Factors influencing long-term settlement of municipal solid waste in laboratory bioreactor landfill simulators

Long-term settlement of municipal solid waste (MSW) in bioreactor and conventional landfills is caused by a number of mechanisms. Laboratory tests in bioreactor landfill simulators allow for a careful assessment of each mechanism and of the factors that affect it. A systematic review and synthesis of 98 tests from 29 mesoscale simulator studies available in the literature are presented. Long-term settlement is divided into three phases: the transitional phase, the active biodegradation phase, and the residual phase. Duration, strain, and long-term compression ratio (equal to the ratio of strain to duration) are calculated for each phase. Statistical analysis of the data is conducted. The majority of the long-term settlement occurs during the active biodegradation phase (9.5% strain on average), and the mean compression ratio is 0.168. The other two phases contribute significantly less to the total long-term settlement. The effects of the initial and operational conditions of simulators on the magnitude and rate of long-term settlement of MSWare explored. External vertical stress application prior to long-term testing is found to reduce the amount and rate of long-term settlement. Aeration of waste during long-term testing increases the settlement rate by promoting aerobic biodegradation. MSW long-term settlement is also affected by waste composition, total unit weight, and simulator size.