Innovative high energy-efficient and cost-effective wastewater treatment concept
Investment in Gdansk aimed to develop and test energy efficient and cost-effective concept uniting processes that require low energy consumption treatment. The investment included a combined anammox-constructed wetland pilot plant and additional energy meters installed at the WWTP. The examined technology focuses on minimizing energy consumption for the removal of organics and nutrients while maximizing recovery of organic matter from wastewater to produce renewable energy.
The concept enabled the possibility of recovering a high fraction of organic carbon. Increased carbon extraction improves energy balance of a plant due to higher production of biogas that can be utilized in a combined heat and power plant (CHP) to generate surplus renewable power. Furthermore, the anammox-based process allows a shortcut in the nitrogen cycle, as anammox bacteria convert ammonium and nitrite directly into nitrogen gas. This enables nitrogen removal at 60% lower oxygen consumption compared to conventional nitrification-denitrification systems. Besides, the process does not require a carbon source for denitrification that enables higher carbon extraction. Therefore, the concept of innovative combined technology considerably improves the energy balance of the plant and allows making it cost-effective and energy-positive. Recovering energy from wastewater treatment brings many environmental and health benefits.
Deammonification has been widely applied at wastewater treatment plants as a cost effective process to treat sidestreams with high nitrogen load. Applying the deammonification process in the mainstream, however, still presents a challenge. Major barriers in this application include low temperature, low ammonia concentration and high COD/N ratio.
The examined process configuration included primary, secondary and tertiary treatment steps. The physical-chemical primary treatment consisted of two stage flocculation tank and primary sedimentation tank. The secondary treatment incorporated integrated fixed-film activated sludge (IFAS) reactor, equipped with mixer and fine bubble air diffuser, and coupled with the secondary sedimentation tank. The IFAS reactor (720 litres) was inoculated with anammox bacteria immobilized on AnoxKaldnes K5 plastic carriers from the Sjölunda WWTP in Malmö and suspended growth activated sludge from the Wschód WWTP in Gdańsk. The tertiary treatment for removal of remaining organic matter and nutrients was demonstrated by two stage horizontal flow constructed wetland (HF-CW) followed by vertical flow constructed wetland (VF-CW).
In order to monitor process performance grab samples were collected and analysed in the laboratory.
Moreover, the process being tested was monitored remotely with WTW on-line instruments for measuring concentration of solids, N-HH4, N-NO3, N-NO2, dissolved oxygen (DO), pH, conductivity and oxidation reduction potential (ORP). Removal efficiency of organic carbon and nutrients were observed after each step of the wastewater treatment process. The effect of C/N ratio and temperature variations on TN removal were investigated, with particular attention to the efficiency and resilience to low temperature of deammonification.
The pilot plant was fed with real wastewater after mechanical treatment of the temperature from 13.3°C to 25.9°C. Performance of the IFAS reactor was evaluated under different hydraulic loading rates between 15.0 and 22.2 l/h. Nitrogen loading rates of the reactor varied in the range of 40–75 g N/(m3·d). The reactor was aerated intermittently 20–25/40 min aeration on/off time. Dissolved oxygen (DO) concentration was kept at the desired set points between 1.20 and 1.75 mg O2/l by means of the PLC control system linked with air valve and oxygen probe.
The study results showed the possibility of applying the combined anammox-constructed wetland system to remove TN and organic carbon from the mainstream efficiently at low wastewater temperature and unfavourable COD/N ratio. After chemical precipitation in primary treatment the ratio of COD/N in wastewater was decreased from 8.0 ± 0.7 to 5.0 ± 0.4, which resulted in higher by 48% recovery of organic compounds compared to that observed at the Wschód WWTP. At hydraulic loading rates of the IFAS reactor between 15.0 and 22.2 l/h, the measured removal efficiency of TN in the examined process configuration was high and averaged 84 ± 9%, while the minimum and maximum removal was 67% and 98%, respectively.
Mainstream treatment with the innovative combined anammox-constructed wetland system maximizes energy recovery from wastewater by directing more organic carbon to anaerobic treatment from which more biogas can be captured and utilized in a combined heat and power plant (CHP) to generate renewable power. Reducing aeration energy consumption and maximizing recovery of organic carbon is the key to achieve energy-positive wastewater treatment plant.