Ultrafiltration is considered as an interesting polishing technique for effluent of wastewater treatment plants (wwtp-effluent). Results in pilot-scale experiments indicated that microparticles might dominate the filtration characteristics. In this article the results of investigations on the role of microparticles on the filterability of effluent is described. Calculation effects on particle size distributions of effluent showed that particles smaller than 2.0 microm predominantly influence the filterability of the wwtp-effluent. Additionally, experiments were performed on the filterability of size fractions of effluent. Effluent was fractionated with laboratory filters (pore diameter 5.0, 1.2, 0.45, 0.2 and 0.1 microm). The filterability of this pre-filtered effluent in applying ultrafiltration was evaluated by measurement of the SUR, a new parameter for measurement of the filterability of effluent. The results showed that filterability of the effluent was mainly influenced by 40% to 57% by the fraction larger than 0.1 microm and smaller than 0.2 microm, which is five to twenty times larger than the ultrafiltration membrane pores. Pre-treatment of the effluent by coagulation or sand filtration showed only minor influence on these results.

Variations in water quality of waste water treatment plant (WWTP) effluent complicate ultrafiltration of this feed water. Traditional parameters do not provide sufficient information to explain the fouling of membranes during ultrafiltration of WWTP effluent. New parameters for measuring and monitoring the fouling potential of feed water for ultrafiltration membranes need to be developed. The normalised membrane fouling index for ultrafiltration membranes (MFI-UFn) can be used as such and is according to the cake filtration theory calculated from the ratio of filtration time and filtration volume as a function of the filtration volume. MFI-UFn can be calculated from both experiments with constant Trans Membrane Pressure (TMP) and from experiments with constant flux. This parameter can also be calculated independent of the scale of the experiment. Results show that differences in fouling potential can be measured for various feed waters using the same membrane type and for various membrane types using the same feed water. Variation in feed water quality leads to a deviation of the MFI-UFn, as was found especially for WWTP effluent. The applied TMP influences the value of the MFI-UFn, indicating cake compression when applying a higher TMP. MFI-UFn can be used to identify the effect of pre-treatment methods, which is useful when using WWTP effluent as feed water for an ultrafiltration processes.

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The application of membrane filtration for the polishing of wastewater treatment plant (wwtp) effluent is more and more widely used. However, fouling problems are still not well understood. In order to investigate the role of particles in dead-end ultrafiltration of wwtp-effluent, Roorda studied the influence of pre-filtrated wwtp-effluent on the filterability. In addition this research investigates the relation between the filterability of the different fractions in the wwtp-effluent and the amount of organic foulants in these fractions. The results show a relation between the filterability and supra-dissolved particles in the fractions <0.45 and <0.2 μm. The amount of COD, colour, proteins and polysaccharides in these fractions are found of minor importance. A significant increase in filterability is found between the fractions <0.2 and <0.1 μm as well as a significant decrease in amount of polysaccharides. It is suggested that an essential feature in understanding membrane fouling during ultrafiltration of wwtp-effluent is to investigate the influence of supra-dissolved particles on the filterability as well as the influence of the amount of organic foulants (total organic carbon, proteins, polysaccharides, colour and humic substances) on the reversibility.