Composting 101

Composting has been defined as the managed microbial decomposition of Organic Matter into a stable soil like product.  

Simply putting organic matter into piles does not reasonably qualify as environmentally and neighbor friendly "composting".

There are some popular MYTHS in the world of composting that seem to persist.  The first of these is the Myth / theory / belief about passive aeration by convection.   Many people believe that because they see hot air / moisture vapor rising up off the top of a pile of compost, that fresh air is being sucked in at the bottom and that will keep the compost aerobic.   Just because a belief is popular, a lot of people believe it, does not make it true. 

That may be logical THEORY.   Let's see what oxygen and CO₂ meters say when we do cross section profile monitoring, of a passively aerated windrow, to get FACTS, quantified data.

The monitoring points are defined below. 

This first graph represents data collected by monitoring 24 hours after the windrow was built.

We monitored this windrow one hour after it was built, 24 hours, and 46 hours after it was built.  The data for one hour and 46 hours are on the Facts vs. Myths page of our website.  We've monitored many windrows at different compost sites, and we almost always come up with this same pattern.   If the oxygen level is good, we question whether there are any oxygen consuming microbes present.

How important is oxygen to the composting process?  

Could YOU function well without oxygen?  Of course not!  Well, the most efficient microbes are aerobic, that is, they function with oxygen.  Just because they are microbes, microscopic, too tiny to be seen with the naked eye, does not mean that they don't consume oxygen.

What about that hot air we see rising off piles of compost?   Isn't that causing air to be pulled in at the sides?  Yes, it is.  And, when you look at the above graph, you see that monitoring points "A" and "C", those points where air is being pulled in, have the highest oxygen levels, even if far below the minimum target level of 15%.

What happens when we 'check up' on our microbial master composter workforce, to see how many active bacteria we have?    Because we are committed to Excellence, we believe that we've got to be able to check up on our microbial workforce.  That involves being able to quantify the number of active bacteria, doing direct estimates, in our own lab.  Since bacteria, the sort of critters that digest our breakfast, lunch and dinner, are so tiny, we use a 1000X microscope.    We figure that ignorance is far more expensive than a $24,000 microscope. 

As the legend says, the red bars represent the population of ACTIVE bacteria, referencing the Y scale on the left which represents billions of active bacteria, and the blue bars represent the CO₂ level at the point where the sample was taken, and reference the Y scale on the right.  The numbers above the red bars also represent billions.  The most interesting part of this graph is that Samples 2536, 2540 and 2541 were all yard waste compost from the Town of Yorktown,  taken from 12" below top, center, of a windrow that had been built about 10 days before sample 2536 was taken.  When we took that sample, we also collected enough to fill one of our compost research silos.  Note that the CO₂ level was off the top of the scale on our meter, and the active bacteria population was down at only 100,019,234, per gram, dry weight, of compost. (the very low red bar for Sample Number 2536.    2540 and 2541 were sampled 6 days later, 2540 from the same spot where 2536 had been taken.  2541 was material from that spot, that had been placed in one of our compost research silos for 6 days, with forced aeration, maintaining CO₂ at 2% or less during the entire 6 days.  Holding the CO₂ at 2% or less seems to have resulted in 45 times more active bacteria than we found in 2540.

There seems to be pretty much an inverse relationship between oxygen and CO₂.  When CO₂ is low, oxygen is high, and when CO₂ is high, oxygen is low. 

How much fresh air does it take to hold the CO₂ at 2% or less?

We've set up our compost research silo aeration system so that we can quantify the volume of air that we are moving through the compost.   Often, with early stage compost, we find that we need over 10cfm of air, per cubic yard of compost.  That translates to a volume of fresh air equal to over 23 times the volume of compost, every hour, 24/7, .    That's not guessing.  That's not theory.  This is based on actually metering the rate of air flow, and the CO₂ level.

We are still amazed that life forms that we can't see with our naked eyes, can consume that much oxygen. 

We did our first oxygen depletion test in January of 1996, and discovered that the microbes depleted the oxygen from 19% down to 2% within 15 minutes.  For us, that was a major wake up call.  Combined with our own cross section profile monitoring of oxygen and CO₂ in passively aerated windrows,  we were convinced that if we are to provide our microbial master composters with good working conditions, which includes oxygen, then we've got to provide more fresh air.

How much pressure or vacuum does it take to move enough air through the compost?  That depends on the porosity of the compost, and how deep the compost is.  Some of our bio-filters have 60" of compost in them, and we are able to move the air through them, at that rate, with as little as 0.193"wc of pressure.  To measure pressure and vacuum in those very low ranges, we use an Omega model HHP-401-1 digital manometer.   We need facts, and tools like this enable us to get facts.  Click on the image of the digital manometer to go to the Omega website page listing the Digital Manometer.

How do we monitor oxygen and CO₂?

We use Fyrite meters, available from Grainger.   Click on the image if you want to go to the Grainger website page listing them.   We have a custom made probe that lets us pull gas samples and measure pressure / vacuum 8' into a pile of compost.  To access the page on the Grainger website, you may have to fall back to their home page, and search for "Fyrite Oxygen".

This page was last updated on Wednesday, November 29, 2006