Cost and Performance of Retention Basins
335
Table 4. Illustration of costs associated with control of
wet-weather pollution loads from a hypothetical combined sewerage system
Size of Sewer District
Sanitary Sewage Flow
1220 acres;
Population
-25,000
40 gpcd;
Infiltration allowance - 820 gal/day/ac.re
Average Sanitary Sewage Flow -
1.0 MGD
Total Infiltration Allowance
-
1.0 MGD
Design Average Sewage Flow
— 2.0 MGD
4 Times Average Sanitary Flow -
4.0 MGD
Cost of Sewage Treatment Hants
-
2.0 MGD
$ 1,700,000
4.0 MGD
$3,000,000
Cost of Retention Basin
- 6.5 MG capacity
$5,800,000
Cost of Combined Sewer Separation @ $16,000/acre
$19,500,000
Alternative
Schemes for
Wet-Weather'
Pollution
Control
Net WetNet Removals
Cost in excess
Weather
in excess of
of Scheme I
Removals
Scheme I
($ million)
(1000 lb./yr).
(1000 lb./yr).
Unit Costs
($ per lb. removed
per year)
SS
BOD
SS
BOD
SS
BOD
Scheme I - Wetweather treatment in plant
designed for
dry-weather
flow
0
Scheme II - Enlargement of
treatment plant
to accommodate
4MGD
1.3
Scheme III -
Enlargement of
treatment plant
and construction
of retention
basin
7.1
Scheme IV —
Separation of
combined
sewerage system
17.8
86
28
0
0
172
56
86
28
15
47
607
161
521
133
14
53
139
134
53
106
335
167