There are three engineered ways to control air emissions from active composting: hold them at the pile surface with a cover, pull process air through the pile and treat it in a biofilter, or put the operation inside a building and treat the building’s exhaust. All three can satisfy a regulator when designed and maintained properly. They differ in where the emission is captured, how much air has to be moved, what has to be maintained, and how much of the site they occupy.

Key takeaways

• A cover works at the pile. A biofilter works at the end of a pipe. A building works at the wall. The farther from the pile you capture, the more air you have to move and treat.
• Air district rules are mostly written around performance, not hardware. South Coast AQMD Rule 1133.3 asks for a tested control efficiency or an emission rate per ton, and leaves the device to the operator.
• The exception matters: South Coast AQMD Rule 1133.2 requires the active phase of co-composting (biosolids, or heavy manure mixes) to take place inside an enclosure as the rule defines it.
• A biofilter is a biological process that needs moisture, even airflow and periodic media replacement. Budget for it as an operation, not a structure.
• Whatever you choose, ask for source test data that states the baseline, feedstock and test method.

What has to be controlled

Active composting releases volatile organic compounds (VOCs), ammonia, odorous compounds, dust and bioaerosols. California air districts regulate the first two directly. South Coast AQMD’s Rule 1133.3, as amended on September 5, 2025, states its purpose as reducing VOC and ammonia emissions from composting operations. Odor is usually handled separately, through nuisance rules, solid waste permit conditions and odor management plans. Keep the two tracks apart when you read vendor data. Our odor management guide covers the odor track. Every option below also sits on top of operating practice: recipe, porosity, moisture and prompt handling of feedstock decide how much is generated in the first place.

Option 1: Covers at the pile surface

A cover treats the pile itself as the containment. There are two kinds.

Biocovers. A layer of finished compost or screened overs is placed over the active pile. Compounds leaving the pile pass through a moist, biologically active layer first. Cornell describes the same effect inside a pile as in situ biofiltration. Air rules recognize it: Rule 1133.3’s windrow practices call for a cover of finished compost or compost overs at least six inches thick within 24 hours of pile formation, and San Joaquin Valley Rule 4566 lists a finished compost cover as a mitigation measure for windrows. A biocover costs little, but it has to be rebuilt with every pile and its thickness varies with the loader operator.

Membrane covers. An engineered fabric is pulled over an aerated static pile and sealed at the edges. In positively aerated systems, blowers push air up through the pile and the membrane is the last thing process air meets before it leaves. The membrane is breathable, so water vapor and carbon dioxide leave the pile while rain stays out. Sustainable Generation’s SG Advanced Composting™ Technology is this type: a GORE® Cover over a positively aerated pile, with aeration controlled by oxygen and temperature feedback, no building and no biofilter. SG’s FAQ describes the cover as a breathable membrane that blocks VOCs, bioaerosols and rain while letting composting vapors escape. SG reports greater than 95% VOC control in third-party testing. Covered ASP with a GORE Cover is also designed to contain odors at the pile.

A membrane cover needs a way to place and remove it (a winder machine on larger sites), edge sealing after every pile build, inspection for tears, and eventual replacement. Piles are uncovered when they are built, moved and broken down.

Fits when the site is outdoors and wants to stay that way, rain or water supply is a concern, the site may grow in stages, or power is limited. Does not fit when a rule or permit specifically requires an enclosure, when receiving and mixing are the main sources (a pile cover does nothing for the tipping floor), or when the operator cannot commit to disciplined covering and sealing.

Option 2: Negative aeration to a biofilter

In a negatively aerated static pile, blowers pull air down through the pile into the aeration floor and push it to a biofilter. Because the pile is under suction, most of what would have left the surface is collected in the ductwork instead.

Cornell’s biofiltration guidance describes the device: a bed of moist organic material that adsorbs odorous compounds and then degrades them biologically. Media is typically compost, soil, peat, or chipped brush and bark, sometimes blended with gravel for porosity. Beds are usually 1 to 1.5 meters deep. Shallower beds risk gas bypassing, and deeper beds are harder to keep uniformly moist. Cornell gives literature airflow rates of 1 to 5 cubic feet per minute per square foot of bed surface, typically 3 to 4, which means the biofilter’s area is set by the volume of air you move. It also notes that exhaust air dries the media and that rewetting from the surface tends to be uneven, so the inlet air is usually humidified.

That is the trade. Capture by suction is effective, and BioCycle’s review of aeration floor design credits negative aeration with the highest emissions and leachate capture efficiencies of the aeration modes. In exchange:

• Blowers handle hot, wet, corrosive process air and condensate instead of ambient air.
• The biofilter adds footprint, back pressure and fan energy.
• Media settles, dries, channels and breaks down. It has to be irrigated, monitored and replaced.
• An uncovered negative pile is still open to rain, so stormwater that touches the pile becomes process water.

SG argues this point from its side: its positive ASP article says negative systems need constant monitoring and maintenance to hold their capture performance. Our guide to positive vs negative aeration takes both sides in more detail.

Fits when the permit calls for collected and treated process air, when a stack or bed that can be source tested at an inlet and an outlet is wanted, or when the system already sits under a roof. Does not fit when space is tight, when staff cannot maintain a biological treatment bed, or when rainfall on open piles is already a water management problem.

Option 3: Enclosed buildings and vessels

Full enclosure puts receiving, mixing or active composting, or all three, inside a structure. Building air is exhausted through a treatment system, usually a biofilter and sometimes a scrubber ahead of it.

