Anaerobic digestion

Refers to the biological degradation of either sludges or solid waste under anaerobic conditions, meaning that no oxygen is present. In the digestive process, solids are converted to noncellular end products.

In the anaerobic digestion of sludges, the goals are to reduce sludge volume, insure the remaining solids are chemically stable, reduce disease-causing pathogens, and enhance the effectiveness of subsequent dewatering methods, sometimes recovering methane as a source of energy. Anaerobic digestion is commonly used to treat sludges that contain primary sludges, such as that from the first settling basins in a wastewater treatment plant, because the process is capable of stabilizing the sludge with little biomass production, a significant benefit over aerobic sludge digestion , which would yield more biomass in digesting the relatively large amount of biodegradable matter in primary sludge.

The microorganisms responsible for digesting the sludges anaerobically are often classified in two groups, the acid formers and the methane formers. The acid formers are microbes that create, among others, acetic and propionic acids from the sludge. These chemicals generally make up about a third of the by-products initially formed based on a chemical oxygen demand (COD) mass balance, and some of the propionic and other acids are converted to acetic acid.

The methane formers convert the acids and by-products resulting from prior metabolic steps (e.g., alcohols, hydrogen , carbon dioxide ) to methane. Often, approximately 70% of the methane formed is derived from acetic acid, about 10–15% from propionic acid.

Anaerobic digesters are designed as either standard- or high-rate units. The standard-rate digester has a solids retention time of 30–90 days, as opposed to 10–20 days for the high-rate systems. The volatile solids loadings of the standard- and high-rate systems are in the area of 0.5–1.6 and 1.6–6.4 Kg/m³/d, respectively. The amount of sludge introduced into the standard-rate is therefore generally much less than the high-rate system. Standard-rate digestion is accomplished in single-stage units, meaning that sludge is fed into a single tank and allowed to digest and settle. High-rate units are often designed as two-stage systems in which sludge enters into a completely-mixed first stage that is mixed and heated to approximately 98°F (35°C) to speed digestion. The second-stage digester, which separates digested sludge from the overlying liquid and scum, is not heated or mixed.

With the anaerobic digestion of solid waste, the primary goal is generally to produce methane, a valuable source of fuel that can be burned to provide heat or used to power motors. There are basically three steps in the process. The first involves preparing the waste for digestion by sorting the waste and reducing its size. The second consists of constantly mixing the sludge, adding moisture, nutrients, and pH neutralizers while heating it to about 143°F (60°C) and digesting the waste for a week or longer. In the third step, the generated gas is collected and sometimes purified, and digested solids are disposed of. For each pound of undigested solid, about 8–12 ft³ of gas is formed, of which about 60% is methane.

[Gregory D. Boardman ]

RESOURCES

BOOKS

Corbitt, R. A. Standard Handbook of Environmental Engineering. New York: McGraw-Hill, 1990.

Davis, M. L., and D. A. Cornwell. Introduction to Environmental Engineering. New York: McGraw-Hill, 1991.

Viessman, W., Jr., and M. J. Hammer. Water Supply and Pollution Control. 5th ed. New York: Harper Collins, 1993.