- remove sludge and scum so they don't accumulate in rivers where the water is discharged,
- prevent oxygen-demanding organic material from entering rivers where it kills aquatic life by stealing oxygen,
- prevent odors created in water lacking enough dissolved oxygen, and
- remove potential disease-causing bacteria and viruses (pathogens).
The preliminary or first step in wastewater treatment removes large objects (roots, rags, cans, body parts, pianos) by screening the water through a grate or bar screen made of parallel bars. Every so often, collected materials are raked off the screen and disposed of by burning or burying. Some plants automatically shred (grind) these materials and leave the pieces in the wastewater flow for later removal.
While the screening process removes large objects, it does nothing about sand and grit, which has to be removed so it doesn't ruin pumps and other equipment. Grit is often removed in grit channels or a grit tank. Grit tanks are know as detritors where water flow is slowed enough to let heavy inorganic materials (grit) settle. Many plants aerate the water to help separate the grit. A grit screw like the one on the right removes such settled grit from the water so it can be removed manually for disposal.
After grit is removed, water begins primary treatment in a large basin or tank (left), called a primary clarifier, where solids (called sludge) settle and light materials (grease, oil, light solids) float.
Primary treatment helps clear the water. In round clarifiers, an arm or "plow" with a scraper blade and squeegees rotates around the bottom to guide sludge into a collection pipe for removal where the sludge is pumped to a digester. Scum on the surface, in the picture at the right, is pushed (skimmed) by a skimmer arm into an effluent trough for ultimate disposal in the digester. The term effluent is used to refer to any wastewater or related liquid leaving a treatment unit.
In a rectangular clarifier, sludge is collected by a continuously moving collector chain that acts like a kind of conveyor belt and drags the sludge to a collection point where it is removed. At the same time, floating scum is skimmed and removed from the surface through scum skimmer troughs. Water leaves the clarifiers by flowing over a weir, a steel "curb" with teeth that allows the water to flow slowly. You could imitate this overflow process if you put a glass under the kitchen spigot and let water flow in at a rate just fast enough so that it flows out over the edge slowly.
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