Tab. 5-4 Typical information on the characteristics and quantities of screenings remo ved from stewater with coarse screens Moisture content,% Volume of screenings(L/m) between bars mm 700-100 37-74 -80 600-1000 600-1000 Combined storm and sanitary collection systems may produce volumes of so s several times the mounts produced by separate systems. The quantities of screenings have also been observed to vary widely, ranging from large quantities during the "first flush"to diminishing amounts as the wet weather flows persist. The quantities of screenings removed from combined sewer flows are reported to range 3.5to84 00 m' of flow Screenings Retained on Fine Screens. Fine screenings consist of materials that are retained on screens with openings less than 6 mm. The materials retained on fine screens include small rags. paper plastic materials of various types razor blades. grit, undecomposed food waste, feces. etc. Compared to coarse screenings. the specific weight of the fine screenings is slightly lower and the moisture content is slightly higher. Because putrescible matter, including fecal material, is contained within screenings, they must be handled and disposed of properly. Fine screenings contain substantial grease and scum. which require similar care, especially if odors are to be avoided. Screenings Handling, Processing, and Disposal. In mechanically cleaned screen installations, screenings are discharged from the screening unit directly into a screenings grinder. a pneumatic eiector or a container for disposal; or onto a conveyor for transport to a screenings compactor or collection he Belt conveyors and pneumatic ejectors are generally the primary means of mechanically transporting screenings. Belt conveyors offer the advantages of simplicity of operation, low maintenance, freedom from clogging and low cost. Belt conveyors give off odors and may have to be provided with covers Pneumatic ejectors are less odorous and typically require less space; however, they are subject to clogging if large objects are present in the screenings Screenings compactors can be used to dewater and reduce the volume of screenings(see Fig 5-5) Such devices, including hydraulic ram and Discharge screw compactors, receive screenings directly from the bar screens and are capable of transporting the compacted screenings to a Feed receiving hopper. Compactors can reduce the water content of the screenings by up to 50 Screenings percent and the volume by up to 75 percent. As with t automatic controls nse iams, automatically reverse the mechanism. and actuate alarms and shut down equipmen Fig. 5-5 Typical device used for compacting screenings Means of disposal of screenings include(D) removal by hauling to disposal areas (landfill solid wastes. (2)disposal onlv). and( 4) discharge to grinders or macerator where they are ground and returned to the wastewater. The first method of disposal is most commonly used. In some cases, screenings are required to be lime stabilized for the control of pathogenic organisms before disposal in landfills. 5-2 Coarse Solids reduction as an alternative to bar screens or fine screens. comminutors and macerator can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shred screenings that are removed from the wastewater. The solids are cut up into a smaller, more uniform size for return to the flow stream for bsequent removal by downstream treatment operations and processes. Comminutors, macerator, and grinders can theoretically eliminate the messy and offensive task of screenings handling and disposal. The use of comminutors and macerator is particularly advantageous in a pumping station to protect the pumps 5-75-7 Tab. 5-4 Typical information on the characteristics and quantities of screenings removed from wastewater with coarse screens Size of openings between bars,mm Moisture content,% Specific weight kg/m3 Volume of screenings(L/m3 ) Range Typical 12.5 60-90 700-1100 37-74 50 25 50-80 600-1000 15-37 22 37.5 50-80 600-1000 7-15 11 50 50-80 600-1000 4-11 6 Combined storm and sanitary collection systems may produce volumes of screenings several times the amounts produced by separate systems. The quantities of screenings have also been observed to vary widely, ranging from large quantities during the "first flush" to diminishing amounts as the wet weather flows persist. The quantities of screenings removed from combined sewer flows are reported to range from 3.5 to 84 L/1000 m3 of flow. Screenings Retained on Fine Screens. Fine screenings consist of materials that are retained on screens with openings less than 6 mm. The materials retained on fine screens include small rags, paper, plastic materials of various types razor blades, grit, undecomposed food waste, feces, etc. Compared to coarse screenings, the specific weight of the fine screenings is slightly lower and the moisture content is slightly higher. Because putrescible matter, including fecal material, is contained within screenings, they must be handled and disposed of properly. Fine screenings contain substantial grease and scum, which require similar care, especially if odors are to be avoided. Screenings Handling, Processing, and Disposal. In mechanically cleaned screen installations, screenings are discharged from the screening unit directly into a screenings grinder, a pneumatic ejector, or a container for disposal; or onto a conveyor for transport to a screenings compactor or collection hopper. Belt conveyors and pneumatic ejectors are generally the primary means of mechanically transporting screenings. Belt conveyors offer the advantages of simplicity of operation, low maintenance, freedom from clogging, and low cost. Belt conveyors give off odors and may have to be provided with covers. Pneumatic ejectors are less odorous and typically require less space; however, they are subject to clogging if large objects are present in the screenings. Screenings compactors can be used to dewater and reduce the volume of screenings (see Fig. 5-5). Such devices, including hydraulic ram and screw compactors, receive screenings directly from the bar screens and are capable of transporting the compacted screenings to a receiving hopper. Compactors can reduce the water content of the screenings by up to 50 percent and the volume by up to 75 percent. As with pneumatic ejectors, large objects can cause jamming, but automatic controls can sense jams, automatically reverse the mechanism, and actuate alarms and shut down equipment. Fig. 5-5 Typical device used for compacting screenings Means of disposal of screenings include (1) removal by hauling to disposal areas (landfill) including co-disposal with municipal solid wastes, (2) disposal by burial on the plant site (small installations only), (3) incineration either alone or in combination with sludge and grit (large installations only), and (4) discharge to grinders or macerators where they are ground and returned to the wastewater. The first method of disposal is most commonly used. In some cases, screenings are required to be lime stabilized for the control of pathogenic organisms before disposal in landfills. 5-2 Coarse Solids Reduction As an alternative to coarse bar screens or fine screens, comminutors and macerators can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shred screenings that are removed from the wastewater. The solids are cut up into a smaller, more uniform size for return to the flow stream for subsequent removal by downstream treatment operations and processes. Comminutors, macerators, and grinders can theoretically eliminate the messy and offensive task of screenings handling and disposal. The use of comminutors and macerators is particularly advantageous in a pumping station to protect the pumps