Fundamental Research On Application Of Forward Osmosis Technology In Treatment Of Watsetwater Of Fuel Gas Desulfurization

With increasingly stringent national environmental standards and policies,more and more domestic thermal power plants urgently need to carry out technical transformation of zero-emission of wastewater of fuel gas desulfurization?FGD?.In this background,forward osmosis?FO?technology is attracting more and more attention due to its low pressure,low pollution and low energy consumption.In particular,the FO systems using NH4HCO3 and NH4HCO3-NH4OH mixed solution as draw solution show a broad application prospects in treatment of wastewater with high salt content.In this paper,the FO syestem was studied using NH4HCO3 and NH4OH-NH4HCO3 mixed solution as draw solution,and the specific pretreatment/preconcentration process route was proposed and then verified by experiments.The effects of influent pretreatment,cross flow rate,influent p H,temperature etc.on performance of FO system were discussed as well as FO membrane fouling and corresponding control and cleaning technology were studied.Moreover,the wastewater pretreatment target for FO was suggessed.Additionally,separation and recovery of draw solution were discussed.These research results can provide useful reference for subsequent pilot application and engineering practice.The main work and conclusions are as follows:?1?A pretreatment/preconcentration process route using softening clarification and ultrafiltration as well as reverse osmosis and electrodialysis was proposed for influent pretreatment for FO and its feasibility was verified by experiments.The specific softening process was Na OH-Na2CO3 softening process with its chemical cost reaching 6.21 yuan/ton.The optimal flux of ultrafiltration was 20 L/?m²·h?.By combing reverse osmosis and electrodialysis,the salt content of FGD wastewater can concentrate to above 60 000 mg/L.?2?The results show that in addition to influent p H,cross flow rate,temperature and draw solution concentration had a remarkable influence on the performance of FO process.Generally the influent p H should keep at 6.0⁸.0.The permeate flux increases with the increase of flow rate,temperature and draw solution concentration.But the flow rate should not be too high and the optimal rate was 19.0 cm/s.Temperature has an important influence on membrane flux,reverse solute permeation flux and solubility of draw solute,and the optimum operating temperature was 35⁴0?.Higher osmotic pressure of draw solution thus leading to higher flux can achieved by mixing NH4HCO3 and NH4OH.However,taking into account reverse solute permeation flux,the appropriate solution was determined to be NH4HCO3 solution with a concentration of 4.5 mol/L.Severe membrane fouling was not observed during the test run,which indicated that pretreatment of inflent can effectively slow down membrane fouling.?3?Membrane fouling tests show that the FO membrane fouling was mainly casused by Ca CO3 scaling as a consequence of HCO3^-in draw solution reversely diffusing into the infuent and then reacting with Ca²⁺.Therefore,it was necessary to strictly control Ca²⁺concentration below 50 mg/L.On the basis of the above,the influent pretreatment target for FO was given in practical application.The experimental results show that the addition of scale inhibitor was beneficial to inhibit the fouling of membrane surface and delay the decrease of flux.For seriously polluted membrane,the membrane flux can be effectively restored to more than 90%through permeate backwashing.?4?The factors that affect separation and recovery of draw solution are mainly the ratio of carbon to nitrogen,dilution bypass ratio and operating temperature.At present,the energy consumption of the FO system is significantly lower than that of traditional evaporative crystallization process,therefore the separation and recovery process still needs to be optimized and the energy consumption needs to be further reduced.