Aerated Static Piles (ASP)

What are aerated static piles?

Aerated static piles (ASP) are a large-scale compost method that uses aeration pipes instead of turning to keep oxygen in the mix. Because the pile does not have to be turned after assembly, it requires less maintenance than some other methods.

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, ASP relies on naturally present decomposers like bacteria, fungus, and bugs to consume organic material and transform it into humus, the organic component of healthy soil. Creating an effective ASP hinges on building the right environment for these decomposers to thrive. 

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. (Manure from omnivorous and carnivorous animals is riskier to compost due to potential pathogens, but can be managed if a large-scale system reaches sufficient temperatures.) These rich materials are the 'fuel' that speeds up compost decomposition. They are mainly broken down by bacteria. See 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. The carbon in these materials balances out excess nitrogen so it does not form into volatile compounds like ammonia. Bulkier materials like wood chips are also extremely helpful in large-scale compost systems for keeping more air spaces open in the pile and lowering its 'bulk density.' They are mainly broken down by fungus. See 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. (Compost will have the highest need for oxygen early in the process. The need will decrease the more mature the compost is. Compost under 0 °C will also use very little oxygen.) ASP uses aeration pipes to supply the pile with oxygen.
4. Water - The microbes need water to be able to digest the material around them. Compost should be wet thoroughly while the pile is being assembled. Depending on weather conditions, it may be necessary to water the pile one or more additional times while it breaks down.
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 when assembling the pile.

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 pile, operators can fine-tune the amount of different feedstocks added to make sure their final mix hits the recommended C:N ratio of 30:1 (or in the range of 25-35:1). There are many C:N ratio calculators available online, many of which come pre-programmed with C:N numbers for common feedstocks like mixed food wasted, pig manure, cow manure, grass clippings, leaves, paper, and woodchips. 

Managing heat: A well-balanced, damp, oxygen-rich pile of compost generates heat from microbial activity. In small-scale compost systems, this heat mostly dissipates into the surrounding area. But piles over about 1 cubic yard tend to insulate themselves enough that the centre of the pile heats up quite a bit. In even larger piles like most ASP, the centre of the pile may easily reach 65-70 °C. These hot temperatures cause the compost to decompose more quickly, and can also be harnessed to kill weed seeds and pathogens. However some care should be taken with large-scale piles, as if a combination of heat from the microbial activity and outside weather conditions are able to heat a pile up to 75 °C or more, it will actually start to sterilize the compost microbes themselves, as well as be at risk for starting a smoldering fire. It is a good idea for people managing large-scape compost piles to invest in a lengthy compost thermometer or sensor system that allow them to track the temperature in the centre of the pile. If the temperature is getting into the danger zone, the compost can be cooled by adding water, reducing airflow, or in drastic situations, opening up the pile so heat can dissipate. 

Compost will go through a dramatic heating spike in the initial phase of composting, and later cool down as most of the softer, nitrogen-rich material is used up. (Adding more air or water may cause a smaller, secondary spike.) The final stage of composting, known as curing, happens at ambient temperature.

Assembly instructions:

• While there are multiple ways to set up an ASP, most start by building a base of about 1 food of coarse woodchips on the ground. The aeration pipe is laid on top of these woodchips. While many kinds of pipe can be used, 6" sewer pipe (with 1" holes every 12") is common. 
• Assemble the compost pile on top of the aeration pipe and woodchip base. This can be done by mixing and watering the feedstocks nearby and then repiling them, or thin layer of different materials can be alternated and wet directly on top of the base. 
• While there isn't a mandatory size for an ASP, larger piles will be more insulated and generate higher compost temperatures. We recommend making and ASP that is at least 6' tall, 6' wide, and 10' long, as a minimum. Make sure that the aeration pipe is long enough for both ends to stay clear of the compost pile.
• Once the pile is complete, cap it with 12" of woodchips to hold in moisture.
• The aeration pipe can be left as-is or hooked up to a blower. On its own, and aeration pipe will pull air into the pile through negative pressure - the hot center of the compost will cause hot air to rise out the top of the pile, and cooler air will be pulled in to replace it through the pipe. However even greater oxygen saturation can be achieved by using a motor to force air into the pipe for short periods of time. 
• As stated above, it is important to monitor the temperature in the center of the pile. In addition to watching out for ignition risk temperatures (~75 °C), this also allows operators to keep track of how decomposition is going. It will take at least several days for a large-scale compost to get up to temperature. If there is no shortage of oxygen or water, the pile can be expected to stay at peak temperature for several weeks at least. When it eventually begins to cool down, the compost will enter the curing phase of slower, final decomposition. After 2-3 months of curing at lower temperatures, the compost should be mature.
• Mature compost can have any remaining large pieces screened out, or it can be used as-is. All nitrogen-rich materials should be fully degraded, and the compost should have cooled off to near ambient temperature. The compost should be brown, crumbly, and smell earthy like soil.

 

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