332
D.H. Waller
is not identified by conventional analysis: the particulate trash and debris in surface
runoff. At two of the surface runoff sampling sites coarse screens were used to protect
the samplers, and after each storm the contents of the screen were air dried and weighed.
The average weight of material collected, per storm, was 0.23 pounds/acre at one site and
0.10 pounds/acre at the other.
The retention basin was not operated in 1969, and advantage was taken of the
opportunity to obtain samples of the receiving water for bacteriological analyses. Samples
were obtained following a number of storms in 1970 for comparison with the 1969 data.
Despite the absence of effective chlorination, the 1970 receiving water samples for storms
when the basin prevented or reduced overflow were markedly better than 1969 results for
storms of similar magnitude. On one occasion when the tank was bypassed, receiving
water quality corresponded to 1969 levels.
A RETENTION BASIN AS PART OF A POLLUTION CONTROL SYSTEM
A retention basin is an element of a system that also includes the sewage treatment
plant, which treats both sanitary sewage and wet-weather flows that are returned from
the retention basin after a storm. The interaction of the retention basin and the sewage
treatment plant, in terms of their effects on suspended solids loads, is illustrated in Fig. 1.
The solids loads used in the figure are based on values, for the Halifax area, shown in
Tables 2 and 3.
Fig. 1 assumes that the sewage treatment plant will achieve 90 percent removal of
suspended solids in dry-weather and wet-weather conditions. It is assumed, based on the
1970 performance of the Halifax basin, that the retention basin will retain two-thirds of
the combined sewage it receives (as noted earlier, this is a conservative assumption), and
that it will remove 50 percent of the solids from combined sewage that is spilled from the
tank. Consideration of rainfall experience at Halifax indicates that the sewage treatment
plant must have a capacity of four times the average dry-weather flow rate if the tank is
to be emptied, after a storm, before the beginning of the next storm.
The total solids load that is discharged from the combined system in Fig. 1 is not
particularly sensitive to changes in retention tank treatment efficiency. If a larger
proportion of the combined sewage flow was retained, the wet-weather solids discharged
from the tank, and the need for effective overflow treatment, would be further reduced;
diversion of a greater load to the treatment plant of course results in somewhat increased
dry-weather effluent loads.
Fig. 2 shows the same system in terms of BOD loads, which have been derived for the
Halifax study area on the basis of measured relative concentrations of BOD and solids in
combined sewage and surface runoff. The system in Fig. 2 is identical to that in Fig. 1,
except that the retention tank is assumed to achieve only 25 percent removal of BOD.
Two points are evident in these figures. The first is that the retention tank-sewage
treatment plant system reduces wet-weather solids by 85 percent and BOD by 80 percent,
and achieves an overall (wet- and dry-weather) solids reduction of 85 percent and a BOD
reduction of 87 percent. The second point is that the separate system produces a
wet-weather load of solids of 500 pounds/acre/year and a total load of 590
pounds/acre/year; the combined system produces a wet-weather solids load of 93
pounds/acre/year and a total load of 218 pounds/acre/year. The separate system yields a
wet-weather BOD load of 92 pounds/acre/year and a total load of 196 pounds/acre/year;
D.H. Waller
is not identified by conventional analysis: the particulate trash and debris in surface
runoff. At two of the surface runoff sampling sites coarse screens were used to protect
the samplers, and after each storm the contents of the screen were air dried and weighed.
The average weight of material collected, per storm, was 0.23 pounds/acre at one site and
0.10 pounds/acre at the other.
The retention basin was not operated in 1969, and advantage was taken of the
opportunity to obtain samples of the receiving water for bacteriological analyses. Samples
were obtained following a number of storms in 1970 for comparison with the 1969 data.
Despite the absence of effective chlorination, the 1970 receiving water samples for storms
when the basin prevented or reduced overflow were markedly better than 1969 results for
storms of similar magnitude. On one occasion when the tank was bypassed, receiving
water quality corresponded to 1969 levels.
A RETENTION BASIN AS PART OF A POLLUTION CONTROL SYSTEM
A retention basin is an element of a system that also includes the sewage treatment
plant, which treats both sanitary sewage and wet-weather flows that are returned from
the retention basin after a storm. The interaction of the retention basin and the sewage
treatment plant, in terms of their effects on suspended solids loads, is illustrated in Fig. 1.
The solids loads used in the figure are based on values, for the Halifax area, shown in
Tables 2 and 3.
Fig. 1 assumes that the sewage treatment plant will achieve 90 percent removal of
suspended solids in dry-weather and wet-weather conditions. It is assumed, based on the
1970 performance of the Halifax basin, that the retention basin will retain two-thirds of
the combined sewage it receives (as noted earlier, this is a conservative assumption), and
that it will remove 50 percent of the solids from combined sewage that is spilled from the
tank. Consideration of rainfall experience at Halifax indicates that the sewage treatment
plant must have a capacity of four times the average dry-weather flow rate if the tank is
to be emptied, after a storm, before the beginning of the next storm.
The total solids load that is discharged from the combined system in Fig. 1 is not
particularly sensitive to changes in retention tank treatment efficiency. If a larger
proportion of the combined sewage flow was retained, the wet-weather solids discharged
from the tank, and the need for effective overflow treatment, would be further reduced;
diversion of a greater load to the treatment plant of course results in somewhat increased
dry-weather effluent loads.
Fig. 2 shows the same system in terms of BOD loads, which have been derived for the
Halifax study area on the basis of measured relative concentrations of BOD and solids in
combined sewage and surface runoff. The system in Fig. 2 is identical to that in Fig. 1,
except that the retention tank is assumed to achieve only 25 percent removal of BOD.
Two points are evident in these figures. The first is that the retention tank-sewage
treatment plant system reduces wet-weather solids by 85 percent and BOD by 80 percent,
and achieves an overall (wet- and dry-weather) solids reduction of 85 percent and a BOD
reduction of 87 percent. The second point is that the separate system produces a
wet-weather load of solids of 500 pounds/acre/year and a total load of 590
pounds/acre/year; the combined system produces a wet-weather solids load of 93
pounds/acre/year and a total load of 218 pounds/acre/year. The separate system yields a
wet-weather BOD load of 92 pounds/acre/year and a total load of 196 pounds/acre/year;