Hot composting

[Dr Mike Oxgreen]

I've always been disappointed with the results of composting. It takes ages to produce anything that can be used, and can't keep up with the rate at which I produce grass clippings.

So I've bought one of these hot composting bins in the hope that I can make compost faster. The claim is that you don't need to turn the compost; you just keep adding material and provided there's some "bulking agent" the bin will stay hot, between 40-60 degrees centigrade. It can supposedly make compost in as little as three weeks, and you can compost any organic matter, including all kitchen food waste.

My main aim is to compost my grass clippings faster, because it always seems a terrible waste putting it into our green bins and letting the council compost it rather than recycling the nutrients myself.

I set it up on Friday and put a few lawnmower bags full of grass clippings in, with some shredded paper, and some of the "bulking agent" that came with it (basically looks like semi-composted bark and wood chippings). I also added some of the less decomposed material from my "normal" compost bin.

It's only about a third full, and after two days the thermometer stuck into the waste is measuring 70 degrees centigrade (about 160 Fahrenheit)!!

Any tips on hot composting? The instructions imply that the "bulking agent" is really important for keeping the mixture open and aerobic. I don't really want to buy it, and don't usually generate much in the way of woody chippings - so I'm wondering whether corrugated cardboard torn up and scrunched into random shapes would perform the same function?

Here's the bin. It has an air inlet at the bottom, and a valve at the top that you crack partly open. It's meant to work a bit like a chimney, with the heat being generated by the aerobic bacteria and creating air movement upwards through the waste. It's made of a material reminiscent of expanded polystyrene:



It has a removable panel at the bottom so you can harvest the compost.

Edited by Dr Mike Oxgreen on Sunday 19th April 16:01

[Dr Mike Oxgreen]

My top thermometer in the lid is currently showing 'only' 40C, but I reckon that's not bad considering the bin is less than a third full and it's only been going for two days. The thermometer sticking into the waste itself, though, is showing 70C, despite my turning the mixture earlier on...



I've been doing a lot more reading on the Hotbin site, and also having looked at prices I realise that you can buy 50 litres of fine bark chips from Homebase for a fiver, which isn't too bad. Certainly cheaper than £15 for 75 litres of "bulking agent" from the Hotbin site, and I reckon it would probably do much the same job. According to the Hotbin site, corrugated cardboard does not do the same job - or are they just trying to push more of their pricey bulking agent? They say that paper and cardboard just has a drying effect on the mix, not an aerating effect - which could still be important to me if I'm mostly going to be composting grass clippings.

I'm really hopeful that this is going to give me a viable way of recycling my grass clippings and keeping the nutrients within my garden. I think if it produces results quickly then I'll find the motivation to put the effort in to keep doing it, whereas I've been a bit lazy with the 'cold' composting because it takes such a long time to see the benefits.

Rude Girl makes an interesting point about not having enough material to keep it going hot in the winter, especially since we're only a household of two as well. There won't be any grass clippings, and if it does cool down it could be difficult to get it going again so that you can compost your kitchen waste.

Edited by Dr Mike Oxgreen on Sunday 19th April 19:28

[Dr Mike Oxgreen]

Esseesse said:

Flipping heck, these hot compost bins are expensive. How do they work, just insulated? Could you build your own for less?

Yes, they're pricey!!

I think two things are important: insulation, and air flow. And I suspect the air flow is the more important, so you'd need to ensure air can enter at the bottom and leave at the top.

You can do hot composting without a bin at all, but you need to build a BIG pile otherwise it won't get hot enough, and you need to keep turning it inside-out. The insulation of the bin is what allows you to do hot composting on a smaller scale, I think.

I opened the hatch at the bottom yesterday to take a look. It's not yet what I would call compost (although it's only been a week!), and quite a lot of steam came out so I assume the bacteria are still very active down there. Smelt okay: a sweet, grassy aroma rather than ammonia or rotten. Possibly a little on the wet side, so I might add more cardboard in future.

I like the idea of using it as a slow cooker!

Edited by Dr Mike Oxgreen on Saturday 25th April 08:53

[Dr Mike Oxgreen]

Just opened the bottom hatch again, now two weeks after starting. And whilst it certainly isn't fully mature compost, and is still a bit wet, you could certainly use it for mulching. I reckon in another week it could be quite nice stuff. It's still quite warm and steamy, so probably still aerobic and active.

I did scoop a potful of it out from the bottom and mixed it into the large-ish quantity of grass clippings I've just added at the top (together with some of the "bulking agent"). Hopefully this'll have two benefits: kick-starting the decomposition of the grass clippings, and de-compacting the lower layer for better aeration.

Temperature has fallen a bit, as you might expect, but has still been around 50-55 Centigrade.

Edited to add: The other thing that has occurred to me is that I could always harvest the stuff out of the bottom and chuck it into my conventional compost bins to mature for a month or two before using it, thus using the hot bin to rapidly break down the clippings to semi-composted material which is also a smaller volume. Then the cold bins will manage to convert it to fully mature compost.

Edited by Dr Mike Oxgreen on Friday 1st May 13:01

[Dr Mike Oxgreen]

An update:

Shortly after my last posting I managed to convince myself that the contents of the bin needed watering, because there was some white mould-like stuff on the surface and I read somewhere that this can mean it's too dry.

So I watered it. Possibly a bit too much. Big mistake!

The temperature started falling over the next few days down to about 30C, and I noticed some 'fruity' smells. I opened the bottom hatch and realised that it had gone anaerobic down there and was very wet - probably not letting much air through. So I decided to dig out the semi-composted stuff at the bottom and bung it into my conventional compost bin to finish it off.

This appears to have fixed the problem, because since then the temperature has gone up to 75C and stayed there, only dipping for a few hours when I add more grass clippings. I've been slightly more generous with the 'bulking agent' (I'm now using ordinary bark chips) and since then there are no smells at all.

So, lesson learnt: Don't water it! (Or at least, if you do, be very cautious.)