The inhibitory effect of phosphate on acetoclastic-methanogens was investigated for three different thermophilic (55 °C) anaerobic consortia. When 70 mM of phosphate was tested, acetoclastic methanogens was completely inhibited in “Eerbeek” sludge which is dominated by Methanosaeta-like methanogens. For the “Hoogezand” sludge the specific methanogenic activity dropped by 79%, indicating that any of the acetate-consuming methanogens present in the sludge was more resistant to the phosphate applied. The results demonstrated that the inhibitory effect of phosphate may affect both methane production rate and final methane concentration and might also be time dependent. This study indicates that the degree of inhibition is species-dependent, and even more resistant species may be affected during long-term experiments. Such inhibition is a matter of concern for researchers since misleading conclusions might be taken from growth and specific methanogenic activity tests when considerable concentrations of phosphate buffer are used and no interference is expected.

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