James G. Speight - Encyclopedia of Renewable Energy

Any organic substance can become subject to anaerobic digestion so long as there are warm, wet, and airless conditions. For example, marsh gas is a product of the anaerobic digestion of vegetation at the bottom of ponds; this gas rises to the surface and bubbles, and the gas is also combustible. With the aid of human intervention, there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are complex. The end result of the process is a well-established sludge in which 40 to 60% of the volatile solids are destroyed. Finally, a combustible gas is produced consisting of 60 to 75% v/v methane and the remainder largely being carbon dioxide. ( Table A-21).

Table A-21Summary of most common chemical reactions in the anaerobic digestion process.

Thus, simplified examples of the chemistry of anaerobic digestion process are as follows:

Organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. Temperature must be maintained within certain ranges – heating increases the activity of the anaerobic bacteria, reducing the required digestion time. A pH of 7.0 to 7.5 is recommended to encourage the methane-producing stage.

The organic content of the sludge is significantly reduced by conversion into gaseous end-products; the obnoxious odor of the sludge is removed, and the final digested sludge has a characteristic tarry odor; fats and greases are broken down by the process. The liquid fraction (supernatant) contains increased levels of ammonia as a result of the breakdown of organic nitrogen (proteins). This makes the digested sludge liquor potentially suitable for agricultural use; the biogas that is formed is a mixture of carbon dioxide (CO 2) and methane (CH 4) that can be used for digester heating or to generate power.

The slow rate of bacterial growth usually requires long periods of time for start-up and limits the flexibility of the process to adjust to changing feed loads, temperatures, and other environmental conditions. In addition, the process is prone to upsets if not regularly monitored and if corrective action is not taken in time.

See also: Acetogenesis, Acidogenesis, Aerobic Digestion, Digester, Digestion, Hydrolysis, Methanogenesis.

A typical gas system for the production of gas by anaerobic digestion comprises the digester cover, pressure and vacuum relief devices, water trap, flame trap, pressure regulator, gas meter, check valve, pressure gauges, waste gas burner, and a gas holder. The continuous stirred tank reactor (CSTR) is an example of a type of anaerobic digester.

The digester is covered to contain odors, maintain temperature, keep air out, and collect the gas. Fixed covers are more usual than floating covers. During normal operation, there is a space for gas collection between the cover and the liquid surface of the digester contents. The cover of a digester has certain unique features that the operating staff must be aware of, for example, how the variation in pressure and the level inside the digester may affect the cover. The biggest danger associated with the operation of fixed cover digesters occurs when the pressure relief device mounted on top of the digester fails or the sludge overflow line blocks and the liquid level in the digester continues to rise. In such a situation, the excess gas pressure inside the digester can exceed the maximum design pressure and damage the cover or its mountings. Fixed covers can also be damaged by excess negative pressure (vacuum) or if the rate of waste sludge withdrawal exceeds the feed rate or the vacuum relief device fails.

Gas leaving the digester is almost saturated with water vapor. As the gas cools, the water vapor condenses causing problems, which are more severe when digesters are heated. It is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices. For this reason, water traps should be located as close to the digester as possible. All piping should be sloped a minimum of 1% toward the water trap, which should be situated at a low point in the gas line.

Flame traps are emergency devices installed in gas lines to prevent flames traveling back up the gas line (flashback) and reaching the digester. The flame trap generally consists of a box filled with stone or a metal grid. If a flame develops in the gas line, the temperature of the flame is reduced below the ignition point as it passes through the trap and the flame is extinguished. Many anaerobic digestion waste treatment plants have a means of storing excess gas, either in the form of a floating roof on the digester or a separate gasholder.

A mixture of biogas and air can be explosive. Methane gas in concentrations of between 5% and 15% in air by volume is explosive and all piping and equipment must be sealed properly to prevent gas from escaping to the outside. In addition, all electrical installations, including light switches, and torches must be of the explosion-proof type, as the smallest spark could ignite escaped gases.

See also: Aerobic Digestion, Anaerobic Digestion.

Animal wastes, or manure, as a source of biomass has the advantage that it is not competitive with other uses for this material. In the broader senses, animal waste can also include, in addition to animal manure, related animal waste (such as bedding and feed), dead animals, and waste from slaughter and meat processing. All such wastes generally fall in to a sub-class of biomass. Animal waste must of necessity come from confined operations such as dairy farms, cattle feedlots, and slaughter houses as well as meat processing plants.

Animal waste from farms and livestock/poultry and dairy production operations can severely threaten water quality if not managed properly and has the potential to contribute excess nutrients, pathogens, organic matter, solids, and odorous compounds to the environment. This pollution can cause eutrophication of surface waters, degradation of ground water quality, and threats to human health.

Historically, manure generated by livestock has been returned to the soil to improve its tilth and fertility. Recently, the use of animal waste to produce an renewable fuel (biogas) has become of interest. Anaerobic digestion is a renewable solution to livestock waste management that offers economic and environmental benefits.

Anaerobic digestion is a process to decompose organic material in the absence of oxygen. Biogas is produced as a waste product of digestion. In the first stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteria (acid formers). In the second stage, these acids are further converted into biogas by more specialized bacteria (methane formers). With proper planning and design, this anaerobic-digestion process can be managed to convert the waste-stream from a farm into an asset and is, currently, the most appropriate option for farms and small-scale operation. The conversion of animal waste to gas by this process could result in animal husbandry operations being self-sufficient in energy.

Biogas produced in an anaerobic digester typically contains methane (60 to 70% v/v), carbon dioxide (30 to 40% v/v), various toxic gases (including hydrogen sulfide, ammonia, and sulfur-derived mercaptans), and 1 to 2% v/v water vapor.