Master of Engineering - Research
School of Civil, Mining and Environmental Engineering
Abousnina, Rajab M., Oily wastewater treatment: removal of dissolved organic components by forward osmosis, Master of Engineering - Research thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3750
Produced water is water brought to the surface with crude oil or natural gas; it is the largest waste stream by volume associated with the production of oil and gas. Some crude oil and traces of organic compounds, particularly organic acids, are known to occur in produced water. Although the current international standard limits the amount of dissolved oil in produced water to less than 30 mg/L prior to environmental discharge, no regulations exist for other dissolved organic constituents. This is mostly because of the lack of low cost, high efficiency technologies capable of removing dissolved organic constituents from produced water. This work investigated the removal of dissolved organics from produced water by the forward osmosis (FO) process, with a particular focus on Libya. In an off-shore platform, seawater can be utilised as the draw solution for the FO process as it allows for a significant reduction in the cost of treatment before discharging produced water into the sea. Two membranes specifically designed for the FO process (namely HTI-Cartridge and HTI-Pouch) provided by Hydration Technology Innovation and two typical NF membranes (namely NF270 and NF90) provided by Dow Chemical were used in this study. Acetic acid was selected as a model organic acid and a synthetic oil-in-water emulsion was prepared using motor cycle oil (Fork w2.5) in Milli-Q. The water flux, reverse salt flux, the rejection of acetic acid, and the effects of concentrated oil in produced water were systematically evaluated. This investigation appears to be the first attempt to study the removal of dissolved components from produced water using an FO membrane.
Water flux and reverse salt flux were investigated at different pH values (un-adjusted pH, pH4, and pH6), and the results showed that the HTI-Cartridge membrane produced a higher permeate flux than the HTI-Pouch membrane when the same draw solution concentration was used in the FO mode (e.g. active layer facing the feed solution). On the other hand, there were no significant differences in the water flux and reverse salt flux at different pH values for each individual membrane. The transport phenomena of the HTI-Cartridge were also investigated since it performed better as a permeate flux than the HTI-Pouch membrane. An HTI-Cartridge membrane was evaluated in the FO and pressure retarded osmosis (PRO) modes (in the PRO mode, the active layer of the membrane is in contact with the draw solution). Higher water flux and reverse salt flux were observed under the PRO mode rather than the FO mode because the internal concentration polarisation (ICP) phenomenon which is considered to be unique in the FO process.
The performance of the FO membranes (HTI-Cartridge and HTI-Pouch) and NF membranes (NF-90 and NF-270) were also investigated under reverse osmosis (RO) mode and the results were compared with the FO mode. The rejection of acetate by the FO and NF membranes was strongly pH-dependent. At near neutral pH (6.7-7.3), acetate rejection by either the HTI-Cartridge or HTI-Pouch membranes was almost 100%. The rejection of acetate decreased dramatically as the feed solution pH decreased to pH 4, although both of them rejected acetate more efficiently under FO mode where the active layer faced the feed solution and the backing layer faced the draw solution. Acetate rejection by the NF-270 and NF-90 membrane was considerably lower than the FO membranes. The rejection of acetate increased from 55% to 92% with the NF-90 membrane, as the feed pH increased from 4 to 9. Similarly, the rejection of acetate by the NF-270 membrane (which has a larger pore size than the NF-90 membrane), increased from as low as 2% to 89% as the feed pH increased from pH 4 to pH 9. In the FO mode, acetate rejection was also strongly pH dependent. More importantly, acetate rejection in the FO mode was at least 10 % higher than in the RO mode. In addition, the allowable oil content (30 mg/L) did not affect acetate rejection in either the FO or RO modes. Furthermore, the allowable oil content of 30 mg/L did not cause any discernible membrane fouling in either the FO or RO modes. The reported results indicate that a highly efficient removal of acetate from produced water can be achieved using the FO process without pH adjustment, because the pH range of the produced water produced from light crude oil is usually from pH 6 to pH 7.7.