**** Ulrich Bretscher's Wastewater Page ****
Swimming and Rescue from aerated basins
Testing on safety measures on activated sludge basins
After some tragic accidents in Swiss wastewater treatment plants, the author made some tests in 1976 with the goal to improve safety measures. I swam
In 1976 most activated sludge basins were aerated by turbines or fine bubble line aerators. There didn't exist any grid arranged aerators as are mostly used today, covering the entire floor, providing an even water-air mix and only little turbulence. For catching up testing this new system too, I'm still looking for "stunt men" among plant operators, prepared to swim in activated sludge.
But I will also report on the latest safety measures taken as a consequence of the latest accidents.
These were the first used aerators in Switzerland since 1962. Their advantage was a simple construction and no clogging of aerator pores, such as in diffuser-aerators. Their disadvantage was a poor oxygenation capacity of only about 1.5 kg O2/m3۰h.
Though the surface of turbine-aerated tanks appear very turbulent, it's easy to swim in these tanks. Air bubbles exist only near the surface and turbulence is generally low. We even swam in a twin turbine tank in-between the turbines and crossed the basin. Even without life jackets this was feasible.
The first fine bubble line aerators were built ca. 1970. They had a better oxygenation capacity and enough turbulence to avoid settling of solids at the bottom of the tank, especially sludge and sand.
As demonstrated in the picture at the left, line aerators have the advantage of an efficient oxygenation and well mixing capabilities. But they pose a threat to swimmers!
Their downward draft at the opposite side of the aerators prohibits any swimming, even when wearing a life jacket.
Floating foam on the surface of any kind of aerated basins poses a particular threat. Foam in an activated sludge tank is usually the result of a mass-growth of filamentous bacteria. Even if it should be possible to swim in a given basin, your nose will be only one or two centimeters above the water level. So you will breath foam and certainly suffocate by inhaling it.
Watch the following Super-8 movie first, made in 1976
And by the way, life-preservers in sewage treatment plants are mere decorations, soothing the concious of the responsible autorities only.
To make waste water treatment plants as safe as possible you have to provide two preliminary measures are necessary, as follows:
1. Equip each well and basin more than one meter in depth with a suitable number of exit ladders!
2. Install emergency switches every 20 meters at least, along all railings of aerated basins. Hitting one of these switches immediatly turns off all blower fans responsible for the aeration of this basin.
3. Fight foam at the surface of basins. That may be achieved by biologic measures or by the addition of chemicals.
And here, a final story with good news: A recently finished waste water plant, known to me, celebrated the "Day of the open door" for the public taxpayer. Here came in a father with his child riding on his shoulders. As a remnant of the construction work, there lay a piece of wood on the ground, unnoticed by the father. He stumbled and could hold himself at the railing. But his child fell headfirst over the railing into the aerated basin. Thanks to emergency switches along the railings the aerators were immediately stopped and the child could be saved.
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