Primary Clarification. The primary clarifier is a large settling tank used to separate solid matter from the liquid wastestream. Solids are removed and pumped to the aerobic digester for further processing. Floatable components, such as grease, are also removed and collected in the primary clarifier. The tank is covered to contain and control any odors that may be generated.   Aeration Basins. This process, generally referred to as "secondary treatment", consists of two parallel tanks in which the conditions for biological treatment are optimized through addition of air. The tanks contain hundreds of millions of actively growing single-celled microorganisms (mostly bacteria, but also protozoa, fungi, and others) which consume the bacteria in the wastewater. The District's aeration basins are normally operated in "extended aeration" mode which allows for conversion of ammonia to non-toxic forms of nitrogen.