Tertiary treatment of biologically pre-treated landfill leachates by non-catalytic wet oxidation

Effluents from the biological treatment of landfill leachates continue to have great environmental impact, and there is still a need to develop feasible tertiary treatments for these aqueous wastes in order to comply with legislation. For this reason, the treatment of biologically pre-treated leachates by non-catalytic wet oxidation was here investigated and the effect of the main operating conditions, such as temperature (423-483 K), pressure (2.0-8.0 MPa) and pH (2.0-11.0), on the degree of mineralization, biodegradability, toxicity and colour were analyzed. These results revealed the presence in the leachate of easily oxidizable compounds (10-25% of the initial COD), which were degraded at the beginning of the reaction, causing a fast reduction in COD and colour number and increasing the pH value. Once the most easily oxidized matter was degraded, COD decreased more slowly over time, showing asymptotic behaviour at high reaction times due to the presence of refractory compounds (35% of the initial COD in the best of the cases). Increases in pressure and/or temperature improved the degradation rate and the final degree of mineralization, although pressure had no significant effect on biodegradability, toxicity or colour reduction. Operating temperatures near 463 K and/or the addition of alkali to the leachate caused the appearance of coloured intermediates in the medium that significantly decreased the biodegradability of the effluent. With the exception mentioned above, the biodegradability of the leachate was greatly enhanced by wet oxidation. Thus, biodegradability indices (BOD5/COD) from 0.16 to 0.49 were obtained, the initial value being 0.012. Finally, the experimental results were successfully fitted to a pseudo-first order kinetic model. (C) 2015 Elsevier B.V. All rights reserved.