Ozonation of biologically treated landfill leachate: efficiency and insights in organic conversions

• Ozonation efficiency of biological landfill leachate was assessed.
• pH significantly impacted the COD removal efficiency.
• Organic matter conversions were assessed using UV–visible measurement calculations.
• Spectral changes are uniform after an initial reaction phase.
• Differences in working mechanism are seen compared to municipal effluent ozonation.

Biologically stabilized landfill leachate contains a significant amount of recalcitrant organic matter, often expressed as Chemical Oxygen Demand (COD). To meet regulatory requirements, (a combination of) advanced treatment techniques such as ozonation and granular activated carbon adsorption must be applied. However, the mechanisms occurring during ozonation of the complex matrix are not well understood. Therefore, transformations were studied (in a lab-scale semi-batch reactor) into detail based on COD measurements and UV–visible spectral calculations. First of all, a declining initial COD (COD0 from 1846 to 112 mg O2 L−1) resulted in a decreasing trend of UVA254 removal efficiency (respectively from 1.5 to 0.42 m−1 absorption decrease mg−1 O3) which was not seen for COD removal efficiency (no clear trend noticeable; average of 0.34 mg COD removed mg−1 O3). For low COD0, the amount of most reactive moieties is less, resulting to further reactions with the formed intermediates. An increased pH probably resulted in a higher production of hydroxyl radicals (HO). Using similar ozone doses, more COD was removed at higher pH values. Secondly, spectral calculations (such as differential absorbance) gave more insights into the reaction mechanism. It is hypothesized that after an initial period during which ozone selectively oxidized some species, ozonation of landfill leachate led to a significant HO production, dominating the ozone reaction mechanism.

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