Study On The Efficiency Of Pressure Delayed Osmosis In Removing Dissolved Organic Matte

Pressure retarded osmosis（PRO）process has attracted much attention as an emerging green membrane separation technology driven by osmotic pressure for efficient salinity energy harvesting,freshwater recovery and low CO₂ emission.Nevertheless,similar to other water treatment membrane separation technologies,membrane fouling,caused by dissolved organic matters（DOMs）,had significantly hindered the commercial viability of the PRO process.In contrast to pressure-driven membrane separation techniques,the impact of DOMs on membrane fouling in PRO processes was more complexity.Hence,it was crucial to conduct a thorough investigation into the influence of DOM on PRO process performance,curating valuable data and theoretical foundations to support future commercial deployment.In this study,the membrane orientation,hydraulic conditions（effective osmotic pressure difference,operating hydraulic pressure and cross-flow shear）and chemical properties of the feed solution（ionic strength and p H）were investigated using fulvic acid as the model foulant.Also,the working conditions with high water purification effect and power density were determined by the variation of performance parameters such as water flux,power density,salt flux,retention rate and adsorption capacity.Based on the determined working conditions,the degree of PRO membrane fouling by four typical foulants,namely sodium alginate,bovine serum albumin,fulvic acid and tannic acid,and their effects on PRO process efficiency were investigated.It was shown that PRO filtration of a single typical organic matter showed the highest flux drop（～60.1%）and the lowest power density（0.82 W/m²）for tannic acid at the end of filtration compared to baseline conditions,followed by bovine serum protein,xanthic acid and sodium alginate.It is worth noting that the PRO membranes had good removal effects（above 97.5%）for all the above four foulants.However,the adsorption of the foulants by the PRO membrane was the lowest for sodium alginate,followed by fulvic acid and bovine serum albumin,and the highest total adsorption was for tannic acid.This order was in agreement with the degree of water flux reduction described above,which suggests that the variation in water flux was closely related to the adsorption behavior of different foulants.The effects of combined organic foulant and natural dissolved organic matter on PRO process efficacy were examined from a hydrophobic perspective,in addition to the effects of single organic fouling on PRO process efficacy.It was found that in the combined fouling system formed by sodium alginate and hydrophobic organic matter（tannic acid or fulvic acid）,the water flux decreased significantly with the increase of the proportion of hydrophobic organic matter concentration,which indicated that the presence of hydrophobic organic matter played a major role in the effectiveness of the mixed system.In the combined system formed by bovine serum albumin and fulvic acid,bovine serum albumin played a dominant role in the mixture system.However,for the combined system formed by bovine serum albumin and tannic acid,in particular,the relative concentration of tannic acid and fulvic acid was about 10%higher than the water flux of single filtration of tannic acid or bovine serum albumin at 1:1.In addition,the addition of calcium ions in the four mixture systems led to an increase in the complexation effect among the organics,which in turn increased the degree of membrane fouling,which adversely affected the filtration of complex organic maters by PRO membranes,while the increase in electrostatic repulsion effect with increasing p H led to a decrease in membrane fouling.The study investigated the effect of natural DOM on the efficiency of the process by using incoming raw water from two waterworks in Jinan City,Shandong Province.Upon analysis,it was observed that the presence of hydrophilic organics in natural water sample 1 resulted in significant water flux reduction（～23.7%）and power density loss to the PRO permeable membrane compared to natural water sample 2.The fouling of the membrane porous support layer was mainly caused by the natural DOM in natural water sample 1,whereas the DOMs in natural water sample 2 mainly fouled the membrane surface with small and medium molecular weight protein-like substances adsorbed on the PRO membrane.In this study,the mechanism of membrane fouling was analyzed by using XDLVO theory.From the calculation results of adhesion free energy into and cohesion free energy,it was evident that the adhesion force between natural DOM and the PRO membrane in the PRO process was the primary reason for the initial flux decrease,and the cohesion force between the foulants presented in feed solution and those adsorbed on the membrane was the main cause of subsequent flux decrease.