Study On The Technological Mechanism Of Advanced Treatment Of Coking Wastewater By Membrane Process And Concentrated Water Enhanced Coagulation

As the acceleration of the industrialization,the coking industry is developing vigorously,resulting in the generation of large amounts of coking wastewater.The problems caused by it have become a key factor restricting the high-quality development of the coking industry.Because of its strong biological toxicity,high corrosivity and high COD/TN ratio,coking wastewater poses high challenges to traditional biological treatment.Under the circumstances of water scarcity,and with the continuous tigntening of sewage/wastewater discharge standards.One of the main goals of the development of the coking industry is to improve the quality of coking tail water through advanced treatment,and explore ways for wastewater recycling.As a promising and stable advanced treatment technology,membrane separation is playing an increasingly important role in wastewater purification.It is necessary to explore a new process mode which could realize an combination of coking wastewater treatment and reuse via advanced membrane technology.This subject of this work is firstly to construct an OHO-MBR combined process.The conversion relationship and removal efficencies of the combined process on COD,NH₄⁺-N,NO₂^--N,NO₃^--N and TN were explored The treatment efficiency and transformation characteristics of typical pollutants in wastewater was analyzed.Secondly,a nanofiltration system was used for advanced treatment of biologically treated coking wastewater.The ion retention efficiency of nanofiltration was investigated by varying the solution environment,pH,operation pressure and water recovery rate.The strengthening effect of monovalent ions on the removal of divalent ions was explored.The retention of ions with different valence states was summarized.Finally,the nanofiltration concentrate was reused in the pre-coagulation treatment unit to explore the influence and promotion effect of divalent ions on the coagulation.Different reflux ratios were set up to investigate the enhanced removal of target pollutants,and explore the ways for resource recycling and re-utilization.Results showed that:(1)Under stable operation conditions,the treatment effect of OHO-MBR progress:under a total hydraulic retention time of 56 h,and with the influent COD,TN,NH₄⁺-N,SCN^--N of3063,146.1,27.35,85.10 mg·L^-1,the combined process can achieve COD,TN,NH₄⁺-N,and SCN^--N removal of 85.9%,65.5%,95.1%,98.6%even at low ambient temperature conditions(10-20?).OHO-MBR may be used as an alternative technology for treatment load enhancement and water quality improvement.It can be used to upgrade existing biological treatment processes without additional infrastructure renovation.(2)A nanofiltration system was constructed as an advanced treatment unit for advanced treatment of the secondary biological treatment effluent of coking wastewater.By comparing the pure water flux and single salt rejection rate of three kinds of nanofiltration membranes,the nanofiltration membrane with the highest mono/divalent ion separation rates was selected for follow-up experiments.In double-salt system with different solute concentration ratios,the ion retention behaviors and mechanisms were analyzed of.With the presence of NaCl,the membrane rejection rate of Mg²⁺and Ca²⁺increased to 98.27%and 81.46%on average,which proved that the existence of valence ions can enhance the retention of divalent ions by nanofiltration.The effects of pH,operation pressure,and water recovery rate on the separation of ions of different valence states were investigated.Results show that:for the actual secondary biological treatment effluent of coking wastewater,when the transmembrane pressure was 0.6MPa and the recovery rate was 90%.The removal rates of Ca²⁺,Mg²⁺and SO₄^2-were 79.4%,95.6%,98.9%,respectively,and the removal rates of Na⁺,Cl^-,and NO₃^-were 44.5%,5.0%,-61.9%,respectively.The nanofiltration process intercepts organic matter and high-valent ions,and showed a significant degree of separation for mono and divalent ions.The enrichment of high-valent ions was achieved in the concentrate.The membrane fouling mechanism and control strategy were explored.After 45 hours of operation,inorganic salt crystals formed on the fouled membrane surface,but did not accumulate in a large amount into a dense fouling layer.The nanofiltration membrane fouling can be alleviated by hydraulic cleaning,and the membrane flux can be restored to 99.1%of the initial flux.(3)The nanofiltration concentrate was re-used for pre-coagulation.By adjusting the raw water pH and PFS dosage,the optimal coagulation operating parameters were determined as pH=9 and PFS=1 g·L^-1.Taking COD,SCN^-,and turbidity removal rates as indicators,the influences of ion/salt type and concentrations on coagulation were analyzed.The concentrate reflux ratio was finally used as an independent variable to explore the enhancement effect on pre-coagulation.When the reflux ratios were 50%,100%,150%,the turbidity removal rate was68.9%,68.9%,69.0%,the SCN^-removal rate was 21.7%,27.1%,32.2%,the COD removal rates was 29.3%,44.4%,38.0%,respectively,which were significantly higher than those of the control group(with turbidity,SCN^-and COD removal rates were 59.3%,19.9%,and 14.0%,respectively).The nanofiltration concentrate has a significant enhancement effect on pre-coagulation,which can be used as a feasible method for its disposal,realizing resource recycling.This work is expected to provide a new alternative process mode to improve the coking wastewater tail water quality for wastewater and waste reuse.