3 Analysis and Selection of Wastewater Flowrates and Constituent Loadings Determining wastewater flowrates and constituent mass loadings is a fundamental step in initiating the conceptual process design of wastewater treatment facilities. Reliable data for existing and proiected flowrates affect the hydraulic characteristics, sizing, and operational considerations of the treatment svstem components. Constituent mass loading. the product of constituent concentration and flowrate, is necessary to determine capacity and operational characteristics of the treatment facilities and ancillary uipment to ensure that treatment objectives are met. 3-1 Components of Wastewater Flows The components that make up the wastewater flow from a community depend on the type of collection system used and may include Domestic(also called sanitary) wastewater. Wastewater discharged from residences and from commercial institutional and similar facilities 2. Industrial wastewater. Wastewater in which industrial wastes predominate 3. Infiltration/inflow(I/D. Water that enters the collection system through indirect and direct means Infiltration is extraneous water that enters the collection system through leaking ioints. cracks and breaks. or porous walls. Inflow is stormwater that enters the collection system from storm drain connections (catch basins), foundation and basement drains, or through access port(manhole)covers 4. Stormwater. Runoff resulting from rainfall and snowmelt Three types of collection sy stems are used for the removal of wastewater and stormwater: sanitary collection systems, storm collection systems, and combined collection systems. Where separate collection systems are used for the collection of wastewater(sanitary collection systems) and stormwater(storm collection systems), wastewater flows in sanitary collection systems consist of three maior components: (D domestic wastewater.(2) industrial wastewater. and (3) infiltration/inflow Where only one collection system(combined) is used, wastewater flows consist of these three components plus stormwater. In both cases, the percentage of the wastewater components will vary with local conditions and the time of the 3-2 Wastewater Sources and flowrates Data that can be used to estimate average wastewater flowrates from various domestic, commercial, institutional, and industrial sources and the infiltration/inflow contribution are presented in this section Variations of the flowrates that must be established before collection systems and treatment facilities are designed are also discussed Domestic wastewater Sources and flowrates he principal sources of domestic wastewater in a community are the residential areas and commercial districts. Other important sources include institutional and recreational facilities. For areas n with collection systems. wastewater flowrates are commonly determined from existing records or by direct field measurements. For new developments. wastewater flowrates are derived from an analysis of Water consumption records may also be used for estimating flowrates. In the United States, on the average about 60 to 90 percent of the per capita water consumption becomes wastewater. The higher percentages( 90%)apply to the northern states during cold weather; the lower percentages(60%)are applicable to the semiarid region of the southwestern United States where landscape irrigation is used extensively. When water consumption records are used for estimating wastewater flowrates, the amount of water consumed for purposes such as landscape irrigation(that is not discharged to the collection system), leakage from water mains and service pipes, or product water that is used by manufacturing establishments must be evaluated carefully Residential Areas. For many residential areas, wastewater flowrates are commonly determined on population and the average per capita contribution of wastewater. For residenti and anticipated population densities. Where possible, these flowrates should be based on actual flow data projections for use in estimating wastewater flowrates was often the responsibility of the engineer b,? from selected similar communities, preferably in the same locate. In the past, the preparation of populatio today population projection data are usually available from local, regional, and state planning agencies Wastewater flowrates can vary depending on various situations such as economic, social. and other
