Sulfate removal from indigo dyeing textile wastewaters

Sulfate is an important parameter especially for discharges to sewer systems. The textile industry is a major source of sulfate. Some sulfate sources in the industry have material replacement alternatives. However in some sources, sulfate or species convertible to sulfate are the main materials. The indigo dyeing process involves sulfur species as main materials. In this study, indigo dyeing wastewaters which contain significant concentrations of oxidized and non-oxidized sulfur components are evaluated in terms of sulfate removal. The approach is a pretreatment at the source before being mixed with other wastewaters. The study is conducted in two steps. In the first step, conversion of species to either sulfide or sulfate is experimentally evaluated. While reduction to sulfide poses problems, oxidation of all species to sulfate is found to be applicable. In the second step sulfate precipitation using calcium, barium and lead is practiced. Calcium precipitation provides up to 30% sulfate removal and these results are supported with existing literature data. Barium sulfate and lead sulfate precipitation provided practically complete removal. Economical evaluation of alternative methods is also given.