Removal of hard COD from biological effluent of coking wastewater using synchronized oxidation-adsorption technology:...

Efficient removal of the non-biodegradable organics from the biological effluent of industrial wastewater is becoming more and more important with the increasing demand for stringent discharge regulation. In this study, a synchronized oxidation-adsorption (SOA) technology was proposed for the removal of hardly biodegradable COD (hard COD) from the biological effluent of coking wastewater, and its performance was verified in a full-scale coking industrial park wastewater treatment plant (Q = 5,000 m³/d). The SOA was performed by coupling oxidation by hydroxyl radical (molar ratio of Fe²⁺ to H₂O₂ of 1:1 and pH = 5.0 ± 0.2) and adsorption by in-situ-formed nano hydrolyzed Fe³⁺ particles (nano-FeOOH). The nano hydrolyzed Fe³⁺ particles formed during the SOA exhibited a much higher specific surface area (22.83 m²/g) than the particles (10.87 m²/g) formed during the polyferric sulfate coagulation (PFSC). In comparison to PFSC, SOA performed better in terms of average COD removal (39% vs 18%) from the biological effluent. Wastewater fractionation result showed that SOA performed better in the removal of the hydrophobic acid matters, which was supported by the experiment using fulvic acid as the model organics. Mechanism studies using both biological effluent and fulvic acid solution showed that more carboxylic substances were adsorbed by the in-situ-formed nano-hydrolyzed Fe³⁺ particles formed by SOA than by PFSC, which was likely due to the generation of carboxylic substances by hydroxyl radical oxidation. In the full-scale, the COD was reduced from 118.5-198.0 mg/L in the PFSC-pretreated effluent to 61.5-104.0 mg/L through SOA treatment. The SOA treatment characterized with a mild pH condition (pH 5) and low molar ratio of Fe²⁺ to H₂O₂ (1:1) is particularly suitable for the polishing purpose to remove limited amount of organic pollutants from wastewater before discharge.