Reject water: treating of process water in large wastewater treatment plants in Germany--a case study

The increase of the hydraulic loading of a municipal wastewater treatment plant caused by the sludge dewatering process is of minor importance. However, the rejected nitrogen load accounts for up to 25% of the nitrogen load in the raw sewage. As a result, the process water significantly influences the stage volume of the biological treatment steps. Rejection management can prevent critical nitrogen concentrations in the effluent of the plant. In some cases, the treating capacity must be increased. Separate treatment of the process water can be an alternative to the enlargement of the main wastewater treatment stages. This article describes the compositions of the process waters from different sludge treatment steps, the influence on the main wastewater treatment steps, possible strategies to handle the rejection problem, and processes for separate treatment of the sludge liquor. Some results of a German case study on large wastewater treatment plants illustrate the situation concerning separate process water treatment in Germany. These results suggest that side stream treatment can be a practical alternative to a classic increase in the length of the main stream stages. Moreover, they prove that many different processes are useful for the separate treatment of sludge liquor.