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.
- Steady-state removal efficiencies of COD under methanogenic digestion was 81-97%, depending upon organic loading rate, and effective treatment occurred up to a volumetric COD loading rate of 3.75 kg COD.m-3.d-1.
- In sulphate-reducing digesters SO4 was converted stoichiometrically to dissolved H2S, driving organic matter mineralization. Sulphate concentrations of 35, 105 and 175 mM in the feed resulted in SO4 removal efficiencies of 92, 52 and 35%, respectively.
- Steady state removal efficiencies of organic COD in the sulphate-reducing digesters averaged 62% at 35 mM, 80% at 105 mM, and 84% at 175 mM feed SO4 concentrations.
- The ratio of COD:SO4 (g/g) utilized was dependent upon the concentration of SO4 present, and at 35 mM SO4 was 0.67. At an organic loading rate <1 kg COD.m-3.d-1 the sulphate-reducing digesters offered organic removal efflciencies comparable to methanogenic digesters, but were less effective at higher loading rates.
- Propionate accounted for the majority of the residual COD in the effluents from the methanogenic digesters, but acetate in the effluent from the sulphate-reducing digesters.
- Removal of sulphide from the reactors was achieved by addition of iron and by gassing off sulphide, but there was no increase in COD removal after sulphide removal which indicated that sulphide toxicity was not significant.
- Both methanogenic and sulphate-reducing reactors completely removed up to 100 ppm Cu, Mn, Ni and Zn added in the feed, indicating that even methanogenic reactors produce sufficient sulphide to precipitate these metals completely.
- The two types of digesters used in this study each had their own advantages and disadvantages. The methanogenic digesters under stress loading accumulated propionate, while the sulphate digesters accumulated acetate (Table below). When combined together as a two-stage process the final effluent fell to a very low COD as the initial sulphate-reducing stage effectively converted C3-C8 fatty acids to acetate, and the relatively high residual acetate from the first stage was removed by the second methanogenic stage. Overall, 97% removal of organic COD was removed. Thus it appears that a two stage process might harness the advantages of both types of reactor to produce a final effluent with an extremely low COD.