Converting a Windrow Site to Covered ASP

Converting a turned windrow site to covered aerated static pile (ASP) composting replaces the turner with blowers, an aeration floor and covers, and it shortens and consolidates the active phase onto a smaller part of the pad. It makes sense when a site is out of room, is adding food waste or biosolids, is drawing complaints, or faces an air or water rule that windrow practices cannot meet. It rarely makes sense for a yard-waste site with plenty of land and no regulatory pressure.

Key takeaways

  • The change is from moving material to moving air. Oxygen comes from blowers under a static pile, so the pile is opened a few times in its life, not every few days.
  • Taller, wider, static piles and a shorter active phase free up pad area. How much depends on your current layout, and should be measured in a pilot, not assumed.
  • Pathogen reduction rules differ by method. Windrows need 15 days at temperature with five turnings, while ASP needs three days at temperature.
  • A method change is a permit event. Expect to notify or amend with the solid waste, air and water agencies before construction.
  • Convert in phases. Keep windrows running on part of the pad while the first covered piles prove the recipe and train the crew.

What actually changes

A windrow is aerated by turning. An aerated static pile is built once on an aeration floor, and blowers supply oxygen and remove heat. In a covered ASP the pile is then sealed under a cover for the active phase.

That one change runs through everything else on the site:

Area Turned windrow Covered ASP What to plan for
Oxygen supply Turning, plus passive air movement Blowers through an aeration floor Power supply, blower stations, controls
Pile shape Long, low rows sized to the turner, with an aisle beside each Taller, wider piles or bunkers, placed side by side New pad layout and traffic pattern
Pile handling Turned many times Built once, moved once or twice between phases Loader hours shift from turning support to pile building
Exposure to weather Open to rain, sun and wind Covered during the active phase Less contact water, less evaporation
Emissions Released at each turn Held at the pile surface under the cover Air permit pathway changes
Pathogen reduction record 15 days at temperature, five turnings Three days at temperature New monitoring points and records

Footprint

Windrow dimensions are set by the turner. Static piles are not, and a shorter active phase means fewer piles on the pad at once. In a CalRecycle demonstration at Tulare in the San Joaquin Valley, reported on its composting emissions page, an aerated static pile system needed a footprint 55% smaller than windrows handling the same material. That project used positive aeration with a biofilter compost cap, not a membrane cover, and CalRecycle also reported water savings of around 20%, so treat it as an indication of direction. Sustainable Generation states that its SG MOBILE® System requires 60 to 80% less space than in-vessel or windrow, and SG’s How It Works page describes a 4 to 8 week treatment time versus 8 to 12 months for open windrow. Both are SG’s figures. Your own number comes from bulk density, pile geometry and residence time, which our facility design guide shows how to calculate.

The freed area can hold more tonnage, more curing and storage, a buffer toward the nearest neighbor, or a second phase.

Pad and aeration infrastructure

Most windrow pads can be reused if they drain well. The additions are:

  • Aeration floor. The two families are pipe laid on grade and channels cast into the pad. BioCycle’s review of aeration floor design calls pipe on grade by far the most common type. Pipes are pulled out before a pile is broken down and re-laid for the next one. In-floor systems are almost always cast in concrete, give a flat working surface and cost more to build. SG’s How It Works page lists both in-ground trenching and on-grade pipe as configurations.
  • Power. Blowers and controls need electricity at the pad. If the grid is far away, see our guide to solar-powered and off-grid composting.
  • Drainage. The floor should drain leachate away from the pile base. BioCycle notes that mixes with significant food waste release a large volume of leachate early in the process, which fills pore space and blocks airflow if it cannot drain.

A small Vermont facility profiled in BioCycle shows the low end of the range: a solid waste district repurposed a former windrow composting site for aerated static piles by building bays from concrete blocks and laying perforated pipe on top of the existing covered concrete pad, manifolded to fans.

Equipment

The windrow turner is no longer the central machine. Loaders, the grinder, the mixer (if you have one) and the screen stay. New equipment is the blowers, controls, probes, covers and, on larger sites, a cover winder. Some sites keep a turner for curing or blending: SG’s profile of Natural Soil Products still lists one in its equipment.

Labor tasks

Turning shifts and turn logs go away. In their place come careful pile building (a static pile is not remixed, so the initial mix, moisture and porosity have to be right), laying and pulling aeration pipe, placing and sealing covers, reading process data, and blower and probe maintenance. Budget time for training before the first full-scale pile, and see our guide to staffing and operator training.

Water

Open windrows are exposed to sun and wind, and California air rules add a watering duty of their own. South Coast AQMD Rule 1133.3 requires water on each active windrow within six hours before turning for at least the first 15 days, checked with a squeeze ball test. A covered pile is shielded from sun, wind and rain. According to SG, once the initial mix reaches 55 to 65% moisture, its process typically carries that moisture through without added water.

Rain that lands on a cover runs off as stormwater and does not pass through active compost. That reduces the contact water to collect and store, which matters under orders such as California’s General Order for commercial composting operations. SG’s article on keeping stormwater and leachate separate explains the approach.

Emissions

A turned windrow releases a pulse of emissions each time it is opened. A static pile under a cover is opened far less often. In the CalRecycle demonstration cited above, the aerated piles showed VOC emission reductions of nearly 99% compared with windrows, along with significant ammonia reductions. For membrane-covered systems, SG reports greater than 95% VOC control in third-party testing. These figures come from different tests and should not be compared directly.

