Ecotoxicological and Treatability Assessment of Dyeing Effluents from Vietnamese Craft Villages Using Biocatalysis and Heat-Activated Oxidation

Abstract

Artisan weaving and dyeing craft villages contribute significantly to both the national and local economies in Vietnam. However, due to limited financial and spatial resources, these decentralized establishments often lack proper treatment facilities. This results in the direct discharge of untreated wastewater containing highly recalcitrant acid dyes and complex organic loads into the environment. Although the Vietnamese government has invested in establishing localized treatment plants, these efforts have proven largely ineffective, primarily due to the severe underestimation of wastewater volumes and chemical complexity during initial planning. Given the profound adverse impacts of these discharges on surrounding ecosystems and local public health, developing a comprehensive contamination assessment and targeted remediation strategies is urgently required. The first part of this study establishes the physicochemical and ecotoxicological baseline of authentic dyeing effluents collected from Ha Nam province, Vietnam. Physicochemical characterization revealed extreme COD paired with persistently low BOD/COD ratios, confirming severe biodegradability limitations that render conventional biological treatment inadequate. Color values exceeding 2,000 Pt-Co were recorded at the point of dyeing discharge, requiring up to 16-fold dilution for color compliance alone under Vietnamese national regulations. Ecotoxicological assessment using a Toxic Unit framework across three aquatic species including Selenastrum gracile, Desmodesmus subspicatus, and Aliivibrio fischeri demonstrated that both locally sourced commercial dyes and simulated effluents exert significant acute toxicity toward green microalgae, confirming that these discharges threaten primary aquatic producers even at substantial environmental dilutions. The second part investigates immobilized oxidase enzyme biocatalysis as a low-resource biological treatment pathway. Laccase and horseradish peroxidase were immobilized onto chitosan beads using glyoxal as a less toxic alternative crosslinker to conventional glutaraldehyde. The immobilized biocatalysts demonstrated robust operational stability across variable pH and temperature conditions, with complementary environmental resiliencies suggesting potential for a dual-enzyme system. Theoretical Toxic Unit analysis confirmed that enzyme treatment successfully reduced acute toxicity to below the safe discharge threshold for moderate-hazard anthraquinone streams across multiple reuse cycles, while highly toxic azo dye streams exceeded the treatment capacity of biological pathways alone, establishing the operational boundary for biocatalysis and motivating the chemical treatment approach of the third chapter. The third part engineers an energy-efficient advanced oxidation strategy utilizing heat-activated peroxydisulfate. Uniquely designed to exploit the residual thermal energy inherently present in hot dyeing effluents through ambient cooling, the system additionally leveraged native groundwater iron as a natural co-activator, enhancing degradation rate constants by 6–9 times relative to iron-free conditions, effectively overcoming radical scavenging by background organic matrix components. Treatment of simulated dyeing wastewater under all tested thermal conditions achieved near-complete decolorization exceeding 99%, COD reduction below the Vietnamese regulatory threshold of 150 mg/L, and reduction of acute A. fischeri toxicity from TU > 1 to TU < 1 crossing the critical safety threshold established in Chapter 1. Importantly, color compliance was achieved significantly faster than ecotoxicological safety, particularly for the recalcitrant anthraquinone structure, demonstrating that visual decolorization alone is an insufficient indicator of true treatment completion and that ecotoxicity monitoring is essential for discharge validation. Residual heat integration eliminated external heating costs entirely, reducing total operational expenditure by approximately 82% compared to grid-powered isothermal operation. Overall, this dissertation establishes a unified Toxic Unit framework as a quantitative ecotoxicological tool for evaluating dyeing effluent safety, revealing that existing Vietnamese physicochemical regulations substantially underestimate the biological risk posed by craft village dyeing discharges. By integrating rigorous ecotoxicological tracking with complementary biological and chemical treatment pathways, this work provides the scientific and engineering foundation for craft-village specific ecotoxicity regulations and practically deployable treatment solutions tailored to the resource constraints of decentralized artisan industries in Vietnam.