Postdoctoral Study: Treatment of Landfill Leachate by Methanogenic and Sulphate-Based Anaerobic Digestion
High concentrations of sulphate in wastewater streams subjected to anaerobic digestion have been considered to be undesirable because of the production of sulphide during digestion, which can inhibit methanogenesis. Appreciable sulphate may be present normally in many industrial effluents, and anaerobic digestion of such effluents results in low biogas formation and high concentrations of sulphide in the effluent.
Sulphate-reducing bacteria (SRB) compete with methanogenic bacteria (MB) for substrates such as acetate and hydrogen, outcompeting MB when sulphate is present in the environment. While the methanogenic bacteria metabolise only a restricted range of substrates (principally acetate and hydrogen), the SRB metabolise a far wider range of substrates, including long chain fatty acids, aromatic compounds etc. The SRB might be expected, therefore, to be more resilient to challenge by novel organic molecules than the MB, and digestion based upon sulphate-reduction therefore more resilient than methanogenic digestion. Furthermore, the production of sulphide during digestion may precipitate many metals as their insoluble sulphides, a process which has been used to reduce high metal content in waste waters. In contrast, methanogenic digestion has a reputation for being sensitive to toxicity by organic inhibitors such as toluene or chloroform and by metals. Inhibition of methanogenesis can also be caused by organic overload when the rates of acid production exceeds methane production, leading to decreased pH and souring of the digester.
This study was designed to investigate the effect of high sulphate concentrations, which may be present in landfill leachates. The study was commissioned by the UK Department of Environment (now Defra) who needed to understand the impact of elevated sulphate levels in landfill leachate, which would be expected to occur if coal containing high sulphate levels was imported from outside of the UK, and the disposal option for the processed coal waste (to remove sulphate) was co-disposal with household and/or industrial waste in landfill. This work investigated the treatability of landfill leachate with elevated sulphate and heavy metals by anaerobic digestion, comparing methanogenic with sulphate-based digestion, in upflow anaerobic digesters where biomass was retained as biofilms on support material.