Study On The Application Of Membrane Technology In The Integrated Citric Acid-methane Fermentation System

In order to solve the problem of fermentation wastewater pollution in citric acid production process,an integrated citric acid-methane fermentation system was proposed and established in our laboratory.In the integrated process,the wastewater was reused and the amount of wastewater and fresh water was reduced.However,the integrated process was still restricted by some problems.Firstly,the temperature of anaerobic digestion and the air stripping treatment was respectively 35 ^oC and 55 ^oC.It meant the temperature rise would cost a lot of heat energy.In addition,the ammonium could also be removed by electrodialysis.Secondly,the electrodialysis design in a previous study was different from the conventional electrodialysis design and could result in some potential problems.Finally,scaling phenomenon took place in concentration solution in electrodialysis treatment.To solve the problems,the air stripping should be removed and ultrafiltration with its pretreatment process was introduced.With electrodialysis treatment,the inhibitors in anaerobic digestion digestion were effectively removed.As a result,these optimization improved the stability and feasibility of the integrated process.The main research contents are shown as follows:?1?By determinating the limiting current density of simulated solution,it was confirmed that when the current density of anaerobic digestion was lower than the highest value of simulated solution effluent,the polarization phenomenon would not happen at the same operational conditions.The electrodialysis would run normaly.According to desalination rate,current efficiency of HCO_%^&,energy consumption and ammonium removal rate,the optimal operation conditions of electrodialysis were at a applied voltage of 10 V,a flow velocity of 2.61cm·s^-1 and a concentrate to diluate volume ratio of 1:4.When the pH was maintained at 6.5,the LSI of the concentration solution was always less than the critical scaling point?LSI=1?,and the scaling phenomenon was eliminated.?2?The turbidity of permeate of 100 kD and 200 kD tube ultrafilatration membrane was higher than the requirement of electrodialysis?highest value of turbidity<3 NTU?.It was confirmed that anaerobic digestion effluent should be pretreated to reduce the turbidity before treated with ultrafiltration.Considering the turbidity removal efficiency,ultrafiltration permeate flux and potential membrane pollution,lime softening was selected as the pretreatment of ultrafiltration,and the turbidity of ultrafiltration permeate complied with the requirements of electrodialysis.The optimal operation pressure and flux velocity were respectively 0.20 MPa and 4.023 m·s^-1.The LSI of concentration solution was less than 0,and scaling phenomenon disappears in eletrodialysis treatment.Diluate solution was reused to citric acid fermentation and the citric acid production was 134.67±4.72 g·L^-1,which was comparable to production with demonized water.?3?In 8 batches of recycling experiment,the citric acid production of 2-8 batches was comparable to control with demonized water.The methane fermentation performed well and the average of removal rate of COD was over 95%.The average value of organic volume load,hydraulic retention time and the methane production rate were respectively 1.17±0.18 kg·m^-3·d^-1?9.29±0.69 d and 276.2±37.4 mL·g^-1 TCOD_removed.The total hardness of anaerobic digestion effluent was lower about 200 mg·L^-1 and achieved the success of lime softening.The average turbidity of permeate of ultrafiltration was 2.7±0.3 NTU,which was higher than the normal turbidity value?<1 NTU?but lower than the highest turbidity value?<3 NTU?.After ultrafiltration and electrodialysis treatment,the conductivity of the anaerobic digestion solution mixed with deionized water was lower than the reuse standard of anaerobic digestion solution.The final conductivity of the mix solution of anaerobic digestion effluent and the additional fresh water met the requirement of reuse in the integrated process.The LSI of concentration solution was lower than the critical scale point?LSI=1?,and scale phenomenon disappeared.The concentrations of main metal ions in anaerobic digestion solution did not accumulate in the recycling batches.As a result of water loss when wastewater treated with lime softening,ultrafiltration and electrodialysis,the average water recovery of the integrated process was54.1±6.5%.