COMPOST OPERATORS FORUM
TROUBLESHOOTING THE COMPOST PILE
This second installment continues our look at key factors to examine when problems arise in the composting process.

Bill Seekins

Troubleshooting exercises for compost piles often converge on two factors — moisture content and C:N ratio. In fact, problems in the compost pile frequently stem from a moisture content or C:N ratio that is either too high or too low. Nevertheless, the root cause of composting troubles often lies elsewhere. Part One covered understanding what emissions come out of a composting pile, failure of the pile to heat, odor management and immature compost. Part Two continues this analysis of common but often overlooked causes of compost process and product problems.

Volatile solids

Volatile solids (VS) are a measure of materials energy content. Lab reports for composting feedstocks often include VS analysis. VS content is the percentage of the material that can be burned up in an oven (not including water). It is nearly the same measure as organic matter content. Both properties indicate how much material is potentially available to the organism as a food/energy source (although food-value resistant organic materials like wood will be overestimated). In my experience, you don’t get adequate heating in the compost pile unless VS content is greater than 40 percent on a dry basis. In other words, the edible part of the material can be so diluted with soil, ash or other mineral substance that there is not enough energy for the pile to heat up. VS can be raised by adding fresh material such as biosolids, manure, grass, green vegetation, etc.

Bulk density

One of the most overlooked process conditions is the bulk density of the composting mixture. Bulk density is the weight of the mixture for a given volume, such as lbs/cubic yard (cy) or kilograms/cubic meter. It indirectly indicates how easily air/oxygen can penetrate the material. If the material is too dense, no matter what system is used, it won’t compost adequately. Composting probably will go well if bulk density is between roughly 800 and 1,000 lbs/cy. If it is over 1,000 lbs/cy, it will be hard to aerate the material. Frequent agitation or turning will be needed to maintain air in the pile. If the bulk density is over 1,200 lbs/cy, the material will be too dense to adequately aerate, even with turning. A bulking agent must be added to lighten the mix. Materials with very low bulk density — below 800 lbs/cy — will compost well with little or no turning, but may lose moisture rapidly.

Loss of structure

Structure is the physical property of the mix that refers to its ability to hold shape in a pile and resist settling and compaction. Stiff, rigid particles with uniform shapes provide good structure. Bulking agents like wood chips, bark and straw are used to establish and maintain structure. However, over time, particular materials used as bulking agent can lose their stiffness as they absorb water and decompose. Paper is a good example of a bulking agent that loses structure quickly. When a pile loses structure, it collapses and becomes dense. It becomes difficult to turn and anaerobic conditions follow. The remedy is to add more bulking agent, or a different one that maintains good structure, like wood chips or shavings. Moisture is also a factor. A given material has more structure as it dries.

Dewatering polymers

The polymers routinely used to dewater biosolids can cause problems in biosolids composting (although some polymers seem to present more difficulty than others). The polymers act as a binding agent to settle the solids out of the wastewater. What they also do is react with water such that it becomes part of this structure. This is great during dewatering, but it creates problems about four or five days into composting. When a bowl of jello heats up to 150° to 200° F, what happens to the jello? It turns to water. The same is true of dewatering polymers. All of the water bound up in the structure of the polymer is released during composting. A biosolids compost pile can look great for the first four days, and then a pool of water starts running from the bottom after five. That is the polymer breaking down and letting its water go. When dealing with polymer-dewatered biosolids, the composting mix should include more bulking agents or dry amendments to handle that extra water. An additional nitrogen source may be needed to compensate for the extra dry (usually nitrogen poor) amendment.

Oily materials

Materials that contain oils, such as some types of fish, can cause problems in the composting process. The first, and perhaps most apparent problem, is the persistence of an oily odor that lasts for weeks into the composting process, even after the material is no longer visible. A second problem is that oily materials generally break down more slowly than expected. In part, the problem is due to the hydrophobic nature of oils — they repel water. A third effect is that oils indirectly impact process control. Oil lends the materials a sheen that can be mistaken for moisture. This “wet look” can give the operator the impression that the pile is moist when in fact, it may be too dry.

Oily materials can be composted, but only in amounts less than ten to 20 percent of the mix by weight. Including a fast-decomposing or “hot” composting ingredient, such as horse manure, in the mix enhances breakdown of the oils. Additional curing time may be necessary to obtain mature compost from oily feedstocks.

Excessive forced aeration

Composting needs air, but too much can be a detriment to the process. Particularly in a forced aeration system, where blowers provide the air, excessive air flow can remove a lot of moisture. The problem is aggravated by the channeling of air that occurs within piles at high air flow rates. It occurs first around the pipes or air inlets of pressurized aeration systems. The material near the pipes dries out first and microbial activity slows, even though the material is still highly degradable. Eventually, the other parts of the pile are affected. In systems that use continuous forced aeration, half of the pile can dry out in 10 to 14 days. The result is a nonuniform compost that contains immature material. The material will remain inactive until it gets wet again. If the compost is sold without additional processing, it can create odors and other problems associated with immature compost.

Another consequence of excessive aeration is ammonia loss, especially with high nitrogen (low C:N ratio) mixes. As the material dries out, more ammonia volatilizes and consequently, more nitrogen is lost.

To correct a pile that has been partially dried out, it may be necessary to break it down, wet it again and repile the material. Otherwise, it may be enough simply to reduce the air flow rate into the pile, particularly as decomposition proceeds and the microorganisms require less oxygen. Other solutions include more technological approaches such as humidifying incoming air and periodically reversing air flow. Generally, concentrated drying occurs less with negative or suction forced-aeration systems because the incoming air enters over the whole exterior surface of the pile rather than at a concentrated point. Overall, smaller pile size, thorough mixing and increased porosity (e.g. lower bulk density) also improve the uniformity of air flow.

Inadequate time

The point at which compost is sufficiently mature depends on how it will be used. Compost used for field applications six months before planting does not need the same level of maturity as compost used for growing seeds in pots. Nevertheless, operators often push the process to get compost off the processing site sooner. In the long run, this is a mistake. For general uses, I believe that it normally takes six weeks of active composting and then two to six months of curing to be mature. At that point, the ammonia and organic acids have effectively disappeared, the available carbon has been stabilized and the heat generation has slowed enough to bag the product. Mature compost should reach a Class IV rating in the Dewar self-heating test.

Bill Seekins is with the Maine Department of Agriculture, Food and Rural Resources, and a faculty member at the University of Maine Cooperative Extension Compost School, an active member of the Maine Compost Team.