Is
the compost aerobic? The only way to be sure is to check with oxygen and /
or CO2 meters.by John Crockett
Is
the compost aerobic? The only way to be sure is to check with oxygen and /
or CO2 meters.
When we first got into composting, we read, went to BioCycle conferences and visited other compost sites. We read, and were told that windrows will passively aerate by convection. We were also told, and read that turning will keep compost aerobic. We were turning close to 1,000 cubic yards of compost a day.
By the Fall of 1995, it was very clear to us that something was very wrong. On landscape jobs we could not get grass to germinate and grow, when we amended the soil with our compost.
Then we started learning about VFA's, Volatile Fatty Acids; that they are phyto-toxic, and byproducts of anaerobic conditions.
Because
we wanted answers, in January of 1996 we built a pile of horse manure &
shavings, 35' x 35' x 8' high, at Old Salem Farm; with a crude, but functional
aeration system, using perforated 4" HDPE drainpipe under the pile, parallel,
about 5' apart, connected to a little electric squirrel cage blower.
It was time to find out whether our compost was really aerobic so we bought a set of Fyrite Oxygen and CO2 meters, and started monitoring our compost.
We inserted our probe, 42 in from the top of the pile, and started monitoring the oxygen level at 5 minute intervals, recording the reading from the Fyrite oxygen meter. After the first reading, which showed 19% oxygen, we turned off the blower.
As the graph shows, within 15 minutes the oxygen level crashed from 19% down to 2%.
When we turned the blower back on, within 5 minutes the oxygen level was 16%.
This showed us that whatever oxygen is stored in the pore space in the pile, and horse manure / wood shavings (stall bedding) is Very porous, the microbes could exhaust that oxygen in a matter of minutes.
Some people claim that 5% oxygen is "aerobic". Our gut feeling was that we ought to be maintaining a minimum of 15% oxygen, and cross section profile monitoring with oxygen and CO2 meters clearly indicated that passive aeration by convection and turning were not yielding the results we wanted.
Our
monitoring pattern was as shown to the right: We will continue to use the
same monitoring pattern in any future cross section profile monitoring.
We've done enough cross section profile monitoring of passively aerated windrows to be convinced that forced aeration is essential. When we first realized that, back in 1996, we had no idea how much air was needed, nor how much "force" it would take to deliver that air.
Our research goals were quickly being defined for us. We needed answers to both of those questions.
We think the primary cause of heating in the compost is surplus microbial metabolic heat, and that the best composters are aerobic microbes. "Aerobic" means they need oxygen, and some people think they might not work well on intermittent oxygen, on / off, massive variations in the percentage of oxygen.
Our
first compost research silo
research demonstrated that we can get temperatures up to 176°F, 80°C, and maintain
oxygen in the 19 - 20% range with full time forced aeration. Lowering the
temperature requires more air flow, which also means more drying, which under certain
conditions needs to be compensated for with the addition of moisture.
We developed those silos back in the Spring of 2000.
Our compost research silo setup has gone through many enhancements and in August of 2006, we significantly upgraded both the aeration manifold and TempScan wiring. We work with this every day, so it's become "routine". And, for others, it may appear complex.
Most industries appreciate the importance of Research & Development. For some reason that I do not understand, most composters apparently don't appreciate the value of R&D. I wasted over $100,000 during my first 2½ years in composting because I was following 'the crowd". When I got my oxygen and CO2 meters, and did my first oxygen depletion test, and then cross section profile monitoring of passively aerated windrows, I realized that forced aeration is essential.
Now we had to figure out how to design a forced aeration system, that was cost effective. We had to know how much air is needed at various stages of the compositing process, for at least the type of feedstocks we're focusing on. Different feedstocks may have different aeration needs.
In 1996 and 1997 we developed our first systems and learned from those experiences. That led to our developing a diesel powered compost aeration system which we put in service in April of 1998. While it was 'crude', it also performed above our highest expectations, and was part of our learning.
That blower, connected to a 500 gallon fuel tank and a 157' manifold, convinced us that diesel powered compost aeration blowers can be very efficient. It's likely that we've developed even more efficient systems that we're not disclosing.
How
Much Air is Required to hold the CO2 level down to 2%? (oxygen level
about 19%)We adapted some of our compost research silos to help us answer that question. We use a Rotometer to measure air flow under silo conditions, where we can easily monitor the CO2 in the off-gas, and have the benefit of computerized temperature monitoring. When we've identified questions which we want answers for, we then develop ways of getting those answers. The picture to the right shows our rotometer setup, with the pressure regulator.
Of course the Rotometer measures the amount of air going through it. We want to convert that to cubic feet of air per minute, per cubic yard of compost, and also, easier for us to relate the volume of air to the volume of compost. In the spreadsheet, "Air Exchanges per Hour" assumes 100% pore space. We know that is not the case, but it gives us a reference that we can relate to, and Minutes per Air Exchange is the same way.
