Composting System Buyer’s Guide for Wastewater Utilities

A wastewater utility should buy a composting system when it needs a second outlet for its solids, can make a Class A product that local users want, and has the space, bulking material and staff to run the process beside the plant. Composting changes what the solids are and who will take them. It does not remove PFAS, so a utility whose outlet problem is PFAS needs to settle that question before it compares composting systems.

Key takeaways

  • Start from outlet risk. List every place your solids go today and what would close each one.
  • Class A under 40 CFR Part 503 is a pathogen standard with a defined time and temperature route for composting. The buying question is whether a system lets you document it batch after batch.
  • The composting pad must hold the mix, not the cake. Bulking material can multiply the volume, and that sets the footprint.
  • Odor at a treatment plant is judged at the fence line, and the plant already has a baseline. Plan for cake handling and mixing as well as the piles.
  • Secure the amendment supply and the compost outlet in writing before you size the system.
  • Composting is not the answer where compost made from sludge cannot legally be land applied, or where PFAS levels rule out distribution.

Decision 1: Which outlets are at risk

According to EPA, about four million dry metric tons of sewage sludge were generated in 2024: roughly 2.39 million land applied, 982,000 landfilled and 558,000 incinerated (EPA basic information on biosolids). Most utilities depend on one or two of those routes, usually through a contractor, and each route has its own pressure: landfills that limit wet loads, farms that sell or change hands, haul distances that grow, and incinerators that age.

PFAS is the newest pressure and the least settled. EPA released a draft sewage sludge risk assessment for PFOA and PFOS on January 14, 2025. It is a draft assessment, not a regulation (EPA, PFAS in sewage sludge). On July 1, 2026 EPA released draft guidance with voluntary recommendations for reducing risk from PFOA and PFOS in biosolids, with a public comment period that EPA extended to October 5, 2026 (EPA draft guidance). This is the position as of October 2026; check both pages for changes. States have moved on their own. Maine banned the land application of sludge and sludge-derived products, effective August 8, 2022 (Maine DEP).

Be plain with your board about this: composting does not destroy PFAS. If your solids carry PFAS at levels that concern your state, compost made from them carries it too. Test first. The PFAS and biosolids guide covers the topic in depth.

Write an outlet register like this before going further:

Current outlet What could close it Would Class A compost help?
Class B land application Loss of fields, public opposition, state restrictions Yes, if compost use is allowed and the product is clean
Landfill Tip fee increases, wet waste limits, diversion laws Yes, it removes solids from the landfill
Incineration Equipment age, air permit renewal Yes, as a replacement or a backup
Contract hauler to a third-party processor Contract ends, processor closes Yes, it brings processing under your control
Any outlet, where PFAS is the constraint State PFAS limits or bans No, composting does not remove PFAS

Decision 2: What product you are making

Class A is the goal for most utility composting projects because it opens distribution to the public. Under Part 503, Class A Alternative 5 requires treatment in a Process to Further Reduce Pathogens (PFRP) and either fecal coliform below 1,000 Most Probable Number per gram of total solids or Salmonella below three Most Probable Number per four grams of total solids, on a dry weight basis (40 CFR 503.32). Appendix B defines the composting PFRP: for the within-vessel or static aerated pile method, 55 degrees Celsius or higher for three days; for windrows, 55 degrees or higher for 15 days or longer with at least five turnings in that period (40 CFR Part 503, Appendix B).

Vector attraction reduction is a separate requirement. The option most composters use calls for an aerobic process of 14 days or longer, with the temperature above 40 degrees Celsius throughout and averaging above 45 degrees (40 CFR 503.33).

Two points for buyers. First, EPA does not certify compost or composting systems. The utility meets the rule by operating the process and keeping records, so ask each vendor how temperatures are measured, logged and reported per batch. SG says its covered ASP process is designed to meet the EPA Part 503 PFRP time and temperature requirements, and that the SG COMPOST CONTROL™ System gathers the data needed for regulatory reporting (control system page). Second, pathogen class is not the whole of product quality. In SG’s interview with a biosolids expert, consultant Dan Collins, P.E. points out that biosolids must meet EPA limits for contaminants such as metals to qualify for land application even before composting, and that poor operations producing a poor product hurt the whole sector. The biosolids composting guide explains the rule in full.

Decision 3: Whether it fits on the plant site

Treatment plants are short of land, and what land they have is often reserved for future process tanks. Size the composting area from the mix, not the cake:

  1. Cake solids. In SG’s interview, Collins gives a typical range of 12 to 30% total solids for biosolids, and a target mix moisture of 45 to 60% with a carbon to nitrogen ratio of 25 to 35:1.
  2. Bulking ratio. Wetter cake needs more wood chips or yard trimmings to reach that moisture and to hold pile structure. Better dewatering shrinks the compost pad.
  3. Residence time. Active composting, then curing. Shorter active time means fewer piles on the pad at once.
  4. Support areas. Amendment storage, a mixing area, screening, and covered or contained product storage.
  5. Water. Where pad runoff goes. A plant has an advantage here: process water can usually return to the headworks, subject to your own permit.

Method matters for footprint. Turned windrows need aisles for the turner and a 15-day high temperature period with five turnings. Aerated static piles are taller, are not turned in the active phase, and need three days at temperature. In-vessel systems are compact but mechanically complex. SG’s covered ASP places a GORE® Cover over a positively aerated pile, with no building and no biofilter, and SG’s biosolids page describes the range of support it offers utilities. The list of biosolids composting technologies compares the options by name.

