Discussion by G. Ainsworth,
Director of City of Manchester Rivers Department, Great Britain
In many countries, including Britain, combined sewers often resulted from the diversion of sewage
into existing old surface-water systems. Once this practice had become established in a district, it was
logical that extensions to that system should still rely on combining foul sewage and surface water,
relying on storm relief overflows on the trunk sewers to prevent surcharging and flooding during really
excessive storm conditions.
The need for surface-water treatment is not so apparent, particularly when balanced against river
pollution which may occur when storm overflows come into operation. Most modern development in
Britain has tended to utilize separate foul and surface-water sewerage, and it is therefore interesting
that Professor Waller's observations have indicated that a combined sewerage system incorporating a
retention basin for excess flows shows a much more favourable benefit/cost relationship than does
mere treatment plant enlargement or separation of the combined sewerage system. Of much more
significance, however, is the more fundamental result that less polluting material found its way into
the watercourse from the cheapest of these options. The calculations have only taken capital costs,
however, ignoring the costs of maintenance tank emptying, etc. Has Professor Waller any estimates of
this additional operational cost and its effect on his overall cost comparisons?
It is unfortunate that the results were influenced by the large volume of infiltration water and their
high content of mineral solids. Settlement occurred in the sewer under dry weather conditions and the
author has assumed that 30 per cent of sanitary sewage solids were so deposited. Was it not possible to
get a more accurate assessment by using known per capita values for solids production and comparing
these with the values actually found in the dry weather sewage samples analysed?
With the much greater water consumption in North America and, in the case of Halifax N.S. at
least, the excessive rainfall intensity, it is difficult to compare British with North American practice. In
inland British towns, whilst it is rare to have storage tanks on overflows on the trunk sewerage systems
which are usually scaled to take about 6 to 8 times average dry weather flow, stormwater storage tanks
are invariably provided at the sewage works even on so-called 'separate' sewerage systems. These
usually have a capacity equivalent to 6 hours D.W.F. and, though they might subsequently overflow in
prolonged or heavy storm conditions, coarse objectionable solids are intercepted and over 50% of the
B.O.D. and suspended solids are removed as a result of the settlement. Chlorination of effluents is not
usually practised in Britain.
In the authoritative British document on this subject, the Final Report of the Ministry of Housing
and Local Government's Technical Committee on Storm Overflows and the Disposal of Storm Sewage,
1970, the committee expressed some reservations about the high cost of separate sewerage systems
and were concerned about the highly polluting character of some surface run-off, particularly around
industrial sites, which they recommended should be connected to the foul sewer. Nevertheless, they
were not in favour of construction of any new sewerage systems incorporating storm overflows.
The Technical Committee carried out a series of in-depth studies on the performance of storm
overflows at several sites and the effect of providing retention basins of different capacities on the
amounts of polluting materials which would be discharged to the watercourse.
At one such sewer overflow, in Northampton, set at 6 times average D.W.F., it was found that
storage tanks of 2 hours and 6 hours dry weather flow capacity reduced the volume discharged to the
river by 21 and 1 per cent respectively, the B.O.D. load by 40 and 63 per cent, and the suspended
solids load by 35 and 59 per cent. To obtain a similar reduction in load passing to the river would have
necessitated increasing the sewer capacity to about 9 D.W.F. and 12 D.W.F. respectively.
Similar researches were undertaken into the performance of storm tanks installed at the actual
sewage works, where the time of concentration of the whole sewerage system was much longer than
on the individual sewers. On one such scheme, receiving a D.W.F. of 1 million gallons per day
(4550m
3 /d) from 28,000 population discharged into a combined sewerage system capable of
transporting over 10 D.W.F, flows in excess of 3 D.W.F. were diverted to storage tanks. In a period of
two years, only 25.8 million gallons (117,400m
3 ) overflowed to the storm tanks and it was shown
that, for storage of 6, 9 and 12 hours, there would be 38, 49 and 60 per cent reduction in this volume
respectively discharged to the river, equivalent to 69, 74.5 and 80 per cent reduction in polluting load
respectively.
The Technical Committee therefore concluded that the existing storage capacity usually provided
(viz. 6 hours D.W.F.) was generally adequate. Any such decision must be somewhat arbitrary, but I
should be pleased to learn why Professor Waller chose 2.7 million gallons (i.e. 1.35 days D.W.F.) rather
than simple fractions or multiples of D.W.F. Has he made any calculations on the economics of smaller
or larger storage capacities?
