334
D.H. Waller
the combined system yields 42 pounds/acre/year of BOD in wet weather and 156
pounds/acre/year in wet and dry weather. If surface runoff treatment were employed in
the separate system, efficiencies would have to exceed those assumed for the treatment of
combined sewage in the retention tank in order to produce comparable overall results.
Figs. 1 and 2 are based on information derived from the Halifax system. It is evident
that in other communities the configuration of the sewerage system, population densities,
and land use will determine solids inputs to the system, that different receiving water
characteristics and treatment cost factors may dictate different rates of effluent discharge
and different treatment plant efficiencies, and that different hydrologic factors will
determine retention basins capacities. The system that is described by these figures can be
manipulated for the conditions in any community in order to achieve an optimum
solution that will satisfy a given quality objective.
It is concluded that a retention basin is an effective method for controlling pollution
due to wet-weather discharges from a combined sewerage system, and that a combined
system that includes a retention basin can discharge smaller pollution loads than a
separate system.
COST IMPLICATIONS
Although a sewage treatment plant-retention tank system can be manipulated to
achieve any desired quality objective, the cost of the required facilities will not
necessarily be lower than that of alternative means of achieving the same objective. The
hypothetical example that is described in Table 4 is intended to illustrate the nature of
the costs associated with the use of a retention basin.
If a separate sewerage system is installed to serve the community described in Table 4
it will require a 2 mgd treatment plant. A treatment plant capacity of 4 times dry-weather
flow (4 MGD) is provided to accommodate return flows from the retention basin. A
hydraulic capacity of this magnitude may not imply a corresponding increase in cost
compared with dry-weather treatment; Escritt (1969) indicates that English treatment
plants can accept combined sewage flows of up to 6 times dry-weather flow and still yield
satisfactory effluents.
The costs shown in Table 4 are based on an Engineering News Record Index of 1600
for the year 1971. Sewage treatment costs are based on those published by the Ontario
Water Resources Commission (1965). Costs of a retention basin are based primarily on
recorded costs of two installations in the Halifax area: the 0.9 million gallon (Imp.)
Halifax tank referred to in this paper was constructed in 1965 (ENR Index 800) at a cost
of $420,000; a 0.2 million gallon (Imp.) tank was recently completed in the adjacent city
of Dartmouth at a cost of $250,000. Melpar (1970) estimated that a 0.2 million gallon
(Imp.) underwater storage facility for Cambridge, Maryland, would cost $159,000. An
average 1971 cost of $900,000 per million gallons has been used in the table. Estimates of
the cost of combined sewer separation have been made by the U.S. Public Health Service
(1964) and by Waller (1966), of $12,427/acre and $8,300/acre respectively. An assumed
1971 cost of $16,000 per acre is used in Table 4.
D.H. Waller
the combined system yields 42 pounds/acre/year of BOD in wet weather and 156
pounds/acre/year in wet and dry weather. If surface runoff treatment were employed in
the separate system, efficiencies would have to exceed those assumed for the treatment of
combined sewage in the retention tank in order to produce comparable overall results.
Figs. 1 and 2 are based on information derived from the Halifax system. It is evident
that in other communities the configuration of the sewerage system, population densities,
and land use will determine solids inputs to the system, that different receiving water
characteristics and treatment cost factors may dictate different rates of effluent discharge
and different treatment plant efficiencies, and that different hydrologic factors will
determine retention basins capacities. The system that is described by these figures can be
manipulated for the conditions in any community in order to achieve an optimum
solution that will satisfy a given quality objective.
It is concluded that a retention basin is an effective method for controlling pollution
due to wet-weather discharges from a combined sewerage system, and that a combined
system that includes a retention basin can discharge smaller pollution loads than a
separate system.
COST IMPLICATIONS
Although a sewage treatment plant-retention tank system can be manipulated to
achieve any desired quality objective, the cost of the required facilities will not
necessarily be lower than that of alternative means of achieving the same objective. The
hypothetical example that is described in Table 4 is intended to illustrate the nature of
the costs associated with the use of a retention basin.
If a separate sewerage system is installed to serve the community described in Table 4
it will require a 2 mgd treatment plant. A treatment plant capacity of 4 times dry-weather
flow (4 MGD) is provided to accommodate return flows from the retention basin. A
hydraulic capacity of this magnitude may not imply a corresponding increase in cost
compared with dry-weather treatment; Escritt (1969) indicates that English treatment
plants can accept combined sewage flows of up to 6 times dry-weather flow and still yield
satisfactory effluents.
The costs shown in Table 4 are based on an Engineering News Record Index of 1600
for the year 1971. Sewage treatment costs are based on those published by the Ontario
Water Resources Commission (1965). Costs of a retention basin are based primarily on
recorded costs of two installations in the Halifax area: the 0.9 million gallon (Imp.)
Halifax tank referred to in this paper was constructed in 1965 (ENR Index 800) at a cost
of $420,000; a 0.2 million gallon (Imp.) tank was recently completed in the adjacent city
of Dartmouth at a cost of $250,000. Melpar (1970) estimated that a 0.2 million gallon
(Imp.) underwater storage facility for Cambridge, Maryland, would cost $159,000. An
average 1971 cost of $900,000 per million gallons has been used in the table. Estimates of
the cost of combined sewer separation have been made by the U.S. Public Health Service
(1964) and by Waller (1966), of $12,427/acre and $8,300/acre respectively. An assumed
1971 cost of $16,000 per acre is used in Table 4.