There seem to be five basics to pay attention to in order to get optimal microbial performance. If you want to make it a six pointed star we can add "Particle Size"... to give the microbes more surface area on which to work.

Temperature wants to be between 131° and 140° F to get good pathogen reduction. The literature suggests that at temperatures above 140° F we start to get microbial kill from the high temperatures. After weed seed and pathogen kill are accomplished temperatures may want to be maintained between 113° and 124° for greater microbial diversity. There are also reports that at temperatures above 150° F some of the aerobic microbes will start to behave like Anaerobes. As with each of the other points... the goal is to have the temperature throughout the composting mass within the target range... not just at a few isolated points. Pile cross section profile checking is a very important part of process management. Varying the volume of air flow on a forced aeration system is an easy way to maintain temperature control. We will soon be doing research on shifting between positive and negative pressure to see if that effectively gets more uniform temperatures.

When considering temperature... we may want to consider whether or not the temperature is uniform throughout the composting mass. The chart to the right illustrates the temperature variation that we have found in doing cross section profile studies on passively aerated windrows. The monitoring points are defined in a diagram below.

Oxygen: The literature suggests that Aerobic thermophilic microbes are the best workers available to us. In my early days in composting I spent over a thousand hours turning my compost under the illusion that I was creating aerobic conditions with some lasting benefit. Then I got an oxygen meter and discovered that the microbes can use up the oxygen in the compost very quickly... and that any benefits of aeration by convection are very limited... as illustrated by the following charts which ought to be quite self explanatory. These are tests that I did myself... they are not something that someone else told me about. You don't need to accept my charts, my word, my data. Take your own oxygen meter and do the test yourself... get first hand experience.

According to some sources windrows are self aerating because of convection. The following is from some windrow cross section profile testing that I did in December, 1996, and I have probed enough windrows at enough different compost sites to believe that the data is reasonably representative.

The windrow cross section profile research involved checking temperature, oxygen and CO₂ at the six monitoring points as shown in the illustration to the right. "A", "D" and "E" were about 18" in from the surface of the windrow, the other points were about 36" deep.


While some may argue the use of minimum oxygen set at 16%... it is clear to me from this data that the oxygen level varies substantially between the points monitored in passively aerated windrows. I am quick to admit that I want to do a lot more research. I also am convinced, based on this research which I did myself... that forced aeration is essential to maintaining oxygen levels above 15%. You don't need to believe this data. I encourage you to do this study, gather the data on your own windrows. I certainly intend on doing a lot more cross section profile oxygen and temperature monitoring as we work to build a system that will provide the conditions that we think the microbes work efficiently at.

One of our future research projects will be to take automatic data logging equipment, and just after a section of compost is turned insert oxygen and temperature probes... and collect data at five minute intervals. We will collect cross section profile information to see what is happening in the different parts of the composting mass... with and without forced aeration... and with different pressures, both positive and negative on the forced aeration. How do different feedstock mixes and pile size and shape compare? How does moisture content affect the data?

In the absence of adequate oxygen Anaerobic reactions will take place.

Moisture: At what moisture levels do the microbes work best? How does moisture content affect maintaining aerobic conditions? Six inches of rain may only moisten the top foot of material in a windrow leaving the core as dry as it was before the rain. Some of our feed stocks require as much as 20 gallons of water to be added, per cubic yard of material, to bring the moisture content up to 60%. The microbes seem to work better under moist conditions. At the same time, the higher the moisture level the faster the composting mass will settle and inhibit air flow through the mass. Uneven moisture levels could promote uneven aeration and anaerobic pockets. At the end of the composting process... anyone with experience screening and trucking compost knows that high moisture levels create severe problems. High moisture material does not screen well... and the higher density moist material means less cubic yards per truckload within legal maximum weight limits. High moisture content material is harder for the end user to work with. Therefore it seems to me that we want to be able to get the material to dry out at the end of the composting process.

Fluffing and Homogenizing: Moist material tends to settle, compact and it appears to us that air would not flow as well though the material as it becomes more compacted. Therefore it is our belief that turning the material is essential to achieve both fluffing and homogenizing if a high percentage of the material is designed to become finished product, not bulking agent expected to be screen separated out at the end of the process. Our experience is that we have not yet mastered achieving uniform moisture and air flow. Where we have greater air flow we also have greater drying of the material. Turning helps to homogenize the material achieving better uniformity while also fluffing it, making it more porous.

Turning is, at best, a very costly process. What is the optimum turning frequency from the point of view of odor control, process turn around time, economics and finished product quality. If working on a concrete composting pad with aeration vanes built into the concrete pad, and a roof sheltering the entire area from rain making the composting area a "zero run off" area, much more acceptable to a major city water shed... then how many batches per year becomes an economic factor to be considered.

C:N Ratio: If the microbes need carbon for energy and nitrogen for protein for multiplying then a reasonable balance between carbon and nitrogen is desirable. The literature suggests a C:N Ratio of about 30:1 is optimal. One challenge that I have found is massive variations in results reported to us by commercial labs. Again, we want to do more research.

A driving question that I have been asking is WHY do In Vessel Systems report such fast turn around times?. The answers that I have guessed at are: Daily, or at least frequent turning, forced aeration to maintain high oxygen levels and controlling the temperature between 131° and 140° and management of the moisture content.

In designing a new composting facility that will serve an area with a population of approximately 2 million people, a facility with an expected capital investment in excess of $5 million we want to test and verify as many of the design elements to minimize the risk of costly modifications afterwards.

1. From The Practical Handbook of Compost Engineering, Roger T. Haug, page 123