Hot Composting, Explained

What is hot composting?

Composting is microbes eating organic matter and leaving behind something stable and dark that plants love. If you want the basics, start with what is composting.

Hot composting is the managed version. Instead of tossing scraps in a heap and waiting a year, you build the pile so the microbes work as hard as they can. When they do, they throw off a lot of heat. The EPA puts the sweet spot for those microbes at 131 to 160 degrees Fahrenheit.

A cold pile still breaks down, just slowly, and it may not get hot enough to kill weed seeds or pathogens. A hot pile does both, and it does it in weeks instead of months.

Where does the heat in a compost pile come from?

From life. Bacteria and fungi break down sugars, starches and proteins and release energy as heat. As the pile warms, the cool-loving microbes slow down and heat-loving ones, called thermophiles, take over. UF/IFAS Extension says thermophiles prefer roughly 105 to 140 F.

Those thermophiles push the pile into the range that matters. When the food runs low, the pile cools, other microbes and fungi move back in, and the material cures into finished compost.

What temperature kills pathogens in compost?

131 F is the number you hear over and over, and for good reason. EPA guidance says pile temperatures must stay at 131 F or above for several days to meet standards for reducing pathogens (EPA).

The federal rule behind that number comes from 40 CFR Part 503, the EPA regulation for treating biosolids. Its "Process to Further Reduce Pathogens" spells it out (Cornell LII):

1. Windrows (long piles): 131 F (55 C) or higher for 15 days or longer, with at least five turnings during that time.
2. In-vessel or aerated static piles: 131 F or higher for 3 days.

Turning matters in a windrow because the outside of the pile is cooler than the core. Each turn moves the outside material into the hot middle so every bit gets its time in the heat. That is why we say our compost is hot-composted to EPA standards: 131 F or higher for the required time.

For how that connects to the food on your plate, read is compost safe for vegetables.

What do you need to make a pile run hot?

Four things, in balance.

Carbon and nitrogen. Food scraps are rich in nitrogen. Wood chips, leaves and shredded paper bring carbon. The EPA calls a starting ratio of about 30 parts carbon to 1 part nitrogen ideal, and warns that much below 25 to 1 can cause odor (EPA).

Water. Microbes need it, but too much drowns the air out. Cornell puts the best moisture range at 40 to 60 percent. Think of a wrung-out sponge.

Air. Hot composting is aerobic. Turning a windrow or blowing air through a static pile keeps oxygen moving. Without air, the pile goes sour and starts to smell.

Size. A small pile loses heat too fast. Big piles hold the heat in the core, which is why commercial operations build long windrows instead of backyard bins.

Can you compost meat and dairy with hot composting?

Yes, and this is the big reason commercial composting takes food that backyard bins should not. A cold pile with meat in it can smell and draw animals. A properly managed hot pile breaks down meat, bones and dairy along with everything else, and the heat takes care of the pathogens.

That is why Melly's accepts all food scraps, including meat, bones and dairy, in our red bins. If it was food, it can go in.

How long does hot composting take?

The hot phase runs for weeks. Then the pile needs time to cool and cure, so the microbes finish the job and the compost becomes stable. Finished compost is dark, crumbly, and smells like soil, not like food.

How long the whole process takes depends on the mix, the method and the weather. Rushing curing is a common mistake. Unfinished compost can tie up nitrogen and stress young plants.

How does hot composting work in South Florida?

Our climate helps and hurts. Heat is on our side: piles warm up fast, and a cold snap that would stall a pile up north barely matters here. But our heavy summer rain can soak a pile and push it past the moisture sweet spot, so drainage and pile shape matter. Food scraps also start breaking down the minute they leave the kitchen in our heat, which is why we pick up often, never less than weekly, and swap in a clean bin every time.

The payoff is local. South Florida soils, sandy in some places and limestone in others, are short on organic matter. Finished compost made from Miami-Dade's food scraps goes back to the region's growers. That is the loop: scraps from the city, heat in the pile, soil for the farm.

Frequently asked questions

What temperature should hot compost reach?

Aim for 131 to 160 F. EPA guidance says 131 F or higher held for several days is needed to reduce pathogens, and federal windrow rules call for at least 15 days at that temperature with at least five turnings.

Is hot composting better than cold composting?

For safety and speed, yes. Hot composting kills pathogens and weed seeds and finishes much faster. Cold composting is fine for a backyard pile of leaves and vegetable peels, but it is not the right method for meat, dairy or large volumes.

Why does my compost pile smell?

Usually too much nitrogen or too much water, which pushes out the air. Add dry carbon like wood chips or shredded leaves and turn the pile to get oxygen back in.

Can I buy hot-composted compost in bulk?

Yes. Melly's sells finished compost wholesale by the cubic yard, with pickup at our yard by appointment. Delivery is coming soon.

If you grow for a living and want compost made from Miami-Dade's food scraps, get wholesale pricing. Pricing depends on volume, and we will walk you through what is in the pile.

Sources

• EPA: Approaches to Composting
• Cornell LII: 40 CFR Appendix B to Part 503, Pathogen Treatment Processes
• EPA: Composting, Chapter 7 (archive)
• Cornell Waste Management Institute: Water Quality Protection, Cornell Composting
• UF/IFAS Extension Sarasota County: Composting Bacteria