Sewage is treated by a variety of methods to make it suitable for its intended use, be it for spraying onto irrigation fields (for watering crops) or be it for human consumption. Sewage treatment mainly takes place in two main stages: primary and secondary treatment. In arid areas, where there is not enough water, sewage also undergoes a tertiary treatment to meet the demands of the drinking water supply. During primary treatment, the suspended solids are separated from the water and the BOD (biochemical oxygen demand) of the water is reduced, preparing it for the next stage in wastewater treatment. Secondary treatment consists of aeration and settling tank. This process removes 75-95% of the BOD. In case of trickling filter, BOD removal is up to 80%-85%. The water is then disinfected, mostly by chlorination, and released into flowing streams or oceans. Therefore, the main objective of this chapter is to provide a deeper insight into preliminary, primary, secondary, and tertiary treatment of wastewater and furthermore provide cognizance concerning design considerations of treatment units.
TopPreliminary Treatment Process
The purpose of preliminary treatment is to ensure a satisfactory quality of final effluent and final sludge product, removal of waste water constituents such as rags, sticks, floatables, grit and grease that may cause maintenance or operational problem with the treatment operations, processes and ancillary systems (Crites & Tchobanoglous, 1998) and to protect the treatment process from malfunction associated with accumulation of screenings, debris, inorganic grit, excessive scum formation or loss of efficiency associated with grease or oil films or fat accumulations (Environmental Protection Agency Ireland, 1995) They are also referred as the physical unit operations. The unit operations used are screening for removing floating grit chambers or detritus tanks for removing grit and sand; skimming tanks for removing oils and grease; and primary settling tank or primary clarifier for removal of residual settle able suspended matter. It helps to reduce the BOD by 15% to 30% (Singh, Anurag).
Screening
In the waste water treatment generally the screening process is adopted as first unit operation. It’s a device of uniform size openings using circular bars or the rectangular bars, use to retain the floating material or solids found in the influent waste water to the treatment plant. The basic principle of screening is to remove or separate the coarse floating material found in the waste water which could damage the electrical or subsequent process equipment (pumps, valves, pipe lines, impellers etc.), can inhibit or reduce the treatment process reliability & effectiveness, or contaminate waste way (Jamal, Haseeb, 2017). It consists of passing the raw sewage through different type of screens systems (Trash Rack, Manually Cleaned Bar Screen and Mechanically Cleaned Bar) with different sizes (U.S. EPA, 2000) (Fine screening, for a spacing under 10 mm, Medium screening, for spacing of 10 to 40 mm, Coarse screening, for spacing of over 40 mm).
Manual Screening
Manually cleaned bar screen (large in size, in order to reduce the frequency of screenings collection operations) at low flow rate for less floating material collecting at the screens, having bar spacing of 20 - 25 mm. Manually cleaned bar screens are used at the beginning of diverse wastewater treatment plants, also used as standby screening for servicing or repairing of the main screen or at the time of power failure, the length of the manually cleaned bar screens shouldn’t be larger than the distance not covered (distance which is not faceable for the workers to clean up the bars manually) or not convenient for racked by hands (3 m or 10ft). Perforated plates attached with the screens are provided at the top for storing the rack which has been collected or stored temporarily for drainage of the excess wastewater in the rack. The influent channel and the screens should be perpendicular to each other for uniform distribution of floating material and solids throughout the flow and on the screen.