Influence of the boron doping level on the electrochemical oxidation of raw landfill leachates: Advanced pre-treatment prior to the biological nitrogen removal

Abstract

The electrochemical oxidative treatment of landfill leachates (LLs) containing high amounts of ammonia nitrogen and organic matter was used as a promising method, prior to biological processes, to achieve the final effluent quality that would be acceptable by current regulations. The deposited boron-doped diamond electrodes (BDDs) with different boron doping concentrations (10000, 5000 and 500ppm of B) were applied as anodes. The results showed that the boron doping level influences the electrochemical activity and selectivity of electrode surface due to a decrease in the sp3/sp2 ratio of the BDD material. Special attention was paid to the oxidation efficiency of organic matter (COD=4225mg O2/L, BOD=366mg O2/L) and ammonia (2270mg N-NH4+/L) in the investigated LLs. Additionally, bisphenol A (BPA; 1539.6μg/L), a suspected endocrine disruptor, was studied as a potential indicator of the removal efficiency of micropollutants.It was found that the oxidation of BPA and BOD are correlated with the sp3/sp2 ratio, while a decrease in the sp3/sp2 ratio of the BDD material was associated with the elevated efficiency of N-NH4+ removal. Low pH and the addition of Fe(II) salts suppressed the oxygen evolution reaction, and overcame the mass transport limitation of organics in the case of OH-mediated oxidation.Regarding the elimination of ammonium nitrogen, lower effectiveness was generally achieved in comparison to the COD removal. The maximum removal of COD and ammonium nitrogen reached 79 and 41%, respectively. These values were much higher than those reported in the previous study involving a single-cell flow reactor. Thus, anaerobic ammonium oxidation (Anammox) processes seem to be a reasonable option as a final step of LL treatment.