Dyeing wastewaters and residual size are textile factory waste streams that can be treated anaerobically. For successful anaerobic treatment of dyeing effluents, a co-substrate has to be added because of their low concentration of easily biodegradable compounds. Starch-based size contains easily biodegradable material, but is too concentrated to be treated without difficulties. Although residual size makes up only a small volume, when mixed with the other textile wastewater streams it has a considerable impact on the overall organic load. Many textile dyes can pass through a conventional aerobic treatment plant without being degraded. Anaerobic pre-treatment of the dyeing wastewaters before discharge to the aerobic plant can solve this problem, as many dyestuffs are partly degradable under anaerobic conditions, rendering aerobically degradable products. In this study, the possibility of using waste size as a co-substrate for the anaerobic pre-treatment of dyeing wastewaters was investigated. It was found that waste size was applicable as co-substrate for the decolourisation of the two textile dyeing wastewaters studied. Adding a redox mediator could enhance decolourisation rates for both wastewaters.

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