338
Discussion
Halifax was close to $10,000, excluding chlorine costs. Most of this cost was for labor and supervision;
the basin is so located that it drains to the interceptor by gravity following a storm, and no pumping
costs are involved. The estimated cost of maintenance of the 0.2 MG tank at Dartmouth, N.S.,
including power and chlorine, is $14,500.
If operation and maintenance costs are included, the costs shown in Table 4 of the paper will be
greater, but the conclusions to be drawn from the table are unchanged. Only the excess costs of
Schemes II and III are increased: Scheme II by the cost of maintaining a 4 MG treatment plant instead
of a 2 MG plant, and Scheme III by this cost plus the maintenance cost of the retention basin. The
extra annual cost to maintain the larger treatment plant should be about $60,000; the cost of
operating and maintaining a 6.5 MG retention basin is estimated to be $30,000. Using a 5% interest
rate, the equivalent extra capital cost is about $1.2 million for Scheme II and $1.8 million for Scheme
III, and the total costs become $2.5 million and $8.9 million. The unit costs, in dollars per pound per
year of SS and BOD removed, are increased to 29 and 89 for Scheme II and 17 and 67 for Scheme HI.
Thus, the result of introducing maintenance costs is to accentuate the benefits of Scheme III, which
includes the retention tank.
The fact that dry weather flows were diluted by high rates of infiltration did not affect calculation
of mass discharge rates, which are the product of measured flows and measured concentrations. The
high proportion of mineral solids in the combined sewage originated with surface runoff, not with
infiltration. Since only 10% of the sewers in the Halifax study area contain deposits, the total amount
of deposited material is small, and information about per capita contributions of solids is not
sufficiently precise to permit a reliable calculation of the difference between solids entering and
leaving the system in dry weather. In Table 3 and Figures 1 and 2, it was assumed that one-third (the
largest proportion reported in the literature) of the dry weather solids are deposited-in the 10% of the
sewers where deposition occurs-in order to illustrate that even with this high rate of deposition sewer
deposits would contribute only a small fraction of the solids in Halifax combined sewage. Reliable
information about the contribution that sewer deposits make to combined sewage has not come to the
author's attention.
In one of the examples described by Mr. Ainsworth, the combined sewage system is capable of
conducting 10 DWF to the treatment plant; wet-weather flows in excess of 10 DWF are presumably
overflowed at an upstream point. In this situation, a 6 hr. DWF tank at the treatment plant will be
considerably more effective than a tank of similar size would be in the example considered in the
paper, where the total storm flow in excess of treatment plant capacity must be handled by the tank.
The size of the 0.9 MG Halifax tank was based on the reduction of frequency and volume of
overflow, and on provision of a minimum chlorine detention time of 15 minutes at design peak flow.
For purposes of the hypothetical example of Table 4, this capacity was simply extrapolated to the
larger area, yielding a capacity of 6.5 MG. Information about 1970 storm rainfall at Halifax supports
the choice of a 6.5 MG basin. The basin was estimated to retain two-thirds of the combined sewage
flow, and the estimate-based on mean rainfall experience and on the performance of the Halifax
tank-was acknowledged to be conservative. In 1970 the total rainfall at Halifax in the 6-month period
May to November (in storms yielding more than 0.03 in.) was 30.4 ins., which occurred in 46 storms
with a total duration of 362.4 hours. Assuming a 30% run-off factor, this rainfall would yield 255 MG
of runoff from the hypothetical 1220 acre drainage area. The excess treatment plant capacity of 2
MGD would eliminate overflow in one storm and would reduce the wet-weather discharge of untreated
wastewater to 236 MG. Addition of 0.5 (6 hour DWF) retention basin would reduce the volume of
overflow to 194 MG, in 34 storms. A 6.5 MG tank would reduce the overflow to 57 MG in 10 storms.
There is no necessary relationship between dry weather flow rates and rates or volumes of storm
flow. The British Technical Committee concluded that a wet-weather storage capacity of 6 hours DWF
is generally adequate, but it is evident that a 6 hour DWF tank would reduce overflow frequency and
volume by only about 25 percent, compared with 78 percent reductions resulting from use of a 3.25
days DWF (6.5 MG) tank. The average annual rainfall at Halifax is 53 ins., compared with 25 to 32 in.
in the communities considered by the Technical Committee, and intensities at Halifax are about twice
those predicted by the British Ministry of Health formulae. If British rainfalls permit smaller retention
basins to be used, the economic advantages of using retention basins will be greater than those cited in
the paper.
C. Gold. USA.
What attention is given to BOD for combined sewer overflows discharging to navigable rivers? What
appurtenances are used in retention basins?
W.F. Lester, UK.
