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 μm 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 μm). 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 μm and smaller than 0.2 μm, 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.

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