Combined AOP/GAC/AOP systems for secondary effluent polishing: Optimization, toxicity and disinfection

Abstract This study reports the use of advanced oxidation processes (AOPs), specifically O3, O3/H2O2 and UV/H2O2, in combination with granular activated carbon (GAC) to polish secondary effluents. For this purpose, three configurations were tested: O3-GAC-O3, O3/H2O2-GAC-O3/H2O2 and UV/H2O2-GAC-UV/H2O2. The efficiency evaluation was based on the ability of the systems to remove aromaticity (UV254), organic matter (chemical oxygen demand - COD), Escherichia coli and total coliforms content as well as toxicity (Artemia salina and Allium cepa). The methods were optimized using central composite design to investigate the influence of O3 and H2O2 concentration in the efficiency of the system and the optimal conditions were subsequently applied to treat urban secondary effluents. High organic matter removal rates were obtained by all the tested configurations, of which O3-GAC-O3 proved to be the most efficient. The systems were also efficient to disinfect the samples, promoting nearby 5-log reduction of E. coli and total coliforms. Acute toxicity, cytotoxicity, and genotoxicity were observed in samples subjected to the UV/H2O2-GAC-UV/H2O2 method. When used alone, GAC removed around 80% of organic matter, nevertheless failed to disinfect the samples, which furthermore exhibited acute toxicity and genotoxicity toward A. cepa. Samples subjected to direct photolysis proved more toxic than untreated samples while uncombined H2O2, O3, GAC, and UV, as well as O3/H2O2 and UV/H2O2 failed to degrade organic matter or to mitigate the toxicity of the samples.