A Hot Composting Saga (Part 5) - Filling in the Gaps

 For previous installments, see here, here, here, and here. Picking up from the last episode, The Super Compost bin had been cleaned and reassembled to improve air ingress from the bottom. After three days it was apparent the contents were not heating up sufficiently for the hot composting process to proceed under its own steam. Photo 1 is an infra-red photograph of the Super Compost bin on 24th of April 2023. The compost heap was at 25-30 ℃ and the outside air temperature was 5 ℃. Clear evidence of heat leakage around the joints and lid seal.

Photo 1: Infra-red heat map of Super Compost bin (24/4/23)

Compare this with my HotBin which was happily running at 50 ℃. As Photo 2 shows, hot air was only escaping from the vent in the lid as it is designed to do.

Photo 2: Infra-red heat map of HotBin (24/4/23)

Maybe fixing the Super Compost bin was just a simple matter of stopping the heat leakage. After emptying the bin (recycling the contents through the HotBin to complete the hot composting process), it was disassembled in readiness for draughtproofing.

Photo 3: Part disassembled Super Compost bin

I had some self-adhesive foam insulation strips left over from draughtproofing duties and used these to fill obvious gaps in the Super Compost bin - essentially along all four vertical joints after cleaning said joints with a damp rag (see Photos 4 and 5) ...

Photo 4: Applying self-adhesive foam to the side panel of Super Compost bin

Photo 5: Finished draughtproofing prior to reassembly

... before, finally, plugging any obvious gaps around the lid's rubber seal (Photo 6)...

Photo 6: Plugging small gaps around the lid seal

It is important to remember the compost bin should not be airtight - cool air needs to enter at the bottom and hot air needs to escape from the top to ensure the circulation of air through the compost pile that will keep the process aerobic.

The first opportunity to try out the 'improvements' was on the 25th of April. This was a relatively short run aimed at testing out the bin while, at the same time, reducing my backlog of garden waste. Consequently, shredded garden waste was added at a faster rate than is my normal practice. Figure 1 shows the cumulative volume (litres) of garden waste added over the 18-day period from empty to full bin - just over 300 litres in total. Note that garden waste was not added on days 12 to 15 as we were on holiday.

Figure 1: Cumulative Garden Waste (L) added to Super Compost bin

The daily compost temperatures measured in the 'active' hot composting region (30 cm and 10 cm depths) are shown in Figure 2. As noted above, we were on holiday between Day 12 and Day 15 (inclusive) hence the temperature decline from Day 11 to Day 16. In my experience, maintaining consistently high (>50 ℃) compost temperatures in the 'active' hot composting region (0 to 30 cm) is best achieved by adding fresh garden waste daily. Do not, however, let perfect be the enemy of good - composting will look after itself and a cooling pile can almost always be brought back into the hot composting zone (50 - 70 ℃).

Figure 2: Compost Temperatures at 30 & 10 centimetres depth

The infra-red photograph below (Photo 7) was taken on Day 3 when the temperature of the compost heap was 40-45 ℃. There is still some heat leakage around the lid seal but it was much reduced compared with the earlier setup (c.f. Photo 1 when the compost heap temperature was 10 - 15 ℃ lower).

Photo 7: Infra-red Heat Map of Super Compost bin (27/4/23)

A few days into the composting period (Figures 1 & 2), I added some extra draughtproofing around the rubber lid seal. Initially, this comprised strips of capillary matting (Photo 8) that were replaced later with thicker strips of recycled denim jeans (previously used as packing material). Note the 28 lb weight in front of the bin because that is an important part of this composter!

Photo 8: Additional capillary matting to seal the lid

The heat map of the Super Compost bin on Day 4/5 - when the compost temperature had reached 60 ℃ - is shown in Photo 9.

Photo 9: Infra-red heat map of Super Compost bin

Despite the extra draughtproofing around the lid there is still a hot spot (31.7 ℃) at the upper front right-hand corner. That is down to an issue with the lid of this Super Compost bin - a slight warp in that corner  creating a small gap and a preferential escape route for the hot vapour. This is where the 28 lb weight (Photo 8) comes in handy - by closing the gap when the weight is positioned on the corner. Photo 10 was taken on the 5th June when the bin was full and the compost temperature at 10 cm depth was 60 ℃.

Photo 10: Heat Map of Super Compost Bin with added weight

The hot spot between lid and base (Photo 9) has moved to the rear of the lid with a lower temperature (23.5 ℃ compared to 31.7 ℃, Photo 10). The rear lid hinge helps to restrict the gap size when the weight is added to the front of the (slightly) warped lid.

Just to confirm the Super Compost Bin was now fully operational, the bin was emptied and a fresh run started on 23rd May. This run is still current although the bin is full as of 6th June. The bin was left unattended for 6 days Days 5 - 10) while we were away dog-sitting (Photo 11):

Photo 11: Belle relaxing after a long walkies

Figure 3 summarises the Super Compost run to date (6/6/23). Total added garden waste was similar to the previous run (Figures 1 & 2) at around 300 litres. Temperatures quickly rose to those required for hot composting - somthing that had not happened before the modifications to the Super Compost bin.

Figure 3: Super Compost Run (23/5/23 to 6/6/23)

During the unattended period (Days 5 to 10 inclusive), the temperatures in the upper ('active') region dropped from over 60 ℃ down to around 40 ℃. Temperatures soon picked up, however, with the addition of more garden waste that included fresh grass cuttings.

In summary, the Super Compost bin was not achieving 'hot composting' temperatures due to poor insulation at the joints and too large an air gap between the lid and base. These defects were fixed with a little bit of draughtproofing to seal the gaps.

Some confounding factors not considered here are the make-up or composition of the added garden waste and the external ambient temperature. I do not believe the type of garden waste available was a factor because there was no shortage of fresh grass cuttings - and there is no better waste input than freshly mown grass. The ambient temperature could have been a factor - it is certainly warmer in May/June than in April but, surely, not by that much.

That just leaves my other Super Compost bin to modify  ...