Effects Of Pollutants In Steel Industry Wastewater On Membrane Distillation Performance

Wastewater of steel industry is a kind of difficult-to-treat industrial wastewater,which contains multiple organic pollutants and inorganic salts.The high concentration of pollutants and poor biodegradability of steel wastewater are the main problems affecting the near-zero emissions of the steel industry.Membrane distillation（MD）is a novel membrane separation technology,which has the advantages of high rejection efficiency,low fouling propensity,strong applicability to wastewater and simple operation.In recent years,it has been considered as a potential technology for the advanced treatment and regeneration of complex industrial wastewater such as coking wastewater,petrochemical wastewater and dyeing wastewater.However,there is a lack of systematic research on the process of MD for the treatment of steel wastewater.Therefore,this work systematically investigated the pollution characteristics of steel industry wastewater,explored the feasibility of MD to treat steel industry wastewater,and analyzed the interaction mechanism between the pollutants and the membrane surface.The main contents and conclusions of this thesis are as follows:（1）The pollution indicators of reverse osmosis concentrated water,neutralization station wastewater and mixed wastewater produced from the steel industry were systematically studied,and pollution characteristics of different wastewater were analyzed by three-dimensional fluorescence spectroscopy（3D-EEM）,ion chromatography,gas chromatography-mass spectrometry（GC-MS）,etc.The results showed that RO concentrateⅠcontained higher concentration of ammonia nitrogen and heterocyclic organic compounds compared with other steel wastewaters,while RO concentrateⅡpossessed higher content of total phosphorus and organic acids.The organic compounds and inorganic salts in neutralization station wastewater were significantly higher than the other wastewaters.The organic matters of neutralization station wastewater were mainly composed of halogenated hydrocarbons,alkanes and lipids,whereas the inorganic salts mainly consisted of Cl^-,SO₄^2-,and Ca²⁺.And the organic matters in the mixed wastewater were mainly alkanes.The pollutants composition and characteristics of these wastewaters may have a significant impact on the performance of MD.（2）The feasibility of MD for wastewater treatment of steel industry was explored.The changes of water flux and conductivity were continuously monitored during the MD process.The results of water quality on the permeate side showed that MD can effectively reject most inorganic and organic pollutants in wastewater of steel industry.However,due to the high content of ammonia nitrogen and heterocyclic organics in RO concentrate I,partial membrane wetting occurred during MD.MD had great potential in treating RO concentrateⅡsince the flux and permeate conductivity was relatively stable.The high concentration of ions such as Cl^-,SO₄^2-,Ca²⁺in neutralization station wastewater can easily block the membrane pores and cause obvious flux attenuation.Membrane wetting and fouling reduced to a certain extent when the mixed wastewater was utilized as the feed solution,which was result from the complexation between organic and inorganic pollutants.（3）The accumulation of pollutants in the wastewater of steel industry on the membrane surface was analyzed,and the physical and chemical properties of the pollutants were determined by scanning electron microscopy-X-ray energy spectrometer（SEM-EDS）,contact angle meter,X-ray photoelectron spectrometer（XPS）,etc.Hermia model was used to fit the pollution type and the fouling resistance during the MD process was calculated so as to analyze the membrane fouling behavior.It was found that the pollution layer of RO concentrateⅠwas dense and mainly composed of hydrophobic organic matters.The resistance of the fouling layer gradually increased with the running time during MD and the pollution type was closed to the middle blockage.The fouling layer formed by RO concentrateⅡwas relatively loose which was similar to complete blockage.The fouling layer caused by the wastewater from the neutralization station was mainly composed of hydrophilic inorganic salt crystals.The flux attenuation was mainly due to the increased resistance of concentration polarization.After membrane cleaning,the flux of neutralization station wastewater was easy to recover.The degree of membrane fouling with mixed wastewater was not significant and the main types were composite pollution with organic and inorganic pollutants.