Composting at Landfills, Adding Organics Processing to an Existing Site

A landfill is often the most practical place to build a compost facility, because the scale house, roads, equipment, staff, buffers and customers are already there. The work is in fitting an aerobic process onto a site designed for burial: choosing ground that can carry the pad, protecting the final cover and gas system, keeping compost water apart from landfill water, and amending permits that were written for disposal. Owners who plan for those four issues can divert organics at the gate instead of sending them to the working face.

Key takeaways

Organics are the reason landfills make methane. EPA estimates that food waste is about 24% of landfilled municipal solid waste and the source of about 58% of fugitive landfill methane.
Diversion laws are moving organics out of disposal. California’s SB 1383 set a 2025 target of 75% less organic waste sent to landfills, and since 2020 green material used as daily cover no longer counts as diversion there.
A compost pad can go beside the landfill, on a closed cell, or on an inactive deck of an active landfill. Each has different load, settlement, gas and permit questions.
Federal rules require 30 years of post-closure care and say post-closure use must not disturb the final cover, liner, containment or monitoring systems.
On top of waste, surface-mounted and relocatable systems are the simplest fit. Permanent in-floor aeration on a closed landfill is possible, but only after geotechnical work and with nothing cut into the cover’s low permeability layer.
Compost leachate, compost pad stormwater, landfill leachate and landfill stormwater are four different waters. Keep them separate in design and in the permit.

Why landfill owners add composting

The methane and diversion case

The EPA Landfill Methane Outreach Program reports that municipal solid waste landfills are the third-largest source of human-related methane emissions in the United States, about 14.4% of the total in 2022. EPA’s research on landfilled food waste estimates that food waste makes up about 24% of municipal solid waste disposed of in landfills and, because it decays quickly, accounts for an estimated 58% of the fugitive methane those landfills release.

States are responding with diversion mandates. CalRecycle summarizes SB 1383’s 2025 targets as 75% less organic waste sent to landfills and 20% of unsold, still-edible food sent to food recovery organizations, and says organics such as food scraps, yard trimmings, paper and cardboard make up half of what Californians landfill. See our list of key SB 1383 requirements and the guide to organics diversion laws by state.

A second California change matters to landfill operators specifically. Under AB 1594, CalRecycle explains, the use of green material as alternative daily cover stopped counting as diversion on January 1, 2020, and is treated as disposal when a jurisdiction’s per capita disposal rate is measured. That green material now needs a recycling outlet, and the nearest one can be a compost pad on the same property.

The business case, in plain terms

Customers already come to the gate. Haulers with separated organics need a destination.
Airspace. Every ton composted is a ton that does not consume permitted disposal capacity.
Shared infrastructure. Scale house, roads, loaders, water trucks, shop and trained operators are in place.
Buffers and zoning. The site is already zoned and buffered for waste handling.
A product. Compost can be used on site for erosion control and cover vegetation, or sold.

There are limits. A landfill with close neighbors and an odor history will be judged harshly for any new odor source, and some sites have no stable ground for a pad.

Where on the site: three options

Location	Fits when	Watch for
Native ground beside the landfill	Undisturbed land exists inside the permitted boundary. Permanent pads, in-ground aeration trenches and buildings are wanted	Gas migration near the waste boundary, buffer distances, and competing uses such as future cells or borrow areas
On top of a closed cell	Land is short and the closed area is large, flat and past its early settlement	Final cover protection, differential settlement, gas wells and headers, post-closure plan amendment, limits on irrigation and structures
On an inactive deck of an active landfill	The owner needs capacity now and expects to move the operation as filling advances	Intermediate cover that is not built as a pad, ongoing settlement, traffic conflicts with disposal, and a system that must be relocatable

Sustainable Generation (SG) has profiles of several landfill-sited projects. The City of San Diego’s Miramar Greenery moved from turned windrows for green waste to an SG MOBILE® System when food waste was added. At the Otay Landfill in San Diego County, SG’s profile says Republic Services needed a system that was modular and portable because the operation would sit on an active landfill less than a mile from homes. In New York, the Staten Island Compost Facility at Fresh Kills required a first project phase devoted to the geotechnical challenges of operating on a closed landfill, before a permanent SG BUNKER® System with positive in-floor aeration was built. In British Columbia, the Grand Forks Compost Facility is located at the regional district’s landfill.

