Compost Bins Statistics

 Facts and Figures central to bins including  number of cycles annually, decay ratios and durations for bin types and sizes

Over a number of complete bin cycles, we can see the impact of key variables such as the type of bins (bottomless, size etc..), seasonality and the available input rate of waste at our disposal for home composting.

Highlights

• A bin gets twice its size in volume  waste  over its Active phase
• The finished compost volume is about 2/3rd of the bin volume
• We complete around 5 compost bins annually
• The Active phase period depends on bin size, decay rates and seasonality

Background

First a recap on the particulars of all the bins in the compost lab to bring into context and  help in understanding the data analysis and charts below: There are five compost bins: 

Volume decay ratios & counts

The table below is an extract from the MCL dashboard (April 2024). It provides long term numbers for key attributes per bin. The key elements are three (volume) ratios between FC (finished compost),  IW (Input Waste) and BinV (the total volume of a bin). The #cycles column  indicate how many compost cycles have been completed for each bin. Bin C being the first one, it has the highest number of cycles where D and E, being more recent acquisitions have the lowest number of cycles (we operate the bins in a round robin fashion). 

The three ratios are computed at the end of every cycle. 

• The FC2BinV ratio tells you the volume of compost you get as a proportion of the bin volume. You can see that roughly speaking, you get 2/3rd of the bin volume. 

• The IW2BinV shows how much waste we can push into a bin; typically double or about of the bin volume (as it decays over time). The bin D with 1.8 being the slowest to decay compared to the other “bottomless” bins. 

• The IW2FC is the conversion (waste/compost) ratio; with all bins more or less in the same range [2.8 to 3.4], that is the compost volume is roughly ⅓ of the total waste loaded. Note we get slightly more FC with the dalek bins (~2.8 or 35%), or the decay rate is better with bins D and E (~3.3, 30%) if what matters is to reduce waste. The three ratios have some dependencies, with any two you can compute the third one.

Cycle phases periods

The chart below shows the average periods for each phase of each compost bin cycle. The cooling period is more or less the same across all bins; the decayed material continues to decompose and stabilise (at a much slower rate). It also allows for the last waste top-ups in the Active phase to fully decompose. The maturing period is really optional and often used for further decomposition or if the compost or the bin is not needed immediately

The main variation for the compost bins is the Active phase. That can easily be explained by two factors: (1) the input waste rate (long term average at ~270L/month) and (2) the IW2BinV ratio above.  So we can see that the greater the bin size the longer the Active period. The exception being bin D (tumbler) where the decay rate is not as fast because it is the only bin that is in contact with the soil (not bottomless).  

In reality, there is at least a third factor - seasonality. The long term (all seasons) IW monthly rate is  270L/m  and seasonally adjusted to 160 L/m and 375L/m for the Cool/Warm season. However, there are enough bin cycles completed in the experimental analysis that long term averages may prevail.

Annual completed bins distribution

The bins are deployed on a round robin basis. A bin should not be started if there is an existing bin in the Active phase. All the waste material ought to be loaded in a bin (s) in the Active phase. There are exceptions to these of course if the “Active” bin happens to be full; typically during the summer when we have single large loads of waste. Then we start a new bin if there are no rooms in the current Active phase bin(s). Below is the chart exhibiting the number of bin cycles finished annually and the breakdown per bin. We can see the alternance of the bind in a round robin fashion on a regular basis.

The 2017 to 2019 surge in completed bins corresponds to the large volume of waste coming from other sources (typically friends, family and neighbours). The Long term average of completed bins per year is 5 (roughly one of each A-E completes annually). This can be seen by the #cycles per bin column in the table above; roughly corresponding on the number of years a particular bin has been in operation.