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Is an Anaerobic Digester Right for You?by: Kyle Teamey
An anaerobic digester can be a great way to manage waste while generating power and other commodities. Digesters convert organic waste feedstocks into methane that can then be used for heating and/or to generate electricity. When properly designed and installed these systems can yield significant economic benefits. Before deciding on a digester you should evaluate how it will fit into your overall operation. Careful planning and maintenance are important to a successful digester and are the keys to making the digester an economically viable source of renewable energy. Here are some considerations before installing a digester:
- Do you have the right kind(s) of organic feedstock? Digesters work well with feedstocks that are mostly water with 5-15% solids. Some of the better feedstocks are dairy cow manure, hog manure, duck manure, food processing wastes, sewage, and animal grease.
- Do you have enough feedstock? Generally speaking, more feedstock means a larger digester and a greater economic return. Large digesters tend to have a shorter payback period than small digesters. Larger digesters can also be easier to operate. If a digester is too small, it will usually not make economic sense to install. For instance, the minimum size for a dairy farm is about 300 dairy cows.
- How often do you collect the feedstock? Many digester designs require a steady supply of feedstock. They work best if new feedstock is added at least once a day. A digester may not be the best solution for you if your waste stream is not consistent.
- How long do you intend to operate the digester? To ensure economic benefit, you should plan to operate the digester for a minimum of 10 years.
- Do you understand the capital costs involved? Designing and installing a digester may cost $3,000 to $10,000 per kilowatt of electric capacity, depending on the type of system and type of feedstock. For a dairy farm, this equates to $500 to $1,000 per milk cow. Grants and other incentives are available from state and federal government to reduce capital costs, but will not pay for everything. For these reasons, those interested in a digester should look at the lifecycle costs of the system to evaluate the economic benefits.
- Do you understand the operating costs and requirements? Operating a digester can be tricky. You must closely monitor and control the substances going into the digester. If the wrong substances are fed into the digester, it can stop methane production. In addition, generators and other parts of the system will require regular maintenance in order to operate properly. A fully functioning digester requires an hour or two of maintenance each day. Generally speaking, a digester will cost anywhere from $200 to $1,000 in annual maintenance per kilowatt of electric generation capacity. Larger digesters tend to be less expensive to operate than smaller digesters.
- What do you intend to gain from owning a digester? Digesters have a number of potential benefits including:
- Electric power generation
- Heat generation
- Nutrient management / pollution control
- Odor and fly reduction
- Economic benefits including; reduced energy costs, reduced waste disposal costs and potential sale of byproducts.
The savings realized from owning a digester will be greatest in areas with high electric prices, high natural gas prices, high propane prices, and high costs for treating or transporting organic wastes.