Building on closed cells: cover, settlement and loads

What the federal rules protect

Under 40 CFR 258.60, a closed municipal solid waste landfill unit has a final cover designed to minimize infiltration and erosion, with an infiltration layer of at least 18 inches of earthen material and an erosion layer of at least 6 inches that can sustain plant growth. Under 40 CFR 258.61, post-closure care runs for 30 years unless an approved state shortens or lengthens it, and includes maintaining the final cover and repairing the effects of settlement, subsidence and erosion. The post-closure plan must describe the planned uses of the property, and post-closure use must not disturb the integrity of the final cover, liners or other containment components, or the function of the monitoring systems. The director of an approved state may approve a disturbance if the owner demonstrates that it will not increase the potential threat to human health or the environment.

A compost facility is a post-closure use, so the plan has to be amended and the design has to show that the cover still does its job with a pad, piles and trucks on top of it.

What a state adds: California as an example

California’s postclosure land use rule, 27 CCR 21190, shows the level of detail to expect. Proposed uses other than non-irrigated open space go to the enforcement agency, the regional water board, the local air district and the land use agency. The enforcement agency reviews and approves projects that involve structures on top of waste or within 1,000 feet of the disposal area, modification of the low permeability layer, or irrigation over waste. Construction has to maintain the final cover, drainage and erosion control systems and gas monitoring and control systems. Buildings and utilities must be designed for differential settlement with flexible connections, utilities may not be installed in or below the low permeability layer, and the agency may require an additional soil layer or building pad on top of the final cover before construction.

Design consequences

Settlement survey first. Review settlement data and waste age before choosing an area.
Add fill, do not cut. Build the pad up over the cover on engineered fill. Do not excavate into it.
Keep aeration and utilities above the cover’s barrier layer. Trenches or foundations cut into the cover conflict with it. Above-ground aeration is the simplest answer, and SG’s Miramar and Otay profiles both list it. A permanent in-floor system has to be designed with the geotechnical engineer and the regulators, as at Staten Island, where SG’s profile lists positive in-floor aeration on a closed landfill.
Plan for regrading. Slopes change as the cell settles, and low spots pond water.
Check loads. Have the engineer of record check bearing and slope stability for trucks, grinders and loaders.
Limit added water. Irrigation over waste is a named trigger for review in California.

Landfill gas and the compost operation

Landfill gas is roughly half methane, according to EPA. 40 CFR 258.23 requires owners to keep methane below 25% of the lower explosive limit in facility structures and below the lower explosive limit at the property boundary, with routine monitoring. For a compost operation that leads to three rules of thumb:

Map every gas well, header and monitoring probe before laying out piles and haul routes, keep access to them, and protect them from loaders.
Avoid enclosed spaces on or near waste. California requires automatic methane sensors in buildings on landfilled areas and prohibits enclosed basements.
Tell the gas system operator about the project, because added loads and changed drainage can affect the wellfield.

Leachate and stormwater: four waters, kept apart

Landfills already manage run-on and run-off. 40 CFR 258.26 requires a run-on control system that prevents flow onto the active portion during the peak discharge from a 25-year storm and a run-off system that controls at least the volume from a 24-hour, 25-year storm. A compost pad adds two new streams: liquid that drains from the piles, and rain that falls on the pad and touches feedstock or compost.

Do not let compost water become landfill leachate by default. It adds volume to the leachate system. If you intend to do it, get it written into the permit.
Do not let it infiltrate the cover. Grade the pad to collection points.
Separate clean from contact water, and reuse process water in the piles where the permit allows.

