least 92. 66 percent (100 - 7. 34) of
the total runoff in a year. This value is dependent, however, on the
availability of maximum storage at the beginning of each storm, a func-
tion of the release rate of the reservoir.
The release rate can be as low as the annual mean daily runoff rate orq
When this rate is assumed with a storage capacity equal to the volume of
a one-year storm, analysis of the five-year synthetic hydrograph for sub-
watershed number 6 reveals that an additional 3. 01 percent of the runoff
can be expected to spill as a result of the occurrence of storms whose >
volumes, although less than the one-year storm, exceed available storage,
When the release rate is increased to 2.0q, the additional percentage of
spill falls to 1.42 percent. Figures 28 and 29 show the approximate re-
lationships between various releases or pretreatment rates and spill per-
centages for a range of storage capacities. It can be seen from these
figures that a release rate of approximately 1. 7q in combination with a
90
-------
CD
CD
S-,
cd
0)
o
4-J
GO
0)
g
0)
M
rt
^
O
50.0
25. 0
20.0
10.0
5.0
2.0
1.0
0.7
0. 5
0. 2
0. 1
1.0
2.0 3.0
Pretreatment Rate (x Mean Annual Daily Runoff)
2%
3%
4. 0
Figure 28. Storage/Pretreatment Rate
Combinations vs. Spill Percentages
-------
14
13
0. 5 yr
12
CD
to
o
a
a
o
H
d
o
S-i
o
o
f-t
co
11
10
9
1.0
_L
2.0 3.0
Pretreatment Rate (x Mean Annual Daily Runoff)
0.7 yr
2.0 yr
5.0 yr
4.0
CQ
0)
a
3
s
!H
O
!-°yr ^
6
O)
bfl
ct)
-------
storage facility having a capacity equal to the volume of a one-year storm
will result in approximately nine percent of the runoff being spilled with-
out treatment: The remaining 91 percent will pass through the pretreat-
ment unit at a controlled rate. Lowering pretreatment rate to approxi-
mately 1. Iq increases the spill to 10 percent and greatly reduces storage
requirements following the pretreatment unit where reuse is being
practiced;
It is important to note that the water which does spill from the storage
facility does so after passing through the storage structure where it has
been detained for sufficient time to settle the larger and heavier solids.
In view of the high quality of the balance of the water after pretreatment
and the very infrequent nature of the spills, it was felt that restricting
spills to 10 percent of total runoff was a realistic limit on the design and
operation of the storage/pretreatment system. It would appear then, that
a reservoir sized at the one-year storm volume operated in conjunction
with a pretreatment unit controlled at a release rate somewhat greater
than the mean runoff rate would process storm water within the water
quality requirements of the system. A tradeoff obviously exists between
spill volume and design criteria. Furthermore, a system having a given
Spill performance consists of a series of complex tradeoffs between storage
capacity, release rate, and intermediate storage following pretreatment.
Analysis of any of these tradeoffs, however, would require a quantitative
estimate of the value of various levels of water quality, a task outside
the scope of this study. Inherent in the study, however, is a method for
inputing a value for water pollution control due to the study of the appli-
cation of conventional methods of pollution control to the same watershed,
reported in Section XVI. Both studies were directed to the same water
quality requirement, and the 10 percent maximum spill requirement was
made of both systems. The method employed to estimate spills was also
common to both studies.
Having'determined the storage requirement for effective water pollution
control, this requirement must be weighed against other reservoir pur-
poses. Figure 30 shows the storage/yield characteristics of sub-watershed
number 16 plotted against the storage equivalent to the one-year storm
volume. It can be seen that the maximum long-term demand of 25, 327
gallons/day (mean runoff less 10 percent spill) can be sustained at the
80 percent reliability level. Similar relationships were found for each
of the other sub-watersheds indicating that the water quality requirements
on reservoir size result in a satisfactory reservoir from the water supply
standpoint. The only other requirements found on reservoir size were
those related to available land area and aesthetic considerations. Since
these can only be determined by examination of a particular site with a
particular size and construction type of proposed reservoir in mind, they
were not considered until after the systems analysis effort was complete
and conceptual designs were being produced from the results of the
analysis/ Sections X and XII.
93
-------
en
a
o
, — I
I — I
OJ
o
o
ni
0,
cd
U
01
tXO
rt
-O
0)
OJ
K
3.0
2.0
1.0
One Year Storm Volume
= 782, 000 Gallons
_L
_L
50
Reliability (%)
28, 140 Gal/Day
25, 000 Gal/Day
20,000 Gal/Day
15,000 Gal/Day
10, 000 Gal/Day
5, 000 Gal/Day
100
Figure 30. Storage/Reliability Characteristics vs.
Yield, Sub-Watershed No. 16
94
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As a result of .'a comparison of the various influences on reservoir and
pretreatment unit design, the following criteria have been selected as
adequate to protect all uses under consideration:
i
1. Net storage capacity of the reservoir shall be equal to the
maximum volume for the sub-watershed or combination of
sub-watersheds of a storm just meeting the one-year
return interval intensity.
2. Release 'from the reservoir into the pretreatment unit will
be controlled at a rate between 1. 2 and 2. 0 times the annual
mean runoff rate.
3. The pretreatment unit will be capable of operating at 2. 0
times the annual mean runoff rate.
4. No storage need be provided following pretreatment.
With respect to Criterion 2 above, it is noted that no discussion has been
devoted to the effect of base flow on these criteria. Due to the use of
the natural floodplains as collecting storm drains, it is likely that a
significant amount of relatively constant flow will occur as a result of
springs and seepage from the ground. This quantity is in excess of the
volumes of runoff described in the analysis, although there is no ready
way of predicting its quantity. Due to the steady nature of the flow, it
places no particular requirement on storage, but will increase the
required release rate and pretreatment rate by an amount equal to its
flow rate. It is considered that in an area such as Columbia, the base
flow would probably not be more than 30 percent of the mean rundff rate
from storm water. Hence the requirement of 2. 0 times the mean runoff
rate for pretreatment design.
CONSTRUCTION, OPERATION, AND MAINTENANCE COSTS
In addition to the design criteriafor storm water storage and pretreatment
facilities discussed above, it was also necessary to develop information on
the cost of constructing, operating, and maintaining these facilities. These
data were developed parametrically in order to permit them to be used in the
computerized system model. Estimates were made of the costs of storage
and pretreatment facilities and of various capacities. These data were
then plotted as smooth curves. The curves were then replaced by a series
of linear approximations between the major infliction points. Tables were
then prepared as inputs to the computer program so that cost data for any
capacity system could be selected by linear interpolation between points
from the tables. A further discussion of this technique and the development
of the cost data used in the study are included in Appendix C. It is noted
that these estimates are based on equipment and construction prices
existing in the 1967-1968 time frame.
95
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SEDIMENT REMOVAL
This study was based on a system of reservoirs operating in an estab-
lished development which has restabilized. Accordingly, the sediment
yields would not be excessive. For this reason, it was assumed that
a majority of the solids would be removed from the pretreatment units
and could be accomplished by pumping to tank trucks for transport and
disposal. Periodically, sediment removal from the storage ponds would
be required; however, the cost on a per gallon of runoff basis would be
approximately the same as the pretreatment unit cleaning. The costs
for storage basin and pretreatment unit cleaning would be considerably
higher if the area was still under development or was undergoing con-
tinuing erosion due to the increased flows resulting from urbanization.
The methods for maintaining storage ponds under these conditions are
being considered as part of the demonstration program discussed in
Section XIX.
96
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SECTION VIII
TREATMENT METHODS
Section VI described the wide range of pollutants and pollutant concen-
trations found in storm water by other investigators. The literature
searches reported in Sections V and VI failed to locate reports of success-
ful treatment of storm water immediately after collection from small
watersheds. In the absence of a comprehensive storm water sampling
and analysis program in the Columbia area, it must be assumed that
storm water pollutants will be similar in number and concentration to
those found in runoff from comparable areas. Table 10 in Section VI
listed the pollutants whose presence is assumed and indicated the expected
range of concentration of each. Specification of unit processes intended
to achieve a given level of treatment must necessarily be conservative,
due to the lack of reported field experience with similar problems. The
use of systems normally used for treating water from conventional sur-
face water impoundments was not considered directly applicable because
of the random nature of certain pollutants and the small capacity of the
individual treatment unit.
Section V described the four levels of treated water quality tobe considered
in the systems analysis. Various unit processes have been examined for
each level, with attention to the maximum pollutant concentrations expected
as well as the ability of the process to withstand severe shockloads. Table
20 lists the processes selected, indicating the treatment levels for which
each is used. Figure 31 is a flow sheet showing the processes associated
with each class of treatment.
Algae control is applicable only to those cases where storm water will be
stored in an open facility. The term is intended to include all treatment
steps necessary to prevent degradation of the stored water or the creation
of a nuisance condition. Under some conditions, this might include steps
to prevent mosquito or other insect breeding as well as the control of
aquatic growths. Further discussion of these considerations can be found
in Appendix C. After thorough analysis of the actual water to be treated,
several of the processes listed might become unnecessary. Examples
are pH adjustment and softening. Limited sampling conducted to date
indicates that total hardness is low under base flow conditions and lower
yet during storm flows, while pH varies between 6. 3 and 8. 3, depending
on flow. Further sampling might indicate a need for some chemical
adjustment of these parameters, however.
Descriptions of the systems that were assumed for treating the collected
storm water as a function of quality and reuse classification are described
in the following paragraphs.
97
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pH Adjustment
Chlorine
Coagulant
Algae Control
Effective Runoff
Coagulant
Yield
Spills
CD
CO
STORM WATER STORAGE
•Algae Control* (all classes)
x
Sediment
Removal
Air
Net Yield
Class "C" Water
to Surface Drainage
PRETREATMENT
*Flocculation (all classes)
* Sedimentation (all classes)
*pH Adjustment (all classes)
•Chlorination (C)
Chlorine
Product
Water
Precoat
Filter Wash
or Strainer
Backwash
FINAL TREATMENT
•Straining (B)
• Softening* (AA, A)
•Flocculation* (AA, A, B)
•Filtration (AA, A)
•Carbon Adsorption (AA)
•Chlorination (AA, A, B)
•Aeration (AA, A)
*If required
Figure SI. Treatment Systems
-------
TABLE 20. PROPOSED TREATMENT PROCESSES
Unit Process Type Water Use Category
Raw Water Storage with Algae Control
Aeration
Sedimentation
Flocculation
Straining (with pre-coat)
CD
<£> Sand Filtration
pH Adjustment
Softening
Chlorination
Carbon Adsorption
*IP required
A-A
X
X
X
X
^
X
X
X
X
X
A
X
X
X
X
fr
X
X
X
X
B
X
X
X*
X
X
X
c
X
X
X*
X
X
-------
TREATMENT TO CLASS "C" QUALITY
Class "C" treatment consists of mechanical trash screening, raw water
storage with algae control, sedimentation, pH adjustment, andchlorination.
Trash is excluded from the storm water collection system to a degree by
•the inlet design, which incorporates gratings and restricted openings.
Smaller objects can enter the system, however, and parts of the subsequent
collection system are open channels and natural watercourses, permitting
further ingress of trash. Due to the difficulty of preventing a stoppage at
the entrance to a storage facility, it is anticipated that no screening be
provided at that point. This choice would depend on the difficulty of re-
moving large objects from the storage basin itself. The point of with-
drawal from the storage facility would then be screened to exclude trash
from subsequent treatment processes. Since this screen would not be re-
quired to operate at storm flows, routine maintenance, as discussed in
Appendix C, should permit satisfactory operation without the installation
of mechanical cleaning devices.
Raw water storage might consist of any type of structure or basin, the only
requirement being that its maximum elevation be at or below inlet eleva-
tion. This will permit storm flows to enter by gravity without adversely
affecting the hydraulics of the storm water collection system. The possi-
bility of pumping storm water at storm flow rates was excluded from the
analysis for reasons of reliability. All available equipment designed for
such intermittent, high rate service is electric motor driven. The corre-
lation between electric power outages and severe thunderstorms is con-
sidered relatively high in an area of this type and such a joint occurrence
would result in spills of large quantities of untreated storm water. Even
though the facility could be designated to avoid flood damage from such an
occurrence, the overall reliability was considered inconsistent with the
water quality objectives of the study.
The decision to employ a specialized sedimentation device, or pretreat-
ment unit, effectively eliminated sedimentation considerations from the
design of the storm water storage facility. The criteria which were
employed in sizing this facility are described in detail in Section VII.
The storm water storage facility precedes the pretreatment unit and
coagulants are introduced as the water is withdrawn from storage. The
pretreatment unit is used as a point of application for pH adjustment
chemicals, if required, and chlorine. Effluent from this combined oper-
ation is directed to the Class "C" water uses or allowed to overflow to
the natural watercourse.
TREATMENT TO CLASS "B" QUALITY
Class "B" treatment consists of all the unit operations of Class "C, "
namely storm water storage with algae control (if necessary) and pretreat-
ment, followed by a straining operation. This operation is the Microstrain-
mg process of the Glenville Kennedy Division of Crane Company, or equiv-
alent. It consists of a fine mesh screen which is coated with an expendable
100
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filter media, or precoat, so that effective removal of all particles 10
microns and larger can be achieved. Additional chlorine is added after
this process in order to maintain an adequate residual throughout .the
distribution system. Following straining, the effluent is' available for
Class "B" or "c" water uses. Excess water will be permitted to over-
flow after the pretreatment process before entering the straining equip-
ment. In this manner, distributed water will be at least Class B, '
while excess water will be at least Class "C. "
TREATMENT TO CLASS "A QUALITY
Class "A" treatment consists of the unit operations of Class "C, " storm
water storage with algae control (if necessary) and pretreatment, fol-
lowed by aeration, additional chemical coagulation, and mixed media
filtration. Chemical additions and filter designs will be selected to en-
sure clarification of the effluent to 1. 0 Jackson Turbidity Unit or better.
This effluent, after rechlorination, will be distributed to Class "A, "
"B, " or "C" uses. Excess water will be permitted to overflow after the
pretreatment unit.
TREATMENT TO CLASS "AA" QUALITY
The treatment processes for Class "A" and Class "AA" are identical
with the single addition of activated carbon adsorption in Class "AA"
applications. This process removes tastes and odors as well as pro-
viding a buffer against occasional excessive concentrations of adsorbable
organics and other dissolved materials. An activated carbon column
utilizing a replaceable charge would be employed. The carbon charge
would be periodically removed and disposed of or regenerated at a cen-
tral facility. The capacity of the carbon column would be set at a value
substantially in excess of anticipated requirements, in the interest of
long life and security. Rechlorination would be practiced following the
carbon treatment. As before, excess water will overflow after the pre-
treatment step.
TREATMENT SYSTEM COSTS
For each type of treatment described above, estimates were made of
the equipment, facility, operating, and maintenance costs. These data
were prepared parametrically using the same techniques used for the
storm water storage facility and operating costs. The basis for the
estimates of the treatment system costs and the input data used in the
system analysis program are contained in Appendix D.
SUMMARY
The foregoing paragraphs do not represent suggested design for every
circumstance. Actual configurations should be determined after suit-
able investigation of the characteristics and variations in the quality of
101
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the storm water actually available. The processes described are only
intended to be representative of adequate treatment under most local
conditions and may well constitute over-design in many applications.
Due to the supplemental nature of the water supply being evaluated, fail-
safe controls can be employed to shut down the treatment facility in the
event of malfunction without loss of supply to the connected demand. In
this manner, considerable redundancy commonly a part of municipal
water plant design can be eliminated and small package-type treatment
units of low cost might be profitably employed. For purposes of the
system analysis, however, no innovations have been considered. Every
process discussed above has known performance characteristics consis-
tent with the most difficult expected requirements.
102
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SECTION IX
REUSE DISTRIBUTION SYSTEMS
The'distribution system for''storm water reuse is comprised of all the
pipes, valves, meters, and appurtenances required to conduct the water
from tHe point of treated water storage to the point of ultimate use. It
includes' connections to the existing public water system for'makeup
water with anti-b'ackflow provisions at the 'interface between the''two
systems.
Pipe sizes throughout the distribution system have been selected for the
estimated peak flows as well as reasonable ease of maintenance. Fire
flows have not been assumed, since the public system will retain that
function, consistent with the discussion in Section V. No pipes smaller
than two inches have been assumed and the largest pipe diameters are
six inches. Where a sub-potable distribution system serves a property,
double meter vaults have been assumed with meters on both potable and
sub-potable systems. Costs for sub-potable distribution systems within
buildings include the additional costs of dual risers, extra fittings, etc.,
but do not include fixture costs. Sub-potable distribution systems have
been designed for each of the distribution service areas described in
Section V and laid out in such a way that any combination of continuous
systems can be interconnected by simply operating valves (see Figure 32).
This permits the evaluation of systems for various combinations of sub-
watersheds without changing distribution costs, as well as providing for
continued operation in the event of temporary failure of one system.
The transmission main from the treatment plant to the distribution piping
has been calculated as a function of plant location only. The cost includes
the public water system connection, which includes provisions for providing
makeup water to sub-potable systems without the possibility of backflow
into the public system. This might be accomplished by introducing public
water into an open tank through a float-controlled valve located above the
overflow, or, local regulations permitting, by one of several types of
backflow preventer valves. In the case of potable reuse systems, the public
connection is the point of injection into the public potable water distribution
system. Some type of backflow preventing device might be employed here
to isolate sections of the distribution system, depending on local conditions.
CLASS "C" AND "B&C" USES
When treated storm water is to be used to flush toilets only or to sprinkle
lawns and flush toilets, a relatively simple dual distribution system is
required. In addition to a sub-potable distribution piping in the streets,
dual service connections and meters are required at each home, along
with dual headers on the individual plumbing. Connections from the sub-
potable system are made to the outside hose bibs for sprinkling use and
to toilet fixtures throughout the building.
103
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\
RECYCLE DISTRIBUTION SYSTEM
WILDE LAKE
. WATERSHED
'/ DISTRIBUTION
SYSTEM
POTENTIAL
TREATMENT SITE
/^^\ '-
/^ \ '
MIDDLE PATUXENT •*• \—^-LITTLE PATUXENT
Figure 32. Distribution System Map - Wilde Lake Watershed
-------
CLASS "B" USES
Reuse of treated storm water exclusively for lawn sprinkling requires
a sub-potable distribution system and dual service connections, but
minimizes the cost of plumbing alterations in each building. The sub-
potable system is confined to the basement or ground level and is
connected to outside hose bibs only.
CLASS "A", "A&B", "A&C", AND "A, B, &C" USES
These use combinations also require a sub-potable distribution system
with dual service connections to each building. The sub-potable plumbing
within the building, however, must be provided to each plumbing fixture
except kitchen and bathroom sinks. In many homes, and particularly in
apartment buildings, this will require virtual duplication of the entire
cold water system. It is assumed that the hot water system could be
split in multifamily units and perhaps in single family residences.
CLASS "AA, A, B, & C" USES
When storm water is treated to potable water quality, the distribution
system is greatly simplified. The transmission line is provided from
the treatment plant to the existing public water supply main where a
metered connection is provided. The existing public system is then
permitted to distribute the potable water to all water uses within the
service area. It was anticipated that the service areas might be divided
by check valves or division valves, but in practice this would probably
not be desirable and the output of the treatment plant would supplement
the supply throughout the system.
CONSTRUCTION COSTS
All costs for the distribution systems were estimated on the assumption
that the necessary piping and plumbing would be installed following
construction of the streets and buildings, and that no particular economies
would be realized from concurrent construction with conventional facil-
ities. Underground pipelines have been assumed to be cast iron and
buried with a minimum of four feet of cover, with conventional valves
and fittings. Individual building connections and plumbing alterations
have been priced on a per unit basis. The in-place cost of cast iron
pipe was taken at $7. 50 per foot of six-inch pipe and $4. 75 per foot of
two-inch pipe. Table 21 lists the estimating figures used for building
service connections and for interior plumbing. All figures include
materials, labor, overhead, engineering,and construction contingencies
and are 1967-68 prices. These prices are applicable to sub-potable
distribution systems only and include provision for identification of
pipes and valves by color coding, tagging, etc.
105
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TABLE 21. CONSTRUCTION COSTS FOR SERVICE
CONNECTIONS AND INTERNAL PLUMBING FOR SUB-POTABLE REUSE
o
en
Type of Occupancy
Single family residential,
medium density
Single family residential,
low density
Town houses
Garden apartments
Mid-rise apartments
Schools, 500 pupils
Commercial uses
"C", "B&C"
$300/unit
$300/unit
$300/unit
$1000/20 units
or fraction plus
$65/unit
$80/unit
$1000/building
$200/unit
$200/unit
$200/unit
$1000/20 units
or fraction
$1000/building
"A", "A&B",
"A&C",
"A, B, & C"
$3 40/unit
$3 40/unit
$340/unit
$1000/20 units
or fraction plus
$100/unit
$100/unit
$1000/building
individually estimated
-------
QPERATION AND MAINTENANCE COSTS
Operation and maintenance of sub-potable distribution systems is pri-
marily composed of occasional repair of distribution mains, the portion
of the service connections in the public right-of-way, and routine inspec-
tion and reading of the individual meters. Only the incremental cost of
reading the, additional meter was computed, since it would be read simul-
taneously with the existing potable water meter. These costs were esti-
mated on a yearly basis as 0. 006 times the construction cost of the sys-
tem. The formula employed to convert these costs to daily costs is:
DAGOS = ATOT (0.006/365) (25)
where;
ATOT - annual total construction cost of distribution
system
DACOS = daily total operation and maintenance cost of
system
The portion of the service connection which is located on private property
and the internal plumbing would be maintained by the individual property
owner in the same manner as the existing conventional plumbing which it
supplements.
INPUT TO SYSTEM MODEL
The construction cost for each service area, or sub-watershed, was
computed for each group of reuse classes as described in the preceding
paragraphs. These values were used as input to the computer program,
permitting the selection of the proper costs for any sub-water shed or,
by adding costs for component areas, the proper value for any combina-
tion of sub-watersheds. The costs of transmission lines from treatment
plants were computed separately for each treatment plant location.
These values are shown as Tables 22 and 23, respectively.
107
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TABLE 22. CONSTRUCTION COST VS. SERVICE
AREA - DISTRIBUTION SYSTEM
o
oo
Sub-
Watershed
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Construction Cost for Reuse Level (Dollars)
"A"
17, 540
20, 990
10, 330
39, 060
45, 800
269, 920
141, 760
54, 500
151, 300
135, 650
80, 890
44, 180
54, 450
110, 820
22,460
82, 760
108, 720
31, 320
56, 360
101, 240
83, 330
33, 860
"B"
14, 600
18, 050
9, 210
27, 300
30, 120
163,400
85, 900
31, 900
136, 600
92, 250
63, 950
36, 200
41, 850
82, 000
18, 120
70, 300
78, 660
24, 840
50, 200
78, 840
50, 910
26, 020
"C"
16, 700
20, 150
10, 010
35, 700
41, 320
238, 930
125, 520
47, 720
147, 100
122, 575
76, 050
41, 900
50, 850
102, 300
21, 220
79, 200
104, 680
29,440
54, 600
93,440
72, 415
31, 620
"A", "B"
17, 540
20, 990
10, 330
39, 060
45, 800
269, 920
141, 760
54, 500
151, 300
135, 650
80, 890
44, 180
54, 450
110, 820
22,460
82, 760
108, 720
31, 320
56, 360
101, 240
83, 330
33, 860
"A" tl/-ill
17, 540
20, 990
10, 330
39, 060
45, 800
269, 920
141, 760
54, 500
151, 300
135, 650
80, 890
44, 180
54, 450
110, 820
22,460
82, 760
108, 720
31, 320
56, 360
101, 240
83, 330
33, 860
II A 11 ll-p II II fill
17, 540
20, 990
10, 330
39, 060
45, 800
269, 920
141, 760
54, 500
151, 300
135, 650
80, 890
44, 180
54, 450
110, 820
22, 460
82, 760
108, 720
31, 320
56, 360
101, 240
83, 330
33, 860
"B", "C"
16, 700
20, 150
10, 010
35, 700
41, 320
238, 930
125, 520
47, 720
147, 100
122, 575
76, 050
41, 900
50, 850
102, 300
21, 220
79, 200
104, 680
29, 440
54, 600
93, 440
72, 415
3, 162
-------
TABLE 23. CONSTRUCTION COSTS VS. SERVICE
ARE A-TRANSMISSION LINES
Treatment
Plant Location
(Sub- watershed)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Construction
Cost
(Dollars)
3000
3000
4500
2250
5550
2630
750
1800
2250
1950
3900
1500
750
1500
2400
3750
4280
2850
1730
1580
4280
1500
109
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SECTION X
SYSTEM MODEL
The system study of the local storage, treatment,and reuse of storm
water concept has been outlined in Section III. The system was defined
in Figure 1. The objective function was stated as:
1. Water released to surface drainage from the Local Storage,
Treatment, and Reuse of Storm Water system shall meet or
exceed stated effluent standards.
2. This condition will be achieved at the lowest net system
cost, or maximum net system benefit.
Maximum net system benefit was, in turn, defined as:
max/SB - EC - a EC.)
\ a a i/
where:
B = marginal annual benefits
cL
C = marginal annual operating and maintenance costs
3.
Op = marginal fixed construction and project costs
a = capital recovery factor (Equation 3)
The effluent standards required by the first part of the objective function
were developed in Section VI and shown as Table 10. The constraints
on the objective function were identified and investigated and the results
reported in preceding sections. All information obtained concerning these
constraints was prepared in a suitable form for input to a digital computer
code for maximization of the objective function subject to the constraints.
A review of the prior sections indicates a large number of possible
physical systems, all of which would satisfy the stated physical, tech-
nological, and institutional constraints. Twenty-two sub-water sheds
were considered, and a local system could be placed in each of them.
In this manner, the total watershed could be served by twenty-two local
reuse systems, making up the overall system for the watershed. This
is only one of many ways to meet the requirements of the watershed,
however. There are many points in the watershed where two, three,
or more sub-watersheds can be collected at one point. Figure 33 shows
the flow pattern of the various sub-water sheds and can be used to deter-
mine feasible combinations of multiple sub-watersheds. In these cases,
the combined sub-watersheds are treated as a single area and have a
single local reuse facility. As many as eighteen sub-watersheds can be
combined into a single collection area, and four which discharge directly
111
-------
2 •* 3
12 *• 11
19 22
13
14
16
15
17
18 ». 9
WILDE
LAKE
20
6 -
21 10
Figure 33. Wilde Lake Drainage Area Sub-Watershed Flow Pattern
-------
into Wilde Lake must remain independent. An exhaustive analysis of the
flow diagram would reveal that more than 10, 000 combinations of various
sized collection areas are possible. Fortunately, many of these are
trivial, and others present varying degrees of difficulty in terms of
physically intercepting the storm flows at the required points. Close
examination of the watershed resulted in 173 workable combinations of
service areas, each comprised of some number of sub-watersheds,
ranging from one to six. In view of the study objective of evaluating
local storage, treatment, and reuse of storm water, an upper limit of
six sub-watersheds, or about 350 acres, was set for the initial attempt
at system optimization. It was planned to reconsider this decision after
the first trial, but the results, reported below, indicated that the optimum
had been located within the range selected.
Each of the 173 overall systems is made up of a number of sub-systems,
namely, the various storage units, pretreatment units, final treatment
units, treated water storage units, distribution systems, etc., that
comprise the facilities in each collection area. The storm water might
be treated to any of four qualities and the distribution system might be
constructed to connect to any of eight different combinations of water
uses. Within a given overall system, the individual facilities might vary
from each other. Each of these possible sub-systems and overall systems
must have construction, operating, and maintenance costs calculated,
adjusted to annual costs, and subtracted from the net benefit that would
be derived from the overall system. When the remainder from this
calculation is available from each of the possible systems, the optimum
system can be identified and described. Due to the great complexity and
magnitude of the calculations required, the system optimization was
performed by a digital computer. The following sections describe the
preparation of inputs to the computer program, the operation of the
program, and the type of outputs obtained. The system optimization
results will be described, and final investigation of alternatives reported.
DESIGN INPUTS
Storage Facilities
In order to insure that the systems under analysis all correspond to the
range of technologic and physical possibilities, the essential design
criteria have been determined and must be included as inputs to the
system model, prior to optimization wherever possible. In the case of
storm water storage, various types of storage were considered which
might have varying applicability in different locations. Steel and concrete
tanks, both buried and at grade, were considered, along with open ponds.
The system model was arranged for a sub-optimization of storage facili-
ties, based on economic factors, which is described later. In the event
that local conditions prohibit a free choice among the alternatives, the
program can be instructed to choose either open ponds, buried tanks, or
nonburied tanks as the only possible construction method. It can also
113
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be instructed to choose only steel tanks or only concrete tanks, when
tanks are being considered.
I
Section VII described the anlysis performed to develop sizing criteria
for reservoirs. It was determined that the various reservoir purposes
could be served adequately by sizing the storm water storage facility at
the maximum expected volume of a storm just meeting the one-year
return interval criteria. The one-year storm volume has been calculated
for each sub-water shed, using the results of the hydrological analysis
reported in Section IV, and these values are listed as Table 24.
TABLE 24. EXPECTED MAXIMUM VOLUME
OF ONE-YEAR STORMS
One-year One-year
Storm Volume Storm Volume
Sub-water shed (gal) Sub-water shed (gal) ;,
1 460,800 12 594,200
2 586,500 13 696,800
3 530,000 14 1,030,000
4 341,600 15 236,400
5 530,500 16 782,000
6 1,104,000 17 1,130,000
7 803,200 18 290,000
8 625,900 19 923,500
9 1,460,000 20 1,203,000
10 1,258,000 21 690,000
11 854,000 22 358,400
Since it was originally anticipated that storage volumes would be a function
of demand, the systems analysis computer program was arranged to test
the service area demand against the available supply before computing
storage volume. The total runoff expected from each sub-watershed was'
calculated by methods described in Section IV. Further analyses described
later disclosed that, given the design criteria selected for the Wilde Lake
watershed, 90 percent of the total runoff will be available for reuse at
114
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release rates of at least 1.13 q. Accordingly, the total demand of any
service area under consideration can be compared to 90 percent of the
total runoff of that same area to determine the availability of the required
volume of treated water. When the demand exceeds the available
supply, it ,is reduced to equal that supply. This insures that later cal- >
culations of system costs, benefits, etc., refer only to the portion of
the demand that will be satisfied by treated storm water. The balance
of the demand will be satisfied by the public water supply system and
will not figure in the calculation of costs or benefits. When the demand
does not exceed the available supply, subsequent calculations will be
based on the actual demand. Table 25 lists the available supply for each
of the sub-water sheds, equal to 90 percent of total runoff.
