In the pre-modelling era, engineers developed various complex criteria, which, as they believed, describe 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 mass balance and 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 (m3·m−2·s−1), typically expressed in m/h, in the US m/d.

Solids loading rate is the overflow rate multiplied by the incoming solids concentration NA = νXa (kg·m-2·s-1), typically expressed in kg/m2 h, in the US kg/m2 d. 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 criterion 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 l/g, 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 into the clarifier (l/m2 h)/1000 = m3/m2 h = m/h.

A131 recommends maximum values of qsv 650 l/m2 h for predominantly vertical clarifiers and 500 l/m2 h for predominantly 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.


Predominantly vertical and predominantly horizontal - what is it?

This is an artefact (artifact) introduced by ATV A131 (see for instance Merriam-Webster Dictionary: an object remaining from a particular period - The caves contained many prehistoric artifacts):

Predominantly horizontal flow tanks are those where the ratio of the distance from the inlet aperture to the water surface (vertical component, hin) to the horizontal distance from inlet to outlet at the height of the water level (horizontal component) is smaller than 1:3. Predominantly vertical flow tanks are those where the ratio is higher than 1:2. For ratios lying between the two, the permitted sludge volume loading rate can be interpolated linearly. It is recommended that the values in the table are used for dimensioning.


Permitted values for the transition area between predominantly
horizontal and predominantly vertical flow secondary settling tanks


≥ 0.33

≥ 0.36

≥ 0.39

≥ 0.42

≥ 0.44

≥ 0.47

≥ 0.5

qSV (l/m2h)

≤ 500

≤ 525

≤ 550

≤ 575

≤ 600

≤ 625

≤ 650

qA (m/h)

≤ 1.60

≤ 1.65

≤ 1.75

≤ 1.80

≤ 1.85

≤ 1.90

≤ 2.00

RV (-)

≤ 0.75

≤ 0.8

≤ 0.85

≤ 0.90

≤ 0.90

≤ 0.95

≤ 1.00

*) Vertical component to horizontal component e.g. 1:2,5 = 0.4

Volumetric loading rate is a useful criterion if not applied as in ATV A131. This is shown in next figure. Resch1 investigated several Dortmund type and one Berlin type clarifiers, all vertical according to ATV A131.

Dortmund type clarifiers:

In 1887 a treatment plant was built at Franzius Street, Dortmund (Germany), with four settling tanks (Dortmundbrunnen) 6,5 m diameter, 12 m deep. The Berlin variety was built at the the treatment plant Berlin - Ruhleben in the period 1957 - 1963 (24 clarifiers 12,5 m diameter, 15 m deep).

Typical values of loading rates

Summary of standard criteria adopted by many states in the U.S.A.2 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.






Surface overflow rate at peak hourly flow (conventional activated sludge) 

1 200 




Surface overflow rate at design peak hourly flow (fixed film)

1 200




Surface overflow rate at peak hourly flow (single-stage nitrification) 

1 000 




Surface overflow rate at peak hourly flow (with chemical addition for P removal)





Solids loading rate, peak day (conventional activated sludge)





Solids loading rate, peak day (single-stage nitrification)





Weir loading (large clarifiers)

30 000 




Weir loading (small clarifiers)

20 000 





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.


Solids loading rate - what is it and where it came from?

According to Richard I. Dick, the criterion appeared first in USA in 19593. Already in 1976 Dick4 questioned the solids loading rate definition and application.

Dick concluded: "There are activated sludge plants operating at solids loadings far less than 20 lb/sq ft/day (98 kg/sq m/day) which have difficulty in thickening solids and, conversely, plants are in operation with solids loadings of 80 lb/sq ft/day (390 kg/sq m/day) or more with good performance. Thus, it seems undesirable to attempt to specify a loading to represent a "typical" design value."


In some recommendations and technical standards the solids loading rate is still wrongly defined as it was in 1959:

From Dick (1976):

where Ga is the applied solids flux (this is correctly total flux, by standard notation Gt = v(1+ R)Xa);

           r is the recycle ratio (standard notation R = Qr/Qe);

          Q is the clarifier outflow rate (Qe);

          cMLSS is the concentration of the operating mixed liquor suspended solids (Xa);

          A is the area of the clarifier.


Wahlberg5 has shown why such definition is wrong in principle.



The explanation is the following:

If a clarifier is overloaded by solids loading rate, the descending straight-line in the solids flux analysis exceeds the maximum permissible solids flux curve. Proper response is to increase the RAS flow (R = Qr/Qe). The recycled sludge suspended solids concentration decreases (from 11 g/l to 7,5 g/l), the new descending straight-line does not exceed the maximum permissible flux curve, the clarifier is loaded to its maximum capacity and stops loosing suspended solids into the effluent.


What happened to Dick´s equation (5)?

The only change is that r increased and thus the total loading rate increased too. According to wrongly defined solids loading rate criterion the situation should be worse. Just the opposite is true!

Who is wrong? Not in 1959, now! Clearly those who still advocated calculating applied solids flux including the term (1+R). 


Solids flux theory is rock solid since 1957 (Yoshioka et al.). Its application does not depend on the geometry of the clarifier (shape, area), scraping  or whatever else. In terms of solids flux theory application all clarifiers are equal. It has to be noted, however, that the solids flux analysis is the same for a clarifier deep 1 m and 10 m (as examples). Obviously, depth is not a parameter of the classic solid flux theory and has to be reviewed by newer modifications of the solids flux theory or by other theories. One of the newer approaches is based on the thickening gradient, as shown in the figure. Solids flux gradient approach can be used in sludge blanket mathematical modeling.




Application of Total Flux as a criterion is wrong in principle! The Applied Flux criterion has little if any value in the mathematical modelling era.



Typical recommendations for operating sludge concentration (MLSS) are 2 to 6 g/l and for the returned sludge 5 to 15 g/l.  Of course, any recommendation neglecting SVI is wrong.


Geometry of Circular Clarifiers

Table: Typical values of minimum clarifier depth in the U. S. A.1






Minimum side water depth (fixed film) 





Minimum side water depth (suspended growth)






A131 recommends minimum side water depth 2,5 m and minimum average water depth 3 m.


Geometry of Rectangular  Clarifiers

Various guidelines recommend certain ratio of the length to the width, typically minimum ratio 5:1. Clarifier design (WEF manual of practice) states the range from 1.5:1 to 15:1. 

Other guidelines recommend maximum length, for instance A131 states that  the length of 60 m should not be exceeded. In reality, however,  much longer clarifiers can be found, for instance Salzburg, Austria, 95 m, Lodz, Poland, 100 m etc.


The criteria of geometry as such are meaningless unless other parameters, such as mixed liquor inlet, water and sludge uptake arrangements etc. are considered.


1 H. Resch: Untersuchungen an vertikal durchströmten Nachklärbecken von Belebungsanlagen; Berichte aus Wassergütewirtschaft und Gesundheitsingenieurwesen, Nr. 29; Technische Universität München; 1981

2 Adapted from Wet Weather Operating Practices for POTWs with Combined Sewers, Technology Transfer Document, New York State Department of Environmental Protection, 2000

3 Sewage Treatment Plant Design, ASCE WPCF Manual of Engineering Practice, Water Pollution Control Federation, Washington, D. C. 375 pp., (1959).

4 Dick Richard I. (1976).  Folklore in the design of final settling tanks. Journal Water Pollution Control Federation, 48, pp. 633-644.

5 Wahlberg Eric J.: Folklore in Activated Sludge Treatment Plant Operations. Rocky Mountain Water Environment Association 65th Annual Meeting, September 17, 2002


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