Effect of oxidized leachate on degradation of lignin by sulfate-reducing bacteria

Abstract

Municipal solid waste materials (MSWs) in landfills need a long period of stabilization because lignin compounds in MSWs and leachate are not readily biodegraded, but inhibit methanogenic metabolism. Recirculation of leachate into the landfill offers the potential advantage of increasing the rate of decomposition of organic matter. However, the degradation of lignin by leachate recirculation alone is quite difficult. Several recent studies have demonstrated that sulfate-reducing bacteria (SRB) were able to degrade lignin compounds. In this study, batch tests were conducted to investigate the impacts of SRB enrichment on lignin decomposition rates as well as the decomposition of other biodegradable organics. Further, the effects of nitrite and nitrate on lignin degradation rates were also studied. A 16S rRNA assay showed that the SRB used herein, which were obtained by enriching solid waste collected from a closed MSW landfill, were Thaurea sp. and Desulfovibrio sp. Lignin was found to be biodegraded by the SRB and the rate of lignin removal per unit of waste volatile suspended solid was 2.9 mg lignin g(-1) VSS day(-1). It was found that the initial degradation rate increased under higher initial lignin concentrations. However, the degradation rate during days 6-19 became slower than that during the initial 9 days because lignin consisted of complexly bonded aromatic compounds that were not readily biodegradable. Adding other organics such as lactate seemed to improve the rate and amount of lignin degradation, probably due to the increase in SRB associated with consumption of the additional organics. The lignin removal percentage decreased with increases in oxidized nitrogen (nitrite or nitrate) concentrations, indicating that oxidized nitrogen could inhibit SRB activity. Conclusively, the study verified the existence of SRB in the landfill and showed that the SRB could be activated for the degradation of lignin by the recirculation of the leachate, which is consistent with other studies showing that leachate recirculation could shorten the stabilization period of the landfill.