TABLE 25. EXPECTED AVAILABLE SUPPLY FROM STORAGE
Available Supply Available Supply
Sub-water shed (gal/day) Sub-watershed (gal/day)
1 8,946 12 18,090
2 11,565 13 24,989
,. 3 10,064 14 38,340
4 11,807 15 9,129
5 19,080 16 25,327
6 43,513 17 50,305
7 33,915 18 , 9,630
8 26,468 19 38,520
9 43, 994 20 50, 193
10 61, 997 21 28, 734
11 30,552 22 13,410
When the service area under consideration consists of more than one
sub'•water shed, storage volume and maximum demand can be found by
adding the values associated with each of the component sub-watersheds
on the tables above.
115
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Pretreatment
Operating criteria developed in Section VIII for the pretreatment unit
stipulated that it must be capable of operating at twice the mean rate of
runoff. This can be accomplished within the system model by inputing
a table of total annual runoffs from each sub-water shed (Table 26) and
using these figures to determine costs for the pretreatment unit. Further-
more, when the total system of sub-watersheds is analyzed, each down-
stream pretreatment unit must process twice the mean runoff from its
own watershed as well as the total of the treatment rates of all the
upstream units. The cost figures used for determining construction
and operating cost of pretreatment units (Tables D-l and D-7) are based
on total runoff and treated water demand, respectively, and take into
account the operating rule suggested for the unit. Construction costs -"--'
are calculated to provide the necessary capacity, using the total annual .,--
runoff from Table 26.
TABLE 26. TOTAL ANNUAL RUNOFF VS. SUB-WATERSHED
Sub - wate r she d
1
2
3
4
5
6
7
8
9
10
11
Total
Annual Runoff
(gallons)
3, 626, 640
4, 693, 608
4,073,400
4, 790, 547
7, 732, 769
17, 653, 330
14, 006,578
10, 740, 789
17, 817, 840
25, 123, 680
12, 377, 880
Sub -water shed
12
13
14
15
16
17
18
19
20
21
22
Total
Annual Runoff
(gallons)
7, 332, 120
10, 133, 568
15, 528, 105
3, 700, 224
10, 259, 712
20, 388,024
3, 905S 208
15, 636, 600
20, 351, 232
11, 652,552
5, 439, 960
116
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Final Treatment
No physical or technological constraints are placed on final treatment
facilities, other than their categorization into four classes of treatment,
as discussed in Chapter VIII. Class "C" consists of pretreatment only
and only a portion of its operating costs are ascribed to final treatment.
Classes "B, " "A, " and "AA" are composed of pretreatment and various
levels of additional treatment. In all cases, demand is selected as the
parameter for determining costs, and the demand is tested prior to
calculation of treatment costs by the method described above in the
paragraph headed "Storage Facilities. "
Pumping Facilities
Pumping facilities are sized by adjusted treated water demand and are
constrained only by cost.
Treated Water Storage
As in the case of stormwater storage, a number of storage configurations
are analyzed. Steel and concrete tanks, above or below grade, as well
as elevated tanks, standpipes, and hydropneumatic tanks are all estimated
for each application, unless instructions are input to do otherwise. The
program can be instructed to select any of the storage configurations
without reference to the others. Treated water storage tanks are sized
as a function of demand by the relationships shown as Equations D-l,
D-2, and D-3.
Distribution System
The various physical constraints on distribution systems were discussed
in Section IX and were taken into account in developing distribution system
costs. Distribution costs are a function of the sub-watershed under study
and the level of reuse being considered. Sub-water sheds can be combined
by simply adding distribution co'sts. The transmission line to the treat-
ment plant itself is listed separately (Tables D-10 and D-ll) and is a
function of treatment plant location only. Where one treatment plant
serves two combined sub-watersheds, the two distribution costs are
summed, but only the transmission line costs for the actual location of
the plant apply.
SYSTEM COSTS
Storage Facilities
Construction, operating, and maintenance costs have been developed for
five different types of storm water storage facilities and presented as
117
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tables of cost/capacity relationships. Capital costs are composed of .
land cost, site preparation costs, burial costs where applicable, and
erection costs. Appendix C lists these costs. Average cost of land in-'-
the Columbia area was taken as $12, 000 per acre. Later discussions.
with the developer of Columbia led to a decision to confine facilities to
land already dedicated to public use, land not usable for other purposes ,
due to its function as natural floodplain. Since the cost of this land
was not chargeable to the local reuse system, land costs were set at
$0.00 per acre for purposes of systems analysis. An auxiliary com- :
putation indicated that total system costs were reduced approximately -
seven percent by this assumption. Land area, for purposes of computing
land costs and site preparation costs, is found from a land area table •.:":
(Table C-2) in the case of open ponds, and computed from capacity and.-/)
H/D ratio (Table C-3) in the case of tanks. Operation and maintenance1^ i
costs are made up of sediment removal costs and metal tank maintenaneei
Methods for computing these costs are given in Appendix G. After 'i-'-'>~t<
evaluation of all component costs, the total system cost of a storm waters.'
storage facility of any desired configuration can be expressed as: ••'-- -M
, 'i:' ','<••
C. = a(L. + P. + B. + E.)/365 + (R. + M.) (26) .:,
J J J J J J J ,-';,".
where:
3'.''"'
C. = total system cost of storage facility of
^ configuration j (dollars/day) •' :
a = capital recovery factor (Equation 3) '•. r
L. = land cost for storage facility of configuration . >
^ j (dollars) , -
P. - site preparation cost for storage facility of
^ configuration j (dollars) < < • i
B. = burial cost, where applicable (dollars) ." ' ;i:
- t
E. = erection cost (dollars)
J t "
R- = silt removal cost (dollars/day) -,:i-
J
M. = tank maintenance cost, where applicable -:
J (dollars/day)
All system costs were calculated as dollars/day to simplify the output
format. The annual rate of interest was taken at the 1967 level recom-
mended by the Water Resources Council for evaluation of water resources
projects - 0. 032 or 3. 2 percent annually. It was recognized that various
persons and agencies interested in the storm water reuse concept will
have a range of interest figures which they consider appropriate to the
analysis. A common approach is to select a rate based on the current
118
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cost of new capital to the agency considering construction of such a
facility. This might range from 4. 5 percent for the Federal Treasury
(1967) to approximately 5. 0 percent for most local governments in late
1967 or early 1968, to perhaps as much as 7. 0 percent for a private
developer.. Since the local reuse system is being compared to a conven-
tional approach which is characterized by relatively high capital costs
and low operating costs as compared to the study concept, the selection
Of a high interest rate would tend to penalize the conventional central
system. Accordingly, the 3. 2 percent figure was selected as the -lowest
interest rate that might be considered for the analysis, a conservative
assumption from the standpoint of evaluating the local reuse concept.
The amortization period is likewise a subject of some difference of
opinion. When such considerations are influenced by financing details,
the life of the bonds issued to fund the project is a common criterion -
frequently about 30 years. From an economic sense, the estimated
useful life of the facilities might be used, preferably each individual
unit but possibly the average useful life of all facilities. Fortunately,
in this case no substantial disagreement existed between the two
approaches. The estimated useful life of all facilities under consider-
ation is considered to average approximately 30 years. For this reason,
30 years was selected as the period of amortization.
The methods employed to calculate the total system cost on a dollar/day
basis of each of five possible configurations of storage have now been
outlined. Following these calculations, a sub-optimization is performed
for storm water storage configuration. Unless constrained to fewer
choices by the options described above, the five system costs are com-
pared and the lowest total system cost is selected. Results of the other
four computations are destroyed and the selected system costs are
retained as the final costs of storm water storage.
Pretreatment
Appendix C described the development of capital costs for the pretreatment
unit. Total annual runoff is obtained for a given watershed from Table 26,
and the pretreatment cost for this capacity is found by linear interpolation
of Table D-l. Sediment removal costs are computed as part of the stor-
age facility costs,and other operating and maintenance costs are combined
with final treatment costs. Total system cost of the pretreatment unit
is defined as:
C - a(S )/365 (27)
where:
C = total system cost of pretreatment unit (dollars/day)
S - construction cost of pretreatment unit (dollars)
119
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Final Treatment
Final treatment costs are composed of construction and operating costs,
each computed vs. daily treated water demand for four different levels
of treatment. Costs for given demand and level of treatment can be
obtained by linear interpolation of Tables D-3 and D-7 with respect to
demand, using the sections pertaining to the treatment level selected.
In this manner a construction cost and an operating cost are selected,
and they are combined in the following manner:
C, = a(S.)/ 365 + 0 (28)
T T I- • ' -
where:
C = total system cost of final treatment (dollars/day)
S = construction cost of treatment unit (dollars)
O, - operating and maintenance cost of treatment unit
(dollars/day)
Pumping Facilities
Construction costs of pumping facilities are obtained by linear inter-
polation of Table D-5. Operating and maintenance costs have been
combined with final treatment costs. Total system costs for pumping
are found by the following expression:
Ch a(Sh)/365 (29)
where:
C, = total system cost of pumping (dollars/day)
S, = construction cost of pumping facility (dollars)
Treated Water Storage l
Seven different configurations of treated water storage facilities were '
analyzed and costs developed for each of them. The types of facilities
examined include standpipes, elevated tanks, hydropnuematic tanks,
ground level steel and concrete tanks both above and below grade. Con-
struction costs for each of these are made up of land cost, site prep-
aration costs, burial costs (where applicable), and erection costs. Land
areas for land costs and site preparation costs are computed from the
tank capacity and the H/D factor. Costs are summarized in Tables C-6,
C-7, and C-9. H/D factors appear in Tables C-3, C-4, and C-5. Tank
120
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maintenance costs consist of periodic painting and maintenance of metal
tanks and are found by the methods described in Appendix C. As in the
case of storm water storage, a sub-optimization was performed for each
case by computing the total system cost for each tank (unless constrained
by the instructions described above) as follows:
Ck = a(Lk + Pk + Bk+Ek)/365+0k (30)
where:
C, = total system cost of treated water storage
facility of configuration k (dollars/day)
L^ = land cost for storage facility of configuration k
(dollars)
Pi - site preparation cost for storage facility
of configuration k (dollars)
B, = burial cost, where applicable (dollars)
E, = erection cost (dollars)
O, = operation and maintenance cost
(dollars/day)
The total system costs for the five possible configurations are compared
and the least cost alternative is selected and its costs retained. Costs
for the other approaches are not retained.
Distribution System
Costs for distribution systems for each sub-watershed and each level of
reuse are input directly to the computer program. These costs are tabu-
lated in Table 21. Service areas made up of several watersheds are
calculated by adding the appropriate entries from the component sub-water-
shed costs. The transmission line to the treatment plant is listed sepa-
rately on Table 22 and is added to distribution cost. Operation and main-
tenance cost is a function of total distribution capital cost and is calculated
by Equation 25. Total system cost for distribution is calculated as follows:
C , = a(S, + S )/365 + O, (31)
d d c u
where:
C , = total system cost of distribution system (dollars/day)
S, = construction cost of distribution system (dollars)
121
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S = construction cost of transmission line (dollars)
c
O = operation and maintenance cost of distribution
d system (dollars/day)
FEASIBLE SYSTEMS
Figure 33 illustrates the flow pattern of the 22 sub-watersheds in the
Wilde Lake watershed. As discussed earlier, preliminary investigation
revealed that a very large number of combinations of sub-watersheds
might be considered as possible collection areas. It was reported that,
in order to limit the number of cases to be calculated, no more than
six sub-watersheds would be considered a single collection area. Also
many trivial and doubtful cases were neglected and it was found that . r
110 different collection areas could be defined which would permit 173
combinations for the entire watershed. The collection areas, whichVare
virtually identical with the corresponding service areas, were determined
by examination of topographic maps of the watershed and include eve'ry
case where runoff from one or more sub-watersheds is apparently avail-
able at a single place. In each case, that point of collection was identified
by the sub-watershed within which it was located and the collection area
thus defined was assigned an index number. Tables 27, 28, and 29 list
the collection areas identified, showing the sub-watersheds which make
up each area, the treatment plant location, and the index number. Each
separate table represents a single computer run and is assigned its own
set of index numbers. The watershed was divided into three separate.
areas for analysis designated by Computer Runs A, B, and C, to simplify
the computer runs and to make the results of the first analysis available
at the earliest possible time, in the event that modifications to the method
would be required. The limitation of six sub-watersheds in a single .col-
lection area permitted the total watershed to be so divided without any
significant penalty in terms of total number of combinations presented for
analysis.
COMBINATIONS OF COLLECTION AREAS
Having defined all the individual storm water storage, treatment, and
reuse facilities possible within the Wilde Lake watershed, subject to the
stated constraints, the combinations of these systems which would serve
the entire watershed remain to be defined. Again, this may be done by
inspection,and Tables 30, 31, and 32 list the combinations so identified,
defined in terms of index number. Each index number, in turn, refers
to the corresponding collection area for the same computer run. In other
words, Combination Number 11 in Run B (Table 31) is made up of Index
Numbers 2, 6, and 24, which are sub-watershed numbers 2, 6, and the
combination of 1, 3, 11, and 12 (from Table 28). Each combination
represents a portion of a complete system with storage, treatment, and
reuse facilities for every area in that section of the watershed. The
various methods of assembling the 173 combinations result in 186, 300
ways of serving the entire watershed.
122
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TABLE 27. COLLECTION AREAS - COMPUTER RUN A
Index
Number
1
2
3
4
5
6
7
8
9
10
ll
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2§
29
30
31
32
33
34
35
36
37
38
39
40
41
Collection Area (Sub-watersheds)
4
5
6
7
8
14
4
4
5
6
6
7
7
4
4
5
6
7
6
4
5
5
4
4
4
4
5
4
5
6
4
4
4
5
4
4
4
4
5
4
4
5
14
14
8
7
8
14
5
7
7
7
8
7
5
6
7
6
7
5
6
6
7
7
7
5
5
6
6
5
5
5
6
6
5
5
14
14
14
14
14
8
7
7
8
7
8
7
7
7
8
8
8
6
7
7
7
6
6
7
7
7
6
6
14
14
14
14
14
14
7
8
8
8
8
7 14
8 14
8 14
8 14
7 8
7 8 14
Treatment
Location
4
5
6
7
8
14
4
14
14
6
7
7
14
14
14
14
14
7
7
7
7
7
7
7
14
14
14
14
14
14
7
7
7
7
6
14
14
14
14
7
14
123
-------
TABLE 28. COLLECTION AREAS - COMPUTER RUN B
Index
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Collection Area (Sub-Watersheds)
1
2
3
11
12
13
1
1
2
3
3
11
11
12
1
1
1
2
2
3
3
11
1
1
1
2
2
3
1
1
1
2
1
2
3
3
11
12
12
13
13
2
3
3
3
3
11
11
12
2
3
3
3
3
11
2
2
3
3
2
3
11
12
11
12
12
13
13
3 11
11 12
11 13
11 12
11 13
12 13
3 11 12
3 11 13
11 12 13
11 12 13
3 11 12 13
Treatment
Lbcatidh
1
2
3
11
12
13
2
3
3
11'
12 !
11
13
13-'
3
11
12
11
12
11
13
13-
11
11
13
11
13
13
11
13
13
13
13
124
-------
TABLE 29. COLLECTION AREAS - COMPUTER RUN C
Index
Nurnber
1
2
3
4,
5
6
7
8
9
10
11.
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Collection
15
16
17
18
9
20
10
19
21
22
15
15
15
16
17
18
9
15
15
15
15
16
17
18
15
15
15
15
16
16
17
15
15
15
16
15
16
17
18
18
18
9
20
16
16
17
18
18
18
9
16
17
16
18
17
18
18
16
17
16
17
16
Area
17
18
18
9
9
9
20
17
18
18
9
18
9
9
17
18
18
18
17
(Sub -water sheds)
18
9
9
20
9
20
20
18 9
9 20
9 20
9 20
18 9 20
Treatment
Location
15
16
17
18
9
20
10
19
21
22
16
17
18
18
18
9
20
17
18
18
9
9
9
20
18
9
9
20
9
20
20
9
20
20
20
20
125
-------
TABLE 30. COMBINATION OF AREAS - COMPUTER RUN A
05
Combination
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
Index Numbers of
Collection Areas
41
40
39
38
37
36
35
35
34
34
33
33
32
31
30
30
29
28
27
26
25
25
24
24
23
23
22
22
21
21
20
20
19
19
6
1
2
3
5
13
4
8
1
9
2
3
5
7
1
1
2
1
2
10
3
9
2
9
2
8
1
8
1
10
3
14
7
6
6
6
6
6
2
3
3
5
5
5
3
3 6
5
5 6
3
3 6
5
5 6
6
5 6
6
Combination
Number
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
. 61
62
63
64
65
66
67
68
69
Index Numbers of
Collection Areas
19
19
19
18
18
17
17
16
16
15
15
14
14
14
14
13
13
13
13
12
12
12
12
11
11
11
11
10
10
10
10
9
, &
- 7-
i
8
9
1
7
1
7
1
10
1
10
2
11
12
10
3
7
7
10
1
7
8
9
1
7
8
9
1
7
8
9
1
1
2
3
2
2
1
2
3
2
5
2
1
3
2
3
5
3
4
4
10
3
1
2
3
2
1
2
5
2
1
2
4
2
1
2
3
3
4
3
6
3
5
5
5
5
5
2
3
6
3
3
3
6
5
5
5
6
4
4
4
4
4
5
4
-------
TABLE 31. COMBINATION OF AREAS - COMPUTER RUN B
CO
Combination
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Index Numbers of
Collection Areas
33
32
31
30
29
28
28
27
26
25
24
23
23
22
22
22
22
22
21
21
20
20
19
19
18
18
17
1
2
5
6
7
1
1
1
2
2
14
5
15
7
8
9
1
7
1
7
1
13
1
14
1
13
2
5
6
5
6
6
3
2
1
2
5
2
6
2
1
4
1
5
2
3
5
6
6
6
Combination
Number
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Index Numbers of
Collection Areas
17
16
16
15
15
15
15
14
14
14
14
14
13
13
13
13
13
12
12
11
11
10
10
9
8
7
6
2
14
2
12
13
14
4
10
9
8
7
1
11
9
8
7
1
7
1
7
1
7
1
1
2
3
5
4
2
5
6
5
4
5
7
4
4
4
2
7
1
2
3
2
4
2
4
2
5
2
4
4
4
4
6
6
6
1
2
3
3
5
5
5
3
6
4
6
4
6
5
5
5
5
3
4
5
6
6
6
6
6
6
2
-------
TABLE 32. COMBINATION OF AREAS - COMPUTER RUN C
Combination
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
36
35
34
33
32
31
31
30
30
29
28
27
26
25
25
24
24
24
24
23
23
22
22
21
20
20
19
19
18
18
18
17
17
17
17
17
17
17
17
16
16
16
15
15
14
14
13
12
11
1
Index Numbers of Collection Areas
7
1
3
2
6
11
1
12
1
1
2
3
2
17
5
18
11
12
1
11
1
12
1
2
17
2
17
3
16
17
4
11
12
13
11
12
14
15
1
11
12
1
11
1
12
1
2
2
3
2
8
7
7
7
7
7
2
7
3
6
3
6
6
7
6
7
3
2
2
6
2
6
3
3
2
5
3
5
6
4
5
15
14
2
3
2
1
1
2
3
2
2
5
2
5
3
3
4
4
3
9
8
8
8
8
8
7
8
7
7
7
7
7
8
7
8
7
7
3
7
6
7
6
6
7
6
7
6
7
7
6
7
7
3
4
4
3
2
3
6
6
3
6
5
6
5
5
5
5
4
10
9
9
9
9
9
8
9
8
8
8
8
8
9
8
9
8
8
7
8
7
8
7
7
8
7
8
7
8
8
7
8
8
7
7
7
7
7
4
7
7
6
7
6
7
6
6
6
6
5
10
10
10
10
10
9
10
9
9
9
9
9
10
9
10
9
9
8
9
8
9
8
8
9
8
9
8
9
9
8
9
9
8
8
8
8
8
7
8
8
7
8
7
8
7
7
7
7
6
10
10
10
10
10
10
10
10
10
9
10
9
10
9
9
10
9
10
9
10
10
9
10
10
9
9
9
9
9
8
9
9
8
9
8
9
8
8
8
8
7
10
10
10
10
10
10
10
10
10
10
10
10
9
10
10
9
10
9
10
9
9
9
9
8
10
10
10
10
10
10
10
9
10
128
-------
Since each section of the complete watershed that was isolated for a com-
puter run is, by the terms of the assumptions, independent of the others,
each can be optimized separately and the optimum system for the water-
shed will be the summation of the individual optimum systems. This
approach reduces by several orders of magnitude the number of cases
which must be examined.
129
-------
SECTION XI
SYSTEM MODEL OUTPUTS AND OPTIMIZATION
Section V discussed the water demands which are under consideration as
possible areas of storm water reuse. It was determined that the four
classes of demands could be served in eight different ways: the storm
water system could be connected to the "AA, A, B, and C" demands, the
"A, B, and C" demands, the "A and B, " "A and C, " or "B and C" de-
mands, or simply be connected to the "A, " "B, " or "C" demands indepen-
dently of the others. In each case, the distribution system and treatment
system required are different from any of the other cases. Tables 27, 28,
and 29 presented various collection areas that couldbe considered, each of
them made up of one or more sub-watersheds, but permitting storm water
to be collected, stored, and treated at a single central location. The
first step in the optimization was, therefore, to compute total system
costs for each feasible arrangement of facilities within each possible
collection area.
TABLE OF NET BENEFITS
These results are displayed as a Table of Net Benefits, a sample of which
is reproduced as Figure 34. The case numbers in the left-hand column
can be observed to correspond to the index numbers of Table 27. Each
line represents a particular collection area, with the sub-watersheds and
treatment plant location listed in successive columns. The entries under
the eight water service level columns correspond to the net system bene-
fit for the facilities which would serve the demands indicated in the col-
lection area listed. The final column reflects the maximum net benefit
appearing on that line and identifies the optimum configuration for that
collection area alone. Negative net benefits are identical with net system
costs. All values are in dollars/day and are computed as follows:
B = B — (_*. — C, — C,, C_>, — C^T — \~s , /oo\
n gjopthkod (32)
where:
B = net system benefits (dollars/day)
B = gross system benefit, computed as discussed
^ (see Section.XI) (dollars/day)
C. = sub-optimized system cost of storm water
3° storage from Equation 33 (dollars/day)
C - system cost of pretreatment from Equation 34
p (dollars/day)
131
-------
**** TABLE OF NET BENEFITS ****
CO
to
CASE
NUMBER
1
b
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
WATERSHED
NUMBER
PLANT
LOCATION
4
5
6
7
8
14
4
4
5
6
6
8
7
4
4
5
t>
8
6
4
6
8
6
8
4
4
5
4
5
6
4
4
4
5
4
4
4
4
5
4
4
0
0
0
0
0
0
5
14
14
8
7
7
14
5
7
7
7
7
7
5
7
7
7
7
5
6
6
8
8
7
5
5
6
6
5
5
5
6
6
5
5
0
0
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
14
8
7
5
5
4
4
7
7
7
7
7
8
6
8
7
7
6
6
a
7
7
6
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
14
14
7
7
8
8
8
7
7
B
e
7
7
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
8
a
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
5
6
7
8
14
4
14
14
6
7
7
14
14
14
14
14
7
7
7
7
7
7
7
14
14
14
14
14
14
7
7
7
7
6
14
14
14
14
7
14
A
-9.18
-10.74
-41.24
-21.72
-10.76
-19.77
-15.58
-24.93
-25.95
-45.92
-58.38
-26.75
-35.28
-31.15
-40.25
-41.19
-69.22
-41.47
-62.49
-31.02
-62.58
-32.32
-62.71
-31.38
-46.15
-73.07
-72.63
-46. 54
-47.48
-72.39
-66.58
-37.28
-66.34
-65.90
-54.44
-76.47
-52.18
-76.20
-75.70
-69.75
-79.40
8
-7.54
-10.09
-27.41
-16.96
-9.07
-15.59
-12.84
-18.65
-20.64
-31.93
-39.64
-21.58
-28.06
-23.69
-31.16
-33.13
-50.75
-32.52
-44.21
-25. 16
-44.71
-26.62
-42.75
-24.63
-36.24
-53.86
-55.82
-35.74
-37.70
-55.32
-47.82
-29.69
-47.32
-49.28
-40.11
-58.93
-40.81
-58.77
-60.44
-52.39
-64.12
C
-5.78
-7. 12
-27.72
-12.08
-6.03
-13.47
-10.71
-17.36
-18.14
-27.75
-37.85
-16.35
-23.79
-22.09
-27.75
-28.55
-43.51
-27.93
-37.99
-20.85
-39.16
-21.07
-40.31
-20.29
-32.55
-46.11
-46.90
-32.03
-32.83
-46.35
-41.57
-25.05
-40.38
-39.44
-33.20
-50.85
-36.83
-50.30
-51.06
-43.39
-54.88
WATER
A,B
-8.60
-9.80
-41.24
-21.72
-8.91
-16.43
-13.43
-21.06
-21.47
-45.92
-58.38
-26.75
-33.97
-25.52
-38.31
-38.17
-69.22
-38.13
-62.49
-31.02
-62.58
-30.95
-62. 71
-31.09
-42.39
-73.07
-72.63
-42.15
-41.71
-72.39
-66.58
-35.28
-66.34
-65.90
-54.44
-76.47
-45.56
-76.20
-75.70
-69.75
-79.40
QUALITY
A,C
-8.60
-9.17
-41.24
-21.72
-8.88
-16.43
-13.43
-21.06
-21.11
-45.92
-58.38
-26.75
-33.97
-25.52
-38.31
-38. 17
-69.22
-38.13
-62.49
-31.02
-62.58
-30.95
-62.71
-31.09
-42.39
-73.07
-72.63
-42.15
-41.71
-72.39
-66.58
-35.28
-66.34
-65.90
-54.44
-76.47
-45.56
-76.20
-75.70
-69,75
-79.40
-
A,B,C
-8.60
-9.17
-41.24
-21.72
-8.88
-16.43
-13.43
-21.06
-21.11
-45.92
-58.38
-26.75
-33,97
-25.52
-38.31
-38.17
-69.22
-38.13
-62.49
-31.02
-62.58
-30.95
-62.71
-31.09
-42.39
-73.07
-72.63
-42.15
-41.71
-72.39
-66.58
-35.28
-66.34
-65.90
-54.44
-76.47
-45.56
-76.20
-75.70
-69.75
-79.40
B,C
-7.20
-8.67
-31.94
-15.83
-6.46
-11.50
-10,-21
-14.10
-15.22
-33.86
-44.91
-16.90
-24.02
-17.70
-28.00
-27.79
-52.99
-27.49
-47. S8
-21.07
-48.68
-20.67
-48.89
-20.87
-31. 3G
-54.98
-53.54
-29.85
-28.41
-52.10
-51.31
-24.64
-49.87
-48.43
-41.52
-55.52
=30.40
-54.04
-52.54
-50.42
-54.38
AA,A,B,C
-6.61
-6.52
-4. 16
-4.57
-5.26
-4.48
-5.20
-3.55
-2,93
-1.39
-0.14
-2.40
-1.00
-1.54
0.53
1.51
5.09
2.71
3.52
-1.95
2.37
0.11
1.39
-0. 87
3.09
6.80
7.89
4.29
5.37
9.09
4.02
1.65
5.22
6.31
2.66
9.61
7.08
10.84
11.99
8.02
13.85
MAX
-5.78
-6.5?
-4.16
-4.57
-5.26
-4.48
-5.20
-3.55
-2.93
-1.39
-0.14
-2.40
-1.00
-1.54
0.53
1.51
5.09
2.71
3.52
-1.95
2.37
0.11
1.39
-0,87
3.09
6.80
7.89
4.29
5.37
9.09
4.02
1.65
5.22
6.31
2.66
9.61
7.08
10.84
11.99
8.02
13.89
Figure 34. Table of Net Benefits - Computer Run A
-------
C^ - system cost of final treatment from Equation 35
(dollars/day)
Ch = system cost of pumping from Equation 36
(dollars/day)
C, = sub-optimized system cost of treated
water storage from Equation 37
(dollars/day)
C, = system cost of distribution from Equation 38
(dollars/day)
TABLES OF CAPITAL COSTS AND DAILY COSTS
Figures 35 and 36 show computer listings of capital costs and daily
operating and maintenance costs for each of the collection area/reuse
level alternatives considered in one of the computer runs. The Table of
Capital Cost indicates the total construction cost, in dollars, for each
alternative. Case Number 11, for example, is a collection area composed
of sub-watersheds Numbers 6 and 7 and for the reuse level "C" (only
Class "C" uses considered) has a capital cost of 0.03958E06, read as
0. 03958 x 106, or $39, 580. This includes all costs associated with the
construction of a storm water storage facility, a pretreatment unit,
pumping facilities, treated water storage, and a ' C" level distribution
system. No final treatment was provided, since only Class "C" water
was required. The next entry on the same line, for reuse level "A, B," in-
dicated a capital cost of 0. 4658E 06 or $46, 580 and includes a Class "A"
treatment plant as well as the additional cost of the "A, B" distribution
system. Pumping and treated water storage facilities maybe sized some-
what differently due to increased total demand.
