To remedy the situation, two things have to happen. First the large solids must be screened out and the fine solids removed by means of a delta separator. The clear effluent can be used either directly for irrigation (pumped into large irrigation dams where it is diluted with run-off water) or it can be ponded.

If the water goes into a pond, micro-organisms have to be added, and oxygen has to be introduced at the bottom of the pond to counter the latent anaerobic conditions.

The selected microbes can be introduced in the form of an air-dried powder, and the water has to be aerated. The simple way of doing this is a fountain that circulates the water and ensures that it absorbs oxygen as it is sprayed into the air.

This certainly prevents stagnation, but it is not very efficient. It does not get the oxygen to bottom where the helpful bacteria really need it.

A better way is to force the air right down to the bottom of the pond to reverse the anaerobic conditions. But that’s easier said than done.

If the air is released in big bubbles it stirs things up, and those bubbles shoot straight back to the surface, in this way giving up little oxygen to the water. "What we have to do," says Bob Hadley of Bio-Systems SA, "is deliver the oxygen in tiny bite-sized bubbles — something the microbes can get their teeth around."

The device that does the trick is an injector that produces such tiny bubbles that the air looks like white foam in the water.

The effect is almost miraculous. If the pond has been seeded with the right microbes, the anaerobic conditions disappear in a matter of days, and so does the unpleasant smell.