A Pilot-Scale Study On Membrane Adsorption Reactor Based On Internally Circulating Fluidized Bed Technology For Defluorination From Groundwater

There commonly exist high-fluorine problems of groundwater in most areas in China to varying degrees and there are a great number of people drinking the high-fluorine water as yet.The present commonly-used fixed-bed defluorination technology has a lot of defects such as low utilization of adsorbent,frequent regeneration,easy caking of adsorbent,low degree of automation,hard to miniaturize,nontransparent cost of running,etc.Hence,developing a new defluorination technology,which is suitable for country areas,is being pressed for.Activated alumina was selected to process fluorine-containing groundwater via a pilot-scale integrated MAR(membrane adsorption reactor)apparatus designed by our research group.The reactor is an internal-circulating fluidized bed in which a set of microfiltration membrane module is imbedded to separate particles of alumina from water.Sequence batch mode and continuous-flow intermission mode were respectively choosed to experiment by programmable logic controller.The rate constant(k)of the pseudo-second order adsorption model was formulated with activated alumina dose(a)and initial sorbate(F-)concentration(C0)by surface fitting with a range of experimental data.And the equilibrium sorbate concentration(ye)was similarily formulated.In continuous-flow progress,k is actually not a constant due to the rising trend of the sorbate concentration resulted from the entering of raw water.The author reached new expressions to calculate k and the realtime potential equilibrium sorbate concentration(ye,t).According to the results of the sequence batch mode running experiment,the best reacting time is 0.5h,and the dosage of activated alumina is 20g/L.In continuous-flow intermission running mode,the pre-reacting time(tp)is determined as 1h.The quiescent reaction time(tq)is properly settled as 2h through calculation and analysis.The dosage of activated alumina and the HRT(hydraulic retention time)are respectively determined as20g/L and 2h via experimenting.The total water yield in both sequence batch mode and continuous-flow intermission mode is almost the same,but the average daily water yield of continuous-flow intermission mode is 8.8m3/d,greater than the sequence batch mode which is only5.434m3/d.However,the amount of aeration per unit volume of permeated water of sequence batch mode is lesser,just 61.4% of sequence batch mode(which means lesserenergy consumption).At this point,continuous-flow intermission mode is regarded as a means of improving average daily water yield exhausting more energy.Data of experimenting with different numbers of alumina regeneration times shows that chemical cost can be sharply reduced through regeneration.Conclusions of one-month’s 24 h continuous-flow intermission running are drawn as follows: The TMP developing rate increases with the water yield in the case of not backflushing and chemical cleaning.In single water-yielding period,membrane fouling can be eliminated largely by no-effluent aeration.Fittings of whole period and single period TMP data indicate the membrane fouling progress are probably dominated by poreblockage filtration model and cake filtration model,respectively.The concentration of Al3+ in permeated water is permissible below the state limit(0.2mg/L).By analyzing the cost of defluorination with this apparatus,the cost of energy consumption and chemical reagents are 0.52 yuan/t and 0.9 yuan/t respectively.A mathematical model of continuous-flow(no intermission)was established to predict the concentration of fluorine in permeated water.Total water yield before exceeding the limit predicted by the model is just 43% of sequence batch mode and continuous-flow intermission mode,which indirectly proves the setting of continuous-flow intermission mode relatively reasonable.