Application of advanced oxidation processes in the reclamation of wastewaters from the oil & gas sector

Resumen

This thesis has been developed within two projects dealing with the reclamation of wastewater generated by the processing of oil & gas industry. The Integroil project (Horizon 2020), in which our research group (Catheter) at Universitat Rovira i Virgili, was one of the participants, aimed at water reuse in the oil & gas sector. Although the project included wide range of technologies to propose an integral treatment solution to the oil and gas sector, including upstream and downstream processes, the research parts that belong to Integroil project within this thesis (Chapters 3-4) deal with the treatment of downstream effluents. For the treatment of downstream effluents, several photo-based and ozone-based advanced oxidation processes (AOPs) were assessed in detail starting from a synthetic refinery wastewater (SRW) with a complex composition. SRW was prepared with the typical organic and inorganic groups found in petroleum refinery effluents according to the literature, to simulate as much as possible the real effluents. Ultimately, this allowed obtaining more reliable information than using a one-component synthetic water. The initial results obtained with SRW allowed the treatment efficiencies to be compared with a stable effluent that has the same nature for all experiments, which would be unstable and changeable for real wastewater. Also, studying the effect of parameters such as the amounts of oxidants, pH and reaction time on SRW allowed understanding each treatment behaviour and establish the working ranges for application and further optimization in real refinery effluents. After the initial screening on SRW, the treatment performance of three different real wastewaters was investigated aiming at reaching the requirements for water reuse in the plant. The different effluents came from a petroleum refinery located in Turkey, one of which was collected after primary treatment while the other two were collected after secondary treatment. In the refineries, the treated water can be used for different purposes depending on the quality obtained after treatment. For the case refinery in this study, the total organic carbon (TOC) content needs to be below 15 mg/L and 4 mg/L for firewater and cooling water, respectively. Among the applied photo-based treatments, although heterogeneous photo-catalysis conducted with TiO2 and a combined process of heterogeneous photo-catalysis and photo-Fenton showed promising results for the treatment of the refinery effluents, photo-Fenton treatment revealed a superior effectiveness for application, both as a secondary and tertiary treatment, considering the improvements on TOC removal, toxicity and biodegradability. Photo-Fenton as secondary treatment resulted as efficient as the biological treatment, presenting ca. 90% of chemical oxygen demand (COD) removal, getting final TOC values of ca. 20 mg/L. Moreover, a marked increase in the BOD5/COD ratio from 0.38 to 0.83 was obtained, indicating an important improvement of biodegradability. That is, wastewater would be more easily treated by biological means after photo-Fenton as secondary treatment. As a tertiary treatment, photo-Fenton process either with H2O2/COD=10 and H2O2/Fe2+=50 or H2O2/COD=4 and H2O2/Fe2+=10 provided a final TOC value <4 mg/L. This result reveals the possibility to reuse the effluent in the refinery plant, thus increasing the sustainability. However, besides the downstream wastewater treatment, the upstream wastewater treatment that was investigated in parallel within the Integroil project, regardless from the content of this thesis, agreed that ozone-based processes were more suitable for both upstream and downstream applications rather than photo-based treatments. Thus, ozone-based treatments were investigated as the possible solution to the refinery as tertiary treatment, to reach the final requirements for reuse and recycle purposes. Optimization studies were also conducted by experimental design. The screening tests by fractional factorial design performed on SRW revealed that the significant parameters for the treatment were ozone feed ratio, H2O2 amount and reaction time, while pH was found insignificant. Based on the box-Behnken, response surface methodology performed for an effluent collected after biological treatment, the significant parameters were optimized, being the ozone ratio of 0.9 g/h, H2O2 amount of 47 mg/L and 60 min duration. However, in case of increasing the H2O2 amount to 80 mg/L, the duration can be minimized to 37.5 min decreasing the energy and reagent consumption costs by a 37%, and reaching a final total organic carbon (TOC) under 4 mg/L, which is the target for reuse possibilities. Chapters 5 and 6 are related to the treatment of oil sands process water (OSPW) currently stored in tailing ponds covering 220 km2 of the land because of the zero-discharge policy in Alberta region, in Canada. This research is part of a second project, Resilient Reclaimed Land and Water Systems leaded by Dr. Gamal El-Din’s at University of Alberta. The accumulation of OSPW in tailing ponds is becoming a serious problem due to the unknown future of its management, as well as the increasing public concern about its environmental effects including the wildlife around the tailing pond. Firstly, catalytic ozone-based and UV-based treatments with different oxidants and operating pH were compared. As catalysts, the catalytic activities of granular activated carbon-based materials previously doped with heteroatom (N or N/S) and Fe (III) were investigated, which were rarely studied for the treatment of OSPW. A catalyst selection was performed based on the fractional factorial design for catalytic ozonation of a synthetic water. Afterwards, the selected materials were investigated further for the treatment of real OSPW by initially ozone-based treatments, afterwards for photo-based treatments. Also, UV/Fenton treatment was considered as an effective method for removing the organic components present in the effluent that cause acute toxicity. Obtained results showed that the treatment efficiency by the AOPs in terms of mineralization was in the order of UV/peroxymonosulfate (PMS) < O3 < UV/H2O2 < UV/Fenton, where dissolved organic carbon (DOC) removal was up to 14%, 21%, 31% and 64%, respectively. The best improvement in acute toxicity tested with V. Fischeri was achieved by UV/Fenton, although the ozone treatments also led to great toxicity reduction. This was in agreement with the reduced concentrations of naphthenic acids, which are considered as the main contributors to the toxicity of OSPW. It was observed that transformation of the toxic compounds rather than mineralization took place for the ozone-based treatment. Afterwards, another study of the ozone-based combined AOPs including H2O2 and UVC to achieve mineralization was performed, while also aiming at reduction in toxicity arising from the organic components, including naphthenic acids present in the effluent. In this part, O3/H2O2 process was tested considering the previous experiences obtained within the Integroil project. Additionally, UVC combination to O3-based treatments was studied for the first time to treat OSPW. Obtained results showed that considerably high DOC removals were obtained after 90 min treatment by O3, O3/H2O2, UVC/O3 and UVC/O3/H2O2 with a 1.8 g/h O3 production rate, which resulted in 45%, 84%, 84% and 98%, respectively. All the treatments removed significantly the acute toxicity on Vibrio fischeri. Although the highest mineralization was achieved by O3/UV/H2O2 treatment, O3/H2O2 treatment without UVC was found more energy-efficient to operate in terms of energy consumption. Thus, UVC combined treatment could only be considered in case of reuse/recycle purposes in the plant that requires a high quality water.