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 Sourees 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 bm,? 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 of flow data for design purposes Tab3-2 Typical wastewater flowrates from institutional sources in the U.S. Unit Flowrate, L/capitad Prisor 300-570 0-60 School( day, with cafeteria 40-80 School(boarding) 280-380 Recreational Facilities. Wastewater 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 llet o Person 95 With central toilet Person 130-190 Employee 38-57 Picnic club with flush toilet Ⅴsior 19-38 Customer Vacation home Person 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 Increases the rinsing 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 dripping faucets Toilet leak detectors blets 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. th e water su upply is onlv 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 ioints. connections, or ss 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/inflo Those of inflow that hay 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, yard and areaway drains, access port covers, cross c eatment ection sy iltration may al may vary in area ing heavy rains, w workma mainte length cracks has bee system into the grour Seepage of untreated waste wat supply. Well contamination has d 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 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 continuousl during both dry and wet weather from the contributing wastewater ources. 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 fows during wet weather can mask completely the dry weather flow patterns. As eeds throu conditions within the system(surcharging results when the pipe capaci exceeded) exceeded. a portion of the flow may be discharged directly into a receiving body through overflows( maybe intentionally most of times) facility. In some cases where the combined system is undersized ooding or surcharging 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 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 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 treatment facility, or being transported to other points in the system are determined htering the downstream system. Finally, the volumes of flow exiting the system through CSO outlets 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
Tab3-6 Terminology used to quantify observed variations in flowrate and constituent concentrations Average dry-weather flow(ADWF) e of the daily flows sustained during dry-weather periods with e average of the daily flows sustained during wet-weather periods when ation is a factor annua The average flowrate occurring over a 24-h period based on annual flowrate data Highest record flowrate occurring for a period consistent with the recording quipment. In many situations the recorded peak flow may be considerably below the actual peak flow because of metering and recording equipment Peak hour The average of the peak flows sustained for a period of l hour in the record Maximum day, Max imum mont Minimum ho Minimum day Variations in Wastewater Flowrates Wastewater flowrates vary during the time of day day of the week, season of the vear, or depending upon the nature of the dischargers to the collection system. Short-term. seasonal. and industrial variations in wastewater flowrates are briefly discussed here Short-Term Variations. The variations in wastewater flows observed at treatment plants tend to follow a diurnal pattern, as shown on Fig. 3-3 Fig. 3-3 Typical hourly variations in domestic wastewater Midnight flowrates Minimum flows occur during the early morning hours when water consumption is lowest and when the base flow consists of infiltration and small quantities of sanitary wastewater. The first peak flow generall ccurs in the late morning when wastewater from the peak morning water use reaches the treatment plant between 7 and 9 PM. The time of occurre and the amplitude of the peak flowrates collection system. As the community size increases, the variations between the high and low flows decrease due to(D the increased storage in the collection system of large communities that tends to equalize flowrates and(2) changes in the economic and social makeup of the community. When Seasonal Variations. Seasonal variations in domestic wastewater flows are commonly observed at resort areas. in small communities with college campuses, and in communities that have seasonal commercial and industrial activities. The magnitude of the variations to be expected depends on both the size of the community and the seasonal activity Industrial Variations. Industrial wastewater discharges are difficult to predict. Many manufacturing facilities generate relatively constant flowrates during production, but the flowrates change markedly during cleanup and shutdown. While internal process changes may lead to reduced discharge rates, plant expansion and increased production may lead to increased wastewater generation. Where joint municipal and industrial treatment facilities are to be constructed, special attention should be given to industrial flowrate projections, whether they are prepared by the industry or jointly with the city's staff or engineering consultant. Industrial discharges are most troublesome in smaller wastewater-treatment plants