The Table of Daily Cost, Figure 36,follows the same format as Figure 35,
except that entries are for total operating and maintenance cost, expressed
in dollars/day. The alternative used in the example above, Case Number 11
at reuse level "C,"can be found to have an operating and maintenance cost
of $19. 70 per day. The "A, B" reuse level for the same case hasanoper-
ating and maintenance cost of $30. 20 per day, largely due to the additional
cost of operating the Class "A" treatment plant. These costs include silt
removal, maintenance costs of storm water and treated water storage,
where applicable, operating and maintenance costs of pretreatment,final
treatment, pumping, and distribution facilities. The total system cost
of each alternative in dollars/day is the sum of the daily costs from
Figure 36 and the capital costs from Figure 35 multiplied by the capital
recovery factor and divided by 365.
133
-------
** TABLE OF CAPITAL COST **
LASt
NUMBtk
1
2
3
4
5
Q
7
a
9
10
ii
12
13
1*
15
16
17
18
19
20
21
22
23
24
25
26
27
2d
29
30
31
32
33
34
35
36
37
38
39
40
41
WATERSHED
NUMBER
PLANT
LOCATION A
4
5
6
7
a
14
4
i.
>3
6
t>
d
7
^
4
5
o
8
o
4
6
8
6
8
4
4
5
4
5
6
4
4
4
5
4
4
4
4
5
4
4
0
0
0
0
0
0
5
14
14
8
7
j
14
5
7
7
7
7
7
5
7
7
7
7
5
6
6
a
8
7
5
5
6
6
5
5
5
6
6
5
5
0 0
0 J
0 0
0 0
0 u
0 0
0 0
0 0
0 0
0 0
0 0
0 u
0 0
14 0
14 C
14 0
14 0
14 0
8 0
7 0
5 0
5 0
4 0
4 0
7 14
7 14
7 14
7 14
7 14
8 14
6 7
8 7
7 8
7 8
6 8
6 7
8 7
7 8
7 8
6 7
6 7
0
0
0
0
0
u
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
C
0
0
0
0
0
0
0
0
0
0
14
14
14
14
8
8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
0
u
0
0
u
0
0
0
0
0
0
u
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
u
0
0
0
u
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
u
0
0
u
0
0
0
0
0
u
0
0
0
0
0
0
0
u
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
i,
5
6
7
8
14
4
14
14
6
7
7
14
14
14
14
14
7
7
7
7
7
7
7
14
14
14
14
14
14
7
7
7
7
6
14
14
14
14
7
14
0.7069E
0.8307E
0. 3145E
0. 180UE
0. 8976E
0. 1505E
0. 1235E
0. 1933E
0. 2019E
0.3779E
0.4658E
0. 2441E
0.3051E
0.2449E
0.3481E
0. 3567E
0.5925E
0. 3663E
0. 5294E
0.2769E
0. 5185E
0.29S9E
0.5092E
0. 2873E
0.3997E
0. 6370E
0. 6470t
0.4101E
0. 4186E
0.6583E
0. 5626E
0.3389E
0. 5740E
0. 5840E
0.4741E
0.6914E
0.4622E
0.7025E
0. 7121E
0.6285E
0.7556E
050.
050.
060.
060.
050.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
060.
B
6113E
6915E
2060E
1233E
6836E
1227E
9704E
1527E
1567E
2427E
2964E
1607E
2148E
1867E
2451E
2489E
3880E
2510E
3335E
1881E
3306E
1947E
3268E
1907E
2793E
4163E
4222E
2822E
2860E
4250E
3610E
2248F
3639E
3677E
3073E
4525E
3164E
4554E
4590E
3981E
4887E
050
050
060
060
050
060
050
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
060
C60
060
060
060
060
060
060
060
060
060
060
060
060
060
WATER QUALITY -
C A, 8 A,C A,B,C B,C AA,A,
.5139E
.6231E
-2642E
. 1454E
.66C9E
. 1246E
.9845E
.1629E
.1700E
.3181E
.3958E
.1992E
.2578E
.2086E
.2964E
. 3035E
.5086E
.3102E
.4497E
.2304E
.4421E
. 2449E
.4347E
.2378E
.3421E
.5474E
.5545E
.3496E
.3566E
.5619E
.4810E
.2835E
.4885E
.4959E
.4030E
.5930E
.3952E
.6004E
.6073E
. 5345E
.6449E
050.7100E 050.7100E 050.7100E 050.6982F 050.5752E 05
050.8357E 050.8391E 050.8391E 050.8081E 050.6405E 05
060.3145E 060.3145E 060.3145E 060.2835E 060.7175E 05
060.1800E 060.1800E 060.I800E 060.1644E 060.6493F 05
050.9073E 050.9075E 050.9075F 050.8495E 050.6257E 05
060.1528E 060.1528E 060.1528E 060.1446E 060.6894E 05
050.1248E 060.1248E 060.1248E 060.1175E 060,6644-E 05
060.1963E 060.1963E 060.1963E 060.1839E 060.7394F 05
060.2053E 060.2056E 060.2056E 060.1910E 060.7686E 05
060.3779E 060.3779E 06C.3779E 060.3382E 060.8194E 05
060.4658E 060.4658E 060.4658E 060.4161E 060.8293E 05
060.2441E 060.2441E 060.2441E 060.2199F 060.7586E 05
060.3060E 060.3060E 060.3060E 060.2792E 06C.8203E 05
060.2490E 060.2490E 060.2490E 060.2305E 060.8181E 05
060.3494E 060.3494E 060.3494E 060.318'4E 060.8698E 05
060.3587E 060.3587E 060.3587E 060.3261E 060.8989E 05
060.5925E 060.5925E 060.5925E 060.5302E 060.1031E 06
060.3686E 060.3686E 060.3686E 060.3332E 060.9143E 05
060.5294E 060.5294E 060.5294E 060.4708E 060.9397E 05
060.2769E 060.2769E 060.2769E 060.2513E 060.7852E 05
060.5185E 060.5185E 060.5185E 060.4630E 060.9080E 05
060.2968E 060.2968E 060.2968E 060.266BE 060.8372E 05
060.5092E 060.5092E 060.5092E 060.4554E 060.8788E 05
060.2875E 060.2875E 060.2875E 060.2591E 060.8081E 05
060.4024E 060.4024E 060.4024E 060.3654E 060.9551F 05
060.6370E 060.6370E 060.6370E 060.5695E 060.1109E 06
060.6470E 060.6470E 060.6470E 060.5772E 060.1156E 06
060.4137E 060.4137E 060.4137E 060.3732E 060.9962E 05
060.4237E 060.4237E 060.4237E 060.3809E 060.1043E 06
060.6583E 060.6583E 060.6583E 060.5851E 060.1197E 06
060.5626E 060.5626E 060.5626E 060.5023E 060.9769E 05
060.3403E 060.3403E 060.3403E 060.3060E 060.8867E 05
060.5740E 060.5740E 060.5740E 060.5102E 060.1018E 06
060.5840E 060.5840E 060.5840E 060.5178E 060.1065E 06
060.4741E 060.4741E 060.4741E 060.4244E 060.9487E 05
060.6914E 060.6914E 060.6914E 060.6164E 060.1233E 06
060.4682E 060.4-682E 060.4682E 060.4202E 060.1121E 06
060.7025E 060.7025E 060.7025E 060.6241E 060.1272E 06
060.7121E 060.7121E 060.7121E 060.6314E 060.1315E 06
060.6285E 060.6285E 060.6285E 060.5572E 060»1143E 06
060.7556E 060.7556E 060'.7556E 060.6696E 060. 1382E 06
Figure 35. Tatsle of Capital Cost - Computer Run A
-------
** TABLE OF DAILY COST **
00
Ol
CASE
NUMBER
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
WATERSHED
NUMBER
PLANT
LOCATION
4
5
6
7
8
14
4
4
5
6
6
8
7
4
4
5
6
a
6
4
6
8
6
8
4
4
5
4
5
6
4
4
4
5
4
4
4
4
5
4
4
0
0
0
0
0
0
5
14
14
8
7
7
14
5
7
7
7
7
7
5
7
7
7
7
5
6
6
8
8
7
5
5
6
6
5
5
5
6
6
5
5
0
0
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
14
8
7
5
5
4
4
7
7
7
7
7
8
6
8
7
7
6
6
8
7
7
6
6
0
0
0
0
0
0
0
0
0
0
0
0
c
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
14
14
7
7
8
8
8
7
7
8
8
7
7
0
0
0
0
0
0
0
c
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
14
14
14
8
8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
5
6
7
8
14
4
14
14
6
7
7
14
14
14
14
14
7
7
7
7
7
7
7
14
14
14
14
14
14
7
7
7
7
6
14
14
14
14
7
14
- WATER QUALITY -
C A.B A,C
A,B,C
B,C
AA.A.B.C
3.63
4.91
17.84
12.81
7.00
9.93
8.53
13.55
14.82
26.62
30.20
21.83
24.93
18.44
28.32
29.52
41.99
31.46
38.36
23.56
36.32
26.72
34.16
25.53
32.91
45.30
47.07
34.85
36.04
48.90
39.85
30.11
41.68
43.44
36.68
50.39
39.08
52.21
53.98
46.76
57.29
2.16
2.21
8.36
5.19
3.26
5.96
4.37
8.11
8.16
11.61
13.54
8.43
11.13
10.31
13.28
13.33
19.48
14.38
16.78
9.54
15.74
10.64
15.69
10.59
15.48
21.63
21.68
16.53
16.58
22.72
17.89
12.79
18.93
18.98
15.96
23.83
18.73
25.22
25.00
21.13
27.83
2.15
2.89
11.54
8.00
4.09
6.44
5.03
8.58
9.32
17.02
19.70
12.07
14.42
11 .46
16.59
17.34
28.30
18.58
25.29
13.02
23.85
14.95
22.43
14.21
19.54
30.90
31.65
20.78
21.53
32.86
26.55
17.13
27.97
29.27
23.33
33.81
23.73
35.02
35.76
31.44
37.93
4.27
5.93
17.84
12.81
9.01
13.63
10.88
17.88
19.83
26.62
30.20
21.83
26.11
24.26
30.07
32.24
41.99
34.48
38.36
23.56
36.32
27.96
34.16
25.79
36.05
45.30
47.07
37.88
39.65
48.90
39.85
31.92
41.68
43.44
36.68
50.39
42.96
52.21
53.98
46.76
57.29
4.27
6.62
17.84
12.81
9.05
13.63
10.88
17.88
20.23
26.62
30.20
21.83
26.11
24.26
30.07
32.24
41.99
34.48
38.36
23.56
36.32
27.96
34.16
25.79
36.05
45.30
47.07
37.88
39.65
48.90
39.85
31.92
41.68
43.44
36.68
50.39
42.96
52.21
53.98
46.76
57.29
4.27
6.62
17.84
12.81
9.05
13.63
10.88
17.88
20.23
26.62
30.20
21.83
26.11
24.26
30.07
32.24
41.99
34.48
38.36
23.56
36.32
27.96
34.16
25.79
36.05
45.30
47.07
37.88
39.65
48.90
39.85
31.92
41.68
43.44
36.68
50.39
4?.Qfe
52.21
53.98
46.76
57.29
3.04
3.82
12.99
9.16
5.79
9.57
7.24
12.69
13.37
20.24
23.85
15.46
20.01
17.32
24.21
26.54
34.68
28.92
32.15
17.27
30.38
21.99
28.05
19.66
30.27
36.89
37.98
31.39
32.49
39.11
33.23
26.19
34.36
35.45
30.89
40.19
34.69
41.31
42.40
37.66
44.61
4.21
6.82
15.57
12.17
9.47
13.72
11.02
17.91
20.52
24.53
26.86
21.61
25.26
24.27
28.87
31.11
37.86
33.39
34.81
22.94
32.71
27.46
30.47
25.22
34.59
40.89
42.76
36.49
38.37
44.67
35.92
31.07
37.83
39.70
33.97
45.79
41.39
47.69
49.57
42.73
52.59
Figure 36. Table of Daily Cost - Computer Run A
-------
TABLE OF BENEFITS
The gross system benefits for the alternatives discussed in the preceding
sections were calculated and displayed on a Table of Benefits, shown as
Figure 37. Benefits were taken as the marginal cost from the alternate
source of the water used. In this case, benefits were the marginal cost,
to the consumer, of an equal volume of water purchased from the Howard
County Metropolitan Commission. Under the rate structure in effect in
July 1968, that cost is $0.45 per 1000 gallons. The use of this very ele-
mentary measure of benefits overlooks the effect of such use on the public
water supply system. Many of the costs associated with providing the
public water supply to the demand area under study would not be reduced
by the introduction of the supplementary source. The distribution system,
the cost of billing and collection, the overhead structure of the Metropolitan
Commission - all of these would be essentially unchanged. Since all of the
water distributed by Howard County is purchased from Baltimore City and
the Washington Suburban Sanitary Commission, the most clear-cut reduc-
tion in true costs would be the reduction in the quantity of water purchased,
which accounts for less than half of the total selling price of water. Other
costs would be reduced in a less easily determined manner, such as booster
pumping costs, storage costs, transmission main costs, etc. To properly
evaluate the total impact of the study system on the public supply system,
a detailed study of the financial structure of the Metropolitan Commission
would be required, a task outside the scope of this work. Since the Wilde
Lake area constitutes a small portion of the county water distribution area,
it was considered that the elimination of a portion of the demand in this
watershed would not have a significant overall effect on the cost of supplying
water to the county. For this reason, the full price of water was taken as
a benefit, although in a proposed large-scale application of the local reuse
concept, this assumption would require careful review in light of local
circumstances. Ideally, the economic benefit due to water supply shouldbe
taken as the marginal reduction in the total long-term costs of the public
water system, due to the reduction in demand on that system.
At the time the computer runs were performed, the marginal cost of water
in Howard County had recently been revised to $0. 50 per 1000 gallons.
Shortly after the runs were completed, however, the rate was reducedto
$0.45 per 1000 gallons. Accordingly, Figure 37 shows benefits calculated
on the basis of the higher figure, and subsequent discussion of net benefits
in this chapter will be in terms of $0.50 per 1000 gallons marginal cost of
water. Prior to the defintion of the conceptual optimum systems described
in Sections XII-XV, however, and prior to the system evaluation reported
in Section XVII,the benefit rate was revisedto $0.45 per 1000 gallons and
economic comparisons made on that basis. Since the benefit is, in this
case, completely linear and independent of other assumptions, manipu-
lating the rate has no effect on system optima. The individual benefits
listed on Figure 37 are calculated as the product of the treated water
demand in gallons per day and marginal cost of water to consumer, from
public system in dollars per gallon. The net system benefits can also
be calculated as the difference between the gross system benefits on
Figure 37 and the total system costs (Capital Cost from Figure 35 -r 365
x Capital Recovery factor + Daily Cost from Figure 36).
136
-------
** TABLE OF BENEFITS **
CASE
NUMBER
WATERSHED MUMBbR
PLANT
LOCATION
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Jl
32
33
34
35
36
37
38
39
40
41
4
5
6
7
8
14
4
4
5
6
6
b
7
4
4
5
fa
8
6
4
6
8
6
8
4
4
i>
4
5
6
4
4
4
5
4
4
-------
TABLE OF NET BENEFITS FOR COMBINATIONS OF WATERSHEDS
Tables 30, 31, and 32, described earlier, defined the various combina-
tions of collection areas which would satisfy the entire section of the
watershed under consideration. The entire watershed was analyzed in
three sections, each the subject of a single computer run. In order to
compare and rank the various alternative systems which would serve the
section under study, it is necessary, for each combination of collection
areas, to add the net benefits associated with the most attractive reuse
levels for each of the component collection areas, obtaining the total net
benefit for the combination. This is done by adding the net benefits
appearing in the final column of the Table of Net Benefits (Figure 34) for
the collection areas which make up a given combination. When this is
done for each possible combination, the results can be ranked and the
optimum system for that section identified.
In order to obtain as much1 information about alternative strategies as
possible, a five-level output was obtained. This output defined optimum
systems under each of the following assumptions:
1. All levels of reuse possible
2. Only reuse up to water quality "A" possible
3. Only reuse up to water quality "B" possible
4. Only reuse up to water quality "C" possible
5. No reuse possible
This output for computer Run A is displayed as Figures 38 and 39.
Figure 38 lists total net benefits for all possible combinations under the
first four assumptions noted above. Column "AA" reflects combinations
of optimum subsystems when all levels of reuse are possible. It should
be noted that the optimum subsystems might be composed of any assort-
ment of reuses. The only requirement is that each collection area be
served by the system having the maximum net benefit for that area. These
subsystems are those identified by the MAX column of Figure 34.
Column "A" follows the same pattern, except that all subsystems involv-
ing the use of "AA" water are excluded from the analysis. Column "B"
excludes all "AA" and "A" uses and Column "C" excludes all but "C. "
The assumption of no reuse required a separate computer run, due to the
limitation of the system to storm water storage andpretreatmentfacilities,
Since no benefits, as defined in the economic analysis, derive from this
application, the net benefits reported are identical with system cost.
Figure 39 lists the totals for the various combinations. This assumption
describes a system which retains only water pollution control as its
purpose and permits direct comparison with the alternative method of
water pollution control described in later chapters.
138
-------
«*** TABLE OF NET BENEFITS FOR COMBINATIONS OF WATERSHEDS ****
COMBINATION
NUMBER
PLANT COMBINATIONSr PERMISSIBLE MAXIMUM QUALITY
AA
4
2
3
t,
5
6
J
8
9
10
11
12
13
14
15
16
17
IB
L9
20
21
22
23
24
^5
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
41
40
39
38
37
36
35
35
34
34
33
33
32
31
30
30
29
28
27
26
25
25
24
24
23
23
22
22
21
21
20
20
19
19
19
19
19
18
18
17
17
16
16
15
15
14
14
14
14
13
13
13
0
6
1
2
3
5
13
4
8
1
9
2
3
5
7
1
1
2
1
2
10
3
9
2
9
2
8
1
8
1
10
3
14
7
a
9
1
7
1
7
1
10
1
10
2
11
12
10
3
7
7
10
0
0
0
0
0
0
0'
6
0
6
0
6
6
6
0
2
3
3
5
5
0
5
3
3
5
5
3
3
5
5
6
5
0
6
2
1
2
3
2
5
2
1
3
2
3
5
3
4
4
10
3
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6
0
6
0
6
0
6
0
6
0
0
0
0
6
0
3
0
5
0
5
0
5
0
0
0
5
0
5
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
a
0
0
0
0
0
0
0
0
0
a
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
o
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
13.85
3.54
6.20
4.33
2.93
4.35
1.66
-6.40
2.77
-3.96
2.29
-5.78
-6.99
-5.72
3.88
-3.21
-4.57
-6.39
-3.15
-4.97
1.69
-6.33
-7.95
-16.02
-6.80
-14.86
-7.59
-14.31
-6.43
-13.15
-7.83
-15.85
1.98
-6.17
-6.55
-5.20
•13.27
-6.65
•13.75
-5.37
-12.46
-5.66
-13.68
-7.37
•15.40
-6.94
-8.09
-7.51
-15.53
-7.60
-15.62
-14.69
-54.38
-54.90
-56.84
-57.42
-57.82
-56.88
-57.00
-56.79
-53.54
-56.72
-55.60
-59.00
-63.55
-59.11
-56.56
-59.25
-61.61
-64.39
-58.71
-59.26
-59.05
-64.74
-62.92
-66.32
-61.56
-64.96
-62.18
-65.36
-59.29
-62.48
-60.11
-65.80
-55.69
-59.70
-59.21
-58.99
-62.40
-65.11
-67.80
-59.75
-62.45
-61.33
-67.02
-62.63
-68.32
-61.58
-61.46
-57.54
-63.23
-61.76
-67.45
-64.45
-54.90
-56.84
-57.42
-57.82
-56.88
-57.00
-56.79
-53.54
-56.72
-55.60
-59.00
-63.55
-59.11
-56.56
-59.25
-61.61
-64.39
-58.71
-59.26
-59.05
-64.74
-62.92
-66.32
-61.56
-64.96
-62.18
-65.36
-59.29
-62.48
-60.11
-65.80
-55.69
-59.70
-59.21
-58.99
-62.40
-65.11
-67.80
-59.75
-62.45
-61.33
-67.02
-62.63
-68.32
-61.58
-61.46
-57.54
-63.23
-61.76
-67.45
-64.45
„g/j „ 86
-56.86
-54.84
-57.42
-6*,55
-57,QQ
-58.75
-56.80
-58.69
-58.53
-60.97
-6^^2-4
-61.08
-57.06
-59.25
-66.34
-66.88
-59.26
-60.30
-66.30
-66.15
-68.60
-64.48
-66.93
-66.15
-68.05
-62.55
-6*.45
-62-07
-68.07
-60.08
-62.17
-62.47
-61.92
-66.36
-68.55
-60.25
-62.45
-62.09
-62.63
-68.63
-65.97
-66.16
-61.92
-61.42
-62.26
-68.26
-64.45
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
13
12
12
12
12
11
11
11
11
10
10
10
10
9
8
7
1
1
7
8
9
1
7
8
9
1
7
8
9
1
1
2
3
2
2
3
2
1
2
5
2
1
2
4
2
1
2
3
3
4
3
3
6
3
3
3
6
5
5
5
6
4
4
4
4
4
5
/t
5
0
0
0
6
0
0
0
6
0
0
0
6
5
5
6
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
-22.71
-16.24
-16.61
-15.27
-23.34
-15.09
-15.46
-14.11
-22.18
-15.66
-16.03
-14.68
-22.75
-22.70
-24.05
-23.68
-30.77
-70.14
-65.47
-64.98
-64.76
-68.17
-65.59
-65.10
-64.88
-68.29
-61.55
-61.06
-60.84
-64.24
-66.53
-66.75
-67.24
-69.93
-70.14
-65.47
-64.98
-64.76
-68.17
-65.59
-65.10
-64.88
-68.29
-61.55
-61.06
-60.84
-64.24
-66.53
-66.75
-67.24
-69.93
-70.45
-63.25
-68.55
-68.00
-70.44
-68.06
-68.36
-67.81
-70.25
-64.OZ
-64.32
-63.76
-66.21
-69.76
-70.32
-70.02
-72.21
Figure 38. Table of Net Benefits of Combinations of Watersheds,
Reuse Computer Run A
139
-------
**** TABLE OF NET BENEFITS FOR COMBINATIONS OF WATERSHEDS
XUMb 1 NA 1 1OTM
NUMBER
1
2-
3
4
5
6
7
—8—
9
l'\
1 1
1 2
13
-t-* —
1 5
16
17
18
1 9
-2f— •
21
22
23
24
25
- 26- -
27
28
29
30
31
-32-
3 3
34
35
36
37
38
3°
4-™-
41
42
43
44
45
46
47
48
49
5-i
51
52
53
54
•35
56
57
55
59
6'.,
61
-6-2-
63
64
65
66
67
6-fl
69
41
-4-9-
39
33
37
36
35
-6 d--
1 <"
2 •;
3 ,)
s a
13 '<
-^5 — *- — 6—
34
34
33
33
32
~tt—
3C-
3^
29
28
27
gO -
25
25
24
24
23
-23-
22
22
21
21
2i.':
8 T
1 6
9 C
2 6
3 6
— 5- — 6 —
7 2
1 2
1 3
2 3
1 5
-2 5 —
It ^
-A r3 *• a »"
'! '1 " V
,-, '1 - (V '.
3 (".' ', f ^ A A
J 0 V '/ -J
_3| £ £ f fj.
.) '.'' •" 0 )
-5 "-' ^ -3 '''
C i: -" C' v
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8 3
1 - 3
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-6 &-:— 7-~ ,• o ; 3
•Sit 3- 5— -6 -n %'— •* i
19
19
19
19
--48-
18
17
17
16
16
-IS
15
14
14
14
14
-13
13
13
13
- 12
12
1 2
12
11
1 1
1 1
1 1
- K- -
1,.
1 ',
1 •
9
fl
--7 -
1
i •+ '.>
7 «
8 2
9 - 1
1 2
-.7. - 3_.
1 2
7 5
1 2
1C1 1
1 3
-1 ^ - - 2
2 3
11 5
12 3
!/>. 4
3 4
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7 3
U- 1
1 2
- 7 - -3— -
8 2
9 1
1 2
7 5
8 2
9 1
1 2
7 A- —
8 2
9 1
1 2
1 3
2 3
- 3- -4 -
2 3
-> K i; fr --fl
j i' " C' />
C/ 1,- -r - 0 i5
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3 >•• C r j
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c- o ;'• •'> i
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5 •- ' o •;
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2 i. ., >;,
3 5 • r* 1
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3 t i V ''
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3 6 <• t 0
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5 • ^ j
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4 '.' • i
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fi r.
-<* — -0-
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- -•> -J
r J
— 1 9-
- 0---5-
i) 0
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' 9
fi — -0-
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t ^
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-
NO REUSE
Figure 39. Table of Net Benefits of Combinations of Watersheds,
No Reuse - Computer Run A
140
-------
OPTIMIZATION OF SYTEM
The discussion of sizing criteria for multiple purpose reservoirs which
appear sin Section VII included reference to various physical and aesthetic
constraints which exist at reservoir sites. Since the application of these
criteria is a function of the size of reservoir required at a given site as
well as the type of reservoir selected, application of these constraints
was not attempted within the systems analysis. In order to define the
final optimum system, however, tentative optima must be tested for the
physical and aesthetic constraints not included in the computer analysis.
Many of these factors are determined by the exercise of judgement on the
part of the engineer, the owner, and the owner's representatives, includ-
ing architects and land planners. For these reasons and others, they
are not considered susceptible to quantitative analysis. The methodused
to test for these constraints was as follows:
1. The total net benefits for a particular section of the watershed
and particular level of output were placed in order of decreasing
net benefit.
2. The highest-ranking system was defined in terms of reservoir
types, sizes, and locations.
3. Each location was examined physically and via topographic and
land use maps for feasibility of installation of the required facility.
4. If any of the reservoirs in a given combination appeared not
feasible due to insufficient usable space, appearance, impact
on the neighborhood, etc., that combination was discarded and
the next-highest ranking one selected.
5. The process was repeated until the optimum feasible system
was located for the section and output level.
6. The same method was applied to each output level and to each
watershed section until all final optimum systems were defined.
These final systems are described and discussed in Sections XII, XIII,
XIV, and XV. Information and guidance were obtained from the planning
and engineering staff of the Howard Research and Development Corporation,
developers of Columbia, with respect to land uses, aesthetic considerations,
and acceptability. The responsibility for the exercise of judgement in
qualitative matters rests, however, solely with the staff of Hittman
Associates, who performed the analysis and made the final decisions.
It is interesting to note that the system actually selected from computer
Run A, all-reuse-level option, was the thirty-first ranking system. Its
total net benefit was $20. 79 per day lower than the highest-ranking sys-
tem. Had the local reuse system been afactorinthe original planning
of this area, it is reasonable to assume that land planning would have
provided for a system of significantly higher net benefit, thus improving
the economics of the concept over those detailed in this report.
141
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SYSTEMS ANALYSIS COMPUTER PROGRAM
The method employed to analyze and optimize the local reuse system has
been described in detail in this and the previous chapter. The actual
computations and organization of data referred to in the report were per-
formed on an IBM 360-65 digital computer system, utilizing a computer
code developed as part of this study.
142
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SECTION XII
DEVELOPMENT OF CONCEPTUAL DESIGNS
The conceptual design and cost estimates of the Local Storage, Treatment,
and Reuse system were prepared for three cases. These are:
1. Local Collection, Storage, and Treatment of Storm Water
for Potable Reuse
2. Local Collection, Storage, and Treatment of Storm Water
for Sub-Potable Reuse
3. Local Collection, Storage, and Treatment of Storm Water
for Pollution Control
The system analysis and optimization studies discussed in the two pre-
vious chapters had provided a complete analysis of the various combina-
tions of components and locations based on least overall cost. The sys-
tem analysis program had not been programmed to evaluate actual site
constraints and other design conditions. Accordingly, the development
of conceptual designs and cost data involved (1) the selection of the sys-
tem configuration and locations which could be constructed and operated
in the Wilde Lake Watershed at the least overall cost, (2) the preparation
of typical designs and drawings, (3) modification of costs based on actual
site conditions, and (4) compilation of performance characteristics and
cost data.
SELECTION OF PLANT LOCATIONS AND SYSTEM CONFIGURATIONS
A typical output of the system optimization program was previously shown
as Figure 38, "Table of Net Benefits for Combinations of Watersheds, "
and was discussed in Section XI. Similar output sheets were produced
for each of the three sectors of the Wilde Lake Watershed for which con-
ceptual designs were desired. These tables provide a listing of the
various combinations of plants and the net benefit (or cost) expressed in
terms of do liars/day.
As a first step it was necessary to examine the topography, boundaries,
and other site conditions to determine the maximum size reservoir that
could be constructed at each possible location. This involved plotting
the contours of the maximum water levels associatedwith various heights
of dams, planimetering storage areas, estimating additional storage
quantities that could be obtained by excavation, etc. In this examination,
the existing land use plans and lot boundaries were considered invariant
to assure a practical design based on actual site conditions.
143
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The next step was to determine the available storage capacities of vari-
ous combinations of reservoirs and to compare the available storage
with the calculated volume for storms of one-year return interval. For
each type of local storage system, the plant combination showing the
maximum benefit or least cost was found from among the first feasible
plant combination in the case of systems for potable reuse for the south-
west section of the Wilde Lake watershed (CombinationNo.46, Figure 38).
Having selected the least cost and first feasible plant combination, the
system configuration was also identified. The computerized system anal-
ysis program automatically calculates the capital and operating costs of
alternative systems and selects the least cost configuration. Accordingly,
it was only necessary to examine the printout of the program to deter-
mine the components that had been selected by the computer program
for the various plants. In all cases, the least cost plant was based on
the use of open storage reservoirs. The system analysis program also
calculates and prints the amount of water supplied for reuse.
PREPARATION OF PLANT LOCATION AND TYPICAL DESIGN
DRAWINGS
The conceptual design drawings showing the plant locations were pre-
pared based on the selected least cost plant combinations. In the case
of the "Local Collection, Storage, and Treatment of Storm Water for
Sub-Potable Reuse, " it was also necessary to lay out a distribution sys-
tem. These drawings are contained in Sections XIII, XIV, and XV, with
discussions of the conceptual designs of the three systems.