3-1 3 Analysis and Selection of Wastewater Flowrates andConstituent Loadings Determining wastewater flowrates and constituent mass loadings is a fundamental step in initiating the conceptual process design of wastewater treatment facilities. Reliable data for existing and projected flowrates affect the hydraulic characteristics, sizing, and operational considerations of the treatment system components. Constituent mass 1oading, the product of constituent concentration and flowrate, is necessary to determine capacity and operational characteristics of the treatment facilities and ancillary equipment to ensure that treatment objectives are met. 3-1 Components of Wastewater Flows The components that make up the wastewater flow from a community depend on the type of collection system used and may include: 1. Domestic (also called sanitary) wastewater. Wastewater discharged from residences and from commercial, institutional, and similar facilities. 2. Industrial wastewater. Wastewater in which industrial wastes predominate. 3. Infiltration/inflow (I/I). Water that enters the collection system through indirect and direct means. Infiltration is extraneous water that enters the collection system through leaking joints, cracks and breaks, or porous walls. Inflow is stormwater that enters the collection system from storm drain connections (catch basins), foundation and basement drains, or through access port (manhole) covers. 4. Stormwater. Runoff resulting from rainfall and snowmelt. Three types of collection systems are used for the removal of wastewater and stormwater: sanitary collection systems, storm collection systems, and combined collection systems. Where separate collection systems are used for the collection of wastewater (sanitary collection systems) and stormwater (storm collection systems), wastewater flows in sanitary collection systems consist of three major components: (1) domestic wastewater, (2) industrial wastewater, and (3) infiltration/inflow. Where only one collection system (combined) is used, wastewater flows consist of these three components plus stormwater. In both cases, the percentage of the wastewater components will vary with local conditions and the time of the year. 3-2 Wastewater Sources and Flowrates Data that can be used to estimate average wastewater flowrates from various domestic, commercial, institutional, and industrial sources and the infiltration/inflow contribution are presented in this section. Variations of the flowrates that must be established before collection systems and treatment facilities are designed are also discussed. Domestic Wastewater Sources and Flowrates The principal sources of domestic wastewater in a community are the residential areas and commercial districts. Other important sources include institutional and recreational facilities. For areas now served with collection systems, wastewater flowrates are commonly determined from existing records or by direct field measurements. For new developments, wastewater flowrates are derived from an analysis of population data and estimates of per capita wastewater flowrates from similar communities. Water consumption records may also be used for estimating flowrates. In the United States, on the average about 60 to 90 percent of the per capita water consumption becomes wastewater. The higher percentages(90%) apply to the northern states during cold weather; the lower percentages(60%) are applicable to the semiarid region of the southwestern United States where landscape irrigation is used extensively. When water consumption records are used for estimating wastewater flowrates, the amount of water consumed for purposes such as landscape irrigation (that is not discharged to the collection system), leakage from water mains and service pipes, or product water that is used by manufacturing establishments must be evaluated carefully. Residential Areas. For many residential areas, wastewater flowrates are commonly determined on population and the average per capita contribution of wastewater. For residential areas where large residential development is planned, it is often advisable to develop flowrates on the basis of land-use areas and anticipated population densities. Where possible, these flowrates should be based on actual flow data from selected similar communities, preferably in the same locate. In the past, the preparation of population projections for use in estimating wastewater flowrates was often the responsibility of the engineer, but today population projection data are usually available from local, regional, and state planning agencies. Wastewater flowrates can vary depending on various situations such as economic, social, and other
characteristics of the community Data on ranges and typical flowrate values are given in Table 3-1 for residential sources in the United States. Beginning in recent years, greater attention is now being given to water conservation and the installation of water-conserving devices and appliances. Reduced household water use changes not only the quantity of wastewater generated but also the characteristics of wastewater Tab 3-1 Typical wastewater flowrates from urban residential sources in the U.S. Household size, number of persons Flowrate.l/capit 25-385 288 194-335 Commercial Districts. Depending on the function and activity unit flowrates for commercial facilities can vary widely because of the wide variations that have been observed every effort should be made to obtain records from actual or similar facilities. If no other records are available. estimates for selected commercial sources, based on function or persons served, may be made using the data presented in some deign manures. Sources( airport, apartment, automobile service station, bar/cocktail lounge, conference center,department store, hotel, laundry, motel, public lavatory, shopping center, theater), unit(passenger bedroom. vehicle serviced, emplovee, seat, guest). In the past, commercial wastewater flowrates were often based on existing or anticipated future development or comparative data. Flowrates were generally expressed in terms of quantity of flow per unit area i.e. m/ha.d Typical unit- flowrate allowances fo commercial developments normally range from 7. 