Methanogenic degradation of p-cresol in batch and in continuous UASB reactors

Two methanogenic cultures were grown in the presence of p-cresol as sole carbon source or together with a mixture of volatile fatty acids (VFA). Both batch cultures produced methane from VFA in the presence of p-cresol, up to concentrations of about 200 ppm, in a first experiment, without significant activity inhibition. After several transfers, one of the cultures tolerated higher p-cresol concentrations. Only that culture, originating from industrial effluents contaminated with phenolic compounds, was able to remove p-cresol which was completely degraded to methane and carbon dioxide with p-hydroxybenzoate, propionate and acetate as detectable intermediate compounds. These intermediates did not accumulate in the medium. The addition of VFA as auxilliary carbon sources very slightly delayed p-cresol removal in batch assays but allowed, on the contrary, the use of higher p-cresol influent concentrations in continuous UASB reactors than in the absence of VFA. With a mixture of p-cresol: VFA at a COD ratio of 2:1, complete COD degradation was reached when operating at an hydraulic retention time (HRT) of 0.67 day and at an influent concentration of 650 ppm p-cresol. When operating under the same conditions in the absence of VFA, around 80% p-cresol was removed although a near complete degradation was reached at influent p-cresol concentrations of 400 ppm working at a HRT of 0.8 day.