Since this system study also involved the preparation of conceptual de-
sign drawings of the facilities for a demonstration program, the loca-
tions for the demonstration facilities were used as the basis for the
preparation of typical design drawings for storage reservoirs, pretreat-
rnent systems, and treatment plants.
MODIFICATION OF COSTS BASED ON ACTUAL SITE CONDITIONS
The typical design drawings of the facilities for the demonstration pro-
gram were also used for another purpose. Detailed unit and material
takeoffs were made and cost estimates based on the actual designs were
prepared. These cost estimates were compared to the capital costs
generated by the system analysis program. The capital cost inputs for
the system analysis program had been based on parametric cost data as
a function of reservoir capacity and treatment rates. Although all the
major components were considered, it was not possible to consider all
of the possible site conditions in this type of analysis. Based on the
comparison of the computer-generated costs and the estimates based on
the typical designs prepared for the demonstration program, the cost
data used for the storage reservoirs and pretreatment units were found
to be low. This was due to the following local conditions.
144
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1. The storage reservoirs would be "Class 3 Impoundments" in
accordance with recent regulations of the State of Maryland.
"Class 3 Impoundments" are required to have spillways de-
signed on the basis of a 100-year storm interval intensity.
This regulation requires a dam of greater height and longer
wing walls to retain the water. Approximately two feet were
added to typical embankment height,and embankment lengths
were generally doubled.
2. The reservoirs for the demonstration programs were
designed to have a minimum pool depth of two feet to pre-
clude weed growth. This, plus the "Class 3 Impoundment"
requirement, increased the amount of excavation required.
3. The demonstration plant estimate includes fencing around
the reservoirs.
4. The allowances for contingencies and engineering the dem-
onstration plant are based on "first of a kind" facilities and
the construction of a limited number of facilities.
The treatment plant designed for the demonstration program would prob-
ably not be typical of those used for the treatment of storm water in
future installations, particularly where the quantity of water is relatively
small. This plant is designed as a general purpose test plant in which it
would be possible to evaluate various treatment processes. Once treat-
ment processes are established for storm water, it is most likely that
packaged treatment plants would be designed for this purpose. These
would be automated plants designed to shutdown automatically uponfail-
ure and could be installed in either underground chambers or small
surface structures.
For purposes of estimating the capital costs of constructing local storage,
treatment,and reuse systems in the Wilde Lake Watershed, the estimated
costs for the demonstration program storage reservoirs and treatment
units were used in order to be conservative. This was done by plotting
the estimates for the three demonstration reservoirs and pretreatment
systems on the parametric cost curves used in the system analysis. New
curves of costs vs. capacity were generated,and the appropriate costs
were determined for each of the reservoirs required. Treatment plant
costs were not modified. The treatment cost data used in the system
analysis program are considered conservative, particularly since packaged
treatment plants would be used.
COMPULATION OF PERFORMANCE CHARACTERISTICS AND COST
DATA
Each of the three local collection, storage, and treatment systems
consists of a number of reservoirs, pretreatment, treatment, and dis-
tribution facilities. Accordingly, the conceptual design of the system
145
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involved the compilation of the performance data for each of the indi-
vidual installations and the evaluation of the overall effectiveness as a
means of pollution control and water supply augmentation for the entire
watershed. Similarly, the capital and operating costs of the individual
installations were compiled to evaluate the overall cost of the system,
the economic benefits of reuse, and the net costs of pollution control.
These costs were then broken down to establish the costs per acre, per
dwelling unit, and per thousand gallons of runoff. The performance and
cost data for each of the three local collection, storage, and treatment
systems is presented in the next three sections.
146
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SECTION XIII
LOCAL COLLECTION, STORAGE, AND TREATMENT
OF STORMWATER FOR POTABLE REUSE
I
The "Local Collection, Storage, and Treatment of Storm Water for Potable
Reuse" system, hereafter referred to as the "Potable Reuse System," con-
sists of a series/parallel arrangement of storage reservoirs which collect
flows from the associated sub-watersheds and treated water and overflows
from the upstream storage reservoirs. Table 33 lists the locations of the
10 reservoirs and the sub-watersheds and upstream reservoirs from which
flow is collected. The locations are shown on Figure 40.
TABLE 33. POTABLE REUSE SYSTEM
STORAGE RESERVOIR COLLECTION AREAS
Source of Flow
Location Sub-Watershed Upstream Reservoir
A 1 2 3
B 11 12 13 A
C 4 5 14 D
D 6 7 E
E 8
F 9 15 16 17 18 20 B, C
G 19
H 22
I 21 J
J 10*
*The only feasible location for a storage reservoir in sub-watershed 10
is Location J. At this location only 40 percent of the flow from the sub-
watershed is intercepted.
A pretreatment unit is installed at each of the storage reservoirs. These
pretreatment units treat the storm water to Class ' C" quality for dis-
charge to Wilde Lake or for further treatment and reuse. Treatment
plants are located at nine of the storage reservoirs. The treatment plants
take Class "C" water from the pretreatment units and treat the water to
potable quality, Class "AA. " Table 34 lists the type of storage, storage
capacity, pretreatment capacity, quality of water produced, and the final
treatment plant capacity. No treatment plant is provided at Location H
because incremental costs of treatment exceeded possible benefits from
reuse. Pretreatment units are sized for maximum flow independent of
water diverted for treatment upstream.
Each of the nine treatment plants includes pumping facilities and connections
to the public water systems. The treated storm water is supplied as a sup-
plementary source of water and is distributed through the public water dis-
tribution systems to meet all types of demands, Classes "AA" "A" "B]1 and ' C.
147
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oo
LOCAL COLLECTION, STORAGE, AND
TREATMENT OF STORM WATER FOR
POTABLE RE-USE
FWPCA Contract No 14-12-20
WILDE LAKE WATERSHED
Columbia, Md.
Figure 40. Potable Reuse System Location Plan
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CO
TABLE 3-4. LOCAL, COLLECTION, STORAGE, AND TREATMENT
OF STORM WATER FOR POTABLE REUSE - CONCEPTUAL DESIGN
Location
A
B
C
D
E
F
G
H
I
J
Storage
POND
POND
POND
POND
POND
POND
POND
POND
POND
POND
Capacity
(Gal)
1, 577, 300
2, 145, 000
1, 902, 100
1, 907, 200
625, 900
5, 101,400
923, 500
358, 400
690,000
503, 200
Pretreatrnent
Capacity
(gpd)
67, 900
231, 400
386, 100
232, 400
58, 900
1, 036, 300
85, 700
29, 800
63, 800
55, 100
Water
Quality
Produced
AA
AA
AA
AA
AA
AA
AA
C
AA
AA
Final
Treatment Capacity
(gpd)
30, 400
81, 800
76, 900
86, 800
29, 500
209, 400
38, 400
31, 900
24, 780*
Connected
Demands
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
AA, A, B, C
percent of watershed intercepted by storage facility.
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COMPONENT DESCRIPTIONS
Figure 41 is a flow diagram of a typical "Potable Reuse System"
installation. Each of the installations (except at Location H) consist of
the following:
Storage Reservoir
These reservoirs are open storage reservoirs, or ponds, formed by
the construction of concrete dams.
Pretreatment Units
The pretreatment units are installed downstream and receive flow by
gravity from the storm retention reservoirs. The pretreatment units
consist of a float-operated valve, chlorination equipment chamber, and
a tube settler. The pretreatment units have a sump for the collection
of sediment and use float valves to control flow.
Treatment Plant
The treatment plant takes flow from the pretreatment unit. The plant
consists of a mixed media filtration unit followed by an activated carbon
column for carbon absorption. Provision is made for chemical feed
prior to filtration and for chlorination prior to discharge to the treated
water storage tank. The piping is arranged for backwashing the mixed
media filter with a connection to the sanitary sewer.
Treated Water Storage
The treated water storage consists of steel tanks at grade. The treated
water storage tank also serves as the filter backwash storage tank, the
distribution pump clearwell, and the chlorine contact tank.
Pumping System
The pumping distribution system consists of a pump with associated con-
trol and a connection for supplying treated water to the public water sys-
tem.
Sewer Connection
A connection is provided to public sewer systems to receive filter back-
wash. Appropriate measures are taken to prevent any possibility of back-
flow from the sewer.
150
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Distribution System
Storage Reservoir
Chlorine
Pressure
High Service
Pump
Chemical
Feed
Overflow to Chemical
Stream
Feed
L...
Sediment
Removal
Pretreatment Unit
Treated Water
Storage
To Sanitary
Sewer
.
Media)
ravel
Acti-
vated
Carbon
-H-f
Mixed Media
Filtration
Carbon
Adsorption
Figure 41. Potable Reuse System Flow Diagram
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PERFORMANCE CHARACTERISTICS
The demands for potable water in the Wilde Lake Watershed will exceed
the amount of storm water runoff available even with the higher runoff
quantities that will result when the area is further developed. In addition,
since the treated storm water would be supplied to and distributed by the
public water system, the demands of any part of the watershed can be
supplied. This is contrasted to systems using a separate distribution
system in which the supply and demand of the individual sub-watershed
must be considered. Table 35 contains a listing of the annual potable
water requirements, the total amount of storm water runoff, and the
amount of treated water supplied to the service areas from the treatment
plants.
Since the demands of the watershed exceed the available supply, the
"Potable Reuse System" was designed to produce the maximum treated
water in order to maximize the reuse benefit. An upper limit of 90 per-
cent of the available storm runoff was used in the design of the individual
reservoirs and treatment plants, in order to assure that a portion of the
storm runoff would still go to Wilde Lake, in addition to the base flow.
As finally designed,the "Potable Reuse System" will provide 202,942,000
gallons per year of calculated annual runoff of 246, 967, 000 gallons. This
represents 82 percent of the available supply. About 29, 000, 000 gallons
of treated water would be released to Wilde Lake and an additional
15, 000, 000 gallons of storm water runoff would go directly to the lake
since the only possible location for a reservoir in sub-watershed 10 is
at Location J which can only intercept some 40 percent of the runoff from
the sub-watershed.
The pretreatment units are designe_d to treat storm water at twice the
mean annual daily runoff rate, 2 x q. In order to maximize the treated
water supply, the pretreatment units would be operated at 1. 1 to 1. 4 x q,
plus the base flow. At this rate, there is an increased probability of not
having the reservoir emptied sufficiently to receive the runoff from a
subsequent storm. In terms of_the design criteria of storms of one-year
recurrence intervals, the 2 x _q discharge rate would retain 98. 5 percent
of the runoff, whereas a 1. 13 x q discharge rate would retain 97. 3 percent
of the flow. Expressed in terms of all storms including those greater than
one-year recurrence interval, the following relationships were calculated.
Percent of Total
Condition Runoff Retained
One-year storm design no residual assumed 92. 6
One-year storm design 2 x q_discharge rate 91. 2
One-year storm design 1. 13 q discharge rate 90. 0
Accordingly, with the "Potable Reuse System" not more than 10 percent
of the runoff would normally overflow the reservoir and not receive the
full Class C treatment. However, even the overflows will be retained
in the storage reservoirs for a time period adequate to allow settlement
152
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TABLE 35. POTABLE WATER REQUIREMENTS, TOTAL STORM WATER
RUNOFF, AND AMOUNT SUPPLIED
(Amounts in Gallons/Year)
Ul
GO
Potable Water Requirement
Treatment Plant Pretreatment
Location Sub-watersheds By Sub-watershed Plant
A 1
2
3
B 11
12
13
C 4
5
14
D 6
7
E 8
F 9
15
16
17
18
20
G 19
H 22
I 21
J 10
4, 519,000
4, 551,000
1, 858, 000
15,283,000
7,234,000
11,852, 550
9,278,000
10,289,000
24,229,000
67,435,000
35,679,000
16, 542,000
22,907,000
3,927,000
15, 830,000
24, 794, 000
6,216, 000
20, 122,000
8, 399,000
7,296,000
22,601, 000
45,045,000
10,928,000
34,369,000
43,796,000
103, 114,000
16,542,000
93,796,000
8, 399,000
7,296,000
22,601,000
45,045,000
Total Storm Water Runoff
By Watersheds
3,627,000
4,694,000
4,073,000
12,378,000
7, 332,000
10, 134,000
4, 791,000
7,733,000
15, 528,000
17,653,000
14,007,000
10, 741,000
17,818,000
3,700,000
10,260,000
20, 388,000
3, 905,000
20, 351, 000
15,637,000
5,440,000
11,653,000
25, 124,000
Per
Treatment Plant
12,394,000
29, 844,000
28,052,000
31, 660,000
10,741,000
76,422,000
15,637,000
5,440,000
11, 653,000
10, 049, 000*
Amount
Supplied
11, 104, 500
26,754, 500
25,155, 800
28, 185, 300
9,621,400
68,612, 700
14,009, 900
10,453,600
9,044, 500
Totals
385,886,000 385,886,000 246,967,000 231,892,000 202,942,200
^Intercepts 40 percent of the sub-watershed.
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of the heavier participate matter. Added to the runoff that cannot be
practically intercepted, this would represent 22, 000, 000 to 40, 000, 000
gallons per year compared to the 247, 000, 000 gallons or greater amount
of runoff that would otherwise be discharged directly to the lake. Neither
estimate includes the base flow from the watershed which is not affected
by individual storms.
Most of the water reused by the "Potable Reuse System" would eventually
return to the river basin as effluent from the wastewater treatment plant.
Since this is downstream from Wilde Lake, the remaining flow of the lake
must also be considered. Water must be supplied to Wilde Lake to provide
for the evaporative losses, seepage from the lake, andthe release require-
ments placed on the Wilde Lake dam by the State of Maryland. The evapora-
tive losses of the lake were calculated to be about 22, 000, 000 gallons per
year. However, the rainfall directly into the lake represents an equal or
slightly greater amount of water. The state requirement on release is a
general requirement that precludes diversion of the water from the lake
itself.
The 29,000, 000 gallons of treated water and the 22, 000, 000 to 40, 000,000
gallons of untreated water that would be discharged to the lake did not
include the base flow from the stream. As discussed in Section II, a tem-
porary gaging station was installed and operated on al30-acre sub-water-
shed in Wilde Lake as part of this study. The measured base flow of
0.05 cfs, extrapolated to the entire 1140-acre watershed, would indicate
an annual base flow to the lake in excess of 104, 000, 000 gallons per year.
The total flow of 150 to 170 million gallons of water per year to Wilde Lake
would correspond quite closely to the natural condition of the area with a
runoff calculated at 43, 000, 000 gallons per year plus the base flow.
DESIGN AND CONSTRUCTION COSTS
The "Potable Reuse System" will consist of 10 storage reservoirs and
pretreatment units and nine storm water treatment plants, pumping sys-
tems, and connections to the public water and sewer system. The major
design and construction items are summarized in Table 36.
As discussed previously, land costs were not included on the basis that
all of the storage reservoirs and treatment plants could be locatedwithin
the floodplain which is dedicated to open space in the case of Columbia.
In order to provide conservative estimates of the design and construction
costs for the "Potable Reuse System" for comparison with other systems,
the cost estimates in Table 36 are based upon the assumptions and criteria
used in the system analysis. As noted previously in Section XII, the esti-
mated costs of the storage reservoirs and pretreatment units have been
scaled up to take full account of local conditions. It is considered that
these costs could be reduced substantially by further design work. Like-
wise, estimates for the treatment plants are based on small size conven-
tional treatment plants. It is envisioned that packaged treatment plants
154
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would be developed for future "Potable Reuse Systems, " and these costs
would also be substaintailly reduced.
OPERATION AND MAINTENANCE COSTS
The operation and maintenance costs for the "Potable Reuse System"
were estimated using the assumptions and criteria used in the system
analysis as discussed in Appendices C and D. Since a number of nearly
identical installations are involved which would be located in a relatively
small area, the parametric cost estimates were modified based on the
actual designs that had been developed. For example, checks and oper-
ations can be perfomed by personnel moving from one location to another,
Likewise, maintenance items such as the cleaning of sediment from res-
ervoirs can be accomplished by a crew handling a number of facilities at
one time. Certain of the costs (i.e., chemicals, pumping power, etc.)
can be directly related to the quantities of water processed. As dis-
cussed in Appendix D, the costs for operation and maintenance,as based
on this work being performed by extension and expansion of an existing
organization and a supporting organizational and overhead structure,has
not been included.
Table 37 contains a summary of the operation and maintenance for the
"Potable Reuse System. "
155
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TABLE 36. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM
WATER FOR POTABLE REUSE - DESIGN AND CONSTRUCTION COSTS
(Amounts in Dollars)
A
B
C
I>
E
Location
F
::-
I.
.
____
Construction
Storage Reservoirs 61,000 71,500 67,500 68,000 40,500 109,000 47,000 32,500 42,000 33,500
Pretreatment Units 8,200 9,600 10,200 9,600 8,000 11,400 8,600 7,300 8,200 7,600
Treatment Plant 35,600 41,800 41,100 42,400 35,400 78,400 36,600 35,800 35,000
Pumping and Distri-
bution
Subtotal
Total
12,400 12,320 12,600 12,800 10,200 19,400 10,800 10,600 12,100
117,200 135,200 131,400 132,800 94,100 218,200 103,000 39,800 96,600 87,600 1,155,900
Design, Inspection and
Field Engineering 17,500 20,300 19,700 20,000 14,100 32,700 15,600 6,000 14,600 13,200 173,700
Contingencies and
Escalation
Total
11,700 13,500 13,100 13,300 9,400 21,800 10,300 4,000 9,700 8,800 115,600
146,400 169,000 164,200 166,100 117,600 272,700 128,900 49,800 120,900 109,600 1,445,200
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TABLE 37. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM:
WATER FOR POTABLE REUSE - ANNUAL OPERATING AND MAINTENANCE COSTS-
Storage Reservoirs
(1) Routine Checks
(2) Trash Removal and
Nuisance Control
(3) Sediment Removal
Pretreatment Units
(1) Routine Checks
(2) Sediment Removal
(3) Chemicals
(4) Preventative Mainten-
ance and Repair
Treatment Plants
(1) Routine Inspections
(2) Water Tests
(3) Routine Operation
(4) Chemicals
(5) Preventative Mainten-
ance
(6) Electrical Energy
(7) Sewer Change
Pumping Facility
(1) Routine Checks
(2) Preventative Mainten-
ance and Repairs
(3) Electrical Energy
(4) Distribution
Miscellaneous
Totals
Labor
(man hours)
260
240
260
610
2, 920
940
1, 880
2, 350
240
700
10,436
Labor
($)
750
690
750
1, 760
8,400
3,060
5,400
7, 640
715
2, 260
120
31, 545
Material Equip. Rental*
($) ($)
250
2,400
3,250
4, 500
880
400
2,500
3, 820
1, 130
14,090
26,840 5,650
Miscellaneous
($> Totals
4,090
10, 110
35,030
3,000
810
6, 390
3, 000
14,090
6., 810 70, 845
^Include Operators
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SECTION XIV
LOCAL COLLECTION, STORAGE,AND TREATMENT OF
STORM WATER FOR SUB-POTABLE REUSE
The "Local Collection, Storage, and Treatment of Storm Water for Sub-
Potable Reuse" system, hereafter referred to as the "Sub-Potable Reuse
System, " consists of a series/parallel arrangement of storage reservoirs
which collect flows from the associated sub-watersheds and treated water
from the upstream storage reservoirs. Table 38 lists the locations of the
10 storage reservoirs and the sub-watersheds and upstream reservoirs
from which flow is collected. The locations are shown on Figure 42.
TABLE 38. SUB-POTABLE REUSE SYSTEM
STORAGE RESERVOIR COLLECTION AREAS
Source of Flows
Location
A
B
C
D
E
F
G
H
I
J
*Reservoir
Sub -water sheds
1
11
4
6
7
9
19
22
21
10
J only
2
12
5
8
15
*
3
13
14
16 17 18
intercepts 40 percent of
Upstream
Reservoirs
-
A
D
E
-
20 B, C
sub -water shed 10.
The "Sub-Potable Reuse System, " like the "Potable Reuse System, " has
pretreatment units which treat the storm water to Class "C" quality for
discharge to Wilde Lake or for further treatment. In the case of the
"Sub-Potable Reuse System" the water from the pretreatment units is
either distributed as Class "B" or Class "C" water. In the case of Class
"B" reuse, a treatment plant is installed with a pumping station which
takes the water from Class "C" to Class "B" quality. In the case of
159
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LOCAL COLLECTION, STORAGE, AND
TREATMENT OF STORM WATER FOR
SUB-POTABLE RE-USE
FWPCA Controct No.14-12-20
WILDE LAKE WATERSHED
Columbia. Md.
Figure 42. Sub-Potable Reuse System - Location flan
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Class "C" reuse, the water is taken from the pretreatment units through
a pumping station. As shown on Figure 42, a distribution system is
provided for the sub-potable water in each sub-watershed. Connections
are provided from the pumping stations to the distribution system in the
sub-watershed or combination of sub-watersheds from which runoff is
collected.
Table 39 lists the type of storage/ storage capacity, pretreatment
capacity, water quality produced, final treatment quality, and the con-
nected demands for each of the reservoirs and treatment plants used
with the "Sub-Potable Reuse System."
Comparing Table 33 and Table 38 it is notedthat the "Potable Reuse System"
and the "Sub-Potable Reuse System" differ with respect to the collection
areas of reservoirs D and E. As shown on Figure 38,the optimal feasible
case for the "Potable Reuse System" is the Combination Number 46
(Cases 14, 11, and 5),whereas the optimal sub-potable combination is
number 48 (Case 14, 10, and 4) for Class "C" water.
COMPONENT DESCRIPTIONS
The components comprising the "Sub-Potable Reuse System" installations
are similar to those of the "Potable Reuse System" described in
Section XIII. The two types of systems differ only with respect to the
treatment plants and the sub-potable distribution system.
The "Sub-Potable Reuse System" has two types of treatment facilities.
Figure 43 shows a flow diagram for a Class "B" treatment plant of the
type used at Locations A, B, C, and F. For this type plant, water from
the pretreatment unit goes through a microstrainer and is pumped to
a treated water storage tank which also serves as a chlorine contact tank.
A high pressure service pump supplies the sub-potable distribution
system. A connection from the public water supply system provides
water when the local treatment plant is not operating.
Figure 44 is a flow diagram for a Class "C" treatment plant of the type
used at Locations D, E, G, H, I, and J. With this type plant, flow from
the pretreatment unit goes directly to a treated water storage tank which
is also a chlorine contact tank and is then pumped to the sub-potable
distribution system. A connection from the public water supply is also
provided to provide water when the plant is not operating.
The sub-potable distribution system consists of underground pipe lines as
shown on Figure 42 and the separate piping systems withinthe houses, town
houses, apartments, schools, and commercial buildings. The design
is based on the construction of the sub-potable distribution system follow-
ing installation of other utilities. In actual practice, such a system might
be placed at the same time as the public water supply and sewer system.
This would appreciably reduce the cost of the sub-potable system.
161
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TABLE 39. LOCAL COLLECTION, STORAGE, AND TREATMENT OF
STORM WATER FOR SUB-POTABLE REUSE - CONCEPTUAL DESIGN
Type of
Location Storage
A
B
C
D
E
F
G
H
I
J
POND
POND
POND
POND
POND
POND
POND
POND
POND
POND
Stora
ge
Capacity
(Gal)
1, 577,
2, 145,
1,902,
803,
1, 729,
5, 101,
923,
358,
690,
503,
300
000
100
000
900
400
500
400
000
000*
Pretreatment
Capacity
(gpd)
67,
231,
386,
232,
155,
1, 036,
85,
29,
63,
55,
900
400
100
400
600
300
700
800
800
100
Water
Quality
Produced
B
B
B
C
C
B
C
C
C
C
Final
Treatment Capacity
(gpd)
23,
54,
65,
33,
77,
160,
4,
5,
20,
20,
500
000
400
600
800
200
300
400
100
700
Connected
Demands
B,
B,
B,
C
C
B,
C
C
C
C
C
C
C
C
#40 percent of watershed intercepted by storage facility.
-------
O5
CO
Auxilllary Source
(Public Supply)
Backflow
Preventer
Valve
Distribution
System "*"
Pressure
High Service
Pump
Chemical
Feed
Overflow
System
Treated Water
Storage
Chemical
Feed
Chlorine
Storage Reservoir
"[Sediment
I Removal
Pretreatment Unit
rf—M-
Microstrainer>
Figure 43. Sub-Potable Reuse System Flow Diagram -
Class "B" Treatment
-------
Backflow
Preventer
Valve
Distribution System
05
Auxiliary Source
(Public Supply)
Pressure
High Service
Pump
Chemical
Feed
Treated Water
Storage
Overflow to
Stream
Chlorine
Storage Reservoir
Sediment Removal
Pretreatment Unit
Figure 44. Sub-Potable Reuse System Flow Diagram -
Class "C" Treatment
-------
PERFORMANCE CHARACTERISTICS
In the case of the "Sub-Potable Reuse System, " the water is distributed
for use to the sub-watersheds from which it is collected. Unlike the
"Potable Reuse System, " the "Sub-Potable Reuse System" must consider
the available supply of water and the demands for sub-potable water on
a sub-water shed by sub-watershed basis. Table 40 contains a listing
of the sub-potable water requirements by class of water to be distributed
for each sub-water shed and treatment plant,the total storm water runoff
available from each sub-watershed at each treatment plant, and the amount
of water supplied.
The amount of water supplied for reuse in the Wilde Lake watershed
would be 167, 849, 000 gallons per year. This represents 46. 5 percent
of the total water requirements of the area.
The pretreatment units would be operated in the same manner as
previously described for the "Potable Reuse System. " The probability
of spills and quantities of water overflowing the reservoirs or going
directly to Wilde Lake without the full Class "C" treatment would be
essentially the same for the "Sub-Potable Reuse System. "
With respect to maintaining adequate flow to Wilde Lake, the "Sub-
Potable Reuse System" would normally discharge about 70, 000, 000 gal-
lons of treated water to the lake. This would be in addition to the base
flow, possible overflows, and the 15, 000, 000 gallons from sub-watershed
10 that cannot be intercepted in a practical manner. Accordingly, more
than adequate flow will be maintained to the lake.
The 167, 849, 000 gallons of water supplied to the area represents an
annual benefit of $75, 532 at the current distribution price of 45 cents
per thousand gallons.
DESIGN AND CONSTRUCTION COSTS
The major construction items for the "Sub-Potable Reuse System" will
be the same as those for the "Potable Reuse System" in Section XIII
with the following exceptions:
1. As discussed above, the treatment plants will have fewer
components
2. The sub-potable distribution system, composed of:
a. Underground piping
b. Dual systems within buildings
Table 41 contains a summary of the estimated design and construction
costs for the "Sub-Potable Reuse System. " As shown, the cost of the
165
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TABLE 40. SUB-POTABLE WATER REQUIREMENTS, TOTAL
STORM WATER RUNOFF, AND AMOUNT SUPPLIED
(Amounts in Gallons/Year)
Sub-Potable Water Requirement
Treatment
Plant Location
A
B
C
E
D
F
G
H
I
J
Totals
Sub-
Watersheds
1
2
3
11
12
13
4
5
14
6
8
7
9
15
16
17
18
20
19
22
21
10
Class
B, C
B, C
B,C
C
C
B, C
C
C
C
C
by Sub-watersheds
Quantities
3, 369,000
3,401,000
1,420,000
8,658,000
4,278,000
6, 924,000
5,212,000
4,923,000
13, 724,000
23, 327,000
5,497,000
12, 238, 000
17, 166,000
2,230,000
10,957,000
12,038,000
3,697,000
12,363,000
1, 540,000
1, 964,000
7,315,000
12, 687,000
174, 928, 000
Per
Treatment
Plant
8, 190,000
19, 860,000
23, 859,000
28, 824,000
12, 238,000
58,451,000
1, 540,000
1,964,000
7, 315,000
12, 687, 000
174, 928,000
Total Storm
by
Watershed
3,627,000
4,694,000
4,073,000
12, 378,000
7, 332,000
10, 134,000
4, 791,000
7, 733,000
15, 528,000
17, 653,000
10, 741, 000
14.007, 000
17, 818,000
3,700,000
10,260,000
20, 388,000
3, 905,000
20, 351,000
15, 637, 000
5,440,000
11, 653,000
25, 124,000
246, 967,000
Water Runoff
per
Treatment
Plant
12, 394,000
29, 844,000
28,052,000
28, 394, 000
14, 007,000
76, 422,000
15,637,000
5,440,000
11, 653, 000
10, 049, 000*
231, 892, 000
Amount
Supplied
8, 190,000
19, 695, 000
23,856, 000
25, 555,000
12, 238, 000
58,451,000
1, 540,000
1, 964,000
7, 315,000
9, 045,000
167, 849. 000
*Based on intercepting 40 percent of sub-watershed
-------
03
TABLE 41. LOCAL COLLECTION, STORAGE, AND TREATMENT
OF STORM WATER FOR SUB-POTABLE REUSE
DESIGN AND CONSTRUCTION COSTS
(Amounts in Dollars)
Construction
Storage Reservoirs
Pr e - Tr eatment
Treatment Plants
Pumping System and
Connection
Subtotal
Distribution
Subtotal
Location
ABCDEFGHIJ
61,000 71,500 67,500 45,000 65,000 109,000 47,000 32,500 42,000 33,500
8,200 9,600 10,200 9,600 9,200 11,400 8,600 7,300 8,200 7,600
15,000 15,900 16,200 18,700
12,700 11,200 13,300 5,700 5,800 27,700 5,500 5,400 5,900 7,800
96,900 106,200 107,200 60,300 80,000 166,800 61,100 45,200 56,100 48,900 828,700
37,400 134,800 143,400 230,000 100,400 380,000 43,600 25,200 57,900 97,100
134,300 241,000 250,600 290,300 180,400 546,800 104,700 71,400 114,000 146,000 2,074,200
Design, Inspection and
Field Engineering 21,100 36,200 37,700 43,500 27,600 81,100 15,800 10,800 17,100 21,900
Contingencies and
Escalation 13,400 24,100 25,100 29,000 18,400 54,700 10,500 7,200 11,400 14,600
Total
168,800 301,300 312,800 362,800 225,400 682,600 136,000 87,900 142,500 182,500 2,598,600
-------
sub-potable distribution system represents a major portion of the cost
of the overall system. The design of the sub-potable system is based
on a system of comparable design to public water systems and assumes
that it is installed as a separate entity. As previously noted, in the case
of new developments, this system would probably be installed concurrently
with the public water and sewer system. In other cases where there is
a large irrigation or industrial demand for sub-potable water,a simpler
or less expensive distribution system could be used. Table 42 illus-
trates the limits and effect of the cost of the distribution system.
