Summary. Accumulation of volatile acids produced during anaerobic fermentation of distillery-spent liquor decreased the activity of Desulphovibrio desulphuricans employed in previously communicated studies; this caused a decrease in the reduction of sulphates to sulphides. An enriched strain of Desulphovibrio rubentschikii made in a synthetic mineral medium (NH4C1, 1.0 g ; MgS04.7H20, 2.0 g ; NaCl, 10.0 g ; CaS04, 1.0 g; calcium acetate, 1.0 g ; K2HP04, 0.5 g; Mohrs' salt, trace in 1 1.) using calcium acetate as the substrate and carbon from the tap-water was gradually built up to a 3-1. volume. The amount of hydrogen sulphide in this medium was found to be 225 mg/l. The fermentation was next carried out in a synthetic medium containing volatile acids from distillery-spent liquor as the only substrate. A 3-1. fermentation set up with diluted distillery-spent liquor (300 ml of raw liquor in 3,000 ml of water), neutralized with NaOH to pH 7.2 and supplemented with calcium sulphate and 0.1 per cent urea, showed sulphide productions (calculated as milligrams of hydrogen sulphide per litre of raw distillery-spent liquid) of 2,000, 4,000 and 6,000 with 0.1, 1.0 and 2.0 per cent calcium sulphate in the charge respectively. The volatile acid content, expressed as acetic acid, was 18,564 mg/litre of raw distillery-spent liquor on an average throughout the study. The ratio of hydrogen sulphide produced per litre of raw distillery-spent liquor to the amount of volatile acids developed expressed in milliequivalents per litre of raw liquor during fermentation increased gradually from 0.1 367 to 0.5567 in the presence of 0.1 to 2-0 per cent of calcium sulphate in the charge.
PrefaceIn a previous communication (Part I, in press), studies carried out with organisms of the type Desulp?wvibrio desulphuricans in the bacterial reduction of sulphates to sulphides using distillery -spent liquor as carbon and hydrogen donor were reported. It was pointed out that increased yields of sulphide might be obtained by employing in the microbial sulphate-reduction a volatile-acid-utilizing 181 182 S. K. BASU AND T. K. GHOSE bacteria of the type Desulphovibrio rubentschickii. Studies subsequently conducted on the enrichment, isolation and fermentation characteristics of a specific volatile-acid-utilizing strain of sulphate reducers isolated from Calcutta City Sewage are reported. A comparative study on the sulphide fermentation with the agency of both D . desulphuricans and D . rubentschickii using distillery-spent liquor as hydrogen-donor has also been made. The influence of volatile acids on the bacterial reduction of sulphates to sulphides is also reported here.