Cost-effective optimization and extension of sewage plants by specific analysis

The principle of the activated sludge process for sewage treatment technology was already published in 1914 by Arden and Lockett. The original purification goal of sewage treatment plants was the elimination of the organic load. In the years from 1969 to 1993, for example, the organic residual pollution of public sewage treatment plants in Baden-Württemberg was reduced from approx. 75,000 tonnes BSB5 to below 10,000 tonnes BSB5. Nevertheless, the nutrients nitrogen (N) and phosphorus (P) contribute to the eutrophication of inland waters and marginal seas, and drinking water production is polluted by nitrate.

Since 1 January 1989, the waste water management regulations of the Water Management Act for municipal waste water treatment plants have therefore set stricter limit values for nitrogen and phosphorus as well. Most sewage treatment plants are affected by this, as they were originally designed only to eliminate carbon and the removal of N and P requires further measures.

Traditional approach

The design of sewage treatment plants is usually carried out by engineering consultants according to the worksheets of the Abwassertechnische Vereinigung (ATV). Although Worksheet A 131 of the ATV points out that an analysis of the operating logs is recommended to determine the design data, in many cases only a measuring program lasting several days is carried out and evaluated with regard to the required design data. Depending on the extent of the necessary extensions of the sewage treatment plant calculated in this way, the resulting plant costs often amount to several million DM up to some 10 million DM.

Solution approach of the IGB

In view of such investment sums, the Fraunhofer IGB carefully evaluates the operating diaries in advance despite the increased time and cost involved. In doing so, not only the design parameters provided for in the ATV are determined, but also the function of the sewage treatment plant is determined by means of process-technical procedures and forms of presentation in the sub-areas of the plant. Frequently, a specific measurement program must also be carried out to determine the performance of the existing plant.

Although this procedure is more expensive than the usual one, it is nevertheless useful in view of the possible cost savings when expanding sewage treatment plants. This is the only way to find individual solutions which bring the sewage treatment plants up to the state of the art at a reasonable price. Overall, considerable sums can be saved by this procedure.

Experience in practice

Fraunhofer IGB has now implemented its concept in numerous municipal and industrial wastewater treatment plants. In all cases the performance of the wastewater treatment plant could be optimized at reasonable cost and adapted to the legally prescribed values.