Biosolids composting mixes dewatered wastewater solids with a carbon-rich bulking agent and holds the pile at high temperature long enough to meet the federal Class A pathogen standard in 40 CFR Part 503. For an aerated static pile or in-vessel system that means 55 degrees Celsius or higher for three days, and for a windrow it means 55 degrees or higher for 15 days with at least five turnings. A utility that also meets the metal limits and a vector attraction reduction option ends up with a product it can distribute in bulk or bags instead of a cake it must haul to a field or a landfill.

Key takeaways

• Part 503 sets two pathogen classes. Class A can go to lawns, home gardens and bagged sale. Class B can be land applied only with site restrictions.
• Composting is listed in Part 503 as a Process to Further Reduce Pathogens (PFRP). The time and temperature differ by method: three days for aerated static pile and in-vessel, 15 days and five turnings for windrow.
• Pathogen reduction alone is not enough. The product must also meet a vector attraction reduction option and the metal limits, and the utility must keep the records.
• Recipe matters more than equipment. Dewatered cake needs a bulking agent for carbon and porosity, and the supply of that agent is often the limiting factor.
• Composting does not destroy PFAS. A utility with elevated PFAS in its solids should resolve that question before investing in any land-based outlet.

Why utilities look at composting

EPA’s use and disposal statistics show where sewage sludge went at the facilities that report to EPA, based on 2024 annual reports: of roughly four million dry metric tons generated, about 59.5 percent was land applied, 24.5 percent was landfilled, 14 percent was incinerated and 2 percent was managed in other ways. Each of those outlets can tighten. A landfill can restrict wet loads or raise its fees, an incinerator eventually needs replacement, and Class B land application depends on willing farms, weather windows and state rules.

EPA’s biosolids technology fact sheet, Use of Composting for Biosolids Management, lists the reasons composting grew: scarce landfill space, rising tipping fees, an emphasis on beneficial reuse, and a product that is easy to store, handle and use. It also notes that the added cost of reaching Class A instead of Class B can be marginal.

What 40 CFR Part 503 requires

Part 503 is the federal rule for the use or disposal of sewage sludge. The requirements below are quoted from 40 CFR Part 503. States can be stricter, so check your state biosolids program as well.

Class A and Class B pathogens

Item	Class A (section 503.32(a))	Class B (section 503.32(b))
Pathogen test	Fecal coliform under 1,000 MPN per gram of total solids, or Salmonella under 3 MPN per 4 grams, dry weight	Geometric mean of seven samples under 2,000,000 MPN or CFU of fecal coliform per gram of total solids
Process route	One of six alternatives. Alternative 5 is treatment in a PFRP, which includes composting	Treatment in a Process to Significantly Reduce Pathogens (PSRP), or an equivalent
Where it can go	Bulk land application, lawns and home gardens, sale or giveaway in a bag or container	Agricultural land, forest, public contact sites and reclamation sites, with site restrictions
Site restrictions	None for pathogens	Yes. For example, food crops whose harvested parts touch the soil mixture cannot be harvested for 14 months after application

Under section 503.15, only Class A material may be applied to a lawn or home garden or sold or given away in a bag.

PFRP time and temperature for composting

Appendix B of Part 503 defines composting as a PFRP in two sentences:

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

The difference is the main regulatory reason utilities choose aerated systems. EPA does not certify compost or composting systems. The utility demonstrates compliance with its own temperature records and lab results. EPA’s pathogen and vector attraction guidance, updated in 2023, explains the alternatives and is described by EPA as a guide, not a regulation.

Vector attraction reduction

Section 503.33 lists the vector attraction reduction (VAR) options. Material sold in bags or applied to lawns and gardens must meet one of options 1 through 8. The one composters normally use is option 5: aerobic treatment for 14 days or longer, during which the temperature is higher than 40 degrees Celsius and the average temperature is higher than 45 degrees. Class A pathogen reduction must be met before or at the same time as VAR.

