Integration of catalytic wet peroxidation and membrane distillation processes for olive mill wastewater treatment and water recovery

The degradation of organic matter present in olive mill wastewater (OMW) and the recovery of water were studied by the integration of catalytic wet peroxidation (CWPO) and direct contact membrane distillation (DCMD) for the first time. The oxidation step was performed in a fixed-bed reactor (FBR) working in continuous mode (pH(0) = 4.0 +/- 0.2, 60 degrees C, Q = 0.75 mL/min, [H2O2]/[COD](feed) = 2.3 +/- 0.1 g H2O2/g O-2). Samples of OMW diluted by 5- and 7.5-fold were used (OMW-5x and OMW-7.5x, respectively), corresponding to inlet chemical oxygen demand (COD) values of 3562 +/- 68 and 2335 +/- 54 mg/L, total phenolic content (TPh) of 177 +/- 17 and 143 +/- 7 mg GA(eq)/L, and total organic carbon (TOC) of 1258 +/- 63 and 842 +/- 45 mg/L, respectively. The FBR was loaded with 2.0 g of a Fe-activated carbon derived-catalyst, prepared by using olive stones as the precursor, in line with a circular economy model approach. The catalyst was selected based on the activity and stability towards polyphenolic synthetic solutions shown in previous works of the team, while actual OMW samples were used in this work. CWPO-treated samples of OMW allowed the operation of the DCMD unit at higher fluxes than with the analogous untreated ones, also showing higher rejections of organic matter from the feed solution upon DCMD, highlighting the beneficial effect of this novel configuration. Using a pre-treated sample of OMW-7.5x as feed solution (Q = 100 mL/min, T-permeate approximate to 18 degrees C, T-feed approximate to 66 degrees C), the produced permeate water stream presented several parameters well-below the legislated thresholds required for direct discharge for crops irrigation, including total suspended solids (TSS < 10 mg/L), TPh (<0.01 mg GA(eq)/L), biochemical oxygen demand (BOD5 < 40 mg/L), and dissolved Fe (<0.06 mg/L). Moreover, the resulting concentrated OMW-retentate streams could be recirculated to the FBR and maintain the same removal efficiencies obtained previously, despite the increased initial organic loadings of the retentate after DCMD.