unit. The cutting or knife blades force screenings through a stationary grid or louver that encloses the rotating assembly. Wash-water is typically used to keep the unit clean and to help transport materials back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen Flow equalization is a method used to overcome the operational problems caused by flowrate variations. erformance of the downstream processes and to reduce the Description/Application Flow equalization simply is the damping of flowrate variations to achieve a constant or nearly constant flowrate and can be applied in a number of different situations, depending on the characteristics of the collection system. The principal applications are for the equalization of (1) dry-weather flows to reduce peak flows and loads, (2)wet-weather flows in sanitary collection systems experiencing inflow and infiltration, or (3)combined stormwater and sanitary system flows The application of flow equalization in wastewater treatment is illustrated in the two flow diagrams given on Fig. 5-8. In the in-line arrangement (Fig. 5-8a) Flowrate varies Flowrate is relatively const fle through the equalization Inner basin. This arrangement Equalization Secondary Emuer can be used to achieve a considerable amount of Controlled-tiow constituent concentration and flowrate damping In间 Flowrate va Flowrate is relatively constant the off-line arrangement Overflow ( Fig. 5-8b), only the flow above Secondary ELuent treatmont predetermined flow is diverted into qualization basin pumping Although pumping Fig. 5-8 Typical wastewater treatment plant flow diagram incorporating flow equalization:(ain-linn requirements equalization (b)offline equalization. Flow equaltation ca be applied after grit remoul afier primary are minimized sedimentation, and aftersecondary treatment where advanced treatment is used arrangement,the amount of constituent concentration damping is considerable reduced. Off-line equalization is sometimes used to capture the"first flush "from combined collection systems. The principal benefits that are cited as deriving from application of flow equalization are: (1 biological treatment is enhanced, because shock loadings are eliminated or can be minimized inhibiting substances can be diluted and pH can be stabilized (2) the effluent quality and thickening performance of secondly sedimentation tanks following biological treatment is improved through improved consistency in solids loading:(3)effluent filtration surface area requirements are reduced. filter performance is improved, and more uniform filter-backwash cycles are possible by lower hydraulic loading; and (4)in chemical mping of mass loading improves chemical feed control and process reliabilitv. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants. Disadvantages of flow equalization include(1) relatively large land areas or sites are needed. (2) equalization facilities may have to be d maintenance is required, and 4)capital cost is increased. Design considerations The design of flow equalization facilities is concerned with the following questions Where in the treatment process flowsheet should the equalization facilities be located What type of equalization flowsheet should be used, in-line or off-lin v What is the required basin volume? 95-9 unit. The cutting or knife blades force screenings through a stationary grid or louver that encloses the rotating assembly. Wash-water is typically used to keep the unit clean and to help transport materials back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen. 5-3 Flow Equalization Flow equalization is a method used to overcome the operational problems caused by flowrate variations, to improve the performance of the downstream processes, and to reduce the size and cost of down- stream treatment facilities. Description/Application Flow equalization simply is the damping of flowrate variations to achieve a constant or nearly constant flowrate and can be applied in a number of different situations, depending on the characteristics of the collection system. The principal applications are for the equalization of (1) dry-weather flows to reduce peak flows and loads, (2) wet-weather flows in sanitary collection systems experiencing inflow and infiltration, or (3) combined stormwater and sanitary system flows. The application of flow equalization in wastewater treatment is illustrated in the two flow diagrams given on Fig. 5-8. In the in-line arrangement (Fig. 5-8a), all of the flow passes through the equalization basin. This arrangement can be used to achieve a considerable amount of constituent concentration and flowrate damping. In the off-line arrangement (Fig. 5-8b), only the flow above some predetermined flow limit is diverted into the equalization basin. Although pumping requirements are minimized in this arrangement, the amount of constituent concentration damping is considerable reduced. Off-line equalization is sometimes used to capture the "first flush" from combined collection systems. The principal benefits that are cited as deriving from application of flow equalization are: (1) biological treatment is enhanced, because shock loadings are eliminated or can be minimized, inhibiting substances can be diluted and pH can be stabilized (2) the effluent quality and thickening performance of secondly sedimentation tanks following biological treatment is improved through improved consistency in solids loading; (3) effluent filtration surface area requirements are reduced, filter performance is improved, and more uniform filter-backwash cycles are possible by lower hydraulic loading; and (4) in chemical treatment, damping of mass loading improves chemical feed control and process reliability. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants. Disadvantages of flow equalization include (1) relatively large land areas or sites are needed, (2) equalization facilities may have to be covered for odor control near residential areas, (3) additional operation and maintenance is required, and (4) capital cost is increased. Design Considerations The design of flow equalization facilities is concerned with the following questions: ✓ Where in the treatment process flowsheet should the equalization facilities be located? ✓ What type of equalization flowsheet should be used, in-line or off-line? ✓ What is the required basin volume? Fig. 5-8 Typical wastewater treatment plant flow diagram incorporating flow equalization: (a)in-line equalization ;(b)off-line equalization. Flow equalization can be applied after grit removal, after primary sedimentation ,and after secondary treatment where advanced treatment is used