Evaluation of an electro-flotation-oxidation process for harvesting bio-flocculated algal biomass and simultaneous treatment of residual pollutants in coke wastewater following an algal-bacterial process

Abstract

This study investigated the feasibility of employing an electro-flotation-oxidation process that employs a pair of boron-doped diamond (BDD) and aluminum (Al) electrodes for electrochemical harvesting of green microalgae (Scenedesmus quadricauda) and treatment of residual pollutants in coke effluent, following an algal-bacterial process. Electro-coagulation-flotation with polarity exchange and with direct electro-flotation at 15 mA cm(-2) or more for 40 min allowed almost complete harvesting of microalgae. Similar harvesting efficiencies were achieved using direct electro-flotation, without electro-coagulation, under different electrical densities because algal biomass formed flocs with the other microorganisms in the activated sludge (AS). These results also indicate that the proposed approach of inducing bioaggregation via floc-forming microorganisms with microalgae is an efficient alternative to chemical flocculation, because it can minimize the release of toxic metal coagulants during electrochemical harvesting. During sequential electro-oxidation, anodic oxidation using the BDD electrode simultaneously mineralized residual soluble chemical oxygen demand (SCOD) and thiocyanate (SCN-), which are not degraded by algal-bacterial mixed cultures. Although the degradation rate of SCN- was much higher than that of SCOD under certain current densities, further investigation is needed to clarify the mechanism of SCN- mineralization during BDD-anodic oxidation. To satisfy the standard level of electrical power consumption for wastewater treatment, an electric current density below 15 mA cm(-2) must be supplied. The proposed electrochemical approach involving bioflocculation could be used as an efficient post treatment of microalgae-mediated process for treating coke wastewater.