Filterability of activated sludge in membrane bioreactors

The filterability of activated sludge is an important factor for the economical operation of membrane bioreactors (MBR). In the literature mainly investigations on sludge dewaterability in respect to further disposal are published. In this study, a procedure for determining filterability in a crossflow test cell is introduced. Its features are: no increase in sludge concentration during batch trials, crossflow conditions, and little impact on the sludge structure. The activated sludge filterability is given as the ratio of permeate flux after 40 min of operation to clear water flux. Sludge samples of eight different MBR and one conventional wastewater treatment plant (wwtp) have been examined and compared. Contrary to the literature, no impact of suspended solids (SS), concentration, sludge viscosity or extractable extracellular polymer substances (EPS) concentration on the filterability was found. Instead, the composition of the liquid phase was found to effect most the filterability of activated sludge, a major influence being the concentration of suspended EPS: the higher the suspended EPS concentration, the lower the filtration index. Suspended EPS concentration increases with high mechanical stress in the MBR and high F/M ratios, if the treated wastewater contains considerable amounts of proteins or polysaccharides.

Publications on activated sludge filterability state a number of influencing parameters with, to a certain effect, even opposite effects in different investigations. This indicates the complex nature of activated sludge. ideally, investigations on filterability should hence be carried out using a standard procedure and consider a combination of parameters such as SRT, suspended EPS, SS, particle size distribution, and operation performance in interpreting the results. In this study a procedure for determining filterability of activated sludge using a crossflow test cell was introduced and successfully applied to the investigation of sludge samples taken from eight different MBR. Although just one specific membrane material was used, it can be expected that the general dependencies as reported as transferable to other membrane materials. SS concentration of the investigated sludge samples was in a range of 2 to 24 gL!1 and revealed only little influence on the filtration index. In contrast to the literature, no impact of sludge viscosity or extractable EPS concentration was observed. A comparison of two MBR (side-stream and submerged modules) supplied with wastewater with a high fouling potential revealed better sludge filterability in the submerged system and for higher SS concentration and thus lower F/M J R η ∆p — — — — Permeate flux, Lm!2h!1 Resistance, m!1 Viscosity, mPas Transmembrane pressure difference, bar Subscripts m p s t — — — — Membrane Permeate Sludge Total