Study On The Effect Of Periodic Backwash On Membrane Filtration Performance And Effective Pore Size

In recent years,microfiltration（MF）membrane technology as an efficient separation unit has been applied in water treatment,reverse osmosis（RO）pretreatment and other fields.However,membrane fouling restricts the wide application of MF technology in industry.Study on membrane cleaning plays an important guiding role in the control of membrane fouling and the actual membrane industrial process,thus promoting the improvement of membrane filtration efficiency and the wide application of membrane technology.In this paper,a plate MF membrane with a pore of 0.2 um polyvinylidene fluoride（PVDF）was used for constant pressure cross-flow filtration and constant pressure backwash of yeast suspension.（1）The effects and contribution of five parameters（filtration TMP,concentration,velocity,flux decline rate,backwash duration and backwash strength）on membrane fouling resistance（R_f）,backwash efficiency（P）and specific energy consumption（E）in the process of constant pressure cross-flow filtration and backwash were studied,and R_f,P and E in the case of backwash were compared.The results showed that the periodic backwash can reduced 39.0%of R_f,increased 37.9%of P and saved 25.0%of E compared with no backwash under the same operating time.The contributions of backwash conditions on irreversible fouling resistance(R_irr)were 39.17%（concentration）,31.98%（backwash strength）,27.16%（flux decline ratio）,1.58%（filtration TMP）and 0.1%（backwash duration）,on P were 38.97%（concentration）,26.37%（backwash strength）,23.76%（backwash duration）,10.24%（filtration TMP）and 0.66%（flux decline ratio）,on E were 76.03%（filtration TMP）,12.79%（concentration）,10.67%（flux decline ratio）,0.31%（backwash duration）and0.20%（backwash strength）.The increase of flow velocity can reduce R_irr and increase P,and the flow velocity of 0.71m/s（Re=4200）was the optimal option.Furthermore,the concentration,filtration TMP,cross-flow velocity,backwash strength and backwash duration must be carefully selected in industrial operation process to balance“trade-off”between P and E.（2）The variations of effective mean pore size of the membrane（d_e）,the shear force（τ）in the membrane pores and the reversible fouling resistance（R_r）during the backwash process were studied.And the d_e changes with and without backwash were compared.The results showed that the d_e,the shear force in the membrane pores,and R_r decrease with increasing period.The d_e increases with TMP,flow velocity,backwash duration and backwash strength,and decreases with increasing concentration.When1700<Re<4200,increasing the flow velocity can effectively increase the d_e.When Re>4200,the de does not significantly improve by increasing the flow rate.The shear force in the membrane pores increases with the increase of TMP,flow velocity,backwash duration and backwash strength,and decreases with the increase of concentration.R_rincreases with increasing TMP,concentration,backwash duration and backwash strength,and decreases with increasing flow backwash.Periodic backwash can effectively increase the d_e,and the de of 2-10 cycles was larger than that of non-backwash one.