Chapter 6 – Preliminary treatment

Publisher Summary This chapter focuses on the preliminary stage of sewage treatment process. Screening is normally the first operation used at sewage treatment plants, and just before urban runoff is conveyed into a wetland system. The general purpose of screens is to remove large objects such as twigs, rags, paper, plastic, metals, and shopping trolleys. These objects, if not removed, may damage the pumping and sludge removal equipment, hang over weirs, and block valves, nozzles, channels, and pipelines, thus creating serious plant operation and maintenance problems. For preliminary treatment, screens normally comprise vertical or inclined bars with openings between 20 and 60 mm for coarse screens, and 10 and 20 mm for medium screens. The second operation constituting preliminary treatment is grit removal. Grit includes sand, dust, and cinders and it is necessary to remove these materials to protect moving mechanical equipment and pumps from unnecessary wear and abrasion; prevent clogging in pipes and heavy deposits in channels; and prevent cementing effects on the bottom of sludge digesters and primary sedimentation tanks. The type of grit removal methods to be used is determined by the quantity and quality of the grit, head loss, space requirements, and the type of equipment used in other parts of the plant.