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Swimming and Rescue from aerated basins

Testing on safety measures on activated sludge basins


Is it possible to swim in an aerated activated sludge basin? Some say it is not possible because of the low density of water caused by air bubbles. Some say there is too much turbulence. Which is the truth?

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 my swim suit at first,
- then fully dressed in overall and rubber boots, with and without lifejacket.
Of course, all tests were done secured by a rope.

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.

Turbine Aerators

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.

Swimming in a turbine aerated sludge tank

Line Aerators

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.

Foam in aerated basins

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.

A risky job! Wali, the operator of this plant, lying atop of the wall of one of his SBR-reactors. Good luck!
Floating sludge caused by a mass growth of fibrouse Nocardia bacteria.

Swimming in different kinds of activated sludge basins

Watch the following Super-8 movie first, made in 1976

Experiences during the last 45 years

In the last 45 years as a chemiscal engineer it was my primary duty to run pilot plant tests for finding new solutions for sewage cleaning. Finding the source of  malfunctions of plants, and training sewage plant managers and operators on the job and through master courses. So I had ample opportunity to observe many accidents.
Our first plants had no railings around the basins and many of them even no fence around the plant. (This is confirmed by the movie you have watched above.) And there were no exit ladders from the basins. About 1985 rails became mandatory and from 2000 entrance doors of facilities had to be locked permanently by law.
But the railings are causing also a continual problem. For cleaning the inside of the walls of the basins and the air supply tubes with high pressure cleaners, the operators must climb over the rails for a good access to the walls. And most drownings (3 in our companies160 plants so far) were caused by such acrobatic and risky operations.
The first accident occurred in 1970 when a sewage operator drowned in his sludge thickener. There were no ladders attached to the wall and the water/sludge level was about 3 meters below the rim. The next morning only the hat of the poor operator was found floating on the surface.
And one operator wrestled with an obstinate hose upon the roof of a digester. Suddenly the hose gave way and the operator fell from the roof into the waste water receiver channel. Badly hurt he drowned there.

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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Last updated: Sept. 4th, 2011