5 to 14 m/ha.d Institutional Facilities. Typical flowrates from some institutional facilities are shown in Table 3-2. Again, it is stressed that flowrates vary with the region, climate, and type of facility. The actual records of institutions are the best Tab 3-2 Typical wastewater flowrates from institutional sources in the U.S. Unit Flowrate, L/capitad Prisor 300-570 20-60 School( day, with cafeteria 40-80 School(boarding) 280-380 Recreational facilities tewater flowrates from seasonal variations. Typical data on wastewater flowrates from recreational facilities are presented Table 3-3 Tab 3-3 Typical wastewater flowrates from recreational facilities in the U.s Facility lowrate.L/capita.d apartment,resort Person vee Person 5-110 95 With central toilet Person 130-190 Country club Member present Emplovee 38-57 Picnic club with flush toilet Ⅴsior 19-38 Customer Vacation home Person Visitor cente Visitor 10-19 Strategies for Reducing Interior Water Use and Wastewater Flowrates Because of the importance of conserving both resources and energy, various means for reducing wastewater flowrates and pollutant loadings from domestic sources are available. The reduction of 3-2
3-2 characteristics of the community. Data on ranges and typical flowrate values are given in Table 3-1 for residential sources in the United States. Beginning in recent years, greater attention is now being given to water conservation and the installation of water-conserving devices and appliances. Reduced household water use changes not only the quantity of wastewater generated but also the characteristics of wastewater as well. Tab 3-1 Typical wastewater flowrates from urban residential sources in the U.S. Household size, number of persons Flowrate,L/capita.d Range Typical 1 285-490 365 2 225-385 288 3 194-335 250 4 155-268 200 5 150-260 193 6 147-253 189 7 140-244 182 8 135-233 174 Commercial Districts. Depending on the function and activity, unit flowrates for commercial facilities can vary widely. Because of the wide variations that have been observed, every effort should be made to obtain records from actual or similar facilities. If no other records are available, estimates for selected commercial sources, based on function or persons served, may be made using the data presented in some deign mannures. Sources( airport, apartment, automobile service station, bar/cocktail lounge, conference center, department store, hotel, laundry, motel, public lavatory, shopping center, theater), unit(passenger, bedroom, vehicle serviced, employee, seat, guest). In the past, commercial wastawater flowrates were often based on existing or anticipated future development or comparative data. Flowrates were generally expressed in terms of quantity of flow per unit area [i.e., m3 /ha·d ].Typical unit-flowrate allowances for commercial developments normally range from 7.5 to 14 m3 /ha·d . Institutional Facilities. Typical flowrates from some institutional facilities are shown in Table 3-2. Again, it is stressed that flowrates vary with the region, climate, and type of facility. The actual records of institutions are the best sources of flow data for design purposes. Tab 3-2 Typical wastewater flowrates from institutional sources in the U.S. Source Unit Flowrate,L/capita.d Range Typical Assembly hall Guest 11-19 15 Hospital Bed 660-1500 1000 Prison Inmate Employee 300-570 20-60 450 40 School (day, with cafeteria only) Student 40-80 60 School (boarding) Student 280-380 320 Recreational Facilities. Wastewater flowrates from many recreational facilities are highly subject to seasonal variations. Typical data on wastewater flowrates from recreational facilities are presented in Table 3-3. Tab 3-3 Typical wastewater flowrates from recreational facilities in the U.S. Facility Unit Flowrate,L/capita.d Range Typical Apartment, resort Person 190-260 230 Cafeteria Customer Employee 8-15 30-45 10 40 Camp With toilet only Person 55-110 95 With central toilet And bath facility Person 130-190 170 Day Person 55-76 60 Country club Member present Employee 75-150 38-57 100 50 Picnic club with flush toilet Visitor 19-38 19 Swimming pool Customer Employee 19-45 30-45 40 40 Vacation home Person 90-230 190 Visitor center Visitor 10-19 15 Strategies for Reducing Interior Water Use and Wastewater Flowrates Because of the importance of conserving both resources and energy, various means for reducing wastewater flowrates and pollutant loadings from domestic sources are available. The reduction of
