1  Water Pollution and Pollutants
The relationship between polluted water and disease was firmly established with the cholera epidemic of 1854 in London, England. Protection of public health, the original purpose of pollution control, continues to be the primary objective in many areas. However, preservation of water resources, protection of fishing areas, and maintenance of recreational waters are additional concerns today. Water pollution problems intensified following World War II when dramatic increases in urban density and industrialization occurred. Concern over water pollution reached a peak in the mid-seventies.
Water pollution is an imprecise term that reveals nothing about either the type of polluting material or its source. The way we deal with the waste problem depends upon whether the contaminants are oxygen demanding, algae promoting, infectious, toxic, or simply unsightly. Pollution of our water resources can occur directly from sewer outfalls or industrial discharges (point sources) or indirectly from air pollution or agricultural or urban runoff (nonpoint sources).
Chemically pure water is a collection of H2O molecules—nothing else. Such a substance is not found in nature—not in wild streams or lakes, not in clouds or rain, not in falling snow, nor in the polar ice caps. Very pure water can be prepared in the laboratory but only with considerable difficulty. Water accepts and holds foreign matter.
Municipal wastewater, also called sewage, is a complex mixture containing water (usually over 99 percent) together with organic and inorganic contaminants, both suspended and dissolved. The concentration of these contaminants is normally very low and is expressed in mg/L, that is, milligrams of contaminant per liter of the mixture. This is a weight-to-volume ratio used to indicate concentrations of constituents in water, wastewater, industrial wastes, and other dilute solutions.
Microorganisms. Wherever there is suitable food, sufficient moisture, and an appropriate temperature, microorganisms will thrive. Sewage provides an ideal environment for a vast array of microbes, primarily bacteria, plus some viruses and protozoa. Most of these microorganisms in wastewater are harmless and can be employed in biological processes
to convert organic matter to stable end products. However, sewage may also contain pathogens from the excreta of people with infectious diseases that can be transmitted by contaminated water. Waterborne bacterial diseases such as cholera, typhoid, and tuberculosis, viral diseases such as infectious hepatitis, and the protozoan-caused dysentery, while seldom a problem now in developed countries, are still a threat where properly treated water is not available for public use. Tests for the few pathogens that might be present are difficult and time consuming, and standard practice is to test for other more plentiful organiama that are always present (in the billions) in the intestines of warm-blooded animals, including humans.
Solids. The total solids (organic plus inorganic) in wastewater are, by definition, the residues after the liquid portion has been evaporated and the remainder dried to a constant weight at 103℃. Differentiation between dissolved solids and undissolved, that is, suspended, solids are accomplished by evaporating filtered and unfiltered wastewater samples. The difference in weight between the two dried samples indicates the suspended solids content. To further categorize the residues, they are held at 550℃ for 1
5 minutes. The ash remaining is considered to represent inorganic solids and the loss of volatile matter to be a measure of the organic content
Suspended solids (SS) and volatile suspended solids (VSS) are the most useful. SS and BOD (biochemical oxygen demand) are used as measures of wastewater strength and process performance. VSS can be an indicator of the organic content of raw wastes and can also provide a measure of the active microbial population in biological processes.
Inorganic constituents. The common inorganic constituents of wastewater include:
1. Chlorides and sulphates. Normally present in water and in wastes from humans.
2. Nitrogen and phosphorous . In their various forms (organic and inorganic) in wastes from humans, with additional phosphorous from detergents.
3. Carbonates and bicarbonates. Normally present in water and wastes as calcium and magnesium salts.
4. Toxic substances. Arsenic, cyanide, and heavy metals such as Cd, Cr, Cu, Hg, Ph, and Zn are toxic inorganics which may be found in industrial wastes.
In addition to these chemical constituents, the concentration of dissolved gases, especially oxygen, and the hydrogen ion concentration expressed as pH are other parameters of interest in wastewater.
Organic matter. Proteins and carbohydrate constitute 90 percent of the organic matter in domestic sewage. The sources of these biodegradable contaminants include excreta and urine from humans; food wastes from sinks; soil and dirt from bathing; washing, and laundering; plus various soaps, detergents, and other cleaning products.
water pollution