Treatment of fruit processing wastewater by electrochemical and activated persulfate processes: Toxicological and...

A fruit processing wastewater was submitted to different advanced oxidation processes, namely, electro-Fenton (EF), electrochemical oxidation (EO), activated persulfate (PS), and combined EF/PS. The performance of the treatment processes, at different experimental conditions, regarding organic load removal, biodegradability increment, toxicity reduction, and specific energy consumption (E_sp), was evaluated. At the experimental conditions studied, EO led to the treated solutions with the highest biodegradability increment, from 0.24 to 0.48, and toxicity reduction towards Daphnia magna, from 5.8 to 1.5 toxic units, without requiring the addition of chemicals. Nevertheless, the highest chemical oxygen demand (COD) removals were obtained for EF and combined EF/PS treatments. For the electrochemical processes, an increase in COD removal rate with applied current density (j) was observed. However, the increase in j substantially raised the E_sp. In PS treatment, COD removals above 80% were only achieved for high amounts of added persulfate and iron, which led to less biodegradable and more toxic solutions. Combined EF/PS attained the lowest E_sp values, mainly due to the conductivity increase originated by the persulfate and iron salts addition. Besides the disadvantage of the chemicals added, this combined treatment led to treated solutions with very acidic pH and significant iron content.