Study On Calcium Sulfate Scaling Tendency And Softening Process Of Saline Wastewater

With the increase of water resource shortage and wastewater discharge standard,saline wastewater must be performed with zero emission treatment.Concentrating through reverse osmosis has become the core technique for zero emission treatment of saline wastewater.However,the formation of scales like calcium sulfate during the concentrating process usually leads to unrecoverable degradation of membrane performance.Therefore,it is of great practical significance to forecast the scaling tendency of calcium sulfate and to prevent the formation of scale through proper approach.In this study,the hybrid process of chemical softening+RO（I）+ion exchange resin softening+RO（II）was adopted to concentrate saline wastewater.Firstly,the saturation index equation for the forecasting of the formation of calcium sulfate scale in saline wastewater was obtained through dissolution equilibrium method and was used to predict the scaling tendency of saline wastewater.Then,the influences of reagent dosage,pH,reaction temperature,mixing time on the softening effect and the influences of resin operation and regeneration conditions on the adsorption effect were systematically studied.Finally,a pilot-scale test was carried out to demonstrate the stability and applicability of the process in concentrating of saline wastewater.The conclusions obtained from the experimental results were as follows:（1）The solubility of calcium sulfate in saline wastewater was mainly determined by temperature,salt effect and co ion effect.Both the increase of temperature and the salt effect promoted the dissolution of calcium sulfate,while the co ion effect inhibited the dissolution of calcium sulfate.In addition,Mg²⁺and SO₄^2-could form a stable magnesium sulfate pair to promote the dissolution of calcium sulfate.The saturation index equation for the forecasting of the formation of calcium sulfate scale in saline wastewater obtained through dissolution equilibrium method was as follow:I_s=logc(Ca²⁺)c(SO₄^2-)+4.9679-2.1857×10^-2T+1.1022×10^-4T²-3.1571S_i^0.5+1.2891S_i+7.6479×10^-3S_i^0.5T.（2）The results of chemical softening showed that the hardness of the saline wastewater with a Ca²⁺concentration of 259.72 mg/L could be effectively removed through using chemicals of CaO-Na₂CO₃,NaOH-Na₂C₂O₄ or NaOH-Na₂CO₃,respectively.Under the optimum softening conditions,the Ca²⁺concentrations of effluent were 9.62,4.92 and 24.32 mg/L,respectively.The method of NaOH-Na₂C₂O₄exhibited the best softening efficiency at the highest reagent consumption.Considering the softening effect and reagent consumption,method of NaOH-Na₂CO₃was selected to soften saline wastewater,the reagent consumption was 1.67 yuan per tone of wastewater,and the quality of the softened effluent could meet the inlet requirement of RO（I）.The removal of Ca²⁺was mainly affected by solution pH in the process of NaOH-Na₂CO₃ chemical softening.Al₂（SO₄）₃ coagulation pretreatment exhibited better promotion effect compared with PAC,when the Al₂（SO₄）₃concentration was 80 mg/L,the Ca²⁺removal increased by 13.45%.（3）The Dow XE830 resin was the most suitable softening resin for the softening of the concentrated wastewater of RO（I）for its good softening efficiency and high adsorption capacity.The optimum conditions for static adsorption of Dow XE830resin were:resin dosage=5.5 g/L,reaction temperature=room temperature,pH=7.00and reaction time=60 min.Both the Langmuir and Freundlich adsorption isotherm equations could be used to describe the adsorption behavior,but Freundlich adsorption isotherm model described more accurately,the correlation coefficient R²was 0.9830 and the adsorption isotherm equation was q_e=15363C_e^0.6990.The pseudo second order kinetic equation could better reflect resin adsorption behavior,its correlation coefficient R² was 0.9999 and the kinetic equation was dq_t/dt=0.01569（14.959-q_t）².The optimum solution velocity for dynamic adsorption was 5 m/h,the optimum HCl concentration,NaOH concentration and regeneration agent dosage were 4%,4%and 2.0 BV,respectively.（4）The results of pilot-scale test demonstrated that the saline wastewater could be effectively concentrated through the hybrid process of chemical softening+RO（I）+ion exchange resin softening+RO（II）.The total concentration factor could be up to10.12.Pilot-scale experiments performed with six types of saline wastewater of different compositions indicated that the hybrid process possessed good stability and applicability.XRD analysis demonstrated that calcium sulfate scale was not formed in the process.The estimated cost of this process was about 7.78 yuan per tone of wastewater,showing obvious economic benefit compared with evaporation crystallization.