Aerated static pile (ASP) composting builds a mixed pile of organic material over an air distribution system and uses blowers, not turning, to supply oxygen and remove heat. The pile stays in place through the active phase while a timer, a temperature sensor or an oxygen sensor decides when the blowers run. Done well, ASP gives an operator direct control of the process on less land than turned windrows, without the building that an in-vessel plant needs.

Key takeaways

• An ASP is not turned during active composting. Air is pushed up through the pile (positive aeration), pulled down through it (negative aeration), or alternated between the two (reversing).
• The pile only works if air can move through it. The mix recipe, the bulking agent and the pile height matter as much as the blower.
• Aeration floors are either pipe laid on the pad or channels built into it. Pipe on grade costs less to install and more to run. In-floor systems are permanent.
• Under the federal biosolids rule, a static aerated pile must hold 55 degrees Celsius or higher for three days, against 15 days and five turnings for a windrow.
• The surface of the pile is where emissions, heat and water are won or lost. That is why ASPs are capped with compost or wood chips, or covered with a fabric or membrane.
• ASP fits between windrow and in-vessel on cost and control. The right choice depends on feedstock, neighbors, climate and the rules that apply to your site.

What an aerated static pile is

The U.S. Environmental Protection Agency describes the method simply: actively aerated static piles use blower systems to move air through the pile, the blowers can be activated by a timer or a temperature sensor, and piles may be covered to help control temperature and moisture. The same EPA overview of composting approaches notes that the method works with large quantities and all types of materials.

Three things separate ASP from a turned windrow:

• Oxygen comes from blowers. A windrow depends on turning and on natural convection between turns. An ASP delivers air on demand.
• The pile is not disturbed. Heat stays in, and emissions are not released at each turn.
• The pile can be bigger. No turner has to straddle it, so piles can be taller, wider and side by side.

Building the pile: recipe, structure and height

Start with the mix

Microbes need carbon, nitrogen, water and air. Cornell Waste Management Institute’s composting pages give working starting points: a carbon to nitrogen ratio of around 30 to 1, and a moisture content of 50 to 60%, with decomposition slowing when the mix is much wetter than that. Wet, dense, nitrogen-rich feedstocks such as food waste, biosolids and digestate need a coarse, carbon-rich bulking agent (ground wood, wood chips, yard trimmings) to reach those ranges and to hold the pile open. The guide to compost process control covers each parameter, and the list of common compost feedstocks covers what each material brings.

In a static pile the bulking agent does the job that turning does in a windrow: it keeps air passages open for weeks. Mix thoroughly, because there is no second chance to blend.

Height and base

The FAO manual gives an initial pile height of about 150 to 245 cm (roughly 5 to 8 feet) for a basic ASP, depending on material porosity, weather and the reach of the loader, and describes building the pile over a base of wood chips or other very porous material that contains the perforated aeration pipe.

A build-day sequence

1. Check incoming feedstocks for moisture and contamination.
2. Grind or shred woody material.
3. Blend feedstock and bulking agent to the target carbon to nitrogen ratio and moisture content.
4. Inspect the aeration floor: clear blocked holes or channels and drain any standing liquid.
5. Lay the porous base layer if your floor design uses one.
6. Build the pile to its design height with a loader, without driving on the placed material.
7. Apply the cap layer or pull the cover, and seal the edges if the cover is designed to be sealed.
8. Insert temperature (and oxygen) probes, start the blowers, and log the batch start time.

Aeration floors and blowers

Pipe on grade or in-floor

A BioCycle design article by Craig Coker and Tim O’Neill, Composting Aeration Floor Functions and Designs, sorts aeration floors into two families.

