Towards energy neutrality by optimising the activated sludge process of the WWTP Bochum-Ölbachtal.

As a result of Ruhrverband's regularly performed energy audits the wastewater treatment plant (WWTP) Bochum-Ölbachtal shows substantial deficits concerning energy efficiency. Due to the energy consumption of internal recirculation, mixers and return activated sludge (RAS) pumping the existing pre-denitrification process configuration offers a specific energy consumption for biological treatment of 23 kWh (PE · a)(-1). In order to optimise the energy situation and to improve the treatment efficiency, the process layout was changed completely to a three-stage step-feed process. By optimising the hydraulic conditions, it was possible to reconstruct the plant with a free flow throughout the whole biological treatment system without any additional pumping. The total investment costs for this process scheme were 3.9 million €. These costs could be partly offset against the wastewater charge paid (2.9 million €). Compared to the overall energy consumption before the process modification, today the energy consumption for biological treatment amounts to 12.4 kWh (PE · a)(-1). The highest saving potential has been achieved by optimising mixing and reducing the energy demand for internal recirculation and RAS pumping. In the case of the WWTP Bochum-Ölbachtal, the modification of the treatment process not only results in an improved energy situation but also increased the treatment efficiency in such a way that the nitrogen concentration in the effluent could be constantly kept below 5 mg L(-1) N(tot), which provides the basis for being exempted from the wastewater discharge for nitrogen. As a result of all these measures, the rate of self-sufficiency by using biogas from the digester in combined heat and power units has been increased substantially from 60% before process modifications to 97%. With the upcoming optimisation measures, a further increase of self-sufficiency is expected to finally achieve energy neutrality based on yearly averages. The example of the WWTP Bochum-Ölbachtal clearly shows that it is worthwhile to consider further measures for improving the energy and treatment efficiency, although most of the machinery was not fully depreciated at the time of implementing the new concept. It is also evident that the regular energy monitoring is essential for identifying potential for improvement. In this context and based on Ruhrverband's experience, it is highly suggested that energy audits are performed regularly for larger WWTPs.