Color and pumice stone problems in denim processing effluents: removal potential by integrated physical–chemical treatment

BACKGROUND: Despite extensive research on textile wastewaters, very few studies have been devoted to denim processing effluents, basically ignoring the major pumice stone and color problems. This article defined an integrated physical–chemical pre-treatment scheme as a sustainable solution, which would achieve full removal of pumice stone and color, and would provide an optimum adjustment of wastewater characteristics for biological treatment. RESULTS: This survey revealed a strong effluent with 1.0 g L⁻¹ pumice stone (> 2 mm in size); 3.4 g L⁻¹ total suspended solids (TSS) (< 2 mm in size); 1.15 g L⁻¹ chemical oxygen demand (COD) and color 630 platinum-cobalt (Pt-Co) units, as 70 percentile values. Then, two representative samples approximating 70 percentile COD value were taken for laboratory-scale evaluation involving a sequence of preliminary settling and chemical settling experiments. Preliminary settling for 1.0 h reduced the TSS levels to 760–520 mg L⁻¹ and COD to 790–810 mg L⁻¹ for samples 1 and 2, respectively. No appreciable reduction was observed for color. Chemical treatment with polyelectrolyte and poly-aluminum chloride (PAC) exerted significant polishing impact on preliminary settling effluent, achieving almost complete removal of TSS, down to 60 mg L⁻¹, additional COD reduction to a final level of 335 mg L⁻¹. Moreover, color could be removed below 7.0 m⁻¹ for all absorbance levels of 436, 525, and 630 nm tested in the experiments. CONCLUSION: The integrated physical–chemical pre-treatment scheme proved to be an effective and reliable approach, yielding an effluent free of TSS and color, with a COD level that can easily be eliminated through biological treatment.