Research On Multi-membrane Treatment Technology Of Coal-Series Gas Produced Water

With the scarcity of fossil fuels,the exploitation of unconventional natural gas in coal-series has received much attention.The coal-series gas reserves in the Linxing block are in large-scale development,while a large number of waste water which include special impurities,organic pollutants,and output of materials such as high salt also can be produced with the process of mining,and direct emissions may pollute the surrounding environment and waste water resources.By analyzing the quality of coal-series gas produced water in Linxing block,it is found that the produced water has the following characteristics:more suspended solids and large turbidity;higher salinity,total salt content is 50000-60000 mg/L;high hardness,calcium,magnesium and other content above 4000 mg/L;more pollutants,high concentrations of organic matter and fluoride.According to the water quality characteristics of the study area,the reverse osmosis is the core demineralization process,and the nanofiltration is the softening process.The pre-membrane pretreatment process selects iron-carbon micro-electrolysis to remove organic matter,and the micro-bubble dissolved-coagulation process removes turbidity.Firstly,the turbidity as the main index and the COD as the secondary index,the microbubble dissolved gas test was carried out to optimize the process parameters.The removal effect of coagulation experiment and loading flocculation experiment on produced water was compared,then the flocculation process parameters were optimized by response surface.The optimal experimental conditions were 28.88 mg/L PFS and 9.96 mg/L APAM.The amount of reflux sludge was 1.155 mL/L,the kept the raw water pH and slowly stir for 10 min.The result shows that the turbidity was 11.3 NTU,the turbidity removal rate was 98.93%,the COD was 1636.7 mg/L,and the removal rate was 37.41%.Secondly,the single factor method was used to investigate the treatment effect of iron-carbon microelectrolysis on the produced water after microbubble dissolved-loaded flocculation treatment,then the experimental parameters were optimized by orthogonal method.The optimum process is iron-carbon ratio 1:2.5,solid-liquid ratio 12%,initial pH 3,reaction time 90 min,the COD removal rate is 66.9%,and conductivity is 52.5 ms/cm.Finally,the process parameters of nanofiltration and reverse osmosis membranes are explored.During the 1.25 MPa operation of nanofiltration,the membrane flux fluctuated within the range of 2.30-2.35 L/(m2·h);the COD varied between 310-330 mg/L,and the removal rate was 39%-42%;the conductivity was in the range of 35-38 ms/cm,the overall salt rejection rate is about 30%,and the retention rate of divalent ions is always above 80%.The physical cleaning method of the nanofiltration membrane adopts the first low pressure and high pressure circulation cleaning to alleviate the membrane fouling;the chemical cleaning method selects“citric acid(2%w)+ammonia,which pH is 2.5-4.0,and run for 30 min;then adjust the pH of the Na2EDTA by sulfuric acid(0.8%w)between 7.5 and 8.0 and washed for 30 min;the membrane flux was restored to the original 98.7%after washing.During RO operation,the operating pressure is better under 4 MPa conditions,and the membrane flux fluctuates within the range of 15.78-15.8 L/(m2·h);the COD is 30.11-30.41 mg/L;the conductivity is 0.97-1.03 ms./cm,the overall RO salt rejection rate is above 97%.After RO treatment,the total salt content of effluent is 853-978 mg/L,and the water quality can reach the relevant standards of farmland irrigation water discharge.