Covered ASP with a GORE® Cover is also designed to contain odors at the pile. Receiving, mixing and screening are not under the cover, so those areas still need their own practices. Our odor management guide covers them.

Pathogen reduction and records

The federal biosolids rule sets different time and temperature requirements by method. Under Appendix B to 40 CFR Part 503, composting by the within-vessel or static aerated pile method must hold 55 degrees Celsius or higher for three days. The windrow method must hold 55 degrees or higher for 15 days or longer, with at least five turnings in that period. California applies the same numbers to compost generally in Title 14, section 17868.3, and adds that aerated static piles must be covered with 6 to 12 inches of insulating material unless the enforcement agency approves an alternative method as equivalent. If a membrane cover will replace that layer, confirm the approval path early.

SG states that its process is designed to meet the EPA Part 503 PFRP time and temperature requirements. The practical change for a converting site is the record: fixed probes logging temperature at set points in each pile, in place of hand readings and a turn count. The SG COMPOST CONTROL™ System logs temperature, oxygen and blower data and exports it for reporting.

Permits: modification or new

A conversion changes the process description in every permit the site holds. Whether that is an amendment or a new application depends on the agency and on whether tonnage or feedstock changes at the same time.

  • Solid waste. The facility’s operating document describes the method, equipment and capacity, so expect to amend it. Adding food waste or biosolids at the same time is a bigger step than changing method alone.
  • Air. Air districts treat a change of method as a change to the emission source. In the San Joaquin Valley, Rule 4566 requires a Facility Emission Mitigation Plan with an Authority to Construct application, and an operator using something other than the listed windrow measures files an alternative mitigation compliance plan. Under Rule 1133.3, an aeration system vented to an emission control system must be source tested within one year of startup and every three years after.
  • Water. California’s General Order requires a revised Notice of Intent at least 90 days before changing material or construction specifications, or an operation that was not described in the approved notice and technical report.

Other states use different instruments, but the sequence is the same: talk to each agency before design is final. SG’s permitting and engineering support page and our permitting guide go further.

Phasing a conversion while operating

Few sites can stop receiving material. A staged approach keeps the gate open.

  1. Baseline. Record current tonnage, pad area in use, residence time, water use, fuel use and complaint history.
  2. Pilot. Set up one or a few covered piles on a corner of the pad and run several batches of your own recipe next to the windrows. SG describes SG MOBILE as installed in days to weeks, depending on the site, and its pilot and demo program is built for this step.
  3. Agency meetings. Take the pilot data to the solid waste, air and water agencies and confirm the permit pathway.
  4. Design the end state first, then decide which block to build first.
  5. Build phase one on cleared pad. Consolidate windrows to free a block, install power and the aeration floor, and commission the first piles.
  6. Shift the highest-risk feedstock first. Move food waste or biosolids into the covered piles and leave clean yard waste in windrows until later.
  7. Train on real piles before the next phase opens.
  8. Repurpose the freed area and retire or reassign the turner.

Several SG project profiles follow this pattern. Prince George’s County, Maryland piloted an SG MOBILE System beside its windrow operation in 2013 to test food waste composting and later expanded to an SG BUNKER® System. The City of San Diego’s Miramar Greenery ran a covered pilot when food waste was being added to a turned windrow green waste operation. Grand Forks, British Columbia composted yard and garden waste in turned windrows before food waste and then biosolids were added to its feedstock. Natural Soil Products in Tremont, Pennsylvania converted an open-windrow biosolids operation after community complaints.

When conversion pays off

Your situation Conversion is likely worth studying Staying with windrows is reasonable
Land Pad is full, or expansion land is unavailable Ample land and setbacks
Feedstock Adding food waste, biosolids, digestate or manure Yard and wood waste only
Neighbors Complaints, encroaching development Remote site, no complaint history
Air rules A VOC or ammonia control tier applies or is coming Windrow best practices satisfy the rule
Water Wet climate, costly contact water storage, or scarce supply Moderate climate and enough pond capacity
Fleet Turner is due for replacement Turner is new
Power Grid or solar is practical at the pad No practical power supply

If most of your answers fall in the middle column, a pilot is the next step. For a neutral side-by-side of the methods, see composting methods compared.

FAQ

Can I reuse my existing windrow pad?

Often, yes. A pad that drains well and carries loaders can take pipe-on-grade aeration with little construction, as smaller retrofits have shown. Check slope, drainage toward the collection system, load capacity and whether your water permit sets a permeability standard for working surfaces. In-floor aeration needs new concrete.

How much space will conversion free up?

It depends on your current windrow size, aisle width, residence time and the pile geometry you convert to. Published figures range widely and are specific to each test. Calculate it from your own bulk density and tonnage, then confirm with a pilot before you commit the freed area to a new use.

Will I need a new permit?

Expect at least an amendment or notification with your solid waste, air and water agencies, because the method, equipment and site plan change. If you are also adding a feedstock or raising tonnage, a fuller review is likely. Ask each agency before design is final.

Can I convert without shutting down?

Yes. Most conversions are phased: consolidate windrows to clear one block of pad, build and commission covered piles there, move the most odorous feedstock first, and repeat. A mobile pilot system lets the crew learn before permanent infrastructure is built.

Next step

The fastest way to test a conversion is a side-by-side pilot on your own pad. See how SG’s pilot and demo program works, or ask the SG consulting team to review your layout, tonnage and permits and outline a phased plan.

Related guides

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