Recovering a 'Cold' Hot Compost Bin

 As a year-round hot composter, some planning is needed when going on holiday. In my experience, adding 6 - 10 litres of compostable material daily helps maintain a healthy & hot compost pile. When going away for a few days, simply add larger volumes and/or fill the bin to capacity. The HotBin will happily maintain high temperatures for 3 - 4 days with no interventions.

Photo 1: HotBin Composter alongside a Maturation Bin

For example, we were planning to be away for 4 days in early October so I started preparing the HotBin a few days earlier:

Day 1: The HotBin was full with a working temperature of 63 ℃ at 10 cm depth. 80L composted matter was extracted from the HotBin hatch and transferred to a maturation bin to make space for fresh compostable material. 40L of shredded garden waste was added to the bin.

Day 2: The upper layer had warmed to 58 ℃ at 10 cm depth. Another 40L of shredded garden waste was added.

Day 3: Working temperature now 70 ℃. 20L of mixed garden waste added in the morning before departing for a well-earned holiday.

Day 7: Arrived back from holiday late afternoon to find the working compost temperature was 60 ℃ at 10 cm depth. Add 20L shredded garden waste and continue ad infinitum.

But what happens for longer periods of absence when the core working temperature may drop below 40 ℃? You could leave a pile of garden waste beside the bin and ask a neighbour, friend, or relative to visit daily and add a little to the HotBin. Maybe that is taking friendship and neighbourliness too far. Otherwise, just follow the procedure above and accept the compost pile will have cooled below the optimum hot composting temperatures by the time you return. You can then either (i) allow the bin contents to continue composting at the lower temperatures (20 - 40 ℃), or (ii) reinvigorate the contents back to full hot composting temperatures (50 - 70 ℃).

I had reasons to put option (ii) into practice following our delayed stay in the Isle of Wight because my two other hot composters were already at the option (i) stage.

I had prepared the HotBin for a 5-day absence by topping up with about 80L shredded garden waste before departure. The HotBin was up to working temperature (>60 ℃ at 10 cm depth) before leaving. Unfortunately, homecoming was 10 days later by which time the working compost temperature had dropped to 35 ℃.

There was still enough heat in the HotBin contents to restart the hot composting process but first I needed to ascertain the 'wetness' of the cooled pile. A cooled pile that is dry is easier to restart than a wet one because it takes a lot of energy to heat that additional water.

If your pile is dry then just mixing fresh lawn mowings (lots of nitrogen and moisture) into the top 15 - 30 cm of the heap should be sufficient to restart the hot composting process. If you do not have grass cuttings you will need another source of nitrogen/moisture such as kitchen waste, green vegetation (including weeds), urine, urea, comfrey/nettle tea, poultry manure, and diluted fertilizer to kick start the process.

If your compost pile is wet, as mine was, you must first add some absorbent material; e.g. shredded card/paper or dry coir pieces. By absorbing 'free' water, you open up the structure of the compost heap and create the essential air pockets needed for the decomposition process to restart and accelerate. You might want to add some bulking agent as well; I don't because my shredded garden waste contains plenty of woody material already.

Once any wet/dry issues have been addressed, it is time to start adding heat and fresh compostable material to restart the hot composting process.

The simplest way to add heat is to use the 'hot water bottle' supplied with your HotBin. My original bottle gave up the ghost a few years ago so I repurposed a couple of 2.5L bottles that had originally contained organic patio cleaner. Any HDPE plastic bottle should be up to the job but be careful as you will be adding boiling water. My standard practice is to use two hot water bottles filled with about 2L each of freshly boiled water. If you only use 1 bottle then the reheating process will take a little longer.

We can calculate the amount of energy (kilojoules, kJ) stored and/or transferred from our two hot water bottles to the compost heap using the formula:

Stored/Transferred Energy (kJ) = c  x m x ΔT            (1)

where c is the specific heat capacity of water (4.18 kJ/kg.⁰C), m is the mass of water (kg), and ΔT is the difference between the initial and final water temperatures (⁰C) of the hot water bottle.

Suppose I fill my two 2.5L hot water bottles with 4L of boiling water and, by the time I have buried them in the top of the compost pile, their temperatures have dropped to 90 ⁰C. Over a period of time (e.g. overnight) the water temperature in the bottles falls to 40 ⁰C (i.e. ΔT = 50 ⁰C). From equation (1) we calculate that 836 kJ of energy stored in the hot water bottles has transferred to the compost heap.

To raise the temperature of 1 kg water by 1 ⁰C requires 4.18 kJ of energy; to raise the temperature by 10 ⁰C will require ten times as much, i.e. 41.8 kJ. Therefore, 836 kJ will increase the temperature of 20 kg of water by 10 ⁰C. If we assume the compost heap has a water content of 50%, then burying two hot water bottles in the top 40-50L of the compost heap should increase its temperature by 10 ⁰C.

In the presence of fresh compostable material, even a relatively modest temperature boost of 10 ⁰C will often be sufficient to restart the heat-generating bacterial decomposition of garden waste. You can speed up this process by replenishing the hot water bottles daily until the hot composting process becomes self-sustaining.

In practice, this is what the process looks like:

Day 1: 80L composted material was removed via the HotBin hatch & transferred to maturation bins (Photo 1) to make room for adding fresh garden waste. After resettling the HotBin contents (Photo 2), the compost temperatures were remeasured at 30 cm depth (26 ⁰C) and 10 cm depth (36 ⁰C).

Photo 2: HotBin Before Adding Hot Water Bottles

Two hot (90 ⁰C) water bottles were buried in the upper layer of the compost and covered with 8L shredded cardboard and 12L garden/kitchen waste. Within three hours, the temperatures at 30 cm and 10 cm had risen to 38 ⁰C and 48 ⁰C respectively.

Day 2: Overnight, the temperatures at 30 cm and 10 cm had settled at 42 ⁰C and 45 ⁰C respectively. Both hot water bottles were emptied, refilled with boiling water, and buried in the top layer with a covering of 8L shredded cardboard and 10L garden waste.

Day 3: Temperatures at 30 cm and 10 cm were now 53 ⁰C and 57 ⁰C respectively and the HotBin was now self-sustaining (Video 1). 4L shredded cardboard and 12L garden/kitchen waste were added.

Day 4: Temperatures at 30 cm and 10 cm were now 57 ⁰C and 56 ⁰C respectively. Add 12L garden waste.

Video 1: Reinvigorated HotBin (Day 4)

Day 5: Temperatures at 50 cm, 30 cm, and 10 cm were now 53 ⁰C, 57 ⁰C and 60 ⁰C respectively. Add 10L garden waste and 8L shredded cardboard.

Day 6: Temperatures at 50 cm, 30 cm, and 10 cm were now 50 ⁰C, 58 ⁰C and 67 ⁰C respectively. The hot composting process was now in supercharge mode!

Note: I always use a breathable mat on top of the compost (see Video 1) to help retain heat and moisture. I find this works for me though it is not recommended by the suppliers of hot composting bins.


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