The City of Florence, Oregon shows what a small plant can do. According to SG’s profile, the city commissioned a study of biosolids treatment options, ran a demonstration with an SG MOBILE® System at its wastewater treatment facility, chose it in part because it needed no permanent construction or hard-wired infrastructure, then moved the system to a dedicated asphalt pad with in-ground aeration and later added a second unit (profile).

Decision 4: How you will manage odor at the fence line

Neighbors do not separate the plant’s odors from the compost pad’s. A new operation is judged against the existing baseline, and the first complaint after startup will be blamed on it. Map the odor sources in order:

  • Cake storage and transfer. Dewatered cake that sits turns septic. Mix it the day it is produced.
  • Mixing. This is the most exposed step with any outdoor method. Schedule it for favorable wind and finish promptly.
  • Active piles. This is where methods differ. Open windrows release odors at each turning. Negative ASP pulls air down through the pile to a biofilter that needs media care. Covered positive ASP with a GORE Cover is designed to contain odors at the pile.
  • Curing and screening. Material that left the active phase too early will smell here.

Write an odor management plan and a complaint procedure before startup, and agree with operations staff who responds. The odor management guide covers measurement and complaint handling, and SG’s Natural Soil Products profile describes a biosolids composter in Pennsylvania that moved from open windrows to covered bunkers after community complaints and regulator scrutiny.

Decision 5: Where the amendment comes from

A biosolids composting project fails without a steady bulking supply. Wood chips are the preferred amendment in Collins’s account because they give the porosity that air needs to move through a dense, nitrogen-rich material. Ask:

  • Does the city’s own yard trimmings or tree crew output cover the need in every season?
  • If not, who supplies chips, under what contract, and at what particle size?
  • Can screened overs be reused as bulking material to cut purchases?
  • Where will a seasonal stockpile sit, and how is it kept from becoming a fire or dust problem?

The Florence public works director, quoted on SG’s site, cites an abundance of yard debris from the community as one reason the city tried composting. A utility without that supply should price out and contract for it before choosing a system size.

Decision 6: Who takes the compost

Decide the outlet before startup: city parks and public works, giveaway to residents, bulk sale to landscapers and soil blenders, or agricultural use. Each has its own quality expectation and its own delivery logistics. SG’s Florence profile reports that the city branded its compost FloGro, first offered it free to residents, and saw demand exceed supply. That outcome followed a consistent product and local outreach, which Collins stresses in the interview, along with environmental management programs such as the National Biosolids Partnership.

Plan storage for the months when nobody is spreading compost, and test the product on a schedule that your state permit and your customers accept. The compost quality metrics list explains the tests.

Decision 7: How to buy it

Checklist before you issue a request for proposals:

  • [ ] Outlet register complete, with PFAS results for your solids
  • [ ] Product goal stated (Class A, intended users)
  • [ ] Cake solids data for at least a year, with the seasonal range
  • [ ] Amendment source identified and quantified
  • [ ] Site area mapped, with drainage to the headworks confirmed
  • [ ] Odor baseline and nearest receptors documented
  • [ ] Staffing plan: who mixes, builds piles, monitors and screens
  • [ ] Pilot plan, if the board or regulator wants local data
  • [ ] Funding route checked with your state revolving fund program

On funding, EPA lists construction of publicly owned treatment works among the project types eligible for Clean Water State Revolving Fund loans (EPA CWSRF). Whether a composting facility at your plant qualifies is your state program’s call, so ask it early. A pilot is the low-commitment way to produce Class A data on your own cake. SG runs pilots with the SG MOBILE System (pilots and demos) and offers it under a leasing program. For permanent, larger installations SG rates the SG BUNKER® System at 5,000 to 500,000 tons per year.

Where composting, covered ASP or SG is not the right fit

  • PFAS-limited solids, or a state that bars sludge-derived compost from land. Composting does not solve this.
  • No amendment within a reasonable haul. Without bulking material the process does not work at any scale.
  • No land at or near the plant and no partner site. Thermal drying or a regional processor may fit better.
  • A plant that already makes a Class A product by another process and has a secure outlet.
  • Energy recovery as the priority. Digestion comes first, and composting can follow for the dewatered digestate.

FAQ

Does composting biosolids always produce Class A material?

No. The process has to meet the Part 503 pathogen requirements, including the PFRP time and temperature conditions and the fecal coliform or Salmonella limit, plus a vector attraction reduction option, and you have to document it. A system helps by making temperatures uniform and by recording them. Poor mixing, wet piles or cold spots can cause a batch to fall short.

Will composting remove PFAS from our biosolids?

No. Composting stabilizes organic matter and reduces pathogens. It does not destroy PFAS. If PFAS in your solids is the reason an outlet is closing, test the solids, talk to your state regulator about limits for compost, and decide on that basis whether a compost product would have a legal and acceptable market.

How much bulking material will we need?

It depends on cake solids. Wetter cake needs more amendment to reach a workable moisture and porosity. SG’s biosolids interview cites a target mix moisture of 45 to 60% and biosolids solids content typically between 12 and 30%. Have a recipe calculated from your own cake and the amendment you can actually get, then confirm it in a pilot batch.

Who buys or uses biosolids compost?

Typical users are city parks and public works departments, residents, landscapers, soil blenders and farms, depending on state rules. Florence, Oregon gave its compost to residents at first, according to SG’s profile, and demand exceeded supply. Outlets depend on consistent quality, clear labeling and public education, so budget for testing and outreach.

Next step

Send SG your cake solids data, annual quantity and a site sketch through the contact page and ask for a preliminary mix recipe and pad layout. If you are still deciding whether composting belongs in your solids plan, the consulting page describes SG’s pre-design support.

Related guides

← All commercial composting guides

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