NAMS 2002 Workshop - ICOM 2008

scale where fouling rates are commonly at least one order of magnitude lower.
The discrimination whether the reason for a sudden permeability decrease in full
scale is a low filterability of the activated sludge or, e.g., module sludging can be
realised on this basis. Hence a regular monitoring of sludge behaviors in full
scale plants is hereby possible. To some extent, data can be used for model
identification to analyse the mechanisms that are responsible for the observed
fouling rates. Due to the interactions of ambient conditions in full scale, defined
lab studies are the only way to independently study influences on biological
kinetics. Cleaning success can be transferred qualitatively to full scale but also
not quantitatively. Thus, it will be demonstrated that lab scale experiments can be
meaningful for full scale operation only if the following preconditions are fulfilled:
Hydrodynamics must be comparable to achieve qualitatively comparable fouling.
Aeration rate, crossflow velocity and geometry (channel width etc.) must be the
same in both scales. Due to the fluid-structure interactions this can hardly be
achieved for hollow fibres. Operating conditions (constant TMP/constant flux)
must be identical to achieve comparable quality of the fouling layer. Even such
alleged banalities like test cell orientation must be carefully considered. Fresh
sludge from the full scale plant must be used or in-situ experiments must be
carried out in order to avoid effects due to starvation/disintegration etc. of the
sludge during sludge storage and shipment. If any of these stipulations is not
met, researchers and operators should be aware that the inherently limited
representativeness of results gained in lab scale is restricted even further.