Southwest Clean Water Plant
The Southwest Plant generates approximately 15 dry tons of biosolids per day. The biosolids are processed and utilized as a soil conditioner and fertilizer. Biosolids hauled from the Northwest Clean Water Plant are also processed at the Southwest Plant. See the biosolids page for more information.
The biosolids or excess microorganisms from the activated sludge biological treatment processes are first pumped to gravity belt thickeners (GBT). In this process, the waste biosolids, which are about 1% solids, are conditioned with water-soluble polymers to flocculate the solids. The GBT process works by filtering free water from the flocculated biosolids by gravity drainage through a porous belt. This produces biosolids with approximately 5% total solids.
The next step is anaerobic digestion. This is accomplished by employing microorganisms in the absence of oxygen in one million gallon covered tanks called "anaerobic digesters." Conditions are carefully controlled in order to produce an environment where anaerobic biological activity can flourish. This breaks down solids, reducing their concentration by about one-half. This process transforms the biosolids into an inoffensive humus type of substance, which can be safely spread on land for soil conditioning and fertilization. Thus, the biosolids produced by the treatment processes are beneficially reused and recycled in an environmentally safe manner.
Anaerobic digestion results in the generation of methane gas, a valuable source of energy. This methane gas is used as a fuel for large engine driven plant equipment and to produce building heat, reducing the use of electricity and other fuels.
The wastewater plants disposal option for biosolids recycling is to dewater the digested biosolids using high-speed centrifuges and then transport the material to area farms to be land applied as fertilizer. Again using polymers to aid in dewatering, the digested biosolids are taken from 3% solids to 23% solids. The biosolids material can be stockpiled at the SW Plant during occasions when it is too wet to land apply. Thus, the biosolids produced by the treatment processes are beneficially reused and recycled in an environmentally safe manner according to the Envioronmental Protection Agency (EPA) Part 503 Biosolids Regulations.