Reply
Mr. Ainsworth has identified an important omission from the information in the paper: costs of
operation and maintenance of retention basins. The 1971 cost of maintaining the 0.9 MG tank at
337
Director of City of Manchester Rivers Department, Great Britain
In many countries, including Britain, combined sewers often resulted from the diversion of sewage
into existing old surface-water systems. Once this practice had become established in a district, it was
logical that extensions to that system should still rely on combining foul sewage and surface water,
relying on storm relief overflows on the trunk sewers to prevent surcharging and flooding during really
excessive storm conditions.
The need for surface-water treatment is not so apparent, particularly when balanced against river
pollution which may occur when storm overflows come into operation. Most modern development in
Britain has tended to utilize separate foul and surface-water sewerage, and it is therefore interesting
that Professor Waller's observations have indicated that a combined sewerage system incorporating a
retention basin for excess flows shows a much more favourable benefit/cost relationship than does
mere treatment plant enlargement or separation of the combined sewerage system. Of much more
significance, however, is the more fundamental result that less polluting material found its way into
the watercourse from the cheapest of these options. The calculations have only taken capital costs,
however, ignoring the costs of maintenance tank emptying, etc. Has Professor Waller any estimates of
this additional operational cost and its effect on his overall cost comparisons?
It is unfortunate that the results were influenced by the large volume of infiltration water and their
high content of mineral solids. Settlement occurred in the sewer under dry weather conditions and the
author has assumed that 30 per cent of sanitary sewage solids were so deposited. Was it not possible to
get a more accurate assessment by using known per capita values for solids production and comparing
these with the values actually found in the dry weather sewage samples analysed?
With the much greater water consumption in North America and, in the case of Halifax N.S. at
least, the excessive rainfall intensity, it is difficult to compare British with North American practice. In
inland British towns, whilst it is rare to have storage tanks on overflows on the trunk sewerage systems
which are usually scaled to take about 6 to 8 times average dry weather flow, stormwater storage tanks
are invariably provided at the sewage works even on so-called 'separate' sewerage systems. These
usually have a capacity equivalent to 6 hours D.W.F. and, though they might subsequently overflow in
prolonged or heavy storm conditions, coarse objectionable solids are intercepted and over 50% of the
B.O.D. and suspended solids are removed as a result of the settlement. Chlorination of effluents is not
usually practised in Britain.
In the authoritative British document on this subject, the Final Report of the Ministry of Housing
and Local Government's Technical Committee on Storm Overflows and the Disposal of Storm Sewage,
1970, the committee expressed some reservations about the high cost of separate sewerage systems
and were concerned about the highly polluting character of some surface run-off, particularly around
industrial sites, which they recommended should be connected to the foul sewer. Nevertheless, they
were not in favour of construction of any new sewerage systems incorporating storm overflows.
The Technical Committee carried out a series of in-depth studies on the performance of storm
overflows at several sites and the effect of providing retention basins of different capacities on the
amounts of polluting materials which would be discharged to the watercourse.
At one such sewer overflow, in Northampton, set at 6 times average D.W.F., it was found that
storage tanks of 2 hours and 6 hours dry weather flow capacity reduced the volume discharged to the
river by 21 and 1 per cent respectively, the B.O.D. load by 40 and 63 per cent, and the suspended
solids load by 35 and 59 per cent. To obtain a similar reduction in load passing to the river would have
necessitated increasing the sewer capacity to about 9 D.W.F. and 12 D.W.F. respectively.
Similar researches were undertaken into the performance of storm tanks installed at the actual
sewage works, where the time of concentration of the whole sewerage system was much longer than
on the individual sewers. On one such scheme, receiving a D.W.F. of 1 million gallons per day
(4550m
3 /d) from 28,000 population discharged into a combined sewerage system capable of
transporting over 10 D.W.F, flows in excess of 3 D.W.F. were diverted to storage tanks. In a period of
two years, only 25.8 million gallons (117,400m
3 ) overflowed to the storm tanks and it was shown
that, for storage of 6, 9 and 12 hours, there would be 38, 49 and 60 per cent reduction in this volume
respectively discharged to the river, equivalent to 69, 74.5 and 80 per cent reduction in polluting load
respectively.
The Technical Committee therefore concluded that the existing storage capacity usually provided
(viz. 6 hours D.W.F.) was generally adequate. Any such decision must be somewhat arbitrary, but I
should be pleased to learn why Professor Waller chose 2.7 million gallons (i.e. 1.35 days D.W.F.) rather
than simple fractions or multiples of D.W.F. Has he made any calculations on the economics of smaller
or larger storage capacities?
Reply
Mr. Ainsworth has identified an important omission from the information in the paper: costs of
operation and maintenance of retention basins. The 1971 cost of maintaining the 0.9 MG tank at
337