Behavior And Control Of Membrane Fouling During Forward Osmosis Treatment Of Reverse Osmosis Concentrate From Coal Chemical Industry

Coal chemical wastewaters have a complex composition with a large number of toxic substances.Reverse osmosis（RO）is normally employed at the end of the treatment process for water reclamation,thus producing a certain amount of concentrated brine.RO concentrate from the coal chemical industry（CCI ROC）is characterized by high salinity,high chroma,and a certain amount of refractory organic matter,which requires proper treatment to mitigate its detrimental effect on the environment and to further recover the water resource.Conventional thermal concentration technologies result in a huge investment and a large energy cost,while the application of high-efficiency reverse osmosis,membrane distillation,and electrodialysis faces the constraints of energy requirement and membrane fouling.In contrast,forward osmosis（FO）,using osmotic pressure difference as the driving force for water transport,has been widely used for wastewater treatment due to the advantages of lower energy consumption and less severe membrane fouling.During the application of FO,membrane fouling is an important limiting factor.However,most studies related to membrane fouling are based on simulated wastewaters and foulants,which couldn’t fully reflect the fouling process and mechanism of real wastewaters.Concerning the above issues,this thesis evaluated the effect of operating parameters and proposed several cleaning methods for fouling removal.Besides,lime-soda ash softening and electrochemical oxidation were investigated to address the problems of inorganic fouling and organic fouling,respectively.Firstly,effect of operating parameters on FO performance was evaluated.The flux was significantly higher under AL-FS mode（active layer facing feed solution）than under AL-DS mode（active layer facing draw solution）,indicating serious concentration polarization and membrane fouling under AL-DS mode.Besides,a dramatic flux decline was observed under AL-FS mode when the water recovery exceeded 60%,which accorded with the inorganic fouling-dominated mechanism.The reverse flux of draw solute could alter the concentration of the species near the membrane surface on the feed solution（FS）side,influencing the fouling process.Among the draw solutes assessed herein,the severity of the inorganic fouling was in the order Ca Cl₂>Mg Cl₂>Na Cl>Na Cl+EDTA.A higher concentration of the draw solution（DS）resulted in a higher flux,but with a slower flux increase because of a stronger diluting effect near the membrane surface on the DS side.The higher flux also led to a stronger concentrating effect near the membrane surface on the FS side so that the inorganic fouling occurred at a lower recovery.The concentrating ability of FO was similar to thermal concentrator and much higher than reverse osmosis and mechanical vapor compression.The energy consumption of FO was lower compared with other brine concentration technologies.With the help of SEM-EDS,FTIR,and Visual MINTEQ,inorganic fouling was confirmed to be the major fouling type during the FO treatment of CCI ROC.Hydraulic cleaning,osmotic backwash,and EDTA cleaning could completely recover the FO flux（cleaning efficiency of approximately100%）,while HCl cleaning was insufficient to restore the membrane performance.To control the inorganic fouling,induced crystallization and lime-soda ash softening were investigated.Smaller seed size and a higher dosage resulted in a faster crystallization process,which,however,also had a negative effect on FO performance since more Ca SO₄ seeds would suspend in the system.Consequently,volume of accumulated permeate（after a flux decline of 15%）was slightly increased from 245m L to 308 m L by induced crystallization.Through lime-soda ash softening,the scaling precursor ions Ca²⁺and Ba²⁺were removed by more than 98.5%,while Mg²⁺,Si,and Sr²⁺were removed by more than 80%.Due to the considerably reduced inorganic fouling propensity,the maximum water recovery of FO was elevated from54%to 86%,and the volume of accumulated permeate（after a flux decline of 15%）was considerably increased to 4490 m L.Meanwhile,the softening sludge was produced with a yield of 5.6 kg/m³ CCI ROC and Ca CO₃ purity of 92.2%,which showed great potential for resource recovery.After lime-soda ash softening,the inorganic fouling was mitigated significantly and the main fouling type was transformed into organic fouling.Then electrochemical oxidation was evaluated for organics removal and organic fouling mitigation.Under the effect of direct oxidation and indirect oxidation mediated by·OH and active chlorine,COD was removed by about 90%,significantly slowing down the FO flux decline.According to EEMs and GPC,the removal of fluorescent organics was higher than 95%,while 62.6%,52.2%and 13.0%for large-molecule（>5000 Da）,medium-molecule（1000-5000 Da）and small-molecule（<1000 Da）organics,respectively.As shown by the economic evaluation results,FO-EO（forward osmosis followed by electrochemical oxidation）required a larger investment than EO-FO（electrochemical oxidation followed by forward osmosis）,mainly because of the serious fouling and large investment of cleaning chemicals under FO-EO.This thesis focused on the FO treatment of real CCI ROC by first analyzing the fouling mechanism and then proposing several effective cleaning methods for the fouling removal.Besides,inorganic fouling and organic fouling were effectively mitigated by lime-soda ash softening and electrochemical oxidation,respectively,which motivated the application of FO in the treatment of CCI ROC.