Treatment of silk production wastewaters by membrane processes for sericin recovery

Abstract

Sericin protein, although a valuable resource for many industries including cosmetics, pharmaceutical and biomedical, has been discarded as a waste in silk industry, causing environmental pollution. This paper describes determination of a membrane-based process for sericin recovery from cocoon cooking wastewaters (CCW) that will enable value-added utilization of waste sericin. The iso-electric point (p I) of sericin was found as 5–6, whose MW was distributed as 180–200, 70–80, 30–40 and 10–25 kDa. Prior to membrane filtration, sericin was separated from other impurities via centrifugation (CFG) followed by microfiltration (MF) in the pre-treatment stage, which also helped minimize post membrane fouling. Ultrafiltration (UF) and nanofiltration (NF) were adopted at a pH equal to p I of sericin. UF achieved partial recovery of sericin polypeptides at 37–60%, which was attributed to increased transmission of uncharged sericin polypeptides at their p I. On the other hand, NF achieved sericin recovery as high as 94–95%, containing all MW fractions. Severe flux decline was the major problem due to protein–membrane interactions and high sericin concentrations, where concentration polarization mainly had a dominant effect. Although flux declines were as high as 58–88% in UF and 70–75% in NF, flux recovery by at least 83% was achieved by chemical cleaning using NaOH and free chlorine.