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Related to Waste Water: Waste water treatment
(or sewage), water that is contaminated with domestic and industrial wastes and is removed from populated areas and industrial plants by means of sewerage systems. Also classified as waste waters are storm sewage and other waters that result from precipitation within populated areas and industrial facilities and are carried away by sewerage systems.
When the organic matter contained in waste water enters bodies of water in substantial quantities or accumulates in the soil, it can rapidly decay and cause deterioration of sanitary conditions in the water bodies and atmosphere by fostering the spread of various diseases. For this reason, the treatment, decontamination, and reclamation of waste waters are considered an integral part of conservation, environmental sanitation, and the provision of good sanitary conditions in cities and other populated areas.
Classification and composition. Three principal categories of waste water are distinguished according to the origin, composition, and qualitative characteristics of the pollutants: (1) domestic waste waters, which carry, for example, household and fecal wastes, (2) industrial waste waters, and (3) atmospheric waste waters, including storm sewage.
Domestic waste waters include sewage discharged from toilet facilities, baths, showers, kitchens, bathhouses, laundries, restaurants, and hospitals. Such waters are contaminated primarily by physiological wastes and wastes from households and nonindustrial establishments.
Industrial waste waters include water used in various industrial processes, for example, water used to wash raw materials and finished products and water used for cooling purposes. Also classed as industrial waste water is the water pumped to the earth’s surface in the extraction of minerals. In a number of industries the effluents contain mainly factory wastes. Here, the pollutants may include substances containing radioactive elements and such toxic substances as hydrocyanic acid, phenol, arsenic compounds, aniline, copper salts, lead salts, and mercury salts. Some wastes have a certain value as secondary raw materials. Two types of industrial waste water are distinguished according to the amount of pollutants present: contaminated sewage, which must undergo preliminary treatment before being discharged into bodies of water or before being reused, and relatively clean, or slightly contaminated, sewage, which can be discharged into bodies of water or reused without treatment.
Atmospheric waste waters include rainwater and meltwater from melting ice and snow. On the basis of the qualitative characteristics of the pollutants, this category also includes water used for cleaning streets and watering greenery. Atmospheric waste waters that contain primarily mineral pollutants present less of a danger to health than do domestic and industrial waste waters.
The degree of pollution of waste waters is determined by the concentration of the pollutants—that is, by their weight per unit volume of water in milligrams per liter or grams per cubic centimeter.
The composition of domestic waste water is more or less uniform. The concentration of pollutants in such waste waters depends on the amount of water used per person—that is, on the rate of water consumption. The pollutants in domestic waste waters may be divided into insolubles, colloids, and solubles. The insolubles may consist of coarse particles, of dimensions exceeding 0.1 mm, in solution, or they may form suspensions, emulsions, or foams, where the particle dimensions range from 0.1 mm to 0.1 micrometer (µm). The particle dimensions of the colloids range from 0.1 µm to 1 nanometer (nm). The solubles are present in solution; the sizes of the dispersed molecular particles are less than 1 nm.
Mineral, organic, and biological pollutants are found in domestic waste waters. Mineral pollutants include sand, slag particles, clay particles, and solutions of mineral salts, acids, and alkalies. Organic pollutants are of plant and animal origin. Examples of pollutants of plant origin are vegetable oils and the remains of plants, fruits, vegetables, and paper. Carbon is the principal chemical element in such pollutants. Pollutants of animal origin include human and animal excreta, the remains of animal tissues, and glues. Such pollutants are characterized by an appreciable nitrogen content. Biological pollutants include various microorganisms, yeast fungi, mold fungi, small algae, and bacteria, including pathogenic bacteria—for example, pathogens of typhoid, paratyphoid, dysentery, and anthrax. This type of pollutant is found not only in domestic waste water but also in several kinds of industrial waste water, such as the effluents of meat-packing plants, slaughterhouses, tanneries, and biologies-manufacturing plants. In their chemical composition biological pollutants are organic pollutants, but they are put into a separate group because of the health hazard presented by their entry into bodies of water.
Of the total amount of pollutants contained in-domestic waste waters, about 42 percent is mineral matter and about 58 percent is organic. Settleable suspended matter constitutes 20 percent, suspensions account for 20 percent, colloids form 10 percent, and dissolved solids represent 50 percent. The quantity of domestic sewage depends primarily on the rate of sewage flow. This rate is determined by the level of public services and amenities in the buildings. For example, according to the standards in use in the USSR, the average amount of waste water produced per person per day—when running water, sewerage, and hot water are provided in the building—is 275–350 liters per 24-hour period.
