Treatment of produced water: potential application of forward osmosis

Dewatering of industrial wastewater
Dr. MR Bilad

Produced water represents an ever increasing wastewater challenge

Produced water (PW) – co-produced during oil and gas exploitation – is one of the largest sources of oily wastewaters. PW contains dispersed oils, suspended particles and dissolved solutes; and constitutes the largest waste stream from oil and gas industries. An estimated amount of PW in the US alone reaches about 6–8 x106 m3/day (data of 2008), let alone globally. Its amount keeps increasing as a result of rapid rise in the numbers of crude oil extraction processes and ever increasing demand of fossil fuels.

PW is typically characterized by high organic and dissolved solid contents, typically oil and grease of 15-300 mg/L, TDS of 1000-15,000 mg/L, COD of 20-2,500 mg/L and TSS of 5–4,000 mg/L. Recently, public concerns are growing for PW treatment to minimize its detrimental health and environmental impacts. The increasingly stringent regulatory standards for discharging PW dictates technological developments for meeting those standards. For instance the upper limits of oil and grease in Australia, USA and Europe are 30, 29 and 10 mg/L, respectively.

Traditional PW treatment methods

Traditional methods for PW treatments such as electro-coagulation, flotation, and hydro‑cyclone often do not adequately satisfy the discharge standards. They fail to fully eliminate low hydrocarbon content in the treated water. They are specifically ineffective in removing finely dispersed oil droplets of smaller than 10 μm. The conventional methods also poorly recover up to 60% of water, while the rest is still contaminated. This prompts the adopting of pressure driven membrane processes for treatment of PW over the last two decades, thanks to its effectiveness in removing fine oil droplets.

Despite its prevalence, pressure driven membrane processes suffer from membrane fouling that diminishes their performances, complicates their operations, and inflates their operational expenditures. Vulnerability of pressure driven membrane from fouling during PW treatment is largely because of the presence of fouling prone chemicals, in particular oil and organic substances, primarily originating from dispersed crude oil components and chemical additives used during oil production processes. Therefore, research on membrane processes for treatment of PW mainly focuses on combating membrane fouling problem. The challenging nature of the PW with respect to its high fouling propensity recently brings about the application of forward osmosis (FO).

Treating produced water with forward osmosis technologies

FO has received intensive attention as a low-fouling membrane process. Compared to the pressure-driven membrane processes, fouling in FO is looser and more reversible. Another advantage of FO is the high rejection to salts and most organics. The high fouling resistance of FO is believed to be originated from its “natural” transport mechanism (osmotic process). Moreover, developments in FO membranes has rendered it to be applicable for different applications, including those for highly challenging feeds. The application of standalone FO for PW treatment using earlier FO membranes has proven successful, but unfortunately still suffers from a low flux (because of too-low allowable draw solute concentration) and it still consumes too much energy for draw solute recovery. More importantly, high cross-flow velocities are required to minimize the impact of concentration polarization that significantly lowers the achievable flux. Despite of those shortcoming, PW treatment is seen as a sweet-spot of FO application due to its complex and difficult nature where fouling resistant property of FO can come to the fore. This area of application remains open for further explorations as a potential large-scale FO implementation because of the large scale and market for this application and emerging superior FO membranes (with respect to high flux and low membrane fouling propensity).

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