Frederikshavn Water Utility

The municipal water utility “Forsyningen” located in Frederikshavn, Denmark has focused on the implementation of methodologies to better estimate and quantify greenhouse gas emissions from a number of different wastewater processes within their municipal wastewater treatment plants (WWTPs).

Determining the Actual N₂O Derived CO₂ Footprint

The utility runs both recirculation and biofilm wastewater processes. Due to increasing uncertainty in using emission key numbers, the utility contacted Unisense Environment for assistance in determining the actual N₂O derived CO₂ footprint. As the foundation for the new initiative, the utility started out with monitoring N₂O emission from a standard recirculation.

First Data Analysis after Two Weeks

In order to save energy and increase the N-removal capacity, the utility operated the aerobic zone with intermittent aeration, allowing denitrification in the aerated zone depending on the ammonium levels in the tank. Unisense Environment installed two N₂O Wastewater Systems, one placed in the anoxic tank and the other in the aerobic tank. To facilitate the understanding of the N₂O formation, the utility logged N₂O sensor data with all other plant data through a standard integration to the SCADA system.

Following a quick 2-point calibration procedure, the sensors started monitoring N₂O levels. After a 14 day measuring period, the utility extracted a full data series from the SCADA system and sent the data to Unisense Environment for analysis. Using different data management tools, Unisense Environment documented the overall correlations and in particular the N₂O production.

Large Variations during a 14-day Measurement Campaign

From the 14-day period, it was clear that the variation in N₂O emission was extensive. The main emission over the period was linked to two high load events that led to 53% of the total N₂O emission during 42 hours out of the 14 days. Furthermore, low dissolved oxygen set points in the beginning of the campaign led to a significant increase in the N₂O concentration, clearly stressing the correlation between low oxygen and N₂O formation. During the analyzed period, the N₂O formation in the anoxic tank was very low and only during a high ammonium period did the N₂O concentration increase to 0.3 N-mg/L for 30 hours. In the aerated part, N₂O was mainly produced during the anoxic phases introduced by the intermittent aeration and subsequently stripped to the atmosphere by the aeration.

Finally, emission calculations were performed using peer reviewed and validated models to assess the N₂O emissions and derived CO₂equivalents. This was compared with the aeration power derived CO₂equivalents and presented to the management of the water utility as part of a 2-day consultancy service. The total carbon footprint from the period was calculated to be 10 ton CO₂equivalents with N₂O derived CO₂ accounting for 59% of the total emission.

A Shortlist of Problems and Possible Solutions

From the short monitoring and data analysis campaign, a shortlist of problems and possible solutions was derived:

The N₂O concentration and emission is highly variable
Dissolved oxygen control is important to avoid elevated N₂O production and emission
Short periods of high ammonium loads should be avoided by balancing the influent load
Control strategies with COD enhanced denitrification can potentially diminish the N₂O emission
Energy optimization using pause-aeration strategies must be coupled with N₂O monitoring to avoid excessive N₂O emissions.

With a small investment, the utility gained a real insight into the source and causes of its N₂O derived emissions and can now act towards minimizing the climate impact from N₂O.