Processing Finished Compost

Emptying a compost bin is always exciting. It is the time to see the fruit of all the hard work. Here we talk about the unloading activities.

Visual checks

Towards the end of the cooling or maturing phases of a bin cycle, a visual check is applied to determine if the compost is ready to be used. Visual checks are in reality performed a couple of weeks earlier on a regular (weekly) basis to ascertain its readiness: The bin is stirred to check:

• Whether there are any worms left: I would expect to have significantly less worms when compost is ready to use; so if there are still enough worms; this would suggest there is still activity taking place. I would then leave it for a week or so and then check again

• A texture test: By picking a handful of compost material  to check & feel that the compost is crumbly and refined (not clumpy)

• Dry/moist test: Dryish or moist compost is easy to handle & sieve. I try to avoid wet compost; unless the target for this compost is to apply it unprocessed (as raw compost). If too wet, I would stir it to create air pockets, let it dry for a week and check again.

We should not need to perform smell checks because by now the compost should smell earthy. Any other smells (ammonium or sogginess) should have been sorted weeks or months earlier. The cycle would not have been given a "ready2use" status otherwise.

Prepare, unload & sieve/store

After passing the visual checks, the bin is prepared for unloading. Depending on the bin type; the preparation is different: For example, for dalek bins (A,B,C), wood panels are placed on the sides of the bin to contain any material that spills over when the bin is removed. The bin is slightly shaken  on the edges as well as carefully moved vertically and side-ways to allow easy removal in an upward sleeve fashion (dalek bins are bottomless). The bin is emptied in full and measured using the 12L bucket as the unit of measurement.

Depending on the period of the year and our compost needs, this compost material is handled in different ways: this compost is then put in different types of containers. There is a priority list:

1. Have a 120L sieved compost reserve (backup) for seed trays, house plants and other eventualities

2. Spread equally the remaining compost on all vegetable raised beds, pots and soil (this is calculated on expected volume of compost for a season)

3. Store in bags: If the vegetable plots are (fully) occupied (typically March to November), then store it in large bags. 

Image below is an extract from back garden layout and it depicts the compost bags area, with six different containers (125L bags).

• In Winter, one of the empty raised beds could be used as a storage container to store the compost, covered by  a (transparent) corrugated PVC panel. This will help the compost to dry further (for sieving). A  sheet or  carpet is layered before the compost is added to separate it from raised bed soil. 

  • There have been occasions where the compost has been returned to its initial bin. The purpose of emptying it is to measure it and validate that it is ready for use (and at the same time, give it an additional good mix).

• If the compost is moist/dry, I will sieve enough of it to fill up the reserve compost bag

• In Autumn and early Spring, the vegetable garden (including raised beds and pots) get a top dressing of compost; so on the day of emptying the bin, the compost is applied directly and mixed with soil.

Green/brown balance (C:N carbon: nitrogen ratio)

Another visual check is used to determine whether the green/brown balance was good enough for this  compost cycle. In my experience, invariably if there is any unbalance, it would be on a high brown/green ratio. If there is too much green, the smell indicator is compelling (ammonia smell) and hence easy to rectify (by adding browns). But the opposite is more difficult to detect. Too much brown will cool off the decomposition activity during the active phase (aka thermophilic):

• So if when emptying the bin, there are still (too many) residual leaves (my main brown source); it means this bin cycle was on a higher brown/green ratio; equally if the compost material is too clunky and I have used shredded cardboard material, it is likely to be a high brown/green ratio.

• I have been using C:N ratio tables  (see below) to try and get the balance right. The target C:N ratio is around 30:1 (by mass or weight) and getting the right proportion and measurement is key. Sometimes estimating this is difficult, not least because of moisture content among other environmental elements in the IW (Input Waste) constituents. Over time, by experience I learnt (more often than not) how to balance green/brown; but early years were quite trial and error.

Full or partial removal

There are different ways to access finished home compost. Some would leave it in the compost bin and take out as needed using for example a slide door. I have opted to fully empty each bin and measure it. This is part of the Mellow lab project, being able to measure output against input and derive ratios (taking into account other parameters).

Removing compost material as and when needed (even if measured) would somehow make the calculation and estimation more difficult because compost continues to reduce/decay with time (albeit at a much slower pace during the cooling and maturing phases then earlier phases). In addition, there is the practicality of accessing compost bins from side doors as opposed to storing in larger open containers.

Estimating the compost volume

One of the other key activities is the measurement part of the compost volume and whether it corresponds to the expected volume. We use three different methods for estimating the volume:

1. Unloading volume: The simplest of the three; based on the volume cleared from the bin; measured by the number of buckets. This is the benchmark measurement

2. Bin depth: Prior to emptying a bin, we measure the depth of compost level relative to the top of the bin and calculate the compost volume (knowing the geometric shape, parameters and equation for each bin)

3. By ratios: Using previous finished compost to input waste (IW) ratios, we estimate the expected volume (knowing the IW volume for this cycle).