OPERATION AND MAINTENANCE COSTS
Table 43 contains a summary of the operation and maintenance costs
for the "Sub-Potable Reuse System. "
The operation and maintenance costs for the "Sub-Potable System" were
estimated on the same basis as the "Potable Reuse System" with treat-
ment plant operating costs being lower and an allowance made for the
maintenance of the distribution system.
TABLE 42. EFFECT OF DISTRIBUTION SYSTEM ON CAPITAL COSTS
System without Distribution Costs
Construction Costs without Distribution $ 828, 700.00
Design, Contingencies, and Escalation (25%) 207,200.00
Subtotal without Distribution 1, 035, 900. 00
Equivalent Cost per Year 54, 220.00
Equivalent Cost per Day 148. 54
System with Distribution Costs
Construction Cost with Distribution 2, 074, 200. 00
Design, Contingencies, and Escalation 524,400.00
Total with Distribution 2, 598, 600. 00
Equivalent Cost per Year 136,013.00
Equivalent Cost per Day ' 372. 64
Value of Sub-Potable Water per Year 75, 532. 00
Value of Sub-Potable Water per Day 206. 94
168
-------
en
CD
TABLE 43. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM WATER
FOR SUB-POTABLE REUSE - ANNUAL OPERATION AND MAINTENANCE COSTS
Storage Reservoirs
(1) Routine Checks
(2) Trash Removal and
Nuisance Control
(3) Sediment Removal
Pretreatment Units
(1) Routine Checks
(2) Sediment Removal
(3) Chemicals
(4) Preventative Mainten-
ance and Repair
Treatment Plants
(1) Routine Inspection
(2) Water Testing
(3) Routine Operations
(4) Chemicals
(5) Preventative Mainten-
ance and Repair
(6) Electrical Energy
(7) Sewer Charge
Pumping Facilities
(1) Routine Checks
(2) Preventative Mainten-
ance and Repair
(3) Electrical Energy
(4) Connections
Distribution System
(1) Maintenance and
Repair
Miscellaneous
Total
Labor
(man hour)
260
240
260
520
1,460
1,040
1,454
406
312
394
36
1,054
69, 361
Labor
($)
750
690
750
1,730
4,200
3,060
4,200
1,320
900
1,280
120
3, 750
22,750
Materials Equipment Utilities
($) Rental ($) ($)
250
2, 400
1
3,250
4,500
850
1
3,000
660
1, 680
500
640
2, 400
3, 750
8,670
22, 320 5, 650 4, 580
Totals
10,050
18, 520
5,340
7, 500
8, 670
55, 300
-------
SECTION XV
LOCAL COLLECTION, STORAGE, AND TREATMENT
OF STORM WATER FOR POLLUTION CONTROL
The "Local Collection, Storage, and Treatment of Storm Water for
Pollution Control" system, hereafter referred to as the "Local Pollution
Control System," consists of a series/parallel arrangement of storage
reservoirs which collect flows from the associated watershed and treated
water and overflows from upstream reservoirs. Each storage reservoir
is connected to a pretreatment unit and the water collected by the reser-
voir is treated to Class "C" quality and discharged into the stream beds
and ultimately to Wilde Lake. Figure 45 shows the locations of the
reservoirs and the pretreatment units. Table 44 lists the annual run-
off from each sub-watershed and the amount annually collected by each
reservoir. Table 44 has been arranged to illustrate the interaction of
the individual installations and the amount of flow coming from upstream
reservoirs, the total flow treated annually and average per day, and
the sizing of the pretreatment units and the reservoirs.
Unlike the "Potable Reuse System" and the "Sub-Potable Reuse System"
described previously, the "Local Pollution Control System" makes no
provision for the treatment of water beyond Class "C" quality or for
distributing the water. The conceptual design for the "Local Pollution
Control System" is intended to provide a basis for comparison with
other pollution control systems and a reference for evaluating the
benefits of reuse. The "Local Pollution Control System" has been opti-
mized and conceptually designed as a system that could be applied for
pollution control independent of reuse consideration.
COMPONENT DESCRIPTIONS
The "Local Pollution Control System" installations consist of two com-
ponents, a storage reservoir and a pretreatment unit. These com-
ponents are essentially identical in design to the storage reservoirs and
pretreatment units previously described for the "Potable Reuse System"
and the "Sub-Potable Reuse System. " The criteria used in sizing the
reservoirs and pretreatment units for the "Local Pollution Control Sys-
tem" are slightly different since none of the water is diverted for reuse.
With the "Local Pollution Control System," flow leaving the pretreatment
unit after receiving treatment to Class "C" quality is discharged directly
to the stream beds.
PERFORMANCE CHARACTERISTICS
Since the sole purpose of the system is pollution control, the pretreat-
ment units would be operated at a higher rate than the two reuse sys-
tems previously discussed. In the case of reuse systems, pretreatment
171
-------
CO
LOCAL COLLECTION, STORAGE, AND
TREATMENT OF STORM WATER FOR
WATER POLLUTION CONTROL
FWPCA Contract No.M-12-20
WILDE LAKE WATERSHED
Columbia, Md.
3?-igUr-e 45. Loca-l Pollution Control System - Location Flan
-------
TABLE 44. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM WATER
FOR POLLUTION CONTROL RUNOFF BY SUB-WATERSHED AND PRETREATMENT UNIT
WITH PRETREATMENT AND RESERVOIR CAPACITIES
Reservoir/
Pretreatrm
Location
A
B
E
D
c
F
G
H
I
J
Sub -Water shed
2nt R unof ±
No. (gpy)
1
2
3
11
12
13
8
6
7
4
5
14
9
15
16
17
18
20
19
22
21
10
3,627,000
4,694,000
4,073,000
12,378,000
7,332,000
10, 134,000
10, 741,000
17,653,000
14,007,000
4, 791,000
7,733,000
15, 528,000
17, 818,000
3,700,000
10,260,000
20,388,000
3, 905,000
20, 351,000
15, 637,000
5,440,000
11,653,000
25, 124,000
Total Runoff Flow from
Per Reservoir Upstream Reservoir Total Flow Treated
(gpy) No. (gpy) (gpy) (gpd)
12, 394,
29,844,
10,741,
31,660,
28,052,
76,422,
15,637,
5, 440,
11,653,
10,049,
000
000 A
000
000 E
000 D
B
000 C
000
000
000
000*
12,
12,394,000 42,
10,
10,741,000 42,
42,401,000 70,
42, 238,000
70,454,000 189,
15,
5,
11,
10,
394,000
238,000
741,000
401,000
454,000
113,000
637, 000
440, 000
653,000
049, 000*
34, 000
116, 000
28, 500
116, 200
193, 000
518, 000
43,000
14, 900
32,000
26,000
Pretreatment Reservoir
Capacity Size
(gpd) (gals)
68,
232,
59,
232,
386,
1,036,
86,
29,
64,
53,
000
000
000
400
000
000
000
800
000
200
1,577,
2,145,
1, 729,
803,
1,902,
5,101,
923,
358,
690,
375,
30fr
000
000
000
000
000
500
400
000
000=1
246, 967,000
''Based on intercepting 40 percent of the watershed
-------
flow rates would be regulated to maximize the water available for reuse.
The "Local Pollution Control System" would be operated at a rate of
twice the mean annual daily flow rate to increase the probability that
the reservoirs will be able to collect the majority, if not all, of sub-
sequent storms. As discussed in Section XIII, with the pretreatment
unit operating at twice the mean annual runoff plus base flow, 98. 5 per-
cent of all one-year storms will be stored. At this rate 91. 2 percent
of all storms, including those with return intervals greater than one
year, will be retained. '
With the "Local Pollution Control System" only that water from sub-
watershed number 10 will enter Wilde Lake untreated. This amounts
to 15, 075, 000 per year and of the remaining 231, 892, 000 gallons of
annual runoff, only 3, 478, 000 gallons will not receive the full Class "C"
treatment for any normal year and only 20, 407, 000 gallons for storms
of greater return intervals. All water except that from sub-watershed
10 will be retained in the storage reservoirs for a sufficient interval to
permit the heavy particles to settle out. Expressed differently, if the
water not receiving full treatment were to have a suspended solids con-
centration of 200 ppm, the resultant overall suspended solids in the lake
would be 43 ppm when diluted with the treated effluent having 30 ppm
suspended solids, assuming no sedimentation in Wilde Lake itself.
DESIGN AND CONSTRUCTION COSTS
The construction items for the "Local Pollution Control System" would
be the same as the two reuse systems previously discussed with respect
to the storage reservoirs and pretreatment units. Table 45 contains a
summary of the estimated design and construction costs for the "Local
Pollution Control System. "
OPERATION AND MAINTENANCE COSTS
The operation and maintenance for the "Local Pollution Control System"
would involve the same items of work as the storage reservoirs and
pretreatment units for the two reuse systems previously discussed.
Table 46 contains a summary of the estimated costs for the "Local
Pollution Control System. "
174
-------
TABLE 45. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM WATER
FOR WATER POLLUTION CONTROL - DESIGN AND CONSTRUCTION COSTS
Location
A_ B_ _?_ IL JL JL _G_ JL ! I Total
Construction
Storage Reservoirs 61,000 71,500 67,500 45,000 65,000 109,000 47,000 32,500 42,000 33,500
Pretreatment Units 8,200 9,600 10,200 9,600 9,200 11,400 8,600 7,300 8,200 7,600
Subtotal 69,200 81,100 77,700 54,600 74,200 120,400 55,600 39,800 50,200 41,100 663,900
Design, Inspection and
Field Engineering 10,400 12,100 11,700 8,200 11,100 18,000 8,400 6,000 7,500 6,100 99,500
Contingencies and
Escalation 6,900 8,100 7,800 5,500 7,400 12,000 5,600 4,000 5,000 4,100 66,400
Total 86,500 101,300 97,200 68,300 92,700 150,400 69,600 49,800 62,700 57,300 829,800
-------
TABLE 46. LOCAL COLLECTION, STORAGE, AND TREATMENT OF STORM WATER FOR
WATER POLLUTION CONTROL - ANNUAL OPERATION AND MAINTENANCE COSTS
Storage Reservoirs
(1) Routine Checks
(2) Trash Removal and
Nuisance Control
(3) Sediment Removal
Labor
(man hrs. )
260
240
Labor
($)
750
690
Material
($)
Equipment
Rental ($)
Utilities
($)
Totals
2,400
Pretreatment Units
(1) Routine Checks
(2) Sediment Removal
(3) Chemicals
(4) Preventative Mainten-
ance and Repair
(5) Electrical Energy
260
520
750
1,730
4,500
1,730
3,250
200
Miscellaneous
3,600
Subtotals
1,280
4, 320
9,830
5,650
200
$20,000
-------
SECTION XVI
CONVENTIONAL STORM WATER TREATMENT SYSTEM
In order to evaluate the systems for "Local Storage, Treatment, and
Reuse of Storm Water" it was necessary to develop a base for compar-
ison. As a parallel effort, a conceptual design was prepared for a
system to control the pollution of Wilde Lake using conventional design
methods. This work was performed by Whitman, Requardt and Asso-
ciates of Baltimore, Maryland. A summary description of the Con-
ventional Storm Water Treatment System, hereafter referred to as the
"Conventional Treatment System, " is contained in this chapter.
DESIGN REQUIREMENTS
The design of the Conventional Treatment System was based on the same
area, runoff quantities, flow hydrographs, and same quality of water
requirements as the Local Storage, Treatment, and Reuse System.
Table 47 contains a summary of the design requirements.
TABLE 47. DESIGN REQUIREMENTS FOR
CONVENTIONAL TREATMENT PLANT
Area Considered: Wilde Lake Watershed
Treated Water Quality: State of Maryland Water Quality
Standards for "Water Contact
Recreation" modified as discussed
in Section V under Class "C" water
quality
Water Quantity: Runoff quantities as calculated
using the synthesized hydrograph
method with storms of one-year
return interval as the basis for
design
SYSTEM DESCRIPTION
The Conventional Treatment System consists, in part, of a central
treatment basin located at the head of Wilde Lake and the necessary
facilities to collect and pump the runoff water to the treatment basin.
As shown on Figure 46, the Conventional Treatment System is divided
by natural boundaries into two parts. "Area A" consists of 954 acres
of the watershed from which water flows by gravity to the treatment basin.
177
-------
co
FWPCA Contract No.U-12-20
WILDE LAKE WATERSHED
Columbia, Mi
WHITMAN. REOUARDT-a ASSOCIATES
' PLAN OF
CONVENTIONAL SYSTEM
SCALE. l'."400'
Figure 46. Plan of Conventional Treatment System
-------
"Area B" consists of an area of 165 acres from which runoff must be
collected and pumped to the treatment basin. The components comprising
the Conventional .Treatment System, are as follows:
1. Holding Pond. This pond is located at the east side of
AreaB and has a capacity of 1, 840, 000 gallons. The pond
receives the runoff from the major portions of sub-water-
sheds 10, 20, and 21 from a gravity interceptor on the west
side of the pond. The holding pond provides storage capacity
in order to avoid pumping runoff water at storm rates.
2. Pumping Station No. 1. This station is located on the south
side of Wilde Lake and pumps the water from the holding
pond to Pumping Station No. 2 for transfer to the treatment
basin. This station also receives flow from gravity inter-
ceptors which collect runoff from the watershed adjacent to
the south side of Wilde Lake. This station has a pumping
capacity of 2.5 MGD.
3. Gravity Interceptors. The northern-most gravity interceptor
collects the runoff from the majority of sub-watersheds 9,
16, and 19, and is located to permit this water to flow by
gravity directly to the treatment basin. The gravity interceptor
located adjacent to the north side of Wilde Lake collects the
runoff from the lower portions of the sub-watersheds, which
flows by gravity to the wet well of Pumping Station No. 2.
A gravity interceptor located at the west of the head of
Wilde Lake intercepts the majority of the flow from sub-water^
shed 17 in order for the water to flow by gravity to the treatn-
ment basin. A second gravity interceptor located adjacent to,
the southwest side of Wilde Lake collects the runoff from the
lower portion of the watershed which flows by gravity to
Pumping Station No. 2.
4. Pumping Station No. 2. This station receives runoff from
the two gravity interceptors located adjacent to the north and
southwest side of Wilde Lake. This station also transfers
the waste from the force main from Pumping Station No. 1
to the treatment basin. Pumping Station No. 2 uses dual
5 MGD pumps for a maximum capacity of 10 MGD.
5. Treatment Basin. The treatment basin is located at the head
of Wilde Lake as shown on Figure 46. Figure 47 is a more
detailed drawing of the treatment basin. The treatment basin
consists of an earth and concrete dam and an excavated
basin sized to retain a one-year storm. The treatment basin
provides retention of the storm water runoff to allow sedi-
mentation of suspended solids and incorporates a rock dam
and gravel filter for filtration of water. Chlorination
equipment is provided to disinfect the effluent and is designed
179
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CO
o
PROFILE
TREATMENT BASIN
Figure 47. Conventional System Treatment Basin
-------
to provide variable dosages to meet the range of overflow
rates. The normal pool elevation is maintained by stop
logs in the outlet structure and the pool may be drained for
silt removal and other purposes by removal of the logs.
The principal features of the treatment basin are as follows:
Dam height
Spillway length
Spillway overflow
elevation
One-year storm maximum
water level
Normal pool storage
Maximum storm storage
High water level
High water volume
Area at high water level
Maximum water level for
dam design
15 feet from existing stream
bed to top of dam elevation 349. 0
60 feet
344. 5 feet
344.0 feet
2. 6 MG
12.4 MG from normal pool
to spillway overflow
346.0 (10-year storm)
16.6 MG from normal pool
to high water level
424, 000 square feet (9. 75 acres)
346. 8 (100-year storm)
The outlet from the treatment basin is a 24-inch conduit
which will drain the basin in 19 hours at an average flow
of 20 MGD varying from 27. 5 MGD at the high water level
elevation to 0. 5 MGD at the low pool elevation.
PERFORMANCE CHARACTERISTICS
The Conventional Treatment System is designed to collect and transfer
storm water runoff to the treatment basin at the runoff rates and quan-
tities of a one-year storm. The system is designed to drain the holding
pond and the treatment basin to the minimum levels within a 24-hour
period in order to minimize the probability that these storage basins
will not be sufficiently empty to receive subsequent storms. The treat-
ment basin is designed to meet the State of Maryland requirements for
a Class 3 impoundment and the dam and spillway design is based on a
100-year storm. However, at storms of greater than one-year return
interval, the capacity of gravity interceptors and the pumping stations
may be exceeded and a portion of the runoff will overflow directly to
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Wilde Lake. All of the runoff flowing directly to the treatment basin
would be retained for some period.
The treatment provided the storm water with the Conventional Treat-
ment System consists of sedimentation, gravel filtration, and chlori-
natioru The removal of suspended solids and resulting reduction of
turbidity in storm water by natural sedimentation is a function of the
water surface area and overflow rate of the reservoir. Table 48
contains a listing of the size particles that would be removed with the
treatment basin used for the Conventional Treatment System.
TABLE 48. PARTICLE REMOVAL WITH
CONVENTIONAL TREATMENT BASIN
Water Size of Particles
Water Effluent Surface Area Overflow Rate Removed
Elevation Q(MGD) (square ft) (gal/day/sf) (Microns)
337 0 10.0 181, 000 55 7. 5
33G.O 14.0 186,000 75 8.8
340.0 19.5 196,000 100 10.1
342.0 24.0 219,000 110 10.6
344.5 28.0 320,000 89 9.5
346.0 278.0 424,000 660 26.0
Table 48 shows that the treatment basin will remove silt particles
(approximately 10 microns) for overflow rates up to the spillway (ele-
vation 344. 5 feet) which includes the one-year storm (elevation 344. 0
feet). However, it is considered that Class "C" water will require a
greater reduction of turbidity and settlement of particles smaller than
10 microns.
In order to provide for treatment to a quality comparable to Class "C, "
the treatment basin incorporates a rock dam backed by a gravel filter-
through which storms of less than 5, 000, 000 gallons of runoff would
pass, as would at least this amount of runoff from storms greater than
5, 000, 000 gallons. Storms of 5, 000, 000 gallons magnitude are exceeded
six percent of the time. In terms of quantity of runoff, the 5, 000, 000
gallon filtration capability would represent about 60 percent of the total.
A storm of 18, 600, 000 gallons per day on the entire drainage will utilize
12, 400, 000 of the storage capacity of the basin without overtopping the
spillway. For the five years of record, storms of 18, 600, 000 gallons
per day were exceeded 0. 25 percent of the time, which represents two
to three percent of the total flow.
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DESIGN AND CONSTRUCTION COSTS
As part of the conceptual design of the Conventional Treatment System,
an estimate of the design and construction costs Was prepared. Table 49
contains a summary of this estimate.
TABLE 49. DESIGN AND CONSTRUCTION COSTS -
CONVENTIONAL TREATMENT SYSTEM
Construction Costs
Holding Pond $ 24, 000
Pumping Station No. 1 i 17, 000
Force Main 30, 000
Interceptor System 205, 000
Pumping Station No. 2 300, 000
Treatment Basin 263,000
Subtotal $ 939,000
Design, Field Engineering,& Inspection 187,800
Contingencies and Escalation 187, 800
TOTAL $1,314,600
The $1, 314, 600 construction cost for the Conventional Treatment System
translates to a daily cost of $198.50 for the system using the same
amortization rate previously used with the Local Storage, Treatment, and
Reuse System.
OPERATION AND MAINTENANCE COSTS
The operating and maintenance costs for the Conventional.Treatment
System were estimated at $25, 000 per year which translates to $68. 49
per day.
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SECTION XVII
ECONOMIC COMPARISON
Table 50 is an economic comparison of the four systems presented in
Sections XIII, XIV, XV, and XVI. To more thoroughly analyze these
results, a benefit/coat evaluation was made of the various system alter-
natives.
POTABLE REUSE
Capital costs associated with the optimum potable reuse system presented
in Section XIII were $1,445,200. Whenthese costs are amortized over the
life of the facilities, capital costs are found to be equivalent to uniform
costs equaling $207.21 per day. Operating and maintenance costs are
estimated at $194. 10 per day. The sum of capital, operating, and main-
tenance costs is the total system cost and can be found to equal $401. 31
per day over the estimated life of the facilities. Offsetting these costs
are tangible benefits from two sources. The treated water which is pro-
duced for reuse has been evaluated at $0. 45 per thousand gallons and
totals $278. 00 per day in the case of potable reuse. The total system
cost of the conventional storm water treatment system was computed as
$256. 99 per day (based on capital costs of $1, 314, 500 and annual operating
and maintenance costs of $25, 000). Taking the imputed value of water
pollution control as the total system cost of the best available alternative,
total benefits can be computed as the sum of the value of reused water
and the cost of conventional treatment, or $534. 99 per day. These com-
putations are summarized as Case 1 on Table 51.
This computation of benefits and costs can be used to evaluate a benefit/
cost ratio of 1. 26 or a daily net benefit of $105. 88 depending on the pur-
pose of the analysis. Several other factors bearing on this analysis
should be considered in the interest of a comprehensive evaluation. The
benefit/cost summary just outlined is appropriate to a water resource
project having a variety of purposes and potential benefits. Two of the
principal benefits, water supply and water pollution control, have been
quantified and compared to total system costs. Other intangible benefits
are discussed in later sections of this chapter and must be considered as
part of the evaluation. The costs and benefits presented above can be
organized somewhat differently in the case of projects intended principally
as water supply facilities or as water pollution controlfacilities. Table 51
includes analyses of this type and lists the cost of potable water after full
credit is taken for the water pollution control aspect of the system, as
well as the cost of water pollution control after full credit is taken for
the value of the potable water. Both of these costs are shown in units
which permit ready comparison to other cost indices.
Critical to the calculation of benefit/cost ratios is the method used to
evaluate benefits. The benefits noted above and on Table 51 are computed
consistent with the assumptions described in other sections of this report.
The sensitivity of the results to these assumptions can be investigated
185
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CO
en
TABLE 50. ECONOMIC COMPARISON OF POLLUTION CONTROL SYSTEMS
Amortized Average Value of
Capital Capital O&M Reuse Net Cost of System _
Costs Costs Costs Water - ^ -
($) ($/day) ($/day) ($/day) ($/day) ($/acre-year) ($/DU-year)
Local Pollution
Control System 829,800 118.99 54.79 173.78 55.25 16.88
Local Potable
Reuse System 1,445,200 207.21 194.10 250.20 151.11 48.05 14.68
Local Sub -Potable
Reuse System 2,598,000 151.51 372.64 206.94 317.21 100.85 30.82
Conventional
Treatment System 1,314,600 188.50 68.49 256.99 81.71 24.96
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TABLE 51. BENEFIT/COST ANALYSIS OF WATER SUPPLY
AND PQ-LLUTIQN CONTROL ALTERNATIVES
Case 1 Case 2 Case 3 Case 4
Costs
Capital Cost of System $ 1,445,200.00 $ 1,445,200.00 $ 2,598,600.00 $ 2,598,600.00
Amortized Capital Cost ($/Day) 207.21 207.21 372.64 372.64
Operating and Maintenance Cost ($/Year) 70,845.00 70,845.00 55,300.00 55,300.00
Operating and Maintenance Cost ($/Day) 194.10 194.10 151.51 151.51
Total System Cost ($/Day) 401.31 401.31 524.15 524.15
Benefits
Water Reuse
Quantity of Reused Water (Gallons/Year) 202,942,000.00 202,942,000.00 167,849,000.00 167,849,000.00
Value of Reused Water ($/Year@$0. 45/1000 Gallons) 91,324.00 91,324.00 75,532.00 75,532.00
Value of Reused Water ($/Day) 250.20 250.20 206.94 206.94
Water Pollution Control
(-JQ Capital Cost of Conventional System 1,314,600.00 829,800.00 1,314,600.00 829,800.00
-j Amortized Capital Cost ($/Day) 188.50 118.99 188.50 118.99
Operating and Maintenance Cost ($/Year) 25,000.00 20,000.00 25,000.00 20,000.00
Operating and Maintenance Cost ($/Day) 68.49 54.79 68.49 54.79
Total System Cost ($/Day) 256.99 173.78 256.99 173.78
Total Benefits ($/Day) 507.19 423.98 463.93 380.72
Multipurpose Water Resource Project
Net Benefits ($/Day) 105.88 22.67 -60.22 -143.43
Benefit/Cost Ratio 1.26 1.06 0.89 0.73
Water Supply Project
Total Cost Less Water Pollution Benefit ($/Year) 52,680.00 83,050.00 97,510.00 127 890.00
Net Cost of Treated Water ($/1000 Gallons) 0.26 0.41 0.58 ' 0^76
Water Pollution Control Project
Total Cost Less Water Supply Benefit ($/Year) 55,160.00 55,160.00 -115,780.00 115 780.00
Net Cost of Pollution Control ($/Acre-Year) 48.05 48.05 100.85 ' lOo! 85
($/DU*-Year) 14.68 14.68 30.82 3o! 82
*DU = Dwelling Unit
Case 1. Potable Reuse System vs. Conventional Storm Water Treatment
Case 2. Potable-Reuse System vs. Local Storage Without Reuse
Case 3. S-ub-Potable Reuse System vs. Conventional Storm Water Treatment
Case 4. Sub-Potable Reuse System vs. Local Storage Without Reuse
-------
rather easily. The value of reused water, for example, has been taken at
$0. 45 per thousand gallons. Should this water be purchased by the local
water utility and distributed and sold by them, the value of the treated
water should conform to the cost of the water utility of an alternative
supply. In the case of the Howard County Metropolitan Commission, this
would be the incremental cost of water purchased from Baltimore City as
well as other incremental costs associated with pumping, storing, and
conveying that water to the Wilde Lake watershed. An exact determination-
of this cost would require a thorough study of the financial structure of the
Metropolitan Commission. Another consideration might be the effect of
higher water values, such as those existing in other areas of the country,
on the economic evaluation. A comparison of this type is of limited value,
since the system cost figures refer to a specific installation at Columbia,
Maryland,and costs might vary widely in different locations. Case 1 on
Figure 48 shows the relationship of the Net Benefit measure developed in
Table 51 to the value of the reused water. It can be seen that the riet bene-
fits are zero, that is, the system can be considered break-even in ,;an eco-
nomic sense when the value of water is approximately $0. 26 per thousand
gallons. When the value is set higher than this level, positive net benefits
result, and when it is lower, a net cost develops which must be compared
to the nonquantified secondary benefits described in later sections.-:
Although the conventional storm water treatment system developed;by
Whitmah, Requardt and Associates can be said to be fairly representative
of the best available method of achieving the water quality objective using
current technology, the sensitivity of the conclusions to pollution control
costs associated with advanced technology should be investigated. 'The
local storage concept itself offers an advanced method of water pollution
control when optimized for that purpose alone, without reuse. This type
of system is described in Section XVI and will be compared to the conven-
tional approach later in this chapter. Using the costs of the local storage
system without reuse as the imputed value of water pollution control also
permits convenient examination of the economics of reuse separately from
the storm water control and treatment aspects of the system. Case 2 on
Table 51 summarizes economic analyses of the potable reuse system as a
multipurpose project, in each case using the costs of the local storage
system without reuse as the imputed value of water pollution control. The
lower value assigned to the water pollution control function results in lower
net benefits in the multipurpose analysis and a higher incremental cost of
treated water. The analysis of the system as a water pollution control
project is, of course, unchanged. Case 2 on Figure 48 shows the effect
of the value of treated water on the net benefit function when the local
storage system is used as the source of water pollution control costs.
SUB-POTABLE REUSE
The optimum system for sub-potable reuse is described in Section XIV.
The cost of this system totals $2, 598, 600, of which $1, 562, 700 represents
distribution system costs. These costs are amortized over the life of the
facilities and added to the operating and maintenance costs, resulting in
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CASE 3
(w/o Distribution
System)
CASE 4
(w/o Dist.
System)
CASE 3
(w/Distribution
System)
1.00
CASE 4
(w/Distribution System)
Value of Treated Water ($/1000 Gal.)
Figure 48. Net Benefits vs. Value of Treated Water
189
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a total system cost of $524. 15 per day. Further details appear as Case 3
on Table 51. Total benefits are calculated at $463. 93 per day, producing
a negative net benefit or net cost of $60. 22 per day. The sub-potable reuse
system then, when considered as a multipurpose project, fails to generate
tangible benefits equal to the costs.
Other comparisons were carried out as described in the previous section
to investigate the effect of water supply and water pollution control consid-
erations. The results of the analysis as a water supply project are
further illustrated by Case 3 on Figure 48, where the effect of various
values of treated water is plotted. Figure 48 also illustrates the influence
of the cost of the distribution system on the overall results. It can be seen
that treated water could be priced at $0. 10 per thousand gallons at the
treatment plant (the zero cost intercept of plot "without distribution sys-
tem") but that the cost of providing the distribution system increases the
break-even price to $0. 58 per thousand gallons. Recomputation of bene-
fits based on the use of the local storage system to establish the value of
water pollution control leads to Case 4. It can now be seen that the portion
of the total system cost allocable to water supply is substantially higher,
but that the apparent cost of water pollution control remains at $30. 82 per
dwelling unit.
NO REUSE
Costs for the local storage system without reuse have been presented in
previous sections as an alternative measure of the value of water pollution
control. Total system cost has been given as $173. 78 per day, compared
to $256. 99 per day quoted for the conventional system. In addition, the
local storage system offers the secondary, unquantified benefits described
later in Section XVIII.
