Hot Composting #4

If you are thinking of going down the hot composting route then one of the first questions you will want to ask yourself is: what size of hot compost bin will I need?


HOTBINs come in 3 sizes: 100, 200 and 450 litres. The Green Johanna is available in only one size (330 litres) whilst the Aerobin comes in a 200- and 400-litre formats. Other hot composters may be available.

I have three 200-litre hot composters - 1 x HOTBIN and 2 x Super Compost Bins*

[* prototype from SoilFixer - Disclaimer: I received the first one gratis for trialling/reviewing and bought the second one. The prototype is now available as the HOTBIN Mega]

Figure 1: My HOTBIN

Figure 2: One of my Super Compost Bins ready for emptying

Modes of Operation for Hot Composting

Generally, there are two basic ways to operate your hot composting bin and this will impact the amount of garden waste that can be processed:

Mode A: Fill the bin with garden waste and ignore for 30 days once you are happy that the operating temperature (60 ℃) has been reached during the initial stages. If your garden waste comes in bulk at certain times of the year then this method involves minimum effort. However, it is not the most efficient way of processing large quantities of organic garden waste unless you have lots of hot composting bins (expensive!). And you will end up with a less than half-full bin of composted matter to spread onto the garden.

With a slight modification, this procedure can be improved upon. Start by filling the bin about two-thirds full with mix of green and brown garden waste and continue to add more daily to rapidly fill the bin. It usually takes a couple of days to reach optimum temperatures (50 to 65 ℃) during these early stages. As the compost breaks down and reduces in volume, continue to add more compost up to about the twentieth day (the time varies from one compost run to another). As soon as the active compost temperature at the top of the bin drops to 50 ℃ or below, stop adding organic waste and leave the bin contents to mature for at least 2-3 months. Compost yield was about 120 litres.

Figure 3

An example of this modified approach using one of my Super Compost Bins is illustrated in Figure 3. Although nominally a 200-litre bin, the internal dimensions are approximately 180 litres. The bin was filled over the first two days and checked to ensure it reached 'operating' temperatures; compost temperatures were measured with probe thermometers at three depths: 50 cm, 30 cm and 10 cm. After a short holiday break, I returned on Day 5 to find everything hunky dory and all bin temperatures within the 'Goldilocks' range (40 -60 ℃). Further garden waste was added daily up to Day 21, then stopped once the temperature in the upper (i.e. most active) part of the compost heap (Temp@10cm) dropped below 50 ℃. The compost bin was left to mature without any more additions; it is currently at Day 80 and will probably continue its maturation process for at least another 6 months.

As Figure 3 shows, the modified Mode A method was able to process 400 litres of garden waste or twice as much as the 'standard' Mode A procedure. It is worth noting here that I shred my garden waste, which effectively reduces its volume by about 50%; all volumes reported here are shredded waste volumes. If you add your garden waste 'as it comes' then you will not be able to process as much; however, if you cut up your garden waste into one-inch (2.5 cm) pieces then you should achieve similar volume reductions in the composting process.

Mode B: By taking a more leisurely route to filling your hot composting bin, you will be able to process even greater quantities of garden waste. And, consequently, you'll need fewer hot composting bins.

Start the new hot composting process by approximately half-filling the bin with your mix of green and brown vegetation. The contents should heat up to 40 - 60 ℃ in a day or so. Continue to add small amounts daily (2 - 10 litres) or when compostable material becomes available. This method is ideal if your garden/kitchen waste comes in small batches. You can start the process with as little as one-third of a bin provided you are using an efficient hot composter such as the HotBin. An advantage of this method is that it gives you more control over the initial start-up to ensure hot composting temperatures (40 - 60 ℃) are reached asap. Should anything go wrong (and no process is perfect), it is easier to clear out a half-full bin than a full one. An example of a Mode B run is illustrated in Figure 4.

Figure 4

Over 1000 litres of shredded garden waste was processed in this particular compost run (Figure 4) with approximately 250 litres of finished compost produced. You will have noticed, I'm sure, that the lengths of these two composting processes (Figures 3 and 4) appear very different. In practice, they were not too dissimilar: 80+ days (modified Mode A, Figure 3) compared with 145 days (Mode B, Figure 4) and double the yield of finished compost in the Mode B process. Furthermore, you would need at least two hot composters to operate in a continuous manner with Mode A. As another example, my current HotBin has been running continuously for 443 days; I will report on that once it has finished.

So there are two basic methods: (i) fill as fast as you can, or (ii) fill regularly and slowly. In fact, there is a third way which is to use a hybrid of both methods and adapt according to the seasons and availability of garden waste. I use the hybrid method, but erring more toward Mode B, because that fits in best with my accessibility to garden waste.

