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 operations3-5 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