SUMMARY
The economic comparison of the three local storage systems and the
conventional storm water treatment system has been conducted in several
different ways. When the local storage systems are treated as multi-
purpose water resource projects, only the potable reuse system yields
a benefit/cost ratio greater than unity, regardless of the base employed
for evaluation of water pollution control benefits. The ratio was as high
as 1.26 when the conventional system was used as the base and fell to
1.06 with the local pollution control system as the base. Investigation
of the value placed on treated water revealed that all values greater
than $0. 26 per thousand gallons resulted in positive net benefits when
the conventional system was employed as a base. Use of the local pol-
lution control system as a base increased the break-even point to $0. 41
per thousand gallons. When the system was analyzed as a water pollution
control project, the net cost of pollution control amounted to $55, 160 per
year, or $14. 68 per dwelling unit-year. This can be compared to the
cost of pollution control by conventional means of $24. 97 per dwelling
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unit-year and $16. 88 per dwelling unit-year by the local storage system
without reuse. Analyzed on another basis, it can be found that if the
system is installed as a pollution control system and the necessary facil-
ities for potable water reuse are added, the additional cost involved in
treating and distributing the potable water will total $0.41 per thousand
gallons of water distributed.
The sub-potable reuse system exhibits benefit/cost ratios as low as
0. 73 and is characterized by a moderately high net cost even after full
credit is taken for all quantified benefits. The cost of treated water is
higher than the cost of water from the present alternative source when
the system is analyzed as a water supply project. Furthermore, when
analysis is based on water pollution control considerations, the cost of
pollution control is higher than any of the alternatives. The excessive
cost of the sub-potable system can be seen to be wholly caused by the
cost of the sub-potable distribution system. The construction, operation,
and maintenance of this facility alone accounts for an additional cost of
approximately $0. 48 per thousand gallons of treated water which is more
i than the total incremental cost of the alternative supply of water.
Operation of the local storage system without reuse as a water pollution
control method was compared to the conventional system and found to
result in a net benefit of $83. 21 per day, somewhat less than the benefit
associated with the operation of a potable reuse system which yields
$105. 88 per day.
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SECTION XVIII
SECONDARY BENEFITS
The primary objectives and benefits of the local storage, treatment, and
reuse concept have already been discussed. Although the system was de-
signed for optimum realization of these primary benefits, other benefits,
both tangible and intangible, do result from the installation of such a system
in an urban watershed. Although these benefits may be considered secon-
dary, they are nevertheless important and might be as economically signif-
icant in many watersheds as pollution abatement and reuse.
STORM WATER MANAGEMENT AND FLOOD CONTROL
As any area begins to urbanize, the hydrology of the watershed alters
drastically. With an increase in imperviousness and improved channels,
the runoff from a given storm increases both in volume and peak flow.
The previously derived regression model and hydrograph analysis clearly
indicate these effects. Such increases generally result in greatly raising
the potential for flooding the low areas adjoining the waterways. Flood-
plains which, in a natural state, were subjected to inundation once every
few years might become flooded several times each year.
To indicate the seriousness of this problem, reference can be made to
Figure 5 which shows that a natural area which is developed to an imper-
viousness of 0. 35 will yield over 400 percent more runoff from an inch of
rainfall. The effects on the peak flow may be just as serious, with 100
percent to 300 percent increases, depending upon the configuration of the
drainage basin.
In areas where the floodplain is developed, flooding resulting from urban-
ization can cause severe economic losses. Even in those areas where the
floodplain is left as dedicated open space, the use potential decreases as
the probability of flooding increases.
A system of small reservoirs located along the stream reaches within
the watershed, as previously described, has the ability to significantly
reduce the frequency of flooding. This has been demonstrated in the past
with large reservoir systems, and the principles underlying these flood
control projects hold true for the local systems discussed in this report.
As a means of graphically portraying the flood control potential of these
reservoirs, an analysis was made of the operation of two small storage
reservoirs'in series during a large storm. For simplicity, the water-
sheds supplying these reservoirs are hypothetical, with approximately
rectangular shapes. It was assumed that initially the watersheds had an
imperviousness of 0. 1 and, using the hydrograph analysis and stream
routing equations, the runoff hydrographs at the outflow points in each
basin were calculated for a five-year storm.
193
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A development of the basins was then assumed to take place with the
imperviousness of the upper basin increasing to 0. 5 and that of the lower
basin to 0. 3. This development also altered the lag time in each basin,
reducing it from 12 minutes to four minutes in the upper basin and from
24 minutes to 12 minutes in the lower basin. Two more calculations of
the runoff hydrographs were performed assuming first that no reservoirs;
existed, and then that reservoirs were installed immediately above the
outflow point in each watershed. The three basin configurations used
for analysis are shown in Figure 49 with the size of the areas and reser-
voirs indicated.
It was necessary to select an initial water level in the reservoirs since
their ability to dampen flood peaks is dependent upon the available storage.
To accomplish this, a frequency distribution of water levels in one of the
proposed Wilde Lake reservoirs was prepared. A probability level of
0. 85 was chosen as a reasonable (and conservative) criterion for selection
of a storage level. The frequency distribution indicated that for 85 per-
cent of the time, during the wettest year, the reservoir was'at one-third
capacity or less.
Using this value for both reservoirs, hydrographs were obtained for the
developed area without reservoirs, and were then routed through the
reservoir storage. This yielded three different hydrographs at the two
outflow points: one from the natural area, one from the developed area
with no reservoirs, and one from a reservoir system.
The results of this analysis are shown in Figures 50 and 51 which are
plots of the runoff hydrographs at points A and B, the outflows of the two
basins. It can be seen that at point A the development increased the peak
flow by 112 percent with no reservoirs and actually reduced it to a level
below the natural peak with a reservoir. At point B the increase was
110 percent with no reservoir and only nine percent with the reservoir.
This analysis clearly indicates the ability of the local storage reservoirs
to dampen the runoff peak for even very large storms. The savings in
economic losses due to flooding in urban areas can be an important
consideration in any analysis of the desirability of such a reservoir
system.
EROSION CONTROL
Any area, whether in a natural condition or undergoing urbanization, is
subject to erosion. This process is entirely a natural one, resulting
from the action of both rainfall and runoff on the ground surface and stream
channels. It may, however, be tremendously accelerated by man-induced
changes in the hydrology of an area. Such accelerations are well docu-
mented, with typical sediment yields from areas under construction or
already developed 10 to 1000 times those from the natural area.
194
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Natural A re
2. Developed Area without Reservoirs
3. Developed Area with Reservoirs
Area 2 100 ac.
I . 3
Area 1 50 ac
t, 12 mm.
Reservoir Capacity =
1.2 mg
Reservoir Capacity
1.7m
Figure 49. Basins Used for Hydrograph Damping Investigation
195
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C/2
fe
u
o
G
d
375
350
325
300 -
275 •
250 -
225 •
200 -
175
150
125
100
75
50
25
0
Developed
Without
Reservoirs
Developed with Reservoirs
10 20 30 40 50 60 70 80
Time from Start of Storm (Min)
90 100 HO 120
Figure 50. Runoff at Point A
196
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Developed without
Reservoirs
Developed vlth
Reservoirs
K
25 '
0 10 20
30 40 50 60 70 80 90
Time from Start of Storm (Min. )
100 110 120
Figure 51. Runoff at Point B
197
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Although much of this increase in sediment and erosion can be attrib-
uted to the stripping of vegetation from the land during development, a
still significant portion results'from the increased runoff discussed pre->
viously. The erosion of either land surface or stream channels is a
function of both the kinetic energy per unit volume and the total volume '-•
of the runoff. Since kinetic energy is proportional to the velocity squared,
a. large change in velocity or flow rate will have an even greater effect
on the kinetic energy and erosion potential of the water. Velocity, flow,
and volume increases are all significant during urbanization, and even
after construction in a watershed has been terminated, the increased
erosion of the stream valley continues.
There are many case histories of once natural streams becoming severely
eroded when their watersheds are developed. The stream bottoms are cut
deeper and the banks begin to become undercut and collapse. Deep gullies
are formed in areas of the floodplain where there previously were small
rivulets. Once a stream approaches this condition, there is generally
public pressure to have the banks and bottom paved, completely destroying
its original character. Improved channels further accelerate peak flows
and create hydraulic and erosion problems downstream.
For the reasons discussed with respect to flood control, this condition
does not have to occur in an area with reservoir-controlled runoff. The
reduction of flows and high volumes of water by reservoir storage can
help to retain the natural stream channels and floodplains. Although
dampening hydrographs do not always lead to a proportionate decrease
in downstream erosion (due to changes in flow regimen), the ability to
control release rate and the diversion of a substantial quantity of water
to reuse should result in significant reduction in channel erosion.
NUTRIENT REDUCTION
An increasing awareness of the problems of nutrient enrichment of lakes
and estuaries is developing throughout the country. Although many
causes of such enrichment are offered, there is general agreement that '
urban runoff can add a significant amount of nitrates and phosphates to
a receiving body.
The operation of the treatment plants associated with the storage reser-
voirs cannot be expected to remove more than a token quantity of the
nutrients present. By returning a sizable portion of the runoff to the
watershed through the reuse system, the nutrients will eventually be
transported out of the area through the sanitary sewers.
While this does not remove the nutrients from the water, it does have
two beneficial effects: it prevents a large portion of the nutrients from
entering the small urban lakes which are prone to eutrophication, and
it transfers them to a large treatment plant where they may be even-
tually removed economically.
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ECOLOGICAL PRESERVATION
As,a watershed changes from rural to urban, there is generally a drastic
alteration of the ecological characteristics of the streams and their flood -'
plains due to the increased storm flows, high silt loads, erosion of natural
banks, and pollution.
Atypical history of a small stream situated, in a watershed undergoing,
urbanization might be as follows:
During the stripping and excavating of the land, which takes
place in the construction phases of development, the stream
begins to become clogged with sediment. Bottom organisms
found in the natural stream cannot live once the thick layer
of silt has settled over the previously rocky bottom. Larger
organisms; including the original fish population which were
•dependent upon these organisms, begin to disappear. As con-
struction proceeds, larger areas of the watershed become im-
pervious and the storm flows greatly increase. The former
banks of the stream fail to hold the increased flow, and mud
flats are deposited along the stream valley. Erosion increases
greatly and the combination of erosion and mud deposition
begins to destroy the natural vegetation along the stream
banks and in the floodplain. Eventually this entire area
changes in appearance with little of the natural flora or fauna
remaining. Trash, debris, and pollutants carried by the
stream help to spawn a new ecology, usually including rats,
leeches, scavenger fish, and mosquitoes. The stream valley
in this condition is shunned by the residents of the watershed.
Such a history, although typical, is not necessary. As was previously
discussed, a series of reservoirs within the floodplains of the streams
can abate many causes of the destruction of a natural ecology. The elim-
ination of most sediment, the dampening of floods, and the reduction of
erosion can assist in the maintenance of the original ecological conditions
not only in the stream but in the surrounding floodplain as well. The
effects of preserving this original appearance on the recreational potential
of the area are discussed in the following section.
RECREATIONAL AND AESTHETIC CONSIDERATIONS
Water is an important part of our natural landscape and environment. Its
value has been recognized and employed by planners and administrators
in projects such as the Baltimore City Inner Harbor Development and the
Columbia City Project in Maryland. The aesthetic and recreational value
of water, whether in a natural or urban setting, cannot be overemphasized.
In the case of the local storage ponds, the setting would be primarily a
natural one with as little excavation as possible and immediate replanting
of natural vegetation. The choice and treatment of construction materials
would become a part of, and possibly enhance, the local natural environment.
199
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The ponds present the opportunity for development of both active and
passive recreation areas adjacent to the water. Aside from creating a
body of water, the development of the ponds produces clear flowing
streams of good quality water, thus improving the entire floodplain.
With the now decreased flood activity, the floodplain can be developed
and treated as useful productive land.
Three possibilities of handling the pond and its floodplain are a com-
pletely natural setting, development of a passive recreational area, and
development of an active recreational area.
In the completely natural setting, care is taken during construction to
preserve the character of the area, leaving as much natural vegetation
and rock formations as possible. After construction, the area is land-
scaped to resemble the surrounding environment. The spillway is masked
with rock or vegetation and the entire area is allowed to be overtaken by
the wild vegetation present. Figure 52 shows the appearance of a typical
pond in a natural setting.
The development of the floodplain as a scenic walk and picnic area with
seating facilities and lighting is another possibility. Incorporating the
pond, spillway, and natural surroundings into a creative, stimulating
design could prove quite valuable to the local community. Such a,design
concept is illustrated by Figure 53.
Simple structures could be developed in more populated areas to turn the
pond and spillway area into a recreational playground facility. In the event
that local conditions dictate fencing of the ponds, the development of these
recreational facilities will produce a positive result by having the fencing
contain a usable function rather than merely guarding the pond. One
recreational design that might be considered for such an application is
shown in Figure 54.
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CO
o
Figure 52. Local Storage Pond in Natural Setting
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DO
O
IND
Figure 53. Local Storage Pond in Wooded Park Setting
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DO
O
CO
Figure 54. Local Storage Pond in Recreation Area Setting
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SECTION XIX
DEMONSTRATION PROGRAM
Upon completion of this study in 1968, the results were reviewed with
the State of Maryland, the Environmental Protection Agency, Howard
County officials, and the developers of Columbia, Maryland. It was
generally agreed that the concept of providing retention of storm water
for pollution control and as an alternative water supply had merit.
However, it was the concensus that the control of sediment generated by
construction and urban development was a greater pollution problem
than post-development pollution control and that these aspects of the
study should be given priority in any subsequent demonstration programs.
During this period, the State of Maryland enacted House Bill No. 1151
for the control of grading, erosion, and sediment during construction
and development throughout the state. In the interest of developing
additional guidelines for the implementation of this law, the Department
of Water Resources of the State of Maryland agreed to serve as the
sponsor of the demonstration project, and Howard Research and
Development Corporation and the Columbia Parks and Recreation
Association agreed to contribute to the project. An application was
made to the Environmental Protection Agency and the demonstration
project was initiated in July 1970. The scope of this demonstration
project, which is now in progress and scheduled for completion in late
calendar year 1972, is described in this section.
OBJECTIVES
The objectives of the Environmental Protection Agency demonstration
grant project 15030 FMZ as specified in the grant application and offer
are as follows:
1. Evaluate the effectiveness and costs of advanced methods
of erosion control in urban areas
2. Evaluate the effectiveness and costs of various methods
for the transport, drying, conditioning, and disposal
of sediment
3. Demonstrate urban storm water pollution control during
construction and following development by the use of a
local storage and treatment system
4. Collect and analyze data on the effects of urbanization
and various control measures on hydrology, water quality,
stream ecology, and channel hydraulics
In addition to the regular reports, an "Erosion and Sediment Control
Manual for Urban Development" is to be prepared covering the erosion
205
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and sediment control practices demonstrated under this project and
other practices that can be used in urban development.
DEMONSTRATION SITE AND FACILITIES
The demonstration project is being conducted on a 200-acre watershed
in the Village of Long Reach in the new city of Columbia, Maryland. At
the beginning of the project in 1970, only the rough grading of access
roads within the watershed had been completed. This rural agricultural
area was selected for the demonstration project because it was scheduled
for complete urbanization over the period of the demonstration project.
Upon the initiation of this project, an earth-filled dam was constructed
at the lower end of the demonstration watershed to form a three-and-
one-half acre lake to serve as a sediment retention pond during the course
of the construction and the project. As discussed later, this pond is
also being used to demonstrate storm water management techniques
and sediment handling and removal methods.
Two sub-water sheds at the upper end of the demonstration area have
been respectively designated as reference and experimental sub-water-
sheds. Gaging and sampling stations have been installed to monitor the
flows and to collect samples of the storm water draining from each of
the two sub-water sheds. Within the reference sub-water shed, the
erosion and sediment controls are those applied by the developers in
accordance with applicable county and state regulations. Within the
experimental sub-watershed,special methods of erosion and sediment
control are being applied and evaluated.
Two other gaging and sampling stations have been installed. One is
located immediately above the storm retention pond to monitor flows
and collect samples of storm water runoff from the entire demonstration
watershed. The other station is located downstream of the storm retention
pond. This station records the overflow from the pond and the flow over
the emergency spillway during major storm events.
Three recording rain gages are installed on the watershed and the clocks
are synchronized with those used in the gaging and sampling stations.
EROSION CONTROL
In the demonstration project, primary emphasis is being placed on
erosion control practices on the basis that the most effective method
of control is the retention of the soil and the elimination of sediment as
a pollutant. For purposes of demonstration, evaluation, and definition
of future applications, erosion control practices have been categorized
on the following pages.
206
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Long-Term Stabilization Using Vegetative Cover
Limitation of grading, excavation, and fill to retain
maximum natural cover
Limitation of period of exposure or denudation, especially
with regard to frequency of high intensity rainfall events
Scheduling of earth-moving operations to utilize germina-
tion and growing seasons, whenever possible
Design for and use of proper equipment in the preparation
of soils and the planting of vegetative cover
Use of natural material, matting, and/or artificial
methods to:
Stabilize areas during' the planting and development
of vegetative cover
Provide long-term root reinforcement for vegetative
cover on critical slopes and drainage ways
Use of special ground cover, shrubs, or trees to supplement
permanent grasses on critical areas
Use of turf to provide expeditious development of permanent
ground cover
Use of vegetative cover to stabilize engineered and'temporary
erosion and drainage control structures
Establishment of vegetation with higher velocity tolerance in
areas of concentrated overland flow
Long-Term Engineered Erosion and Drainage Controls
Chutes, flumes, and energy dissipators to convey runoff in
critical areas where volume and/or velocity is beyond
vegetative tolerances
Erosion checks and level spreaders for reduction of flow
velocity
Structural stabilization using rubble, stone, or grouted
riprap
Sediment traps that may be converted to beneficial use
(recreation, wildlife habitat, etc.) after development and
construction operations have been completed
207
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Trickle channel and outfall protection using prefabricated
or field-fabricated devices
Interim Erosion and Drainage Controls
Temporary diversion and interceptor dikes
Level spreaders
Existing naturally vegetated waterways
Temporary chutes, flumes, and downdrains
Bituminous concrete
Reinforced synthetic fabric
Chftmical stabilizers
Temporary sedimentation basins
Temporary seeding of denuded areas
Temporary mulching of denuded areas
Straw and hay
Chemical soil stabilizers
Chemical soil flocculants
Wood fiber
Fiber glass
Temporary stabilization of denuded areas
Mats
Natural material
Synthetic material
Nets
Plastic
Fiber glass
Natural material
Turf
208
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Temporary erosion checks
Weirs (log, stone, chemically stabilized aggregate)
Natural fibers
Plastic materials
Fiber glass products
Chemical products
Emergency Erosion and Drainage Controls
Diversion ditches and berms
Diversion with nylon bags filled with soil or sand
Interceptors built of nylon bags filled with soil or sand
Chutes and flumes of rigid or semirigid plastic material
Chutes and flumes of chemical sprays
Chutes and flumes of synthetic fiber
Level spreaders of synthetic material
Chemical soil stabilizers, blankets, and flocculants
Interceptors built of rock and gravel
As a part of this project, a large number of manufacturers and vendors
of erosion control and landscaping materials and equipment are providing
products for demonstration and evaluation purposes. These include:
chemical soil stabilizers and mulches, blanketing products, fabric form
for bank and stream stabilization, hydroseeders,plus other materials.
SEDIMENT HANDLING AND DISPOSAL
A survey was made of methods for the removal of deposited sediment
from storm retention ponds early in the project. The removal methods
considered included the use of conventional construction equipment, use
of underwater roads with conventional equipment, draglines, underwater
scoops, barge-mounted back hoes, and small dredges. Based on the
equipment available at the time, the use of an underwater scoop operated
off deadmen and the use of conventional front loaders were selected for
demonstration.
209
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An "engineered forebay" has been constructed at the upper end of the
storm retention pond. This consists of an underwater dam and a flow
spreader which form a widened section in the channel entering the pond.
The forebay serves two purposes. First, it provides an area in which
the heavier sediment can be trapped before entering the pond and an
area in which this material can be removed by front loaders with minimal
disruption of the area around the pond. Secondly, the forebay provides
an area to which the deposited sediment from the pond proper can be
transported by the underwater scoop for dewatering and subsequent
removal.
Laboratory tests have been conducted on the dewatering characteristics
of various types of sediment with and without chemical additives. Test
beds are to be constructed to demonstrate selected techniques in the field.
For the demonstration, conventional dump trucks will be usedto transport
the sediment to the dewatering and disposal site. As necessary, plastic
liners will be used to stop leakage from the trucks.
STORM WATER MANAGEMENT
The key features of the local storage and treatment concept of pollution
control considered in the original system study are being demonstrated.
Using the gaging and sampling stations located upstream and downstream
of the storm retention pond, data are being developed on the effectiveness
of this method of storm water management. Special tests are being
conducted in which the level of the retention pond is lowered between
storm events. The effects of the additional available storage capacity
on the downstream hydrographs are being determined. This will also
provide information on the need to provide bank stabilization on ponds
operated with variable water levels.
The size of the storm retention pond makes it a relatively effective
settling basin for most suspended solids. Accordingly, it is being
operated without supplemental treatment to determine the extent to
which pretreatment and treatment would be required to use water from
storm retention basins as a supplemental water supply.
EFFECTS OF URBANIZATION
The data from the rain gages and the gaging stations are being analyzed
to determine the extent to which storm water runoff is being changed by
the installation of storm sewers and the urbanization of the area. Actual
site and rainfall data are being used as input to the Environmental
Protection Agency, Storm Water Management Model (49), and the cal-
culated runoff will be compared to actual field data.
Aerial photographs of the demonstration watershed area are being
acquired quarterly to document the status of construction and development
210
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and other site data. This information is being retained to allow gener-
ation of input data for future runoff models for comparison with actual
runoff. These aerial photographs are also being used to monitor changes
in stream and other drainage channels in the area.
In addition to the aerial photographs, ground surveys are being made
quarterly of the retention pond and the major stream flowing to the pond.
Profiles are being prepared from previously established bench-marks
to determine the extent of stream erosion and the deposition of the
sediment in the stream and pond.
An inventory of the aquatic life existing in the area was established early
in the project and periodic ecological surveys are being made to monitor
the changes that have occurred in the stream and pond during development.
Extensive photographic coverage of the erosion controls, sediment
removal operations, facility construction, etc., is being maintained,
and slides are being accumulated to show sequential effects.
211
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SECTION XX
ACKNOWLEDGEMENTS
Hittman Associates wishes to acknowledge the assistance of the various
individuals and organizations who made important contributions to the
study, .design, and evaluation of the storage, collection, and use of
storm water. Hittman Associates wishes to especially acknowledge the
efforts of the organizations which participated directly in this effort:
Whitman, Requardt and Associates, Baltimore, Maryland,
who participated as a subcontractor in the review of water
usage, public codes and regulations, and performed the
conceptual design work on the "conventional system" and
the facilities for the demonstration program.
The Land Planning and Management and the Construction
Management staffs of The Rouse Company, The Howard
Research and Development Corporation, and the Columbia
Association, who provided detailed data on the projected
development of the Wilde Lake watershed and assistance
in the planning of the demonstration program.
Department of Health, State of Maryland, who provided
guidance on the general health aspects of storm water use
and reviewed the plans for the demonstration program.
Howard County Metropolitan Commission, who reviewed
the concept for application to the County.
Department of Water Resources, State of Maryland, who
provided assistance on the pollution control aspect of the
program and sponsored the demonstration project based
on this study.
Hittman Associates also wishes to thank George Kirkpatrick and William
Rosenkranz of the Environmental Protection Agency for their comments
during the course of the program which provided valuable guidance in
the evaluation of the system; Marshall T. Augustine of the Department
of Water Resources, State of Maryland, who assisted in the formulation
of the demonstration project; and Ernst P. Hall and John J. Mulhern of
the Environmental Protection Agency for their guidance and support in
the conduct of the demonstration program. The assistance of Sidney
Beeman of EPA in the editing of this revised report is also gratefully
acknowledged.
213
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SECTION XXI
REFERENCES
1. Linsley, R. K. , M. A. Kohler, and J. L. H. Paulhns, Hydrology
for Engineers. McGraw-Hill, 1958, pp. 193-209.
2. Holtan, H. N., andD.E. Overton, Storage-Flow Hysteresis in
Hydrograph Synthesis. Journal of Hydrology #2, 1964.
3. Espey, W. H. , and others, A Study of Some Effects of Urbanization
on Storm Runoff From a Small Watershed, Texas Water Development
Board Report 23, August 1966.
4. Horton, R. E. , An Approach Toward a Physical Interpretation of
Infiltration Capacity, Soil Science Society, American Proc. 5, 1939.
«
5. Horn, D. R. , and N. Dee, Synthesis of the Inlet Hydrograph from
Small Pervious and Combination Pervious - Impervious Drainage
Areas, Technical Report 47, Storm Drainage Research Project,
Johns Hopkins University, 1963.
6. Ongerth, Henry J., and Judson A. Harmon, "Sanitary Engineering
Appraisal of Waste Water Reuse," Journal AWWA, Vol. 51,
May 1959, pp. 647-658.
7. Haney, Paul D., and Carl L. Hamann, "Dual Water Systems, "
Journal AWWA, Vol. 52, May 1960, pp. 599-606.
8. Berger, Bernard B. , "Public Health Aspects of Water Reuse for
Potable Supply. " Journal AWWA, Vol. 52, May 1960, pp. 610-615.
9. Bonderson, Paul R. , "Quality Aspects of Waste Water Reclamation,"
Journal of the Sanitary Engineering-Division, ASCE, Vol. 90,
October 1964, pp. 1-8.
10. Merz, Robert C. , "Direct Utilization of Waste Waters, "Water and
Sewage Works, September 1956, pp. 417-423.
11. McGauhey, P. H. , "The Why and How of Sewage Effluent Reclama-
tion, " Walej^nd_Sejvagj_Works, June 1957, pp. 265-270.
12 Marks R. H. , "Waste Water Reclamation: A Practical Approach
for Many Water-Short Areas," Power, November 1963, pp. 47-50.
13 Viessman, Warren, Jr., "Developments in Waste Water Reuse, "
Public Works, April 1965, pp. 138-140.
14 Spiewar I. , and W. F. Schaffer, Jr., "Survey of the Potential Use
of Nuclear'oerived Energy Sources in Waste Water Treatment, "
Atomic Energy Commission, Unpublished Report, Date Unknown.
215
-------
15. Brandel, A. J. , "Recirculation of Cooling Water in Petroleum
Refining. " Industrial and Engineering Chemistry, Vol. 48, No. 12,
December 1956, pp. 2156-2158.
16. Biladeau, Archie L. , "Reuse of Cooling Water in an Atomic Energy
Commission Installation, " Industrial and Engineering Chemistry.
Vol. 48, No. 12, December 1956, pp. 2159-2161.
17. Maguire, John J. , "Biological Fouling in Recirculating Cooling
Water Systems, " Industrial and Engineering Chemistry, Vol. 48,
No. 12, December 1956, pp. 2162-2167.
18. Brown, Howard B. , "Conservation of Water in the Pulp and Paper
Industry Through Recycle, Reuse, and Reclamation, "industrial
and Engineering Chemistry, Vol. 48, No. 12, December 1956,
pp. 2151-2155.
19. Noll, D.E. and H. M. Rivers, "Reuse of Steam Condensate as
Boiler Feedwater, " Industrial and Engineering Chemistry, Vol. 48,
No. 12, December 1956, pp. 2146-2150.
20. Powell, Sheppard T. , "Adaption of Treated Sewage for Industrial
Use, " Industrial and Engineering Chemistry, Vol 48, No. 12,
December 1956, pp. 2168-2171.
21. "industry Utilizes Sewage and Wastes Effluents for Processing
Operations, " Wastes Engineering, September 1957, pp. 444-467.
22. Connell, C. H., and E. J. M. Berg, "industrial Utilization of Munic-
ipal Wastewater, " Sewage and Industrial Wastes, Vol. 31, No. 2,
February 1959, pp. 212-220.
23. Derby, Ray L. , "Water Use in Industry, " Journal of the Irrigation
and Drainage Division, ASCE, Vol. 83, September 1957, pp. 1364-1
to 1364-9.
24. Dykes, D. R. , T. S. Bry, and C. H. Kline, "Water Management: A
Fashionable Topic, " Environmental Science and Technology. Vol. 1,
No. 10, October 1967, pp. 780-784.
25. Reid, G. W. , "Projection of Future Municipal Water Requirements, "
Southwest Water Works Journal. Vol. 46, No. 18, 1965.
26. Linaweaver, F. P. , Jr., and Jerome B. Wolff, Residential Water
Use, Report I on Phase Two of the Residential Water Use Project,
Department of Sanitary Engineering and Water Resources, The
Johns Hopkins University, Baltimore, Maryland, May 1964.
216
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27. Linaweaver, F. P. , Jr., James C. Beebe, and Frank A. Skrivan,
Residential Water Use. Report IV on Phase Two, Data Report on
the Residential Water Use Project, Department of Sanitary Engi-
neering and Water Resources, The Johns Hopkins University,
Baltimore, Maryland, June 1966.
28. Wolff, Jerome B. , F. P. Linaweaver, Jr., and J. C. Geyer,
Residential Water Use. Report V on Phase Two of the Residential
Water Use Project, Department of Sanitary Engineering and Water
Resources, The Johns Hopkins University, Baltimore, Maryland.
29. Public Health Service Drinking Water Standards - 1962, U.S.
Department of Health, Education, and Welfare, Public Health
Service, Washington, D.C., August 1962.
30. Pollutional Effects of Storm Water and Overflows from Combined
Sewer Systems: A Preliminary Appraisal, U. S. Department of
Health, Education, and Welfare, Public Health Service, Washington,
D.C., November 1964.
31. Weibel, S. R. , R. J. Anderson, and R. L. Woodward, "Urban Land
Runoff as a Factor in Stream Pollution, " ^Journal Water Pollution
Control Federation, 36, July 1964, pp. 914-924.
32. Weibel, S. R. , R.B. Weidner, J. M. Cohen, and A. G. Christiansen,
"Pesticides and Other Contaminants in Rainfall and Runoff," Journal
AWWA, Vol. 58, August 1966, pp. 1075-1084.
33. Weibel, S. R. , R.B. Weidner, A. G. Christiansen, -and R. J.
Anderson, "Characterization, Treatment, and Disposal of Urban
Storm Water, " Paper Presented at Third International Conference
on Water Pollution Research, Munich, Germany, Section 1,
Paper No. 15, 1966, p. 15.