In California, compost operations also fall under the State Water Board’s Composting General Order, adopted in 2015 and revised in 2020, which sets tiered requirements to protect water quality. Covered systems simplify the water balance. SG states that a GORE® Cover sheds rain so that stormwater and pile leachate stay separate without a roof or building, and explains its reasoning in an article on keeping stormwater and leachate separate. See also our lists of leachate management methods and stormwater practices.

Power on top of a landfill

Blowers and controls need electricity, and the top of a landfill rarely has a service drop. Trenching conduit through a final cover is the kind of disturbance the post-closure rules restrict, so the alternatives are overhead or surface-run power, generators, or solar with battery storage. SG’s SG SOLAR® Power page describes solar panels, an inverter and battery storage packaged for its systems and names the top of a landfill as an intended use. The Otay facility was announced by Republic Services as California’s first fully solar-powered compost facility. Our guide to solar-powered composting covers when off-grid power makes sense.

Permit interactions

Adding composting touches most of the landfill’s existing approvals.

Approval	Typical question
Solid waste facility permit	Is composting a new activity that needs a permit revision or its own compost permit? What are the tonnage, feedstock and hours limits?
Closure and post-closure plan	Does the planned use protect the final cover, drainage, and gas and groundwater monitoring?
Air permit	Do grinding, composting and the landfill count as one stationary source? Which compost VOC and odor rules apply?
Water permits	How are compost leachate and pad stormwater kept out of landfill systems and off site?

State practice differs, so meet the solid waste agency early with a concept layout. Our compost facility permitting guide walks through the permit families, and SG’s consulting page lists permitting assistance among its services.

A step sequence for landfill owners

Count the organics by month from scale data.
Screen locations against settlement data, gas infrastructure, drainage, traffic and distance to neighbors.
Meet the regulators before design. Ask which permits are revised and which are new.
Choose a technology that suits the ground. On an active deck, prefer surface-mounted, relocatable systems. On a closed cell, permanent construction is possible once the geotechnical questions are answered. SG describes SG MOBILE® as installed in days to weeks, depending on the site.
Run a pilot on the proposed area to test the recipe, the pad, odor at the property line and the water balance. Miramar and Otay both began with pilot-scale projects, according to SG’s profiles.
Amend the plans and permits with pilot data attached.
Build in phases and leave room to expand or move.
Keep monitoring the cover and gas system, and record any regrading or repairs.

FAQ

Can you build a compost facility on top of a closed landfill?

Yes, if the design protects the closure system and regulators approve the use. Federal rules say post-closure use must not disturb the final cover, liners, containment or monitoring systems. That usually means building up with fill, keeping aeration and utilities above the cover’s low permeability layer, allowing for settlement, and amending the post-closure plan.

Does composting at a landfill count as diversion?

Material that is composted and used as compost is recycled, not disposed, so it supports diversion goals. Using unprocessed green material as daily cover is different. In California, since January 1, 2020, green material used as alternative daily cover is counted as disposal when a jurisdiction’s per capita disposal rate is measured.

How does landfill settlement affect a compost pad?

Uneven settlement changes pad slopes, creates low spots that pond water, and can strain fixed structures and buried utilities. Plan for periodic regrading, use flexible or surface-run connections, avoid rigid foundations and choose equipment that can be moved while a section of pad is rebuilt.

Can compost leachate go into the landfill leachate system?

Only if the permits allow it, and it is rarely the best first choice. It adds volume to a system that is costly to operate. Most designs keep clean stormwater away from the piles, collect the liquid that does contact compost, and reuse it in the process where permitted.

Next step

For the process itself, read how covered aerated static pile composting works. To see how other owners have fitted composting onto disposal sites, browse SG’s project profiles, starting with Miramar and Otay.

Related guides

← All commercial composting guides

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