Now, however, many such systems are operating successfully in the southeastern United States and elsewhere. Essentially, the system consists of four cells in series. For municipal wastewater treatment in the southeastern United States, the system will have, at design flow, a total HRT of 4.5 to 5 days, and a depth of at least 3 m. The following three cells, each with a HRT of approximately 1 d, serve the functions of sedimentation, solids stabilization, and sludge storage. The aerobic layer reduces feed-back of nitrogen and CBOD to the water column, and maintains a stable deposit.
Aeration also reduces the dead-space volume of the cells.
Since the control of algal growth is crucial in the reduction of effluent suspended solids, careful attention is paid to factors influencing such growth. The turbidity created in the first cell by maintaining all settable solids in suspension reduces light in the water column to the extent that very little algal growth occurs in that cell. The focus of concern, therefore, centers on factors in the remaining three cells. Those factors include HRT, multicellular configuration, surface area, and mixing. Solids stabilization rates are another consideration in the design of DPMC systems.