Metals and the “exceptional quality” shortcut

Section 503.13 sets ceiling concentrations and, in Table 3, monthly average pollutant concentrations: arsenic 41, cadmium 39, copper 1,500, lead 300, mercury 17, nickel 420, selenium 100 and zinc 2,800 milligrams per kilogram, dry weight. Under section 503.10, bulk material that meets the ceiling and Table 3 concentrations, Class A pathogens and one of VAR options 1 through 8 is exempt from the general requirements and management practices that apply to other land-applied sludge. EPA’s fact sheet calls this material Exceptional Quality (EQ) biosolids.

Monitoring and records

Section 503.16 ties monitoring frequency to how much material is applied or prepared each year, on a dry weight basis:

Dry metric tons per 365 days	Monitoring frequency
More than zero, less than 290	Once a year
290 to less than 1,500	Once a quarter
1,500 to less than 15,000	Once every 60 days
15,000 or more	Once a month

Section 503.17 requires the preparer to keep the pollutant data and signed certification statements for five years.

Building the mix: bulking agents and recipe

Dewatered cake is dense, wet and nitrogen-rich. On its own it will not let air through. EPA’s fact sheet says each composting method mixes the solids with a bulking agent to provide carbon and increase porosity, and that the carbon and nitrogen content must be balanced to reach a carbon to nitrogen ratio between 25 and 35 to 1. It names wood chips, sawdust and other materials as common bulking agents, and cites the US Composting Council’s list of suitable materials, which includes yard trimmings.

In an interview published by Sustainable Generation, biosolids consultant Dan Collins, P.E., makes the same points from the operator’s side: the interview gives a typical range of 12 to 30 percent total solids for biosolids, a target mix moisture of 45 to 60 percent, a pH between 6 and 8, and wood chips as a preferred additive because they provide the porosity that airflow needs.

Two practical consequences: a utility needs a reliable supply of wood chips or ground yard trimmings every week of the year, and wetter cake needs more of it, which means more pile volume per ton of solids. EPA notes that the agent is often screened out after composting and reused.

Choosing a composting method

EPA’s fact sheet describes three common methods and says a fourth, the unaerated static pile, is not recommended for wastewater solids because it lacks operational control.

• Turned windrow. Lowest infrastructure, largest footprint. Meeting PFRP takes 15 days at temperature with five turnings, and every turning releases odor.
• Aerated static pile (ASP). Blowers move air through a mix stacked over pipes or trenches. PFRP is three days at temperature. Uncovered ASP usually relies on a finished-compost blanket for insulation, and negative-aeration designs draw exhaust to a biofilter.
• Covered ASP. The same aeration with a membrane cover over the pile, which sheds rain and holds heat and moisture in. Covered ASP with a GORE® Cover is designed to contain odors at the pile, without a building or biofilter.
• In-vessel. Material moves through a silo, tunnel or channel with forced air. EPA notes that in urban and suburban settings in-vessel technology can be more suitable because exhaust air can be contained and treated, and it needs relatively little land. It carries the most mechanical equipment.

Our neutral list of biosolids composting technologies compares specific options, and the odor control solutions list covers the fence-line question that usually decides the method at a treatment plant.

Composting against the other outlets

Outlet	What it needs	Main exposure
Composting to Class A	Bulking agent supply, pad space, aeration, temperature records, a distribution plan	Odor at the plant, product marketing, PFAS in the finished product
Class B land application	Permitted fields, haulers, storage for wet seasons	Site restrictions, farm availability, state PFAS limits
Landfill	A landfill that accepts wet solids, hauling	Tipping fees, capacity, no beneficial use
Incineration	An operating sewage sludge incinerator and air permit	Capital and energy, ash disposal, air emissions

EPA’s June 2026 draft guidance on PFOA and PFOS in biosolids describes land application as often preferred by treatment works because it tends to be less costly and benefits soil. Composting keeps that beneficial use while moving the product up to Class A.