where there is limited capacity to absorb shock loa Wastewater Flowrate Factors Quantifying the variations in flowrates is important in the design and operation of wastewater-treatment lants. One of the measures used in determining the"peak"or maximum flows is the peaking factor Peaking factors can be developed based on maximum hour. maximum day. maximum month or other time periods. Tile peaking factor is particularly useful in estimating the maximum hydraulic conditions that might occur and have to be accommodated Peaking factors can also be applied to mass loadings. Peaking factors are applied most frequently to determine the peak hourly flowrate. Because it is difficult to compare numerical peak flow values from different wastewater-treatment plants, peak hourly flowrate values are normalized by dividing by the long-term average flowrate. the resultant ratio. known as a
3-6 Tab 3-6 Terminology used to quantify observed variations in flowrate and constituent concentrations Item Description Average dry-weather flow(ADWF) The average of the daily flows sustained during dry-weather periods with limited infiltration Average wet-weather flow(AWWF) The average of the daily flows sustained during wet-weather periods when infiltration is a factor Average annual daily flow(AADF) The average flowrate occurring over a 24-h period based on annual flowrate data Instantaneous peak Highest record flowrate occurring for a period consistent with the recording equipment. In many situations the recorded peak flow may be considerably below the actual peak flow because of metering and recording equipment limitations Peak hour The average of the peak flows sustained for a period of 1 hour in the record examined Maximum day, Maximum month, Minimum hour, Minimum day, Minimum month… … Variations in Wastewater Flowrates Wastewater flowrates vary during the time of day, day of the week, season of the year, or depending upon the nature of the dischargers to the collection system. Short-term, seasonal, and industrial variations in wastewater flowrates are briefly discussed here. Short-Term Variations. The variations in wastewater flows observed at treatment plants tend to follow a diurnal pattern, as shown on Fig. 3-3. Fig. 3-3 Typical hourly variations in domestic wastewater flowrates Minimum flows occur during the early morning hours when water consumption is lowest and when the base flow consists of infiltration and small quantities of sanitary wastewater. The first peak flow generally occurs in the late morning when wastewater from the peak morning water use reaches the treatment plant. A second peak flow generally occurs in the early evening between 7 and 9 P.M. The time of occurrence and the amplitude of the peak flowrates vary with the size of the community and the length of the collection system. As the community size increases, the variations between the high and low flows decrease due to (1) the increased storage in the collection system of large communities that tends to equalize flowrates and (2) changes in the economic and social makeup of the community. When extraneous flows are minimal, wastewater discharge curves resemble water consumption curves, but with a lag of several hours. Seasonal Variations. Seasonal variations in domestic wastewater flows are commonly observed at resort areas, in small communities with college campuses, and in communities that have seasonal commercial and industrial activities. The magnitude of the variations to be expected depends on both the size of the community and the seasonal activity. Industrial Variations. Industrial wastewater discharges are difficult to predict. Many manufacturing facilities generate relatively constant flowrates during production, but the flowrates change markedly during cleanup and shutdown. While internal process changes may lead to reduced discharge rates, plant expansion and increased production may lead to increased wastewater generation. Where joint municipal and industrial treatment facilities are to be constructed, special attention should be given to industrial flowrate projections, whether they are prepared by the industry or jointly with the city's staff or engineering consultant. Industrial discharges are most troublesome in smaller wastewater-treatment plants where there is limited capacity to absorb shock loadings. Wastewater Flowrate Factors Quantifying the variations in flowrates is important in the design and operation of wastewater-treatment plants. One of the measures used in determining the "peak" or maximum flows is the peaking factor. Peaking factors can be developed based on maximum hour, maximum day, maximum month, or other time periods. Tile peaking factor is particularly useful in estimating the maximum hydraulic conditions that might occur and have to be accommodated. Peaking factors can also be applied to mass loadings. Peaking factors are applied most frequently to determine the peak hourly flowrate. Because it is difficult to compare numerical peak flow values from different wastewater-treatment plants, peak hourly flowrate values are normalized by dividing by the long-term average flowrate. The resultant ratio, known as a
