Forced Aeration:

Our Non-unionized microbes are working 24 hours a day, day and night, 7 days a week, Sundays and holidays included.  They want fresh air, oxygen, on the same schedule.:  The least we can do is provide them with fresh air, oxygen, on a full time basis.  If we don't, they may raise a big stink.

 Our first diesel powered blower is working like a charm.  We estimate it had a capacity of about 10,000 cubic feet per minute.  It worked on the same schedule as our microbes, full time.  We shut it down once a week just long enough to  check the oil, and once every five weeks to change the engine oil and oil filter which took about 11 minutes.  The engine was connected directly to the 500 gallon fuel tank.

This blower did an excellent job of proving to us that diesel powered blowers of this type are a very cost efficient way to provide the microbes with oxygen and also for limiting the temperature to not higher than 160° F.

Fuel consumption was about 8.2 gallons per day.  At 1.599 per gallon... the energy costs to provide full time aerobic conditions is about $13.11 per day.  We estimate that electric blowers would cost $24.96 per day to do the same job, that the cost of operating the diesel powered blower is 53% of what an electric blower would cost... for energy. 

This blower was powered by a 23 horsepower 3 cylinder Kubota diesel engine which we retrofitted to an an old municipal leaf vacuum which we had modified to discharge on the horizontal.  Since then we've developed much more efficient systems.

  Hayward 4" PVC True Union Ball Valves are our choice for new valves.  The tower with the blue fluid in it is a manometer, telling us just how much pressure is in the system at the point where it is installed.  At the beginning of the process oxygen and cooling demands are so great that 6" diameter valves are more appropriate.

The manometer pictured was our version 4.2.   The blue fluid is common automobile windshield washer fluid.

The valve enabled us to control the air flow into the vane, thus enabling us to control the amount of cooling and also oxygen level.  Our own standards call for maintaining oxygen at 17% or higher, as determined by using an oxygen meter.

WHY go to the work of providing Forced Aeration?

The most effective microbes for converting organic residuals (wastes) to premium top soil and soil conditioners are aerobic microbes.  "Aerobic" means they work in the presence of oxygen, that they consume oxygen.  In the absence of oxygen the entire chemistry changes and foul odors and phyto-toxic VFA's can be produced.  All fungi are strict aerobes.     Commitment to being a good neighbor and producing a high quality end product mandate the use of forced aeration, and enough oxygen monitoring to verify that the compost is, in fact, aerobic, with oxygen levels ≥ 17%.

The data on the above chart is from The Practical Handbook of Compost Engineering by Roger Haug, page 123.

Preventing Foul Odors:

Many of the chemical byproducts of Anaerobic conditions are foul odor gases. 

Thus the key to preventing foul odors and VFA's is to maintain oxygen levels ≥ 17% throughout the composting mass, and sending all of the off-gas from early stage compost through Dynamic Bio-Filters^tm.  Dynamic Bio-Filtering is a technology that we developed in 2002.

During the beginning of my composting career I spent thousands of hours turning compost under the illusion that I the turning would keep it aerobic.  Then I invested in an oxygen meter and discovered that the microbes can exhaust the oxygen in 15 minutes or less, crashing the oxygen level down from 19% to 2%.... in just 15 minutes. 

This is research that we did at Mother Nature's Farms on forced aeration piles that were over 200 cubic yards, over 35' long, 35' wide, 8' high with 4" diameter aeration vanes under the pile with aeration pressure of 2/3"wc.  That aeration pressure is about the same pressure you use when you blow out the candles on the dinner table.  It is very low pressure and massive volume.

    When we repeat this research with more dynamic compost feed stocks,  the oxygen depletion can be much faster,  as quick as 2 minutes.

Turning compost is still important to homogenize and fluff the material.  Turning will facilitate more uniform air flow through the compost with a lower air pressure and thus less energy to drive the blowers.  Furthermore, bacteria are so tiny, 0.3 - 1.0 µm in diameter, so tiny that it would take 25,000 bacteria, shoulder to shoulder, to span a single inch.  Bacteria insist on a 'balanced diet' of carbon and nitrogen, needing one unit of nitrogen for every five units of carbon.  And like people, they tend to 'eat' what they like first.   When they use up the nitrogen within their reach, they stop working.  Turning the compost helps homogenize it, getting more nitrogen within their 'reach'.    This is why "Static Piles" don't compost as well.  Fungi are different in that they can grow their hyphae out in different directions, getting their different needs from different places. 

Short Circuiting:

Our research to date suggests that the people who alledge that forced aeration is ineffective because of short circuiting may have little first hand experience with adequatedly sized / designed forced aeration systems.  Even when we open up an unused vane valve, creating a 4" wide open unrestricted air escape the pressure in our main manifold drops very little.  When we monitor the oxygen level at dozens of different places in the pile of compost we continue to find our oxygen levels greater than 15%. 

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This page was last updated: Thursday, April 24, 2008, 08:55 PM