Organic Substances

Proteins are nitrogenous organic substances of high molecular weight found in the animal kingdom and to a lesser extent in the plant kingdom. The amount present varies from a small percentage found in tomatoes and other watery fruits and in the fatty tissues of meat to a high percentage in lean meats and beans. All raw foodstuffs, plant and animal, contain proteins. Proteins consist wholly or partially of very large numbers of amino acids. They also contain carbon, hydrogen, oxygen, sulfur, phosphorous, and a fairly high and constant proportion of nitrogen. The molecular weight of proteins is quite high.

Coakley (1975) noted that proteinaceous materials constitute a large part of the wastewater biosolids, and that the biosolids particles, if they do not consist of pure protein, will be covered with a layer of protein that will govern their chemical and physical behavior. Moreover, the protein content ranges between 15 and 30% of the organic matter present for digested biosolids and from 28 to 50% in the case of activated biosolids. Proteins and urea are the chief sources of nitrogen in waste-water. When proteins are present in large quantities, microorganisms decompose, producing end products that have objectionable foul odors. During this decomposition process, proteins are hydrolyzed to amino acids, then further degraded to ammonia, hydrogen sulfide, and simple organic compounds.

Oils and grease are other major components of foodstuffs. They are usually related to spills or other releases of petroleum products. Minor oil and grease problems can result from runoff from highways during wet weather or improper disposal of motor oil in storm drains. Oils and grease are insoluble in water but dissolve in organic solvents such as petroleum, chloroform, and ether. Fats, oils, waxes, and other related constituents found in wastewater are commonly grouped under the category of grease. Fats and oils found in domestic wastewater generally include butter, lard, margarine, and vegetable fats and oils. Fats, which are compounds of alcohol and glycerol, are among the more stable of organic compounds and are not easily decomposed by bacteria; however, they can be broken down by mineral acids, resulting in the formation of fatty acids and glycerin. When these glycerides of fatty acids are liquid at ordinary temperature, they are considered to be oils; those that are solid are considered to be fats.

The grease content of wastewater can cause many problems in wastewater treatment unit processes; for example, a high grease content can cause clogging of filters, nozzles, and sand beds (Gilcreas et al., 1975). Moreover, grease can coat the walls of sedimentation tanks and decompose and increase the amount of scum. Additionally, if grease is not removed before discharge of the effluent, it can interfere with the biological processes in the surface waters and create unsightly floating matter and films (Rowe and Abdel-Magid, 1995). In the treatment process, grease can coat trickling filters and interfere with the activated sludge process, which in turn interferes with the transfer of oxygen from the liquid to the interior of living cells (Sawyer et al., 1994).

Carbohydrates, which are widely distributed in nature and found in wastewater, are organic substances that include starch, cellulose, sugars, and wood fibers; they contain carbon, hydrogen, and oxygen. Sugars are soluble but starches are insoluble in water. The primary function of carbohydrates in higher animals is to serve as a source of energy. In lower organisms (e.g., bacteria), carbohydrates are utilized to synthesize fats and proteins as well as energy. In the absence of oxygen, the end products of decomposition of carbohydrates are organic acids, alcohols, and gases, such as carbon dioxide and hydrogen sulfide. The formation of large quantities of organic acids can affect the treatment process by overtaxing the buffering capacity of the wastewater, resulting in a drop in pH and a cessation of biological activity.

Detergents, or surfactants, are large organic molecules that are slightly soluble in water and cause foaming in wastewater treatment plants and in the surface waters into which the effluent is discharged. Probably the most serious effect that detergents can have on wastewater treatment processes is in their tendency to reduce the oxygen uptake in biological processes. According to Rowe and Abdel-Magid (1995), detergents affect wastewater treatment processes in that "they lower the surface, or interfacial, tension of water and increase its ability to wet surfaces with which they come in contact; emulsify grease and oil, deflocculate colloids; induce flotation of solids and give rise to foams; and may kill useful bacteria and other living organisms." Since the development and increasing use of synthetic detergents, many of these problems have been reduced or eliminated.

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