Cost and Performance of Retention Basins
331
THE ROLE OF A RETENTION BASIN
The basic function of a retention basin is to retain excess wet-weather flows that
would otherwise be discharged untreated and to return them, following a storm, to a
sewage treatment plant. The volume of combined sewage produced by large storms is so
great that a retention basin is ordinarily sized to reduce — rather than eliminate —
overflow, by reducing the frequency of overflow events and by reducing the amount of
combined sewage that is spilled during such events. A retention basin may also be
designed to provide treatment for combined sewage that is spilled from the basin.
PERFORMANCE OF HALIFAX RETENTION BASIN
The retention basin that serves the 168 acre combined sewer drainage area in Halifax is
a covered concrete tank with a storage volume of 0.9 million gallons. The tank was
designed to provide treatment, by chlorination and sedimentation, for combined sewage
that is discharged to the receiving water if the tank is filled.
In the period mid-June to mid-November of 1970, rainfalls large enough to result in
overflow in the absence of the retention basin occurred on 34 days. In the same period
overflows actually occurred on only 10 days, all of which had daily rainfalls in excess of
0.9 inches. A frequency analysis of 20 years of daily rainfall records at Halifax indicated
that a daily rainfall of 0.9 inches occurs, on the average, on 10 percent of days when
rainfalls are large enough to produce overflows. It was concluded that the Halifax
retention basin would eliminate an average of 90 percent of all overflow events.
In the June to September swimming period daily rainfalls large enough to produce
overflows occur, on the average, on 37 days. Bacterial analyses for the receiving water to
which the basin discharges indicate that increased bacterial counts are apparent for about
48 hours following a storm in which overflow occurs. In the absence of the basin the
quality of the receiving water would be impaired on 76 days, i.e. for more than one-half
of the swimming season.
The volume of combined sewage spilled from the Halifax basin in 1970 was estimated
to be about one-half of the overflow that would have been spilled, in the same storms, in
the absence of the tank. The volume spilled corresponded to about one-third of the total
volume that would have spilled in all storms. The fact that the tank prevented the
discharge of two-thirds of the potential overflow in a year when the number of overflow
events due to large storms was about three times the long-term average indicates that in
an average year the effect of the tank would be considerably greater.
Samples of effluent from the retention basin in 1970 were difficult to interpret
because they were relatively few in number and because of the difficulty in making
comparisons with a tank influent that varied widely in composition. It was tentatively
concluded, based on the limited data that was available, that the basin accomplished
suspended solids removal of 30 to 70 percent for detention periods of one-half hour to 3
hours (6500 to 1000 US gpd/sq. ft.). Evaluation of the disinfection efficiency of the tank
was not possible because the chlorination system did not function properly in many
storms.
The inlet to the Halifax retention basin is fitted with a coarse screen, and the basin was
designed so that the roof beams would be submerged when the tank was full in order to
function as skimmers. These devices affected one characteristic of combined sewage that
331
THE ROLE OF A RETENTION BASIN
The basic function of a retention basin is to retain excess wet-weather flows that
would otherwise be discharged untreated and to return them, following a storm, to a
sewage treatment plant. The volume of combined sewage produced by large storms is so
great that a retention basin is ordinarily sized to reduce — rather than eliminate —
overflow, by reducing the frequency of overflow events and by reducing the amount of
combined sewage that is spilled during such events. A retention basin may also be
designed to provide treatment for combined sewage that is spilled from the basin.
PERFORMANCE OF HALIFAX RETENTION BASIN
The retention basin that serves the 168 acre combined sewer drainage area in Halifax is
a covered concrete tank with a storage volume of 0.9 million gallons. The tank was
designed to provide treatment, by chlorination and sedimentation, for combined sewage
that is discharged to the receiving water if the tank is filled.
In the period mid-June to mid-November of 1970, rainfalls large enough to result in
overflow in the absence of the retention basin occurred on 34 days. In the same period
overflows actually occurred on only 10 days, all of which had daily rainfalls in excess of
0.9 inches. A frequency analysis of 20 years of daily rainfall records at Halifax indicated
that a daily rainfall of 0.9 inches occurs, on the average, on 10 percent of days when
rainfalls are large enough to produce overflows. It was concluded that the Halifax
retention basin would eliminate an average of 90 percent of all overflow events.
In the June to September swimming period daily rainfalls large enough to produce
overflows occur, on the average, on 37 days. Bacterial analyses for the receiving water to
which the basin discharges indicate that increased bacterial counts are apparent for about
48 hours following a storm in which overflow occurs. In the absence of the basin the
quality of the receiving water would be impaired on 76 days, i.e. for more than one-half
of the swimming season.
The volume of combined sewage spilled from the Halifax basin in 1970 was estimated
to be about one-half of the overflow that would have been spilled, in the same storms, in
the absence of the tank. The volume spilled corresponded to about one-third of the total
volume that would have spilled in all storms. The fact that the tank prevented the
discharge of two-thirds of the potential overflow in a year when the number of overflow
events due to large storms was about three times the long-term average indicates that in
an average year the effect of the tank would be considerably greater.
Samples of effluent from the retention basin in 1970 were difficult to interpret
because they were relatively few in number and because of the difficulty in making
comparisons with a tank influent that varied widely in composition. It was tentatively
concluded, based on the limited data that was available, that the basin accomplished
suspended solids removal of 30 to 70 percent for detention periods of one-half hour to 3
hours (6500 to 1000 US gpd/sq. ft.). Evaluation of the disinfection efficiency of the tank
was not possible because the chlorination system did not function properly in many
storms.
The inlet to the Halifax retention basin is fitted with a coarse screen, and the basin was
designed so that the roof beams would be submerged when the tank was full in order to
function as skimmers. These devices affected one characteristic of combined sewage that