Investigation On Scaling Potential Prediction And Scale Prevention In NF-SRO Integrated Membrane System Operation

With the rapid development of the national economy, the demand of fresh waterfor domestic and industrial use continues to increase. As an important strategicsolution to water shortage, seawater desalination has becoming playing anincreasingly momentous role. The relative high concentration of hardness in seawatercauses low water recovery and high process cost in current seawater desalinationprocess, especially seawater reverse osmosis （SWRO） desalination process.Nanofiltration （NF） membrane can effectively remove divalent calcium, magnesium,and sulfate ions from seawater, and is capable of reducing hardness to significantlylow level, so it is a potential pretreatment method in seawater desalination. However,with the operation of NF seawater softening process at high water flux, there is alsoscaling potential of some scale-prone inorganic salt on NF membrane surface.Therefore, the scaling tendency prediction and scaling prevention of inorganic salt onNF membrane surface is of great significance.In this study, pilot-scale experiments were carried out on a NF–SWROintegrated membrane system （IMS） with a capacity of5m³·d^-1to evaluate scalingpotential of inorganic salt on NF and RO membrane surface. A kind ofultra-low-pressure and high selectivity NF membrane was investigated extensivelyunder once-through mode and semi-closed-loop operation, respectively. Theoreticalprediction of scaling potential of sparingly soluble inorganic salts in NF/RO brine andon membrane surface was also extensively investigated. The results are shown asfollows:（1） Based on Pitzer’s electrolyte solution theory and concentration polarizationmodel, a method was established to predict the scaling potential of sparingly solubleinorganic salts in NF/RO brine and on membrane surface at different NF permeate recovery （RNF）.（2） Once-through NF experiments were carried out at two kinds of operationconditions, constant cross-flow velocity of0.05m·s^-1at the inlet end of the module（termed as “Scheme I” for short） and that at the concentrate end of the module（termed as “Scheme II” for short）, respectively. All the operation conditions of the NFmembrane were kept at temperature of16±1°C, pH of7.85±0.05. The variation ofNF performance over RNFwas investigated. The effect of RNFincreasing on thescaling tendency of inorganic salt in the NF module and in the consequent RO modulewas investigated in term of concentration polarization modulus （CP） of divalent ions,Stiff and Davis Stability Index （S&DSI）, as well as Supersaturation Index （SI）. Theresults show that the NF membrane has excellent separation performance for divalentions. Take RNFof25%for example, Scheme I operation yields rejection of52.5%,30.5%,97.3%, and77.4%for Mg^2-, Ca^2-, SO₄^2- and CO₃^2-, respectively. Thedifference of NF permeate quality between the two schemes was slightly. Consideringthe virtue of convenient operation and easy regulation, Scheme I operation wascommonly used in commercial application and was adopted here to illustrate theexperimental results. When RNFincreased from10%to35%, the salt rejection（calculated by conductivity） by NF membrane increased gradually from10.9%to13.0%, while the total hardness in the NF permeate decreased from1960mg·L^-1to1650mg·L^-1gradually. The extent of concentration polarization between the twoschemes is quite different and CPSO₄^2- is always much higher than CPCO₃^2-. At RNFofhigher than30%, CaSO₄scale would preferentially form and precipitate on NFmembrane surface other than CaCO₃based on SI prediction. However, the S&DSIvalue on the NF and RO membrane surface always keeps negative, which indicatesthat CaCO₃scaling could not form in the experimental operating range.