peaking factor, is defined as follows Sustained peaking factor, PF-/peak flowrate(eg, houry, daily average long-term flowrate The most common method of determining the peaking factor is from the analysis of flowrate data. Where flowrate records are available, at least 3 years of data should be analyzed to define the peak to average day 3-5 Analysis of Constituent Mass Loading Data The analysis of wastewater data involves the determination of the flowrate and mass loading variations The analysis may involve determining the concentrations of specific constituents, mass loadings, or sustained mass loadings, loadings that occur over a defined period of time From the standpoint of treatment processes. one of the most serious deficiencies results when the design of a treatment plant is based on average flowrates and average BOD and TSS loadings, with little or no can reach two or more ust also flowrates and BOD and TSS mass-loading rates do not occur at the same time Analysis of current records is the best method of arriving at appropriate peak and sustained mass loadings The principal factors responsible for loading variations are (1)the established habits of community esidents, which cause short-term(hourly, daily, and weekly) variations; (2) seasonal conditions, which usually cause longer-term variations; and(3)industrial activities, which cause both long- and short-term Wastewater Constituent Concentrations The physical, chemical, and biological characteristics of wastewater vary throughout the day. An adequate determination of the waste characteristics will result only if the sample tested is representative. Typically, omposite samples made up of portions of samples collected at regular intervals during a day are used. The amount of liquid used from each sample is proportional to the rate of flow at the time the sample was collected. Adequate characterization of wastewater is of fundamental importance in the design of treatment and disposal processes Quantity of Waste Discharged by Individuals in the United States. Typical data on the total quantities of waste discharged per person per day(dry weight basis) from individual residences are reported in Table 3-7. The data have been gathered from numerous sources (primarily the United States). The total number of pathogenic organisms discharged will depend on whether an individual is ill and is shedding pathogens. If one or more members of a family are ill and several orders of magnitude Tab. 3-7 Quantity of waste discharged by individual on a dry weight basis Range Typical without ground-up kitchen Typical with ground-up kitchen waste CODcr tkn as N Rgp as p 09-18 Composition of Wastewater in Collection Systems. Typical data on the composition of untreated domestic wastewater as found in wastewater-collection systems (in the United States)are reported in Table 3-8. The data presented in this table for medium-strength wastewater are based on an average flow of 460 L/capita.d and include constituents added by commercial, institutional, and industrial sources Typical concentrations for low-strength and high-strength wastewater, which reflect different amounts of infiltration, are also given. Be ere is no"typical"wastewater, it must be emphasized that the typical data presented in Table 3-8 should only be used as a guide. Tab. 3-8 Typical composition of untreated domestic wastewater Contaminants Low strengt High strength TDS 3-7
3-7 peaking factor, is defined as follows: Sustained peaking factor, PF=[peak flowrate(e.g., hourly, daily)]/(average long-term flowrate) The most common method of determining the peaking factor is from the analysis of flowrate data. Where flowrate records are available, at least 3 years of data should be analyzed to define the peak to average day peaking factor. 3-5 Analysis of Constituent Mass Loading Data The analysis of wastawater data involves the determination of the flowrate and mass loading variations. The analysis may involve determining the concentrations of specific constituents, mass loadings, or sustained mass loadings, loadings that occur over a defined period of time. From the standpoint of treatment processes, one of the most serious deficiencies results when the design of a treatment plant is based on average flowrates and average BOD and TSS loadings, with little or no recognition of peak conditions. In many communities, peak influent flowrates and BOD and TSS loadings can reach two or more times average values, it must also be emphasized that, in nearly all cases, peak flowrates and BOD and TSS mass-loading rates do not occur at the same time. Analysis of current records is the best method of arriving at appropriate peak and sustained mass loadings. The principal factors responsible for loading variations are (1) the established habits of community residents, which cause short-term (hourly, daily, and weekly) variations; (2) seasonal conditions, which usually cause longer-term variations; and (3) industrial activities, which cause both long- and short-term variations. Wastewater Constituent Concentrations The physical, chemical, and biological characteristics of wastewater vary throughout the day. An adequate determination of the waste characteristics will result only if the sample tested is representative. Typically, composite samples made up of portions of samples collected at regular intervals during a day are used. The amount of liquid used from each sample is proportional to the rate of flow at the time the sample was collected. Adequate