As a general rule, I start a bin by filling between one-third and one-half full with a 50:50 (approx) mix of green and browns. The bin reaches its operating temperature (40 - 60 ℃) within 24-48 hours after which I will add 6 - 10 litres per day on average. I am in the garden doing jobs most days so this routine is convenient for me. I keep an 80-litre bin filled with shredded garden waste close to the hot bins for these daily top-ups. If I have a large amount of garden waste to compost then I will add greater quantities or fill the bin - I will often do this if we are going away for a week. I monitor the temperatures during the hot composting process at depths of 50 cm, 30 cm and 10 cm so I can check the hot composting process is proceeding satisfactorily and identify any problems early.

When considering how many hot composters we require, these are the things we need to think about:

  • how much garden waste do we want to compost?
  • which hot composting method are we going to use?
  • do we want to compost all year round or just in the warmer high-growth months?
  • do we have an area/space where we can finish the composting process outside of a hot composting bin?
Let's start by working out how much compostable matter we will produce by looking at the example of our garden.

Estimating the Number of Hot Composters Needed from the Size of your Garden

I operate 3 x 200L hot composters: two or three of them will be in action during summer and autumn when vegetative growth is fastest but only one will be kept operational during winter. Apart from my own garden waste, I collect and process material from four neighbours.

  1. Our rear garden is just under a tenth of an acre (370 ㎡) of which about half comprises hard surfaces (patio, paths, sheds, greenhouse). So just under 200 ㎡ for growing flowers, trees, bushes, vegetables and fruit. There is no lawn. On its own, the garden would provide sufficient garden waste for between one and two 200-litre hot composting bins.
  2. My next-door neighbour has a small lawn (25 ㎡) and donates all their grass mowings plus a few clippings, mainly the overhangs of the hedge, clematis and jasmine that border both properties.
  3. My other next-door neighbour donates about 300 litres of garden waste per annum (mainly dried palm leaves and cherry tree prunings).
  4. Our neighbours/friends across the road supply about 1500 litres of, mainly, conifer and holly hedge cuttings once a year.

Hedge trimmings from a neighbour
    5. Our next-but-one neighbour had a garden of similar size to ours (200 ㎡) but mainly laid to lawn with a number of mature trees, including several old apple trees, plus some flower borders and a large pond. I did not get the lawn cuttings but did get all the other prunings, fallen apples and garden waste. They recently moved into a new house with a much smaller garden (~75 ㎡) a few miles away so I no longer get the garden waste from the larger garden. However, they still bring me their garden waste in addition to a monthly 60-litre bag of spent church flowers. Strangely, I now receive the lawn clippings although they are usually yellowing and part-composted and matted by the time I get them.

In summary, my three hot compost bins (total nominal volume = 600 litres) were in constant use and worked hard processing garden waste from just over a fifth of an acre. Since our neighbour's move, we only receive garden waste from between 0.1 to 0.125 acres and my three bins are not worked as hard. Indeed,  I could probably get by with just two bins.

Estimating the Number of Hot Composters Needed from the Volume of Garden Waste

Garden waste, as received, is often bulky (e.g. whole plants including roots, hedge cuttings, fruit tree prunings). Ideally this needs to be chopped or shredded to increase the ease and speed of the composting process.

Apart from grass mowings and kitchen waste, pretty much everything else will go through my shredder to reduce the bulk by about 50%. [note: shredding efficiency depends on the sharpness of the rotating blades  in my Bosch Shredder and I'll admit to not changing the blades often enough].

Shredded hedge trimmings

Shredded material is stored in an 80L bin sited near the hot composters ...
 
80L Bins for Garden Waste

If you have a good feel for the quantities of garden and kitchen waste you produce each week, month or year then you can have a pretty good guess at how many hot composting bins you need. As discussed earlier in this post, a single HotBin can handle up to 2000 litres of chopped or shredded garden waste  even if you defer from composting over winter. Since 2000 litres of shredded compost equates to about 4000 litres of 'raw' garden waste (hedge trimmings, hard- & softwood prunings, old bedding plants, fallen leaves, deadheaded flowers, etc), it means a single HotBin can process 50 x 80L standard plastic dustbins or 6/7 one-tonne bulk carriers of raw garden waste per year. On a somewhat more human-scale, a single 200L HotBin needs, on average, a minimum of 2 litres of chopped kitchen/garden waste per day and can comfortably handle up to 10 litres per day.

Summary

A 200-litre Hotbin is sufficient for processing between 15 and 70 litres per week of chopped kitchen and garden waste. In terms of garden size (i.e. the area vegetatively planted and excluding hard surfaces), I estimate something of the order of 100 square metres (10 m x 10 m) or, in old money, 1000 square feet (30 ft x 35 ft) could be handled by a single HotBin (200 L). Smaller and larger Hotbins are available should you need to scale up or down.

This has been a rather rambling post. I started it some time ago. It was meant to be short but just grew and grew. If anything is not clear, please get in touch. Hopefully, it contains some useful information. 


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