Reporter 425, 19 October 1998

Sewage has never smelt as good

What does it take to mask the smell of raw sewage? Two Leeds researchers, trying to give sewage sludge a more pleasant aroma, held their noses and found out.

Smelly sewage is nothing new, in 1880 Parliament was forced to close because of the stink from the polluted Thames, but image-conscious privatised water companies are now seeking suitable ways of removing its anti-social odour.

“They’ve tried scrubbing it and have even added pine-smelling perfume to mask the smell,” said Dr Nigel Horan of the School of Civil Engineering, “but generally that just makes the whole thing smell ten-times worse.”

Raw sludge is first treated by removing the water and disposing of the remaining solids. This treated sludge starts to smell as it ‘goes off’, when the air is exhausted and bacteria which release hydrogen sulphide gas begin to grow. Adding the fertiliser-based chemical, calcium nitrate prevents the growth of these microbes, and stops the generation of the smelly gas.

The calcium nitrate is too expensive to use all of the time, so the Leeds team was working on a dosing procedure which could be applied to especially foul-smelling outbreaks. “The first step is to calculate just how smelly the sewage is,” said Dr Horan, “then we can add the right amount of chemical to take the smell away.”

Water companies will be prohibited from dumping raw sewage in the sea from 2001, significantly increasing the volume of sewage which must be treated. “Sewage works used to have acres of empty land around them,” explained Dr Horan, “but increased treatment and pressure for house building means many are now virtually on people’s doorsteps.”

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