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A digester is a power plant

Thomas J. Lawson Published on 31 March 2014

While the process of anaerobically digesting organic wastes has long been present in the U.S., its effective use is often not realized since the basics of its operation is not well understood.

Often on a dairy farm the driving force for considering digestion is for the effective handling of dairy cow manure. While this is a very good application for digestion, dairy cow manure is sometimes not the single source organic waste input to the digester.

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Although cow manure has good biogas potential and is a good source of bacteria, as expected a good portion of the biogas potential of silage fed to the cow is lost in the digestion process.

Frequently because of this, the U.S. digester operator will feed the digester a mixed stream of organic waste inputs. Typically these inputs will vary over the expected design life for the digester facility (some 20 years). Availability of organic wastes, changes in dairy operations and the desire to have a balanced blend of organic waste inputs all contribute to this operational reality.

Knowing this, the digester operator must understand that not all organic waste is created equal. When the decision is made to build a digester, what is really being built is a power plant. The organic wastes fed into the digester are actually fuel for the bacteria within the digester’s oxygen-absent environment.

The bacteria consume the carbon matter in the organic waste inputs. Consider these wastes to be “current carbon” as opposed to the fossil fuels (gasoline, diesel, fuel oil, lubricants) that we use, which could be considered to be refined products of “old carbon.”

Old carbon has been buried deep within the earth for millions of years where old organic matter has decomposed and been transformed into what we recover from the ground or under the ocean in the form of oil and natural gas.

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When we design digestion systems, they are configured to handle input materials considered to be both wet (15 percent or less solids) and dry (more than 20 percent solids) while ensuring that what enters the digester is pumpable.

Throughout the life of any digester facility, the operator will likely have many options for accepting both wet and dry organic waste materials. The operator may also have the opportunity to pick and choose among a variety of potential organic waste inputs.

The digester itself is not meant to be the primary location for mixing incoming organic wastes. Rather, process systems upstream of the digester are expected to deliver a consistent homogenous supply of inputs.

These upstream systems are designed to prepare the organic wastes, much the same as preparations within a kitchen, so the inputs upon entering the digester are optimally available to the bacteria.

Then they can consume the organics in the least amount of time and produce the most biogas as a byproduct of the biological destruction process. This affords maximum biogas production and minimal hydraulic capacity and retention time within the digester, thus minimizing cost.

Biogas produced within a digester is generally composed of about 55 percent methane but the percentage varies with the quality of the organic wastes fed into the digester. This biogas can be considered to be a weak or dilute natural gas, which is generally composed of about 98 percent methane.

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Therefore, since natural gas is considered, for design purposes, to have a heat value of about 1,000 BTU per cubic foot, biogas has a heat value of about 550 BTU per cubic foot. To get scientific for a moment, a British Thermal Unit (BTU) is defined as the amount of energy required to raise the temperature of one pound of liquid water by 1°F at a constant pressure of one atmosphere.

Generally, organic wastes are valued, in descending order of preference by bacteria, based on ease of access to the nutrients. First are sugars, then fats/oils/grease, carbohydrates and proteins, down to grasses and cellulosic materials.

As digester owners consider taking other organic wastes as fuels, the internal bacteria dynamics must be considered. Therefore, it is recommended that each organic waste (fuel) considered for digestion should be analytically tested to determine its actual biogas potential.

The cost per sample for each test, including shipping, is generally between $100 and $125. This information affords the digester operator the knowledge of how quickly the bacteria will consume the intended fuels and how much biogas will be produced.

The internal bacteria within the digester can accept changes in organic wastes, but they don’t like to be shocked by quick transitions in the materials they are asked to consume. Such quick transitions within the digester can lead to reduced or increased biogas production and possible chemistry upsets.

Gradual changes in organic waste inputs are expected and normal. The digester operator must determine how changes in organic waste inputs will alter the biogas potential of the blend of materials being fed.

As long as the digester is producing sufficient biogas (in quantity and quality) to fuel the demand load of the combined heat and power (CHP) or biogas upgrading system, then the facility is balanced. This is much the same as electrical utilities producing sufficient electrical energy to meet the power grid demands of its paying customers.

Each of us reading this article can understand the feeding of anaerobic digesters if we realize that each of us has our own digester (our stomach). There is no direct oxygen introduced to our stomach – much the same as a digester. Each of us knows how we feel if we quickly change our diet or eat too much at any one sitting, such as at Thanksgiving dinner. The result is often not pleasant – and another kind of biogas can be produced.

Dairy farmers considering an anaerobic digester for their farm should look for a design that produces a steady and consistent biogas supply. Digesters are more than another method for handling manure; they are power plants and must be treated as such to maximize a farm’s return on investment. PD

Thomas J. Lawson, P.E., is a technical director and vice president of EnviTec Biogas USA Inc. Its parent company, EnviTec Biogas AG, based in Lohne, Germany, has built approximately 600 digester facilities worldwide.

Tom Lawson
  • Thomas J. Lawson

  • Technical Director, Vice President
  • EnviTec Biogas USA Inc.

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