Nitrophenols are among the most important and versatile industrial organic compounds and are widely used in the chemical industry and are listed as priority pollutants by the U.S. EPA. The toxic effects and degradability of three selected nitrophenols (2‐nitrophenol, 4‐nitrophenol and 2,4‐dinitrophenol) in anaerobic acetate and propionate enrichment systems were studied using batch serum bottles. The toxicity to both propionate‐ and acetate‐fed systems decreased in the following order: 2,4‐dinitrophenol > 4‐nitrophenol > 2‐nitrophenol. An up‐flow fixed film, acetate‐fed reactor was able to acclimate to 20 mg/L of 2‐ and 4‐nitrophenol showing a 95% removal of each nitrophenol. The effect of biomass was studied using serum bottles for anaerobic toxicity assays with the design volatile suspended solids (VSS) concentrations of 500, 1000, and 1500 mg/L. With higher VSS the toxic effects of nitrophenols on methanogenesis were less severe. Nitrophenols were more inhibitory to acetate utilization than propionate utilization. Under anaerobic conditions, 2‐nitrophenol and 2,4‐dinitrophenol were transformed both abiotically and biotically to 2‐aminophenol and 2‐amino,4‐nitrophenol, respectively. The presence of propionate in propionate enrichment culture enhanced the removal rates of all three nitrophenols studied.

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The complex process of propionate oxidation in anaerobic systems in the presence of excess sulfate is based on combinations of six biological reactions. Laboratory batch and continuous experiments were conducted to study which of these reactions were predominant in propionate oxidation by sulfate-reducing bacteria in the presence of acetogens and methanogens. The engineering significance of the feed propionate to sulfate ratio was studied. In batch serum bottle experiments, sulfidewas more toxic to sulfate-reducing bacteria (SRB) than to acetogens and methanogens. Acetate was the least favored substrate for sulfate reduction. Two mechanisms appear possible under sulfate-limited conditions: (I) propionate use by both SRB and non-SRB acetogens and acetate and hydrogen use by methanogens and (2) propionate use by non-SRB acetogens followed by acetate use by methanogens and hydrogen use by both SRB and methanogens. In the chemostats, at a feed propi· onate:sulfate ratio of 2.2 (sulfate-limited condition), the acetate formed was primarily used by methanogens. With a gradual decrease in feed propionate:sulfate ratio from 2.2 to 0.44 (sulfate-rich condition), SRB outcompeted methanogens for acetate. In the chemostat study un-ionized H2S concentrations of up to 178 mg S/L (total sulfide 464 mg S/L) did not inhibit sulfate reduction. Water Environ. Res .. 67,330 (1995).

The ability of sulfate-reducing bacteria (SRB) to effectively degrade several substituted and non-substituted aromatic and aliphatic compounds offers a promising alternative for the treatment of sulfate-containing industrial wastewaters and landfill leachates. For the design of sulfate-reducing anaerobic treatment systems for such wastewaters, it is important to determine the effects of toxicants on the kinetics of sulfate-reducing and methanogenic reactions. Batch kinetic experiments were conducted in the presence and absence of selected organic toxicants with a sulfate-reducing propionate enrichment culture. Fourteen chlorophenols and four nitrophenols were selected for this study. Toxicity due to dichlorophenols and trichlorophenols to both propionate and acetate degradation was dependent on the substitution position of chlorine atoms on the benzene ring. Among the dichlorophenols, 2,3-dichlorophenol and 2,6-dichlorophenol were the least toxic and 3,5-dichlorophenol was the most toxic. Among the trichlorophenols, 2,3,5-trichlorophenol and 2,4,5-trichlorophenol were more toxic as compared to 2,3,6-trichlorophenol and 2,4,6-trichlorophenol. Toxicity due to the mononitrophenols was also dependent on the substitution position showing 4-nitrophenol as the least toxic. All of the selected nitrophenols were more toxic to acetate utilization by methanogens than propionate utilization by the SRB.

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