Bidirectional diffusion of functional draw solutions in an osmotic photobioreactor coupling wastewater treatment and microalgal growth

The osmotic photobioreactor (OsPBR) technology has shown great potential by integrating forward osmosis with algal wastewater treatment. However, the reverse salt flux remains a key challenge. In order to reveal the diffusion behaviors and the transport mechanisms of reverse salt flux (RSF) in an OsPBR, the NH4HCO3 and (NH4)2HPO4 (DAP) were compared as draw solutions. The results showed that the biomass growth in OsPBR with NH4HCO3 as draw solution was higher than that of DAP initially, reached up to 0.78 g L−1. Although the reverse salt flux of NH4HCO3 was twice higher than that of DAP, the OsPBR using NH4HCO3 as draw solution exhibited better N & P removal rate than that of DAP, reached at 75% and 100%, respectively. A higher diffusion of cation to anion was further found for DAP as draw solution. This work showed the feasibility to use NH4HCO3 as draw solution in OsPBR, which allows the upgrade of microalgal osmotic photobioreactor technology not only in water treatment but also its carbon neutralization potential.