Compost Process Control, Oxygen, Temperature, Moisture and C:N

Compost process control means keeping four things in range so that aerobic microbes can do the work: oxygen in the pore space, temperature, moisture, and the carbon to nitrogen (C:N) ratio of the mix. Two of them, C:N and starting moisture, are set on the day the pile is built. The other two, oxygen and temperature, are managed every hour after that, by turning in a windrow or by blowers in an aerated pile. Records of those readings are also how a facility proves pathogen reduction to its regulator.

Key takeaways

  • Cornell Waste Management Institute’s working targets are a starting C:N ratio around 30 to 1, moisture of 50 to 60%, oxygen above 10% in the pore space, and temperatures in the thermophilic range without running much past 65 degrees Celsius.
  • The four parameters are linked. More air raises oxygen but also removes heat and water. More water supports microbes but fills the pores that air needs.
  • Recipe problems cannot be fixed with a blower. Get C:N, moisture and structure right at the mixer.
  • Aeration can be run on a timer, on temperature feedback or on oxygen feedback. Each step up matches the air supply more closely to what the microbes are using.
  • For biosolids, 40 CFR Part 503 sets the time and temperature record a facility must be able to show, and records are kept for five years.
  • Sensors only help if someone checks them. Calibrate probes, place them where your rule or permit specifies, and read the trend, not just the latest number.

The four parameters and their target ranges

The ranges below come from Cornell’s composting science pages on compost chemistry and compost physics. They are starting points, not set points.

Parameter Working target Below the range Above the range
C:N ratio at the start Around 30 to 1 Excess nitrogen is lost as ammonia gas The mix is short of nitrogen and will not heat up
Moisture content 50 to 60% by weight Under about 30%, bacterial activity is inhibited Over about 65%, pores fill with water and decomposition slows
Oxygen in the pore space Greater than 10% Under about 5% average, anaerobic regions develop Not a biological problem, but it usually means more air (and more heat and water loss) than needed
Temperature Thermophilic, 40 to 60 degrees Celsius for the fastest decomposition, and 55 or higher for the pathogen reduction period The pile is too dry, too wet, short of nitrogen or too small Past about 60 to 65 degrees, most microbe species cannot survive
pH 5.5 to 8.5 Acids from anaerobic zones can pull pH down to 4.5 Outside the range Cornell gives as optimal for compost microorganisms

Carbon to nitrogen ratio

Microbes use carbon for energy and nitrogen to build protein. Cornell gives about 30 to 1 as the usual recommended starting ratio and notes that it falls to between 10 and 15 to 1 as composting proceeds and carbon leaves as carbon dioxide. With too little nitrogen the mix will not heat up. With too much, nitrogen is lost as ammonia, and the pile can overheat or turn anaerobic.

Treat the calculation as the first draft of a recipe and the first week’s temperatures as the test of it. The list of common compost feedstocks summarizes what each material contributes.

Moisture

Water is where the biology happens: microbes live in the film of water on each particle. Cornell’s physics page gives 50 to 60% as the generally accepted optimum. Too little water stops bacterial activity. Too much fills the pore space, and Cornell’s page on factors leading to anaerobic conditions names excess moisture as the most common cause of them.

Moisture is measured by drying a sample. Cornell’s moisture content method is to weigh a small sample, dry it for 24 hours in an oven at 105 to 110 degrees Celsius, and reweigh it. The weight lost, divided by the wet weight, is the moisture content.

Oxygen

Air is 21% oxygen. Cornell reports that the pore space of a fresh pile starts at about 15 to 20% oxygen, that concentrations greater than 10% are considered optimal for aerobic composting, and that when the average falls below about 5%, anaerobic regions develop. Anaerobic zones are where foul-smelling compounds form, and Cornell notes that acid accumulation in an anaerobic system can lower the pH to 4.5.

