Model-based dynamic simulation study to boost the WWTP performance in winter tourism regions

Wastewater treatment based on activated sludge processes faces a major challenge due to an increased number of people contributing to wastewater and changes in wastewater characteristics in regions with seasonal winter tourism. The combination of significant increases in carbon (COD) and nitrogen (N) loads by a factor of four to five within a short periode of time and low wastewater temperatures (<12 °C), represents a bottleneck in meeting essential effluent requirements. According to literature research, however, little attention has been paid to these special framework conditions for wastewater treatment. Based on this fact, a model-based simulation study of the wastewater treatment plant (WWTP) in Montafon (Austria) evaluates operational optimization approaches for the above-mentioned framework conditions and is intended to provide new insights for nitrification-optimized and energy-optimized wastewater treatment. The WWTP Montafon is designed for a COD load of 6250 kg d−1, a daily dry weather water discharge of 12,700 m3 d−1 and is a typical plant in a winter tourism region. The 5 studied smart operation approaches shown are a combination of two different aeration control strategies based on a time-controlled (i) or ammonia-based feedback aeration control (ABAC) (ii) for intermittent nitrification-denitrification and sludge removal control strategies based on a constant sludge concentration to 3 g L−1 (iii), a constant sludge retention time (SRT) to 20 days (iv) and a constant sludge retention time to 20 days combined with a higher level sludge concentration control to max. 3 g L−1 (v). The simulation results show that a combination of a time-controlled aeration strategy and controlling the sludge concentration to a constant 3 g L−1 is not recommended in order to comply with the emission limits. Under the mentioned influent conditions, ABAC control alone does not inevitably lead to a nitrification-optimized process and reduction in energy consumption. Sufficient nitrifying bacteria in the activated sludge are the key to a nitrification optimized process and reduction of energy consumption for aeration. Hence, a combination of an intermittent aeration control based on ammonium measurement and a constant sludge retention time to 20 days is preferable for nitrification capacity and is the most energy-efficient control strategy to obtain required COD and N effluent quality. An average energy consumption of 57–63 Wh PE120−1 d−1 for the aeration is to be expected for combinations of (i,iv), (ii, iii) (ii,iv) and (ii,v) which is low in relation to the influent challenges and to plants with higher wastewater temperatures (>12 °C) between 27 Wh PE120−1 d−1 and 121 Wh PE120−1 d−1 according to literature. The approach with a time-controlled intermittent aeration and a SRT to constant 20 days was implemented at the WWTP Montafon and confirms the results of the associated simulation. According to this simulation study, a 35 % increase in nitrification capacity in existing WWTPs designed as activated sludge processes can be achieved through optimized operating conditions without additional high investments.