	Pipe on grade	In-floor (trench or sparger)
What it is	Perforated pipe laid on the pad under each pile	Channels or pipes cast into a concrete or asphalt pad
Typical use	Small to medium piles, pilots, temporary sites	Permanent, higher-throughput facilities
Installation	Low cost, no pad construction	Higher cost, needs civil work
Operation	Pipe is pulled and relaid at every build and is easily damaged by loaders	Loader works on a flat surface; channels need periodic cleaning
Service life	Heavy-walled HDPE pipe typically lasts 12 to 24 months, per the BioCycle authors	Can last decades with suitable materials, per the same article

The same article gives two rules of thumb for pipe on grade in modest piles (under about 70 feet long): keep the spacing between pipes under two-thirds of the pile height, and keep the ratio of pipe length to internal diameter under 150. Both are aimed at even airflow along the pile.

Blowers and control

Blowers can be dedicated (one per pile or zone) or centralized (one larger fan feeding several zones through ducts and dampers). A second BioCycle article, Compost Aeration System Design, explains the trade: centralized fans can be sized near average demand and run efficiently, while dedicated fans are sized for peak demand but, in positive systems, need no distribution ductwork or zone dampers. The author puts total aeration energy at about 3 to 13 kWh per ton processed, depending on design efficiency and oxygen demand.

Control is the other half. The simplest systems run the blower on a time clock. Better ones cycle it from pile temperature. Oxygen feedback goes one step further and runs the blower when the microbes have used up the oxygen in the pore space.

Positive, negative and reversing aeration

EPA’s overview defines the two directions: in positive aeration, air is blown through pipes beneath or inside the pile; negative aeration draws air down through the pile.

• Positive pushes clean ambient air through the blower and up through the pile. Process air leaves at the surface, so the surface needs a cap or a cover to treat or contain it.
• Negative pulls air down through the pile and sends the exhaust to a treatment device, usually a biofilter. The blower handles hot, wet air, and the floor collects condensate and leachate that need drainage.
• Reversing alternates the two to even out temperature and moisture through the depth of the pile.

According to the BioCycle aeration design article, negative systems tend to need 20% to 30% more energy than positive systems to move the same volume of air, because of the added ductwork and biofilter. Negative aeration with a biofilter has a long compliance record of its own. The full engineering comparison is in the guide to positive vs negative aeration.

Covered and uncovered piles

An uncovered ASP exposes its whole surface to rain, wind and sun. Most ASPs therefore get one of three surface treatments.

Biolayer cap. A layer of finished compost or wood chips over the pile. The FAO manual suggests about 15 cm of finished compost or bulking agent, which protects the surface from drying, insulates it, discourages flies and filters ammonia and odors. California’s composting regulation builds the cap into its pathogen rule: all active compost in an aerated static pile must be covered with 6 to 12 inches of insulating material (14 CCR 17868.3), unless the enforcement agency approves an alternative method as giving equivalent pathogen reduction. A cap has to be applied evenly, and it does not shed rain.

Membrane covers. A BioCycle article on covered ASP design describes covers built around an expanded PTFE membrane laminated between polyester layers. Moisture rising from the pile condenses on the underside of the cover, and that film of water absorbs odorous compounds, while the waterproof outer face keeps rain off the material.

Sustainable Generation (SG) is the exclusive GORE® Cover partner for North America and Oceania. Its SG Advanced Composting™ Technology pairs the cover with positive aeration controlled by oxygen and temperature feedback through the SG COMPOST CONTROL™ System, with no building and no biofilter. SG describes the cover as a breathable membrane that blocks odors, VOCs, bioaerosols and rain while letting composting vapors escape. SG reports greater than 95% VOC control in third-party testing. For a wider view of the category, see the list of aerated static pile composting systems.

Pathogen reduction (PFRP) in an ASP

For biosolids, the federal standard is 40 CFR Part 503. Its Appendix B lists composting as a Process to Further Reduce Pathogens (PFRP) on these terms:

• Within-vessel or static aerated pile: the temperature is maintained at 55 degrees Celsius or higher for three days.
• Windrow: 55 degrees or higher for 15 days or longer, with a minimum of five turnings during that period.

