The Study On New "Zero Liquid Discharge" Process For Treating Wet FGD Wastewater From Coal-fired Boilers

Aiming at the limestone-lime wet flue gas desulfurization(FGD)wastewater in coal-fired boiler power plants,this paper presented a new wet FGD wastewater treatment process,in which an improved coagulation-sedimentation method was used as the pretreatment process and “mechanical filtration+ microfiltration + nanofiltration” was used as the advanced treatment process.The new process could get the goal of zero liquid discharge(ZLD)of FGD wastewater.In this work,the raw water which was from the hydrocyclone overflow of the wet FGD system in a power plant was used.The pretreatment experiment consisted of single factor(SF)optimization experiments and response surface methodology-central composite design(RSM-CCD)optimization experiments.The optimal condition of SF optimization experiments was that the mixing speed of 160 rpm,mixing time of 60 min and dosage of 2500mg/L.Under the optimal condition,each index met the requirements regulated in effluent standard of DL/T997-2006.The RSM-CCD optimization experiments were conducted with numerical model optimization calculation and experimental verification,and the calculated optimal condition was that the dosage of 3018.0mg/L,the temperature of 40.5? and the pH value of 5.7.And under this calculated condition,the maximal removal efficiency of heavy metals(RHM),maximal removal efficiency of fluoride(RF)and minimal supernatant turbidity(ST)were 76.589%,71.627% and 0.80 NTU,respectively.The optimal results of the verification experiment showed that the concentration of nickel,zinc,fluoride were 0.640 mg/L,1.68 mg/L and 14.8 mg/L,respectively,and the ST was 2.1 NTU.Accordingly,the RHM,RF and ST were 75.86%,73.95% and 2.1 NTU,respectively,and they matched the calculated values well.In addition,these values met the requirements regulated in effluent standard of DL/T997-2006.The results of advanced treatment experiments showed that the operation pressure of 2.5MPa and operation temperature of 25? was the most suitable operation condition during the nanofiltration process.Under this operation condition,the pH value of the effluent was 7.79,and the concentration of TDS,Cl-,F-,SO42-,Ca2+,Al3+ and Zn2+ were 2720 mg/L,719.9 mg/L,1.97 mg/L,213 mg/L,24.8 mg/L,0.02 mg/L and 0.05mg/L,respectively.Besides,the concentration of Fe3+ Cr6+ Ni2+ and Cu2+ were all less than 0.01mg/L.The quality of effluent met the requirements of process water for reusing in FGD system.And it also showed that,the cleaning with interval of 5.3 hours,reducing the residual pretreatment reagent in influent,controlling the production of gypsum and removing the produced gypsum in time would contribute to lessening the membrane fouling and improving the efficiency.The improved coagulation-sedimentation method overcame the shortcomings exited in traditional coagulation-sedimentation method,and it needed only one kind of powder purification reagent to add and the effluent met the requirements of discharge standard of wastewater from limestone-gypsum flue gas desulfurization system in power plants(DL/T997-2006).The “mechanical filtration + microfiltration + nanofiltration” process was able to remove most sulfate and chloride in wet FGD wastewater and the effluent could be reused as the process water in FGD system.Compared with other common desalination process,such as,reverse omission,evaporation condensation and so on,nanofiltration had advantage of less investment,lower cost,less energy consumption and more safety and stability.The new ZLD process presented and studied in this paper met the environmental demand of ZLD of wet FGD wastewater.And it had great significance on promoting solving the existed problems in wet FGD wastewater treatment in our country and had a good application value.