Facts versus Myths
Seeking Truth

During our first three years in composting we relied on what we had read and what we 'learned' at BioCycle conferences regarding turning compost to keep it aerobic, and that windrows aerate by convection.   We estimate that we spent about $100,000 turning our compost, under the illusion that the turning was significantly helping to keep it aerobic.

In the Fall of 1995 we started to learn about VFA's, volatile fatty acids; that they are byproducts of anaerobic decomposition, and that they are phyto-toxic.  Since we were depending on our compost performing well, anything phyto-toxic was totally unacceptable.   Was our compost aerobic or anaerobic?  

In January of 1996 we bought an oxygen and CO₂ meter set, with a probe that let us pull samples from 40" in from the surface of the compost pile.  Since then we've probably done well over 1,000 oxygen monitorings, which let us find out just what the oxygen level was at the location monitored.  We knew we needed facts. 

Based on Dr. Jean VanderGheynst's work at Cornell, we figured we want to maintain oxygen levels at or above 15%.   The above chart reflects data from a passively aerated windrow of horse manure and wood shavings, one hour after the windrow was first built.  Note that the blue bars are temperature, and relate to the "Y" axis scale on the right.    The monitoring locations are indicated below, and we always use the same pattern when monitoring windrows.

Note that within a single hour, while the temperature was rising nicely, the oxygen levels at all locations were significantly below our goal of a minimum of 15%.  In FACT, our Oxygen Meter indicated that 4 of our 6 monitoring points had oxygen levels at or below 10%, just an hour after building the windrow.

We monitored the same windrow a day, about 24 hours after building it.  The above chart reflects what our oxygen and CO₂ meters told us; and what the thermometer indicated the temperature to be at each of the monitoring points.  Convection may have been helping a little bit.  Our logic says that monitoring points "A" and "C" would show the greatest benefits of convection, and, indeed, those two points had the highest oxygen levels, even if only 7 and 8%; far below our desired target range.

Did "Aeration by Convection" simply need more time to get established?

The above chart reflects the data we collected at about 46 hours after the windrow was built.  As you can see, the oxygen levels continued to drop, and the CO₂ levels continued to climb.  The temperature at five out of six locations monitored was above 131°F.    "Aeration by Convection" may have made monitoring point "C" have the best oxygen level, even if far below our target range. 

We want to know what the actual ACTIVE bacterial biomass and populations are.  That   requires an on site microbiology lab capable of doing the microbial assays.      We are developing a database on the total and active bacteria and fungi biomass in real active compost under varying conditions, and at varying stages in the composting process.  The number of ACTIVE aerobic bacteria, we believe has a lot to do with the rate of oxygen consumption; and the rate at which the compost completes the process. [We try to clearly identify what we know, versus what we believe, without having hard data.]

Okay, the above charts told us that "Aeration by Convection" doesn't work in the real world as well as many people believe.    So... what's the answer?  We started playing with forced aeration.   Before long we developed a diesel powered blower which proved very efficient, consuming only 8.2 gallons of fuel per day, running full time.     The blower was connected to a 157' long distribution manifold with 27 ports on it.   Pressure on the tail end of the manifold was within ¼" of pressure at the head of the manifold.  Even if we opened two ports with no resistance, the manifold pressure remained close to the same.  This told us that while short circuiting wants to be prevented, it is not as fatal as some have claimed. 

While this blower was crude in some respects, its performance convinced us that diesel powered aeration can be highly efficient.

We've heard people say that forced aeration provides too much cooling.   It can.  The rate of cooling is a factor of how much surplus microbial metabolic heat is being generated, and how much air is being "forced" through the compost.  We are convinced that the rate of flow has to be controllable, and that various parts of the composting mass may well have different needs, so each aeration veins needs to be valved.

This graph shows that forced aeration can very easily keep the oxygen up in the target range.  In this case, once we developed the skills for showing temperature on the same graph with the oxygen and CO₂ data, it was obvious that the air flow must be regulated to maintain the oxygen and the temperature both in their target zones. 

Our experience with our compost research silos has taught us that in the early stages of the composting process much more air is required to provide adequate cooling than to simply maintain oxygen in the 20% - 21% range.  We had silos that were staying above 160°F, with their oxygen at 19 - 20%.  We had to increase the size of the blower serving the manifold that serves our research silos.    The original blower was capable of only delivering one eighth of an inch of pressure.  The new blower delivers 3/4" of pressure. 

We're looking for facts that enable us to improve the composting process.   We've come to the conclusion that we must be able to do the microbial assays on site, that we must develop a database on both total and active bacteria and fungi populations at various stages of the composting process, and under other variables.

Our silo research has taught us that compost can get too dry in 18 hours or less, so we must be prepared to re-hydrate the compost.   It has also taught us that bio-filtering is essential, and can very effectively eliminate foul odors.  It has taught us that responsible management includes extensive monitoring, and the ability to adjust the air flow to the compost in different stages of the process.  While research is expensive, ignorance is far more and unacceptably expensive. 

This information has been provided by:
    John A. Crockett, CEO; a.k.a. Dr. Mike Robe^tm
    Mother Nature's Farms, Inc.
This page was last updated: 11/29/06 

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