Kids Research Express

The processes involved in municipal wastewater treatment plants are usually classified as being part of primary, secondary, or tertiary treatment. Primary Treatment The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal. The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later. Secondary Treatment Having removed 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic—that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelera

Solid Waste Disposal, disposal of normally solid or semisolid materials, resulting from human and animal activities, that are useless, unwanted, or hazardous. Solid wastes typically may be classified as follows: Garbage - decomposable wastes from food Rubbish - nondecomposable wastes, either combustible (such as paper, wood, and cloth) or noncombustible (such as metal, glass, and ceramics) Ashes - residues of the combustion of solid fuels Large wastes - demolition and construction debris and trees Dead animals Sewage-treatment solids - material retained on sewage-treatment screens, settled solids, and biomass sludge Industrial wastes - such materials as chemicals, paints, and sand Mining wastes - slag heaps and coal refuse piles Agricultural wastes - farm animal manure and crop residues. Learn more about: Waste Disposal Methods Recycling Hazardous Wastes

Disposal of solid wastes on land is by far the most common method and probably accounts for more than 90 percent of the nation's municipal refuse. Incineration accounts for most of the remainder, whereas composting of solid wastes accounts for only an insignificant amount. Selecting a disposal method depends almost entirely on costs, which in turn are likely to reflect local circumstances. Landfill Sanitary landfill is the cheapest satisfactory means of disposal, but only if suitable land is within economic range of the source of the wastes; typically, collection and transportation account for 75 percent of the total cost of solid waste management. In a modern landfill, refuse is spread in thin layers, each of which is compacted by a bulldozer before the next is spread. When about 3 m (about 10 ft) of refuse has been laid down, it is covered by a thin layer of clean earth, which also is compacted. Pollution of surface and groundwater is minimized by lining and contouring the fill,

The practice of recycling solid waste is an ancient one. Metal implements were melted down and recast in prehistoric times. Today, recyclable materials are recovered from municipal refuse by a number of methods, including shredding, magnetic separation of metals, air classification that separates light and heavy fractions, screening, and washing. Another method of recovery is the wet pulping process: Incoming refuse is mixed with water and ground into a slurry in the wet pulper, which resembles a large kitchen disposal unit. Large pieces of metal and other nonpulpable materials are pulled out by a magnetic device before the slurry from the pulper is loaded into a centrifuge called a liquid cyclone. Here the heavier noncombustibles, such as glass, metals, and ceramics, are separated out and sent on to a glass- and metal-recovery system; other, lighter materials go to a paper-fiber-recovery system. The final residue is either incinerated or is used as landfill. Increasingly, municipaliti

Hazardous wastes have been defined by the federal Environmental Protection Agency as wastes that pose a potential hazard to humans or other living organisms for one or more of the following reasons: (1) Such wastes are nondegradable or persistent in nature; (2) their effects can be magnified by organisms in the environment; (3) they can be lethal; or (4) they may cause detrimental cumulative effects. General categories of hazardous wastes include toxic chemicals and flammable, radioactive, or biological substances. These wastes can be in the form of sludge, liquid, or gas, and solid. Radioactive substances are hazardous because prolonged exposure to ionizing radiation often results in damage to living organisms, and the substances may persist over long periods of time. Management of radioactive and other hazardous wastes is subject to federal and state regulation, but no satisfactory method has yet been demonstrated for disposing permanently of radioactive wastes. See also Air Pollutio

Groundwater, water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth's water, or about 20 times more than the total of surface waters on continents and islands. Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where th

Climate, the long-term effect of the sun's radiation on the rotating earth's varied surface and atmosphere . It can be understood most easily in terms of annual or seasonal averages of temperature and precipitation. Land and sea areas, being so variable, react in many different ways to the atmosphere, which is constantly circulating in a state of dynamic activity. Day-by-day variations in a given area constitute the weather , whereas climate is the long-term synthesis of such variations. Weather is measured by thermometers, rain gauges, barometers, and other instruments, but the study of climate relies on statistics. Today, such statistics are handled efficiently by computers. A simple, long-term summary of weather changes, however, is still not a true picture of climate. To obtain this requires the analysis of daily, monthly, and yearly patterns. Investigation of climate changes over geologic time is the province of paleoclimatology, which requires the tools and methods of g