Removal of recalcitrant organic matter content in wastewater by means of AOPs aiming industrial water reuse

Abstract

This paper comes out from the need to provide an improvement in the current oil refinery wastewater treatment plant (WWTP) aiming to generate water for reuse. The wastewater was pretreated and collected in the WWTP after the biological treatment unit (bio-disks) followed by sand filtration. Ozonation (ozone concentration from 3.0–60 mgO₃ L⁻¹), UV (power lamp from 15 to 95 W), H₂O₂ (carbon:H₂O₂ molar ratio of 1:1, 1:2, and 1:4), and two advanced oxidation processes (UV/O₃ and UV/H₂O₂) were investigated aiming to reduce the wastewater organic matter and generate water with suitable characteristics for the reverse osmosis operation and subsequent industrial reuse. Even after the biological and filtration treatments, the oil refinery wastewater still presented an appreciable amount of recalcitrant organic matter (TOC of 12–19 mgC L⁻¹) and silt density index (SDI) higher than 4, which is considered high for subsequent reverse osmosis due to membrane fouling risks. Experiments using non combined processes (O₃, H₂O₂, and UV only) showed a low degree of mineralization after 60 min of reaction, although the pretreatment with ozone had promoted the oxidation of aromatic compounds originally found in the real matrix, which suggests the formation of recalcitrant compounds. When the combined processes were applied, a considerable increase in the TOC removal was observed (max of 95 % for UV/O₃ process, 55 W, 60 mgO₃ L⁻¹), likely due the presence of higher amounts of reactive species, specially hydroxyl radicals, confirming the important role of these species on the photochemical degradation of the wastewater compounds. A zero-order kinetic model was fitted to the experimental data and the rate constant values (k, mgC L⁻¹ h⁻¹) ranged from 4.8 < k _UV/O3 < 11 ([O₃]₀ = 30–60 mg L⁻¹), and 8.6 < k _UV/H2O2 < 11 (C:H₂O₂ from 1:1 to 1:4). The minimum and maximum electrical energy per order (E _EO) required for 60 min of treatment were calculated as 5.4 and 81 Wh L⁻¹, respectively, for UV/O₃ (15 W, 60 mgO₃ L⁻¹) and UV/H₂O₂ (95 W, 1C:1H₂O₂). Good results in terms of water conditioning for reverse osmosis operation were obtained using UV/H₂O₂ process with initial molar ratio of 1 C:2 H₂O₂ (UV lamp 55 W) and 1 C:4 H₂O₂ (UV lamp 95 W), and total organic carbon (TOC) removals of 62 % (SDI₁₅ = 1.8) and 74 % (SDI₁₅ = 2.0) were achieved, respectively, after 60 min. The treated wastewater followed to the reverse osmosis system, which operated with an adequate flux of permeate, was very efficient to remove salt and generate a permeate water with the required quality for industrial reuse.