characterization of wastewater is of fundamental importance in the design of treatment and disposal processes. Quantity of Waste Discharged by Individuals in the United States. Typical data on the total quantities of waste discharged per person per day (dry weight basis) from individual residences are reported in Table 3-7. The data have been gathered from numerous sources (primarily the United States). The total number of pathogenic organisms discharged will depend on whether an individual is ill and is shedding pathogens. If one or more members of a family are ill and shedding pathogens, the number of measured organisms can increase by several orders of magnitude. Tab. 3-7 Quantity of waste discharged by individual on a dry weight basis constituents Value,g/capita·d Range Typical without ground-up kitchen waste Typical with ground-up kitchen waste BOD5 50-120 80 100 CODcr 110-295 190 220 TSS 60-150 90 110 NH3 as N 5-12 7.6 8.4 Org N as N 4-10 5.4 5.9 TKN as N 9-21.7 13 14.3 Org P as P 0.9-1.8 1.2 1.3 Inorg P as P 1.8-2.7 2.0 2.2 TP 2.7-4.5 3.2 3.5 Oil and grease 10-40 30 34 Composition of Wastewater in Collection Systems. Typical data on the composition of untreated domestic wastewater as found in wastewater-collection systems (in the United States) are reported in Table 3-8. The data presented in this table for medium-strength wastewater are based on an average flow of 460 L/capita·d and include constituents added by commercial, institutional, and industrial sources. Typical concentrations for low-strength and high-strength wastewater, which reflect different amounts of infiltration, are also given. Because there is no "typical" wastewater, it must be emphasized that the typical data presented in Table 3-8 should only be used as a guide. Tab. 3-8 Typical composition of untreated domestic wastewater Contaminants Unit Concentration Low strength Medium strength High strength TS mg/L 390 720 1230 TDS Fixed mg/L 270 160 500 300 860 520
Volatile CODer NH3-N 12 4 0 0 POJ-P 400 Fecal coliform No /100mL 10-10 Mineral Increase Resulting from Water Use. Data on the increase in the mineral content of wastewater resulting from water use, and the variation of the increase within a collection system, are especially important in evaluating the reuse potential of wastewater. Typical data on the incremental increase in mineral content that can be expected in municipal wastewater resulting from domestic use are reported in Table 3-9. Increases in the mineral content of wastewater may be due in part to addition of highly mineralized water from private wells and groundwater and from industrial use. Domestic and will(because of its high quality ) serve to dilute the mineral concentration in wastewater ev industrial water softeners also contribute significantly to the increase in mineral content and, in some areas, ay represent the major source. Occasionally, water added from private wells and groundwater infiltration Tab. 3-9 Typical mineral increase from domestic water use Constituent ncrement range(mg/L) Constituent Anions Other constituents Bicarbonate(HCO3) Aluminum(Al) Chloride(ci) alcium( Ca) otal alkalinity (as CacO3) Magnesium(Mg) 150-380 Variations in Constituent Concentrations Several types of constituent concentration variations can occur depending upon the characteristics of the contributors to the wastewater-collection system. Types of variations are discussed below Short-Term Variation in Constituent Values. Constituent concentration variations may change significantly during the course of a day. An example typical variations in domestic wastewater strength is shown on Fig 3-4. The bod variation generally follows the flow c matter) concentration often occurs in lara seasonal Variation in Constituent Values. For domestic flow only, and neglecting the effects of infiltration, the unit (per capita) loadings and the strength of the wastewater from most seasonal sources, such as resorts, will remain about the daily basis throughout the total flowrate varies. The total mass of bod and tss of the wastewater, however, will increase directly with the population served Fig. 3-4 Typical hourly variations in flow and strength Time ot domestic wastewater
3-8 Volatile 110 200 340 TSS Fixed Volatile mg/L 120 25 95 210 50 160 400 85 315 Settlebale Solids mg/L 5 10 20 BOD5 mg/L 110 190 350 TOC mg/L 80 140 260 CODcr mg/L 250 430 800 TN Org-N NH3-N NO2-N NO3-N mg/L 20 8 12 0 0 40 15 25 0 0 70 25 45 0 0 TP Org-P PO4-P mg/L 4 1 3 7 2 5 12 4 10 Cl mg/L 30 50 90 SO4 2- mg/L 20 30 50 Oil and grease mg/L 50 90 100 VOCs mg/L 400 Total coliform No./100mL 106 -108 107 -109 107 -1010 Fecal coliform No./100mL 103 -105 104 -106 105 -108 Mineral Increase Resulting from Water Use. Data on the increase in the mineral content of wastewater resulting from water use, and the variation of the increase within a collection system, are especially important in evaluating the reuse potential of wastewater. Typical data on the incremental increase in mineral content that can be expected in municipal wastewater resulting from domestic use are reported in Table 3-9. Increases in the mineral content of wastewater may be due in part to addition of highly mineralized water from private wells and groundwater and from industrial use. Domestic and industrial water softeners also contribute significantly to the increase in