Temperature

Temperature is both a result and a control target. Heat is produced by microbial activity, so a pile that warms up within a few days confirms the recipe had enough nitrogen and moisture. Cornell notes that decomposition is fastest in the thermophilic range of 40 to 60 degrees Celsius, and that most microorganisms cannot survive above 60 to 65 degrees, which is why managers try to keep piles below about 65.

The pathogen reduction requirement sets the floor: 55 degrees or higher for the required period. The operating window during that period is therefore narrow.

The parameter nobody lists: porosity

None of the targets can be held if air cannot move through the pile. Particle size, bulking agent and pile height decide how much free air space there is. Sustainable Generation (SG) puts the trade-offs plainly in its article Speed Gets Attention. Consistency Keeps the Facility Running: more aeration can improve oxygen but can also remove heat and dry the pile, and smaller particles add surface area while too many fines compact the pile.

How the parameters push on each other

  • Air and heat. Moving air is the main way an aerated pile sheds heat. Aerating hard enough to cool a pile can supply more oxygen than the microbes need.
  • Air and water. Warm air leaving the pile is saturated. Every blower cycle exports water, so heavily aerated, uncovered piles dry out and need rewetting.
  • Water and air. Each pore is filled with either water or air. A wet mix has less room for oxygen and needs more structure.
  • Nitrogen and carbon. Low C:N mixes release their surplus nitrogen as ammonia. That is lost fertilizer value as well as an emission.

Because of these links, chasing a single number causes trouble.

Measuring: probes, sensors and where to put them

Temperature. Cornell’s temperature monitoring guidance is to use a probe that reaches deep into the compost, leave it long enough for the reading to stabilize, and take readings at several locations and depths. In aerated systems, it notes, the hottest locations tend to be two-thirds or more of the way up. For compliance, take readings at the locations and depths your rule or permit specifies, not where the pile is hottest.

Oxygen. Oxygen is read with a gas probe drawn from the pore space or with an in-pile sensor. A logged trend tells you more than a spot check.

Moisture. Moisture is checked by sampling. Plan for oven-dried samples at the mix stage and whenever material is moved.

Blower data. Blower run time and pressure show how hard the system is working.

Feedback aeration: timer, temperature and oxygen control

In an aerated static pile the blower is the control lever. There are three common ways to run it. The guide to aerated static pile composting covers the floors and blowers themselves.

Timer. The blower runs on a fixed cycle. A BioCycle article on ASP design notes that early systems used clock timers, usually 20 minutes on and 40 minutes off each hour, and calls that still a valid strategy. It is simple, and it delivers the same air on day 2 and day 20 although demand differs.

Temperature feedback. A probe in the pile starts the blower when the temperature passes a set point and stops it when the pile cools. This protects the biology from overheating. One supplier of temperature-controlled covered piles, quoted in a BioCycle article on covered ASP design, says this typically works out to 3 to 5 minutes of blower run time every 30 minutes.

Oxygen feedback. An oxygen sensor starts the blower when pore-space oxygen falls to a set point. In the same BioCycle article, the manufacturer of one membrane-covered system describes oxygen set points chosen by mix recipe and says its fans generally run about 25% of the time.

A separate BioCycle article on aeration system design recommends continuous-duty aeration or very short duty cycles, on the order of minutes, not hours, so that supply follows demand. It also notes a limit of variable-speed drives: fan performance drops off quickly below about 20% speed, which is one reason many systems cycle fans on and off instead.

How one supplier does it

The SG COMPOST CONTROL™ System is SG’s cloud-based monitoring and control platform for its covered piles. According to SG’s product page, it tracks temperature, oxygen and blower metrics in real time, lets the operator configure pile settings for different feedstocks and climates, sends alerts by email, text or dashboard when thresholds are exceeded, and exports hourly, daily, weekly or regulatory reports. SG’s systems use positive aeration controlled by oxygen and temperature feedback under a GORE® Cover. SG also states that once the initial mix is at 55 to 65% moisture, its covered system typically carries that moisture through the process without added water. Several SG project profiles, including the Florence, Oregon wastewater treatment facility, which composts biosolids, list oxygen control mode as the control parameter.

