Impact of two different excess sludge removal strategies on the performance of a membrane bioreactor system

For small membrane bioreactor (MBR) plants, in order to save investment for infrastructure, it could be beneficial not to withdraw excess sludge on a daily basis, but to store it in the biological reactor and only withdraw it every 2 to 4 weeks. This study aimed at investigating the effect of such an excess sludge removal strategy on the performance of an MBR plant in terms of permeate quality, nutrients removal rates and fouling. An MBR pilot plant, fed with domestic waste water from a remote area, was operated with enhanced biological phosphorus removal and post-denitrification without carbon dosing. 50% of the reactor volume was withdrawn when around 13 g l-1 TS was reached in the membrane reactor. This sludge removal strategy did not lead to failure of neither the biological phosphorus removal, nor the post-denitrification. Higher specific denitrification rates (DNR) were observed during higher organic loading of the anaerobic zone. The average DNR at 20°C was 1.5 mgN(gVSS h)-1. Nitrification was influenced by the discontinuous excess sludge removal. During that period the nitrification rate varied in a wide range between 1.8 and 5 mgN(gVSS h)-1, with a trend to lower rates right after a sludge removal. Fouling was not effected by the excess sludge removal strategy. For both withdrawal strategies the fouling rate was around 5*1010(md)-1. The EPS concentration did not affect the fouling behaviour.

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