German guideline A131 recommends to calculate the sludge blanket height from the bottom concentration but fails to offer methodology how to calculate the bottom concentration. Interpretation in relation to short circuiting is shown in the figure. The value of z depends on a number of factors. It can be seen from the figure that the main discrepancies are at low recycle ratios. G nthert's prediction is too low, that of A131 too high.
More research is needed.
The smaller z , the shallower the sludge blanket and the higher the limiting load.