**** Ulrich Bretscher's Wastewater Page ****



Density Currents in Final Clarifiers

In the late 1960s the following, simple rules governed the design of final clarifiers:

- The surface load : < 1.6 m3/m2·h (at storm water flow)
- Depth of the basin: > 2.2 m
- Load of saw-tooth weirs: < 8 l/s·m (at storm water flow)
- Effluent quality: < 20 mg/l SS (suspended solids, on dry weather only)

The rest was up to the designer.

Predominantly with activated sludge clarifiers, it was hard to comply with the demanded 20 mg/l SS rule. Whereas final clarifiers of trickling filters or rotating biological contactors RBC caused no problems.

Then I discovered the unexpected fast flows inside the water body of up to 15 cm/s in the mainstream where the average velocity was expected to be less than 0,1 cm/s! Was it's cause the hydraulic impulse of the inflow, was it unequal heating by sunshine or was it the wind?? What I didn't take it into considerations then, was the difference between the specific weight of the sludge flocs and the water, seemingly so small, causing density currents.

The village of Unterwasser is a rather small ski resort in Switzerland. During the summer season of the 70s it was inhabited by a population of 1,500. In wintertime there were an additional 2,500 guests, but on weekends quite often a lot more, since Unterwasser is in the vicinity of some large cities. That posed a problem designing a suitable sewage treatment plant. Designing it large enough would make an inefficient plant in summer. And we feared bulking sludge during the skiing season, too.

So we built two parallel, different biological units, - an RBC (then called bio-discs) and an activated sludge unit, designed for 2,000 pop. equivalents each. And this behind one single primary clarifier.

During the summer season we operated the RBC only, since this required only little maintenance and little energy. In the winter season we put the activated sludge unit behind the RBC. This way we got a very flexible two-stage biology. And after our experience such a two-stage plant was less prone to sludge bulking.

Fig. 1:
The sewage treatment plant of the skiing resort Unterwasser in 1968.
On the left side we find the primary clarifier, followed by the activated sludge basin with Kessener brush aerators.
On the right side, barely visible, one of the two final clarifiers. One for the activated sludge and one for the RBC unit.
The facility behind the basins was home of the RBC plus a small office and work shop.
The sludge was digested by two unheated digesters (what caused frequent problems)
Fig. 2:
The three slowly rotating RBCs (then called "bio-discs"). The diameter of the 420 discs is 3.00 meters what makes a total activ surface of 5,900 m2

The array of this plant gave me the unique opportunity to investigate the effect of two very different kinds of biological sludge on the flow in final clarifiers on otherwise identical conditions.

In 1982, at the end of the skiing season, I exactly split the effluent of the primary clarifier and fed one half of it to the RBCs and the other halve directly to the activated sludge unit. Then I waited two weeks for adapting the two units to the new situation, where the activated sludge soon started bulking. After this I made flow-measurements under exactly identical conditions. There is to mention the two final clarifiers were "identical twins" both equipped with chain scrapers, moving at 2 cm/s.

Fig. 3:

Activated sludge clarifier

Tank surface:..36 m2
Flow: .................6.0 l/s
Return sludge: .7.5 l/s
Surface load::::.0.6 m/h
Sludge conc.:...2.65 g/l
Sludge index:...260 ml/g

Fig. 4:


Tank surface:...36 m2
Flow: ...............6.0 l/s
Return sludge: 7.5 l/s
Surface load::::0.6 m/h
RBC sludge:....0.18 g/l

(The digits along the doted lines are indicating the concentration of suspended solids.)

The inflow of the two clarifiers is baffeld by a wall from concrete, ending at 0.5 meters beneath the surface, but prolonged by fingers of asbestos cement, 12 cm broad with 10 cm gaps in between. This according to the drawings above.


The flow inside a clarifier is definitely caused by the mass-transport of the settling sludge. All other causes may be neglected. (As numerous later measurements confirmed)

Due to the reverse of the water flow at the end of the activated sludge clarifier there appeared the characteristic floc-clouds in the vicinity of the discharge weirs. In the mixed effluent 45 mg/l suspended solids were analysed. In the main stream flow generally speeds of 2 cm/s were observed. As I learned later, the speed would have been faster by far, if the sludge had a lower index

Not so the clarifier of the RCB. There I measured sinificanly slower currents of only a few millimeters per second. The mixed sample of the effluent showed 14 mg/l suspended solids, only.


Last updated: Febr. 2006