O

fatty acids. (Esters have the general formula of rC -OR , where R is another hydrocarbon group.) Examples of fats are butter, lard, and margarine; and examples of oils are the vegetable oils cottonseed oil, linseed oil, and palm oil. Fats and oils are abundant in meat and meat products.

Glycerol may also derive, along with phosphoric acid and the fatty acids, a third class of compounds called phosphoglycerides or glycerolphosphatides. In these glycerides, one of the fatty acids is substituted by organic phosphates attaching to the glycerol backbone at one of the ends. The organic phosphates are the phosphates of choline, ethanolamine, and serine. Phosphoglycerides, fats, and oils are collectively called complex lipids. Phosphoglycerides are phospholipids.

Certain alcohols look and feel like lipids or fats; thus, they are called fatty alcohols. Fatty alcohols are also called simple lipids, which are long-chain alcohols, examples of which are cetyl alcohol [CH3(CH2)14CH2OH] and myricyl alcohol [CH3(CH2)29CH2OH]. Therefore, the two types of lipids are: complex lipids and simple lipids. Simple lipids do not have the fatty acid "component" of the complex lipids. The simple lipids can react with fatty acids to form esters called waxes. In environmental engineering, waxes and complex lipids (fats, oils, and phospholipids) and mineral oils such kerosene, crude oil, and lubricating oil and similar products are collectively called grease. The grease content in wastewater is determined by extraction of the waste sample with trichlorotrifluoroethane. Grease is soluble in trichlorotrifluoroethane.

Grease is among the most stable of the organic compounds that, as such, is not easily consumed by microorganisms. Mineral acids can attack it liberating the fatty acids and glycerol. In the presence of alkali, glycerol is liberated, and the fatty acids, also liberated, react with the metal ion of the alkali forming salts called soap. The soaps are equally resistant to degradation by microorganisms.

2.1.25 Surfactants

Surfactants are surface-active agents, which means that they have the property of interacting with surfaces. Grease tends to imbed dirt onto surfaces. In order to clean these surfaces, an agent must be used to loosen the dirt. This is where surfactants come in. Surfactant molecules have nonpolar tails and polar heads. The grease molecules, being largely nonpolar, tend to grasp the nonpolar tail of the surfactant molecules, while the polar water molecules tend to grasp the polar head of the surfactant molecules. Because of the movement during cleansing, a "tug of war" occurs between the water molecules on the one side and the grease on the other with the surfactant acting as the rope. This activity causes the grease to loosen from the surfaces thus effecting the cleansing of the dirt. Detergents are examples of surfactants. They are surface-active agents for cleaning.

Surfactants collect on the air-water interface. During aeration in the treatment of wastewater, they adhere to the surface of air bubbles forming stable foams. If they are discharged with the effluent, they form similar bubbles in the receiving stream.

Before 1965, the type of surfactant used in this country was alkyl benzene sulfonate (ABS). ABS is very resistant to biodegradation and rivers were known to be covered with foam. Because of this and because of legislation passed in 1965, ABS was replaced with linear alkyl benzene sulfonate (LAS). LAS is biodegradable.

The laboratory determination of surfactants involves using methylene blue. This is done by measuring the color change in a standard solution of the dye. The surfactant can be measured using methylene blue, so its other name is methylene blue active substance (MBAS).

2.1.26 Priority Pollutants

Before the 1970s, control of discharges to receiving bodies of water were not very strict. During those times, discharge of partially treated wastewaters were allowed; but although facilities could be built to treat discharges by, at least, partial treatment, several communities and industries were discharging untreated wastewaters. This practice resulted in gross pollution of bodies of water that had to be stopped.

The 1970s show pollution control starting in earnest. Industries were classified into industrial categories. These resulted in the identification of priority pollutants and the establishment of categorical standards for a particular industrial category. These standards apply to commercial and industrial discharges that contain the priority pollutants identified by the EPA. Since industries are allowed to discharge into collection systems, these priority pollutants find their way into publicly owned treatment works (POTWs).

The following are examples of priority pollutants: arsenic, selenium, barium, cadmium, chromium, lead, mercury, silver, benzene, ethylbenzene, chlorobenzene, chloroethene, dichloromethane, and tetrachloroethene. The priority pollutants also include the pesticide and fumigant eldrin, the pesticide lindane, the insecticide methoxychlor, the insecticide and fumigant toxaphene, and the herbicide and plant growth regulator silvex. There are a total of 65 priority pollutants.

2.1.27 Volatile Organic Compounds

Generally, volatile organic compounds (VOCs) are organic compounds that have boiling points of <100°C and/or vapor pressures of >1 mmHg at 25°C. VOCs are of concern in wastewater engineering because they can be released in the wastewater collection systems and in treatment plants causing hazards to the workers. For example, vinyl chloride is a suspected carcinogen and, if found in sewers and released in the treatment plants, could endanger the lives of the workers.

2.1.28 Toxic Metal and Nonmetal Ions

Because of their toxicity, certain metals and nonmetal ions should be addressed in the design of biological wastewater treatment facilities. Depending on the concentration, copper, lead, silver, chromium, arsenic, and boron are toxic to organisms in varying degrees. Treatment plants have been upset by the introduction of these metals by killing the microorganism thus stopping the treatment. For example, in the digestion of sludge, copper is toxic in concentrations of 100 mg/L, potassium and the ammonium ion in concentrations of 4,000 mg/L, and chromium and nickel in concentrations of 500 mg/L.

Anions such as cyanides, chromates, and fluorides found in industrial wastes are very toxic to microorganisms. The cyanide and chromate wastes are produced by metal-plating industries. These wastes should not be allowed to mix with sanitary sewage but should be removed by pretreatment. The fluoride wastes are normally produced by the electronics industries.

Concentration (mg/L)

Constituent

Strong

Medium

Weak

Biochemical oxygen demand, BOD5 at 20°C

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