Evidence of organic–metal decoupling in textile wastewater from Bangladesh's largest export processing zone

<title>Abstract</title> <p> Textile wastewater contains both organic pollutants (biochemical oxygen demand (BOD ₅ ), chemical oxygen demand (COD), and colour) and toxic heavy metals, including lead (Pb) and chromium (Cr). Although organic parameters are routinely monitored in Bangladesh's export processing zones, heavy metals often receive comparatively limited attention. This screening-level study investigated whether compliance with organic and heavy metal parameters is coupled or decoupled in effluents from six textile facilities (five individual effluent treatment plants (ETPs) and one central effluent treatment plant (CETP) in the Chattogram Export Processing Zone (CEPZ). Single grab samples (n = 1 per facility) were collected from Jeans 2000, Premier 1888, Pacific Jeans, Kenpark, Industry X, and the CETP in December 2025, with one additional CETP sample from September 2025 providing limited temporal comparison. Organic pollution was frequently non-compliant: BOD ₅ exceeded the regulatory limit of 30 mg L⁻¹ in 50% of facilities (CETP, 95 mg L⁻¹; Industry X, 90 mg L⁻¹; Jeans 2000, 50 mg L⁻¹), whereas COD (240 mg L⁻¹) and colour (190 Pt-Co) exceeded permissible limits only in the CETP. In contrast, all facilities complied with the regulatory limits for Pb (0.1 mg L⁻¹) and Cr (0.5 mg L⁻¹). Strong positive correlations among BOD ₅ , COD, and colour (Spearman's r _s = 0.83–0.94, <italic>p</italic> &lt; 0.05) contrasted with negligible correlations between organic and metal parameters (r _s = −0.24 to −0.15), indicating decoupling of organic and heavy metal compliance and suggesting distinct pollutant sources and treatment pathways. These findings suggest that tertiary treatment upgrades may merit consideration for the CETP, while Pacific Jeans represents a potential operational benchmark. Because the study was based on single grab samples, the findings should be regarded as exploratory and hypothesis-generating rather than definitive. Well-powered investigations incorporating temporal replication are warranted to validate the observed compliance patterns and the proposed decoupling hypothesis. </p>