A sequential process to treat a cashew-nut effluent: Electrocoagulation plus electrochemical oxidation

Abstract

The treatment of a real cashew-nut effluent has been studied using a pioneering sequential process involving electrocoagulation (EC) and electrochemical oxidation (EO) approaches. The performance of the system was analyzed in terms of chemical oxygen demand removal, by-products evolution, toxicity and energy consumption. Preliminary EC tests with Fe electrodes were performed at different current densities (8.33–100 mA cm−2), yielding COD removals among 17–51%. The residual iron concentration detected was below the Brazilian legal limits of discharge. The sludge obtained from the experiments at 8.33 and 100 mA cm−2 revealed a heterogeneous morphology with interconnected laminar plates. Regarding their structure, the powders showed a mixture of poorly crystalline phases. The main elements identified were oxygen, carbon and iron, which allow proposing an alternative use of this powder for construction and building materials. Sequential experiments, using EC at 8.33 or 100 mA cm−2 plus EO, intensified COD removals up to 51% and 80%, corresponding to 5 and 65 kWh m−3 of energy consumption, respectively. To evaluate the by-products generated during EO treatment, the evolution of carboxylic acids and inorganic species (PO4 3−, NH4 +, NO3 −, NO2 −) was followed. After that, phytotoxicity analyses were performed in order to assess water quality and its reuse in irrigation activities. Based on these results, the sequential experiment with EC by applying 8.33 mA cm−2 plus EO (35.5 mA cm−2) was considered the most efficient and viable to depurate the real cashew-nut effluent.