AbstractPhthalate, dimethyl phthalate, diethyl phthalate, dibutyl phthalate and di(2‐ethylhexyl) phthalate were evaluated for their anaerobic biodegradability and toxicity to methanogenesis. Two anaerobic bioassays (the biochemical methane potential and the anaerobic toxicity assay) were used to monitor the stoichiometric conversion of added substrate to carbon dioxide and methane. Each phthalic acid ester was the only added carbon source (20 to 200 mg/L) prepared in prereduced defined medium using a 10 (v/v) inoculum of municipal digester sludge. Over an extended incubation period (50 to 100 d), all concentrations of phthalate and dibutyl phthalate and low concentrations (20 mg/L) of dimethyl phthalate and diethyl phthalate achieved 75 to 100 of their theoretical methane potentials. Higher concentrations (100 to 200 mg/L) of dimethyl phthalate and diethyl phthalate achieved 25 to 50 of their theoretical methane potentials. Di(2‐ethylhexyl) phthalate showed little mineralization at any concentration, although biotransformation of this substrate was indicated. Acclimation for most of the compounds took several days to weeks before degradation commenced. In the anaerobic toxicity assay, only concentrations greater than 20 mg/L diethyl phthalate produced any significant suppression of methanogenesis over time. All other substrates produced little inhibition of methanogenesis during the initial period of incubation. Generally, increased incubation led to increased levels of methane production above active control levels. Thus, many of these compounds were metabolized completely under methanogenic conditions and were observed to produce little inhibition of methane production. Their fate, therefore, may be significantly influenced by their concentration and residence time in methanogenic habit
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