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PRACTICAL DESIGN CRITERIA
In the pre-modeling era, engineers developed various complex criteria, which, as they believed, described the state and the maximum acceptable loading of clarifiers. In addition, they developed geometrical and other recommendations. Some of the many criteria proposed and contained in technical standards and guidelines are presented and reviewed in this chapter.
Four criteria are most commonly used:
- the surface overflow rate (hydraulic loading);
- the solids loading rate (sludge mass loading);
- the volumetric loading rate (sludge volume loading);
- the weir loading rate (hydraulic loading of the overflow weir).
In terms of the process kinetics, all the loadings are rates or fluxes. Flux is defined as the amount that flows through a unit area per unit time. Volumetric flux is the rate of volume flow across a unit area (m3·m−2·s−1). Mass flux is the rate of mass flow across a unit area (kg·m-2·s-1).
Hydraulic loading rate is actually the overflow rate ν = Qe/A, typically expressed in m/h.
Solids loading rate is the overflow rate multiplied by the incoming solids concentration NA = νXa, typically expressed in kg/m2 h. It is actually the applied solids flux. The bulk flux is the applied solids flux multiplied by (R+1). The solids flux theory recognizes also the gravity flux G, the flux caused by gravity.
Volumetric loading rate is the newest of criteria discussed here. It was probably first introduced in the German guideline ATV A131. It is the solids loading rate multiplied by the sludge volume index SVI. If the units of SVI are in liters/gram, then the units of the volumetric loading are l/m2 h. The symbol introduced in the guideline is qsv. Dividing qsv by 1000 converts qsv to the height of sludge introduced in one hour (l/m2 h)/1000 = m3/m2 h = m/h.
A131 recommends maximum values of qsv 650 l/m2 h for vertical clarifiers and 500 l/m2 h for horizontal clarifiers.
Weir loading rate is the flow rate of the clarifier effluent per the length of the overflow weir. It is argued that if the weir loading rates exceed the recommended values, the velocity of currents approaching the weirs may be such that excessive solids are carried over the weir.
Summary of standard criteria adopted by many states in the U.S.A. [1] is shown in the table. The source explains: “When the above overflow rates, or solids loading rates are exceeded, removal efficiency will decrease significantly. If sludge blanket depths are high, removals may suffer at even lower overflow rates“
Table: Typical values of loading rates in the U. S. A.
|
Criterion |
Values |
Units |
Values |
Units |
|
Surface overflow rate at peak hourly flow (conventional activated sludge) |
1 200 |
gpd/ft2 |
2,04 |
m/h |
|
Surface overflow rate at design peak hourly flow (fixed film) |
1 200 |
gpd/ft2 |
2,04 |
m/h |
|
Surface overflow rate at peak hourly flow (single-stage nitrification) |
1 000 |
gpd/ft2 |
1,70 |
m/h |
|
Surface overflow rate at peak hourly flow (with chemical addition for P removal) |
900 |
gpd/ft2 |
1,53 |
m/h |
|
Solids loading rate, peak day (conventional activated sludge) |
50 |
lb/d/ft2 |
10,17 |
kg/m2.h |
|
Solids loading rate, peak day (single-stage nitrification) |
35 |
lb/d/ft2 |
7,12 |
kg/m2.h |
|
Weir loading (large clarifiers) |
30 000 |
gpd/ft |
15,52 |
m3/m.h |
|
Weir loading (small clarifiers) |
20 000 |
gpd/ft |
10,35 |
m3/m.h |
It is interesting to see the change of approach to the weir loading rate. Many large clarifiers operate successfully with significantly higher weir loading rate than the traditional 10 m3/m h. The response was pragmatic. The value for large clarifiers was increased by 50 % rather than questioning the criterion as such.
Typical recommendations for operating sludge concentration (MLSS) are 2 to 6 g/l and for the returned sludge 5 to 15 g/l.
Table: Typical values of minimum clarifier depth in the U. S. A. [1]
|
Criterion |
Values |
Units |
Values |
Units |
|
Minimum side water depth (fixed film) |
10 |
ft |
3,05 |
m |
|
Minimum side water depth (suspended growth) |
12 |
ft |
3,66 |
m |
A131 recommends minimum side water depth 2,5 m and minimum average water depth 3 m.
Various guidelines require certain ratio of the length to the width. A131 recommends minimum ratio 5:1. Numerous other recommendations vary typically between 3.5 to 4, some even 10 and more. The criterium as such is meaningless unless other parameters, such as mixed liquor linlet and water uptake arrangements are considered.
[1] Adapted from Wet Weather Operating Practices for POTWs with Combined Sewers, Technology Transfer Document, New York State Department of Environmental Protection, 2000.
