Leachate treatment before injection into a bioreactor landfill: Clogging potential reduction and benefits of using methanogenesis

Abstract

In this study, an anaerobic sequencing batch reactor (ASBR) was operated with leachate from Brady Road Municipal Landfill in Winnipeg, Manitoba, Canada. Leachate was collected twice from the same cell at the landfill, during the first and 70th day of the study, and then fed into the ASBR. The ASBR was seeded at the start-up with biosolids from the anaerobic digester from Winnipeg’s North End Water Pollution Control Center (NEWPCC). Due to the higher COD and VFA removal rates measured with the second batch of leachate, an increase of approximately 0.3 pH units was observed during each cycle (from pH 7.2 to 7.5). In addition, CO 2 was produced between cycles at constant temperature where a fraction of the CO 2 became dissolved, shifting the CO 2/bicarbonate/carbonate equilibrium. Concurrent with the increase in pH and carbonate, an accumulation of fixed suspend solids (FSS) was observed within the ASBR, indicating a buildup of inorganic material over time. From it, Ca 2+ and Mg 2+ were measured within the reactor on day 140, indicating that most of the dissolved Ca 2+ was removed within cycles. There is precedence from past researches of clogging in leachate-collection systems ( Rowe et al., 2004) that changes in pH and carbonate content combined with high concentrations of metals such as Ca 2+ and Mg 2+ result in carbonate mineral precipitants. A parallel study investigated this observation, indicating that leachate with high concentration of Ca 2+ under CO 2 saturation conditions can precipitate out CaCO 3 at the pH values obtained between digestion cycles. These studies presented show that methanogenesis of leachate impacts the removal of organic (COD, VFA) as well as inorganic (FSS, Ca 2+) clog constituents from the leachate, that otherwise will accumulate inside of the recirculation pipe in bioreactor landfills. In addition, a robust methanogenesis of leachate was achieved, averaging rates of 0.35 L CH 4 produced/g COD removed which is similar to the theoretical removal of 0.4 L CH 4/g COD. Therefore, using methanogenesis of leachate prior to recirculation in bioreactor landfills will help to (1) control clog formation within leachate pipes and (2) produce an important additional source of energy on-site.