Case Study: Influence of positioning of N2O Wastewater Sensors
Kralingseveer WWTP implemented N2O monitoring in their aeration tanks. One sensor was placed in the aerated zone, and another was placed in the unaerated zone at each compartment (Fig. 1). N2O emissions predominantly occur in aerated zones due to air stripping, and the N2O emission was assumed to be more variable in the aerated zone compared to the unaerated zone. To quantify and understand the variation in N2O emissions in the aerated zone, the WWTP monitored various spots within the aerated zone for a week. Meanwhile, they kept the sensor in the unaerated zone in the same position (Fig. 1).
Scounting Campaign to Identify Sensor Placements
The WWTP chose the best representing spots by monitoring four different spots from the beginning to the end of the aerated zone in both the primary aeration tank and the aeration tank. Monitoring showed that N2O emissions were highest at the beginning of the aerated zone and lowest at the end (Fig. 2A). In the aerated tank, they observed that the N2O emission was highest in the second half of the aerated zone (Fig. 2B). Overall, emission was higher in the aerated zone compared to the unaerated zone.
The scouting campaign led to a position representing an average value for long term monitoring in each aerated zone. In the primary aeration tank, they chose a position between P2 and P3, and a position between A2 and A3 in the aeration tank.
The Aerated Zone Accounted for about 90% of N2O Emissions
The primary aeration tank with a higher load produced higher N2O emission compared to the secondary aeration tanks. In both sections, the N2O emissions were higher in the aerated zone and accounted for about 90% of the N2O emissions, highlighting the importance of focusing on the aerated zone. The scouting campaign showed that the position of the N2O Wastewater Sensor in the aerated zone yielded differences in the measured N2O emissions. Ideally, it would be useful to have one sensor as a reference sensor when another sensor changes position.
The N2O emissions varied by only 23% in the primary tank, and all positions provided a good estimate of the N2O emission. However, the variation in the aeration tank was higher (50%), and the trend of increasing emission towards the end of the aerated zone could indicate a COD limitation, which has been shown to increase N2O emission. In that case, a representative position needs to be more carefully selected. Learn more in our technote on carbon dosage.
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