Treatment of waste water from silk degumming processes for protein recovery and water reuse

Abstract

Large volumes of wastewaters containing proteins (mainly sericin), salts and tensioactives are produced in the degumming process (washing) of both yarn and tissues made from natural silk. These wastewaters bearing a COD above 6000 mg/l are generally treated in biological plants with increasing costs. Sericin represents, on the contrary, a valuable by-product that can be used in cosmetics and pharmaceutical production. To reduce treatment costs and at the same time to recover sericin and reuse process water, a hybrid membrane process is under study. Laboratory and field experiments show that ultrafiltration of the degumming waste solution with different membranes (with molecular weight cut-off in the 20–30,000 dalton range) allows recovery of more than 97% of the sericin but the permeate COD level exceeds 800 mg/l whose value is still too high for water reuse. A final reverse osmosis treatment on the ultrafiltration permeate allows reaching a low COD level (50 mg/l) and recovery of more than 70% of the wastewater for process reuse. Alternative process flow sheets for the silk degumming solution treatment are discussed on the basis of the obtained results.