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

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.

Evolution of clog formation with time in columns permeated with synthetic landfill leachate.

Laboratory column tests conducted to gain insight regarding the biological and chemical clogging mechanisms in a porous medium are presented. To seed the porous medium with landfill bacteria, a mixture of Keele Valley Landfill and synthetic leachate permeated through the column under anaerobic conditions for the first 9 days of operation. After this, 100% synthetic leachate was used. The synthetic leachate approximated Keele Valley Landfill leachate in chemical composition but contained negligible suspended solids and bacteria compared with real leachate. The removal of volatile fatty acids (VFAs), primarily acetate, in leachate as it passed through the medium was highly correlated with the precipitation of calcium carbonate (CaCO(3(s))) from solution. The columns experienced a decrease in drainable porosity from an initial value of about 0.38 to less than 0.1 after steady state chemical oxygen demand (COD) removal, resulting in a five-order magnitude decrease in hydraulic conductivity. The decrease in drainable porosity prior to steady state COD removal was primarily due to the growth of a biofilm on the medium surface. After steady state COD removal, calcium precipitation was at least equally responsible for the decrease in drainable porosity as biofilm growth. Clog composition analyses showed that CaCO(3(s)) was the dominant clog constituent and that 99% of the carbonate in the clog material was bound to calcium.

Influence of landfill leachate suspended solids on clog (biorock) formation.

Laboratory column tests were performed to evaluate the role of leachate-suspended-solids in clogging a granular material permeated with Keele Valley Landfill leachate. The development of the clog material was a result of biological, chemical, and physical processes occurring within the column. The increase in volatile solids, which contributed to clog development over time, was primarily due to the retention of volatile suspended solids and growth of a biofilm capable of removing acetate, propionate, and butyrate from the leachate. Acetate fermentation was primarily responsible for precipitation of calcium within the column. The precipitated calcium and retention of inorganic suspended solids contributed to the increase in clog inorganic solids. Over the duration of the experiment, 3.7 times more calcium was precipitated in the column (due to acid fermentation) than was retained with inorganic suspended solids. Clogging resulted in a greater than 60% reduction in drainable porosity and a six-order magnitude decrease in hydraulic conductivity. The potential practical implications with respect to pipe cleaning and leachate recirculation were discussed.

Predicting biogeochemical calcium precipitation in landfill leachate collection systems.

Clogging of leachate collection systems within municipal solid waste landfills can result in greater potential for contaminants to breach the landfill barrier system. The primary cause of clogging is calcium carbonate (CaCO3(s)) precipitation from leachate and its accumulation within the pore space of the drainage medium. CaCO3(s) precipitation is caused by the anaerobic fermentation of volatile fatty acids (VFAs), which adds carbonate to and raises the pH of the leachate. An important relationship in modeling clogging in leachate collections systems is a yield coefficient that relates microbial fermentation of VFAs to precipitation of calcium carbonate. This paper develops a new, mechanistically based yield coefficient, called the carbonic acid yield coefficient (Y(H)), which relates the carbonic acid (H2CO3) produced from microbial fermentation of acetate, propionate, and butyrate to calcium precipitation. The empirical values of Y(H) were computed from the changes in acetate, propionate, butyrate, and calcium concentrations in leachate as it permeated through gravel-size material. The theoretical and empirical results show that the primary driver of CaCO3(s) precipitation is acetate fermentation. Additionally, other non-calcium cations (e.g., iron and magnesium) precipitated with carbonate (CO3(2-)) when present in the leachate. A common yield between total cations bound to CO3(2-) and H2CO3 produced, called the calcium carbonate yield coefficient (Yc), can reconcile the empirical yield coefficient for synthetic and actual leachates.