Composting in Wet, Cold and Arid Climates

Climate does not decide whether a site can compost. It decides where the water goes, how fast piles lose heat, and how much water the operator has to add or remove. Wet sites are designed around keeping rain off active material and containing what does touch it. Cold sites are designed around holding heat and handling frozen loads. Arid and hot sites are designed around water supply, evaporation and fire prevention.

Key takeaways

  • Composting organisms work best in a narrow moisture window. Cornell puts the optimum at 50 to 60%, and every climate pushes piles out of that window in a different direction.
  • In wet climates the main design question is how much water becomes contact water. Pads, ponds and conveyances are sized for a design storm, and anything that keeps rain off the pile shrinks that volume.
  • In cold climates the pile makes its own heat. The task is to lose less of it: larger pile cross-sections, insulation or covers, controlled airflow, and a plan for frozen feedstock.
  • In arid climates evaporation is the largest water loss. Sites need a reliable water source, a way to add water at mixing and during the process, and a fire prevention routine.
  • Most sites have more than one season to design for. Test the method in the season that worries you most before you commit.

How climate acts on a compost pile

Three physical facts explain most of what follows.

First, moisture. The Cornell Waste Management Institute describes a moisture content of 50 to 60% as generally optimum, with microbial activity inhibited below about 30% and problems above 65%, when water fills the pore spaces that air needs.

Second, heat. Cornell explains that the temperature at any point in a pile depends on how much heat the microorganisms produce, balanced against what is lost through conduction, convection and radiation. Decomposition is fastest in the thermophilic range of 40 to 60 degrees Celsius.

Third, the link between the two. Cornell notes that much of the energy leaving a pile is latent heat, the energy used to evaporate water. Moving air through a pile cools it and dries it at once. That is useful in a wet climate and costly in a dry one.

Climate comparison at a glance

Wet Cold Arid and hot
Main risk to the process Saturated piles, anaerobic zones Slow heating, frozen material, heat loss Piles drying below the active range
Main risk to the site Leachate and contact stormwater volume Frozen pads, snow management, iced conveyances Water supply, dust, fire
Design response Covers or roofs, sloped hard pads, separate clean and contact water, storage sized for a design storm Larger pile cross-section, insulation or covers, protected mixing area, buried or drainable air and water lines Water piping to mix and process areas, storage, wind protection, pile height limits

Wet climates: rain, leachate and stormwater

What goes wrong

An open pile in steady rain takes on water faster than it can evaporate it. Once moisture passes the upper end of the working range, pore spaces fill, oxygen falls, and the pile turns anaerobic in zones. Anaerobic material is a common source of odor, and it also slows the process. At the base of the pile, the excess drains out as leachate.

The second problem is volume. Rain that falls on feedstock, active compost or a dirty working surface becomes contact water that has to be captured, stored and reused or treated. Rain that falls on clean roofs and clean pavement is ordinary stormwater. The more of the site that is exposed active material, the larger the contact water volume.

What regulators expect

California’s Composting General Order shows how a water regulator approaches this. Under State Water Board Order WQ 2020-0012-DWQ, which amends the 2015 order, areas used for receiving, processing or storing feedstock and compost must be protected from inundation by surface flows from a 25-year, 24-hour peak storm event at a minimum. Detention ponds, where used, must be able to hold all runoff from the working surfaces plus direct precipitation from the same storm without overtopping. Berms and drainage conveyances are held to the same design storm.

The depth of that design storm differs widely from place to place. In the United States, the NOAA Precipitation Frequency Data Server delivers NOAA Atlas 14 precipitation frequency estimates by location for the areas the atlas covers, and it is where a design engineer starts. Other states set their own design storms, so confirm the requirement with your own water agency.

Design and operating responses

  1. Keep rain off active material. A roof, an enclosed building, or a cover placed on the pile all reduce the contact water volume. Each has different cost and airflow consequences, compared in our guide to covers, biofilters and enclosed buildings.
  2. Separate clean water from contact water. Grade the site so run-on is diverted around the working area, and so roof and clean-pavement runoff never mixes with pad drainage. Sustainable Generation’s article on keeping stormwater and leachate separate covers the reasoning.
  3. Adjust the recipe ahead of wet periods. Start piles toward the dry end of the working range, with more coarse bulking material for structure.
  4. Reuse contact water. Wetting incoming dry feedstock with stored contact water reduces the volume to manage, where the permit allows it.

Our lists of leachate management methods and stormwater practices for compost facilities go further on each option.

One SG project where water separation was a stated selection factor is the West Maui GreenCycle facility in Lahaina, Hawaii, which composts source separated organics and yard waste using an SG MOBILE® system with a GORE® Cover. SG’s profile lists clear separation of leachate from stormwater and moisture management among the capabilities that mattered to the operator’s choice.

Cold climates: heat retention and frozen feedstock

What goes wrong

Cold air does not stop composting, because an active pile generates its own heat. What changes is the rate of heat loss. Small piles, with more surface for each unit of volume, lose heat fastest. Wind strips heat from exposed surfaces. Cold air drawn or blown through a pile by an aeration system removes heat with every cycle.

Feedstock is the second issue. Food scraps and wet yard trimmings can arrive frozen in blocks. Frozen material does not mix evenly, holds its water as ice until it thaws, and absorbs heat from the pile while it does. A pile built largely from frozen loads can sit for days before it warms.

The third issue is the site itself: snow on pads, ice in drainage channels, frozen water lines and condensate, and loaders working on slick surfaces.

