Primary clarifiers allow further slowing of the wastewater so that settleable organics precipitate to the bottom while fats, oils, and greases float to the top. These physical processes remove approximately half of the contaminants in wastewater. Large objects such as sticks and rags are removed from raw sewage as it passes through bar screens. Wastewater is slowed so that settleable organics settle to the bottom while fats, oils, and greases float to the top. Biological Systems Biological processes remove most of the rest of the contaminants.
Water flows into aeration basins where oxygen is mixed with the water.
Microorganisms consume the organic material as food, greatly reducing the BOD in the water. They convert non-settleable solids to settleable solids and are later themselves captured in final clarifiers, ending up in wastewater biosolids. Since the process is biological, any chemical or substance harmful to life can interfere with the operation of a water recycling plant. This is why cities prohibit discharges of untreated industrial wastes to sewers and promote education among citizens regarding the harmful effects that dumping household chemicals. When the water recycling plant cannot operate properly because chemicals are killing the microorganisms, water reuse programs are jeopardized and the quality of water discharged to receiving streams is lowered.