With the objective of destruction of organic toxic or recalcitrant compounds by a microbial anaerobic mixed population, a new concept has been devised : the Destox concept. It has been applied here to the destruction of a toxic mixture of about 30 polychlorinated aliphatic compounds (PAC-MIX 1), including 51 % of hexachloro-l,3-butadiene. The basic step for initiating the degradation is thought to be reductive dechlorination by microbial interspecies hydrogen transfer in a system using at all times a non-toxic co-substrate as major source of carbon and energy. In an upflow laboratory-scale reactor with a fixed-film stationary-bed, fed with a co-substrate, amounts of 48 mg/litre working volume per day of PAC-MIX 1 have been added during intermittent periods of time. This paper presents, over a period of 421 days, the evolution from a situation of complete inhibition of all microbial activity, from fermentative to methanogenic, into a situation of partial acclimatization, the fermentative and hydrogenotrophic methanogenic bacteria having become resistant to the action of the toxic mixture while acetogenic bacteria seem in the process of acclimatization. The methane production rate was maintained at 30% of the reference value, together with continuous accumulation of mainly acetate and to a lesser extent propionate.

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