Sludge blanket height is the most difficult parameter to estimate. At the same time, it is one of the most important parameters for clarifier design. Without reliable prediction of sludge blanket height neither maximum clarifier loading nor appropriate design of clarifier configuration and equipment are possible. Thus many clarifiers seen at treatment plants are far from optimum design and operation.

Variables and factors influencing sludge blanket

Sludge quality parameters


Process parameters

Xa, v, R, incoming concentration of nitrate, temperature

Clarifier design parameters

Bottom slope, frequency of arms rotation, number of arms, scraper blade height and shape

Available formulae for calculation of sludge blanket height, such as that in A131, include sludge parameter and some of the process parameters.  Clarifier design parameters are frequently missing. Thus all clarifiers, good or bad in design, would be calculated according to the same formula.

Zones in clarifier are defined as in A131:

h1    Clear water zone

h2    Separation zone

h3    Accumulation zone

h4    Thick blanket zone

The two figures below show calculation of sludge blanket, according to two different approaches. The left one is based on AquaNova design procedure, the right one according to A131. Data are from GŁnthert (1984), collected at Buchloe II clarifier. One point was omitted (SVI = 373 ml/g).

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Points falling into the gray area (outside the common spread of data) are influenced by denitrification in the sludge blanket. Thus denitrification does not only cause sludge clusters to rise but also expands the blanket. For details on rising sludge, see Henze et al. (1993).

In this case, A131 overpredicts the sludge blanket height. Further, it does not include any of clarifier design parameters.

Denitrification in final clarifiers has to be avoided by proper design of the nitrification-denitrification system!

At clarifier underloading conditions, sludge blanket height follows the clarifier effluent flow (with time delay).

Sludge blanket height trajectory. Same clarifier and interval as in the left figure.

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Sludge blanket behavior is also influenced by the sludge recycling pattern. In principal, there are two options (left and right figures):

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