Because
knowing how much air is required is vital to designing aeration systems,
we needed to quantify the volume of air required. Our compost
research silos enable us to measure the rate of air flow.
While at first it was hard for me to believe that the microbes could require a volume of air > 20 times the volume of compost, every hour, I've replicated this research many times since starting it about 2004, and I've been doing the CO2 monitoring of the off-gas, so now I believe it. Whether others want to believe it is not my concern.
There are management practices that affect the microbial population, including the nature of the feedstock the particle size and moisture level. There appear to be other variables, some of which we haven't identified yet.
While I've heard and read that 5% oxygen is "aerobic", since my income is very dependant upon the profitability of the entire system, I am more concerned with how the microbes respond to various levels of oxygen. In July of 2004, we started looking at the Active Bacteria population compared to the CO2 level in the compost. We're set up to be able to do the microbial assays in our own lab, so samples can go through the staining process within minutes of being pulled from the pile of compost. Our interest is in what's happening in the pile of compost, not in a Zip Lock bag in transit with Fed Ex or UPS overnight, to an outside lab.
Without
going into the history of the various samples, the data, once put into graph
form, suggests there is an inverse relationship between Active bacteria and CO2.
We appreciate that there are other factors that affect the population of active
bacteria, and this still convinced us that we want to hold CO2
between 1 - 2%.
We've also looked at moisture content and active bacteria, and plan on doing a lot more work in these areas.
In
2001 we developed a QuattroPro spreadsheet template, so the number crunching is
"automatic". We did our homework on moisture balancing about the same
time.
Our
templates for doing the moisture content number crunching have been available to
anyone who asks, since 2001, and are available in both QuattroPro and Excel
formats.
Doing moisture content requires
having a lab balance, and a drying oven. A conventional toaster over works
fine, as long as you have a hood and send the odors through a bio-filter.
The drying oven is a major point source for foul odors. We vent our drying oven,
sending the air through our Dynamic Bio-Filters.
We developed our Dynamic Bio-Filter technology back in 2002. It costs nothing, and takes up no extra space, and is very efficient at scrubbing foul odors. At the same time it overcomes the problems that too often occur with conventional bio-filters. We would not ask neighbors or regulators to permit a composting facility that did not send all of the off-gas from early stage compost through Dynamic Bio-Filters. We've been using Dynamic Bio-Filters in our compost research silo lab for years, because they are very efficient. We figure that composting food residuals in an enclosed space is as rigorous a system test as there is. For us, the bottom line is results, in this case, no foul odors!
Capturing
Heat from the Composting Process:We did significant research on capturing the surplus microbial metabolic heat from compost, in the winter of 2005-2006 with delightful results. In the Fall of 2006 we enhanced our heat capturing setup in our lab. We've captured heat at a rate of over 750 BTU's per hour, per cubic yard. We've also identified weaknesses in our system, and expect that 1,500 BTU's, if not more, is readily possible. As of November, 2006, we have three heat capturing compost research silos, and have just begun working them. Fifteen thermocouples are monitoring the heat capturing system, with computerized data longing.
The Green and Purple lines refer to the Y scale on the right. Graphing the data is an important part of being able to make some sense out of the data. It's also fun.
For our compost research silo work, we use an S&S compost turner. "S&S" stands for shovel & shoulders. Our compost research silo work is a lot of work. And, the information we gain is invaluable.
In August of 2002, John figured out the underlying logic for the key formulas for determining (direct estimating) the population and bio-mass of the total and active bacteria. Then he proceeded to create the template for doing the number crunching. The spreadsheet shown to the right is from an assay we did June 14th, 2004, and is the largest population of active bacteria that we've assayed so far, 5.48 billion, per gram, dry weight, which is about a teaspoonful.
In most industries management pays considerable attention to providing working conditions that are conducive to productivity. In composting, our primary production workers are the microbes. We believe that we must provide those microbes with good working conditions, including enough fresh air and moisure.
While passively aerated windrows are popular, that does not mean that it is good technology. Popularity does not mean that something is good. In 1492 Christopher Columbus had a hard time raising money for his voyage, and was almost locked up in an insane asylum, because the popular belief at the time, was that the world was flat.
All truth passes through three stages:
First, it is ridiculed.
Second, it is violently opposed.
Third, it is accepted as being self-evident.
--Mahatma Gandhi
As of 11/28/2006, when doing a Google search for "compost + active bacteria" the #1 & 2 listings from Google direct people to the our website.
When doing a Google search for "compost + forced aeration" our website came up as the #2 listing.
This page was last updated on January 26, 2010 at 07:13 PM