Design and operating responses

  • Build larger cross-sections. Taller, wider piles hold heat better. Stay within the height your aeration design and your fire plan allow.
  • Insulate or cover. A layer of finished compost or wood chips, or an engineered cover, reduces surface heat loss and keeps snow and meltwater out of the pile.
  • Control aeration by feedback. Systems that run blowers according to measured oxygen or temperature move only as much cold air as the pile needs. Timer-based systems can over-cool a pile on a cold night.
  • Protect the mixing area. A three-sided shelter or a building for receiving and mixing keeps feedstock workable and gives staff a place to break up frozen loads.
  • Mix frozen loads with active or warm material. Blending incoming loads with dry bulking material and a portion of hot, active compost helps a new pile start.

SG has two cold-region project profiles. Happy Trash Can in Bozeman, Montana composts source separated organics, wood chips and yard waste with an SG MOBILE system. SG’s profile says that when the operator adopted the system, its nearest neighbor was less than 300 feet from the site. The Grand Forks Compost Facility in British Columbia, owned by the Regional District of Kootenay Boundary, composts biosolids, source separated organics and yard and clean wood debris in an SG BUNKER® system with in-ground aeration trenches. SG’s page on winter composting operations has more on cold-weather practice.

Arid and hot climates: moisture loss, water supply and fire

What goes wrong

In dry heat, evaporation removes water from piles faster than the feedstock supplies it. Turning exposes moist interior material to dry air, and forced aeration carries water vapor out with every cycle. A pile that drops toward 30% moisture slows sharply. It can look finished while it is only dry, and it may reheat when it is wetted later.

Fire is the second risk. A Cornell Cooperative Extension fact sheet on compost health and safety states that organic material can ignite spontaneously at moisture contents between 25 and 45%, after heating to over 93 degrees Celsius (200 degrees Fahrenheit), which typically requires a pile over 12 feet high. A 2021 BioCycle article on spontaneous combustion gives a similar critical range of 25 to 40% and advises against pile depths over 12 feet. The BioCycle article says the precursor conditions are more likely in storage piles of unprocessed feedstock, curing piles and product storage piles than in active composting piles.

Design and operating responses

  1. Secure a water source early. Estimate water demand by batch and confirm supply (well, reclaimed water, stored contact water) before selecting a site.
  2. Pipe water to where it is used. Mixing areas and active piles both need it. Water trucks work at small scale and become a labor burden at large scale.
  3. Wet at the mix. It is far easier to get water into material during mixing than into a built pile. The BioCycle article notes that compost piles shed water after the first 2 to 3 inches become saturated.
  4. Reduce evaporation. Fewer turns, wind breaks, shade, and covers all cut water loss. Feedback-controlled aeration avoids moving more dry air than the process requires.
  5. Limit pile height in storage and curing, and walk the piles. The BioCycle article recommends weekly monitoring, looking for vents where steam escapes, and breaking apart piles when temperatures reach 82 to 93 degrees Celsius.

The Mid Valley Disposal facility in Kerman, California, in the San Joaquin Valley, composts source separated organics and yard waste in an SG BUNKER system. According to SG’s profile, a pilot at the site showed the need for water addition in the hot, arid climate, and dedicated water piping was built into the permanent design.

How the method changes the climate problem

No method removes climate from the design. Each one shifts where the exposure sits.

Turned windrows are the most exposed. They fit when rainfall is moderate, land is plentiful and feedstock is mostly yard trimmings. They do not fit well where long wet seasons saturate piles or where water is scarce and every turn costs moisture.

Enclosed buildings and in-vessel systems take weather out of the active phase. They fit when the budget and the permit support a building and its air handling, and they still need outdoor curing and storage to be designed for the climate.

Covered aerated static piles put the weather barrier on the pile. SG’s Cómo funciona page says its covered system “does not need a roof or an enclosed building with a biofilter” and has been applied in hot, wet, dry and subfreezing conditions. The same page says that once a starting moisture of 55 to 65% is achieved in the mix, the system typically carries moisture through the process without added water. “Typically” is the operative word. At Kerman, described above, SG’s pilot showed that a hot, arid site needed water addition, and the permanent design included water piping for it. Confirm water demand for your own climate in a trial, and plan a water supply at an arid site. Mixing, curing and storage areas still sit in the open.

The practical test is a trial in the hard season. Our guide on how to run a composting pilot explains how to set one up.

FAQ

Can you compost through a freezing winter?

Yes. Active piles generate heat, and commercial sites operate through winter in cold regions of the United States and Canada. Success depends on pile size, insulation or covers, controlled airflow, and how frozen feedstock is handled at receiving. Expect batches to take longer to reach temperature, and keep records so you can adjust the recipe when warm-up time grows.

Do compost piles need added water in dry climates?

Usually. Evaporation is the main water loss, and it rises with heat, wind, turning and airflow. Plan to bring the mix to the working range at build and to check moisture during the process. Covers and feedback-controlled aeration reduce the loss, but an arid site should still have piped water available at the mixing and active areas.

What causes compost pile fires?

Most are spontaneous combustion in large, dry-ish piles. Cornell’s fact sheet gives the at-risk moisture range as 25 to 45%, and notes the material must first heat past 93 degrees Celsius, which typically takes a pile over 12 feet high. Limiting pile height, keeping material moist, and checking stockpiles weekly are the standard precautions.

Does a cover or roof remove the need for stormwater controls?

No. It reduces the volume of contact water from the covered area. Receiving, mixing, curing, screening and product storage areas, along with roads, still produce runoff that the permit will address. The design storm requirement applies to the whole working surface, so the benefit shows up as smaller ponds and less water to manage, not as an exemption.

Next step

If your site has a difficult season, test the method in that season before you fix the design. SG’s pilots and demos page describes how its mobile system is used for on-site trials, and the consulting team can review climate data alongside your feedstock plan.

Related guides

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