Anaerobic sulphate-reducing microbial process using UASB reactor for heavy metals decontamination

This study was conducted to investigate the possibility of using sewage as an organic substrate for the growth of sulphate reducing bacteria (SRB) and to acclimatise anaerobic sludge to produce sulphide from sulphate reduction, with a view to metal precipitation. The experiments were carried out in a continuous bench-scale bioreactor (13 1 UASB reactor) operated with hydraulic retention times (HRT) of 11 and 19 hours. The feed solution used was composed of the liquid part of the sewage (organic matter) supplemented with nickel sulphate and sodium sulphate. The results showed that it was possible to acclimatise anaerobic sludge for production of sulphide by sulphate reduction. A relation between the available COD and the concentration of sulphate reduced by SRB was observed. High nickel removal efficiency (96%) was obtained during the whole operation (320 days). However, the process required very strict control of the organic load available (sewage) in the solution and, if necessary, the addition of a complementary organic carbon source, in order to maintain a constant level of metal removal. The SRB were not affected by the HRT values studied and were installed and maintained in the system; however, granular sludge was not observed. The micrographs confirmed the presence of iron and nickel sulphides and also a mixed bacterial culture in the anaerobic sludge. The EDS spectrum of the sludge showed that nickel was retained in the sludge predominantly as a nickel sulphide.