| In order to reduce water consumption and sewage discharge,power plant P of 2×1000MW expected to recover and reuse cooling tower blowdown(CTBD)by the combined process of softening-clarification and reverse osmosis.Experimental study was carried out in this paper.Simulated wastewater was softened and clarified by lime-Sodium carbonate method,and the optimal dosage was investigated.Clarified water was advanced treated by reverse osmosis system.(1)The concentration ratio of circulating cooling system in power plant P was 12.The CTBD contained high salinity but less organic pollutant,high hardness,alkalinity and silicon,with certain scale inhibitor and bactericide.The total hardness was 8.55 mmol/L,and the alkalinity was 7.48 mmol/L,and the conductivity was 1045 μ S/cm.(2)The optimum conditions for the treatment of five simulated effluents were studied by the coagulation test.According to the optimal removal rate of hardness,and alkalinity,the optimal removal rate of the scale.inhibitor from high to less was:PAAS>AA/AMPS>formula E>formula A>HEDP.Different amount of softener resulted in different effects.The interference of scale inhibitor could be overcomed by adjusting the amount of lime and soda.The main factor that influenced the clarified water was the dosage of lime.(3)Two kinds of actual sewage containing formula A and formula E in power plant P were softened and clarified by lime-soda method and single lime method.In general,the lime-soda method is more effective than single lime method for removing hardness and calcium hardness,but less effective for removing alkalinity,total silicon,active silica and COD.For sewage A treated by lime-soda method,when CaO was 181mg/L,Na2CO3 was 108mg/L,and the softening pH was 11.35,92%of turbidity,76%of hardness and 60%of alkalinity was removed.As for single lime method,when CaO was 178mg/L,90%of turbidity,51%of hardness and 68%of alkalinity was removed.For sewage E treated by lime-soda method,when CaO was 163mg/L,Na2CO3 was 105mg/L,and the softening pH was 11.21,81%of turbidity,65%of hardness and 46%of alkalinity was removed.As for single lime method,when CaO was 181mg/L,89%of turbidity,41%of hardness and 66%of alkalinity was removed.(4)The highest recovery rate of RO using different feed water was calculate by ROSA software.Maximum recovery rate was only 33%when treating original CTBD without pretreatment,while maximum recovey rates treating clarified water A and E were respectively 68%and 63%.The necessity of softening clarify pretreatment was proved.By comparing the LSI and SiO2 of concentrated water under the maximum recovery rate,it was found that the main factor restricting recovery rate was the silicon scale.The 6h reverse osmosis desalination test treating clarified water was carried out.The drainage gradient method was used to continuously concentrated the feed water and improved the recovery rate of the system.In this method,the fresh water was discharged into the fresh water tank by steps,while the concentrated water was fully returned to the feed water tank.The inlet pressure of the RO system was 0,35-0.53MPa,and the inflow was around 1000L/h,and the fresh water flow maintained 100L/h.Maximum recovery rate reached 79%in dynamic RO test treating clarified water A,while the silicon was 302 mg/L in concentrated water.After 6h-test,the average conductivity of fresh water was 162 μ S/cm,and the turbidity was 0 NTU.Total desalination rate was 91.43%.Maximum recovery rate reached 70%in dynamic RO test treating clarified water E,while the silicon was 288 mg/L in concentrated water.After 6h-test,the average conductivity of fresh water was 143 μ S/cm,and the turbidity was 0 NTU.Total desalination rate was 92.06%. |
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