Been just dying to hear how composting with our HotRot has been going? It’s still a work in progress, but here’s what we’ve learnt so far!

We’ve been composting with our HotRot machine at Cairns St for three months now, since our auspicious beginning in Matariki 2024. This has meant lots of experimentation and learning by trial and error - quite fascinating for compost nerds like us!

The HotRot is quite a simple machine: it turns the material inside to keep it moving through and make it break down faster, blows pressurised air through to keep the material oxygenated (and stop it going anaerobic and smelly), and extracts the exhaust air, which passes through a biofilter to remove odour. While the body of the HotRot is insulated, it doesn’t heat the material inside - all the heat comes from the composting process.

Our challenge is finding the perfect balance of ingredients - food scraps, arborist mulch and coffee chaff - and the right amount of aeration and turning by the machine, to achieve:

The right amount of moisture
A good carbon-to-nitrogen ratio
The thermophilic peak (hottest part of the composting process) happening inside the machine - rather than after the compost comes out.

Food scrap to mulch ratio, moisture content, temperature and aeration all affect each other, and it can take at least a few days to see the effect of any change, so it’s quite a slow process finding this perfect balance!

We’re doing highly technical “squeeze tests” (take a small handful and squeeze hard to see how much water comes out) most days to check the moisture balance of our recipe. This is our best tool for checking if we’re on the right path, as it takes 3 weeks for us to see the end results when the compost comes out the other end of the HotRot.

Checking out compost coming out of the HotRot after 3 weeks of composting

We started out with a high proportion of mulch to food scraps (1.5:1) for our first month - much more mulch than we’d been using for composting at the farm. Erring on the carbon-heavy side is safest, as the worst thing that can happen is a slow composting process. On the other hand, having a mixture too high in food scraps (nitrogen-heavy) can lead to bad smells and sludge - not ideal.

With the 1.5:1 ratio, the compost coming out was pretty mulchy, unsurprisingly! It was also quite dry. After moving a bunch of this compost into a big box to mature, we added some water and saw the temperature shoot up by about 30 degrees. If the compost gets too dry, bacterial activity slows right down and compost temperature also goes down, which can make it seem like it’s ready to use… but adding water sparks that activity right back up, which can damage plant roots if it’s been applied to the garden already. Adding water to the maturing compost was also a bit of a time suck. Efficiency is super important to making this operation financially viable.

So for August, we trialled a 1:1 ratio of mulch to food scraps. This also worked pretty well, but the mixture still seemed on the dry side. We started adding plenty of water to our mulch wheelie bins before tipping the wheelies into the machine, and let our mulch pile get well soaked by all the rain. Turns out hosing into the bins of mulch wasn’t a good idea… the unabsorbed water went straight to the bottom of the machine and got into the air injection system (which would cause corrosion over time). And a few weeks later, compost coming out of the Hotrot was too moist, potent-smelling and compacted - and very steamy!

We stopped adding extra water stat, switched to using dry mulch, and started trying out a 2:3 ratio of mulch to food scraps.

Up to this point, we’d only measured temperature a few times, finding we were getting to around 50 degrees - the low end of our ideal range, 50-65 degrees. Temperature of at least 50 degrees for at least 3 days is needed for pasteurisation - killing off any nasty bugs.

Then one fateful day when we were taking temperature readings, the machine started up on an automatic rotation cycle, pushing compost along and bending the long thermometer, getting it irretrievably stuck in the machine… a very silly $200 mistake, in a week of silly mistakes!

The very stuck thermometer... (with the 60 cm long probe bent inside the machine). The inbuilt faulty temperature probe visible below.

Our HotRot is meant to measure compost temperature at three points along the machine as well as exhaust air temperature. But up to this point, all the temperature probes were reading 855 degrees Celsius - meaning an open circuit. Kate’s dad Jim, who’s handy with electronics, kindly stepped in to help solve this mystery. We knew that temperature readings had stopped making sense sometime in 2021, in the machine’s previous life in Auckland. Eventually we figured out the problem - that the probe wiring inside the legs of the HotRot had been chewed through by rats! Jim rewired the probes and we calibrated them, and we could at last start measuring temperature constantly - important info for getting our compost mix right.

There was plenty of room for improvement: the temperatures were too low, the mix was too wet, and the smell wasn’t great. So we started experimenting with machine parameters - turning more frequently and blasting a lot more air through - alongside going back to 1:1 mulch to food scraps and adding coffee chaff to soak up excess moisture.

This saw our temperatures go up by 10 degrees in the middle and end sections of the machine, and compost output get less moist and compacted - great progress!

We’ll be continuing to try and bring down the mulch proportions, leaving more room for food scraps - meaning we can take on more businesses and households as composting customers. Another focus is reducing smell as much as possible in the warehouse, especially before we get into summer, and the new set of challenges the heat will bring… This project has suffered huge delays, but turns out that starting operations in winter was pretty ideal!