N2O Sensor Controls Emissions from Deammonification Processes

N₂O Sensor Controls Emissions from Deammonification Processes

Tech Notes & References

Research has shown that highly loaded processes based on Anammox are a risk for large emissions of N₂O.

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 side stream processes, such as deammonification using anammox bacteria.

Anammox as Alternative Process

Anammox is a nitrogen removal process where N₂ is formed from NH₄⁺ and NO₂^- (Fig. 1). Controlling the substrate availability and balance is important for the successful implementation and operation of the process.

Two substrate control strategies that have been suggested are PNA (Partial nitrification/anammox) and PdNA (Partial denitrification/anammox). The PNA process aims at partial nitrification by inhibiting NOB (nitrite-oxidizing bacteria) to supply NO₂^- for anammox. On the other hand, PdNA aims at partial denitrification to supply NO₂^- (Fig. 1).

While it might appear as a detour, PdNA for anammox is often operationally easier to achieve. In PdNA, the relationship between COD/N and N₂O can be used for controlling the partial denitrification process to accumulate nitrite for anammox by keeping the COD/N ratio below 1.

The N₂O sensor can monitor the N₂O formation and decrease the carbon loading to achieve partial denitrification to NO₂^-, which can be used for anammox. The carbon can then be harvested and used for biogas production.

NO₂^- is the key substrate, besides NH₄⁺, for anammox, but it is difficult to measure NO₂^- with present sensors. The measurement will be indirect as NO₂^- is formed and consumed inside biofilm or granules and autosampler-based technologies do not deliver real-time data. Unlike NO₂^-, N₂O is not consumed by the anammox bacteria and therefore online N₂O monitoring will provide a more precise measure of the substrate balance in real time. As N₂O is tightly linked to NO₂^- concentration through both nitrification and denitrification, the N₂O sensor can be used as a proxy for NO₂^-.

N₂O Mitigation from Pilot to Full-Scale

The main Belgian utility for sewer transport and wastewater treatment, Aquafin, has set ambitious targets for climate neutrality. To better understand their key emission sources, Aquafin monitors N₂O in a pilot-scale deammonification plant, a twin of the full-scale process at the WWTP in Dendermonde, Belgium. The reactor is equipped with off-gas nitrous oxide measurement as well as a Unisense Environment N₂O wastewater sensor for dissolved N₂O. With the combination of the two measurements, Aquafin can fine-tune the effect of different aeration regimes.

The figures show the results of short-pulsed aeration (left) vs. longer aeration periods (right). In order to reduce emissions, it was advantageous to aerate in shorter pulses, avoiding the build-up of nitrite, minimizing dissolved N₂O formation, and promoting subsequent biomass selection. Short pulses of aeration resulted in a drop in the emission factor from 3.3% to 1.3% kg N₂O/kg N_removed. With these results in hand, Aquafin was able to implement short aeration periods at the WWTP in Dendermonde to start minimizing N₂O in full-scale. A simulated full control implementation produced a further drop in the N₂O emission factor to 0.9% kg N₂O/kg N_removed, while the load capacity dropped as well.

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.”

In another example, researchers from a leading utility in Denmark have developed a control strategy where aeration and loading are based on the online N₂O value instead of controlling it based on pH. In essence, an N₂O setpoint is used to reduce aeration, tuning down nitrification when there is too much NO₂^- present. This effectively uses N₂O as a proxy for NO₂^- and reduces the likelihood of a substrate imbalance of NH₄⁺/ NO₂^- and subsequent N₂O formation.

Avoid N₂O emission and document your CO₂ footprint

In conclusion, the N₂O Wastewater Sensor can be used as a tool for process control in anammox by tightly controlling the substrate ratio between Nitrite and Ammonium. Specific control strategies depend on the reactor design and its limitations. Future-oriented utilities and technology providers have the ability to document CO₂-footprints from sidestream processes, allowing direct comparison with conventional activated sludge processes. This allows making informed decisions about process control and future investments with overall climate neutrality as a target.

References

Parravicini, V., Nielsen, P. H., Thornberg, D., & Pistocchi, A. (2022). Evaluation of greenhouse gas emissions from the European urban wastewater sector, and options for their reduction. Science of The Total Environment, 838

Vasilaki, V., Massara, T. M., Stanchev, P., Fatone, F., & Katsou, E. (2019). A decade of nitrous oxide (N2O) monitoring in full-scale wastewater treatment processes : A critical review. Water Research, 161, 392–412.

Fenu A., Smolders S., De Gussem K., Weemaes M., (2019). Conflicting carbon footprint and energy saving in a side-stream Anammox Process. Biochemical Engineering Journal, 151, 107336.

High N₂O Emissions from Side Stream Processes

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).

Conventional wastewater treatment typically has emissions in the range of 0.05 - 1.5% kg N₂O/kg N_inlet. Side stream treatment processes have been reported in the range of 2-6% or more.

Signal can be used to control deammonification

N₂O can be used as a proxy value for NO₂^- and is often easier to measure in real time. This enables control strategies, in which an N₂O setpoint is used for aeration and loading, balancing the substrate availability and reducing the emissions from deammonification.