Harvesting of cyanobacteria and phosphorus by electrocoagulation-flocculation-flotation: Role of phosphorus precipitation in cell separations and organics destabilization.

A high level of phosphate triggers the excretion of algogenic organic matter (AOM) during algae blooming, leading to disinfection by-products (DBPs) formation. The presence of phosphate could impact cyanobacteria harvesting and AOM separations by electrocoagulation. This study aims to investigate the role of phosphate in cell separations and AOM destabilization by Al-based electrocoagulation-flocculation-flotation (EFF) for harvesting of cyanobacteria and phosphate. The Al-based EFF was conducted to harvest Microcystis aeruginosa (MA) with varied phosphate (0-10 mg/L) at 5 mA/cm2 and pH 8. Fluorescent organic fractions, molecular weight distributions, the properties of flocs and DBPs formation potential were fully investigated. The results showed that the EFF at a low level of phosphate (1 mg/L) effectively improves the harvesting of MA cells, phosphate and the reduction in dissolved organic matter (DOC) up to 99.5 %, 95 % and 50 %, respectively. However, the presence of concentrated phosphate (10 mg/L) alleviates cell harvesting and worsens AOM separations due to ineffective floc formation induced by the fast formation of inactive AlPO4 precipitates along with limited Al(OH)3. At such a condition, it worsens DBPs precursors minimization owing to AOM release from MA cells. The increase in the current density during EFF can compensate for cell harvesting efficiency even though at concentrated phosphate, but it further induces AOM release. It is concluded that Al-based EFF demonstrates an efficient harvesting of cyanobacteria, phosphorus and AOM separations from algae-laden water under phosphate impact.

Simultaneous removal of organic and nitrogenous compounds in mature landfill leachate by a hybrid electro-oxidation-dialysis (EOD) system.

Electrochemical process has been widely applied to eliminate recalcitrant contaminants (i.e., organic and nitrogenous compounds) in landfill leachate. This study aimed to evaluate the performance of a hybrid electro-oxidation-dialysis (EOD) system to minimize organic and nitrogenous compounds through a synergistic process of electrochemical oxidation (EO) and electrodialysis (ED) as well as the dissolved organic matter was characterized in terms of fluorescent component and molecular weight distribution. The EOD was carried out using boron-doped diamond (BDD) and Pt alternately. The results have shown that pH adjustment to acidic conditions is beneficial to EO. At optimal pH (pH 4), BDD-based EO is superior to removing COD and NH4+ up to around 56% and 64%, respectively. During EOD process, the lower current density at 20.83 mA cm-2 is preferred for the recovery of nitrogenous ions (i.e. NH4+ and NO3-), especially for BDD-EOD. In addition, the dominant humic acid-like (HAL) and soluble microbial products-like (SMPL) substances in the mature leachate are mostly degraded to smaller molecules from 105 Da to 103 Da in both EOD processes. Overall, BDD-EOD favours indirect oxidation and has a higher energy consumption efficiency than Pt-EOD induced by direct oxidation for simultaneous removal of organic and nitrogenous compounds. BDD-EOD requires a lower total operation cost of around $2.33/m3 compared to Pt-EOD. It is concluded that the hybrid BDD-EOD process is technically feasible as a powerful pre-treatment approach to mature landfill leachate for refractory organics degradation and nitrogenous nutrients recovery.