EPA does not certify compost or composting systems. A facility demonstrates compliance with its own temperature records, batch by batch. Many states apply the same time and temperature standard to food waste and green waste. California does, and it also sets how the readings are taken: at least one reading each day per 200 cubic yards of active compost during the pathogen reduction period, measured 12 to 18 inches from where the insulation cover meets the active compost. Check your own state’s rule. SG states that its process is designed to meet the EPA Part 503 PFRP time and temperature requirements.

Where ASP fits against windrow and in-vessel

Question	Turned windrow	ASP (capped or covered)	In-vessel or enclosed building
How is oxygen supplied?	Turning and convection	Blowers under a static pile	Blowers, often with mechanical mixing
Land needed	Most	Less	Least for the process itself
Emissions at the surface	Released at the surface and at each turn	Treated by a cap, a cover, or a biofilter	Captured and treated
Rain on the material	Yes	Yes if uncovered or capped; no under a waterproof cover	No
Federal PFRP for biosolids	15 days, five turnings	3 days	3 days
Infrastructure	Pad and turner	Pad, aeration floor, blowers, controls	Building or vessels, air handling, biofilter

EPA notes that windrows are the most common composting method in the United States, and they fit well where there is land, buffer distance and low-odor feedstock. In-vessel systems fit where full enclosure is required. ASP fits the wide middle: sites that need more control than a windrow gives but cannot justify a building.

On timing, SG’s How It Works page describes a 4 to 8 week treatment time versus 8 to 12 months for open windrow. Ask any vendor what curing follows its cycle figure. The neutral side-by-side is in composting methods compared.

Running an ASP day to day

• Read temperatures daily during the pathogen reduction period, keep the records with the batch, and watch for uneven temperatures along the pile.
• Keep floor channels, pipe perforations and condensate drains clear.
• Keep stormwater away from the pile base and manage leachate separately (see leachate management methods).

ASP systems run at very different scales. SG, for example, lists its SG HEAP® System at 1,000 to 100,000+ tons a year and its SG BUNKER® System at 5,000 to 500,000 tons a year. The New York City Department of Sanitation’s Staten Island facility, operated by Denali Water, composts source-separated food waste and yard waste in SG Bunker units with positive in-floor aeration under oxygen control (project profile).

FAQ

How long does aerated static pile composting take?

It depends on the feedstock, the recipe, the control method and what you count as finished. The FAO manual says the active period of a well-built basic ASP is complete in about three to five weeks. SG’s How It Works page describes a 4 to 8 week treatment time for its covered system. Curing follows the active phase in almost every design, so compare total time to a marketable product.

What is the difference between ASP and covered ASP?

A covered ASP (CASP) is an aerated static pile with an engineered cover in place of, or in addition to, a cap of compost or wood chips. The cover’s job is to control what crosses the pile surface: heat, moisture, rain and emissions. Covers range from simple tarps to laminated membranes, so ask for test data and its test conditions.

Can an ASP handle food waste and biosolids?

Yes, with the right recipe and surface control. EPA says the method works with all types of materials. Wet, nitrogen-rich feedstocks need enough bulking agent to hold the pile open, and the cap, cover or biofilter matters more than it does for yard trimmings. Biosolids must also meet Part 503 pathogen and vector attraction requirements.

Does an aerated static pile need a biofilter?

Only if it uses negative or reversing aeration, where exhaust air is collected in ducts and has to be treated. A positive pile releases its process air through the surface, so it relies on a cap of compost or wood chips, or on a membrane cover, in place of a separate biofilter bed.

Next step

To see how one covered ASP design handles aeration, covers and monitoring, read how SG Advanced Composting Technology works. If you are still weighing airflow direction, continue with the guide to positive vs negative aeration.

Related guides

• Compost Process Control: Oxygen, Temperature, Moisture and C:N
• Positive vs Negative Aeration in ASP Composting
• Converting a Windrow Site to Covered ASP
• Compost Facility Design Guide: Layout, Pads, Aeration and Water
• SG BUNKER® vs SG HEAP® vs SG MOBILE®: Which System Fits Your Site

← All commercial composting guides

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