May 20 SPB Program to Feature Anaerobic Digestion and Composting of Organics

by Tim Truscott


ALBANY, NY: Save the Pine Bush will host a program at 7:00 PM on Wednesday evening, May 20 on composting and anaerobic digestion of organics, including food scraps. The guest speaker will be Gary Feinland, Environmental Program Specialist at the New York State Department of Environmental Conservation in Albany.

New York State’s organics, including food scraps and yard trimmings are often disposed in landfills. NYSDEDC views these materials as resources and encourages managing them according to a hierarchy featuring reduction, donation and animal feeding (in the case of food) followed by recycling (including composting and anaerobic digestion), and finally disposal.

Now let’s talk about some basics of composting and anaerobic digestion. This discussion is aimed at medium-to-large scale composting volumes.

With regard to recycling and disposal of organic wastes, the first thing to understand is that there is a hierarchy of sustainable solutions to discarded materials, whether it is food, paper, plastics, metals or any other materials:

1) The first or most desirable is to reduce consumption or reduce creation of the excess which results in “waste”.

2) The second level of the hierarchy is to re-use objects or materials without needing to remanufacture them.

3) The third level is to recycle the materials and manufacture them into new objects.

Recycling requires the most energy consumption of the three options, while reduction requires the least.

There seem to be basically two types of technologies for processing waste food (food that cannot be redistributed for use by other people or animals) and other organics (manure, leaves, soiled cardboard and paper, etc.):

1) Composting

2) Anaerobic digestion or biodigestion

Both composting and anaerobic digestion occur because of the activity of many different kinds of microorganisms (bacteria, fungi and other kinds of microorganisms) working to decompose the organic material.

Composting is by definition aerobic (meaning that it requires oxygen in order for the microbial processes to break down the materials) and produces carbon dioxide as a gaseous byproduct from the breakdown of the organic materials. Organic materials are usually arranged in long windrows (like a windrow in a hayfield) for composting. The organic materials can be aerated by periodically turning the windrow and thereby introducing oxygen into the material. Or the windrow can have perforated pipe in its base and have air forced through the windrow.

In the composting process, microorganisms break down organic matter and produce carbon dioxide, water, heat, and humus, the relatively stable organic end product. Under optimal conditions, composting proceeds through three phases: 1) the mesophilic, or moderate-temperature phase, which lasts for a couple of days and has temperatures somewhat over 100 degrees Fahrenheit, 2) the thermophilic, or high-temperature phase, which can last from a few days to several months and has temperatures in the range of 120-150 degrees Fahrenheit, and finally, 3) a several-month cooling and maturation or “curing” phase.

It is important that the compost stay in the range of 120-150 degrees F. for a period of time so that human pathogens are killed by the heat.

Temperatures much above 150 degrees F. are inhospitable to bacteria and may kill the bacteria. If the temperature of the compost pile gets too high, there is always the possibility of spontaneous combustion occurring and a fire. The temperature of the compost pile can be regulated by manipulating the aeration of the pile, or by adding water.

There are some composting systems which capture the heat generated by the composting process and put that thermal energy to a beneficial use. So far, these systems have been used in agricultural applications. One firm working one this idea, Agrilab Technologies, claims that 1,000 btu’s per ton per hour can be captured from cow manure and animal bedding.

Composting requires less infrastructure and equipment than anaerobic digestion, and therefore has lower start-up costs.

Anaerobic digestion or biodigestion is a process which does not use oxygen and produces methane as a gaseous byproduct from the breakdown of the organic materials. Anaerobic digestion is the process which occurs in a landfill where organics do not have the opportunity to receive oxygen, and methane is formed as a consequence.

In an anaerobic digester, this process takes place in the controlled environment of an “enclosed vessel”, a tank with a bladder arrangement on top which traps the methane generated by the process. Typically, the methane is used to power an engine which drives a generator and produces electricity, but sometimes the methane gas is compressed for use in vehicles or for other purposes.

Anaerobic digesters can be of various sizes (some quite large and some quite small) and take in a variety of organic wastes (waste food, manure, etc.). In addition to methane gas, the other product of anaerobic digestion is an organic solid which contains many nutrients and can be used as a fertilizer.

There are perhaps a few hundred anaerobic digesters in the United States. In Germany, which is only a fraction of the size of the United States, there are thousands of anaerobic digesters, some large and others relatively small which might serve an individual dairy farm.

The most difficult challenge of developing a waste food program is collecting and transporting the food waste. Food waste is usually mostly water, so collection and transport containers must be water-tight. Food waste also putrifies easily, so it cannot sit around at the curb, in a truck or in any other uncontrolled environment for very long.

Generally, developing composting facilities requires a much lower level of capital than does developing anaerobic digesters. However, the anaerobic digesters may be more profitable in the long run.

Both methods of processing organics usually rely on “tipping fees” (charges to the party wanting to dispose of the organic material) for part of their income. These charges, together with transportation costs, must be somewhat lower than landfill tipping fees in order to attract business. Otherwise, the party disposing of the organics will be inclined to simply send the organic material to a landfill. However, some states (e.g. Massachusetts, Connecticut and Vermont) are implementing regulations which will prohibit organics from being placed in landfills. This change should favor composting and anaerobic digestion.

There are two prime reasons for diverting organics from landfills and treating them by either composting or anaerobic digestion: 1) To conserve space in landfills, which are expensive to construct and maintain; and 2) To minimize the volume of methane (a potent greenhouse gas) which is produced by landfills and released into the atmosphere.

There are a lot of fertile opportunities in organics, and these opportunities are just beginning to open up. Once things start rolling, they’re going to roll fast.



Published in May/June 2015 Save the Pine Bush Newsletter