A combined water management approach based on river water quality standards

The objective of the research project described was to develop a combined water management approach for a catchment area, including the sewage system, the wastewater treatment plant (WWTP), and the receiving water. The combined approach was designed to provide both ‘good water quality’ – as defined in the European Water Framework Directive – and cost savings. The investigations were carried out in Odenthal, a small municipality (12,500 p.e.) in the river Dhünn catchment in the western part of Germany. Emphasis was placed on investigation of the impact on the receiving water of pollution resulting from discharges at stormwater overflow tanks (SOTs), combined sewer overflows (CSOs) and the WWTP. These structures are the interface between the urban wastewater system and the receiving water bodies. The river water quality standards applied were deduced from an intensive monitoring program relating to the sewer system, the WWTP and the river and creeks. Specific threshold concentrations for ammonia, dissolved oxygen and nitrite were defined, depending on the frequency of occurrence and the duration of the overflow from SOTs and CSOs. The higher planning costs, primarily the cost of the monitoring program, were compensated for by cost savings relating to the construction of SOTs and the WWTP. In total, considerable cost savings were achieved in comparison to the previous separate design of the sewer system and the WWTP. The present emission-based regulations imposed – in this case study – more restrictive requirements and resulted in higher final systems costs than would have been the case if the costs had been on the requirements deduced from the biological analyses of the receiving water data.