Treatment of silk and lycra printing wastewaters with the objective of water reuse

This paper concerns the treatment of wastewaters produced during printing matrix washing and by dyestuff leakage in the silk and Lycra printing industry. The objective was to obtain effluent of a quality that would allow water reuse. The raw wastewaters contained a high concentration of ammonia nitrogen (from urea, ammonium sulphate and tartrate used for the dyestuff preparation together with organic compounds. A high BOD₅COD ratio determined the possibility of a biological treatment of the wastewaters by nitrification-denitrification. To achieve complete denitrification, however, it was necessary to add an external carbon source. A mixture of methyl and ethyl alcohols from pharmaceutical industry wastes was used to achieve a BOD₅N ratio of about 3.5. High temperature (23-26°C) and low inhibition enabled good rates of treatment to be achieved in a single-sludge system, equal to 0.042 kgN/kg MLVSS × day and 0.12 kgN/kg MLVSS × day, respectively for nitrification and denitrification. After this stage the effluents were characterized by COD <120 mg/L, BOD₅ <25 mg/L, N-NH₄ <10 mg/L, N-NHO₃ <12 mg/L, N-NO₂ <0.3 mg/L values which meet Italian discharge standards. A subsequent quartz filtration and UV sterilization allowed the wastewaters to be reused for washing purposes within the factory. At the final stage of the wastewater treatment it was possible to economize by reusing industrial wastewaters by 30-40%. No problems connected with a gradual accumulation of the recalcitrant or toxic compounds were encountered in the reuse of treated wastewaters and their subsequent biological treatment during 10 months of full scale plant operation. Residual colour removal was however necessary if the wastewaters were also to be used for the preparing of dyes. Laboratory experiments to remove colour using ozone treatment were conducted. A dose of 20 mg/L of ozone was introduced prior to filtration. This step eliminated not only the residual colour but also the residual non-ionic surfactants. As the final result of the treatment it will be possible to use wastewaters not only for washing purposes but also for preparation of the printing colours; it is calculated that economies of 60-70% of process water will be possible.