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.
With adoption of the European Council Directive on urban wastewater treatment in 1991 (EU, 1991), a number of new emission standards were set for wastewater discharge. In sensitive areas, nutrient removal is mandatory for all plants with more than 10,000 population equivalents (PE). To ensure compliance with European legislation, the German effluent requirements for municipal wastewater treatment plants were laid down in the German Wastewater Ordinance (2002). For treatment plants with more than 10,000 PE, effluent criteria for nitrogen and phosphorus have been set to 18 mg/l N and 2 mg/l P respectively. While more stringent standards (13 mg/l N and 1 mg/l P) are applicable for treatment plants with a capacity of more than 100,000 PE. In contrast to most other European states, effluent standards in Germany are monitored on the basis of the so-called qualified grab samples or 2-h composite samples. This results in more stringent requirements with regard to process stability. As implementation of the Urban Wastewater Directive (UWWD) had to be finalized by the end of 2005, nearly all existing wastewater treatment plants (WWTPs), for which nutrient removal now is mandatory, had to be extended or at least optimized by that deadline. Athough today a number of European countries are being able to fulfill their obligations under the Directive, there is still a substantial backlog demand when taking into account all EU member states.
To improve treatment quality, either conventional measures, like e.g. the construction of new tanks, or approaches geared to process optimization can be used. As far as nitrogen elimination is concerned, a separate treatment of return flows from sludge treatment is a promising option because substantial nitrogen loads are usually recycled from the sludge treatment facility when sludge digestion and dewatering are being used. Figure 1 summarizes a Germany-wide survey on the nitrogen loads recycled to the inflow of the treatment plant. Calculated average return loads, under normal conditions, are in the range of 10 to 15% of the inflow nitrogen load. Therefore, it can be expected that with a treatment technology utilizing separate elimination of this specific nitrogen load it is possible to substantially improve the loading situation for the main or central wastewater treatment process.