What is an introduction to anaerobic digestion of organic wastes?

Anaerobic digestion is a process through which bacteria break down organic matter&#;such as animal manure, wastewater biosolids, and food wastes&#;in the absence of oxygen. Anaerobic digestion for biogas production takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions (learn more about AD system design and technology). These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process) which is discharged from the digester.

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Multiple organic materials can be combined in one digester, a practice called co-digestion. Co-digested materials include manure; food waste (i.e., processing, distribution and consumer generated materials); energy crops; crop residues; and fats, oils, and greases (FOG) from restaurant grease traps, and many other sources. Co-digestion can increase biogas production from low-yielding or difficult-to-digest organic waste.

The following figure illustrates the flow of feedstocks through the AD system to produce biogas and digestate.

Anaerobic Digester Outputs

Anaerobic digestion produces two valuable outputs: biogas and digestate.

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Biogas

Biogas is composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage (50 to 75 percent), carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems, among other uses. Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.

Digestate

Digestate is the residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each have value that can be realized with varying degrees of post processing.

With appropriate treatment, both the solid and liquid portions of digestate can be used in many beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics), organic-rich compost (solids), and/or simply as soil amendment (solids), the latter of which may include the farm spreading the digestate on the field as fertilizer. Digestate products can be a source of revenue or cost savings, and are often pursued to increase the financial and net-environmental benefit of an AD/biogas project.

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The problem of waste disposal from a myriad of industries, is becoming increasingly acute, the world over. The burning of such wastes in open dumps or in poorly designed incinerators could be a major source of air pollution (Ndegwa and Thompson. Bioresour Technol 76:107&#;112, ). On the other hand, open dumps and poorly designed sanitary landfills can pollute surface and ground waters causing public health hazards. Meanwhile, the unavailability and rising cost of land near urban areas have made dumps and landfills increasingly expensive and impractical. The production of both livestock and grain on the other hand has increasingly relied on enormous chemical and energy inputs, leaving soils depleted of indigenous nutrients and organic matter, and resulting in wide-scale surface and groundwater contamination. As discussed earlier, recycling and utilization of organic wastes and by products through development of an economically viable, socially accepted and eco-friendly technologies are required. Over the years an array of innovative ideas for the utilization of these wastes have been put forward (Callaghan et al. Bioresour Technol 67:117&#;122, ) to increase productivity and to meet the heavy demand for food of the growing population (Jeyabal and Kuppuswamy. Eur J Agron 15:153&#;170, ). But these wastes could not be fully exploited without a viable technology for their economic recycling. It is well demonstrated that both fresh and composted amendments over these waste materials are potent to stimulate soil biological activities. Fresh wastes produces an initial burst of biochemical activity by the releasing easily degradable organic compounds whereas compost induces lower biochemical activities but more resistance to soils (Masciandaro et al. Soil Biol Biochem 32:&#;, ). Biological treatments plays a pivotal role in treating organic wastes these days. Among them, anaerobic digestion is frequently the most cost effective method because of the high energy recovery and its limited environmental impacts. Biogas production throughout Europe, could reach over 15 million m3/day of methane reported during (Tilche and Malaspina. Biogas production in Europe. Paper presented at the 10th European conference biomass for energy and industry, Wurzburg, 8&#;11 June, ). Presently, biogas production is considered to be an inevitable way of energy production.

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