wastewater flowrates from domestic sources results directly from the reduction in interior water use ates for various devices and appliances are reported Table 3-4 Tab 3-4 Typical rates of water use for various devices and appliances in the U.S. Device or appliance Automatic home-type washing machine Top loading Lload 30-216 automatic home-type dish washer Lload L/use Kitchen food-waste grinder Devices and appliances that can be used to reduce interior domestic water use and wastewater flows described in Table 3-5 Tab 3-5 Flow-reduction devices and appliances in the U.s Faucet aerators nsing power of water by adding air and concentrating flow, thus educing the amount of wash water used Restricts and concentrates water passage by means of orifices that limit and e by the bather Low-flush toilets Reduces the amount of water per flush Pressure-reducing valves maintains home water pressure at a lower level than that of the water distribution system. Decreases the probability of leaks and d g faucets Toilet leak detectors ables that dissolve in the toilet tank and release dye to indicate leakage of the flush valve Vacuum toilets A vacuum along with a small amount of water is used to remove solids from Water Use in Developing Countries The typical flowrates and use patterns presented in Tables 3-1 through 3-4 are based on water use and wastewater flowrate data from communities and facilities in the United States. Many developed countries have flowrates in similar range. Water use and, consequently wastewater-generation rates in developing countries. however. are significantly lower. In some cases. the water supply is only available for limited periods of the day. Sources and rates of Industrial(Nondomestic) Wastewater Flows Nondomestic wastewater flowrates from industrial sources vary with the type and size of the facility, the degree of water reuse, and the onsite wastewater-treatment methods, if any. Extremely high peak flowrates may be reduced by the use of onsite detention tanks and equalization basins. Tvpical design values for estimating the flows from industrial areas that have no or little wet-process-type industries are 7.5 to 14 m /had for light industrial developments and 14 to 28 m/had for medium industrial developments. For cling or reuse programs. it can be assumed that about 85 to 9 percent of the water used in the various operations and processes will become wastewater. For large industries with internal water-reuse programs, separate estimates based on actual water consumption records must be made. Average domestic(sanitary) wastewater contributed from industrial facilities may ary from 30 to 95 L/capita.d Infiltration/Inflow Extraneous flows in collection systems, described as infiltration and inflow, are illustrated on Fig 3-l and are defined as follows. od pr measured Infiltration Water entering a collection system from service connections and from the ground through such means as defective pipes. pipe joints, connections, or access port(manhole) walls. drains from springs and swampy areas. This type inflow is steady and is identified and measured along Intration(inctudrgsteadyirfow) with infiltration ig 3-1 Graphic identification of infiltration/inflow Direct inflow. Those types of inflow that have a direct stormwater runoff connection to the sanitary collection system and cause an almost immediate
3-3 wastewater flowrates from domestic sources results directly from the reduction in interior water use. Representative water use rates for various devices and appliances are reported in Table 3-4. Tab 3-4 Typical rates of water use for various devices and appliances in the U.S. Device or appliance Unit Range Automatic home-type washing machine Top loading Front loading L/load 130-216 45-60 Automatic home-type dish washer L/load 36-60 Bathtub L/use 114 Kitchen food-waste grinder L/load 4-8 Shower L/min·use 9-11 Washbasin L/min·use 8-11 Devices and appliances that can be used to reduce interior domestic water use and wastewater flows described in Table 3-5. Tab 3-5 Flow-reduction devices and appliances in the U.S. Faucet aerators Increases the rinsing power of water by adding air and concentrating flow, thus reducing the amount of wash water used Flow-limiting showerheads Restricts and concentrates water passage by means of orifices that limit and divert shower flow for optimum use by the bather Low-flush toilets Reduces the amount of water per flush Pressure-reducing valves Maintains home water pressure at a lower level than that of the water distribution system. Decreases the probability of leaks and dripping faucets Toilet leak detectors Tablets that dissolve in the toilet tank and release dye to indicate leakage of the flush valve Vacuum toilets A vacuum along with a small amount of water is used to remove solids from toilet Water Use in Developing Countries The typical flowrates and use patterns presented in Tables 3-1 through 3-4 are based on water use and wastewater flowrate data from communities and facilities in the United States. Many developed countries have flowrates in similar range. Water use and, consequently, wastewater-generation rates in developing countries, however, are