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144 Chapter 4 Quantities of Water and Wastewater Flows
runoff. Total amounts vary with the effectiveness of enforcing regulations and conducting
countermeasures. Allowances for illicit stormwater flow are as high as 70 gpcd (265 Lpcd)
and average 30 gpcd (114 Lpcd). A rainfall of 1 in./h (25.4 mm/h) may shed water at a rate
2
3
2
of 12.5 gpm (47.3 L/min) from 1,200 ft (111.5 m ) of roof area, or 1.008 ft /s from an acre
3
(0.0705 m /s from a hectare) of impervious surfaces. Leaky manhole covers may admit 20
to 70 gpm (75.7 to 265 L/min) when streets are under an inch (25.4 mm) of water. The vol-
ume of illicit stormwater approximates the difference between normal dry-weather flows
(DWFs) and flows during intense rainfalls.
4.6.4 Industrial, Commercial, and Institutional Wastewaters
Industrial wastewaters may be discharged into municipal wastewater systems at conven-
ient points, provided they do not overload them or damage the collecting and treatment
works. Nevertheless, it may be advantageous to lead spent process and cooling waters into
separate disposal systems where they exist or can conveniently be built or otherwise pro-
vided. Pretreatment before discharge to the municipal sewer is also a matter for decision.
Within manufacturing plants themselves there may be rigid separation of different
process waters and other wastewaters that can be isolated as such. A metal-finishing shop,
for example, may install separate piping for each of the following: (a) strong chromic
acid; (b) other strong acids; (c) weak acid wastes, including chromium; (d) strong alkalis,
including cyanide; (e) weak alkalis; and (f) sanitary wastes. In addition, separate lines
may carry copper rinses and nickel rinses for their individual recovery. Not all lines need
be laid as underground gravity-flow conduits. Relatively small volumes of wastewaters
may be collected in sumps and pumped through overhead lines instead.
When there is good promise of reasonable recovery of water or waste matters or of
treatment simplification, spent industrial waters may be segregated even if collecting lines
must be duplicated. Examples are the pretreatment of strong wastewaters before admixture
with similar dilute wastewaters and also the separation of cyanide wastewaters for destruc-
tion by chlorine before mixing them with wastewaters containing reaction-inhibiting
nickel. However, it may also pay to blend wastewaters in order to (a) dilute strong wastes,
(b) equalize wastewater flows and composition, (c) permit self-neutralization to take place,
(d) foster other beneficial reactions, and (e) improve the overall economy. As a rule, it pays
to separate wastewaters while significant benefits can still accrue. After that they may well
be blended to advantage into a single waste stream.
Wash waters may require special collection and treatment when they differ from other
process waters. Thus, most wastewaters from food processing contain nutrients that are
amenable as such to biological treatment. However, they may no longer be so after strong
alkalis; soaps; or synthetic detergents, sanitizers, and germicides have been added to them
along with the wash waters of the industry.
4.7 VARIATIONS IN WASTEWATER FLOWS
Imprinted on flows in storm and combined sewers is the pattern of rainfall and snow and
ice melt. Fluctuations may be sharp and high for the storm rainfall itself and as protracted
and low as the melting of snow and ice without the benefit of spring thaws. As shown in
Fig. 4.6, (a) flows of spent water normally lie below and lag behind the flows of supplied
water; and (b) some of the water sprayed onto lawns and gardens is bound to escape into
yard and street drains.
The open-channel hydraulics of sewers allows their levels to rise and fall with the volume
rate of entrant waters. Rising levels store flows; falling levels release them. The damping
effect of storage is reinforced by the compositing of flows from successive upstream areas