Hot Composting - A Cover Up?


Photo 1: HotBin next to Conventional Garden Composter

There are three main reasons why you should hot compost your garden and kitchen waste using, for example, a HotBin:

  1. to speed up the multi-year composting process
  2. to kill off weeds and pathogens
  3. to allow cooked food, including meat, to be composted without attracting vermin
Although the availability of garden waste decreases in the winter months, you may want to keep composting through the colder weather with kitchen scraps, etc. Hot composting over the winter presents its own challenges such as having sufficient quantities of waste and maintaining compost temperatures (50 ℃ to 60 ℃) in sub-zero conditions.

A well-insulated bin is top priority (Photo 1). A little ventilation is necessary because air (i.e. oxygen) is necessary for the aerobic decomposition of vegetative waste. Too much ventilation, especially in sub-zero winter temperatures, might cool the composting material too much. You will not be surprised to hear that the HotBin has all these requirements covered.

In my experience, the HotBin and Super Compost Bin often benefit from a cover laid over the compost surface, especially when: (i) the bins are only partially full, and (ii) during cold weather.

Photo 2: Fleece Cover in Super Compost Bin

If the bin is only part-filled then the cover helps retain heat within the compost pile rather than heating the air space above (before escaping via the lid vent). When the bin is at full working temperature (60 ℃ to 70 ℃), it has a tendency to dry out quickly and the mat helps retain some of the moisture. [Note: remove the mat if the compost is too wet and let it dry out before replacing it].

Up until recently, I have always used a fleece-type material, typically capillary matting - the thicker the better - cut to fit loosely over the top of the compost (Photo 2). As the weather started to get colder (late November), I thought I'd try a more reflective non-adsorbent material that would reflect heat (infra red) back into the compost pile but allow moisture to escape.

Photo 3: Reflective Indoor Grow Sheeting

The material is easy to cut with scissors and position on top of the compost ...

Photo 4: New Reflective Sheet in HotBin

The following day everything seemed to be working fine; some of the hot water vapour had condensed out on top of the sheeting; this can be discarded if the compost was wet enough or added back if it was too dry.

Photo 5: Reflective Sheet in Action

Figure 1 plots compost temperatures at 50, 30 and 10 centimetre depths; for comparison, the minimum overnight temperatures in the garden are also plotted. The cold frosty period at the end of November/beginning of December (Day 3 to Day 12, Figure 1) coincides with lower compost temperatures. The temperature of the upper active layer (top 10 cm) maintained its temperature reasonably well - it did, however, receive extra help in the form of two daily hot water bottles on days 8, 9, 10 & 11 (overnight subzero temperatures). Fresh compostable material was added every day. The arrival on Day 10 of a neighbour's green hedge trimmings (Photo 6) greatly helped.

Figure 1: Daily Compost Temperatures at 50, 30 & 10 cm & Minimum Daily Temperature

Photo 6: Fresh Supplies of Garden Waste

It is too early to say whether the reflective mat is working as I hoped but initial results are encouraging. Compost temperatures drop noticeably when the outside temperature drops below 0 ℃. This effect is smaller in the upper layer provided sufficient fresh easily composted material (e.g. kitchen waste) is added regularly. The addition of hot water bottles helps especially when the outdoor temperatures fall well below zero (Days 10 & 11, Figure 1).

Since a plentiful supply of green compostable material arrived (Day 10, Figure 1) in the form of a neighbour's hedge trimmings, there has been a significant improvement in the overall performance of the HotBin. The temperatures at 30 cm and 50 cm depths have, in particular, been boosted. Whether this is due to the mat reflecting heat, generated in the active top layer, back into the heap is difficult to say. But I'd like to think it is a factor. The dip in temperature on Day 21 (Figure 1) came about because I removed 80 litres of already composted material from the HotBin hatch; the measured temperature at 50 cm depth was very close to the baseplate of the HotBin.

The conditions on Day 23 (Figure 1) are close to ideal with the top 50 cm of the compost pile operating at 50 ℃ or above. A slightly hotter upper layer (top 10 cm), say 60 ℃, would be nice but one mustn't be greedy.

The new reflective mat is easier to use than the old fleece mat which would absorb water and need to be wrung out everyday. The experiment continues ... 


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