A planning sequence for a utility

1. Characterize the solids: percent solids after dewatering, volatile solids, nutrients, Part 503 metals and PFAS.
2. Confirm the outlet for finished compost first. Giveaway to residents, city parks and landscapers are common starting points.
3. Secure the bulking agent and test recipes at bench or pile scale.
4. Choose the method from the site: available area, distance to neighbors, rainfall and winter conditions.
5. Talk to the state biosolids program and the air and stormwater permit writers before design is fixed.
6. Run a pilot through at least one full batch and collect temperature, pathogen and stability data.
7. Write the monitoring plan: probe locations, logging interval, sampling frequency under section 503.16, and five-year record storage.
8. Size the full facility from pilot bulk density and residence time, with room for curing and storage.

Our list of key metrics for compost quality covers the stability and maturity tests that buyers ask for beyond Part 503.

An example: Florence, Oregon

SG’s profile of the Florence, Oregon wastewater treatment facility describes a small coastal utility that followed roughly that sequence. According to the profile, the City commissioned a study of treatment options, ran a pilot demonstration with an SG MOBILE® System, and chose it for its modular design with no permanent construction or hard-wired infrastructure. SG reports that the pilot produced Class A biosolids compost and met the Part 503 pathogen reduction and vector attraction reduction standards. The City later moved the system to a dedicated asphalt pad with in-ground aeration, added a second unit, and distributes the product locally under the FloGro name. In a testimonial on SG’s site, the City’s public works director says the plant uses yard debris from the community in the mix.

SG states that its covered ASP process is designed to meet the EPA Part 503 PFRP time and temperature requirements, and its SG COMPOST CONTROL™ System logs temperature and oxygen data for reporting. The details of what SG offers wastewater utilities are on its biosolids composting page.

A note on PFAS

Composting is a biological treatment process, not a destruction technology. It does not destroy PFAS, and PFAS in the incoming solids will be present in the finished compost. Part 503 has no PFAS limits as of October 2026, but several states do, and at least one bans sludge-derived compost outright. Read the PFAS and biosolids guide and test your solids before committing to any land-based outlet.

FAQ

What temperature does biosolids compost have to reach?

For Class A under the PFRP route, Part 503 requires 55 degrees Celsius or higher for three days in an aerated static pile or in-vessel system, or 55 degrees or higher for 15 days with at least five turnings in a windrow. Vector attraction reduction option 5 then requires at least 14 days of aerobic treatment above 40 degrees with an average above 45 degrees.

Is composted biosolids automatically Class A?

No. Class A status depends on meeting the time and temperature requirement throughout the pile and passing the fecal coliform or Salmonella test at the point the material is used, sold or given away. The product must also meet a vector attraction reduction option and the metal limits. A batch that misses any of these is not Class A, whatever equipment produced it.

Does EPA approve or certify composting systems?

No. EPA does not certify compost or composting systems. Part 503 sets performance requirements, and the preparer of the biosolids demonstrates compliance with temperature records, laboratory results and signed certification statements kept for five years. Vendors can design a process to meet the requirements, but responsibility for compliance stays with the utility and its permit.

What can be used as a bulking agent?

EPA’s biosolids composting fact sheet names wood chips and sawdust as common choices and cites yard trimmings, agricultural by-products and wood processing residues as suitable. The agent has to supply carbon and create pore space for air. Coarse wood chips can often be screened out of the finished compost and reused in the next batch.

Next step

To see how a covered aerated static pile handles temperature, oxygen and moisture through a batch, read how SG’s composting technology works. If PFAS is the open question for your solids, start with the PFAS guide linked below.

Related guides

• PFAS and biosolids: what utilities need to know
• Aerated static pile composting: the complete guide
• Compost process control: oxygen, temperature, moisture and C:N
• How to run a composting pilot
• Composting system buyer’s guide for wastewater utilities

← All commercial composting guides

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