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Des Moines Metropolitan
Wastewater Reclamation Authority
3000 Vandalia Road
Des Moines, IA 50317

Secondary Treatment

Back to Plant Processes

The aeration system consists of six adjacent aeration tanks. Each tank is divided into four passes called basins; each basin is 300 feet in length, 35 feet wide, 20.85 feet deep. The overall capacity of each aeration tank is 6.41 MG for a total secondary capacity of 38.46 MG.

Two air diffusion systems (fine and coarse) are used to deliver process air to and around the aeration tanks. The 26,688 fine bubble diffusers are a porous ceramic disk, which divide air into fine bubble diffusion, which is used to supply oxygen to the microorganisms and provide mixing of the suspended solids within each aeration tank.

Process air is also delivered to the influent and effluent channels of the aeration basin and the influent channel to the final clarifiers. This process air, known as channel air, provides coarse bubble diffusion for two primary reasons: agitation, keeping the solids in suspension and the addition of air helps maintain a higher level of DO keeping the sewage fresh minimizing obnoxious odors.

During normal operation two flow streams enter the aeration system, primary effluent (secondary influent wastewater) and Return Sludge (RSL) from the final clarifiers, called return activated sludge. When the RSL and the wastewater are mixed together, the combination is known as mix liquor and often referred to as mixed liquor suspended solids (MLSS). The mixed liquor volatile suspended solids (MLVSS) is an approximation of the viable microorganism in the system, which do the actual wastewater purification. Most organisms in activated sludge are obligate aerobes; they need free oxygen to stay alive. The fine bubble diffusers located on the bottom of the aeration basins provide the oxygen.