South Coast AQMD’s Rule 1133.2 shows what a regulator means by enclosure. It applies to co-composting, defined as composting any amount of biosolids or more than 20 percent manure by volume. It defines an enclosure as a completely walled, floored and roofed structure or vessel, and requires the active phase to take place inside one. Air must flow inward through every natural draft opening at 100 feet per minute or more (unless the opening has a closure device that seals it if the airflow reverses), and those openings may total no more than two percent of the enclosure’s surface area. Exhaust from the enclosure and the aeration system goes to an emission control system, and curing must also be aerated and vented to control. It offers alternatives for certain small operations.

Enclosure captures what the other two options miss: tipping, mixing and pile handling. It also keeps rain off. The costs are structural and permanent:

• The whole building volume has to be ventilated, so air treatment is sized for far more air than the piles alone produce.
• Warm, humid, ammonia-laden air corrodes steel, wiring and equipment.
• People and loaders work inside that air.
• Capacity is fixed by the building. Expansion means another building.

Fits when homes or businesses are very close, when the rule requires it, when receiving is the dominant source, or when the climate makes outdoor work impractical. Does not fit when capital is limited, tonnage is uncertain or growing in steps, or the organization cannot support building mechanical systems for decades.

Side-by-side comparison

	Membrane cover on positive ASP	Negative ASP to biofilter	Enclosed building with air treatment
Where emissions are captured	At the pile surface	In the aeration floor and ducts	At the building envelope
Air volume to move	Process air only, pushed through the pile	Process air, pulled and then pushed through the bed	Process air plus building ventilation air
Added footprint	Little beyond the piles and cover handling lanes	Biofilter bed, ducts and blower stations	Building, plus biofilter or scrubber
Rain on the pile	Kept off by the cover	Lands on the pile unless roofed	Kept off by the roof
Receiving and mixing emissions	Not addressed	Not addressed	Addressed if those areas are inside
Main upkeep	Cover handling, seals, cover replacement	Media moisture, back pressure, media replacement, corrosion	All of the biofilter upkeep plus building, fans and corrosion
Expansion	Add piles and covers	Add piles and biofilter area	Add building

Energy use follows the air volume row. Ask each vendor for metered energy data from an operating site of similar size.

What regulators accept

Read the rule that applies to your feedstock and district before you compare hardware. Three examples, as of October 2026 (check the current text):

1. South Coast AQMD Rule 1133.3 covers composting that is not co-composting. Sites processing more than 5,000 tons of food waste a year must run the active phase of piles with more than ten percent food waste on an aeration system vented to an emission control system. That system must reach 80% control for VOC and for ammonia, measured against the inlet, a baseline emission factor or an approved site-specific factor, or stay under a set emission rate per ton. A source test is due within one year of startup and every three years after. The rule does not name a device.
2. South Coast AQMD Rule 1133.2 covers co-composting and does name a configuration: an enclosure for the active phase, vented to an emission control system at the same 80% level, with source testing every two years after the first. If you compost biosolids there, ask the district how it treats your proposed system under the enclosure definition.
3. San Joaquin Valley Rule 4566 scales with throughput. Windrow sites under 200,000 wet tons a year use turning and watering measures or an approved alternative showing at least a 19% VOC reduction. Sites from 200,000 to under 750,000 wet tons add a finished compost cover or show 60%. Only sites at 750,000 wet tons a year and above must show 80%.

So 80% is the top tier in these rules, not a universal requirement, and SB 1383 is an organics diversion law run by CalRecycle, not an air rule. SG’s systems have been tested against air district rules, and SG’s article on VOC compliance in California gives its account of those tests. See also the best covered composting systems for California air rules.

How to choose

Work through these in order.

1. Check for a mandated configuration. If the rule or permit requires an enclosure or a collected exhaust, the choice narrows at once.
2. Rank your sources. If tipping and mixing drive complaints, a pile-level control will not be enough without changes at receiving.
3. Map rain and water. Wet climates and sites with strict stormwater permits favor options that keep rain off active material.
4. Count what you will maintain. Covers, media beds, fans, ducts and buildings each have a replacement cycle. SG’s three-part series on long-term technology cost, starting with Part 1, covers this from SG’s perspective.
5. Plan for growth. Decide what each added step of capacity requires.
6. Test on your feedstock. Run a pilot and collect emissions, temperature and product data under your conditions before committing capital.

FAQ

Which option controls emissions best?

Each can meet top-tier air district limits when designed and run well, and each can fail when neglected. Compare tested results that state the baseline, feedstock, method and test conditions. Then ask which one your staff can keep performing through weather, turnover and feedstock changes.

Do membrane covers need a biofilter too?

Not in a positively aerated design, where the cover is the control device and no process air is collected. Sustainable Generation describes its covered ASP as needing no building and no biofilter. A regulator may still ask for source test data to confirm performance under the applicable rule.

How long does biofilter media last?

It depends on the media, the air loading and how well moisture is managed. Cornell’s guidance stresses that beds dry out from warm exhaust air and that uneven wetting leads to bypassing. Plan for routine irrigation, back pressure checks and periodic media replacement, and ask operators of similar systems what replacement interval they actually see.

Is a building required for biosolids composting?

It depends on the jurisdiction. In the South Coast district, Rule 1133.2 requires the active phase of co-composting to occur in an enclosure, with alternatives for certain small operations. Many other jurisdictions set performance or nuisance standards without prescribing a structure. Federal Part 503 pathogen rules do not require a building.

Next step

If two options still look workable, test rather than guess. An SG MOBILE® pilot runs your feedstock under a GORE Cover on your site and produces the data a permit application or board needs. SG’s consulting team can also review an existing layout and permit conditions.

Related guides

• Positive vs negative aeration in ASP composting
• Compost facility odor management
• Compost facility permitting guide: solid waste, air and water
• Compost facility lifecycle cost
• Composting technology RFP guide

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