Efficiency is all about getting the most output from the least input. One of the best ways to do this is to look at waste and find cost-effective ways of putting it to use. Aside from sending organic material straight to the landfill, the status quo has been to throw it into a compost bin and let the waste decompose into fertilizer. This involves both aerobic (with oxygen) and anaerobic (without oxygen) digestion, in which microorganisms break down biodegradable material. Anaerobic digestion (Figure 1) produces biogas, which is primarily methane and carbon dioxide along with other trace gases. Biogas can be used for heating in boilers and furnaces; it can also generate electricity through a process known as cogeneration (also called combined heat and power). The gas can also be sold, allowing for additional revenue streams, and may qualify for carbon credits. Many industries produce organic material such as plant matter, manure, yeast, and food wastes as part of their waste streams. These are potentially valuable resources that can be recovered and returned to on-site processes for substantial efficiency gains and energy savings.
What are the options?
A wide range of technologies are available for anaerobic digestion. Digester types are subcategorized based on the type of waste stream being processed: manure, municipal and industrial wastewater treatment, and organic solid waste.
Anaerobic digestion systems for manure are designed to produce biogas and reduce methane emissions, odors, pathogens, and unwanted seeds. Manure digestion systems fall into four categories:
- Covered anaerobic lagoon digester. A sealed lagoon with a flexible cover and piping to transport the recovered methane to be combusted.
- Plug flow digester. A long, narrow tank with a rigid or flexible cover, typically built below ground for insulation (thus requiring less supplemental heat); ideal for facilities like dairy farms that can scrape up their manure and place it directly into the tanks.
- Complete mix digester. Enclosed, heated tank with a mechanical, hydraulic, or gas mixing system; works best when manure is slightly diluted with wastewater.
- Dry digester. Vertical, silo-style tank made of concrete and steel, with rigid covers; has the advantage of operating at a much higher solids content (20 to 42 percent) and requires less liquid for dilution or cosubstrates.
Stored manure can be odorous and may become a breeding ground for pathogens. Biogas is also flammable and must be managed properly. In 2009, the US Department of Agriculture Natural Resources Conservation Service published Anaerobic Digester technical guidelines (PDF) for covered lagoon, plug flow, and complete mix digesters in order to address these issues.
Municipal and industrial wastewater
Anaerobic digesters are used at municipal wastewater treatment plants to break down sewage sludge and eliminate pathogens before effluent is returned to the environment. For industrial facilities, both food and beverage manufacturing facilities typically generate waste streams with high energy potential. They’re known to have high chemical oxygen demand and solids loading. Wastewater treatment digesters fall into three main subcategories:
- Mesophilic. The most common type, mesophilic bacteria digesters operate between 70° and 100° Fahrenheit (F), the temperature range in which mesophilic bacteria thrive. Research has shown that mesophilic systems are more stable due to a wider variety of bacteria that grow at mesophilic temperatures, and they’re more adaptable to varying environmental conditions.
- Thermophilic. These digesters are run at 122° to 140°F. Thermophilic digestion offers advantages of faster reaction rates and a faster overall process, and it’s also more effective at killing pathogens. This is notably less important if the waste stream goes through pasteurization prior to digestion. Drawbacks include higher expenses because additional heat is needed to maintain the reaction, as well as greater environmental sensitivity and a need for careful temperature control.
- Temperature-phased. These digesters offer a combined approach by applying thermophilic digestion for a short period, followed by a mesophilic period. The short thermophilic period helps to kill off pathogens, but these systems do not require as much oversight as a pure thermophilic system.
Organic solid waste
When a municipality is looking at ways to deal with its own waste streams, the solid organic portion (for example, food scraps, yard waste, or paper) becomes a good candidate for anaerobic digestion. This process is a more highly controlled way of capturing methane when compared with landfill gas capture. The organic solid waste stream can be presorted into separate sources to produce a more energy-dense feedstock. System types ideal for handling municipal solid waste include:
- Single-stage wet digesters. Simpler and less expensive, these systems are limited by the capability of the organisms to handle the sudden change in pH that occurs during the reaction.
- Dry fermenters. A subtype of single-stage digesters, dry fermenters use feedstocks in a solid state, can be handled via front-end loaders without needing additional water, and operate in two modes. In batch mode, all the materials are processed at once; in continuous mode, fresh waste is constantly added and the digested gas is constantly removed.
- Two-stage digesters. More complex and expensive, two-stage digestion separates the acid-producing part of the digestion process to allow for more loading of gas from waste that contains lots of nitrogen. Feedstock is normally diluted with water to achieve the right amount of solids content.
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