Page 181 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal
P. 181

JWCL344_ch04_118-153.qxd  8/2/10  9:18 PM  Page 143







                                                                                            4.6 Wastewater Flows  143
                                                            Table 4.16  Residential Wastewater Flows

                                                                                     Average Flow
                                                            City                        (gpcd)
                                                            Denver, Colorado             69
                                                            Los Angeles, California      90
                                                            Milwaukee, Wsconsin          64
                                                            Phoenix, Arizona             78
                                                            Riyadh, Saudi Arabia         70
                                                            San Diego, California        58
                                                            Seattle, Washington          57
                                                            Stamford, Connecticut        80
                                                            Tampa, Florida               66
                                                            USA                          65
                                                            Conversion factor: 1 gpcd   3.785 Lpcd.

                                         so broad because of the great uncertainty regarding the magnitude of groundwater flows in
                                         leaky sewers. Where poor construction permits entrance of groundwater, the flows may be
                                         expected to vary with rainfall, thawing of frozen ground, changes in groundwater levels,
                                         and nature of the soil drained. As suggested by the diurnal flow variations shown in Fig. 4.6,
                                         seepage makes up most of the early morning flows. Tightness of sewers may be measured
                                         by blocking off a portion of the system, usually with balloons designed for the purpose,
                                         then subjecting the tested portion to an appropriate pressure and observing the rate of
                                         water loss. Various other tests have been devised. If contracts call for such tests and they
                                         are made, even occasionally, the estimates of the rate of groundwater seepage into street
                                         sewers can be greatly reduced and will have validity.
                    4.6.3  Stormwater

                                         In well-watered regions, runoff from rainfall, snow, and ice normally outstrips spent water
                                         in intensity and annual volume. Spent water reaching combined sewers is indeed so small
                                         a fraction of the design capacity that it may be omitted from calculations for combined sys-
                                         tems. Relatively large volumes of raw wastewaters and large amounts of pollutional solids
                                         are, therefore, discharged in times of heavy rain into receiving waters through overflows
                                         from combined sewers.
                                             Although stormwater runoff is seldom allowed to be introduced into spent-water sys-
                                         tems, it is difficult to enforce the necessary ordinances. Even when connections to the
                                         wastewater system can be made only by licensed drain layers, illicit connections to sanitary
                                         sewers must be expected to add to them some of the runoff from roof, yard, basement
                                         entrance, and foundation drains. Poorly sealed manhole covers permit further entrance of

                                            600                      of sprinkling water
                                           Rate of flow, per service  400  Water Wastewater
                                                                     Secondary effects




                                            200



                                              0       4      8     12     16     20     24  Figure 4.6 Flow Variations
                                                              Time of day, h                of Water and Wastewater.
   176   177   178   179   180   181   182   183   184   185   186