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N2O Sensor Controls Emissions from Deammonification Processes

N2O Sensor Controls Emissions from Deammonification Processes

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.

Aquafin logo
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-.

Denitrification O2 and COD demand
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.

Figure 1a Aquafin

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 Nremoved. 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 Nremoved, while the load capacity dropped as well.

Figure 1b Aquafin

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.

Figure 1c Aquafin

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 Ninlet. 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.

Tech Notes & References

Tech Notes
N2O Sensor
Tech Notes References
N2O sensor matches 91% of measured off-gas emissions

Case Study: Full-scale comparison of N2O emissions determined by liquid sensors and off-gas measurement

Tech Notes References
Direct Effect of Activated Sludge Concentration on N2O Emission and CO2-equivalents at Full-scale

Significant 3-fold increase in IPCC2019 wastewater N2O emission factor supported by Danish studies.

Figur 1_featured_700x400
Tech Notes References
Positioning of the N2O Wastewater Sensor

A case study from Kralingseveer WWTP in the Netherlands explores the influence of sensor placement.

N2O mechanisms
Tech Notes
National N2O Mapping and Reduction of N2O Emission through Advanced Online Control

Based on data from Danish WWTPs through advanced online-control.

Figure 1: Activated Sludge Tank
Tech Notes References
N2O Monitoring Highlights Potential for GHG Emissions Reduction

Learn about the results from N2O monitoring in the activated sludge tanks at Severn Trent’s Spernal sewage treatment plant

Tech Notes
A Tool for Carbon Dosage Control

Monitor the N2O concentration in the liquid and use N2O as a control parameter for carbon dosage in the denitrification process.

N2O Sensor Controls Emissions from Deammonification Processes
Tech Notes References
N2O Sensor Controls Emissions from Deammonification Processes

Learn how the water utility Aquafin controls emissions from deammonification processes using the the N2O Wastewater Sensor

Nitrous Oxide Measurement
Tech Notes
Nitrous Oxide Measurement as Key Step towards Climate-Neutral Wastewater Treatment

Greenhouse gas emissions at wastewater treatment plants are coming into focus as the water industry works to reduce its climate footprint

WWTP-Flensburg_credit-photo-Andreas Große
Identifying N2O at the Flensburg wastewater treatment plant

The investment costs had initially deterred us somewhat. However ...

Tech Notes
Characterizing N2O emissions from WWTPs

A study of three very different WWTPs in Denmark: Bjergmarken (125,000 PE), Holbæk (60,000 PE), and Hvalsø (11,570 PE)

Tech Notes
N2O Emissions from Danish WWTPs – a two year monitoring project

The available data show that the nitrous oxide emission varies in time and between wastewater treatment plants.

Trickling Filters Fig. 1
Tech Notes
Nitrous Oxide Emissions from Trickling Filters

Information regarding N2O emissions from trickling filters is limited, partly caused by the difficulties in capturing off-gases.

Tech Notes
IPCC Greenhouse Gas Emission Factors

Significant 3-fold increase in IPCC2019 wastewater N2O emission factor supported by Danish studies.

N2O Monitoring at Vandcenter Syd bassiner_700x400
N2O Monitoring Puts VCS Denmark at the Technological Forefront

VCS Denmark, one of the largest and oldest water and wastewater companies in Denmark, is actively committed to resource optimization

Frederikshavn Water Utility

Frederikshavn Water Utility contacted Unisense Environment for assistance in determining the actual N2O derived CO2 footprint