Treatment of sludge return liquors: Experiences from the operation of full-scale plants

More stringent effluent criteria with regard to nitrogen call for improved nutrient removal techniques in wastewater treatment plants (WWTPs). Besides optimisation of the liquid treatment train of the plants, over the last years, attention has increasinly centred on the problem of return flows from sludge treatment. Depending on sludge handling and treatment, some 15 to 25 % of the influent nitrogen load are usually returned from the sludge dewatering facility to the inlet of the WWTP. By minimising this extra nitrogen load, it can be expected to substantially improve the effluent quality. On a full-scale basis, mainly ammonia stripping and different biological processes have been applied, in Europe, for the treatment of process water streams with the overall goal to reduce the return nitrogen load. A recently performed survey on fullscale plants shows that only eight plants use ammonia stripping. Whereas the majority of WWTPs have been upgraded by implementation of biological measures for the treatment of return flows. Most of these biological systems use classical nitrification and denitrification or – to reduce the consumption of energy and organic substrate – nitritation and denitritation. One of the most recent developments in this field is deammonification which has so far been applied in three full-scale plants. Based on the experience gained from operation of two of these plants, it can be said that a stable nitrogen elimination of no less than 80 % is possible irrespective of the process configuration used, like the fixed film system at the Hattingen WWTP or the Sequential Batch Reactor (SBR) process at the Strass WWTP. While, in both cases, the operating costs are relatively low, the investment costs vary significantly as these strongly depend on the specific site conditions, i.e. the possibility to use existing reactors and machinery of the WWTP.