Introduction

1. Characteristics of particle deposit and resistance build-up on ultrafiltration membranes of different MWCOs: application of a PDA online monitoring method Introduction In many cases, removal of suspended particles from water is the main task in drinking water treatment or tertiary treatment of secondary effluent for wastewater reuse. Therefore, a matter of concern for UF membrane application is the behaviour of higher quantity of suspended particles on flux decline. UF membranes often have a broad MWCO range from several thousand to several hundred thousand Da, but few literatures have dealt with the effect of MWCO on particle deposit and resistance build-up on the membrane surface during filtration, though this is a factor to be considered in MWCO selection. In order to evaluate the weight of particles deposited on the membrane surface and its variation during a filtration run, an online optical technique, namely particle diffusion analyzer (PDA) was introduced in the study. Experimental System Fig. 1 shows the schematic diagram of the experimental system used in this study. The feed solution was boosted to the membrane cell which was constructed in transparent plastics housing a piece of flat sheet membrane. The membrane was a round sheet of 80mm diameter. Polyethersulfone (PES) membranes of three MWCOs were applied for this study: 100k, 50k and 10k Da. In the experiments, the feed solution was pressurised and controlled to a constant value of 0.2MPa in the process of filtration. The membrane cell could be operated in either crossflow or dead end mode. An electric magnetic stirrer was placed in the membrane cell so that the feed solution could be fully mixed and a shear force on the membrane surface could be provided. A steady concentrate flow was delivered to a particle diffusion analyser (PDA2000, Rank Brothers Co. Ltd.) where the variation of suspended particles in the rejected flow could be monitored online. The permeate quantity was online monitored by an electronic balance on which the permeate tank was mounted. Both the PDA and electronic balance were connected with a personal computer for data processing. Figure 1 The schematic diagram of the experimental system Results & Discussion Analysis of membrane resistance Variation of membrane deposit weight The decline of permeate flux is in any sense resulted from an increase of membrane resistance and in this study because kaolinite clay particles were the only material to deposit on the membrane surface or penetrate into membrane pores, an analysis of the relationship between membrane resistance and the deposit weight on the membrane can provide useful information about the behavior of particles on membranes of different MWCOs. An estimation of the total resistance can be performed by using Darcy’s law. The calculation results are shown in Fig. 4. The initial resistance Rm0, i.e. the calculated Rm at t=0 was evaluated as 1.20×1013, 1.43×1012, and 5.75×1011 m-1 for the PES membranes of MWCO=10k, 50k and 100k, respectively. As filtration time elapsed, Rm increased in each case, but the increasing speed was greater for larger MWCO membranes. The membrane resistance Rm can be considered as the initial membrane resistance Rm0, i.e. the hydraulic resistance of clean membrane itself plus an increment ΔRm during the filtration process. As a result, Fig. 5 was obtained to show the variation of ΔRm/q=(Rm–Rm0)/q with time for PES membranes of MWCO=10k, 50k and 100k, where q is the deposit weight per unit membrane surface. It can be seen that a PES membrane of smaller MWCO not only tends to have more suspended matter deposited (Fig. 3) but is also easy to build up higher hydraulic resistance from unit weight deposit. The PDA online monitoring results are shown in Fig. 2 for PES membranes of MWCO=10k, 50k and 100k. With kaolinite suspensions of 40NTU, the average CI value should be about 1.9 according to the preliminary experiment results shown in Fig. 2. However, as membrane filtration started, quick decrease of the CI value was noticed from Fig. 7. In the case of MWCO=10k (curve a), the initial CI value became about 1.0 and it decreased continuously as filtration time elapsed. In the cases of membranes with larger MWCOs (50k and 100k), the initial CI values did not depart from 1.9 so much but they decreased quickly with time. After about 2000 sec filtration, the three CI curves almost coincided with each other and reached a final value about 0.6. Based on the PDA monitoring results, the membrane deposit weight was calculated. Fig. 3 shows the variation of the deposit weight per permeate volume with filtration time. Generally speaking, the membrane deposit weight resulted from the filtration of unit volume permeate decreased with increasing MWCO of the membranes. However, the pattern of variation with time differed with different MWCOs. In the case of MWCO=10k, the initial deposit weight was above 200g/m3, and it kept increasing till the end of filtration. Contrarily, the initial deposit weights for larger MWCO membranes were much lower (near 100g/m3 in the case of MWCO=50k and about 50g/m3 in the case of MWCO=100g/m3) and they tended to decrease as filtration time elapsed. Fig.2 CI curves recorded by PDA Fig.3 Deposit weight on membranes Fig. 4 Membrane resistance Fig. 5 deposited kaolinite clay to the with filtration time build-up of hydraulic resistance Conclusions The method of PDA online monitoring and mass balance analysis used in this study provided an indirect way to monitor particle concentration change in the concentrate flow and then to evaluate the deposit weight, which may assist the investigation of membrane fouling by suspended particles. Acknowledgement: This study is supported by the National Natural Science Foundation of China (Grant No. 50138020)