Usually multiple devices or structures can be placed upstream before going to the primary wastewater treatment process and these unit of operations involved in the pretreatment step is generally constructed and maintained for primary purpose of protecting costly equipment from damage. They play a small part towards reduction of BOD and usually chemical plants which receive wastewater in soluble form free from large contaminants will not have these devices or structures present. Generally there are two main components that serve as pretreatment step, one is the bar screen and the other one is the grit chamber.
Bar Screens or sometimes called Bar Racks are normally installed as the first line of defense to remove large objects primarily, rags, woods and rocks that manage to escape and find their way through the sewer systems. Usually these objects can be of extreme nuisance because if left around going to the main treatment plant, they pose a significant risk of damaging pumps and blocking valves. The racks require constant attention to clean it manually or mechanically and whatever collections of materials are stored in a waste bin before sent to sanitary landfill for disposal.
The screens can be categorized depending on its channel opening and usually it can be something like trash racks which are either cleaned manually or the other one cleaned mechanically. Manually cleaned racks have openings between 25 to 50 mm and they are usually not used in normal wastewater treatment plants as it will require a lot of time and work intervention by operators to keep it working well. They are often used in bypass channels and thus the designed velocity is usually in the range of 0.3 – 0.6m/s. On the other hand, mechanically cleaned racks have smaller openings between 5 – 25mm to trap unwanted waste and for it to function properly; high speed velocity of wastewater is needed to drive through the smaller sized channels. Thus, the designed velocity is at least minimum 0.6m/s so that grit will not accumulate in it.
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