Abstract
A facultative hybrid reverse osmosis (RO) – pressure retarded osmosis (PRO) system can switch its operation between i) RO mode, where high quality water is produced from waste-water treatment plant effluent and ii) PRO mode, where the salinity difference between seawater desalination brine and wastewater is converted to energy. This system takes advantage of fluctuating energy pricing, by consuming low cost energy to produce water and producing high value energy.
The system was modeled by a local concentration polarization and solution diffusion model, considering several geometries. The system was optimized and for different ratios of the economic value of electricity vs water, to find process settings and switch points. A necessary condition for the feasibility of the facultative hybrid RO-PRO concept is that the permeate value satisfies: i) low enough, to justify electricity production and ii) high enough, to justify water production. With a maximum energy price of 0.25 USD/kWh, the first condition translates into <0.10–0.17 USD/m3 (ALFS) or <0.20–0.28 USD/m3 (ALDS). The facultative hybrid RO-PRO concept is only feasible as improvement to a PRO system, where there is a limited demand for purified WWTP-effluent. It is expected that it is usually preferred to produce purified WWTP-effluent.
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•Facultative hybrid RO-PRO produces water or energy depending on varying energy cost.•Process optimization found operating mode switching points and operational settings.•Transverse flow element has best concentration polarization vs crossflow trade-off.•Water production is generally more attractive than energy generation.