Effects of different additives on evaluation of maturity and gaseous emissions during sewage sludge composting

Abstract: Sewage sludge is an unavoidable by-product of the wastewater treatment process. In recent years, the amount of sewage sludge generated has increased dramatically with the upgrading and expansion of wastewater treatment plants in China. Composting is an effective and economical method for the treatment of solid or semisolid biodegradable waste prior to land application. Stable and mature compost can be applied to soil as an organic amendment to improve plant growth and soil fertility, as well as enhancing the function of soil for carbon sequestration. However, the application of unstable and immature compost may fix nitrogen in the soil and restrict plant growth by competing for oxygen in the rhizosphere and releasing toxic substances. Bulking agents are always required to modify the properties of sewage sludge during composting because of the high moisture contents, low C/N ratio and high density of the material. Emissions of gases such as ammonia (NH3), methane (CH4) and nitrous oxide (N2O) can cause secondary pollution and reduces the environmental benefits of compost. In addition, these substances contribute to nutrient loss from the compost product and therefore lower the quality of the compost product. This study investigated the effects of different additives on maturity and gaseous emissions (NH3, CH4 and N2O) during sewage sludge composting. Two different additives (cornstalks and woody peat) were used to co-compost sewage sludge under aerobic conditions in 60 L reactors for a 35 d period. To create aerobic conditions, the air pump installed with the reactor was operated by an automatic controller with an air flow rate of 0.2 L/(kg?min). In this study, dewatered sewage sludge without anaerobic digestion was collected from a sludge dewatering workshop. The additive amount of both cornstalks and woody peat were 15% raw materials (wet weight). During the experiment, maturity indexes such as temperature, pH value, Electric conductivity, C/N ratio, and germination index were determined, and continuous measurements of gaseous emissions (CH4, N2O, and NH3) were taken. The results showed that adding cornstalks in sewage sludge composting reached the required maturity standard and met the compost sanitation requirements. For the adding of cornstalks, NH3 and CH4 mainly occurred in the mesophilic and thermophilic phase of composting, process, while N2O occurred predominantly in the later period of composting. The cumulative emission of NH3, CH4 and N2O for CS treatment (added cornstalks) was 2.2, 0.14 and 0.09 g/kg, respectively. Over 35 days of composting, the compost temperature of adding woody peat treatment almost approached the ambient temperature, and could not meet the compost sanitation requirements. There was no NH3 emission and little CH4 emission during composting, but lots of N2O occurred in the early period of composting for the treatment of adding woody peat. For the treatment of adding cornstalks, the contribution of NH3, CH4 and N2O to the total greenhouse gas (GHG) emissions was 45%, 36% and 19%, respectively.