Anaerobic biodegradation of 2,4,6-trichlorophenol by methanogenic granular sludge: role of co-substrates and methanogenic inhibition

The influence of several co-substrates in the anaerobic biodegradation of 2,4,6-trichlorophenol (246TCP) by methanogenic granular sludge as well as in methanogenesis inhibition by 246TCP has been studied. 4 g-COD·L⁻¹ of lactate, sucrose, volatile fatty acids (VFA) acetate:propionate:butyrate 1:1:1, ethanol, methanol, yeast extract (YE), and 2 g-COD·L⁻¹ of formate and methylamine were tested. Two concentrations of 246TCP: 80 mg·L⁻¹ and 113 mg·L⁻¹ (this last corresponding to the EC₅₀ for acetotrophic methanogenesis) were tested. Three consecutive co-substrate and nutrient feedings were accomplished. 246TCP was added in the second feed, and the 246TCP removal rate increased considerably after the third feed. Accumulated metabolites after ortho-dechlorination, either 4-chlorophenol (4CP) (when methanol, ethanol or VFA were used as co-substrates) or 2,4-dichlorophenol (24DCP) (with lactate) avoided the complete dechlorination of 246TCP. With methylamine and formate this compound was degraded only partially. Monochlorophenols biodegradation was partially achieved with YE, but both 24DCP and 2,6-dichlorophenol (26DCP) were accumulated. In the presence of sucrose para-dechlorination was observed. 246TCP was better tolerated by methanogens when ethanol and methanol were added because of the highest specific methanogenic activity achieved with these co-substrates. Methanol and ethanol were the best co-substrates in the anaerobic biodegradation of 246TCP.