D 2.5: Options for nitrogen removal after advanced carbon extraction
POWERSTEP aims to demonstrate energy-positive wastewater treatment, which requires the utilization of the internal carbon in the wastewater to produce biogas. An increased carbon extraction for biogas production challenges conventional nitrogen removal, in which denitrifying bacteria depend on an easily accessible source of carbon. Hence, POWERSTEP focuses on novel concepts for nitrogen removal in the mainstream line, with a minimum requirement of carbon. Within work package (WP) 2 of POWERSTEP, Mainstream nitrogen removal, three different tasks have been performed that represents three different options for nitrogen removal after advanced carbon extraction. In task 2.1 Advanced control strategies, it was demonstrated in Case study Westewitz WWTP that, with an advanced control system where polymer addition in the primary treatment was based on minimum carbon source requirement for denitrification, a high degree of carbon extraction could be achieved while still meeting the effluent demands for nitrogen, utilizing the conventional nitrification-denitrification pathway. In task 2.2 Mainstream deammonification, the concept using a specific group of autotrophic bacteria, commonly referred to as anammox bacteria, for removal of ammonia to nitrogen gas was demonstrated in full scale prototype in Case study Sjölunda WWTP. Since anammox bacteria are not dependent on carbon for nitrogen removal, the full potential of carbon recovery for biogas production can be reached. In task 2.3 Mainstream duckweed reactor, the potential of using duckweed for high production of vegetal organic biomass for biogas production and simultaneously achieve nitrogen removal, was demonstrated in Case study Westewitz WWTP. This deliverable provides a guideline, where the different options to remove nitrogen within municipal wastewater after advanced carbon extraction are presented based on the performed tasks in WP2 of POWERSTEP, and in comparison with conventional processes. Special emphasis is made on resources (energy, footprint, chemicals) and performances (removal stability, flexibility, sludge production). The outcome from POWERSTEP (tasks 2.1.-2.3) and comparisons with conventional processes showed that in order to meet the full potential of carbon recovery and turning the wastewater treatment plant truly energy positive while still meeting high nitrogen removal requirements, there is a need to implement anammox removal technology. However, the full scale demonstration showed that even if the potential is clearly there, the technology is not yet mature enough to be commonly implemented during cold (<15°C), diluted (low NH4N concentrations) and unfavourable (high) COD to N conditions in the wastewater, why further full scale demonstrations are highly recommended. Under more favourable, and especially warmer wastewater conditions, the anammox technology is today ready for the early frontrunners. Finally, the power of an advanced control strategy for conventional nitrification and denitrification should not be underestimated. With an optimised extraction of primary organic carbon, a large increase of biogas and energy recovery can be obtained without jeopardizing the nitrogen limits. This strategy is ready for implementation and should be evaluated on all wastewater treatment plants.