Oxygen Realities in Compost

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Mother Nature's Farms, Inc.

Fact or Belief?  The following information about oxygen levels is based on our own actual monitoring with oxygen and CO₂ meters.  When we went to forced aeration we needed less than 3" of air pressure to bring the oxygen levels up to above 18%.

We have repeatedly needed to provide over 10cfm of fresh air, per cubic yard of active compost, to hold the CO₂ level at or below 2%.    That is a volume of fresh air more than 23 times the volume of early stage active compost.  We're interested in optimizing the productivity of our microbial master composters.   That we have to provide that much air to hold the CO₂ down to 2% suggests to us that there are a lot of busy microbes consuming the oxygen.

We're assaying the population of active bacteria, and so far our research suggests that when CO₂  is high, there are very few active bacteria.  That research is on-going.

In the graph below, Samples 2536 and 2540 were from 18" below top, center of a passively aerated windrow of yard-waste.  The rest of the samples were from compost on forced aeration.

All of the Cross section profile graphs showing oxygen, CO₂ and temperature, are from passively aerated windrows.

Just one hour after building the windrow, which was about 8' high, 16' across at the base, the oxygen at all of the six cross section profile monitoring points was below our target minimum of 15%.  At the core the oxygen level was down to 5.6%.  This was with horse manure and wood shavings which we have found to not be very biologically active, compared to grass clippings and leaves, or food residuals and shavings, where there is generally much more bacterial activity and oxygen consumption.  While many books and people claim that windrows will adequately aerate by convection,  the passively aerated windrows that we have monitored using oxygen and CO₂ meters showed that convection was not providing adequate oxygen.

The ONLY way to know what the oxygen level is, is to monitor it using an oxygen meter.   Our beliefs about the need for forced aeration are based on our having done over a thousand oxygen and CO₂ tests on real compost.

The above chart shows that within two days the oxygen level at all six cross section profile monitoring points had dropped to below 10%, with the core being below 1% and top center being only 1.1% oxygen.

Why is Oxygen Important?

There are two very important reasons to keep compost aerobic, with oxygen levels greater than 15%:

1. When the oxygen is depleted ANaerobic bacteria take over, and some of their byproducts are phytotoxic, toxic to plant growth, toxic to  germinating seeds.  Some of these intermediate microbial metabolites are VFA's, Volatile Fatty Acids.

Many of the byproducts of anaerobic decomposition are foul odors.




















The above chart illustrates that 6 molecules of oxygen are needed for each molecule of cellulose.  This is a simplification of the reactions, and yet it ought to make it clear that oxygen is vital.












The above chart shows that with as little as 3" of forced aeration (though massive volume) that oxygen levels can be maintained well above 15%, with forced aeration.   Even the core of the pile had an oxygen level of 20.0%.

How fast can the microbes consume the oxygen in a compost pile?  Two critical factors are:

1. How much pore space there is in the compost, space that stores air, which of course starts out containing about 21% oxygen.
2. The other key factor is how many ACTIVE bacteria and other microbes are working in the pile, consuming whatever oxygen is there.

We have done oxygen depletion monitoring, measuring the oxygen content every five minutes to see how fast it can drop when forced aeration is turned off, and recover when the blower is turned back on.  The following chart shows the results of one of the studies we did. 

As you can see, the oxygen crashed from 19%, down to 2%, in just 15 minutes from the time the blower was turned off.  Once the blower was turned back on, at 20 minutes, the oxygen level recovered to 18% in just 5 minutes.  This is not "text book" theory, this is based on oxygen monitoring that we did.   Our belief in the importance of full time forced aeration in large scale composting is based on our own research, thousands of oxygen monitorings.

Before we got an oxygen meter we spent thousands of hours turning compost under the illusion that the turning would significantly help to keep the compost aerobic.  When we got the oxygen and CO₂ meters we learned this vital information, first hand, that we hadn't found in either the many books that we had read, or at the many conferences that we had gone to.

How much air does it take to stay aerobic?  First let's appreciate that with CO₂ much higher than 2 -3%, there will likely be some anaerobic activity; and the foul odors and toxic byproducts associated to anaerobic activity.

We wanted solid answers to the question of how much air is required, so we set up a compost research silo, with forced aeration, with the air going through an Rotameter with a range of 0.5 - 5.0 cfm, enabling us to know within 2% of how much air was going into the bottom of that silo.  The silo has a sealed lid, and we monitored the oxygen and CO₂ in the off-gas.  The Rotameter is pictured to the right.

We believe (and don't know for sure) that the air requirements are proportional to the population of active bacteria, and active fungi, working in the compost, the presumed consumers of oxygen and producers of the Carbon Dioxide.  Our first hand research shows that with the particular compost that we used in our research, that over 3.39 cfm per cubic yard of compost was necessary to hold the CO₂ level down to 2.0%.  That translates to 181 times the entire volume of the pile of fresh air every day.  That's not theory... that is the result of sound research, that we did with real compost, measuring the air flow and monitoring the CO₂ level in the off-gas.  Less fresh air with the same level of active microbes will mean higher CO₂ levels, that will likely suppress the active aerobic bacteria.   We will repeat this research, including with different feedstocks, and during different stages of the composting process, to develop a greater base of information, understanding. 

Compost Research Silos

Because we want facts, we've developed our own compost research silos, with computerized temperature monitoring.  We can provide either up-draft or down-draft aeration.  ALL of the silos that we use for early stage compost, are on "negative pressure" aeration, where we PULL the air through the compost.  That air goes in the inlet of the blower, and the outlet from the blower sends the air through our Dynamic Bio-Filters.  Often we can move fresh air, over 23 times the volume of compost, through the compost, with less vacuum than 0.025"wc, about the 'force' that we use when we blow the candles out after dinner, as measured using a 3 decimal place digital manometer,

We've designed our compost research silo system to enable us to measure the rate of air that we are moving though the compost. We can also easily monitor the oxygen and CO₂ level in the off-gas.  Each Silo has three thermocouples in it providing us with temperature data, one monitoring the temperature of the off-gas, another monitoring the temperature 12" below the lid, and the other monitoring the temperature 24" below the lid.  The Thermocouples are connected to an Omega TempScan, which is connected to a computer. 

This page was last updated on Tuesday, January 26, 2010.