Electrocoagulation and advanced oxidation as intermediate effluent treatment steps for water reuse in a textile dyeing plant

Abstract

Dyehouse effluents are highly colored and contain significant amounts of dissolved and suspended solids so that in most cases the use of membrane separation technologies as a final treatment stage becomes inevitable to achieve high quality process water for reuse purposes. The inherent disadvantage associated with most chemical treatment processes is that they increase the salinity of the final, treated effluent and hence chemical intense methods are generally considered as impractical or impossible to prepare water for reuse. A successful reuse plant for textile dyehouse effluent should ideally involve a system, which will compensate for all sudden changes in the biotreated effluent in order to ensure safe and stable operation of the membrane units. Consequently, the treatment unit that is going to be implemented has to be flexible enough to comply with the existing treatment units. The present experimental work aimed at proposing advanced chemical treatment schemes (electrolysis and ozonation) for the effective and economical remediation of biologically pre-treated textile industry wastewater (average CODo = 370 mg/L; BOD5 < 20 mg/L; Suspended Solids (SS) = 130 mg/L; Dissolved Solids (DS) = 10,000 mg/L; Color as absorbance at 436 nm = 0.280 1/cm; pH = 7.5; SO42- = 1250 mg/L) instead of the more commonly applied "phase - transfer" - limited multi-stage filtration processes. In addition, a more recently developed advanced oxidation process (sonochemical treatment) was employed for the degradation of the most problematic effluent stream of the textile dyeing and finishing sector, namely the dyebath effluent. Spent dyebaths were simulated to explore the effect of ultrasound on decoloration and degradation of dyes in auxiliaries present in spent dyebaths under varying reaction conditions.Electrolysis of biotreated dyehouse effluent appeared to be effective in terms of COD, color and SS once optimized for operating conditions (pH, current, contact time) and electrode materials (iron, aluminum, iron/aluminum). It was also shown that different color removal mechanisms existed for different electrode materials. Indirect redox reactions were found to be responsible for color abatement during electrolysis using iron electrodes, whereas color removal with aluminum electrodes occurred via physical adsorption. A continuous electrochemical system was also developed to simulate real electrochemical treatment conditions and estimate operating costs. The system enabled satisfactory floc formation, coagulation, flotation and sludge separation in one single reactor with iron electrode. However, it did not qualify when Al was used as the electrode. Results indicated that the efficiency of the electrochemical systems did not change when the biotreated effluent was of much lower quality, emphasizing the robustness of electrochemical process as a "buffer" stage between secondary and tertiary treatment.A detailed cost evaluation in terms of operating expenses was also undertaken for all investigated systems.