9.4 Composting, Thermal Drying & Alternative Solids Disposal
Key Takeaways
- Composting is an aerobic thermophilic process controlled by carbon-to-nitrogen ratio (about 25:1 to 35:1), moisture (50 to 60 percent), oxygen (above 5 percent in the pile), porosity, and temperature.
- The federal Class A time-and-temperature requirements for composting are 55 degrees Celsius or higher for three days in an in-vessel or aerated static pile, or 55 degrees Celsius for 15 days with at least five turnings in a windrow.
- Bulking agents such as wood chips and sawdust supply carbon and structure; screened bulking agent is recycled, which is one of the main cost controls in a composting operation.
- Heat drying to at least 90 percent solids, alkaline stabilization to pH 12 for two hours with residual pH 11.5, and incineration are the principal alternatives to composting for producing Class A material or eliminating volume.
- Landfilling remains the fallback disposal route and requires passing the paint filter liquids test for free liquids, which is why dewatering performance governs disposal cost.
9.4 Composting, Thermal Drying & Alternative Solids Disposal
The Grade III/IV biological needs-to-know lists Composting, Solids Handling & Disposal, and Sludge Disposal as separate topics. This section covers the disposal end of the solids train.
1. Composting
Composting is aerobic thermophilic decomposition: the same bacteria and fungi that stabilize sludge generate heat faster than the pile can lose it, driving temperatures into the range that destroys pathogens and weed seeds.
The five control variables
| Variable | Target | If it goes wrong |
|---|---|---|
| Carbon-to-nitrogen ratio | About 25:1 to 35:1 | Too little carbon: ammonia odor and nitrogen loss. Too much: slow, cold pile |
| Moisture | 50–60 percent | Too wet: anaerobic and odorous. Too dry (below ~40 percent): biological activity stalls |
| Oxygen | Above 5 percent in pile voids | Anaerobic pockets produce odor and organic acids |
| Porosity / structure | Maintained by bulking agent | Compaction blocks airflow no matter how much air is supplied |
| Temperature | 55–65 °C in the active phase | Above ~70 °C the microbes that do the work are inhibited |
Process types
- Windrow. Long piles turned mechanically. Cheapest; needs land and turning equipment; odor control is harder.
- Aerated static pile (ASP). Pile built over an aeration grid and covered with finished compost or a biofilter layer; air is blown or drawn through. No turning; better odor control.
- In-vessel. Enclosed reactor with controlled aeration and mixing. Highest capital cost, smallest footprint, best odor and process control.
Class A time and temperature
To qualify as Class A under the pathogen reduction alternatives:
- In-vessel or aerated static pile: maintain the composting mass at 55 °C or higher for three days.
- Windrow: maintain 55 °C or higher for 15 days or longer, during which the windrow is turned at least five times.
Temperature must be measured and recorded at multiple depths and locations — the compliance record is the temperature log, and a probe reading taken only at the warm center of the pile will not survive an audit.
Curing and screening
Active composting is followed by a curing period of several weeks, during which the material stabilizes and matures. Screening separates finished compost from oversized bulking agent, which is recycled back into the process — a major operating cost control. Finished compost must be stored and handled to avoid re-contamination, which would defeat the Class A designation.
2. Heat drying
Rotary drum, belt, paddle, and fluidized bed dryers evaporate water to produce a granular product at 90 percent solids or higher, which is one of the recognized Class A processes. Heat drying eliminates most of the hauling volume and produces a marketable fertilizer product, but it is energy intensive and introduces real hazards: dust explosions in the dryer and product handling system, high surface temperatures, and carbon monoxide. Dryer systems require explosion protection, inert or oxygen-controlled operation where specified, and disciplined housekeeping.
3. Alkaline stabilization
Adding quicklime or hydrated lime raises pH and, with quicklime, generates heat through hydration:
- Class B: raise pH to 12 or higher for 2 hours, then maintain pH 11.5 for 22 additional hours without adding more alkaline material — which is also Vector Attraction Reduction option 6.
- Class A (advanced alkaline): pH and temperature combinations specified by the process, typically pH 12 for 72 hours with a temperature hold and subsequent drying.
Advantages: fast, simple, effective, and the product has agricultural liming value. Disadvantages: substantial mass increase, ammonia release at high pH (an odor and worker exposure issue), and caustic dust hazards — quicklime burns and reacts violently with water.
4. Incineration
Multiple hearth and fluidized bed incinerators oxidize the organic fraction, leaving ash. Volume reduction is dramatic, and land application is not required. The trade-offs are capital and fuel cost, air permitting, emissions control (particulates, metals, NOx, and the federal sewage sludge incinerator standards), and ash disposal, which must meet the applicable solid waste requirements.
5. Landfilling
Most utilities retain landfilling as a fallback. Key operational points:
- The landfill will require the material to pass the paint filter liquids test — no free liquids — which is a direct function of dewatering performance.
- Cake solids content drives cost: going from 18 percent to 24 percent solids removes roughly a quarter of the hauled mass.
- Odor and spillage on the haul route are a public relations problem long before they are a compliance problem — use covered, sealed trailers.
6. Choosing among the options
| Route | Capital | Operating | Product value | Main constraint |
|---|---|---|---|---|
| Class B land application | Low | Low | Agronomic | Site availability, setbacks, public acceptance |
| Composting | Moderate | Moderate | Good | Land, odor management, bulking agent supply |
| Heat drying | High | High (energy) | High | Energy cost, dust explosion safety |
| Alkaline stabilization | Low–moderate | Moderate (lime) | Liming value | Mass increase, ammonia odor |
| Incineration | Very high | High | None | Air permitting, ash disposal |
| Landfill | None | Tipping fees | None | Long-term cost escalation, free liquids |
[!NOTE] Keep an alternative available. Utilities that depend on a single outlet — one land application contractor, one landfill — discover the risk when weather closes fields for a month or a landfill changes its acceptance criteria. A written contingency for solids disposal belongs in the plant's emergency plan alongside power and chemical supply.
A utility composts biosolids in windrows and wants Class A pathogen reduction. What time, temperature, and turning conditions must be met?
A compost pile is wet, dense, and giving off a sour, ammonia-like odor, and internal temperatures have fallen. What is the most likely combination of causes?
Why does a landfill require dewatered biosolids to pass the paint filter liquids test?