Abstract
Pressure-retarded osmosis (PRO) is a promising membrane technology for harnessing the osmotic energy of saline solutions. PRO is typically considered with seawater/river water pairings however greater energy can be recovered from hypersaline solutions including produced water (PW) from the petroleum industry. One of the major challenges facing the utilization of hypersaline PW is its high fouling propensity on membranes. In this unique experimental evaluation, real PW from different sites was pretreated to varying degrees: i) minimal, ii) intermediate, and iii) extensive. The treated effluent was subsequently used for PRO testing and fouling rates were assessed for different membrane configurations over multiple cycles. Commercial grade flat sheet (FLS) coupons and novel hollow fiber (HF) modules were compared to validate the lower fouling propensity of HF membranes in PRO application. When minimally pretreated PW (10-micron cartridge filtration (CF)) was tested in FLS mode, severe membrane fouling occurred and the PRO flux decreased by 60%. In contrast, HF modules showed <1% flux decrease under both minimal and intermediate pretreatment schemes. Extensive pretreatment (1-micron CF, dissolved air flotation (DAF), powdered activated carbon, and microfiltration) reduced FLS PRO flux decline to <1%. These results confirm that PW can be treated to suitable levels for PRO application to avoid membrane fouling. Further validation of these pretreatment methods requires long term pilot testing and techno-economic assessment.
Highlights
A new membrane was used for each pretreatment evaluation cycle and a 5 mg/L biocide concentration was maintained in the system throughout all stages to prevent biofouling
A single-cycle evaluation consists of an initial baseline, actual produced water (PW) fouling test, and a second baseline, while a multiple-cycle evaluation would continue for a second actual PW fouling test and a third baseline
The total dissolved solids (TDS) were calculated as the summation of dissolved ions measured through ion chromatography and Inductively coupled plasma (ICP)
Summary
Several technologies are available for harnessing potential osmotic energy and converting salinity gradients into useful work including; pressureretarded osmosis (PRO) [5,6,7], reverse electrodialysis [8,9], and capacitive mixing [10,11]. Among these technologies, PRO has been the most widely investigated. In PRO, a low salinity feed solution (FS) is separated from a higher salinity draw solution (DS) by a semi-permeable membrane which allows only solvent molecules to pass through [12]. The resulting pressurized flow of diluted DS can be used to generate power via hydroturbines in a way similar to conventional hydro-power plants or converted directly into mechanical work by pressure exchangers [13,14]
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