Preliminary Treatment

Preliminary treatment removes the materials contained in wastewater that may damage or interfere with later treatment steps. This usually consists of two steps.

1. Removing large solids such as canvas shoes, dead animals, caps, balls, sticks, logs, etc. that get into the sewer system due to open access covers or unprotected storm drains. These large solids would otherwise clog piping systems and pumps. Collection is usually by a set of steel bars (or screens) placed in the water's path at the inlet to the treatment plant or sometimes a pumping station. An automatic system rakes the solids into a dumpster for later disposal at a landfill.

2. Removing sand and grit that will cause excessive wear on the pumps and piping system in the treatment plant is also a part of preliminary treatment. This grit results from cracks in the underground piping system that allow sand gravel to enter the system, and from runoff from city streets, parking lots, etc. It is removed by passing the water into a small stilling basin (or tank) where the grit falls to the bottom and the water flows out. The grit is then removed and taken to a landfill.

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