Fraunhofer Institute for Interfacial Engineering and Biotechnology IGBMany wastewater treatment plants still lack the technical equipment for nitrogen elimination. In the example presented here, the municipal sewage treatment plant of the city of Tauberbischofsheim was unable to reliably comply with the required discharge values for ammonium and total nitrogen. An engineering office proposed expansion measures amounting to about DM 22 million. This was too expensive for the municipality. Therefore, Fraunhofer IGB was commissioned to develop proposals for a more cost-effective sewage treatment plant extension.
Economic extension of the sewage plant in Tauberbischofsheim by a denitrification step
System analysis and measurement
The first step was to analyse the values recorded in the operating logs of the sewage treatment plant. This analysis showed that the engineering office had planned a clearly too large extension of the plant. As a basis for assessment, the office had only carried out a measuring programme lasting several days. Due to the considerably higher number of measured values from the operating logbook, the average values calculated by the IGB were considerably more reliable than when evaluating the measuring program, which had also accidentally produced dry weather values for the hydraulic load that were too high. It was also found that the proportion of extraneous water in the plant was very high due to the ingress of groundwater into the sewage system.
The planning of nitrogen elimination was another reason for the highly estimated expansion costs. As only a small amount of data for N elimination had to be obtained via the operations log, IGB also carried out a specific measurement programme at the existing plant. Among other things, measurements with local resolution were carried out along an aeration basin under different operating conditions on three days, with different numbers of blowers in operation to supply the aeration equipment. Measurements of dissolved oxygen and ammonium concentration showed that with sufficient oxygen supply the existing plant is quite capable of complete nitrification.
Measures
The treatment plant was provided with an upstream denitrification zone, about two thirds of which was implemented in the generously dimensioned primary clarification basins. After the retrofit, our measuring program was continued to check the success and to determine the most favourable operating parameters.
In addition, the aeration candles in the aeration tanks were cleaned, which improved the gassing of fine bubbles and the utilisation of oxygen.
The proportion of extraneous water could be significantly reduced by means of suitable remediation measures. Further potential for safe operation lies in the decentralised pre-treatment of the waste water from two plants. This would be more cost-effective than an additional expansion of the municipal sewage treatment plant.
Success and cost savings
As expected, the sewage treatment plant achieved the legally prescribed effluent values solely through the measures described. Due to the step-by-step approach, the total costs for the expansion amounted to less than DM 1 million. Compared to the original cost estimate of more than DM 20 million, this represents a considerable saving.