mineral content and, in some areas, may represent the major source. Occasionally, water added from private wells and groundwater infiltration will (because of its high quality) serve to dilute the mineral concentration in wastewater. Tab. 3-9 Typical mineral increase from domestic water use Constituent Increment range(mg/L) Constituent Increment range(mg/L) Anions: Bicarbonate(HCO3) Carbonate(CO3) Chloride(Cl) Sulfate(SO4 2- ) 50-100 0-10 20-50 15-30 Other constituents: Aluminum(Al) Boron(B) Fluoride(F) Manganese(Mn) Silica(SiO2) Total alkalinity(as CaCO3) TDS 0.1-0.2 0.1-0.2 0.2-0.4 0.2-0.4 2-10 60-120 150-380 Cations: Calcium(Ca) Magnesium(Mg) Potassium(K) Sodium(Na) 6-16 4-10 7-15 40-70 Variations in Constituent Concentrations Several types of constituent concentration variations can occur depending upon the characteristics of the contributors to the wastewater-collection system. Types of variations are discussed below. Short-Term Variation in Constituent Values. Constituent concentration variations may change significantly during the course of a day. An example of typical variations in domestic wastewater strength is shown on Fig. 3-4. The BOD variation generally follows the flow. The peak BOD (organic matter) concentration often occurs in the evening. Seasonal Variation in Constituent Values. For domestic flow only, and neglecting the effects of infiltration, the unit (per capita) loadings and the strength of the wastewater from most seasonal sources, such as resorts, will remain about the same on a daily basis throughout the year even though the total flowrate varies. The total mass of BOD and TSS of the wastewater, however, will increase directly with the population served. Fig. 3-4 Typical hourly variations in flow and strength of domestic wastewater
Infiltration/inflow, as discussed earlier in this chapter, is another source of water flow into the collection system. In most cases, the presence of this extraneous water tends to decrease the concentrations of BOD and TSS, depending on the characteristics of the water entering the sewer. In some cases, concentrations of some inorganic constituents may actually increase where the groundwater contains high levels of dissolved constituents ariations in Industrial Wastewater. The composition of wastewater from industrial operations varies widely depending on the function and activity of the particular industry. In some cases, flow values and this variation, it is often difficult to define"typical operating conditions"for industrial activities. For example, the BOD and TSS concentrations contributed from vegetable-processing facilities during the noon wash-up period may far exceed those contributed during working hours. Problems with high short-term loadings most commonly occur in small treatment plants that have limited reserve capacity to such as canneries can cause both the flow and bod loadings to If industrial wastes are to be discharged to the collection system for treatment in a municipal wastewater facility, it will be necessary to characterize the wastes adequately to identify the ranges in constituent concentrations and mass loadings. Such characterization is also needed to determine if pretreatment is collection system. If pretreatment is needed, the effluent from the pretreatment facilities must also be characterized. Further, any proposed future process changes should also be assessed to determine what effects they might have on the wastes to be discharged. Where data are not available, every effort should be made to obtain information from similar facilities. With sufficient characterization of the wastewater from industrial discharges. suitable Variations in Constituent Values in Combined Collection Systems. Flowrates, constituent concentrations, and mass loads emanating from combined collection systems can vary widely. Typical factors influencing the characteristics of wastewater from combined collection systems are shown in Table 3-10. Example variations of BOD, TSS, and fecal coliform measured in a combined system are shown on Fig. 3-5, during and after a storm event As shown, the BOd and fecal coliform bacteria concentrations are low during the storm when runoff flows are high. After the storm when runoff subsides and the flow consists primarily of wastewater, concentrations rise significantly When this rise occurs. it can be concluded that the bod and fecal coliform concentrations in the stormwater are significantly lower than in the wastewater component. Unlike BOD and fecal coliform rm. The combined sewer systems known as the"first flush The first flush has often been observed following the initial phase of a rainfall event in which much of the accumulated surface contaminants are washed into ds. Together, the res ded material and contaminants washed off surfaces result in high contaminant concentrations. Factors known to contribute to the magnitude and frequency of the first-flush effect include combined sewer slopes; street and catch basin cleaning frequency and design; rainfall intensity and duration; and surface buildup of debris and contaminants Wastewater from combined collection systems usually contains more inorganic matter than wastewater from sanitary collection systems because of the larger quantities of storm drainage that enter the combined sewer system Tab. 10 Typical factors influencing the characteristics of combined wastewater Parameter Quantity-related factors Quality-related factors P Rainfall depth and volume Regional atmospheric quality Wastewater sources Flowrate and variability f contributin Type of contributing ces(residential commercial. etc Drainage basin Pollutant buildup and wash-off characteristics Land-use type Wastewater management practices Soil characteristics Runoff control practices Sewer system, Pipe size, slope, and shape Chemical and biological transformations Inter Quantity of infiltration 9
