Some Inorganic Pollutants of Concern

Cyanide ion, CN‐, is probably the most important of the various inorganic species in wastewater. Cyanide, a deadly poisonous substance, exists in water as HCN which is a weak acid. The cyanide ion has a strong affinity for many metal ions, forming relatively less toxic ferrocyanide, Fe(CN)₆⁴⁻, with iron (II), for example. Volatile HCN is very toxic and has been used in gas chamber executions in the United States. Cyanide is widely used in industry, especially for metal cleaning and electroplating. It is also one of the main gas and coke scrubber effluent pollutants from gas works and coke ovens. Cyanide is widely used in certain mineral processing operations.

Ammonia is the initial product of the decay of nitrogenous organic wastes, and its presence frequently indicates the presence of such wastes. It is a normal constituent of some sources of groundwater and is sometimes added to drinking water to remove the taste and odor of free chlorine. Since the pK_a (the negative log of the acid ionization constant) of the ammonium ion, NH₄⁺, is 9.26, most ammonia in water is present as NH₄⁺ rather than NH₃.

Hydrogen sulphide, H₂S, is a product of the anaerobic decay of organic matter containing sulfur. It is also produced in the anaerobic reduction of sulphate by microorganisms and is developed as a gaseous pollutant from geothermal waters. Wastes from chemical plants, paper mills, textile mills, and tanneries may also contain H₂S. Nitrite ion, NO²⁻, occur in water as an intermediate oxidation state of nitrogen. Nitrite is added to some industrial processes to inhibit corrosion; it is rarely found in drinking water at levels over 0.1 mg/l. Sulphite ion, SO₃²⁻, is found in some industrial wastewaters. Sodium sulphite is commonly added to boiler feed‐waters as an oxygen scavenger:

Organic Pollutants of Concern

Effluent from industrial sources contains a wide variety of pollutants, including organic pollutants. Primary and secondary sewage treatment processes remove some of these pollutants, particularly oxygen‐demanding substances, oil, grease, and solids. Others, such as refractory (degradation‐resistant) organics (organochlorides, nitro compounds, etc.), salts, and heavy metals, are not efficiently removed. Soaps, detergents, and associated chemicals are potential sources of organic pollutants. Most of the environmental problems currently attributed to detergents do not arise from the surface‐active agents which basically improve the wetting qualities of water. The greatest concern among environmental pollutants has been caused by polyphosphates added to complex calcium functioning as a builder.

Bio‐refractory organics are poorly biodegradable substances, prominent among which are aromatic or chlorinated hydrocarbons (benzene, bornyl alcohol, bromobenzene, chloroform, camphor, dinitrotoluene, nitrobenzene, styrene, etc.). Many of these compounds have also been found in drinking water. Water contaminated with these compounds must be treated using physical and chemical methods, including air stripping, solvent extraction, ozonation, and carbon adsorption.

First discovered as environmental pollutants in 1966, polychlorinated biphenyls (PCB compounds) have been found throughout the world in water, sediments, and in bird and fish tissues. They are made by substituting between 1 and 10 Cl atoms onto the biphenyl aromatic structure. This substitution can produce 209 different compounds (Rouessac and Rouessac 2007).

Thermal Pollution

Considerable time has elapsed since the scientific community and regulatory agencies officially recognized that the addition of large quantities of heat to a recipient possesses the potential of causing ecological harm. The really significant heat loads result from the discharge of condenser cooling water from the ever‐increasing number of steam electrical generating plants and equivalent‐sized nuclear power reactors. Large numbers of power plants currently require approximately 50% more cooling water for a given temperature rise than that required of fossil‐fuel plants of an equal size. The degree of thermal pollution depends on thermal efficiency, which is determined by the amount of heat rejected into the cooling water. Thermodynamically, heat should be added at the highest possible temperature and rejected at the lowest possible temperature if the greatest amount of effect is to be gained and the best thermal efficiency realized. The current and generally accepted maximum operating conditions for conventional thermal stations are about 500 °C and 24 MPa, with a corresponding heat rate of 2.5 kWh, 1.0 kWh resulting in power production and 1.5 kWh being wasted. Plants have been designed for 680 °C and 34 MPa; however, metallurgical problems have kept operating conditions at lower levels.

Nuclear power plants operate at temperatures from 250 to 300 °C and pressures of up to 7 MPa, resulting in a heat rate of approximately 3.1 kWh. Thus, for nuclear plants, 1.0 kWh may be used for useful production, whereas 2.1 kWh is wasted. Most steam‐powered electrical generating plants are operated at varying load factors, and consequently the heated discharges demonstrate wide variation with time. Thus, the biota is not only subjected to increased or decreased temperature, but also to a sudden or “shock,” temperature change. Increased temperature will cause remarkable reduction in the self‐purification capacity of a receiving water body and cause the growth of undesirable algae (Krenkel and Novotny 1980). The addition of heated water to the receiving water can be considered equivalent to the addition of sewage or other organic waste material, since both pollutants may cause a reduction in the oxygen resources of the receiving waters. Also, elevated temperatures in the receiving water could cause undesirable algae bloom (Hauser 2018; USEPA 2018).