34. Riis-Carstensen, E. , "improving the Efficiency of Existing Inter-
ceptors, " Se_wa^e_j.ndJndjis_trJ^JJA[ajtes, 27, 10, 1115, October 1955.
35. Sylvester, R. O. , "An Engineering and Ecological Study for the
Rehabilitation of Green Lake," University of Washington, Seattle,
Washington, 1960.
36. Wilkinson, R. , "The Quality of Rainfall Runoff Water from a
Housing Estate, " Journal Institute Public Health Engineers,
London, 1954.
37. Shigorin, G. G. , "The Problem of City Surface Runoff Water, "
Voodsnabzhenie i Sanitarnayo Tekhnika, 2, 19, 1956.
38. Akerlinch, G. , "The Quality of Storm Water Flow, " Nordisk
Hygienisk'Tidskrift (Stockholm), 31, 1, 1950.
217
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39. Stander, G. J. , "Topographical Pollution - The Problems of the
Water and Sanitary Engineer, " 40th Annual Conference, Institute
Municipal Engineers, National Institute Water Research, 1961.
40. "Water Resources Regulation 4. 8: General Water Quality Criteria
and Specific Water Quality Standards," Water Resources Commission
and Department of Water Resources, Annapolis, Maryland, May 1967.
41. "Criteria for the Classification of Maryland Streams," Department
of Water Resources - State of Maryland, Annapolis, Maryland,
February, 1949.
42. Fair, G. M. , and J. C. Geyer, Water Supply and Waste Water
Disposal, Wiley, New York, 1954.
43. "Runoff, " Hydrology Handbook, Chapters, Am. Soc. Civil. Eng. ,
New York, 1949.
44. Private Communication, Vaden Haddaway, Whitman,Requardt and
Associates, August 1968.
45. Private Communication, Donald E. Strickhouser, Assistant Director
of Public Works, Fairfax County, Virginia, August 1968.
46. Hansen, S. P. and G. L. Gulp, "Applying Shallow Depth Sedimenta-
tion Theory, Journal AWWA. Vol. 59, p. 1134 (1967).
47. Hazen, Allen, On Sedimentation, Trans. Am. Soc. Civil Engineers,
Vol. 53, 1904, p. 63.
48. Hazen, Allen, "Water Supply, " American CivilEngineer's Handbook,
Wiley, New York, 1940, p. 1474.
49. Storm Water Management Model, Vols. 1, 2, 3, and 4, Metcalf
and Eddy Engineers, Palo Alto, California, 1971.
218
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SECTION XXII
APPENDICES
•rage
A. Derivation of Hydrology Equations 221
B. Field Gaging and Sampling 225
Figure B-l: Temporary Stream Gaging Station
Drainage Area 226
Figure B-2: Temporary Stream Gaging Station Site
Plans 227
Figure B-3: Observed Hydrograph - Storm 1 229
Figure B-4: Observed Hydrograph - Storm 2 230
Figure B-5: Observed Hydrograph Storm 3 231
Table B-l: Comparison of Gaged and Calculated
Runoff 228
Table B-2: Water Quality Versus Runoff Rate,
June 26 to 27, 1968 (Storm 1) 232
Table B-3: Water Quality Versus Runoff Rate,
July 12, 1968 (Base Flow) 233
Table B-4: Water Quality Versus Runoff Rate,
July 15, 1968 (Storm 3) 234
C. Storm Water Storage and Pretreatment Costs 237
Figure C-l: Land Area Versus Capacity - Open
Ponds 239
Figure C-2: H/D Versus Capacity - Ground Level
and Elevated Tanks 241
Figure C-3: H/D Versus Capacity - Standpipes . . . 241
Figure C-4: Site Preparation Costs Versus Capacity -
Storage Tanks 244
Figure C-5: Burial Costs Versus Capacity - Storage
Tanks 244
Figure C-6: Erection Costs Versus Capacity -
Concrete and Steel Above Ground Tanks,
Elevated Tanks, and Standpipes 247
Figure C-7: Construction Costs Versus Capacity -
Open Ponds 247
Table C-l: Reservoir Construction Types 237
Table C-2: Land Area Versus Capacity - Open
Ponds 242
Table C-3: H/D Versus Capacity - Ground Level
Tanks 242
Table C-4: H/D Versus Capacity - Elevated Tanks. . 242
219
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APPENDICES (Continued)
Page
Table C-5: H/D Versus Capacity - Standpipes 242
Table C-6: Site Preparation Costs Versus Capacity . . 243
Table C-7: Burial Costs Versus Capacity ....... 245
Table C-8: Summary of Tank Erection Cost
Estimates 246
Table C-9: Erection Costs Versus Capacity - Storage
Tanks 248
Table C-10: Summary of Construction Cost Estimates -
Open Ponds 250
Table C-ll: Construction Cost Versus Capacity -
Open Ponds 251
Table C-12: Summary of Sediment Removal Costs . . . 251
D. Treatment System Costs 254
Figure D-l: Pretreatment Capacity Versus Capital
Costs 255
Figure D-2: Final Treatment Capacity Versus Capital
Costs 255
Figure D-3: Daily Demand Versus Capital Cost of
Pumping Facilities 259
Figure D-4: Operation and Maintenance Cost Versus
Capacity - Final Treatment 265
Table D-l: Summary of Construction Cost Estimates -
Pretreatment 254
Table D-2: Construction Cost Versus Capacity -
Pretreatment 254
Table D-3: Summary of Construction Cost Estimates -
Final Treatment 256
Table D-4: Construction Cost Versus Capacity -
Final Treatment 257
Table D-5: Summary of Construction Cost Estimates -
Pumping Facilities 258
Table D-6: Construction Cost Versus Capacity -
Pumping Facilities 260
Table D-7: Summary of Operating Cost Estimates -
Treatment 263
Table D-8: Operation and Maintenance Cost Versus
Capacity - Treatment 264
220
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APPENDIX A
DERIVATION OF HYDROLOGY EQUATIONS
Section IV of this report "Storm Water Hydrology," contains a discussion
of the methods Used td describe the storm flow characteristics of the
Wil'de Lake drainage area. This appendix contains the derivation of the
equations used to modify the regression model for calculating the runoff
for individual storms which are contained in Section IV. These deri-
vations are as follows.
Alteration of the Regression Equation
Let Qrp ~ runoff /unit area from Herring Run
Qp = runoff /unit area from the pervious portion of
Herring Run or Wilde Lake
Q = runoff/unit area from the impervious portion
of Herring Run
Q p = BR. + CR.2 + DR. (neglecting base flow)
Q = Qp(l-I) + QTI (I = Fractibn Impervious)
Qp(l-I) + Qjl = BRt + CRt2 + DR.3
2 3 KXR1
2 3
% = TT (BRi+ CRi + DRi> - —
Q = J_ [(B-KXI)R. + CR,2 + DR.3]
P 1 -I L l l L s
P
Let Q' , = runoff/unit area from the impervious portion of
11 Wilde Lake
Q . = runoff/unit area from Wilde Lake
^••T I LJ
Qr
QIlp = KXR.I' + (^) [(B-KXI)Ri
221
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Adding the base flow term:
A + KXI'R. + Y-J (B-KXDR + CR.2 + DR.3
Derivation of the Expression Defining X
QT-Q
(I+1)0.339(R)1.75(ApI)0.0741
0.0973 - (Ref' 3)
D
where:
I = percent imperviousness
R = precipitation in inches
API = antecedent precipitation index
D = duration of storm, minutes
Qj = runoff volume in inches from an area
with I percent imperviousness
Let Q0 = runoff for 1 = 0, and let X = fraction of rainfall on
impervious areas that actually runs off. Then:
Q = XRI + ^ /100-I
•I 100 ^o \ 100
XRI
I ^o 100 TM
X - 10° ^
x - -RT Q-
"RT^i \L ' 07 I1 Too
). 339
x _ j~L |1QO + / i-ioo
[ Ui+i)°-339
222
-------
Derivation of the Equation Defining P
Let P = fractional increase in unit yield of the developed
area over the natural area
Qp
whe re:
Qrp = A + KXI'R. + (jZf)(B -KXI)R. + CR.2 + DR.3
Q = A
Let [(B-KXI)Ri + CR.2 + DR.3] = Y
KXI'Ri + Y - A +
I-JKXR.-Y
Simplifying:
2
I' KX - B - CRt - DRt
-=,— -KXI+B+CR.+DR.2
rl. 11
223
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APPENDIX B
FIELD GAGING AND SAMPLING
GAGING STATION RESULTS
A temporary stream gaging station was installed in one of the sub-water-
sheds on which little development had taken place. This gaging station
had two primary objectives:
To provide streamflow data to verify the regression model
which was used to calculate runoff
To provide water quality data to compare with the data
provided by the literature search
The watershed that was selected for gaging was a 130-acre area in the
upper end of the Wilde Lake drainage basin scheduled for devel-
opment during the end of 1968. At the time of installation, several
roads had been completed, giving the area an imperviousness of six
percent. The undeveloped portion of the area consists of woods and
a meadow with two main stream channels joining several hundred feet
above the gage site (Figure B-l).
The instrumentation (Figure B-2) consisted of a compound weir located
at the end of a long reach of a straight channel approximately six feet
wide and two and one-half feet deep. When the weir plate was put into
place, the water formed a distinct pond for a distance of 40 feet upstream.
Stage measurements of discharge over the weir were made using a
stilling well, float, and Stevens A35 water level recorder. This recorder
was located in an instrument box and was powered by a storage battery
and power inverter. It provided a continuous record of water level with
both good time and stage resolution.
The stage measurements were related to discharge over the weir by the
use of an equation derived by model studies conducted by the Bureau of
Reclamation. Field measurements were made of the flow by timing the
rate of fill of known-volume containers, and these measurements closely
agreed with the values calculated by the equation.
Approximately 40 feet upstream from the end of the weir pond, an auto-
matic water sampler was installed. This took water samples at preset
time intervals and was usually hand-started whenever rain was forecast.
These samples were collected and returned to the laboratory for analysis.
A University of Maryland weather station located a short distance from
the watershed provided total rainfall information to be used in the regres-
sion model.
225
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IN3
CO
UNIVERSITY OF MARYLAND
WEATHER STATION -
-TEMPORARY GAGING STATION
LIMIT OF DRAINAGE AREA
ROADWAY —-
Figure B-l. Temporary Stream Gaging Station Drainage Area
-------
CO
CO
STEVfMS Type A3S
•LEVEL FLOAT
SECTION A-A
Hittman Associate*, Inc.
SfCTION C-C
SECTION
Figure B-2. Temporary Stream Gaging Station Site Plans
-------
During the time of operation of the gaging station, three storms occurred
from which runoff hydrographs were obtained. These three hydrographs
are shown in Figures B-3 through B-5. From the hydrographs, the
volume of runoff was determined and compared to the runoff volume
calculated by regression model for the same amount of rainfall. Table
B-l gives the results of this comparison.
TABLE B-l. COMPARISON OF GAGED AND CALCULATED RUNOFF
Rainfall Measured Runoff Calculated Runoff
Storm Date (inches) (cfs-days) (cfs-days)
1 6/26-6/27 0.79 0.242 0.352
2 6/27-6/28 0.66 0.236 0.254
3 7/19 1.16 0.70 0.60
Correlation between measured and calculated = 0. 92
For the first and last storms, water samples were also collected. The
results of the analysis of these samples, ag well as the analysis of base
flow, will be presented in a subsequent section.
FIELD SAMPLING RESULTS
In order to permit early verification of the expected pollutant concen-
trations listed in Table 10, a sampling point was established in the
Wilde Lake Watershed in conjunction with the gaging station described
above. An automatic sampler was used to collect runoff samples at
one-hour intervals from a point immediately upstream from the weir
pond. These samples were correlated with measured stream flow at
the time of sampling and analyzed for various pollutants by Hittman
Associates. The results of this analysis are presented as Tables B-2,
B-3, and B-4. Tables B-2 and B-4 reflect water quality during two of
the storms whose hydrographs were analyzed above and identified as
Storms 1 and 3. Table B-3 lists data on water quality during a typical
period of minimum flow, when no storm runoff was present.
All samples were excessively high in suspended and total solids, due
to a high level of construction activity in the watershed during the period
under study. A large portion of the floodplain itself had been denuded
as a result of the construction of a sanitary sewer, and erosion was
severe. For this reason, the results obtained cannot be treated as
typical of this area. Furthermore, little urbanization has occurred
on the gaged watershed at the present time. At the time of sampling
228
-------
2. 0
to
CO
0. 2 _
2000
Storm 1
6/26/68 - 6/27/68
Rainfall = 0.79"
2200
2400 0200
^-f-^- 6/27/68
0400 0600
Time
0800
1000
1200
1400
Figure B-3. Observed Hydrograph - Storm 1
-------
1. 2
to
CO
o
1.0 \-
•Z 0.6 (—
0.2 |—
0
1800
2000
Storm 2
6/27/68 - 6/28/68
Rainfall = 0. 66"
2200 2400 0200 0400
6/27/68 ^ | »- 6/28/68 Time
0600
0800
1000
1200
Figure B-4. Observed Hydrograph - Storm 2
-------
CO
<«-<
o
o
s
0
Storm 3
7/15/68
Rainfall = 1.16"
1400
1430
1500
7/15/68
1530
Time
1600
1630
1700
Figure B-5. Observed Hydrograph - Storm 3
231
-------
1X3
oo
to
TABLE B-2. WATER QUALITY VERSUS RUNOFF RATE
JUNE 26 TO JUNE 27, 1968 (STORM 1)
Sampling
Time
16:30
17:30
18-30
19:30
20 30
21-30
22-30
23:30
24-30
01 30
02-30
03:30
04-30
05:30
06:30
07:30
08:30
09:30
10-30
11:30
12-30
13:30
14:30
Runoff
Rate
cfs
0.04
0. 04
0.04
0. 04
0.04
0.04
0. 04
1. 00
1.65
0.60
0.44
0.33
0.23
0. 20
0. 17
0. 16
0. 15
0. 14
0. 15
0. 13
0. 10
0. 15
0. 14
Turbidity
JTU
190
125
78
130
82
103
85
4, 500
3,500
1, 500
1,050
820
325
270
151
235
103
1, 050
310
105
1, 300
Suspended
Solids
mg/1
140
50
75
80
11, 450
5, 780
1,950
1, 105
823
290
150
238
925
310
1, 500
Settable
Solids
mg/1
10, 830
5, 250
1, 550
920
1, 220
Total
Solids
mg/1
252
185
11,650
2, 100
970
255
1, 110
1,680
Chemical
Oxygen
Demand
mg/1
20
__
__
20
__
-_
30
40
--
30
20
30
--
--
--
_-
_-
30
--
-_
_-
-_
30
PH
8. 3
8. 3
8. 3
7.9
7. 1
7.2
7. 3
7.6
8.0
8.2
7.2
Alkalinity
(CaCO3 )
55
__
„_
55
__
__
55
20
20
25
-_
25
--
--
__
30
-_
35
_-
-_
__
__
10
Hardness
(CaCO3)
50
_-
__
50
__
__
45
30
25
25
--
30
__
--
__
35
__
40
--
__
__
30
Chloride
(CD
mg/1
8
7. 5
7. 5
24
12
11.5
12
10
9.5
10
Nitrate
(Nitrite)
mg/1
0. 7
0.5
0.6
0. 1
0.0
0. 2
0.3
0.6
0.5
Phosphate
(PO4-)
mg/1
3.4
0.3
1.1
1.7
-------
TABLE B-3.
WATER QUALITY VERSUS RUNOFF RATE
JULY 12, 1968 (BASE FLOW)
to
to
CO
Sampling
Time
02:00
04:00
06:00
08:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
24:00
Runoff
Rate
cfs
0.04
0.04
0.04
0.04
0.04
0. 04
0.04
0.04
0.04
0.04
0. 04
0.04
Turbidity
JTU
32
35
35
32
38
45
38
40
35
38
30
35
Suspended
Solids
mg/1
15
12
10
10
14
21
16
18
11
8
12
16
Settable Total
Solids Solids
mg/1 mg/1
130
129
140
138
122
130
Chemical pH
Oxygen
Demand
mg/1
15
--
20
--
15
--
10
--
15
--
10
7.55
7.55
7.55
7. 55
7.60
7.55
7. 55
7. 55
7.60
7.55
7.60
7. 60
Alkalinity
(CaCO,)
mg/11*
45
--
40
--
40
--
40
-_
43
--
40
Hardness
(CaCO,)
mg/ld
30
--
30
--
33
--
30
__
35
--
30
Chloride Nitrate
(Cl~) (Nitrite)
mg/1 mg/1
7.5
.__
7.5
7.5
7. 5
7.5
7.5
0.32
0. 30
0.29
0. 27
0. 25
0. 27
Phosphate
(P04=)
mg/1
0.10
0.10
0.08
0. 12
0. 10
0.09
-------
TABLE B-4.
WATER QUALITY VERSUS RUNOFF RATE
JULY 15, 1968 (STORM 3)
Sampling
Time
13:00
14:30
15:10
15:20
15:30
15:45
16:00
16:15
16:30
CO 17-30
CO
^ 18:30
19:30
20:30
i i.^n
Runoff
Rate
Q
cfs
0.05
0.05
19. 50
12:50
8.20
4. 70
2. 70
2.20
1.00
0. 30
0. 30
0. 20
0. 20
n ns
Turbidity
JTU
30
35
5, 200
4, 600
4, 300
4,000
3, 600
2,600
2, 300
1, 100
1, 150
750
1, 900
Rft
Suspended
Solids
mg/1
23,
14,
10,
7,
6,
3,
3,
1,
1,
10
20
800
600
500
750
000
900
040
955
030
725
510
sn
Settable Total Chemical pH
Solids Solids Oxygen
mg/1 nag/1 Demand
mg/1
23, 300 23, 950 10
1 3 Rfto ?n
9, 800 10, 680 30
5,380 6,200 40
3,500 20
3 170 30
- - 40
- - 20
1 n
I 680 10
s
7.50
6.95
6. 55
6.35
6. 25
6.50
6.80
7.00
7. 10
7.00
7. 05
7. 15
7. 25
7 A*.
Alkalinity
(CaCCU
mg/1*
40
43
15
15
10
8
10
11
10
10
18
20
25
•}«;
Hardness Chloride Nitrate Phosphate
(CaCO3) (Cl~) (Nitrite) (PO4=)
mg/1 mg/1 mg/1 mg/1
30
30
30
25
20
25
25
20
25
23
25
28
35
qn
7.5
7/5
15.0
12.5
22.5
10
22. 5
12.5
10
9
9
9
14
7 ^
0.28
0. 3-1 -,
0. 15
0. 34
0.60
0. 20
0.60
0. 35
0. 34
0.45
0. 27
0. 28
0.32
ft O«
0. 10
0.30
0.07
1.90
4.60
0.50
5.30
2. 20
0. 15
0.18
0.27
0. 20
0. 15
ft 9£
-------
facilities were not available for determining BOD, and a GOD test was
employed instead. Due to the atypical conditions existing, however, the
results of the tests during storm flows are judged to have little value
with respect to estimating future pollution loads. The data presented
on Table B-3 is of some interest in that the base flow water quality
might be expected to be relatively independent of watershed conditions.
As the data indicate, this flow was found to be of excellent quality,
well within the expected limits. The continuation of this sampling
activity throughout the period of urbanization of this watershed would
be of considerable value in assessing the effect of construction and
subsequent population on runoff quality.
235
-------
APPENDIX C
STORM WATER STORAGE AND PRETREATMENT COSTS
RESERVOIR CONSTRUCTION COSTS
Construction costs were calculated for the five types of storm water
Storage reservoirs listed in Table 12. All types except the open ponds
have an alternate application as treated water storage facilities and the
same construction costs were employed for both applications. In addition,
other types of storage were considered for treated water only, namely,
standpipes, elevated tanks, and hydropneumatic tanks. Table C-l lists
all types of storage considered for either application. This section will
describe the methods employed to compute parametric costs for each
of these storage types. The following section will outline operation and
maintenance costs for the storm water application only. Similar costs
for treated water storage applications can be found in Appendix D.
TABLE C-l. RESERVOIR CONSTRUCTION TYPES
Application
Construction Type Storm Water Treated Water
Natural Storage
Open Pond X
Ground Level Storage
Steel Tank - Above Grade X X
Steel Tank - Below Grade X X
Concrete Tank - Above Grade X X
Concrete Tank - Below Grade X X
Elevated Storage
Standpipe x
Elevated Tank x
Pressure Storage
Hydropneumatic Tank x
237
-------
Construction costs for storage facilities were considered to be made up
of the following:
1. Land costs, applicable wherever the purchase of the land is a
necessary cost of the system.
2. Site preparation costs, including clearing and grubbing, rough
grading, finish grading, and seeding or sodding after
construction.
3. Burial costs, including excavation and backfill for tanks which
are installed below grade.
4. Erection costs, which include foundations, tank erection,
painting of metal parts, and in the case of ponds, necessary
excavation, fill, embankment, spillway construction, ;'and
bottom and slope preparation. -,'•'
In each case, actual estimates were made of construction costs for a
number of hypothetical installations of various types and- sizes in the
Columbia area. Prices of 1967 to 1968 were used and,allowances made
for engineering, inspection, and construction contingencies. The
estimates were then plotted against the appropriate size parameter and
a smooth curve extended between the points. This curve was then
replaced by a series of linear approximations which closely followed
it and the inflection points reduced to a table of sizes and costs. This
table was input to the computer in the systems analysis, so that the
cost of any size facility could be estimated by a linear interpolation
between two points on the table. In general, the original estimates
were made at sizes which permitted the curve to be well defined in the
range of possible application in the systems analysis.
LAND COSTS
In most applications, land can be purchased for a per-acre price which
is relatively uniform throughout the project area. Methods for computing
the required land area for each type and size of tank or pond were
developed,and costs were determined by simply permitting the computer
program to multiply the land area in acres by the cost in dollars per acre,
which was input separately. Computation of required land area for open
ponds required the conceptual siting of a variety of pond sizes in the
Wilde Lake Watershed and the computation of the land area. In every
case, a 30-foot border was provided beyond the high water line to protect
slopes and permit decorative fringing with trees and shrubbery. No
attempt was made to force areas into regular shapes except for ease of
computation. Figure C-l shows the resulting land area/capacity curve,
with the linear approximation overlaid. Table C-2 lists the inflection
points of the linear approximation which were input to the computer for
pond land area computations. All other types of storage were treated
as cylindrical tanks and their land requirements were computed on the
238
-------
DO
CO
CD
tn
0)
tn
O
OS
o!
CD
-a
c
cd
15 -
10 -
5 -
Estimates
A Linear Approximation
40
50 60 70
Capacity (million gallons)
80
90
100
Figure C-l. Land Area vs. Capacity - Open Ponds
-------
TABLE C-2. LAND AREA VS. CAPACITY - OPEN PONDS
Capacity Land Area
(gallons) (acres)
0.0 0.0714
10,000 0.0714
100,000 0.340
1, 000, 000 1. 690
4, 000, 000 4. 110
10,000,000 7.645
20,000,000 12.250
40, 000, 000 19. 550
100, 000, 000 36. 370
basis of height/diameter (H/D) ratios. These ratios, found by dividing
tank height by tank,diameter, were determined by plotting H/D for
standard designs of a variety of tanks. Figure C-2 shows such a plot
for ground level tanks and elevated tanks. Figure C-3 shows H/Ds for
standpipes. The corresponding linear approximations can also be seen
on these figures, and Tables C-3, C-4, and C-5 list the ratios used in
the computer program.
Required land area is found by adding 30 feet to the diameter, thus
providing for a minimum 15-foot border around the tank. This calcu-
lation is performed as follows:
BAR = 0.000018 (DIAM + 30. O)2 (C-l)
where:
BAR = required land area, acres
Since the land to be used in the Wilde Lake Watershed was entirely
public open space, the initial systems analysis computer runs were
made with land cost set at zero. Should the value of land to be used
for system construction become a significant consideration, insertion
of the appropriate per-acre cost into the computer input will result
in revised estimates.
240
-------
Elevated
Ground Level
0 1 23 4 5
67 8 9 10 11
Capacity (million gallons)
12 13 14 15
Figure C-2. H/D vs. Capacity - Ground Level and Elevated Tanks
i.o
0.5
Capacity (million gallons)
Figure 03. H/D vs. Capacity - Standpipes
241
-------
TABLE C-3. H/D VS. CAPACITY - GROUND LEVEL TANKS
Storage Capacity
(gallons) H/D
0.0 1.0.00
50,000 0.900
200,000 0.759
500,000 0.640
1,000,000 0.512
2,000,000 0.366
3,000,000 0.285
5,000,000 0.210
7,000,000 0.180
10,000,000 0.165
100,000,000 0.0348
TABLE C-4. H/D VS. CAPACITY - ELEVATED TANKS
Storage Capacity
(gallons) H/D
0.0 0.995
250,000 0.995
300,000 0.900
500,000 0.710
700,000 0.588
1,000,000 0.490
1, 250, 000 0. 442
1,500,000 0.410
TABLE C-5. H/D VS. CAPACITY - STANDPIPES
Capacity
(gallons) H/D
0.0 17.25
180,000 3.00
1,500,000 2.20
SITE PREPARATION COSTS
Since the costs treated under this heading are primarily related to land
area and not to type of construction, they were evaluated on a per-acre
basis and applied to all tanks. The single exception to this is the case
242
-------
of open ponds, where site preparation costs were considered as part of
erection costs and included in that tabulation. Figure C-4 shows the
curve plotted from estimates and the linear approximation. The table
of values used in the computer program is shown as Table C-6. Land
area is computed as described in the previous paragraph on land costs.
Per-acre site preparation costs corresponding to various capacities
are obtained by linear interpolation of the table. The interpolated value
is then multiplied by the land area to yield total site preparation cost.
TABLE C-6. SITE PREPARATION COSTS VS. CAPACITY
Capacity Site Preparation Costs
(gallons) ($/acre)
0.0 34,500
100,000 27,750
300, 000 20, 300
700,000 14,200
1,150,000 11,000
1,500,000 9,500
2, 000, 000 8, 200
2,500,000 7,400
10,000,000 3,400
100,000,000 3,400
BURIAL COSTS
Burial costs are computed only for the ground level storage tanks installed
below grade In these cases, the total cost of excavation and backfill
associated with the construction of various underground tanks was esti-
mated and the results plotted as Figure C-5. This figure also shows
the linear approximation of the curve which is represented by Table C-7.
The tank capacity is used to interpolate between points on the table,
yielding burial costs in dollars per gallon. When this figure is multiplied
by the capacity, the total burial costs are obtained.
243
-------
Estimates
Linear Approximation
Figure C-4. Site Preparation Costs vs. Capacity - Storage Tanks
0.06
Estimates
Linear Approximation
0.02
0.01
1 1 1 1
1 I 1 1 1 1 1 1 1 1
7 8 9 10 11 12 13 14 15
Capacity (million gallons)
Figure C-5. Burial Costs vs. Capacity - Storage Tanks
244
-------
TABLE C-7. BURIAL COSTS VS. CAPACITY
Capacity Burial Costs
(gallons) ($/gallon)
0.0 0.0510
100,000 0.0480
500,000 0.0390
1,000,000 0.0310
1,500,000 0.0269
2,000,000 0.0230
2,500,000 0.0206
3,000,000 0.0188
° 3, 500, 000 0.0176
4,500,000. 0.0162
10,000,000 0.0134
100,000,000 0.0134
ERECTION COSTS
Estimates were obtained from tank fabricators and erectors for in-place
costs of a variety of tank types and sizes. Table C-8 is a summary of
that information. Figure C-6 shows curves for various types of above
grade and elevated tanks. Tanks designed for burial must be constructed
somewhat differently to account for the additional stresses, and infor-
mation from fabricators indicated an approximate 15 percent increase in
the tank cost. When the burial option is selected in the program, then
15 percent is added to the tank erection cost in addition to the computation
of the burial cost. Table C-9 lists the program inputs used to describe
the erection cost capacity relationships for various tank types.
In the case of open ponds, the erection cost is highly site-dependent.
Furthermore, a tradeoff exists between excavation of the site to increase
storage and providing additional spillway elevation. In addition to being
dependent upon the relative costs of excavation and spillway and embank-
ment construction, the optimum combination is a function of the site
topography and the spillway-embankment width. In order to arrive at
an approximate parametric relationship between pond capacity and cost,
a site was selected from the Wilde Lake watershed (sub-watersheds
number 4 and 5) and a dam location determined. Using a large-scale
topographic map with one-foot contours, pond capacity was determined
for each foot of elevation of the dam up to the maximum feasible elevation
at the site. These capacities were those of completely natural ponds with
245
-------
TABLE C-8. SUMMARY OF TANK ERECTION COST ESTIMATES
Ground Level Tanks Steel
Ground Level Tanks-Concrete
Elevated Tanks
Standpipes
(1) Pittsburgh-DesMoines Steel Co.
(2) Chicago Bridge and Iron
(3) The Crom Co.
(4) The Preload Co.
