Peroxymonosulfate promoted wet air oxidation of a real wastewater from a biodiesel production plant

Wastewater from a real biodiesel plant located in the province of Badajoz (Spain) has been treated by means of the Wet air oxidation (WAO) process. The effluent was slightly acidic (pH = 5.77) with a high content of chemical oxygen demand (COD) (43.1 g L^-1) mainly of organic nature (C_TOC = 10.5 g L^-1). The low biodegradability of this effluent (relation biological-chemical oxygen demand BOD/COD = 8.6 × 10^-3), suggests the suitability to be treated by WAO oxidation. The main WAO operational variables (oxygen pressure, temperature and pH), the use of homogeneous catalysis (Cu²⁺), or the addition of oxidant promoters (H₂O₂ or peroxymonosulfate (PMS)) were explored. From the results obtained, oxygen pressure exerted a positive effect while temperature was a crucial variable in the oxidation of the most recalcitrant organics. Although alkaline pH led to an improved behavior in terms of COD and TOC removals, the lack of aromatic compounds elimination (absorbance at 254 nm) suggests the preference to oxidize non-aromatic compounds. Addition of radical promoters resulted in a positive effect. Peroxymonopersufate was more efficient than H₂O₂ in the promoted experimental series. Under the optimum oxidation conditions used in this study, removals of 55% in COD, 45% in TOC and 86% in aromatic compounds were experienced.