Effect Of Three Pretreatment Methods On Fouling Of Ultrafiltration Membrane And Involved Mechanisms

The significance of ultrafiltration (UF) technology applied in water treatment has been gradually recognized in recent years. However, when UF membrane is employed in the water purification process, it will be inevitably fouled. On the premise of water quality ensurance, how to mitigate the membrane fouling effectively is a technical difficulty in membrane technologies. Pretreatment methods are considered to be effective ways to reduce membrane fouling. In this study, three feasible, practical and inexpensive pretreatment methods were selected: coagulation pretreatment, adsorption pretreatment and oxidation pretreatment. In addition, commonly used reagents were selected as representatives. The process performance of a submerged polyvinyl chloride ultrafiltration membrane was investigated at different pretreatment strategies and different operating conditions, with focus on membrane fouling degree.Using aluminum sulfate as coagulant at neutral pH, the removal efficiencies in terms of UV absorbance at 254 nm (UV₂₅₄) and dissolved organic carbon (DOC) were achieved 82% and 67% respectively by coagulation-UF system at optimal coagulant dosage. Moreover, coagulation pretreatment could alleviate membrane fouling effectively. In the same constant flow of 10 L/(m²h), trans-membrane pressure (TMP) of UF system without pre-coagulation was 13.5 kPa more than that of UF system with pre-coagulation after 280 h operation period. It was found that a certain amount of particulate matter (turbidity of 8¹0NTU) had little influence on organic matter removal and TMP increase rate of UF system, and coagulant dosage was determined by the concentration of organic matter.Although a shorter treatment duration and similar permeate quality can be achieved by a micro-flocculation with UF system, it caused a TMP gap of 11 kPa, that is more serious membrane fouling, after 250 h operation period in the constant same flow of 10 L/(m²h) compared to a conventional coagulation process with flocculation. It was mainly because that micro-flocs (smaller than 20μm) took approximately 50% volume percent, which resulted in a denser foulant layer and more severe fouling. Although under-dosing coagulation saved coagulant and produced less floc sludge, 8% and 20% lower removal efficiencies of UV₂₅₄ and DOC were achieved compared with the case of optimal dosage of conventional coagulation. After 300 h operation period, TMP of under-dosing system was 10 kPa higher than that of optimal dosage system. This was mainly due to a decreased flocculation rate resulted from insufficient coagulant dosage. The average size of flocs decreased about 25% and fractal dimension of flocs increased from 2.69 to 2.79 at the end of flocculation stage, which was unfavourable for membrane fouling control. This study found that two-stage coagulant addition exhibited obvious advantages to mitigate membrane fouling. The TMP of two-stage coagulant addition system was 16 kPa higher than that of conventional one-stage addition system after 330 h operation period. It was found that the average size of flocs was increased nearly 25% and fractal dimension of flocs decreased from 2.68 to 2.51. Internal fouling in membrane pores was also reduced by two-stage addition strategy. This was attributed to freshly formed precipitates in the second coagulant dosage which could improve the floc properties to produce preferable flocs for filtration performance.Powdered activated carbon (PAC) was used as adsorbent to study the adsorption pretreatment on ultrafiltration performance. It was found that pre-adsorption of PAC could significantly reduce concentration of organic matter in permeate. Removal effeciency increased with the increasing of PAC dosage, especially for the target micro-pollutants with small molecule which could be hardly removed by UF membrane. The removal efficiencies of two target pollutants, i.e. acetochlor and metolachlor, were achieved 85% and 90% at PAC dosage of 50 mg/L.However, at a low dosage, TMP increase rate seemed to have little change compared to that without PAC addition. While at a high dosage, membrane fouling could be actually aggravated. At higher PAC dosages, humic acid may help to connect carbon particulates to the membrane surface, and carbon layer which deposited on the membrane surface could be also filled with humic acid. The joint effect of humic acid and powdered carbon together could form a denser foulant layer which was adverse to fouling control. The results of addition mode indicated that there was no significant difference in TMP increase rate between adding into mixing tank and directly into membrane reactor. It was showed that TMP of continuous dosing system was 7 kPa lower than that of intermittent dosing after 300 h operation period, which may be due to uneven distribution of PAC in membrane reactor and more PAC deposition on membrane surface in the intermittent dosing mode. The PVC membrane after PAC pre-coating was investigated. The results exhibited that the effect of pre-coated carbon was not significant, which indicated that the effect of pre-coating was related with the property of membrane material.Oxidation process may be beneficial to both organic matter removal and membrane fouling reduction. There was accumulation of microorganism in a submerged UF membrane system as the filtration process continued. It was found that removal efficiencies of UF system in terms of COD_Mn, DOC and UV₂₅₄ were increased 17%, 13% and 30% respectively by pre-ozonation. After 55 d operation period, TMP of UF system without pre-ozonation was 12.5 kPa higher than that of UF system with pre-ozonation.Three-dimensional fluorescence spectroscopy was utilized to analyze the mechanism of membrane fouling control by pre-ozonation. The results showed that about 30⁴0% of humic-like substances and 60⁷0% of protein-like substances could be decreased by pre-ozonation. Pre-ozonation induced a significant reduction of protein-like substances in extracellular polymeric substance, which closely related with membrane fouling. Both external and internal fouling could be effectively mitigated by the pre-ozonation. The content decrease of protein-like substances and structural changes of humic-like substances were observed in external foulants from EEM fluorescence spectra due to pre-ozonation. At the same time, ozonation pretreatment resulted in remarkable reduction of both protein- and humic-like substances in internal foulants but relatively little structural changes. On the basis of above, the effectiveness of ozone pre-oxidation technology to improve UF performance was further confirmed with Songhua River as feed water.