Can I add urine to the HOTBIN?

Friday, 6 January 2012  |  HOTBIN Composting

Many gardening and composting sites recommend adding urine onto compost heap to help speed things up. With cold composting this provides the bacteria with a source of food which can be digested quickly producing heat rapidly.

WHY IS THE HOTBIN DIFFERENT?

To answer this we need to look at why urine is added to a compost heap/pile and what benefits it provides.

Urine is about 98% water and the rest is mainly a chemical called urea. (Although there can sometimes be small amounts of amino acids and other things like antibiotics if you are taking certain medications). Urea is a source of food (nitrogen) for the bacteria, it is made up of lots of small molecules in a water solution – these properties allow urine be digested very quickly which means fast growth and heat.

Almost all the advice regarding adding urine is for cold composting to provide a heat source and get the waste decomposing quicker.

Composting bacteria digest the sugars and carbohydrates leaving carbon dioxide and water. This water then has to be removed to create crumbly rich compost and there are two places the water can go – it can drain into the soil or evaporate into the air.

In cold composting, you are accepting the water will either drain into the ground or evaporate and hence you are only adding the nitrogen rich food (2% = a few grams) to the compost pile/bin.

However in hot composting, specifically HOTBIN, significant heat energy has to go into removing the excess water from waste, especially food waste and grass both of which are >80% water. Adding additional liquid (urine) should therefore be avoided because you are just adding more water to get a few grams of urea (food) which will have very minimal value to the bacteria in comparison to the kitchen/garden waste being added.