Role of Operating Conditions in a Pilot Scale Investigation of Hollow Fiber Forward Osmosis Membrane Modules

Abstract

Although forward osmosis (FO) membranes have shown great promise for many applications, there are few studies attempting to create a systematization of the testing conditions at a pilot scale for FO membrane modules. To address this issue, hollow fiber forward osmosis (HFFO) membrane modules with different performances (water flux and solute rejection) have been investigated at different operating conditions. Various draw and feed flow rates, draw solute types and concentrations, transmembrane pressures, temperatures, and operation modes have been studied using two model feed solutions—deionized water and artificial seawater. The significance of the operational conditions in the FO process was attributed to a dominant role of concentration polarization (CP) effects, where the selected draw solute and draw concentration had the biggest impact on membrane performance due to internal CP. Additionally, the rejection of the HFFO membranes using three model solutes (caffeine, niacin, and urea) were determined under both FO and reverse osmosis (RO) conditions with the same process recovery. FO rejections had an increase of 2% for caffeine, 19% for niacin, and 740% for urea compared to the RO rejections. Overall, this is the first extensive study of commercially available inside-out HFFO membrane modules.

Forward Osmosis: A Critical Review

“…One study looked at the possibility of removing foulants from coal seam gas by FO and found that the dissolved organic carbon, denosine tri-phosphate and major inorganic scalants (Ca, Mg and silica) in the coal seam gas feed were removed substantially [94]. Moreover, FO showed almost 7.5 times higher rejection of urea as compared to the RO rejections [105]. In another study [66], FO was investigated as a potential process for rejection of micropollutants; a higher rejection of bisphenol A in FO mode (99.5 ± 0.1%) than in the RO mode (97.3 ± 1.6%) were obtained.…”

Forward Osmosis as Concentration Process: Review of Opportunities and Challenges

“…Specific rejection studies regarding low MW organics by FO were limited. Even if, in general, rejection remains high, some compounds were poorly rejected (sometimes below 50%) under specific operating conditions (pH, membrane and draw type) especially for compounds below 100 Da (ethanol, acetic acid, urea, phenol) or some uncharged micropollutants even larger than 200 Da [ 82 , 110 , 212 , 222 , 223 , 224 ].…”

“…Very importantly, the reduction in the rejection with increasing recovery was observed, especially for compounds, such as urea, that were imperfectly rejected by FO membranes. This was expected since high recovery results in an increase in the solute concentration in the module [ 224 ]. Such observation is key for concentration process and reinforces the need for a high rejecting FO system.…”

“…One recommendation is to operate FO in counter current mode to maintain a more constant osmotic pressure over the module(s) ( Figure 5 ; [ 224 ]). Such an approach of operating offers the double benefit of (i) avoiding elevated flux at the start of the concentration process, limiting the fouling propensity and (ii) to still maintain a flux at the end of the concentration process instead of having difficulties when reaching the osmotic equilibrium [ 248 , 249 ].…”

“… Description of osmotic pressure of feed and draw stream in along a FO module, depending on operation in co- and counter-current (adapted from [ 224 ]). …”