（3） Semi-closed-loop operation was performed with partial NF concentraterecycling back to the feed water supply tank to investigate the possible maximum NFpermeate recovery corresponding to the occurrence of scaling. We enlarged the NFrecovery to40%,45%,50%,55%,60%and65%, respectively. The NF pilot testswere operated under feed temperature of16±1°C and constant inlet cross-flow velocity of0.05m·s^-1, and feed total dissolved solids （TDS） in the range of34.5–44.7g·L^-1. Three schemes of operation condition were investigated. For Scheme1, neitheracid nor antiscalant was dosed in the feed solution. For Scheme2, only HCl acid wasdosed to adjust the pH of the feed to5.0. For Scheme3, both acid and antiscalantwere dosed to investigate their synergetic effect on scale inhibition. The experimentalresults indicate that for the NF module at high recovery and large flux, specific energyconsumption is about0.77to0.95kWh·m^-3NF permeate. For NF stage, it can be seenthat for all the three operation schemes, the increase of the NF recovery results in anincrease of CP_SO4^2- and CP_Ca^2-, while CPCO₃^2- always keeps close to1.0. At operationscheme I, an increase of NF recovery from40%to65%results in an increase ofCP_SO4^2- and CP_Ca^2-from1.87and1.23to3.95and1.48, respectively. At NFpermeate recovery of less than40%, SI of CaSO₄on the NF membrane surfacealready exceed1.0, which indicates that CaSO₄scale already forms under thiscondition; When the NF permeate recovery exceeds45%, the values of S&DSI on theNF membrane surface changes to positive, which indicates that CaCO₃scaling wouldform on the NF membrane surface. At operation scheme II, when adding hydrochloricacid to adjust pH of the NF feed water down to5.0, the S&DSI values are much lowerthan zero, which means that CaCO₃scaling can not form in this situation; but CaSO₄scaling would form when NF permeate recovery exceeds45%. At operation schemeIII, the SI of CaSO₄is less than1.0in the NF brine and on the NF membrane surfaceat NF recovery of less than60%, which indicates that antiscalant addition couldsuccessfully prohibit CaSO₄scaling formation. The result demonstrates that usingloosen NF membrane as seawater softening pretreatment in NF–SWRO integratedsystem could increase NF permeate recovery in single-vessel unit up to60%whilemitigate scaling with the existence of antiscalant dosage, pH adjustment and NFconcentrate recycling.（4） Comparison test on NF seawater softening was carried out with two tight NFmembranes modules （BDX-N90and NF90） to investigate the scaling potential ofinorganic salt on membrane surface. The NF permeate recovery increasing from10%to35%is achieved by an increase of the recycling ratio from0and1.69to2.22and 9.88, respectively. When NF permeate recovery is above35%, the SI value of CaCO₃in the NF90brine solution increases to great than1.0, which indicates that CaCO₃scaling could form in the experimental operating range. This phenomenon is justdifferent from that from the ultra-low-pressure loose ESNA3NF membrane. Specificenergy consumptions for BDX-N90and NF90membranes were much larger thanESNA3membrane, which fluctuate in2.31-2.82kWh·m^-3,5.91-7.04kWh·m^-3,respectively.（5） According to the previous pilot data, long-term operation was carried outwith the absentence of antiscalant dosage, pH adjustment and NF concentraterecycling, to examine the fouling of the polyamide ESNA3NF membrane and theefficiency of chemical cleaning in seawater desalination pretreatment. The resultsshow that with operating time increasing, the NF permeate flux and salt rejectiondecreased gradually. Long-term effects of IMS on organic matter removal had alsobeen investigated using TOC and UV₂₅₄, respectively. The removal efficiency oforganic matters by the NF membrane decreased with operating time gradually. Therejection variation of TOC, UV₂₅₄by NF membrane showed similar trends, whichimplies that UV₂₅₄, the much more cheaper and convenient method, could be used toindicate the membrane fouling during long-run. A combination of citric acid cleaningand NaOH cleaning may achieve an optimal cleaning efficiency. Two chemicalcleaning procedures were performed during the long run, and both yield highefficiency in term of flux recovery, which were97.5%and95.8%, respectively.Aming the flux recovery effieiency, about80%was fulfilled by acid cleaning, whichindicates that inorganic fouling should be preferentially paid attention to duringchemical cleaning.This research work of inorganic scaling potential prediction and scale preventionin NF-SWRO integrated membrane system at high permeate flux is of greatsignificance for effective scaling prevention, maximization of NF permeate recovery,and mininization of seawater desalination cost.