Where there's muck there's brass. And grass: farmers spread millions of tons of cow manure on fields each year to enrich soil. But animal waste is often contaminated with bacteria dangerous to human health. Now a team of US researchers are suggesting that stored manure can be sterilized cheaply and easily with carbonates -- the chemicals that give fizzy drinks their bubbles. Where there's muck, in this case, there's gas.

"Farm-scale testing will be needed before the procedure is recommended to the cattle industry," says James Russell, a microbiologist at the University of Cornell, New York, and the US Department of Agriculture. "But laboratory tests indicate that carbonate is a highly effective antibacterial agent."

Cattle urine can kill some of the bugs in cow faeces when the two mix naturally to form manure. An faecal enzyme breaks down urea to carbon dioxide. Much of this carbon dioxide converts to carbonate because of the alkaline conditions in manure. Russell's team have traced urine's antibacterial action to this carbonate.

But the manure reaction alone is an unreliable carbonate source. "The ratio of faeces to urine has to be approximately 1:1 [to kill bacteria]," Russell's team explain in the journal Environmental Science and Technology, and cattle produce more than twice as much solid as liquid waste. This means less carbonate and more bacteria.

Lacing manure with sodium carbonate (commonly called 'soda') is the answer, Russell's team suggest. As little as 4 grams of this powder kills almost all of the harmful bacteria in a kilo of cow dung, they find. (But this would not eliminate the risks, as cows will still spread fresh -- and more contaminated -- waste directly on fields.)

Some sodium hydroxide ('caustic soda') is required to keep manure alkaline enough to trap the carbonate. Both chemicals could be added to the storage tanks that hold liquid slurry manure, the group suggests. Russell's team suggest that potassium carbonate is an effective, but more expensive, alternative if there prove to be pollution problems with sodium carbonate.

Carbonate is an unlikely antibacterial agent, and Russell agrees that his team were surprised when they discovered that it was the active ingredient in cattle urine. "But a literature search indicated that bicarbonate and carbonate are toxic to bacteria, especially at alkaline pH," he says. "We are currently working on the mechanism of action, but it is too early to say much," he adds.

Previous research has suggested that chemical sterilization of cow manure is too expensive to be practical. But Russell estimates that it could now be done for as little as $10 per cow per year.

A cheap way of making manure safer could be welcome. Virulent bacterial strains like Eschericia coli 0157 are on the increase, and cattle waste is one source of infection. For example, a mini-outbreak of E. coli 0157 occurred at a 1997 music festival at Glastonbury, England. The festival is held on a working dairy farm, and over 500 cows grazed on the site a week before the festival. Heavy rain turned the fields to mud, which people were unable to remove from their hands and faces before eating.

James Stuart, an epidemiologist at the UK's Public Health Laboratory Service who studied the Glastonbury outbreak is cautious about the value of sterilizing manure. "It sounds a reasonable idea but I think you have to question what the risk reduction would be," he says. "Glastonbury was slightly different as people were directly exposed to fresh cow waste. I'm not sure how many E. coli cases are caused by manure that is stored and then spread out over fields."