The SRB process consists of a bed of porous media including organic material that serves as an electron donor for anaerobic microbial metabolism. As water passes through the SRB bed, sulfate is biologically reduced to sulfide and dissolved metals precipitate as insoluble metal sulfides. Two SRB media mixtures were tested: SRB-A with composted manure and SRB-B with emulsified vegetable oil. The SRB systems were operated for 5 months at hydraulic retention times of approximately 2, 1, and 0.5 d. The HDS process uses lime addition to increase pH and precipitate metals as oxyhydroxides, solids/liquid separation in a thickener, and sludge recycling in a specific reactor configuration designed to densify sludge and substantially reduce waste sludge volume. The HDS pilot plant was operated for 1.5 months using a treatment pH of 9.0-9.5.
On average, SRB-A (manure-based) achieved >99% reduction of dissolved zinc (average effluent Zn = 120 µg/L) and dissolved cadmium and lead concentrations that were less than the analytical reporting limits (5-10 µg/L). SRB-B performance deteriorated rapidly, apparently due to substrate washout. The HDS system removed >99.5% of the dissolved Zn (average effluent Zn = 45 µg/L), and effluent dissolved cadmium and lead concentrations were less than reporting limits. Other process selection considerations such as costs, land requirements, residuals disposal, existing facilities, and others were also evaluated.