The forward osmosis-pressure retarded osmosis (FO-PRO) hybrid system: A new process to mitigate membrane fouling for sustainable osmotic power generation

Pressure retarded osmosis (PRO) is an emerging technology to harvest the renewable salinity-gradient energy. However, its performance can be significantly limited by severe membrane fouling if a real waste effluent is used as the feed solution. To tackle this, we have experimentally demonstrated that forward osmosis (FO) can be used as a pretreatment means to extract water from the wastewater stream to the inter-loop solution. Subsequently, the diluted inter-loop solution could serve as a clean feed to the PRO unit at negligible fouling tendency. As a result, the newly developed FO-PRO hybrid system is capable of sustaining osmotic power generation with the advantages of FO such as (1) low fouling propensity, (2) easy membrane cleaning, and (3) minimal external energy required. Model simulations of full-scale analyses reveal that, by choosing the salinity of the inter-loop solution to PRO as 0.1 M, it is possible to reach a power density greater than 5 W/m(2) set by Statkraft as the commercially viable benchmark. Meanwhile, the concentrated brine draw solution of PRO can be diluted back to the seawater level for easy disposal or reuse. Under this condition, FO possesses a comparable water flux with conventional pretreatment methods using pressure driven membrane processes. A simple design strategy is also provided in detail for the integration of FO and PRO units.