Anaerobic Options

The use of anaerobic digesters on a small scale could provide localized energy sources while reducing the negative effects of greenhouse gases.
By Barnett Koven
Today's volatile energy economy necessitates investment in viable, sustainable sources of energy. While many technologies appear to answer some of these requirements, anaerobic digestion is an especially promising technology as it is efficient, inexpensive and can be quickly scaled and implemented. In addition, anaerobic digestion is extremely environmentally friendly. All of these aspects make anaerobic digestion an ideal technology for our renewable energy future.

Anaerobic digestion is a naturally occurring biological process that uses microbes to break down organic material in the absence of oxygen. In engineered anaerobic digesters, the digestion of organic waste takes place in a special reactor, or enclosed chamber, where critical environmental conditions such as moisture content, temperature and pH levels can be controlled to maximize gas generation and waste decomposition rates.

Landfills generating noxious odors demonstrate the impact of organic waste digestion in a semi-enclosed environment with little or no oxygen. However, by using anaerobic digestion technology, odors are greatly reduced because the gases are captured. Commercial anaerobic digestion systems can replicate this natural process in an engineered reactor that produces methane gas much more quickly, in as little as two to three weeks compared to the 30 to 100 years required by the anaerobic conditions in a landfill.

Digester Prevalence
Anaerobic digestion systems designed to process animal manure have been in widespread use for years in parts of the developing world. Several hundred thousand digester systems are estimated to operate in India, and several million are in use in China. In Europe, government incentives in the form of grants, low- and no-interest loans, and mandates that utility companies purchase the energy produced at a premium (often two to four cents per kilowatt above market value), combined with rising energy prices have encouraged the development of anaerobic digestion plants, with more than 1,000 now in place. These digesters mostly serve waste management and odor control needs and provide limited energy generation, though several in Europe and Asia are net suppliers of energy to utility companies. Examples include the Kompogas plants in Kyoto, Japan, and Rostock, Germany, as well as the Valorga International plants in Barcelona, Spain, and Hanover, Germany.

The use of anaerobic digestion technology is rapidly growing in the U.S. It is already a developing market within the agricultural industry. The technology is economically and environmentally beneficial. The country's high demand for energy coupled with a concern for reducing its dependence on imported oil has driven the expansion in the use of electric power generated from methane. Other incentives include the desire to redirect organic waste from landfills. Anaerobic digestion optimizes the benefits of organic waste used for methane production and helps with the landfill shortage problem. Anaerobic digesters have a financially attractive payback period (dependent on energy prices, subsidies and a number of other factors) when the methane gas is used to generate energy in the form of heat, steam or electricity. A proposed 10,000 tons per year plant servicing the industries at the Brooklyn Naval Yard had an anticipated return on investment of just seven years as a result of significant subsidization by the New York Sustainable Energy Research and Development Authority. Larger plants can be even more profitable.

The Anaerobic Process
When using a thermophilic process (a higher temperature and more efficient bacteria), digestion takes place in four stages (Figure 1) plus a preliminary stage over 10 to 14 days.

Prior to digestion, the feedstock enters the buffer or pretreatment tank where its temperature is raised and microbial activity begins. After one day of pretreatment, the feedstock is released into the main digestion tank where the first of the four steps-hydrolysis-occurs, during which complex organic molecules are broken down into simple sugars, amino acids and fatty acids with hydroxyl groups. The second stage is known as acidogenesis, during which further breakdown occurs producing ammonia, carbon dioxide and hydrogen sulfide.

The third stage is acetogenesis during which the products of acidogenesis are further digested to produce carbon dioxide, hydrogen and acetates, along with some higher-molecular weight organic salts.

Methanogenesis, the fourth and final stage, produces methane, carbon dioxide and water. Methane and carbon dioxide are the main components of biogas (Figure 2). Approximately 55 percent to 70 percent of the gas composition is expected to be methane.

