Greenhouse gas emission from drying and rewetting stored sewage sludge

Sludge produced from wastewater treatment plants is not only rich in nutrients such as nitrogen (N) and phosphorus (P), but also contains toxic organic contaminants and pathogenic organisms which can be potentially hazardous to health of plants, animals and humans. In order to minimize the pathogen contamination in sludge before land application, a minimum of one year of sewage sludge storage has been proposed by Swedish Environmental Protection Agency. Emission of greenhouse gas (GHG) carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) during storage of sludge has been observed, however, studies on the influence of moisture change in the stored sludge caused by natural evaporation and precipitation on the amount of gas emitted are hardly found. A laboratory study was performed to determine the effect of drying and rewetting of sewage sludge on greenhouse gas emission during the storage phase. Emissions from two types of digested sewage sludge were investigated; mesophilically digested sludge and thermophilically digested sludge. For each substrate, three types of treatments were applied; (1) drying/rewetting (17 days/15 days), (2) drying/rewetting (17 days/15 days) with sludge sampling for chemical analysis at the end of the drying phase and (3) Control treatment for which moisture content of sludge during the whole storage phase (32 days) was kept as constant as possible. Results from the experiment showed that drying slightly reduced the accumulated emission of CO2 from both substrates and emission of N2O from thermophilically digested sludge. After the irrigation was applied, a substantial increase in the daily emission of CO2 from mesophilically digested sludge and daily emission of N2O from both substrates was observed. Drying and rewetting hardly made any impact on CH4 emission. Higher carbon emissions from mesophilically digested sludge than that of the thermophilically digested sludge were observed. Reasons to the observations could be due to the difference in: (1) carbon/nitrogen ratio between the two substrates; (2) different moisture content of the two substrates and (3) different digestion treatments applied when producing the sewage sludge