In Fig. 7 for each phase average surplus sludge production
versus sludge loading rate is depicted. Sludge yields between
0.01 and 0.1 g MLSS/g CODremoved were found. It can be seen
that under the condition of nutrient limitation sludge production
was significantly lower. In other words, at a similar sludge
loading rate specific sludge production rates were approximately
50% higher when nutrients were added.
For MBR systems generally low sludge production rates are
reported [6]. A comparably high sludge age due to elevated
sludge concentrations is presumed to be the main reason. It is
commonly known that high sludge ages causes sludge yield to
decrease due to emphasised endogenous metabolism.With this
respect, the low sludge yield can be attributed to the high sludge
age as the theoretical sludge ages ranged from 43 to more than
300 days. Comparable results were found by Ref. [10], who
obtained sludge yields between 0.14 and 0.16 g TSS/g
CODdegraded for an MBR fed with laundry wastewater. In
contrast to these observations [7] report an unexpectedly high
sludge yield of 0.29 at a F/M ratio of 0.1 g COD/gTSS. But they
also note that the sludge yield might be influenced by an
appreciable concentration of non-biodegradable solids in the
influent, probable polyester fibre fragments, causing an
increment of inert solids in the bioreactor