Upgrading of a wastewater treatment plant utilizing existing trickling filters and a new filter stage

The upgrading of wastewater treatment plants with the objective of enhancing the elimination of nitrogen and phosphorus is a demanding undertaking as regards the choice of process technology, especially if the basic function of existing treatment systems is to be retained. With reference to an existing two-stage biological sewage plant with trickling filters in the second stage, the present paper illustrates how the volume of the activated sludge stage that is being enlarged can be minimized by exploiting the benefits of a primary treatment stage with high BOD5 elimination and by integrating the trickling filters in the main stream of the treatment chain. This requires that the extremely costly filtration stage that is essential for eliminating the phosphorus also fulfils other tasks, in this case residual denitrification.