Inhibition of methanogenesis by chlorophenols: a kinetic approach

Chlorophenols exert a crucial effect on the methanogenesis, considerably reducing both maximum methane potential and methanogenic rates. However, there is not enough information about the kinetic mechanism of chlorophenols toxicity on the methanogenesis, which is a key aspect for the control of the anaerobic digesters because of the sensitivity and the potential for energy recovery derived from methane release. The International Water Association-Anaerobic Digestion Model No. 1 (IWA-ADM1) can be adapted to a wide range of situations by updating or changing the equations in the model. The present study proposes a general kinetic model for methanogenesis. This model has been applied to predict the inhibition of methanogenesis by chlorophenols, and it can be used for updating the IWA-ADM1 when treating inhibitory compounds. The model was calibrated and validated using a wide broad of experimental sets of data of methane production by granular sludge in the presence of 2,4-dichlorophenol (24 DCP), 2,4,6-trichlorophenol (246TCP) and pentachlorophenol (PCP) in batch assays. A lag-phase of the effect of chlorophenols on the methanogenesis by non-adapted sludge was detected and modeled by the kinetic model proposed. In addition, the inhibitory effect of PCP was more pronounced on the acetoclastic methanogenesis than on the hydrogenotrophic one. Non-competitive and uncompetitive inhibition types were detected using 24 DCP and 246 TCP, whereas a suicide or irreversible inhibition type was observed in the case of PCP. Values of inhibition constants considerably varied depending on the chlorophenol used, between 45 mg24DCPL(-1), 41-51 mg246TCPL(-1) and 0.9-7.8 mgPCPL(-1). The higher toxicity of PCP is related with its hydrophobicity, which was determined by adsorption tests and using partition coefficients n-octanol/water. Modeling was accompanied by high statistical support in all cases, which confirmed the validation of the model proposed.