Wastewater treatment plays an important part in the global ambition to reduce the climate impact of human activities. The potent greenhouse gas nitrous oxide (N2O) is often the most important fraction of emissions in a wastewater treatment plant (WWTP) (Parravicini et al., 2022). Sludge reject water from the dewatering processes returns high nitrogen (N) loads to the WWTP which increases the need for aeration. To reduce aeration energy, WWTPs often install intensified sidestream deammonification processes using anammox bacteria.
Increased Emissions from Sidestream Processes
Scientific lab and full-scale research over the last decade have shown that these processes are at risk of large emissions of N2O (e.g. Vasilaki et al., 2019). Conventional wastewater processes have emissions in the range of 0.05 - 1.5% kg N2O/kg Ninlet. However, sidestream treatment processes are regularly reported in the range of 4-6% or more.
This demonstrates the need to optimize the process in accordance with climate neutrality goals. Any improvements should be weighed against the total environmental impact of the process including the total greenhouse gas emissions.
N2O Mitigation from Pilot to Full-Scale
In Belgium, one of the leading utilities for sewer transport and wastewater treatment, Aquafin, has set ambitious targets for climate neutrality. Therefore, they monitor the emissions in a pilot-scale plant, a twin of the full-scale deammonification process at the WWTP in Dendermonde, Belgium (Fenu et al., 2019). The reactor is equipped with off-gas nitrous oxide measurement as well as the N2O Wastewater Sensor for dissolved N2O. With the combination of the two measurements, Aquafin can fine-tune the effect of different aeration regimes.
In figure 1 (click to enlarge), you can see the results of short-pulsed aeration (left) vs. longer aeration periods (right). The results show that to decrease emissions from deammonification processes, it is advantageous to aerate in shorter pulses, avoiding the build-up of nitrite, minimizing dissolved N2O formation, and the subsequent biomass selection. With these results in hand, Aquafin implemented short aeration periods at the WWTP in Dendermonde to start minimizing N2O in full-scale.
Short pulses of aeration resulted in a drop in the emission factor from 3.3±0.6% to 1.3±0.2% kg N2O/kg Nremoved. Furthermore, a full control implementation produced a further drop in the N2O emission factor to 0.9±0.2 % kg N2O/kg Nremoved, but the load capacity dropped by 19.79±3.1%.
According to Alessio Fenu, senior R&D Engineer at Aquafin, the dissolved measurement of N2O is a very useful tool for investigating any process in view of N2O mitigation, and crucial in setting up a real-time process control.
N2O Measurements as a Control Strategy
In another example, researchers from a leading utility in Denmark have developed a control strategy where continuous aeration and loading are based on the online N2O value instead of controlling it based on pH. This is effectively using N2O as a proxy for NO2-, severely reducing the likelihood of a substrate imbalance of NH4+/NO2- and subsequent N2O formation.
Using a methodical and fact-based approach these developments show the potential to actively reduce greenhouse gas emissions from key wastewater processes.