Die Bedeutung der Filtrationseigenschaften von belebten Schlämmen beim Betrieb von Membranbioreaktoren

  • The meaning of filtration properties of activated sludge for the operation of membrane bioreactors

Thiemig, Christoph; Pinnekamp, Johannes (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2011)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2011


Membrane bioreactors (MBR) for industrial and municipal applications show advantages at space demand and effluent quality compared to conventional activated sludge (CAS) systems. Their main disadvantages are a higher energy demand and a limited lifetime of the expensive membrane modules. Hence, the challenge to make MBR competitive against CAS contains the minimization of both, membrane surface and energy demand for crossflow aeration. Both aspects are in a close interaction with the sludge properties which affect the formation of the cake layer and an organic fouling of the membranes. The fouling effects are determined by the concentration of extracellular polymeric substances (EPS), the soluble microbial products (SMP) respectively. The maximum specific flux that can be maintained for a longer period (critical flux) depends strongly on the filterability of the activated sludge. If the filterability is poor the transmembrane pressure (TMP) will increase extensively. Up to now there is no general accepted method to measure this important sludge property. The first part of this thesis describes the development and validation of a new simple method to measure the MBR-relevant sludge filterability. The newly developed "sludge filtration index" SFI is measured by commonly used lab devices and produced more reliable data compared to other methods. Temperature effects which influence the sludge viscosity are eliminated by tempering the sludge samples to 20°C before measurement. The SFI showed very well the seasonal changes of sludge filterability in two different municipal MBR. There was a correlation between SFI and the SMP concentration. The second part of this thesis is about trials to improve the sludge filterability in MBR. Three different commercially available polymers (membrane performance enhancer) and other chemicals were tested in beaker tests and large-scale trials. All tests showed a significant improvement of the sludge filterability by the tested membrane performance enhancer. Conventional polymers or precipitation agents showed a much lower improvement. Beside the changes filterability which was quantified by the SFI also the membrane TMP was lowered by the addition of the membrane performance enhancers. The critical flux could be increased significantly. The biological performance was not inhibited by the special polymers shown by long time online measurements of the effluent quality and specific nitrification- and denitrification performance beaker tests. A detailed economic feasibility analysis was presented to define the circumstances under which a polymer addition is profitable for the operator of an MBR. The improvement of filtration-relevant sludge properties in MBR by any measure with the target of increasing the specific flux is the main challenge to enhance the performance of MBR. One way to achieve this target is the addition of specially developed polymers. The hereby presented SFI method is a suitable analytical tool to control and monitor the sludge properties and to judge any measures to improve these properties. The SFI is already used in a couple of different municipal MBR in Germany. An increasing database will help to improve the economical operation of MBR which will make this technology more competitive to conventional technologies.