Anaerobic digestion of print pastes: A preliminary screening of inhibition by dyes and biodegradability of thickeners

The paper reports the results of a screening laboratory- and pilot-scale investigation into the anaerobic inhibition/toxicity of dyes and anaerobic biodegradability of textile print-paste thickeners. Removal and anaerobic toxicity of six dyes, belonging to different classes (azo, anthraquinone, cyanine, etc.) were evaluated, at different concentrations (250-2400 mg/l), in tests spanning 72 or 96 h. The anaerobic biodegradability of a mixture of thickeners was evaluated in an up-flow filter adding milk whey as co-substrate, during a test spanning 70 days. A low degradation was found, in the order of 30-35% on COD (chemical oxygen demand). Moreover, increasing the thickener load to the filter (thickener/milk whey > 0.5 on a COD basis) caused failure of the process. Possible reasons for the malfunction were thought to be inhibition of the methanogenesis due to chemicals present in the thickeners (e.g. surfactants, chelating agents, etc.) and the high ammonia concentration reached in the filter, derived from hydrolysis of urea in the thickeners.