5 Physical Unit Operation Operations used for the treatment of wastewater in which change is brought about by means of or through the application of physical forces are knov hysical unit operations. Because physical unit operations were derived originally from observations of the physical world. they were the first treatment methods to be used. Today, physical unit operations, as shown on Fig. 5-1, are a major part of most wastewater treatment systems ude(D) screening. (2) coarse solids reduction(comminution maceration, and screenings grinding)(3) flow equalization. (4)mixing and flocculation.(5) grit removal. (6) sedimentation,(7 high-rate clarification. (8)accelerated gravity separation(vortex separators).(9) flotation(10) oxvgen transfer(1)packed-bed filtration, membrane separation(12) aeration(12)biosolid dewatering and (13) volatilization and stripping of volatile organic compounds (vocs) Off-line flow equalization (used to dampen peak flows water storage Waste backwash water Prima Se Chlorine Ellen Screens and Effluent Chlorine Chlorine contact Recycled biosolids Waste biosolids Legend Thickener retum flow Waste biosolids ○ Unit operations M Unit processes Thickened biosolids Recycle or solids streams processing facilities Fig 5-1 Location of physical unit operations in wastewater treatment plant flow diagram 5-1 Screening The first unit operation generally encountered in wastewater-treatment plants is screening. A screen is a device with openings, generally of uniform size. that is used to retain solids found in the influent wastewater to the treatment plant or in combined wastewater collection systems subject to overflows, especially from stormwater. The coarse materials from the flow stream that could (D damage subsequent process equipment. (2) reduce overall treatment process reliability and effectiveness, or( 3)contaminate waterways. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to (1)protect process equipment or(2)eliminate materials that may inhibit the beneficial reuse of biosolids All aspects of screenings removal, transport, and disposal must be considered in the application of downstream processes.(2)health and safety of the oLE removal required because of potential effects on screenings contain pathogenic organisms and attract insects. ( 3) odor potential, and(4) requirements for handling, transport. and disposal. i.e., removal of organics(by washing) and reduced water cor pressing), and (5)disposal options. Thus, an integrated approach is required to achieve effective screenings management Classification of screens The types of screening devices commonly used in wastewater treatment are shown on Fig. 5-2. Two general types of screens, coarse screens and fine screens, are used in preliminary treatment of wastewater Coarse screens have clear openings ranging from 6 to 150 mm; fine screens have clear openings less than5-1 5 Physical Unit Operation Operations used for the treatment of wastewater in which change is brought about by means of or through the application of physical forces are known as physical unit operations. Because physical unit operations were derived originally from observations of the physical world, they were the first treatment methods to be used. Today, physical unit operations, as shown on Fig. 5-1, are a major part of most wastewater treatment systems. The unit operations most commonly used in wastewater treatment include (1) screening, (2) coarse solids reduction (comminution, maceration, and screenings grinding), (3) flow equalization, (4) mixing and flocculation, (5) grit removal, (6) sedimentation, (7) high-rate clarification, (8) accelerated gravity separation (vortex separators), (9) flotation, (10) oxygen transfer, (11)packed-bed filtration, membrane separation, (12 ) aeration, (12)biosolid dewatering, and (13) volatilization and stripping of volatile organic compounds (VOCs). Fig. 5-1 Location of physical unit operations in wastewater treatment plant flow diagram 5-1 Screening The first unit operation generally encountered in wastewater-treatment plants is screening. A screen is a device with openings, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment plant or in combined wastewater collection systems subject to overflows, especially from stormwater. The coarse materials from the flow stream that could (1) damage subsequent process equipment, (2) reduce overall treatment process reliability and effectiveness, or (3) contaminate waterways. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to (1) protect process equipment or (2) eliminate materials that may inhibit the beneficial reuse of biosolids. All aspects of screenings removal, transport, and disposal must be considered in the application of screening devices, including (1) the degree of screenings removal required because of potential effects on downstream processes, (2) health and safety of the operators as screenings contain pathogenic organisms and attract insects, (3) odor potential, and (4) requirements for handling, transport, and disposal, i.e., removal of organics (by washing) and reduced water content (by pressing), and (5) disposal options. Thus, an integrated approach is required to achieve effective screenings management. Classification of Screens The types of screening devices commonly used in wastewater treatment are shown on Fig. 5-2. Two general types of screens, coarse screens and fine screens, are used in preliminary treatment of wastewater. Coarse screens have clear openings ranging from 6 to 150 mm; fine screens have clear openings less than