Environmental, Other Benefits of Anaerobic Digestion
From an environmental standpoint, anaerobic digestion has three main benefits. First, it is a waste-to-energy technology, meaning that it converts waste materials and not food supplies or other usable products into energy. As a result, demand for power generated from anaerobic digestion will not affect resource markets or lead to poor land management practices as producers attempt to produce more of a resource on a given area of land to satisfy increased demand. In addition, as anaerobic digestion uses waste as its fuel source, it has the potential to divert large quantities of biodegradable waste away from landfills.

Figure 1. The anaerobic digestion process typically consists of four steps.

Second, anaerobic digestion is considered carbon neutral by the U.S. EPA. Even though anaerobic digestion results in greenhouse gas emissions from the use of the methane portion of the biogas it creates, the effect is zero-sum. This occurs because the same amount of greenhouse gases would be emitted as the waste materials rotted in a landfill.

Finally, anaerobic digestion results in a fertilizer-like byproduct rich in nitrogen and phosphorus, making it ideal for land application. When created through a thermophilic process, the fertilizer-like byproduct receives the U.S. EPA Class A Pathogen-Free designation. The high operating temperatures and 10-plus day retention time mean that it is safe for immediate land application even on fields that are growing crops for human consumption. The byproduct or effluent can be separated, using even a simple dewatering screw, into liquid and solid fractions. The liquid fraction can be applied using a farm's existing irrigation system while the solid fraction must be applied manually. A study conducted by Cornell University's Manure Management Program examined J.J. Farber Dairy, a farm located in the New York City watershed that produces approximately 11,000 pounds of effluent per year and is using both fractions of the effluent, saving an estimated $13,000 per year. The byproduct could potentially curb demand for commercial fertilizers, the production of which requires large amounts of energy and the use of which puts out greenhouse gases.

Figure 2. Biogas properties as compared to the properties of pipeline-quality natural gas

In addition to the environmental benefits of anaerobic digestion, the technology benefits from being an extremely simple means of harnessing energy which is easily scalable.

From an economic standpoint, larger units that serve municipalities or large farms are more cost effective. Larger units benefit from economies of scale for two reasons. First, the material costs for a plant only double as a result of a fourfold increase in capacity. For example, a digester that's 10 feet tall and 10 feet in diameter requires approximately 314 square feet of construction material and has a volume of approximately 785 cubic feet. A digester 10 feet tall and 20 feet in diameter requires approximately 628 square feet of construction material and has a volume of approximately 3,140 cubic feet.

Second, as a result of automation, even an extremely significant increase in plant size requires minimal additional labor. Therefore, the marginal cost for each additional unit of capacity is much less than the marginal revenues resulting from the additional unit of capacity.

However, units can be built efficiently to serve individual households. Regardless of the size, the general design is similar, the main difference being the level of automation. A municipal unit would likely be fully automated while a household unit would be manually operated.

Because of the simplicity of the reactor design, a household-sized unit can be built by anyone with a basic knowledge of plumbing and access to a CNC router and sonic welder.

Household-sized units would run on food and garden waste (sewage could be viable but would be less efficient because of the high moisture content and will likely not be permissible under most health codes) and could provide for a small portion of the home's electrical demand as the biogas can be combusted in a slightly modified reciprocating engine and easily converted into electrical energy. More significantly, the unit could provide for household heating requirements. The liquid fraction of the effluent, which comes off the process at approximately 125 degrees Fahrenheit (for thermophilic systems), is hot enough to be pumped under the floor of a small house to provide radiant heating. This has already been done on farms to heat livestock barns. It could also be used in a closed system to heat clean water. I anticipate that it would be possible to construct a household digester for approximately $1,000, not including the cost of the reciprocating engine or other generation equipment.

Small-scale digesters would be especially valuable in parts of the developing world where grid access is limited or nonexistent.

Barnett Koven is a representative to the United Nations for World Information Transfer, an environmentally focused non-governmental organization. Reach him at