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Can I add urine to my HOTBIN?

We agree urine  works well for cold composting, but it is not good in the HOTBIN. This is due to the nature of removing excess water during the hot composting process.

Many gardening and composting sites recommend adding urine onto compost heaps to help speed things up. This basically means ‘taking a pee on the compost heap’. (If you are looking for advice on composing baby nappies search for our nappy post or contact us).

Why is the HOTBIN different?

To answer this we need to look at what urine is and why it works for cold composting but not for hot composting.

Urine is about 98% water and the rest is mainly a chemical called urea. (There are sometimes small amounts of amino acids and other things like antibiotics if you are taking medication for urinary tract infections). Urea is a source of ‘food’ and nitrogen for bacteria – they eat it. It is lots of small molecules in a water solution – these properties of urine allow it be digested very quickly which means fast growth and heat – it is good news so far!

There are two main home composting methods – hot and cold. Almost all the advice regarding urine is for cold composting. Composting is the reverse of plant life where carbon dioxide and water are combined in photosynthesis to make sugars and carbohydrates. In composting the reverse happens – and you end up (eventually) with carbon dioxide and water. Composting releases the water – there are two places the water can go – it can drain into the soil or evaporate into the air.

In hot composting (and specifically in the HOTBIN), significant heat energy has to go into removing excess water from food waste and grass (both are >80%water). Adding more water (via urine) should therefore be avoided because you are just adding 500 ml of water to get a few grammes of urea (food). The extra water in urine is just a huge negative in the HOTBIN.

In cold composting, you are accepting the water will drain in to the ground, and hence you are adding 2% (a few grammes) of nitrogen.

Although saying that if you have exceptionally dry materials such as bagged straw, wetting it with urine and then placing in the HOTBIN would work very well! Which is why it is OK to put soiled pet bedding into the HOTBIN.

If you are struggling, and it can be a little harder in winter as there is less waste, your best bet is to try a handful of chicken pellets. You will add more nitrogen and more food (poo) for the bacteria and as pellets are dry so it avoids the issue of excess water. You can also kick start it in winter using the hot water bottle.

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