Fouling behaviours and mitigation in pressure-retarded osmosis processes with geothermal water/brine-based draw solutions

In this study, we examined pressure-retarded osmosis (PRO) performance using naturally high temperature NaHCO₃-enriched geothermal water (i.e., capturing CO₂ waste gas) and geothermal brine. The results showed that increasing draw solution temperature from 20 °C to 60 °C, NaHCO₃-enriched geothermal water facilitated improving water flux (J_w) without compromising reverse salt flux (J_s), leading to lower J_s/J_w ratios compared to NaCl-based solution. With NaHCO₃-enriched geothermal water (60 °C) as draw solution, periodically physical cleaning (clean water flushing with/without air sparging, osmotic backwashing) displayed ∼17 % lower permeability recovery ratio (p < 0.05), dissimilar foulant morphology and inorganic compositions compared to chemical cleaning (acid, base, NaClO); while the average permeability levels under both cleaning conditions were comparable (p > 0.05). However, the recovery ratio decreased with extending cleaning cycle, resulting in an almost comparable permeability to that without cleaning after 60-h operation. In addition, the presence of Ca²⁺ at 5 mM in the NaHCO₃-enriched geothermal water (60 °C) led to more significant water flux drop and less reverse flux decrease compared to those with Ca²⁺ at 0 and 30 mM, possibly due to its dense fouling layer formed by smaller-sized CaCO₃ precipitates. When geothermal brine (low or high salinity, pH = 3 or 9.7, 60 °C) was used as draw solution, it was noticed that (1) with low salinity brine, basic pH levels led to higher water fluxes; (2) with high salinity brine, the water flux was impacted by the ionic compositions of draw solutions, instead of pH levels.