Anaerobic treatment of polyethylene glycol may represent an alternative to integrated chemical-biological treatment for the elimination of polyethylene glycol. Anaerobic biodegradation of synthetic waters are made with a mix of polyethylene glycol of molecular weights 400, 600 and 1000 and another made with polyethylene glycol of molecular weights 1500, 3000 and 10 000, was studied using an inoculum adapted to polyethylene glycol 10 000. The critical and minimum retention times were estimated and ranged from 13.2 to 14 and from 18 to 20 days, respectively. Under these operational conditions, the removal percentage ranged from 85 to 90%. Thus, anaerobic degradation of synthetic water containing polyethylene glycol produces removal percentages and hydraulic retention times similar to those of biodegradable wastewater tested at the same operational conditions. (C) 2003 Society of Chemical Industry.