Primary treatment

Sewage treatment is a sequence of steps, each removing a different kind of pollution. Preliminary and primary treatment are the first stages and deal with physical removal: rags and plastics that would jam pumps, grit that would wear machinery and fill tanks, and the settleable organic solids that make up a large share of the pollution load.

Preliminary treatment starts with screens. Coarse screens with large openings protect pumps; fine screens, often mechanically raked, remove smaller debris. Screenings are washed, compacted and sent to landfill or incineration. Grit chambers slow the flow to around 0.3 m/s so that heavy sand and grit settle while lighter organic matter stays in suspension; velocity is controlled with a proportional flow weir or by aerated or vortex designs. Oil and grease traps may follow where needed.

Primary clarifiers are sedimentation tanks, rectangular or circular, that hold sewage for about two hours. Settleable solids sink to form primary sludge, scraped to a hopper and pumped to sludge treatment, while floating scum is skimmed off. A well-designed primary clarifier removes roughly 50 to 70 percent of suspended solids and 25 to 40 percent of BOD, reducing the load on the biological stage. Design is based on surface overflow rate, detention time and weir loading. Some modern plants skip primary clarifiers and send screened, degritted sewage straight to biological treatment such as sequencing batch reactors or extended aeration.