Stricter regulations regarding nitrogen and phosphorus removal push conventional activated sludge or SBR biological processes to their limits. The cyclic activated sludge technology process could be an alternative, which proved to push the technological limits further down. The process is characterized by simultaneous carbon, suspended solids, nitrogen and phosphorus removal in one single step. Buffer tanks, anoxic tanks or final clarifiers are not necessary. The process has already been applied in hundreds of municipal wastewater treatment plants (WWTP) worldwide. At an 18,000 PE municipal WWTP in Austria a case study has been made to investigate the potential of full biological nutrient removal. This plant in the Austrian Alps is further subject of low ambient raw water temperature of minimum 6°C and high seasonal change due to skiing tourism. The case study included low load season in autumn 2019 and high load season in winter until spring 2020. The results of the case study show that the process is highly robust in terms of low wastewater temperature and high load variations. The total nitrogen (T-N) removal achieved was in average over 90% (< 6 mg/l). The study also showed that the T-P effluent could be reduced with ferric top up dosing far below < 0.5 mg/l, even 0.25 – 0.30 mg/l are possible without further post-treatment.
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