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 degrees C to 60 degrees 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 degrees C) as draw solution, periodically physical cleaning (clean water flushing with/without air sparging, osmotic backwashing) displayed similar to 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 degrees 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 degrees 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.

Automated summary

In this study, the performance of pressure-retarded osmosis (PRO) using NaHCO3-enriched geothermal water and geothermal brine was examined. The results showed that increasing the draw solution temperature improved water flux without compromising reverse salt flux, leading to lower salt-to-water flux ratios compared to NaCl-based solutions. Physical cleaning methods were found to be more effective than chemical cleaning methods in terms of permeability recovery ratio and foulant morphology. The presence of Ca2+ in the draw solution resulted in a significant drop in water flux and reverse flux decrease due to the formation of a dense fouling layer. The water flux in PRO using geothermal brine as the draw solution was influenced by ionic compositions rather than pH levels.