The first stages of waste water ( sewage ) treatment are commonly physical and aimed to remove larger suspended solids from waste water, primarily using gravity to allow larger (heavy) particles to settle while allowing the remaining liquid to continue through the plant. Primary waste water treatment uses simple mechanical & physical processes to remove nearly half of the solid contaminants from waste water. Primary treatment consists of units like coarse screening, grit removal, communicator, flow equalization and primary sedimentation tanks. The schematic diagram of primary treatment with commonly used units.
The first step in primary treatment is coarse screening to remove large materials such as rags, logs, plastics and other large objects from the waste water that could damage or interfere with the treatment process and equipment. Coarse screens (also known as bar racks) are made up of vertical or inclined parallel bars of mild steel or stainless steel with openings 25 to 75 mm wide to enable waste water to pass through while large objects are caught on the bars . These screens are raked and cleaned from time to time. Some waste water treatment plants have automated equipment that cleans the screens automatically
In mechanical cleaning arrangement a horizontal rake on a toothed gear drive rakes the bars and removes the captured material to a conveyor that deposits the material into a dumpster for removal to the sanitary landfill. The Fig. shows the arrangement of mechanically cleaned bar screen. Mechanical cleaning arrangements are provided in larger capacity waste water treatment plants (> 20 MLD capacity). In smaller waste water treatment plants, the screens are manually raked.