The composition and degree of pollution of industrial waste waters are varied and depend mainly on the type of industry and the conditions of water usage in the manufacturing process.
The quantity of atmospheric waste water varies considerably, depending on, for example, climatic conditions, topography, the manner in which the cities are built up, and the types of road surfaces present. In cities in the European part of the USSR, on the average, the runoff of rainwater can reach once a year 100–150 liters per second per hectare. The annual rainwater runoff in built-up areas is smaller than the quantity of domestic effluents by a factor of 7–15.
Protection of bodies of water. The pollution of bodies of water is mainly due to the discharge of waste waters from industrial plants and populated areas. When untreated sewage containing considerable quantities of organic matter and microorganisms enters a body of water, such as a river or lake, it disrupts the natural conditions: it absorbs oxygen dissolved in the water, lowers the quality of the water, and contributes to the formation of sediment on the bottom. The bodies of water become unfit to supply drinking water and, sometimes, industrial water, and the fish in them are killed. In addition, the pollution of bodies of water by sewage spoils their aesthetic appearance and restricts their recreational use for swimming, water sports, and tourism.
In the USSR, the required degree of sewage treatment and the conditions for discharge of sewage into bodies of water are regulated by the Regulations for Protection of Surface Waters from Pollution by Waste Waters. Two types of standards have been established for water quality according to the manner in which the water is used: one is for bodies of water that supply drinking water and water for domestic and general municipal use, and the other is for bodies of water used for fisheries. The maximum permissible concentrations of substances in bodies of water have also been established. These limits are the basis for determination of the conditions under which waste waters can be discharged into a body of water. The discharge of untreated sewage into bodies of water is forbidden in the USSR by the Law on Conservation and by water use legislation. Sewage disposal and the treatment or decontamination of waste waters are supervised by units of the public health epidemiologic service of the Ministry of Public Health of the USSR and by the Basin Inspectorates of the Ministry of reclamation and Water Use Management of the USSR.
Treatment and reclamation. In the sewerage systems of populated areas, the waste waters are treated in sewage treatment plants before being discharged into bodies of water. Suspended solids (seeSAND TRAP and SETTLING RESERVOIR), colloids, and dissolved materials (seeBIOLOGICAL SEWAGE TREATMENT) are removed during sewage treatment. The settled sludge in primary settling tanks and the excess activated sludge formed in the process of biological purification are treated and decontaminated for subsequent use. In present-day practice, the most complete removal of pollutants is attained through biological purification.
After suitable treatment, industrial waste water can be reused in manufacturing processes. Many industrial enterprises have developed for this purpose recirculating water-supply systems or closed (undrained) water supply and sewer systems that eliminate discharge into bodies of water. Of great importance to the national economy is the introduction of comprehensive wasteless processing of raw materials, particularly in the chemical industry, the pulp-and-paper industry, and the mining and ore-dressing industry. Physicochemical methods of water treatment, such as coagulation, sedimentation, and filtration, are promising as independent means of treatment or as methods to be used in conjunction with biological treatment. Also promising are such supplementary treatment processes as adsorption, ion exchange, hyperfiltration, and the removal of nitrous materials and phosphates. Such methods provide a very high degree of purification of the waste water before discharge or use in recirculating water-supply systems of industrial enterprises. Other effective methods of dealing with waste waters are thermal decontamination, the processing of highly concentrated effluents into secondary raw materials, and the pumping of effluents into deep, safely isolated underground horizons.
Waste waters, particularly domestic sewage, carry considerable quantities of substances containing such elements as nitrogen, potassium, phosphorus, and calcium. Since these substances are valuable as fertilizers for agricultural crops, waste water is used to irrigate agricultural land (seeSEWAGE FARM). It is good practice to treat the waste water at biological treatment plants before delivering the water to the fields. After adequate processing, such as fermentation and drying, the sediment from the waste water is usually used as fertilizer.
REFERENCESKanalizatsiia, 4th ed. Moscow, 1969.
Kanalizatsiia promyshlennykh predpriiatii. Moscow, 1969.
Z. A. ORLOVSKII