- Department of Energy and Technology, Swedish University of Agricultural Sciences
Freezing/thawing cycles can significantly increase greenhouse gas emissions due to physical and biological mechanisms. Therefore this research was done to see if this is also the case for compost and sewage sludge. The main focus was on nitrous oxide. The set-up of the experiments was based on the outcomes of several trial experiments. 4 substrates were tested: non-nitrified mesophilic sewage sludge, nitrified mesophilic sewage sludge, 2 weeks old compost and 1 month old compost. The two composts had a clear difference in ammonium content. The substrates were first frozen at a temperature of -27 °C, and then thawed at a temperature of 5 °C. Controls were kept at a temperature of 5 °C during the whole experiment. All substrates were tested in triplicates. Emissions of carbon dioxide, methane and nitrous oxide were measured. No clear effect of freezing/thawing was observed on the greenhouse gas emissions from non-nitrified sewage sludge. From the nitrified sewage sludge the emissions of both carbon dioxide and nitrous oxide increased during thawing. The cumulative emission of carbon dioxide was 17500 µg g-1 initial C from the frozen and thawed sewage sludge, and 11170 µg g-1 initial C from the controls. The cumulative emission of nitrous oxide in the sewage sludge that had been frozen and thawed was about 3500 µg g-1 initial N, for the controls this was 3000 µg g-1 initial N. Both the increase carbon dioxide and nitrous oxide emissions were probably due to increased substrate availability, caused by the die-off of micro-organisms during freezing. The nitrous oxide probably came from denitrification. In both of the composts no effect of the freezing and thawing was observed on the emissions of carbon dioxide and methane, but it had a clear effect on the emission of nitrous oxide: in the 1 month old compost a clear emission peak was observed during the first days of thawing. For the one month compost this was probably due to a physical mechanism of trapped nitrous oxide inside the compost which could escape during the thawing period. An unexpected result was the large difference between the two composts considering the amounts of nitrous oxide emissions, which were expected to be approximately the same due to equal nitrate amounts. However, the cumulative nitrous oxide emission from the frozen and thawed 2 weeks compost was 3,00 µg g-1 initial N at the end, while from the 1 month compost this was 1220 µg g-1 initial N. Since the ammonium in the 1 month compost had decreased with 32,5%, and only 17,5% for 2 weeks compost, the large amount of nitrous oxide probably came from the conversion of ammonium. However, it is still not clear whether this comes from nitrification of the ammonium, or from denitrification of the nitrate formed during the experiment. The conclusion was that freezing/thawing increases emissions of carbon dioxide and nitrous oxide from nitrified sewage sludge during thawing, and the emissions of nitrous oxide from compost during thawing. Recommendations for further research are to find out which processes, nitrification or denitrification, are responsible for the nitrous oxide production. Also research to compost with different nitrate amounts is recommended.
ammonium; compost; denitrification; freezing/thawing; greenhouse gas emissions; nitrate; nitrification; sewage sludge
Rapport(Institutionen för energi och teknik, SLU)
2013, number: 2013:062
Publisher: Department of Energy and Technology, Swedish University of Agricultural Sciences