SLUDGE BLANKET
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
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Sludge quality parameters
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SVI
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Process parameters
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Xa, v, R, incoming concentration of
nitrate, temperature
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Clarifier design parameters
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Bottom slope, frequency of arms rotation, number of
arms, scraper blade height and shape
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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:
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h1
Clear water zone
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h2
Separation zone
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h3
Accumulation zone
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h4
Thick blanket zone
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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 Günthert
(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).
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In this case, A131 overpredicts the
sludge blanket height. Further, it does not include any of clarifier
design parameters. |
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Denitrification
in final clarifiers has to be avoided by proper design of the
nitrification-denitrification system! |
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At clarifier
underloading conditions, sludge blanket height follows the clarifier
effluent flow (with time delay).
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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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