BIO-DEGRADATION OF SILK INDUSTRY WASTEWATER: A CASE STUDY

Industrial wastewater treatment represents a major environmental issue to protect water bodies quality, both for human health and ecosystem, as testified by international regulations, placing restrictive limitations for wastewater discharge. Since industrial wastewaters are rarely segregated on the basis of their composition or contamination level, wastewater treatment plants are facing not only heavily, but even variously contaminated media [1]. This element causes increased fragility of treatment performance and working conditions, followed, consequently, by rising management cost to be incurred by customers. The present study has been performed with the aim of modifying the wastewater treatment methodology applied on an Italian silk manufacturing, in order to answer an existing need in the textile industry, where high costs and poor environmental performance put at risk, in the European economic framework, their very survival. Silk processing includes several steps [2]: reeling, weaving, degumming, dyeing or printing, and finishing. The degumming process is necessary to separate the two major proteins composing raw silk: fibroin and sericin, which is mainly discharged into wastewater, but could become a valuable by-product, if properly separated. Sericin, together with Sulfur compounds, Nitrogen, Chloride and surfactants deriving from silk treatment steps, represent major concerns for wastewater management. The system applied is based on stand-alone bio-oxidizers, working in batch on wastewater to be treated, coupled with a free surface tank to provide additional oxygenation to the system and with a twin bio-oxidizer treating air and possible stripped contaminants. Biological oxidation is enhanced by the design of the internal spiral wound plastic sheets, providing a lower pressure drop and resulting in less channeling than packed-bed bioreactors. When wastewater is treated with immobilized-cell spiral bio-reactions, clean air adds oxygen to the centrate containing bacteria, and enzymes. As the wastewater and biomass mix re-circulate via an internal pump through the bioreactor, the biomass, a consortium of bacteria and oxidative enzymes not genetically modified, but, still, under proprietary recipe, attacks the contaminants, degrading them to their components. An extensive monitoring campaign have been performed on system's performance, verifying both the natural degradation of contaminants through mechanic oxygenation (provided by simple recirculating wastewater into the reactor and tank) and the bio-degradation, feeding biomass to the system and trying different operational conditions. Temperature and pH have been tested during the whole trial period, as well as target contaminants. On the basis of the case wastewater characteristic and remediation target, nitrogen profile, chloride, sulphates and COD have been selected as performance indicators. A HI 98172 Portable pH/ORP/ISE Meter and a Hach-Lange spectropho-tometer DR 2800 and specific cuvette tests for different contaminants have been used for monitoring. In a cost-effective perspective, the implementation of this system appears to be suitable within a multistage treatment process. As marked by many authors, in fact, the sericin recovery should be pursued and ultrafiltration has been identified as a feasible technology [1]. Since an additional reverse osmosis stadium would be necessary to deal with the residual COD, which is, in general, still non compliant with discharge regulations, the substitution with RO with the Immobilized cell bioreactor system presented could be desirable to improve environmental performance of the wastewater treatment.