Frequently Asked Questions (FAQ): Compost Engineering

Blog ECS Staff 04/10/2023

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Compost Process Design

How long does your compost process take? How much retention time?

This is a fundamental question to designing a compost process, and of course – the answer depends on several factors such as:
• what target stability is required?
• What is the feedstock?
• What are the potential process conditions?

Let’s unpack these further. Compost stability, or maturity, is often measured by Solvita or CO2 respiration. These analyses look at how much available volatile solids still exists in the material. For modest stabilization suitable agricultural use, a Solvita Compost Maturity Index (CMI) target might range from 5-6.

This can often be achieved with a single step process, such as ~2 weeks on a well-controlled aeration floor.
A more stable finished product, suitable for residential use or horticulture, might require a Solvita CMI score of 7+. This likely requires a multi-step process, such as 2-3 weeks of primary aerated static pile composting, followed by an additional 2-4 weeks of secondary composting. One key for success is ‘re-wetting’ material after the first ~20 days of composting. Since the compost process release moisture, rapid biodegradation can quickly lead to dry material. Moisture levels below ~40-45% begin inhibiting the biological process. So re-wetting, or adding water at the end of primary compost followed by moving the material in a front end loader, can help introduce water and rehomogenize the mix. Surface irrigation without the material handling step can tend to result in channelization, without limited water absorption. With rewetting complete, the microbes can continue breaking down the more complex organic molecules, albeit at a much slower rate than primary composting.

The feedstock properties also have a tremendous impact on compost rates. Feedstocks are commonly made up of water, volatile solids (organic compounds of carbon, hydrogen, oxygen, nitrogen), and inerts (inorganic compounds or elements). Of the volatile solids, a fraction is more ‘bioavailable’ (think sugars and simple starches that microbes rapidly consume), and the remaining fraction is less bioavailable (ie more fibrous and less easily broken down). For example, a high food waste feedstock can have very high volatile solids, perhaps >90% with a high fraction of the volatiles solids as bioavailable. These can compost extremely fast.
By contrast, we have also seen digestate with very low VS, commonly 50-60%. Furthermore, the digestion process already consumed much of the bio-available fraction. Hence, the biodegradation process can be very slow and struggle to heat up from the biological process.

Process conditions will also impact the rate of biodegradation. For example, a modest climate using positive aeration will experience faster composting than an arctic climate using -20F air for positive composting. Additionally, a recirculating aeration system, which enables some of the warm process air to recirculate between the zones to create a more uniform temperature gradient through the pile, can achieve much faster composting than using single direction airflow. Furthermore, highly aerated and controlled systems which create optimized conditions will achieve much faster rates of stabilization. While the specifics will vary by feedstock, we generally see a 3-6x reduction in retention time required for an optimized system vs one with poor process conditions (potentially a several weeks vs several months). And of course, this plays a significant role in the size of facility, surface water management, cost, and overall odor generation.

Do you recommend using moisture sensors to measure compost pile moisture?

Water management plays a tremendous role in compost, so it is very important to understand how water flows through the process. While measuring it real-time is a challenge, we fortunately do not need to.

First, moisture content plays an important role in establishing a best management practice feedstock mix. We’ve mentioned aiming for a 55-62% starting point in other papers. More water, and the material may struggle to heat up. Less water, and you may run into moisture inhibition sooner than intended. The squeeze test is a great option for real-time spot checks, and seasonal lab tests are a good way to quantify if your material is starting in the target range.

But what about moisture sensors? We face two challenges. First, it is very difficult to get an average pile condition at a single point in a huge, non-homogenous pile. Second, the sensors are not very accurate. They rely on dissolved salts conducting electricity to measure a signals strength, which does not provide very accurate data. Stick with frequent squeeze tests, and periodic lab analysis and you will be off to a great start.

Do we need to measure oxygen levels in the compost pile?

Oxygen levels are very important, since higher O2 levels facilitate more O2 absorption into the liquid film where microbes can access it. And of course, well oxygenated microbes lead to faster composting.

However, oxygen is difficult to measure and challenging to control to. The concentration can vary widely between different locations in the pile. In addition, oxygen is much less dense than compost, which means the O2 concentration can be more fleeting and transient than a temperature reading from a large mass of compost.

So are temperature reading enough to control the process? Yes! Temperature is easily measured, and a great process indicator to control to. In addition, several scientists such as Tom Richards, R.T. Haug, David Notton, have shown that with sufficient aeration rates to provide cooling, we have ~50-100x the air we need for the biological process. Their analysis looks at the heat released by volatilizing organic molecules, how much ambient airflow is required to move this heat, and how much oxygen is required to maintain this reaction. While this can vary based on the type of reaction and ambient air conditions – the airflow required for cooling is consistently much higher than that required for stoichiometrically meeting the microbe O2 demands. So, we control based on pile temperature, knowing that oxygen levels will be sufficient in an aerated system.

David Notton
Tom Richards

How should we measure compost stability?

Compost stability can be measured using a variety of methods, some of which include:

1. Respiration test: This involves measuring the amount of carbon dioxide (CO2) that is released as microorganisms break down the compost. The higher the respiration rate, the more unstable the compost is.
2. Solvita test: This is a commercially available test that measures the carbon dioxide and ammonia levels in compost samples, as well as the pH and temperature. These parameters are used to calculate a compost stability index.

Here are the general steps to perform a Solvita test:
1. Collect a representative compost sample: Take a composite sample of the compost to ensure that the test results are representative of the entire pile. The sample should be about 500 grams and should be collected from multiple locations within the pile.
2. Prepare the sample: Remove any debris, rocks, or large particles from the sample. Then, mix the sample thoroughly to ensure homogeneity.
3. Fill the Solvita test kit: Fill the Solvita test kit with the compost sample according to the manufacturer’s instructions.
4. Incubate the kit: After filling the test kit, incubate it at a specific temperature (usually around 75°F or 24°C) for a set amount of time (usually 24 hours). This allows the microbes in the compost to generate CO2, which will be measured in the next step.
5. Read the results: After incubation, read the results according to the manufacturer’s instructions. The test kit will usually display a color change that correlates with the amount of CO2 and ammonia produced by the microbes in the compost. These values are then used to calculate a compost stability index.

It is important to note that Solvita test kits may have different instructions and procedures depending on the specific kit being used. It is always recommended to follow the manufacturer’s instructions carefully to ensure accurate and consistent results.