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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
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                                                                                                     3
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                                         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
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                                         (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
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