The River Tame has a population of 2Vi millions and all sewage and industrial effluents are treated
Discussion
Halifax was close to $10,000, excluding chlorine costs. Most of this cost was for labor and supervision;
the basin is so located that it drains to the interceptor by gravity following a storm, and no pumping
costs are involved. The estimated cost of maintenance of the 0.2 MG tank at Dartmouth, N.S.,
including power and chlorine, is $14,500.
If operation and maintenance costs are included, the costs shown in Table 4 of the paper will be
greater, but the conclusions to be drawn from the table are unchanged. Only the excess costs of
Schemes II and III are increased: Scheme II by the cost of maintaining a 4 MG treatment plant instead
of a 2 MG plant, and Scheme III by this cost plus the maintenance cost of the retention basin. The
extra annual cost to maintain the larger treatment plant should be about $60,000; the cost of
operating and maintaining a 6.5 MG retention basin is estimated to be $30,000. Using a 5% interest
rate, the equivalent extra capital cost is about $1.2 million for Scheme II and $1.8 million for Scheme
III, and the total costs become $2.5 million and $8.9 million. The unit costs, in dollars per pound per
year of SS and BOD removed, are increased to 29 and 89 for Scheme II and 17 and 67 for Scheme HI.
Thus, the result of introducing maintenance costs is to accentuate the benefits of Scheme III, which
includes the retention tank.
The fact that dry weather flows were diluted by high rates of infiltration did not affect calculation
of mass discharge rates, which are the product of measured flows and measured concentrations. The
high proportion of mineral solids in the combined sewage originated with surface runoff, not with
infiltration. Since only 10% of the sewers in the Halifax study area contain deposits, the total amount
of deposited material is small, and information about per capita contributions of solids is not
sufficiently precise to permit a reliable calculation of the difference between solids entering and
leaving the system in dry weather. In Table 3 and Figures 1 and 2, it was assumed that one-third (the
largest proportion reported in the literature) of the dry weather solids are deposited-in the 10% of the
sewers where deposition occurs-in order to illustrate that even with this high rate of deposition sewer
deposits would contribute only a small fraction of the solids in Halifax combined sewage. Reliable
information about the contribution that sewer deposits make to combined sewage has not come to the
author's attention.
In one of the examples described by Mr. Ainsworth, the combined sewage system is capable of
conducting 10 DWF to the treatment plant; wet-weather flows in excess of 10 DWF are presumably
overflowed at an upstream point. In this situation, a 6 hr. DWF tank at the treatment plant will be
considerably more effective than a tank of similar size would be in the example considered in the
paper, where the total storm flow in excess of treatment plant capacity must be handled by the tank.
The size of the 0.9 MG Halifax tank was based on the reduction of frequency and volume of
overflow, and on provision of a minimum chlorine detention time of 15 minutes at design peak flow.
For purposes of the hypothetical example of Table 4, this capacity was simply extrapolated to the
larger area, yielding a capacity of 6.5 MG. Information about 1970 storm rainfall at Halifax supports
the choice of a 6.5 MG basin. The basin was estimated to retain two-thirds of the combined sewage
flow, and the estimate-based on mean rainfall experience and on the performance of the Halifax
tank-was acknowledged to be conservative. In 1970 the total rainfall at Halifax in the 6-month period
May to November (in storms yielding more than 0.03 in.) was 30.4 ins., which occurred in 46 storms
with a total duration of 362.4 hours. Assuming a 30% run-off factor, this rainfall would yield 255 MG
of runoff from the hypothetical 1220 acre drainage area. The excess treatment plant capacity of 2
MGD would eliminate overflow in one storm and would reduce the wet-weather discharge of untreated
wastewater to 236 MG. Addition of 0.5 (6 hour DWF) retention basin would reduce the volume of
overflow to 194 MG, in 34 storms. A 6.5 MG tank would reduce the overflow to 57 MG in 10 storms.
There is no necessary relationship between dry weather flow rates and rates or volumes of storm
flow. The British Technical Committee concluded that a wet-weather storage capacity of 6 hours DWF
is generally adequate, but it is evident that a 6 hour DWF tank would reduce overflow frequency and
volume by only about 25 percent, compared with 78 percent reductions resulting from use of a 3.25
days DWF (6.5 MG) tank. The average annual rainfall at Halifax is 53 ins., compared with 25 to 32 in.
in the communities considered by the Technical Committee, and intensities at Halifax are about twice
those predicted by the British Ministry of Health formulae. If British rainfalls permit smaller retention
basins to be used, the economic advantages of using retention basins will be greater than those cited in
the paper.
C. Gold. USA.
What attention is given to BOD for combined sewer overflows discharging to navigable rivers? What
appurtenances are used in retention basins?
W.F. Lester, UK.
The River Tame has a population of 2Vi millions and all sewage and industrial effluents are treated