3-9 Infiltration/inflow, as discussed earlier in this chapter, is another source of water flow into the collection system. In most cases, the presence of this extraneous water tends to decrease the concentrations of BOD and TSS, depending on the characteristics of the water entering the sewer. In some cases, concentrations of some inorganic constituents may actually increase where the groundwater contains high levels of dissolved constituents. Variations in Industrial Wastewater. The composition of wastewater from industrial operations varies widely depending on the function and activity of the particular industry. In some cases, flow values and water quality measurements may vary by several orders of magnitude over a period of a year. Because of this variation, it is often difficult to define "typical operating conditions" for industrial activities. For example, the BOD and TSS concentrations contributed from vegetable-processing facilities during the noon wash-up period may far exceed those contributed during working hours. Problems with high short-term loadings most commonly occur in small treatment plants that have limited reserve capacity to handle "shock loadings." The seasonal impact of industrial wastes such as canneries can cause both the flow and BOD loadings to increase from two to five times average conditions. If industrial wastes are to be discharged to the collection system for treatment in a municipal wastewater facility, it will be necessary to characterize the wastes adequately to identify the ranges in constituent concentrations and mass loadings. Such characterization is also needed to determine if pretreatment is required before the waste is permitted to be discharged into the collection system. If pretreatment is needed, the effluent from the pretreatment facilities must also be characterized. Further, any proposed future process changes should also be assessed to determine what effects they might have on the wastes to be discharged. Where data are not available, every effort should be made to obtain information from similar facilities. With sufficient characterization of the wastewater from industrial discharges, suitable pretreatment facilities can be provided and plant upsets can be avoided. Variations in Constituent Values in Combined Collection Systems. Flowrates, constituent concentrations, and mass loads emanating from combined collection systems can vary widely. Typical factors influencing the characteristics of wastewater from combined collection systems are shown in Table 3-10. Example variations of BOD, TSS, and fecal coliform measured in a combined system are shown on Fig. 3-5, during and after a storm event. As shown, the BOD and fecal coliform bacteria concentrations are low during the storm when runoff flows are high. After the storm, when runoff subsides and the flow consists primarily of wastewater, concentrations rise significantly. When this rise occurs, it can be concluded that the BOD and fecal coliform concentrations in the stormwater are significantly lower than in the wastewater component. Unlike BOD and fecal coliform bacteria, TSS concentrations rise slightly during the storm, and remain unchanged after the storm. The slight rise in the TSS concentration during the peak flow may be due to a phenomenon common to many combined sewer systems known as the "first flush." The first flush has often been observed following the initial phase of a rainfall event in which much of the accumulated surface contaminants are washed into the combined system. In combined collection systems, the increased flows may be capable of resuspending material deposited previously during low-flow periods. Together, the resuspended material and contaminants washed off surfaces result in high contaminant concentrations. Factors known to contribute to the magnitude and frequency of the first-flush effect include combined sewer slopes; street and catch basin cleaning frequency and design; rainfall intensity and duration; and surface buildup of debris and contaminants. Wastewater from combined collection systems usually contains more inorganic matter than wastewater from sanitary collection systems because of the larger quantities of storm drainage that enter the combined sewer system. Tab. 10 Typical factors influencing the characteristics of combined wastewater Parameter Quantity-related factors Quality-related factors Precipitation Rainfall depth and volume Storm intensity Storm duration Regional atmospheric quality Wastewater sources Flowrate and variability Type of contributing sources(residential ,commercial, etc) Type of contributing sources Drainage basin characteristics Size, time of concentration Land-use type Impervious area Soil characteristics Runoff control practices Pollutant buildup and wash-off Wastewater management practices Sewer system, interceptor design Pipe size, slope, and shape Quantity of infiltration Chemical and biological transformations Quality of infiltration