significantly lower. In some cases, the water supply is only available for limited periods of the day. Sources and Rates of Industrial (Nondomestic) Wastewater Flows Nondomestic wastewater flowrates from industrial sources vary with the type and size of the facility, the degree of water reuse, and the onsite wastewater-treatment methods, if any. Extremely high peak flowrates may be reduced by the use of onsite detention tanks and equalization basins. Typical design values for estimating the flows from industrial areas that have no or little wet-process-type industries are 7.5 to 14 m3 /ha·d for light industrial developments and 14 to 28 m3 /ha·d for medium industrial developments. For industries without internal water recycling or reuse programs, it can be assumed that about 85 to 95 percent of the water used in the various operations and processes will become wastewater. For large industries with internal water-reuse programs, separate estimates based on actual water consumption records must be made. Average domestic (sanitary) wastewater contributed from industrial facilities may vary from 30 to 95 L/capita·d. Infiltration/Inflow Extraneous flows in collection systems, described as infiltration and inflow, are illustrated on Fig. 3-1 and are defined as follows: Infiltration. Water entering a collection system from a variety of entry points including service connections and from the ground through such means as defective pipes, pipe joints, connections, or access port (manhole) walls. Steady inflow. Water discharged from cellar and foundation drains, cooling-water discharges, and drains from springs and swampy areas. This type of inflow is steady and is identified and measured along with infiltration. Fig. 3-1 Graphic identification of infiltration/inflow Direct inflow. Those types of inflow that have a direct stormwater runoff' connection to the sanitary collection system and cause an almost immediate
increase in wastewater flowrates. Possible sources are roof leaders. vard and areaway drains, access port covers. cross connections from storm drains and catch basins. and combined systems Total inflow. The sum of the direct inflow at any point in the system plus any flow discharged from the system upstream through overflows, pumping station bypasses, and the like Delayed inflow. Stormwater that may require several days or more to drain through the collection system Delayed inflow can include the discharge of sump pumps from cellar drainage as well as the slowed entry of surface water through access ports(manholes) in ponded areas The initial impetus in the United States for defining and identifying infiltration/inflow was the Federal Water Pollution Control Act Amendments of 1972. By correcting infiltration/inflow problems and fits to the commu and overflows in the collection system. (2) increasing the efficiency of operation of wastewater-treatmen facilities, and(3) improving the utilization of collection system hydraulic capacity for wastewater Infiltration into Collection Systems. One portion of the rainfall in a given area runs quickly into the stormwater systems or other drainage channels, another portion evaporates or is absorbed by vegetation; and the remainder percolates into the ground, becoming groundwater. The proportion of the rainfall that percolates into the ground depends on the character of the surface and soil formation and on the rate and r frost, decreases the opportunity for precipitation to become groundwater and increases the surface runoff correspondingly. The amount of ground water flowing from a given area may vary from a negligible amount for a highly impervious district or a district with a dense subsoil to 25 or 30 percent of the rainfall for a semi-pervious district with a sandy subsoil permitting rapid passage of water. The percolation of water through the ground from rivers or other bodies of water sometimes has considerable effect on the groundwater table, which rises and falls continually The presence of high groundwater resul ts in leakage into the collection systems and in an increase in the quantity of wastewater and the expense of disposing of it. The amount of flow that can enter a collection system from groundwater, or infiltration, may range from 0.01 to 1.0 m/d. mm-km or more. Infiltration may also be estimated based on the area served by the collection sy stem and may range from 0. 