Avgång av lustgas från luftningsprocessen på Käppalaverket
The requirements for wastewater treatment plants (WWTP) are gradually increasing, partly because of higher incoming loads, which in turn require a more efficient treatment process. In parallel with environmental and economic goals there are also desires to lower energy consumption without compromising the wastewater treatment. Furthermore, there is a demand on WWTP´s to alter eutrophication in aquatic systems through better nitrogen and phosphorus removal. An increasing number of WWTP´s have acquired biological nitrogen and phos-phorus removal. A by-product of this type of biological treatment is the formation of nitrous oxide, which is a potent greenhouse gas. But how large is the WWTP´s share of nitrous oxide to the total emissions? To answer this question an increasing number of WWTP´s around the world have begun to study the emission of nitrous oxide. Today it is still not entirely clear where in the process nitrous oxide forms and why. Studies have shown that the emissions are different from different sewage treatment plants, partly because the plants have different loads and treatment processes. It is therefore essential that more studies are made to get a better understanding of the emission mechanism. In the autumn 2011, measurements of nitrous oxide emissions were conducted on Käppala-verket on Lidingö in cooperation with IVL the Swedish Environmental Research Institute. The purpose of the measurements was mainly to get an indication of how much nitrous oxide Käppalaverket emits each year. In addition tests were also conducted on how emissions could be reduced by comparing two different aeration strategies, by keeping different constant levels of oxygen and by using ammonium-feedback. In this latter test correlations between nitrous oxide emissions and various process parameters were studied. The experiments with different control strategies are based on previous American studies that have demonstrated that the oxygen supply of nitrifiers may be crucial for how much nitrous oxide will be emitted. The results of this study show that Käppalaverket´s biological treatment process emits 5.5 tones N2O/year representing 0.3% of the incoming nitrogen. Attempts to compare two different aeration strategies did not fall out as expected due to process disturbances. Collected data have also been analyzed and compared, with focus on causal relationships of nitrous oxide emission. Any clear correlation between nitrous oxide and the studied process changes have not been detected. The study shows that low oxygen levels do not seem to produce more nitrous oxide.Keywords: Nitrous oxide, activated sludge process, nitrification, denitrification, greenhouse gas, sewage treatment