Experimental Research On The Treatment Of Natural Gas Produced Water With Chemical Coagulation And Fenton Oxidation Process

Gas-field produced water is a kind of special industry wastewater that generatedwith the production of natural gas, which contains many poisonous and harmfulsubstances such as dissolved and dispersed oil compounds, heavy metals, radioactivematerials, organic and inorganic chemical treatments, production solids. Compared withoil-field produced water, gas-field produced water features even higher concentration oforganics and volatile components and more toxic substance. Direct discharge ofgas-field produced water without purification would cause many disadvantages tonatural ecosystem, especially to surface and underground environment. Conventionalbiological treatment method is not suitable to treat gas-field produced water directly,and it often needs physical chemistry pretreatment to improve biodegradability andlower organic loading. The objectives of this subject are utilizing chemical coagulationand Fenton oxidation （conventional Fenton oxidation, ultrasonic Fenton oxidation andultraviolet Fenton oxidation） to treat gas-field produced water, finding out optimizedreaction conditions and investigating the variation tendency of organic contaminant ineach treatment process, and analyzing reaction mechanisms.Chemical coagulation experiments demonstrated that polymeric ferric sulfate （PFS）and polyaluminium chloride （PAC） both could remove organic contaminant andsuspended solid of produced water effectively. It compared the influencing factor ofcoagulation inclouding coagulant dosage, initial pH value, coagulant aid dosage andstirring time. The optimized reaction condition of PFS coagulant could be determinedthat dosage of20g/L, pH value of11, polyacrylamide dosage of30mg/L, rapid stirringtime and rotate speed was2min and250r/m, and slow stirring time and rotate speed was15min and50r/m, besides the overall removal efficiencies of COD, BOD₅, TOC,petroleum oil, color and SS were86.06%,45.54%,86.05%,98.88%,98.16%and99.03%respectively. While the optimized reaction condition for PAC coagulant wasthat dosage of25g/L, pH value of11, rapid stirring time and rotate speed was2min and250r/m, slow stirring time and rotate speed was10min and50r/m, and withoutpolyacrylamide, which achieved final removal efficiencies of90.09%for COD,49.04%for BOD₅,89.41%for TOC,95.25%for petroleum oil,99.92%for color and99.03%forSS.Conventional Fenton oxidation experiments demonstrated that conventional Fenton oxidation could remove COD, TOC, BOD₅, color and UV254effectively. It investigatedthe influencing factor of conventional Fenton oxidation process contained H₂O₂/CODmass ratio, H₂O₂/Fe²⁺mol ratio, initial pH value and reaction time. The optimizedreaction condition of conventional Fenton oxidation process can be determined thatH2O2/COD mass ratio of8, H₂O₂/Fe²⁺mol ratio of25, initial pH value of3, andreaction time of60min, finally the overall removal efficiencies of COD, BOD5, TOCand color were68.9%,47.96%,55.74%and89.78%respectively.Ultrasonic Fenton （US/Fenton） oxidation and ultraviolet Fenton （UV/Fenton）oxidation experiments achieved better organics removal efficiency than conventionalFenton oxidation. It investigated the influence of ultrasonic power on US/Fentonoxidation treatment efficiency, and analyzed the variation tendency of organiccontaminant in US/Fenton and UV/Fenton oxidation. Thtrough US/Fenton oxidation theoverall removal efficiencies of COD, BOD₅, TOC and color were78.77%,65.59%,70.34%and94.67%respectively. While UV/Fenton oxidation attained better removalefficiencies of81.77%for COD,67.9%for BOD5,71.19%for TOC and95.11%forcolor.Chemical coagulation could remove most of organic contaminant and suspendedsolid of produced water. In oxidation experiments, the sequence of organics removalefficiency is UV/Fenton oxidation＞US/Fenton oxidation＞conventional Fentonoxidation. Through chemical coagulation and Fenton oxidation, persistent organicpollutants could be decomposed to be small molecule organic substances, carbondioxide and water, besides the biodegradability was also increased from0.08to0.76.