Treatment of Landfill Leachate by Methanogenic and Sulphate-based Anaerobic Digestion
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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 effleuents 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 metabolize only a restricted range of
substrates (principally acetate and hydrogen), the SRB
metabolize 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 sulfides, 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.