2 to 28 m /ha.d. The variation in the amount of infiltration encompasses a wide range because the lot sizes may vary in area, which in turn affects the length and extent of the collection system network. During heavy ains, when there may be leakage through access port covers or inflow as well as infiltration, the rate may exceed500m3/ha·d Infiltration/inflow is a variable part of the wastewater, depending on the quality of the material and workmanship in constructing the collection systems and building connections, the character of the maintenance,and the elevation of the groundwater compared with that of the collection system. The rate and quantity of infiltration depend on the length of the collection system. the area served. the soil and topographic condi nd. to a certain extent the population density( which affects the number and total length of house connections). Although the elevation of the water table varies with the quantity of rain and melting snow percolating into the ground, the leakage through defective joints, porous concrete, and cracks has been large enough, in some cases, to lower the ground water table to the level of the collection stem Most of the piping systems built during the first half of the 20th century were laid with cement mortar joints or hot poured bituminous compound ioints. Access ports were almost always constructed of brick masonry. Deterioration of pipe joints, pipe-to-access port joints, and the waterproofing of brickwork has resulted in a high potential for infiltration into these old sewers. The use of high quality pipe with dense walls, pre-cast access port sections, and joints sealed with rubber or synthetic gaskets is standard practice in modern collection-sYstem design. The use of these improved materials has greatly reduced infiltration into and exfiltration from newly constructed collection systems, and infiltration rates with time are expected to be much slower than with older sewers. Inflow into Collection Systems. The direct inflow can cause an almost immediate increase in flowrates In sanitary systems Exfiltration from Collection Systems Collection systems that have high infiltration rates and are in need of rehabilitation also may exhibit his exfiltration. When exfiltration occurs, untreated wastewater leaks out of pipe joints and service connections. If the piping and ioints are in poor condition significant quantities of wastewater may see into the ground travel through the gravel bedding of the piping system, or even surface in extreme cases. Seepage of untreated wastewater into the ground near shallow wells can result in pollution of the water supply. Well contamination has occurred in urban areas such as Los Angeles, California, where collection
3-4 increase in wastewater flowrates. Possible sources are roof leaders, yard and areaway drains, access port covers, cross connections from storm drains and catch basins, and combined systems. Total inflow. The sum of the direct inflow at any point in the system plus any flow discharged from the system upstream through overflows, pumping station bypasses, and the like. Delayed inflow. Stormwater that may require several days or more to drain through the collection system. Delayed inflow can include the discharge of sump pumps from cellar drainage as well as the slowed entry of surface water through access ports (manholes) in ponded areas. The initial impetus in the United States for defining and identifying infiltration/inflow was the Federal Water Pollution Control Act Amendments of 1972. By correcting infiltration/inflow problems and "tightening" the collection system, benefits to the community include (1) reducing wastewater backups and overflows in the collection system, (2) increasing the efficiency of operation of wastewater-treatment facilities, and (3) improving the utilization of collection system hydraulic capacity for wastewater requiring treatment instead of for infiltration/inflow. Infiltration into Collection Systems. One portion of the rainfall in a given area runs quickly into the stormwater systems or other drainage channels; another portion evaporates or is absorbed by vegetation; and the remainder percolates into the ground, becoming groundwater. The proportion of the rainfall that percolates into the ground depends on the character of the surface and soil formation and on the rate and distribution of the precipitation. Any reduction in permeability, such as that due to buildings, pavements, or frost, decreases the opportunity for precipitation to become groundwater and increases the surface runoff correspondingly, The amount of ground water flowing from a given area may vary from a negligible amount for a highly impervious district or a district with a dense subsoil to 25 or 30 percent of the rainfall for a semi-pervious district with a sandy subsoil permitting rapid passage of water. The percolation of water through the ground from rivers or other bodies of water sometimes has considerable effect on the groundwater table, which rises and falls continually. The presence of high groundwater results in leakage into the collection systems and in an increase in the quantity of wastewater and the expense of disposing of it. The amount of flow that can enter a collection system from groundwater, or infiltration, may range from 0.01 to 1.0 m3 /d·mm-km or more. Infiltration may also be estimated based on the area served by the collection system and may range from 0.2 to 28 m3 /ha·d. The variation in the amount of infiltration encompasses a wide range because the lot sizes may vary in area, which in turn affects the length and extent of the collection system network. During heavy rains, when there may be leakage through access port covers or inflow as well as infiltration, the rate may exceed 500 m3 /ha·d. Infiltration/inflow is a variable part of the wastewater, depending on the quality of the material and workmanship in constructing the collection systems and building connections, the character of the maintenance, and the elevation of the groundwater compared with that of the collection system. The rate and quantity of infiltration depend on the length of the collection system, the area served, the soil and topographic conditions, and, to a certain extent, the population density (which affects the number and total length of house connections). Although the elevation of the water table varies with the quantity of rain and melting snow percolating into the ground, the leakage through defective joints, porous concrete, and cracks has been large enough, in some cases, to lower the ground water table to the level of the collection system. Most of the piping systems built during the first half of the 20th century were laid with cement mortar joints or hot poured bituminous compound joints. Access ports were almost always constructed of brick masonry. Deterioration of pipe joints, pipe-to-access port joints, and the waterproofing of brickwork has resulted in a high potential for infiltration into these old sewers. The use of high quality pipe with dense walls, pre-cast access port sections, and joints sealed with rubber or synthetic gaskets is standard practice in modern collection-system design. The use of these improved materials has greatly reduced infiltration into and exfiltration from newly constructed collection systems, and infiltration rates with time are expected to be much slower than with older sewers. Inflow into Collection Systems. The direct inflow can cause an almost immediate increase in flowrates in sanitary systems. Exfiltration from Collection Systems Collection systems that have high infiltration rates and are in need of rehabilitation also may exhibit high exfiltration. When exfiltration occurs, untreated wastewater leaks out of pipe joints and service connections. If the piping and joints are in poor condition, significant quantities of wastewater may seep into the ground, travel through the gravel bedding of the piping system, or even surface in extreme cases. Seepage of untreated wastewater into the ground near shallow wells can result in pollution of the water supply. Well contamination has occurred in urban areas such as Los Angeles, California, where collection
systems are within 300 m of water wells. Exfiltration in collection systems near surface water bodies can also contribute to ongoing high Reduction of inflow/filtration in collection systems may serve to limit exfiltration and remove potential threats to water supplies and public health Combined System Flowrates Flow in the combined system is composed mainly of rainfall runoff and wastewater. Flow enters the combined system continuous during both dry and wet weather from the contributing wastewater sources. This flow may include domestic, commercial, and industrial astewater and infiltration During a rainfall event the amount of completely the dry weather flow patterns As modified by hydraul conditions within the system( surcharging results when the pipeline capaci exceeded) exceeded. a portion of the flow may be discharged directly into a receiving body through overflows maybe intentionally most of times rflow(CSO) treatment looding or sure cases where the combined system is undersized. ccur within the system. Either condition(untreato overdo waters or flooding) is undesirable and most likely will result in a violation of the discharge permit and/or public health regulations Fig. 3-2 Flow variations in a combined collection system during The effects of combined system flowrates are illustrated on Fig. 3-2 The catchment hydrograph(flow versus time)resembles that of the variations in rainfall intensity. The short response time between the rainfall event and the increase in the flowrate can be taken as an indication of a short travel time for flow from all points in the upstream combined system. In contrast. the hydrograph at the treatment plant shows less distinct flow peaks and a lag time of several hours for flows to return to normal drv- weather levels following rainfall cessation the higher flows at this location are due to the larger contributing combined system, and the smoothed peaks result from loss of flow through overflows and hydraulic routing effects. The peak flowrates and accompanying mass loadings. however. must be accounted for in the hydraulic d of the treatment plant and in the selection of appropriate unit operations and processes. Calculation of flowrates in a combined system is a complicated and challenging task. The first step in the process involves quantifying wastewater, rainfall runoff, other sources of flow such as groundwater infiltration These sources of flow are then combined and routed through the various components of the treatment facility, or being transported to other points in the system are determina htering the downstream system. Finally, the volumes of flow exiting the system through CSO