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Process monitoring and troubleshooting

Effective control of composting and troubleshooting with confidence require the following:

  1. Some knowledge of the biology and the mechanics of composting.
  2. Applying this knowledge in practice. Through practice one can predict the outcome when different management actions are taken and can also anticipate potential problems.
  3. Having effective process monitoring protocols and using them.

Fortunately, monitoring of composting is a relatively simple process. Pile temperature is the best indicator of progress. Temperature must be monitored and recorded for three reasons:

  1. To provide information on the progress of composting
  2. To support operational decisions about how to manage the composting process (e.g. when to turn and water)
  3. To ensure that temperatures are high enough to eliminate pathogens and/or weed seeds.

The vast majority of pathogens are highly susceptible to composting (see Wilkinson (2007) for specific details to poultry mortality composting). The general pathogen-reduction guidelines stipulate that the entire compost mass has been subjected to temperatures above 55°C for a minimum of 3 consecutive days (USEPA, 2003). This guideline should generally be achieved within about 21 days, provided the compost has been turned a minimum of 5 times.

In a new operation, a thorough temperature profiling is needed to establish the ‘typical’ temperature pattern for the entire composting process of a given feedstock. A typical pattern should emerge after only 2 or 3 batches have been composted. When this is established, an operator can follow a simplified monitoring procedure to ensure that a given batch proceeds according to the typical pattern.

A hand-held temperature probe with a 1-m long stainless-steel shaft can be used, or a temperature reader with a thermocouple probe (Figure 18).

Photos of a bimetal thermometer and a digital thermocouple thermometer.
Figure 18. Options for monitoring temperature include (left) a bimetal thermometer (Source: REOTEMP Instrument Corporation), and (right) a digital thermocouple thermometer (Source: Cole-Parmer Instrument Company).

For routine temperature monitoring, follow these general procedures:

– Insert the temperature probe horizontally into a compost pile at half-height in at least 3 different places. Windrows longer than 10m may need more readings, e.g. every 4 or 5m of windrow length.

– Ideally, at each location, take temperature readings at three depths in the horizontal plane (e.g. 40, 70 and 100cm).

– Monitor temperatures every 2 days at the start of composting to ensure the heap reaches thermophilic conditions quickly. When thermophilic conditions are reached, twice weekly monitoring is usually enough because temperature changes slowly.

– In on-farm operations, composting for extended periods (e.g. 9–12 months), temperature monitoring can be much less frequent (e.g. every few weeks until the process is complete).

When windrows dry out and start overheating, temperature measurements alone can be misleading. Therefore, monitor moisture levels alongside the prevailing temperature regime.

With a little practice, moisture content can be estimated quite accurately with the feel of the compost in the hand. This method is used routinely to estimate moisture content in the raw mix, as well as for monitoring moisture content during composting[1].

A good analogy for compost at 50% moisture content is that of a wrung-out moist sponge – it is obviously moist, but water should not run from it if it is squeezed in the hand.

Follow this procedure for monitoring compost moisture content (Figure 19):

  1. Take a handful of compost and close your hand around it to make a fist.
  2. Tightly hold your fist closed. Observe whether any water runs between the fingers or falls to the ground. If this happens, the compost is too moist.
  3. Open your hand. If the compost has not formed into a ball on the hand, it is too dry. If it forms a ball on the hand and breaks up under light pressure, it will be at the correct moisture content.
Four photos of compost with different moisture levels being squeezed in a person's hand.
Figure 19. Method for estimating moisture content of compost. Compost is squeezed in the hand: if moisture runs through the fingers, it is too wet; if a ball in the palm does not easily form, it is too dry; if a ball forms and it breaks up under light pressure, it is at the right moisture content (about 50%).

