Lime and manure a dangerous cocktail

The hot lime used in some parts of the 
world to treat raw manure poses a fire risk

If you’ve had that rare experience of watching your outhouse burn down because some fool dumped too much hot lime down the hole, then you know liming raw manure can be dangerous.

None-the-less, thermal drying of manure using hot lime (CaO), sometimes known as quick lime, is used in treating raw manure in the Flanders area of Belgium, and maybe for good reasons.

This small area, with only 1.65 million acres of arable land, is known as one of the worst polluting areas of Europe. Flanders is characterised by intensive hog and poultry operations, which produce an annual phosphorus pentoxide (P2O5) surplus of 27 million pounds and an annual N surplus of 48 million pounds for the region.

The EU and the Belgium government have ruled that the region has far more livestock waste than the ground can accept. The government has decreed that no animal based nutrient, meaning manure or animal by-products, can be returned to Flemish arable land.

That’s where thermal drying and liming enter the scene. Thermal drying is expensive, using wood or gas as the fuel to heat manure. The process employs belt dryers, rotary drums, fluidized beds and other such machines.

While the process is expensive, the result is that solid manure is brought down to a favourable dry matter content of 85 percent. In addition to that, it has been rendered hygienic in the process so it can be transported to any European destination.


At that low moisture content, it’s light enough that it can also be made into pellets for long-distance export. The downside is that thermal drying is expensive because it consumes a lot of energy. There is also a high fire risk.

Liming is used to treat manure solids, despite the obvious risk. When quick lime is added to solid manure, the pH quickly rises to 12 and the temperature quickly rises to 70 C. This 70 C mark is important to manure exporters because this is the temperature at which manure solids must be held for one hour to kill pathogens, thus making those solids eligible to cross the border to northern France.

However, in the case of outhouse maintenance, too much hot lime quickly pushes that temperature far beyond 70 C and you can kiss your outhouse goodbye.