outlets, er 3-3 Analysis of Wastewater Flowrate Data Because the hydraulic design of both collection and treatment facilities is affected by variations in wastewater flowrates, the flowrate characteristics have to be analyzed carefully from existing records. In flowrates may differ somewhat from the flowrate entering the treatment plant because of the flow-dampening effect of the sewer system. Peak hourly flowrates may also be attenuated by the available storage capacity in the sewer system. Definition of terms Before considering the variations in flowrates and constituent concentrations, it will be helpful to define some terminology that is used commonly to quantify the variations that are observed. The principal term used to describe these observed variations are defined in Table 3-6. These terms are also of importance in the selection and sizing of individual unit treatment processes and operations
3-5 systems are within 300 m of water wells. Exfiltration in collection systems near surface water bodies can also contribute to ongoing high coliform counts in those water bodies that may be difficult to correct. Reduction of inflow/filtration in collection systems may serve to limit exfiltration and remove potential threats to water supplies and public health. Combined System Flowrates Flow in the combined system is composed mainly of rainfall runoff and wastewater. Flow enters the combined system continuously during both dry and wet weather from the contributing wastewater sources. This flow may include domestic, commercial, and industrial wastewater and infiltration. During a rainfall event, the amount of storm flow is normally much larger than the dry-weather wastewater flow, and the observed flows during wet weather can mask completely the dry weather flow patterns. As flow proceeds through the combined system to the interceptor, it is modified by hydraulic routing effects as well as any surcharged conditions within the system (surcharging results when the pipeline capacity is exceeded). When the collection system capacity is exceeded, a portion of the flow may be discharged directly into a receiving body through overflows(maybe intentionally most of times), or routed to a special combined sewer overflow (CSO) treatment facility. In some cases where the combined system is undersized, flooding or surcharging may occur at various upstream locations within the system. Either condition (untreated overflow to receiving waters or flooding) is undesirable and most likely will result in a violation of the discharge permit and/or public health regulations. Fig. 3-2 Flow variations in a combined collection system during wet weather The effects of combined system flowrates are illustrated on Fig. 3-2. The catchment hydrograph (flow versus time) resembles that of the variations in rainfall intensity. The short response time between the rainfall event and the increase in the flowrate can be taken as an indication of a short travel time for flow from all points in the upstream combined system. In contrast, the hydrograph at the treatment plant shows less distinct flow peaks and a lag time of several hours for flows to return to normal dry-weather levels following rainfall cessation. The higher flows at this location are due to the larger contributing combined system, and the smoothed peaks result from loss of flow through overflows and hydraulic routing effects. The peak flowrates and accompanying mass loadings, however, must be accounted for in the hydraulic design of the treatment plant and in the selection of appropriate unit operations and processes. Calculation of flowrates in a combined system is a complicated and challenging task. The first step in the process involves quantifying wastewater, rainfall runoff, other sources of flow such as groundwater infiltration. These sources of flow are then combined and routed through the various components of the system. Finally, the volumes of flow exiting the system through CSO outlets, entering the downstream treatment facility, or being transported to other points in the system are determined. 3-3 Analysis of Wastewater Flowrate Data Because the hydraulic design of both collection and treatment facilities is affected by variations in wastewater flowrates, the flowrate characteristics have to be analyzed carefully from existing records. In cases where only flowrate data in the collection system is available, it must be recognized that the flowrates may differ somewhat from the flowrate entering the treatment plant because of the flow-dampening effect of the sewer system. Peak hourly flowrates may also be attenuated by the available storage capacity in the sewer system. Definition of Terms Before considering the variations in flowrates and constituent concentrations, it will be helpful to define some terminology that is used commonly to quantify the variations that are observed. The principal terms used to describe these observed variations are defined in Table 3-6. These terms are also of importance in the selection and sizing of individual unit treatment processes and operations