| Forward osmosis(FO)transports water from the low-concentration side across a semi-permeable membrane into the high-concentration side spontaneously under the osmotic pressure gradient.As a novel membrane separation technology with low energy consumption,low membrane fouling and high water recovery,FO process has been widely used in many fields including brackish water and seawater desalination,sewage concentration,wastewater treatment,food processing,emergency rescue and pharmaceutical industry.In view of water shortage and energy conservation,FO combined with reverse osmosis(RO)is proposed for municipal wastewater treatment and clean water production,which is of great significance.In this paper,the bench-scale FO equipment and RO device were used.The real municipal wastewater was treated by FO process using the synthesized seawater concentrate as draw solution(DS).The basic FO performances in the sewage treatment were evaluated in terms of water flux and reverse salt flux.The effects of operating conditions such as temperature,flow velocity and mechanical pretreatment were also discussed.Furthermore,the sewage concentration,pollutants rejection,membrane fouling,physical cleaning and membrane flux recovery were investigated in detail.Second,RO technology was used as the follow-up step to separate clean water from the diluted DS after FO operation.Water flux,salt rejection rate,pollutants removal and product water quality were decided.Moreover,the economic cost and energy consumption of this hybrid system was analyzed.The investigation studied the feasibility and overall performances of FO-RO process for municipal wastewater treatment and clean water production.Results revealed that,water flux was high by using seawater concentrate as DS in the FO-based sewage treatment.During 29 h of filtration,water flux dropped from 15.3 to 11.6 L m-2·h-1 with the decline rate of 24.2%.However,the conductivity in sewage tank increased gradually due to volume reduction and reverse salt diffusion.Water flux increased dramatically with temperature,but high temperature aggravated the reverse salt transport from DS to sewage.Water flux decreased as flow velocity decreased.Moreover,the high decline rate was observed at low flow velocity.Water flux increased when municipal sewage was pre-treated,as the suspended solids in sewage were removed by 0.45 ?m filtration.Although the volume of municipal wastewater was reduced to 1/4,concentration factors of total phosphorus(TP),chemical oxygen demand(COD),total nitrogen(TN)and ammonia nitrogen(NH4+-N)were 3.28-3.50,2.38-2.67,1.58-1.94 and 1.31-1.75,respectively.The rejection rates of TP,COD,TN and NH4+-N by FO process were 96.8±0.8%,69.5±4.5%,49.5±4.8%and 41.4±4.6%,respectively.Membrane fouling caused by municipal wastewater was severe as the dense cake layer was formed in the membrane surface.However,water fluxes of the fouled membranes could be recovered to 94.0-98.47%after physical cleaning.After FO operation,the diluted DS was treated by RO process for clean water production.The concentrations of COD,TP,TN and NH4+-N for RO product water were 2.90,0.001,1.23 and 1.07 mg/L,with the removal rates of 91.7%,97.5%,89.7%and 90.1%,respectively.Excitation and emission matrix fluorescence spectra showed the weak fluorescence response in the EX230-300/EM250-390 region,which indicated that a small amount of protein,polypeptide or amino acid-like soluble organic matters were present in product water.In FO-RO hybrid system,the equipment investment,energy cost,membrane materials and operating expenditure occupied 40.83%,26.2%,24.46%and 8.51%,respectively.More specifically,RO device,energy used for RO pump and FO membrane were the most expensive parts.In conclusion,the FO-RO hybrid process can concentrate municipal wastewater efficiently and then produce clean water,but better FO and RO membranes are needed to obtain good performances and high-quality product water in the future study. |
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