There are many taboos associated with human excrement. Most of us try not to mention the subject. And when we do we tend to avoid plain, long-established, four-letter words. Instead we hide behind the Latin (excrement), fall back on baby-talk (poo), or resort to one of the wide variety of euphemisms. This attitude makes it difficult to have a reasonable discussion about the use of sewage sludge in agriculture.

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I was introduced to the debate about sewage sludge by Countryfile on BBC TV. The basic issue is what to do with the solid by-products of the treatment of waste water. Up until a few decades ago, even in highly-developed societies, sewage was often pumped into rivers or dumped in the sea. These days, we are more aware of the damage this can cause to the environment and to human health. Incineration and landfill are also recognised as unsustainable approaches.

So, water companies are spending huge sums of money on sophisticated equipment for processing the solid residues that are left over after water purification. They prefer to call the end product biosolids rather than sewage sludge. Euphemisms are important in this field. This super-clean sludge can be recycled as fertilizer for agriculture. But some people don't like the idea of spreading fertilizer made from human waste on fields that will grow crops for human consumption. Tabloid aditors play on these fears with headlines such as, 'Human sewage used for cereals'.

Big studies in the US and in the UK have shown that the risks to humans from recycling sewage sludge as fertilizer are extremely small. However, supermarkets are still nervous about customer perception. The British Retail Consortium and Water UK have reached an agreement known as the Safe Sludge Matrix, which specifies the conditions under which processed sludge can be used as fertilizer for all sorts of crops. The Safe Sludge Matrix states that 'enhanced treated sludge' will be free from salmonella and that 99.9999% of other pathogens will have been destroyed.

Risk Communication

This very low risk is not being communicated to the public. The first figure I heard was in the BBC's Countryfile, where a spokesperson for Anglian Water told us that their new £27 million sludge treatment plant kills 99.99% of pathogens. I thought, 'OK, that's good, but not impressive'. I now know that the treatment plant is over 100 times more impressive than the spokesperson claimed!

The tabloid 'Human sewage used for cereals' story mentioned a figure of 99.9%. Another press report had only 99% of pathogens being killed. That's a factor of 10,000 away from the correct figure! The remarkable thing about all these media stories is that the text they use is very similar, which suggests to me that they were all working from the same press release (or copying one another). And yet, the efficiency of the treatment plant varies widely from one story to another. It looks as if an editor decided that 99.9999% was too complicated for their readers so the figure was 'simplified'.

How many decimal places can a typical person cope with? It's a valid question for risk communicators to ask. I suggest that the answer is one - or maybe none at all. We need to bear this in mind when formulating risk communications. Is biosolid fertilizer safe? We'll never know if we can't even talk about it!