A daily and weekly control routine

  1. At the mixer: confirm the recipe by weight or bucket count, and check moisture on the blended material before it is placed.
  2. At pile build: record the batch ID, date, feedstocks, pile location and probe positions.
  3. Daily during the pathogen reduction period: record temperatures at the required locations and depths, and confirm each is at or above 55 degrees Celsius.
  4. Daily for the rest of the active phase: review temperature and oxygen trends and blower run time. Look for zones that lag the others.
  5. When a reading is out of range: find the cause before changing a set point.
  6. At each move or turn: sample moisture, add water if needed, and note the condition of the material.
  7. At batch close: file the complete temperature record with the batch and sample the product for the tests your permit and market require. The list of key metrics for compost quality explains those tests.

What records regulators ask for

Biosolids: 40 CFR Part 503

EPA does not certify compost or composting systems. The facility that prepares the biosolids compost certifies its own compliance and keeps the records that support it. Three parts of the rule drive process records:

  • Pathogen reduction. Appendix B to Part 503 defines composting as a Process to Further Reduce Pathogens when the temperature is maintained at 55 degrees Celsius or higher for three days in a within-vessel or static aerated pile system, or at 55 degrees or higher for 15 days or longer with a minimum of five turnings in a windrow.
  • Vector attraction reduction. One option, 40 CFR 503.33(b)(5), is an aerobic process of 14 days or longer during which the temperature stays above 40 degrees Celsius and averages above 45 degrees.
  • Recordkeeping. Under 40 CFR 503.17, the preparer keeps a description of how the Class A pathogen requirements and the vector attraction reduction requirement were met, with the certification statement the rule prescribes, for five years.

Other feedstocks: state rules

States set the rules for food waste, green waste and manure composting, and many borrow the Part 503 time and temperature standard. California is a clear example. Under 14 CCR 17868.3, each day during the pathogen reduction period an operator takes at least one temperature reading for every 150 feet of windrow or every 200 cubic yards of active compost. Windrows are read 12 to 24 inches below the pile surface, and aerated static piles 12 to 18 inches from the point where the insulation cover meets the active compost. California’s recordkeeping section requires records to be kept in one location and accessible for five years. Check your own state’s composting rule and your permit, since monitoring points and frequency differ.

Whatever the rule, keep the batch ID, build date and feedstocks, the daily temperature readings with location and depth, dates of turns or moves, out-of-range events and the response, and product test results together as one batch record. Automated logging removes the clipboard, not the responsibility: someone still has to review the record and sign for it.

FAQ

What is the ideal C:N ratio for composting?

Cornell Waste Management Institute gives about 30 parts carbon to 1 part nitrogen as the usual recommended starting point. Lower ratios lose nitrogen as ammonia and can overheat or go anaerobic. Higher ratios heat slowly or not at all. The ratio falls to roughly 10 to 15 to 1 as composting proceeds.

What oxygen level should a compost pile have?

Cornell’s guidance is that oxygen above 10% in the pore space is optimal for aerobic composting and that anaerobic regions develop when the pile average falls below about 5%. Fresh piles start at about 15 to 20%. In a system with oxygen feedback, the set point is chosen for the recipe, and the blower runs when readings fall to it.

How hot is too hot for compost?

Cornell notes that most microorganisms cannot survive above about 60 to 65 degrees Celsius, so operators generally try to hold piles below about 65. The pathogen reduction period requires 55 degrees or higher, which leaves a working band of roughly ten degrees. Sustained temperatures well above that band slow decomposition, because fewer organisms survive to do the work.

How long do compost temperature records have to be kept?

For biosolids compost, 40 CFR 503.17 requires the preparer to retain the supporting records for five years. California’s composting regulation also sets five years for facility records. Other states and individual permits set their own periods, so read the permit.

Next step

To see how these parameters are held in a covered aerated pile, read how SG Advanced Composting™ Technology works. For the monitoring side, the SG Compost Control System page shows what is logged and reported.

Related guides

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