The effect of elevated sulfate and sulfide levels on the anaerobic utilization of lactate, butyrate, propionate and acetate was investigated in this thesis. The primary objective was to determine the substrate utilization pathway which was the most sensitive to elevated sulfide concentrations. Four experimental phases were completed, utilizing ten semi-continuous breeder reactors and one hundred and fifty single-run batch reactors. The COD to sulfate ratios investigated ranged from 0.8 to 3.7 g g⁻¹.

Acetate removal was facilitated by methanogens, not sulfate reducers at the COD to sulfate ratios investigated (3.7 to 0.8 g g⁻¹). Acetate, propionate, and butyrate utilizations were more sensitive to aqueous hydrogen sulfide rather than total dissolved sulfide. The aqueous hydrogen sulfide concentration, which resulted in fifty percent inhibition of propionate, acetate, and butyrate utilization were 2.5,3.4, and 7.3 mM (85, 115, and 250 mg L⁻¹).

The lactate degradation pathway was dependent upon the feed COD:​sulfate ratio. A ratio of 3.7 g g⁻¹ resulted in a non-sulfate reducing pathway producing propionate and acetate as products, while a ratio of ≤ 1.6 g g⁻¹ resulted in a sulfate reducing pathway producing only acetate as an organic product. The lactate utilization pathways were by far the least sensitive to sulfide. The fifty percent inhibition thresholds were 11.1. rnNI (377 mg Ll) aqueous hydrogen sulfide and 65.6 mM (2100 mg S L⁻¹) total dissolved sulfide for the former and latter pathways, respectively.

The utilization of propionate was correlated to sulfate reduction. This sulfate reduction pathway was slightly more sulfide sensitive than acetoclastic methanogenesis. The sulfate reduction pathway responsible for lactate degradation, however, was much less sulfide sensitive than acetoclastic methanogenesis. Hydrogen did not build up during inhibition of propionate degradation, suggesting that the inhibition of obligate hydrogen forming acetogens was not the cause of propionate inhibition.

An inoculum acclimated to high lactate and high sulfate conditions developed a completely new lactate degradation pathway when compared to an inoculum taken from an unacclimated biomass source. This demonstrated the importance of acclimation when using biological processes.