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
Sludge quality parameters 
SVI 
Process parameters 
X_{a}, 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:
h_{1} Clear water zone 
h_{2} Separation zone 
h_{3} Accumulation zone 
h_{4} 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 Günthert (1984), collected at Buchloe II clarifier. One point was omitted (SVI = 373 ml/g).
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 nitrificationdenitrification 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. 
Sludge blanket behavior is also influenced by the sludge recycling pattern. In principal, there are two options (left and right figures): 