Capacity
(gallons)
50, 000
100, 000
200, 000
500, 000
1,000, 000
2, 000, 000
5, 000, 000
10, 000, 000
100, 000
250, 000
;. 500, 000
750, 000
1, 000, 000
2,000, 000
5, 000, 000
10,000, 000
50,000
100, 000
200, 000
500, 000
1, 000, 000
2,000, 000
100, 000
500, 000
1, 000,000
2,000, 000
Erection
($)
10,000(1)
8,400(2)
13, 000(1)
11, 000(2)
15, 500(2)
30, 000(1)
27, 000(2)
53, 000(1)
44, 000(2)
110, 000(1)
77, 000(2)
175, 000(2)
340,000(2)
18, 500(3)
28,000(3)
45, 000(4)
45,000(3)
68, 000(4)
80,000(4-)
69,000(3)
100, 000(4)
125, 000(3)
150,000(4)
250, 000(3)
325, 000(4)
500, 000(3)
500, 000(4)
38,000(1)
30, 000 < >
45, 000(1)
38, 500 < >
60,000(1)
42, 500 < >
110,000(1)
85,000 < — >
220, 000(1)
170,000 < >
375,000(1)
290, 000 <— ->
19,000(1)
45, 000(1)
80, 000(1)
130, 000(1)
Costs
33,000(2)
44, 000(2)
52, 500(2)
95,000(2)
186,000(2)
330, 000(2)
246
-------
1000
800
» 600
400
200
Concrete
Steel
Estimates
Linear
Approximation
12 13
15
012 345 67 89 10 11
Capacity (million gallons)
Figure C-6. Erection Costs vs. Capacity - Concrete and Steel
Above Ground Tanks, Elevated Tanks, and Standpipes
300
250 -
~ 200 -
- 150 -
a
O
50 -
Estimates
Linear Approximation
10
20
30
40 50 60 70 80
Capacity (million gallons)
100
Figure C-7. Construction Costs vs. Capacity - Open Ponds
247
-------
TABLE C-9. ERECTION COSTS VS. CAPACITY - STORAGE TANKS
Ground Level Storage
Steel
Ground Level Storage
Concrete
Elevated Tanks
Standpipes
Capacity
(gallons)
10,
50,
200,
400,
700,
1, 000,
2, 000,
5, 000,
100, 000,
10,
50,
100,
300,
900,
1, 500,
2, 500,
3, 500,
5,000,
10,000,
100,000,
10,
50,
100,
200,
350,
500,
1, 000,
1, 500,
10,
50,
200,
400,
700,
1,500,
0. 0
000
000
000
000
000
000
000
000
000
0. 0
000
000
000
000
000
000
000
000
000
000
000
0. 0
000
000
000
000
000
000
000
000
0. 0
000
000
000
000
000
000
Erection Cost
($)
5,
5,
11,
21,
32,
48,
65,
122,
263,
5, 260,
15,
15,
20,
25,
51,
100,
139,
197,
259,
355,
625,
6, 250,
26,
26,
45,
55,
72,
102,
132,
258,
367,
15,
15,
20,
32,
51,
82,
137,
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
000
248
-------
no excavation. Next, capacities were recalculated for each elevation
with excavation of the basin until the incremental cost of excavation
(in dollars per gallon) equaled the incremental cost of the next change
in dam elevation. These final capacities were used to develop the cost
versus capacity curve which appears in Figure C-7. In general, exca-
vation accounted for about one-third of the final pond volume. A sum-
mary of the cost estimates appears in Table C-10. Table C-ll has the
final values used in the computer input.
OPERATION AND MAINTENANCE
In the preparation of the preliminary parametric cost data for use in
the computer program, the operation and maintenance costs for the
storm water storage facilities were considered to be under three
headings. These were:
1. Routine checks of storage and pretreatment facilities
2. Trash removal from storage facilities
3. Sediment removal from storage and pretreatment facilities
In this initial analysis, it was assumed that the routine checks of the
facilities would be conducted approximately once per week and would
require one-half man-hour per reservoir. It was assumed that trash
removal would be required on half of the inspections and an additional
two-tenths of a man-hour were allotted for these inspections.
In the generation of the parametric cost data, it was assumed that sedi-
ment would be removed by pumping from the tanks and pretreatment
facilities using tank trucks. One hour was allotted for this operation
requiring a semiskilled laborer and an equipment operator. It was
assumed that this operation would be required 25 times per year and
that the tank truck rental would be $100 per day or $8. 33 per hour or
per operation.
For the disposal of the sediment, it was assumed that a satisfactory
dumping site would be available within two miles of the reservoir. No
allowance was made for the operation and maintenance of a sediment
disposal facility in this study.
The labor rates used in this initial analysis were based on the use of
semiskilled laborers at the rate of $2.50 per hour, plus 15 percent
for fringe benefits, or $2. 88 per hour, for the routine checks, trash
removal, and as a helper on the sediment removal operations. The
labor rate for the equipment operator was estimated at $3. 50 per hour,
plus 15 percent, or $4.03 per hour.
Table C-12 summarizes the estimated cost of the routine checks and
trash and sediment removal used in the development of the parametric
249
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TABLE C-10. SUMMARY OF CONSTRUCTION COST ESTIMATES - OPEN PONDS
CO
01
o
Capacity
(gal)
52,
91,
159,
238,
339,
465,
630,
858,
1,015,
1,105,
1,780,
950
950
450
800
300
000
000
000
000
000
000
Concrete
x-unu
Excavation Area
Yards
16.
20.
23.
39.
42.
52.
70.
87.
102,
110.
151.
02
11
89
22
77
03
37
00
00
00
00
1,
1,
1,
1,
2,
Yards
87.9
161.9
263.8
393.4
560. 1
768.4
037. 0
416.7
660.0
800.0
850.0
(sq.ft.)
2, 500
4, 000
5, 500
7, 000
9, 000
11, 250
15, 000
20, 500
23,000
24,400
35,000
(Cost Items in Dollars)
Embank-
Concrete ment Excavation Clear
Cost
1, 153
1, 448
1, 720
2, 820
3, 078
3, 746
5, 063
6, 226
7, 344
7,920
10,872
Cost
30
30
30
54
60
71
101
497
520
529
840
1
1
1
1
2
3
Cost
160
360
535
810
, 050
, 320
, 300
608
,850
,,040
, 358
Rip
& Grub Rap
30
40
55
70
90
112
150
207
230
244
350
109
402
740
786
1, 142
1, 031
970
1, 878
1, 536
1, 187
3, 500
Net
Total
1,482
2, 280
3, 080
4, 540
5, 420
6, 280
7, 584
9,454
11, 480
11, 920
18, 920
Cont.
+25%
371
570
775
1, 135
1, 355
1, 570
1, 896
2, 364
2, 870
2, 980
4,730
Gross
Total
1, 853
2, 850
3, 855
5, 675
6, 775
7, 850
9, 480
11, 818
14, 305
14,900
23, 650
-------
C-ll. CONSTRUCTION COST VS. CAPACITY OPEN PONDS
Capacity Construction Costs
(gallons) ($)
0-0 1,765
50,000 1,765
200,000 5,000
1,000,000 13,290
4,000,000 33,800
10,000,000 62,600
20,000,000 99,800
TABLE C-12. SUMMARY OF SEDIMENT REMOVAL COSTS
Number of Total
(
Routine
Check
Trash
Removal
Sediment
Removal
Jccurrences
per Year
50
25
25
; Man-
hrs
0.
0.
1.
1.
5
2
0
0
Rate
2.
2.
2.
4.
88
88
88
03
Labor
Cost
1.
0.
2.
4.
44
58
88
03
Equip.
Cost
0.50
8. 33
Per
Occurrence
1
0
11
. 94
.58
.47
per
Year
$ 97.00
14.50
381.00
Total trash and sediment removal, cost per year $492. 50
Trash and sediment removal, cost per gallon runoff $0. 00004925
cost data for the computer program. These costs are based on a storage
facility receiving 10, 000, 000 gallons of runoff per year.
It is noted that this study was based on reservoirs operating in an estab-
lished and stabilized community. As such, the estimates were not based
on the quantities of sediment that would be generated during construction.
This is further discussed in Section VII.
251
-------
The parametric cost data selected and used in the computer program were
$0.00005 per gallon for runoff for both the tank type storage and open ponds,
independent of size. As discussed in Section XII, the operating and main-
tenance costs for the storage reservoirs and pretreatment unit were fur-
ther refined in making the cost estimates for the various local storage and
reuse systems. In these estimates, the costs for sediment removal from
the storage reservoirs and pretreatment unit were separately estimated
and the frequency of the various operations and man-hour estimates were
revised based on the actual design. These later operation and maintenance
costs also include the cost of chemicals used in the pretreatment units and
the labor and materials for preventive maintenance and repair.
In addition to the costs of the routine checks and cleaning of the storage
reservoir and pretreatment unit, the parametric cost input included pro-
vision for painting and repair of metal surfaces, hatchways, doors, etc,,
as well as repair to concrete structures. Ground maintenance was not
included since the normal maintenance that would be given to the open
space where the facilities are located is expected to be adequate. Minor
repairs and painting can be accomplished as part of the routine inspec-
tions noted above. Concrete structures are not expected to require sig-
nificant maintenance during the time span of the economic analysis. ., t
Metal tanks, however, must be periodically painted and repaired. Buried
tanks require maintenance of their internal surface, while tanks installed
above grade have two surfaces to be maintained. Repair and maintenance ,
experiences with a number of tanks in the Baltimore area were supplied, ,
by Whitman, Requardt and Associates, along with the number of square
feet maintained in each case. The total cost, taking into account painting
and repairing exposed metal surfaces, has been found to average $0. 05
per square foot per year.
The inside surface area of buried tanks was computed as follows:
SURI = 3. 14159 DIAM (HIGH + 0. 5 DIAM) (C-2)
where:
HIGH = tank height tank diameter times H/D ratio, feet
SURI total inside surface area, square feet
This area includes inside walls, floor, and ceiling. The outside area of
above-grade tanks was computed to include outside walls and roof as
follows:
SURO- 3. 14159 DIAM (HIGH + 0.25 DIAM) (C-3)
where:
SURO = total outside surface area, square feet
Total surface area of tanks above grade was taken as inside area plus
outside area, or SURI + SURO. Maintenance costs are then computed by
multiplying tank area by $0. 05 per square foot per year.
252
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APPENDIX D
TREATMENT SYSTEM COSTS
EQUIPMENT AND FACILITY COSTS
Construction costs for pretreatment, final treatment, pumping, and
treated water storage were determined by computing estimated costs
for each of a series of design capacities and assuming that a continuous
function exists between the calculated points. In each case, a minimum
capacity was determined, below which it was assumed that costs would
not decrease with decreasing capacity. Underground construction within
the floodplain area was assumed for pretreatment and final treatment
units, both to minimize pumping costs and overflow provisions and to
permit unobtrusive installations in residential neighborhoods. Under-
ground, ground level, and elevated options were considered for treated
water storage.
PRETREATMENT
Estimated construction costs for the pretreatment unit are listed in
Table D-l. The pH adjustment process was not included in the para-
metric costs, due to its nominal cost and the high probability of a
satisfactory pH at the Columbia location. Costs are based on a solution
feed of both coagulant and chlorine, obtained from hypochlorite. Use
of chlorine gas was also investigated, but not included in the parametric
cost analysis due to the wide range of capacities being investigated
and the probable inapplicability of chlorine gas feed at the smaller size.
The complete pretreatment construction cost/capacity function was
approximated by fitting a curve to the points developed in Table D-l.
This curve was, in turn, approximated by a series of inflection points
and straight lines, permitting the data to be input to a computer
program as a table of cost/capacity relationships. This table is shown
as Table D-2 and the two curves are illustrated on Figure D-l.
FINAL TREATMENT
Capital costs for the final treatment phase were developed in the same
manner as pretreatment costs. Facilities for each treatment level
were estimated for several capacities within the range of those being
investigated. A capital cost/capacity curve was approximated and this
curve in turn, approximated by a series of straight lines. The inflec-
tion points of this last curve were reduced to a table for input to the
computer program, permitting calculation of a cost for any size plant
by a simple linear interpolation between the appropriate points on the
table The basic cost data are summarized on Table D-3, and Figure D-2
shows the developed curves for each of treatment levels. Since no final
253
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TABLE D-l. SUMMARY OF CONSTRUCTION COST ESTIMATES - PRETREATMENT
(Costs in Dollars)
Engineering,
Excavation Structure, Contractor,
Capacity and Pipes, Tube Chemica] Construction Etc.
(gal/day) Backfill Etc. Settler Feeders Cost (@25%)
21, 600 150 315 125
130,000 280 650 750
650, 000 400 3, 350 3, 750
1,300,000 750 5,450 7,500
DO
01
TABLE D-2. CONSTRUCTION COST
Pretreatment
Capacity
(overflow rate -gallons /day)
15, 000
60, 000
150, 000
600, 000
1, 500, 000
5,500,000
250 840 210
400 2, 000 520
500 8,000 2,000
500 14, 200 3, 550
VERSUS CAPACITY - PRETREATMENT
Capital Cost
(dollars)
1, 050 -
1, 680
2, 800
9, 400
22, 200
42, 400
Total
Cost
1, 050
2, 600
10, 000
17, 750
-------
6
O
KstimatcH
Linear
Approximation
10 L
0 100 200 300 400 500 600 700 800 900 1000
Annual Runoff (million gallons /year)
500.000
400.000 I—
„ 300, 000 -I—
200,000 I—
100,000 I—
Figure D-l. Pretreatment Capacity vs. Capital Costs
Class AA
Class A
6 100 a 00 300 400 SCO 600 700 800 300 1000 1100 1200 1300 1400 IFi'&O
Finnl Treatment Plant Capacity (thousands gal/day)
Figure D-2. Final Treatment Capacity vs. Capital Cost
255
-------
TABLE D-3. SUMMARY OF CONSTRUCTION COST ESTIMATES - FINAL TREATMENT
DO
On
Capacity
gal /day
100, 000
500, 000
1, 000, 000
Capacity
gal /day
100, 000
500,000
1, 000,000
Capacity
gal /day
100, 000
500, 000
1, 000, 000
Excavation
and
Backfill
500
1, 000
1,400
Excavation
and
Backfill
5,000
14, 000
22, 000
Excavation
and
Backfill
8, 000
23, 000
35, 500
(Cost in Dollars)
TREATMENT
CLASS "B"
Structure,
Elec. , Chemical Const.
Piping, Etc. Feeders Strainer Cost
3,500
4, 800
6,000
Structure,
Elec.,
Piping,
Etc.
9, 800
38, 000
89, 350
Structure,
Elec.,
Piping,
Etc.
15, 500
61,000
125,000
400 12,400 16,800
500 22, 500 28, 800
650 31,950 40,000
CLASS "A"
Chemical
Feeders Filter Monitors
400 3, 800 1, 000
500 12,000 1,000
650 27,000 1,000
CLASS "AA"
Engr., Cont. ,
Etc. (@25%)
4,200
7, 200
10, 000
Engr. ,
Cont. ,
Const. Etc.
Cost (@25%)
20,000 5,000
66,000 16,500
140, 000 35, 000
Engr. ,
Activated Cont.,
Chemical Auto. Carbon Const. Etc.
Filter Feeders Monitors Unit Cost (@25%)
3,800 400 5,300 11,000
12,000 500 6,500 45,000
27,000 650 6,500 65,000
44,000 11,000
148,000 37,000
260,000 65,000
Total
Cost
21,000
36,000
50,000
Total
Cost
25,000
82,000
175,000
Total
Cost
55,000
185,000
325,000
-------
treatment plant is required for Class "C" water, costs are shown for
Classes "B," "A," and "AA" only. Table D-4 shows the final costs
input to the computer program.
TABLE D-4. CONSTRUCTION COST VERSUS CAPACITY -
FINAL TREATMENT
Final Treatment Capacity
(gal /day)
CLASS "B" 0.0
100,000
200,000
400,000
600, 000
800, 000
1, 000,000
1, 200, 000
1, 500, 000
CLASS "A" 0.0
100, 000
200, 000
400, 000
600, 000
800, 000
1, 000, 000
1, 200, 000
1, 500, 000
CLASS "AA" 0.0
100, 000
200, 000
400, 000
600, 000
800, 000
1, 000, 000
1, 200, 000
1, 500, 000
Capital Cost
(dollars)
18, 000
21, 000
25, 000
34, 000
37, 500
40, 000
50,000
60, 000
75, 000
15, 000
25, 000
45, 000
65, 000
100, 000
150, 000
175, 000
215, 000
265, 000
40, 000
55, 000
95, 000
155, 000
215, 000
285, 000
325, 000
385, 000
440, 000
257
-------
As in the case of the pretreatment unit, underground construction and
solution feed of all chemicals are assumed. Softening and secondary
coagulation are not included in these costs. Chlorine costs are not
shown separately, based on the assumption that the facility installed
in the pretreatment unit would provide chlorine to both injection points.
Both Classes "A" and "AA" treatment facilities will include continuous
effluent turbidity monitors to insure proper operation of the filters.
This monitor will operate recorders, producing a permanent record of
plant operation,as well as alarm systems which will be arranged to shut
down the plant in the event of malfunction. It is anticipated that the
maximum effluent turbidity can be maintained under 1. 0 Jackson Turbidity
Unit by the use of this control system.
Costs for Class "AA" treatment include more generous allowances for
equipment and instrumentation. It is anticipated that product water from
this type of installation would be introduced into an existing potable water
distribution system, thus imposing a higher standard of reliability and
fail-safe operation.
PUMPING
Costs were developed for the high-lift pumping requirement only. The
below-grade installation of the treatment units permits gravity feed
between processes and the cost of backwash or flushing pumps has been
included in the cost of the related piece of equipment. Pump costs include
control and switchgear costs and are based on a single pump for each
application. Redundant facilities were not considered due to the supple-
mental and decentralized nature of the facilities under study. Similarly,
it was not considered appropriate to cost premium-quality, continuous-
duty equipment. Table D-5 summarizes the pump cost estimates and
Figure D-3 shows the development of the final cost table. This final
table is shown as Table D-6.
TABLE D-5. SUMMARY OF CONSTRUCTION COST ESTIMATES
PUMPING FACILITIES (Cost in Dollars)
Capacity
(gal/ day)
1, 000
28, 800
72, 000
144, 000
720, 000
1, 440, 000
Pump &
Motor
110
315
570
1, 220
4, 940
10, 000
Controls
Switchgear
& Wiring
30
50
300
400
700
1, 400
Valves &
Appurt.
20
35
250
300
600
1, 000
Const.
Cost
160
400
1, 120
1, 920
6, 240
12,400
Eng. ,
Cont. , Etc.
(@25%)
40
100
280
480
1, 560
3,100
Total
Cost
200
500
1, 400
2, 400
7, 800
15,500
258
-------
CO
CJ1
CD
-69-
TJ
M
O
m
o
U
a,
a
U
2 -
O Estimates
Linear Approximation
0 100
200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Daily Demand (thousand gallons/day)
Figure D-3. Daily Demand vs. Capital Cost of Pumping Facilities
-------
TABLE D-6. CONSTRUCTION COST VERSUS CAPACITY -
PUMPING FACILITIES
Pumping Capacity
(gal /day)
0.0
1,000
10,000
30,000
50,000
100,000
200,000
400, 000
700,000
1, 000,000
1, 500, 000
Capital Cost
(dollars)
200
200
250
600
1, 000
1, 840
3,100
5, 300
8, 000
11, 000
16, 000
TREATED WATER STORAGE
Seven different configurations of treated water storage were considered
and costs developed for each of them. These alternatives can be divided
into three different modes of operation, namely:
1. Ground level storage with direct pumping into
distribution system, using storage as suction well
2. Elevated storage supplied by pumping, storage floating
on the distribution system
3. Hydropneumatic tank floating on system and supplied
by pumping
The seven storage configurations related to these modes are:
1. Ground level - steel tank below grade
2. ground level concrete tank below grade
3. Ground level - steel tank above grade
4. Ground level - concrete tank above grade
5. Elevated - elevated tank on legs or pedestal
260
-------
6. Elevated - standpipe
7. Hydropneumatic steel tank partly below grade
The development of construction costs for all except the last configuration
was discussed in Appendix C. Hydropnuematic tanks were treated as
steel tanks constructed above grade and sized at four times the required
storage. This approach simplified programming requirements and proved
to be relatively accurate.
Treated water storage facilities were sized as a function of the total daily
treated water demand. A mass diagram of hourly water demands was
analyzed to determine storage requirements. In each case, the storage
requirement was taken as 0. 35 times the total daily demand. This can
also be stated as an eight-hour storage requirement. Due to the various
configurations considered, the actual tank size should be larger than the
storage requirement. The following sizing relationships were used to
obtain tank sizes:
For ground level storage and elevated storage by elevated
tank
CAPT = 0.35 (DEMC) (D-l)
For elevated storage by standpipe
CAPT = 1.05 (DEMC) (D-2)
For hydropneumatic tanks
CAPT = 1.40 (DEMC) (D-3)
where:
CAPT = tank size in gallons
DEMC - daily treated water demand in gallons
OPERATING AND MAINTENANCE COSTS
Operation and maintenance costs were computed for each element of the
system to include the time and material necessary to insure continued
reliable operation. System operation at all levels of scale was treated
as an incremental addition to a larger system. This assumes that the
necessary labor, equipment, and materials would be provided or procured
by an existing organization whose overall size and organizational structure
would not be significantly changed by the largest system under consider-
ation in this study. Depending on the nature of the final system, the oper-
ation responsibility might devolve upon the Columbia Park and Recreation
Association, a quasi-governmental body which holds title to the public
261
-------
space in the Columbia area, including Wilde Lake and its tributaries, or
to the Howard County Metropolitan Commission, a public water and sewer
authority. In either case, the assumption of a small incremental addition
to an existing organization is valid.
Using this approach, labor costs are taken as direct cost plus an allowance
for labor fringe benefits. Materials and supplies are costed at procure-
ment cost and equipment at established local rental rates. No provision
is made for general or administrative overhead, for billing and collection,
or other nondirect items, since it was considered that the addition of the
system under study would not appreciably change any of these costs.
Wage rates and material costs are estimated on the basis of 1967-1968
prices.
Parametric costs were developed in the same manner as the construction
costs, extrapolating a curve from several calculated points. The curves
were reduced to tables of points for linear interpolation by the computer
program. All operating and maintenance costs are expressed in dollars
per day, based on yearly averages of flows and demands.
PRE TREATMENT
The principal costs associated with the operation of the preatreatment
unit are the replenishment of the various chemicals employed and the
periodic removal of accumulated sediment from the sump. Typical
chemical requirements for this process are aluminum sulfate (alum)
and calcium hypochlorite. Dosage rates have been estimated at 10 mg/4
Al2(SC>4)3 and 0. 35 mg/ & available chlorine. This requires feed rates
of 85 pounds of alum and 4. 25 pounds of calcium hypochlorite per million
gallons of runoff. Other chemicals, such as lime, polyelectrolytes,
soda ash, etc. , might be required to adequately treat waters in certain
locations, but published storm water analyses indicate that the chemicals
named will be satisfactory in most applications. All chemicals will be
batch-mixed in solution tanks and fed by metering pumps. The feeders
will be checked daily and chemicals replenished as required. Compu-
tation of operating costs did not include removal of sediment from the
pretreatment unit sump, as this operation was combined with cleaning
and inspection of the storm water reservoir and described with para-
metric costs in Appendix C. Chemical costs, electric power costs, and
costs associated with regular attention to proper operation are calculated
for the pretreatment unit and shown in Tables D-7 and D-8, as well as in
Figure D-4. These costs can be identified as operation and maintenance
costs for Class "C" treatment and are included in the costs shown for
higher levels of treatment.
262
-------
TABLE D-7. SUMMARY OF OPERATING COST ESTIMATES - TREATMENT
CLASS
"C"
Capacity
Gal /Day
100, 000
500, 000
1, 000, 000
Operation
Man/Hrs Rate
0.65
2. 00
5. 00
3. 50
3. 50
3. 50
Chemical
Cost
2.25
7. 00
17. 50
$/Day
0.45
2. 25
4. 50
Electric
Power
$/Day
8. 50
40. 50
81. 00
Misc. Main,
Supplies
$/Day
0.
4.
35.
50
25
00
. Misc.Oper.
Supplies
$/Day
0.
2.
25.
20
00
00
Contingency Total O&M
$/Day $/Day
0.
2.
5.
50
50
00
12.
55.
168.
00
00
00
CLASS
"B"
100,000 1.50 3.50 5.25 0.75 9.00 1.00
500,000 3.50 3.50 12.25 3.75 42.50 1.50
1,000,000 3.50 3.50 12.25 4.25 84,00 1.50
1. 00
1. 50
1. 50
1. 00
1. 50
1. 50
18.00
63. 00
105. 00
DO
OJ
CO
CLASS
"A"
100,000 3.00 3.50 10.50 0.85 9.00 1.15
500,000 10.00 3.50 35.00 4.25 42.50 2.75
1,000,000 13.00 3.50 45.50 8.50 85.00 3.50
1. 50
3. 50
4. 50
1. 50
2.00
3. 00
25.00
90.00
150.00
CLASS
"AA"
100, 000
500, 000
1,000,000
3. 50
11.00
14.00
3. 50
3.50
3. 50
12.25
38. 50
49.00
1. 50
7. 00
14.00
10. 00
43. 50
86. 00
1. 50
5. 00
20. 00
1. 75
6. 00
31. 00
3. 00
4. 00
5. 00
30. 00
105.00
205.00
-------
FINAL TREATMENT
Operation of the final treatment facilities consists of routine inspection
of the various items of equipment, daily tests of water quality to verify
chemical feed rates and to check automatic monitors, backwashing of
filters or strainers, replenishment of chemical feeders, etc. Preventive
maintenance is carried out on the various pieces of equipment and minor
replacement of components is performed as indicated. Solution-type
chemical feeders are used to add a precoat material to the straining
operation for Class "B" treatment and to provide a chemical coagulant
where required to the Class "A" and Class "AA" plants. It was assumed
for costing purposes that this coagulant would be aluminum sulfate
(alum), although in practice other materials might be employed and
the addition of alkalinity and pH adjusting chemicals, such as lime or
soda ash, might be required. Rechlorination will be practiced at all
treatment points, using calcium hypochlorite. Feed rates are estimated
at 50 pounds AL^SO^s and 8.5 pounds Ca(OCl)2- 4H2O per million gallons
of treated water. Daily attention to all treatment equipment has been
assumed.
Estimates also include energy cost for electric power to mixers, metering
pumps, sump pumps, etc. , as well as power for high service pumping.
Table D-7 lists the detailed costs used to develop operating and mainte-
nance costs for various sizes of treatment facilities, and Figure D-4
shows the extrapolated cost curve, as well as the approximated curve.
The table of values input to the computer program is shown as Table D-8.
TABLE D-8. OPERATION AND MAINTENANCE
COST VERSUS CAPACITY - TREATMENT
Treatment Capacity Operating & Maintenance Cost ($/day)
(gal/day) Class "C" Class "B" Class "A" Class "AA1
0.0 0.00 0.00 0.00 0.00
60,000 7.00 9.00 15.00 18.00
100,000 12.00 19.00 24.00 28.00
200,000 24.00 27.00 41.00 48.00
400,000 46.00 52.00 74.00 86.00
600,000 64.00 73.00 103.00 119.00
800,000 80.00 92.00 126.00 146.00
1,000,000 90.00 108.00 145.00 168.00
1,200,000 97.00 119.00 159.00 186.00
1,500,000 106.00 132.00 174.00 206.00
264
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02
01
cti
-O
^
M
-t-J
O
u
o
ni
OJ
~H
'cti
T5
CtJ
O
O
240
220 -
200 -
180 -
160 -
140 —
120
100
80
60 —
40 —
20
Class AA
Class A
Class B
Class C
100 200
Estimates
Linear Approximation
300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Final Treatment Plant Capacity (thousands gal/day)
Figure D-4. Operation and Maintenance Cost vs.
Capacity - Final Treatment
-------
PUMPING
Operating and maintenance costs of the high service pumping facilities
have been included in the Final Treatment costs detailed in Table D-8.
TREATED WATER STORAGE
Operation and maintenance costs were computed separately for each of
the seven storage options described previously. These costs consist of
periodic painting and repair of the tanks and are a function of the surface
area to be maintained. Appendix C describes the methods employed to
calculate the surface area for various types of cylindrical tanks. Ele-
vated tanks may be cylindrical, but are more frequently ellipsoidal. In
addition, they stand on a pedestal or on legs which have appreciable
surface area themselves. A variety of tank shapes and sizes were
analyzed. ' ' • '• '' "
In calculating total maintenance cost for an elevated tank, a cost of
$0. 05 per square foot per year was assumed.
-HJ.S. GOVERNMENT PRINTING OFFICE: 1973 514-151/1501-3 266
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
7. Report Vv.
A/. (?.•;:'ioa ?!<:••
w
Title •
$. Rep&rttyate
THE BENEFICIAL USE OF STORM WATER,
Mallory, C. W.
Hittman Associates, Inc.
9190 Red Branch Road
Columbia, Maryland 21045
8.
, - Report No.
11030DNK
: Cont'ract'/Granttfo.
68-01-0173
13. Type f Repp;,t and
Period Covered
12.
IS.
, . .
Environmental Protection Agency report
number, EPA-R2-73-139, January 1973.
This report covers work originally performed by Hittman Associates in 1968
under Contract 14-12-20. Only a limited number of reports were prepared and the pur-
pose of this report makes this information available for general distribution.
A system study was conducted to determine the technical and economic feasibility
of using small storage reservoirs throughout an urban community as a means of storm
water pollution control. Facilities were provided to treat the water prior to release or
to provide sub-potable or potable water for use in the community. A conventional
approach to controlling storm water pollution was defined for comparative purposes.
Computerized system analysis was used to select the optimal combinations of
reservoir locations, type of treatment, and type of reuse on a least cost per day basis.
Alternatives were ranked and the optimal practical solution determined considering the
constraints. It was determined that the use of local storage and treatment does repre-
sent a feasible and economical method for storm water pollution control. Further, the
use of the treated water can supply a large portion of the fresh water demands of a
typical urban residential community.
A demonstration program was planned and subsequently implemented to evaluate
erosion and sediment control practices which includes a three-and-one-half-acre lake,
evaluation of cleaning and sediment handling methods, and sampling and gaging stations
to monitor changes in water quality and hydrology during urban development.
17 a Descriptors
* Storm Runoff, "'Reservoir Storage, * Water Pollution Treatment, *W'ater Supply,
*Water Conservation, Synthetic Hydrology, Regression analysis, Water Demand,
Water Distribution, System Analysis, Optimum Development Plans, Erosion Control,
Sediment Control
* Storm Water Management, Columbia, Maryland, *Storm Water Reuse
17c. COWRRField& Group 05G
IX A •'3:l-j?iil:ty
28. Security C/asv.
(Repo.1 ••
."9. SP -nty C ;s.
21,
No. of
Pages
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
US DEPARTMENT OF THE INTERIOR
WASHINGTON D C 2O24O
Charles W. Mallory
Hittman Associates, Inc.
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