Anaerobic treatment of sulphate-enriched wastewaters

Anaerobic treatment is increasingly being used by industry in preference to aerobic systems to overcome more stringent legislation regarding the discharge of pollutants into the environment. As high-rate anaerobic systems such as the anaerobic baffled reactor (ABR) gain recognition as effective effluent treatment processes, greater demands are made of the systems. Previous work has shown that the ABR can achieve high carbon removal rates through biomass retention and process stability. However, limited information is available on the treatment of wastewaters containing sulphate in the ABR. As such, the aim of this work was to examine the effects of treating wastewaters with high levels of sulphate in a bench-scale ABR. Experimental work involved the use of an ABR with a 10 litre working volume and eight discrete compartments, run under mesophilic conditions (at 35°C). A baseline feed of 4 g chemical oxygen demand (COD)/1 was supplemented with 0·4 g/l sulphate (a COD: sulphate ratio of 10). This allowed the development of biomass in the reactor that was accustomed to sulphate. Once the reactor was operating at a 20 h hydraulic retention time, the sulphate content of the feed was increased stepwise, resulting in a decrease in the COD: sulphate ratio to a minimum level of 1. Microbiological studies were carried out to evaluate the effect of the extreme sulphate loading conditions on the biomass. The ABR was found to perform well at low influent COD: sulphate ratios. In the front of the ABR where sulphate reduction is expected, specific sulphate reduction rates (kg $SO42−/kg$ volatile suspended solids per day) were significantly higher than the values suggested by the literature. The results of this work showed the structure of the ABR to be well suited to the treatment of wastewaters with low COD: sulphate ratios.