**** Ulrich Bretscher's Wastewater Page ****
In the late 1960s the following, simple rules governed the design of final clarifiers:
The rest was up to the designer.
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.
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.