and condition Surcharging or backwater conditions sediment load resuspended from Type of flow regulation or diversion collection system acton Calculation of Mass loadings Effect of Mass Loading Variability on Treatment Plant Performance antal immensity duo mouring the course of a day, the mass loading that is received by the treatment plant can vary widely. An xample of a diurnal mass loading curve is illustrated on Fig. 3-6. The variation in loading rates and the ompounding effects during the high-flow and concentration periods is illustrated clearly on Fig. 3-6. The variations are more pronounced in small collection systems provide a significant dampening effect. The impact of thes load variations is seen most dramatically in the effects hourly BOD loading may vary as much as 3 to minimum hourly BOD load in a 24-h period. Over longer periods of time, the mass loadings can also vary widely (see Fig. 3-7). These types of variations have to be accounted for in the design of the biological treatment system Fig. 3-5 Typical variations offlowrate, BOD, TSS, and fecal coliform in a combined collection system during a storm event 0246610121411225 Day of the month Fig 3-6 Illustration of diurnal Fig 3-7 Example variations of TSS and BoD Wastewater flow, BOD and concentrations and mass loadings Mass loading variability over a monthly period concentrations and mass loadings 3-6 Selection of Design Flowrates and Maa Loadings The rated capacity of wastewater-treatment plants is normally based on the average annual daily fl at the design vear plus an allowance for future growth. As a practical matter, hoy wastewater-treatment plants have to be designed to meet a number of conditions that are influenced by fiowrates, wastewater characteristics, and constituent concentrations, and a combination of both(mass loading). Conditions that must be considered include peak and minimum hydraulic flowrates and the maximum, minimum and sustained process constituent mass loading rates. Additionally, periods of initial operation and low flows and loads must be taken into consideration in design. The importance of wastewater flowrates and mass loadings in process design and operation is considered in this section Typical flowrate and mass loading factors that are important in the design and operation of wastewater-treatment facilities are described in Table 3-11. The overall objective of wastewater treatment is to provide a wastewater-treatment system that is capable of coping with a wide range of probable 3-10
3-10 and condition Surcharging or backwater conditions Type of flow regulation or diversion Capacity reduction from sediment buildup Sediment load resuspended from collection system Calculation of Mass Loadings Effect of Mass Loading Variability on Treatment Plant Performance During the course of a day, the mass loading that is received by the treatment plant can vary widely. An example of a diurnal mass loading curve is illustrated on Fig. 3-6. The variation in loading rates and the compounding effects during the high-flow and concentration periods is illustrated clearly on Fig. 3-6. The variations are more pronounced in small collection systems where the collection system storage capacity does not provide a significant dampening effect. The impact of these load variations is seen most dramatically in the effects on biological treatment operating conditions. The maximum hourly BOD loading may vary as much as 3 to 4 times the minimum hourly BOD load in a 24-h period. Over longer periods of time, the mass loadings can also vary widely (see Fig. 3-7). These types of variations have to be accounted for in the design of the biological treatment system. Fig. 3-5 Typical variations of flowrate, BOD, TSS, and fecal coliform in a combined collection system during a storm event Fig. 3-6 Illustration of diurnal Fig. 3-7 Example variations of TSS and BOD Wastewater flow,BOD and concentrations and mass loadings Mass loading variability over a monthly period concentrations and mass loadings 3-6 Selection of Design Flowrates and Maa Loadings The rated capacity of wastewater-treatment plants is normally based on the average annual daily flowrate at the design year plus an allowance for future growth. As a practical matter, however, wastewater-treatment plants have to be designed to meet a number of conditions that are influenced by fiowrates, wastewater characteristics, and constituent concentrations, and a combination of both (mass loading). Conditions that must be considered include peak and minimum hydraulic flowrates and the maximum, minimum and sustained process constituent mass loading rates. Additionally, periods of initial operation and low flows and loads must be taken into consideration in design. The importance of wastewater flowrates and mass loadings in process design and operation is considered in this section. Typical flowrate and mass loading factors that are important in the design and operation of wastewater-treatment facilities are described in Table 3-11. The overall objective of wastewater treatment is to provide a wastewater-treatment system that is capable of coping with a wide range of probable