Pressure retarded osmosis: A membrane process for environmental sustainability

Salinity Gradient Power (SGP) based on the controlled mixing between two solutions at different salinities is a viable alternative to produce power from renewable sources. Pressure Retarded Osmosis (PRO) is one of the most promising technologies proposed so far for the exploitation of such energy source. Apart from the typical source of salinity gradients, namely river water and seawater, more and more interest has been raised recently towards the use of non-conventional saline solutions. In this work, water originating from a sewage treatment plant is used as the dilute solution (feed solution), while brine exiting from a desalination plant is used as the concentrate (draw solution), thus being efficiently diluted before the discharge into the sea. Aim of this work is to investigate the performance of PRO modules arranged in series via a purposely developed process model based on transport and balance equations. Detrimental effects due non-ideal phenomena as salt diffusion through the membrane, concentration polarization were taken into account. Pumping power was also accounted for. Geometrical data were derived from existing spiral wound membrane modules typically employed in Reverse Osmosis. The effect of solution velocity, number of PRO units and operative hydraulic pressure (applied on the draw solution channel) on the process performance was investigated. Results show that the dependence of the net power on velocity and applied pressure is not monotonic thereby exhibiting a maximum: under optimized conditions, a maximum net power of ∼1.5kW corresponding to a net power density of ∼6.6 W/m2 can be obtained with six identical PRO modules. Along with this power production, a simultaneous brine dilution of more than 22% was found at the same conditions. However, despite the potential shown by the model, the main limitation for a real process might be the real performances of the osmotic membranes in terms of mechanical robustness and resistance against fouling phenomena. In this regard, valuable contributions may come from nanotechnologies ad-hoc developed.