The Performance And Fouling Behavior Of Cross-flow Ultrafiltration With Mixed NOM And Inorganic Salts

In recent years,ultrafiltration separation technology has been widely used in the field of wastewater treatment due to its high efficiency and low consumption,stable effluent quality,and no secondary pollution.However,membrane fouling is one of the biggest challenges for the further development and application of ultrafiltration technology,because membrane fouling leads to increased energy consumption and decreased permeability of membrane separation operations,which seriously affects the practicability and economy of membrane technology.At present,most studies on ultrafiltration membrane fouling focus on single natural organic matter(NOM)fouling,but the actual wastewater to be treated is usually a complex system with multiple pollutants coexisting,unfortunately,there is a lack of systematic research on organic-inorganic mixed membrane fouling during ultrafiltration.Therefore,it is of great practical significance to study the organic-inorganic mixed membrane fouling under the condition of the coexistence of inorganic salts and NOM.In this study,sodium alginate(SA)and bovine serum albumin(BSA)were used to represent non-humic NOM,and humic acid(HA)was used to represent humic NOM.By adding various inorganic salt ions such as Na+,K+,Ca2+,Mg2+,Al3+ and Fe3+,an organic-inorganic mixed system was formed.The membrane fouling behavior and characteristics of various organic-inorganic mixed systems under cross-flow operating conditions were systematically investigated.In this study,by monitoring the changes of membrane flux and rejection rate under different operating conditions(cross-flow velocity,transmembrane pressure,flow pattern,etc),the membrane fouling characteristics and main influencing factors of different fouling systems were clarified from the macro level.The membrane fouling of organic-inorganic mixed system was characterized by SEM,EDS and other means at the microscopic level;At the same time,the classical membrane fouling model was used to fit the experimental data,and the membrane fouling mechanism of different fouling systems was further analyzed.In addition,on the basis of the experimental study,a computational fluid dynamics(CFD)model of cross-flow ultrafiltration was established by numerical simulation technology,the filtration process under different operating conditions was explored,and the flow field characteristics such as velocity distribution,pressure drop difference,and the shear force were clarified.The main results of this study are as follows:(1)The operating conditions during the cross-flow ultrafiltration have an important impact on the NOM membrane fouling:the increase of the feed concentration will aggravate the membrane fouling of NOM;the increase of the transmembrane pressure has little effect on the SA and BSA membrane fluxes,however,it can aggravate the membrane fouling of HA;in addition,the increase of the cross-flow velocity can effectively alleviate the membrane fouling of NOM.The CFD simulation results also show that the cross-flow velocity is positively correlated with the system pressure drop and the membrane shear force.When the flow state changes from laminar flow to turbulent flow,the membrane shear force increases significantly,which can effectively alleviate membrane fouling during the filtration process.(2)In the HA-inorganic salt mixed system,the ionic strength has a negative impact on membrane fouling.For different mixed systems,under the condition of low concentration ions,the fouling degree is HA-K+>HA-Mg2+>HA-Na+>HA-Fe3+>HA>HA-Ca2+>HA-Al3+;Under the condition of high concentration ions,the fouling degree is HA-K+>HA-Na+>HA-Fe3+>HA>HA-Mg2+>HA-Al3+>HA-Ca2+.Through model fitting,it is found that,except for the HA-A13+and high concentration HA-Ca2+systems,the complete blocking mechanism is the main mechanism of HA-inorganic salt mixed pollution.In addition,the intermediate blocking mechanism also has a good correlation for the HA-divalent inorganic salt system,and the intermediate blocking mechanism and the standard blocking mechanism are also suitable for the HA-Fe3+system.(3)In the SA-inorganic salt mixed system,the ionic strength will aggravate the membrane fouling.Both SA-monovalent inorganic salt and SA-divalent inorganic salt system produced more serious Wembrane fouling,meanwhile,Mg2+ caused more serious membrane fouling than Na+.When the SA concentration is high,the fouling mechanism of the SA-inorganic salt mixed system is complete blocking mechanism.When the SA concentration is low,the complete blocking and intermediate blocking models all have good correlation.(4)In the BSA-inorganic salt mixed system,the effect of ionic strength on membrane fouling is small.For different mixed systems,under the condition of low concentration ions,the fouling degree is BSA-Al3+>BSA-Ca2+>BSA-Na+>BSA-Mg2+>BSA>BSA-K+>BSA-Fe3+;Under the condition of high concentration ions,the fouling degree is BSA-Al3+>BSA-Ca2+>BSA-Mg2+>BSA-Na+>BSA>BSA-K+>BSA-Fe3+.The BSA-monovalent inorganic salt system is dominated by the intermediate blocking mechanism;the BSA-divalent inorganic salt system is dominated by the complete blocking mechanism;and the BSA-trivalent inorganic salt system has a good correlation for both complete blocking and intermediate blocking.and are independent of concentration.