In-Vessel Compost Systems

Image courtesy of www.prairiehogcountry.com.

What are in-vessel compost systems?

In-vessel compost systems are a less common form of large-scale composting that decomposes organic waste inside of an enclosed tank or container under tightly controlled conditions. They come in a wide variety of sizes and designs, and can handle different amounts and kinds of feedstock. 

Large-scale compost methods used by municipalities or businesses should make sure to abide by all relevant provincial and federal regulations. 

How do they work?

Like other forms of aerobic composting, in-vessel composters focus on creating the ideal nutrient, temperature, moisture, and oxygen conditions for decomposer bacteria and fungus to thrive. These bacteria and fungus consume organic material and transform it into humus, the organic compost of healthy soil. 

The basic 5-step recipe for balancing aerobic compost is the same in a large-scale recipe as a regular compost bin:

1. Nitrogen - Aka 'green material,' this includes food waste, green plant trimmings, grass clippings, and herbivore manure. In-vessel composters may also be specialized to deal with challenging nitrogen-rich materials like livestock mortalities. See the note on balancing C:N ratios, below.
2. Carbon - Aka 'brown material,' this includes paper products, cardboard, brown plant trimmings, fall leaves, and wood chips. In-vessel composters typically use wood pellets or wood chips at their carbon source. See the note on balancing C:N ratios, below.
3. Oxygen - Aerobic microbes breathe oxygen, so we want to keep fresh air in the pile as long as it is processing. In-vessel composters typically have electric fans and vents to control airflow. 
4. Water - The microbes need water to be able to digest the material around them. Compost should be wet thoroughly while the windrow is being assembled. In-vessel composters are able to closely monitor the humidity of the material and add water as needed.
5. Microbes - Microbes are the star of the show. Some will already be present on the compost material, but more can be added by incorporating a small amount of soil or previously finished compost into the mix. Some in-vessel composters also keep a reservoir population of microbes.

On balancing C:N ratios: With small-scale composting, simply aiming for a mix of about 40% nitrogen-rich material by volume and 60% carbon-rich material by volume is enough to keep a compost pile in balance. While this volume-based method can work as an approximate measure, large-scale systems achieve greater control by switching to measuring carbon-to-nitrogen ratios (C:N ratios) in their feedstocks. By testing or looking up the C:N ratios of different materials in the mix, operators can fine-tune the amount of different feedstocks added to make sure their final mix hits the recommended C:N ratio of around 30:1. Because in-vessel composters allow the highest level of control, some may aim for a different specific C:N ratio.

Managing heat: Large-scale compost systems tend to heat up due to microbial activity. These hot temperatures cause the compost to decompose more quickly, and can also be harnessed to kill weed seeds and pathogens. Since in-vessel compost systems are able to monitor compost temperature more closely, they are often used to ensure that particularly challenging feedstocks are processed at a safe temperature to kill pathogens.

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Got more composting questions? Email the Compost Hotline at [email protected].