As discussed, compost operators should become quickly familiar with how their process performs. The general pattern of performance typically looks something like this (Figure 20):

  1. When compost starts out at near ambient temperature, it reaches thermophilic conditions (>45°C) usually within 12–24 hours. The pile heats from the inside out.
  2. In the core of the pile, the hot zone gradually expands over a few days, reaching peak temperatures often above 65°C. At this point, the core temperature may start to fall as the supply of oxygen to the microbes becomes limiting. The pile is typically turned at this point.
  3. After turning, there is often a short period (a few hours) of lower temperatures before high temperatures are quickly re-established due to oxygen being replenished.
  4. Temperatures around 45–65°C can be maintained for three months or more (depending on feedstock).

At this point, peak temperatures slowly begin to decline, and the maturation phase of composting begins. Technically, composting might not be complete for another 12 weeks or more. But the compost can usually be safely stored (with little further management) when peak temperatures are within 10°C of ambient.

A graph of a typical temperature profile for turned windrows showing temperature over time and turning events.
Figure 20. A typical temperature profile for turned windrows in relation to turning events and stage of composting.

The performance pattern for mortalities can be a little different (Figures 21 and 22):

  1. In contrast to conventional composting, mortality compost piles heat up first in the outside layers. The edge of a fully constructed pile should reach >45°C within the first 2–3 days of composting. The core might take a few days to a week longer. Temperatures tend to converge after about 7–10 days.
  2. Peak temperatures well above 55°C should be expected after about 7–10 days composting.
  3. The compost just above the base layer will almost always be cooler than the upper layers of the pile. The coolest part is therefore typically at the base in the core of the pile, which could take 10 days to reach >45°C.
  4. In the upper layers, temperatures may fall gradually after about 7–10 days composting, even before the base layer temperatures begin to plateau.
  5. After the pile has been turned for the first time, the pattern of temperature development should begin to look more like a conventional compost pile.

These performance indicators were developed after extensive Australian research (Wilkinson et al., 2014).

Graph showing the pattern of temperature development for mortality composting at the edge, mid-way and core of the pile.
Figure 21. Pattern of temperature development for mortality composting. Edge – temperature taken just below capping layer, halfway up pile height; Core – in the centre of the pile in the same horizontal plane; Mid-way – between edge and core. Based on research by Wilkinson et al. (2014).
Figure 22. Schematic showing what a typical temperature profile could look like for poultry mortality composting (Wilkinson et al., 2014).

Keep in mind that these performance patterns are most relevant for mortality composting piles set up to completion in one hit. Piles that are constructed in layers over time may behave a little differently.

When a simple monitoring process has been established for temperature and moisture content, many common problems can be diagnosed and rectified by examining pile conditions (Table 4).

Table 4. Guide for diagnosing and treating temperature-related problems during composting (adapted from Rynk et al. (1992).

ProblemPossible reasonOther cluesRemedy
Pile fails to heat upMaterials too dryCan't squeeze water from materialsWater or add wet ingredients
Materials too wetPile looks soggy and slumpsAdd dry ingredients and remix; form windrows to shed rainfall
Not enough N, or slowly degrading or stable materialsVery high C:N ratio; wood feedstockAdd high N ingredients
Poor structurePile compacted; few large particles; not too wetAdjust size-reduction equipment for larger particle size; add bulking agent
Small pile sizeHeight <1mEnlarge piles or add highly degradable ingredients
Temperature falls too soonLow oxygen; need for aerationTemperature declines gradually, not sharplyTurn pile
Low moistureCan't squeeze water from materialsAdd water
Uneven temperature in pileMaterials poorly mixedVisible differences in materials alone pileTurn or remix pile
Temperature falls gradually; doesn't reheat after turningComposting nearing completionApproaching expected composting time period; adequate moisture availableNone required
Low moistureCan't squeeze water from materialsAdd water
Pile overheating (>65°C)Insufficient aerationPile is moistTurn pile
Moderate to low moisturePile damp but not wet or dryAdd water and turn
Pile too largeHeight >2.5mShorten pile
Very high temperatures (>75°C)Pyrolysis or spontaneous combustionMaterials dry; interior looks or smells charred, smell of ammoniaDecrease pile size; maintain proper moisture content; break down pile and add water to charred or smouldering sections

[1] It will not be possible to use this method in the early stages of mortality composting – only when composting is well advanced.

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