28 July, 2021
Forward osmosis (FO) as a novel membrane separation technology has recently been investigated in various water treatment applications. The natural mass transfer process between two solutions driven by the osmotic pressure difference leads to many operational advantages in the FO process, such as low energy consumption and minimal fouling problems. It makes FO a feasible technology for the treatment of produced water (PW). Although previously, the treatment of PW using FO has been investigated, osmotic backwashing (OB) is not systematically examined for water flux recovery of the PW-fouled FO membranes. Moreover, the cleaning of FO membranes used for the simultaneous treatment of different PW streams was never previously attempted. In this study, OB was thoroughly investigated for the cleaning of PW-fouled FO membranes. Also, FO membrane chemical cleaning using SDS and NaOH solutions was examined too. To investigate OB, the cleaning efficiency of a 60 min OB cleaning protocol was examined under different FO operating modes in (5 x 20 h) experiments using synthetic desalter effluent as FO feed solution (FS) and 1.2 M NaCl solution or water-oil separator outlet (WO) as draw solutions (DS). The AL-FS (active layer facing FS) mode outcompeted the AL-DS (active layer facing DS) mode, achieving a flux of 12.9 LMH and 80.1% water reclamation when using WO as a DS. Therefore, this FO configuration was selected when evaluating the cleaning protocols. Moreover, after evaluating different OB methods, the 30 min OB protocol achieved the highest system efficiency rate of 95% and was studied for the treatment of real PW streams. The SDS and NaOH chemical cleaning methods achieved flux recovery rates of 99% and 98% by the end of the third treatment cycle, respectively, outperforming the 89% flux recovery rate of the optimized OB protocol. Although the investigated cleaning methods were able to restore the system performance, a substantial increase in RSF was observed due to mainly irreversible colloidal fouling. This study demonstrates the feasibility of OB and chemical cleaning in restoring FO system performance for the simultaneous treatment of PW streams.
Membrane distillation (MD) is an emerging thermal separation technology which proved its efficiency in desalination of highly saline waters, including seawater, brines and impaired process waters. In a long-term prospective, MD can reinforce sustainability of the clean water production and mitigate the water-energy stress caused by lacking suitable freshwater recourses. However, just like in any other membrane separation process, MD membrane is susceptible to biofouling which presents a significant challenge by substantially reducing its performance and deteriorating permeate quality. This study evaluated different cleaning methods aimed at controlling biofilm development on a surface of hydrophobic MD membrane in a direct contact MD (DCMD) process fed by the Red Sea water. This was achieved by applying physical (hydraulic) cleaning and chemical cleanings with a range of chemicals utilized in membrane separation processes including citric acid (mineral acid), ethylenediaminetetraacetic acid (EDTA, metal-chelating agent) and sodium hypochlorite (NaOCl, oxidant). Flux recovery and changes in biofilm morphology, including its thickness and structure as well as microbial and extracellular polymeric substances (EPS) contents before and after cleanings have been analyzed to elucidate cleaning mechanisms and suggest effective strategies of biofilm removal. The results showed that 0.3% EDTA exhibited the best cleaning performance resulting in the highest permeate flux recovery (93%), followed by 0.3% NaOCl (89%), 3% citric acid (76%), and hydraulic (66%) cleanings. Application of EDTA and NaOCl has also resulted in the lowest number of bacterial cells and substantial reduction of the peak intensities caused by protein-like compounds and tyrosine-containing proteins present on the membrane surface after its treamtent. The observed trends are in a good correlation with the optical coherence tomography (OCT) observations which revealed substation changes in biofilm morphology leading to a significant reduction of biofilm thickness which followed the order of hydraulic cleaning < citric acid cleaning < NaOCl cleaning < EDTA cleaning. This study suggests that selection of an appropriate cleaning type and formulation is critical for achieving sustainable MD plant operations, both technically and economically.
The simultaneous treatment of different produced water streams with the forward osmosis-membrane distillation hybrid system (FO-MD) has been suggested recently. This work investigates the need for pretreatment of produced water prior to filtration with FO-MD in order to reduce the level of fouling and scaling in the system. The desalter effluent (DE) stream was selected as FO feed solution, and the water oil separator (WO) stream was used as FO draw solution/MD feed solution, and a significant flux decline was observed in FO and MD within the first 5 hours of operations. SEM and EDX analysis indicated that the formation of scale layer on both membranes was the main reason for the sharp flux decline. Silica was the major contributor to the scaling of the support layer of the FO membrane. While the scaling layer on MD membrane consisted mainly of CaSO4 crystals with some deposition of Silica. Therefore, electrocoagulation (EC) was selected for the pretreatment of produced water to target the removal of Ca, SiO2 and SO4 ions in order to reduce the likelihood of inorganic fouling in FO-MD. The different parameters of EC, namely, the current density, electrolysis time, and initial pH were tested at a wide range of values of 7-70 mA/cm2, 10-60 minutes, 5-9, respectively. calcium and sulfate ions were not effectively removed at the relatively high applied current density of 70 mA/cm2, while high removal of silica was achieved even at low applied current densities. The optimum conditions of EC for silica removal were found to be 7 mA/cm2 for the current density and 10 minutes for the electrolysis time which resulted in a 97% removal of silica. it was found that due to pretreatment, the average FO and MD fluxes increased by 49% (9.93 LMH) and 39% (8.55 LMH), respectively. Therefore, even though EC did not show promising results in terms of the removal of calcium and sulfate, efficient silica removal was achieved with minimum energy requirements which suggests that it could have a potential to be integrated with the FO-MD hybrid system for the treatment and reclamation of produced water.