Conventional Pollutant

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Conventional pollutant

Conventional pollutants fall into five categories; the presence of these pollutants is commonly determined by measuring biochemical oxygen demand , total suspended solids, pH levels, the amount of fecal coliform, and the quantity of oil and grease.

Biochemical oxygen demand (BOD) is the quantity of oxygen required by microorganisms to stabilize five-day incubated oxidizable organic matter at 68°F (20°C). Hence, BOD is a measure of the biodegradable organic carbon and at times, the oxidizable nitrogen . BOD is the sum of the oxygen used in organic matter synthesis and in the endogenous respiration of microbial cells. Some industrial wastes are difficult to oxidize, and bacterial seed is necessary. In certain cases, an increase in BOD is observed with an increase in dilution. It is hence necessary to determine the detection limits for BOD.

Suspended solids interfere with the transmission of light. Their presence also affects recreational use and aesthetic enjoyment. Suspended solids make fish vulnerable to diseases, reduce their growth rate, prevent successful development of fish eggs and larvae, and reduce the amount of available food. The Environmental Protection Agency (EPA) restricts suspended matter to not more than ten percent of the reasonably established amount for aquatic life. This allows sufficient sunlight to penetrate and sustain photosynthesis . Suspended solids also cause damage to invertebrates and fill up gravel spawning beds.

The acidity or alkalinity of water is indicated by pH. A pH of seven is neutral. A pH value lower than seven indicates an acidic environment and a pH greater than seven indicates an alkaline environment. Most aquatic life is sensitive to changes in pH. The pH of surface waters is specified to protect aquatic life and prevent or control unwanted chemical reactions such as metal ion dissolution in acidic waters. An increase in toxicity of many substances is often observed with changes in pH. For example, an alkaline environment shifts the ammonium ion to a more poisonous form of un-ionized ammonia. EPA criteria for pH are 6.5–9.0 for freshwater life, 6.5–8.5 for marine organisms and 5–9 for domestic consumption.

Fecal coliform bacteria are yardsticks for detecting pathogenic or disease causing bacteria. However, this relationship is not absolute because these bacteria can originate from the intestines of humans and other warm blooded animals. Prior knowledge of river basins and the possible sources of these bacteria is necessary for a survey to be effective. The strictest EPA criteria for coliforms apply to shellfish, since they are often eaten without being cooked.

Common sources of oil and grease are petroleum derivatives and fats from vegetable oil and meat processing. Both surface and domestic waters should be free from floating oil and grease. Limits for oil and grease are based on LC₅₀ values. LC₅₀ is defined as the concentration at which 50% of an aquatic species population perishes. EPA criterion is for a 96-hour exposure, and during this period the concentration of individual petrochemicals should not exceed 0.01 of the LC₅₀ median. Oil and grease contaminants vary in physical, chemical, and toxicological properties besides originating from different sources.

[James W. Patterson ]