To destroy the main causes of illness and avoid epidemics, scientists at France's research centre for agricultural and environmental engineering, Cemagref, use heat exchangers for thermal treatment of the slurry, a well-known technique that they are trying to make more energy efficient.

The higher the temperature, the more effectively the micro-organisms are destroyed, so an easy solution would be to set up sterilisation programmes, e.g. treatment at over 120°C for at least 30 minutes.

But the cost of such treatments would be much too high for most farmers.

The initial goal of the research is, therefore, to create treatments that are less costly, but sufficient to meet sanitation targets.

The slurry is heated to temperatures between 55°C and 96°C and maintained at a high temperature for some time before cooling and storage.

To determine treatment effectiveness, the researchers selected a set of seven bacterial and viral indicators, representing various levels of thermo-resistance.

The results showed that heating to 60°C for 10 minutes is sufficient to make the slurry acceptable for spreading purposes. But none of the treatments up to 55°C for three days or 70°C for one hour (protocols based on those required for composting) were sufficient to destroy the most resistant pathogens.

Heating to 96°C for 10 minutes is required to destroy some of them and temperatures over 100°C would appear necessary to completely remove pathogens from the slurry.

These results induced the researchers to look for less costly solutions. For example, a second heat exchanger is used to recycle the calories from the hot slurry as it cools.

This technique recovers between 55 and 70 per cent of the energy used to heat the slurry.

Another important factor is the progressive clogging of the system, which reduces heat exchange and thus augments the cost of the process.

It will be necessary to determine the best treatment strategies for the future development of the process and the best cleaning techniques, whether mechanical or chemical.

Finally, there are plans to couple the process with a methanisation system to produce biogas as an energy source.

This research is attempting to devise more than a simple process, but rather a general method applicable to all types of liquid waste.

Reduced energy consumption will enable farmers, agri-food companies and local governments to comply with regulations and sanitation standards in the future.

December 2010