against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings They are particularly useful in cold climates where collected screenings are subject to freezing screenings at wastewater-treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater. they should not b thought maintains that once cut up the solids are ly handled in the downstream processes. Shredded solids often present downstream problems, particularly with rags and plastic bags. as they tend diffusers and clarifier Plastics and other non-biodegradable material may als so advers bio- solids that are to be beneficially reused Approaches to using comminutors, macerator, and grinders are applicable in many retrofit situations Examples of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Alternative approaches may also be possible, such as using chopper pumps at pumping stations or installing grinders ahead of sludge pumps Comminutors Comminutors are used most commonly in small wastewater-treatment plant less than 0.2 m/s Comminutors are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm without removing the shredded solids from the flow stream. a typical comminutor uses a stationary horizontal screen to intercept the flow(see Fig. 5-6)and a rotating or oscillating arm that contains cutting teeth to mesh with the screen. the cutting teeth and the shear bars cut coarse material. The small sheared particles pass through the material, namely, rags, that can collect on downstream treatment equipment screen and into the down-stream channel. Comminutors may create a string Fig. 5-6 Typical communitors used for particle size reduction of solids diverter screen Macerator DIMe Macerator are slow-speed grinders that typically consist of two sets of counter-rotating blies with blades( see F The assemblies are mounted vertically in the flow Counter channel. The blades or teeth on Counter- the rotating assemblies have a rotating close tolerance that effectively Patting chops material as it passes through the unit the F Fig. 57 Tpical mcerators: (alsdiemaic of i-dhanel tpe sbw-speed grinder/macerator; (b view of a maceratr ropes mounted in an open channeli(schematic of linked-screen macerator installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at smaller wastewater-treatment plants. Sizes for pipeline applications typically range from 100 to 400 mm in diameter Another type of macerator used in channel applications is a moving, linked screen that allows wastewater to pass through the screen while diverting screenings to a grinder located at one side of the channel(see Fig. 5-7c). Standard sizes of this device are available for use in large channels ranging from widths of 750 to 1800 mm and depths of 750 to 2500 mm. The headloss is lower than that of the units with counter-rotating blades shown on Fig 5-7a Grinders High-speed grinders, typically referred to as hammer-mills, receive screened materials from bar screens The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the5-8 against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings. They are particularly useful in cold climates where collected screenings are subject to freezing. There is a wide divergence of views, however, on the suitability of using devices that grind and shred screenings at wastewater-treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater, they should not be returned, regardless of the form. The other school of thought maintains that once cut up, the solids are more easily handled in the downstream processes. Shredded solids often present downstream problems, particularly with rags and plastic bags, as they tend to form ropelike strands. Rag and plastic strands can have a number of adverse impacts, such as clogging pump impellers, sludge pipelines, and heat exchangers, and accumulating on air diffusers and clarifier mechanisms. Plastics and other non-biodegradable material may also adversely affect the quality of bio-solids that are to be beneficially reused. Approaches to using comminutors, macerators, and grinders are applicable in many retrofit situations. Examples of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Alternative approaches may also be possible, such as using chopper pumps at pumping stations or installing grinders ahead of sludge pumps. Comminutors Comminutors are used most commonly in small wastewater-treatment plants, less than 0.2 m3 /s. Comminutors are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm without removing the shredded solids from the flow stream. A typical comminutor uses a stationary horizontal screen to intercept the flow (see Fig. 5-6) and a rotating or oscillating arm that contains cutting teeth to mesh with the screen. The cutting teeth and the shear bars cut coarse material. The small sheared particles pass through the screen and into the down-stream channel. Comminutors may create a string of material, namely, rags, that can collect on downstream treatment equipment. Fig.5-6 Typical communitors used for particle size reduction of solids Macerators Macerators are slow-speed grinders that typically consist of two sets of counter-rotating assemblies with blades (see Fig. 5-7a). The assemblies are mounted vertically in the flow channel. The blades or teeth on the rotating assemblies have a close tolerance that effectively chops material as it passes through the unit. The chopping action reduces the potential for producing ropes of rags or plastic that can collect on downstream equipment. Macerators can be used in pipeline installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at smaller wastewater-treatment plants. Sizes for pipeline applications typically range from 100 to 400 mm in diameter. Another type of macerator used in channel applications is a moving, linked screen that allows wastewater to pass through the screen while diverting screenings to a grinder located at one side of the channel (see Fig. 5-7c). Standard sizes of this device are available for use in large channels ranging from widths of 750 to 1800 mm and depths of 750 to 2500 mm. The headloss is lower than that of the units with counter-rotating blades shown on Fig. 5-7a. Grinders High-speed grinders, typically referred to as hammer-mills, receive screened materials from bar screens. The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the Fig. 5-7 Typical macerators: (a)schematic of in-channel type slow-speed grinder/macerator;(b)view of a macerator mounted in an open channel;(c)schematic of linked-screen macerator Horizontal rotating diverter screen Counter￾rotating cutting assembly Counter￾rotating cutting assembly