Why is wastewater aerated?
One of the single most significant costs of running a wastewater treatment plant is the power used to aerate the water. There are several stages in the process flowchart that require air. Right out of the gate (or bar screen in this case) is the aerated grit chamber. Air is added to the grit chamber not for biological reasons, but for physical separation reasons. The lower density water achieved by adding air makes the solid particles sink faster, in less space. It also serves to agitate the liquid, and break up any organic solids that can be treated downstream. So before any biological treatment really begins, there has already been a need for air.
Dissolved organics pass from the grit chamber, through the primary clarifier, and on to the aeration basin. This is the activated sludge process. Within the aeration basin, the biology works its magic. The conversion of nitrogen to ammonia or ammonium happens effortlessly when the nitrogen is introduced to the water. Specialized autotrophic bacteria then convert the ammonium to nitrite then to nitrate. This process requires the wastewater to be aerated, and is surprisingly not as foul smelling as one might think. The wastewater is then denitrified by creating an anoxic zone where heterotrophic bacteria strip off the oxygen from the nitrate, and release nitrogen gas to the atmosphere. By providing our favorite bacteria an advantage with aerated zones, and then anoxic zones, the natural purification process is sped up dramatically.
Wastewater can then be further cleaned by filtration, and the solids can be thickened by gravity, or dissolved air floatation. These step help produce an effluent that is reusable, and solids that can be removed by land application or incineration.
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