Perhaps no factor is more useful in the control of disease than the science of sewage disposal. It safeguards a community’s water supply by removing water-carried wastes including microscopic dissolved material, solid matter such as human waste, and harmful chemicals and bacteria.

Sewage is generally divided into two classes: domestic, or sanitary, sewage and industrial waste. Domestic waste water includes the used water of businesses and homes; industrial waste water is that discharged during industrial operations. Both the strength and volume of waste water may be markedly influenced by industrial wastes, which constitute about 80 percent of the sewage in the United States.

Sewage Treatment

Sewage systems collect waste water and treat it before discharging it back into the environment. These systems consist of intricate networks of underground conduits, or sewers, that convey the sewage through the treatment process to the point of disposal.

Sewage systems also handle the flow of rainwater, either separately or as part of a single system. Separate systems are generally preferable because, in single systems, heavy rainfall can overload treatment plants, with the result that untreated overflow can become a source of pollution. In separate systems, rainwater is often allowed to flow into streams untreated because it is assumed to be relatively clean.

Sewage is processed in three major steps, called primary, secondary, and tertiary treatments. Most areas do not use all three, and different areas use the treatments in different ways.

Primary treatment. The initial, and sometimes the only, method of cleaning waste water is primary treatment, which consists of removing floating chunks and fine particles of solid waste. The simplest form of primary treatment is a cesspool, now found primarily in rural areas. A cesspool is a big tank with a porous bottom and sides that lets the liquid waste water filter into the ground while holding the solid waste. Periodically the tank must be cleaned; the solid matter, called sludge, is sometimes used for fertilizer or landfill. Septic systems are somewhat similar, though the tank is connected to a drainage field so that more waste can be dispersed over a wider area.

In larger communities, sewer water passes first through a screen, which filters out the larger debris. It then runs through a grit chamber, a long, shallow trough with a dip in the bottom that acts like a trap. As water moves through the trough, small, hard materials in the water drift down to the bottom and fall into the trap. Grease floats to the surface and is skimmed off. The trap, like a cesspool, is periodically scraped clean.

After going through the screen and grit chamber, the sewage still contains small suspended solids about 1 ton per million gallons (3,790,000 litres) of waste water To remove some of these, the sewage is trickled into a sedimentation tank, or settling basin. The water enters through a pipe, then circulates slowly while the suspended particles settle to the floor. The top layer of water continually runs out through exit holes.

The sludge from sedimentation tanks may be sent through a tank called a digester, where bacteria digest it, producing carbon dioxide and methane gas and other by-products. Any combustible gases may then be collected and used to heat the digestion tanks and buildings and to fuel gas engines in the plant. The sludge may also be buried or dumped as landfill, burned, or dried in sludge drying beds for use as fertilizer.

Primary treatment removes about half of the suspended solids and bacteria in sewage, and about 30 percent of the organic wastes. Sometimes chlorine gas is added to the effluent (the liquid remaining after sedimentation) to kill most of the remaining bacteria. Some cities use chemicals that coagulate some of the solids into particles of a size and weight that will settle, so that they can be separated in a settling tank. The use of chemicals makes it possible to remove 80 to 90 percent of the suspended solids.

Secondary treatment. Today, large cities are usually required to put their wastes through both primary and secondary treatment because primary treatment alone removes so little organic material. Secondary treatment uses aerobic, or oxygen-breathing, bacteria to decompose organic wastes. The main object is to put the waste water in contact with as many bacteria as possible while keeping it aerated so that the bacteria have an adequate supply of dissolved oxygen.

One of the most common secondary treatments of this type is the activated-sludge method, so called because it uses sludge that is activated, or teeming with micro-organisms After going through primary treatment, the sewage is put into the activated-sludge tank, where it is aerated by pumps or blasts of compressed air. The compounds produced by the bacteria remain mostly suspended in the water and flow out with it into a secondary sedimentation tank.

The sludge from the bottom of the tank is handled in much the same way as the sludge from the primary sedimentation tank, except that about a quarter of it is recirculated back into the activated-sludge tank. This recirculation serves to seed the activated-sludge tank with fresh bacteria. The activated-sludge method permits almost any desired degree of treatment by varying the period of aeration. It removes about 95 percent of bacteria and more than 90 percent of suspended solids and organic matter.

Another method of secondary treatment is the trickling-filter method. Generally, rotating arms slowly spray the sewage over a shallow circular tank containing a layer of gravel or crushed rock. The rocks are covered with a slimy coating of micro-organisms that break down the organic wastes in the sewage. After this process, as in the activated-sludge method, the water that has been filtered is passed into a secondary sedimentation tank for removal of organic matter that has sloughed off from the stones of the filter. Trickling filters, together with primary treatment and final sedimentation, will remove most suspended solids.

Tertiary treatment. Waste water that has received primary and secondary treatment still contains dissolved materials that make it unsuitable for almost all uses except irrigation. Tertiary treatments, which depend largely on artificial chemical processes, are designed to remove these materials in order to make the effluent safer to discharge into waterways and safer for industry to use. A number of methods may be used, including radiation treatment, discharging the effluent into lagoons, and chlorination.

Sewage may also be passed through filters made of activated carbon, which consists of finely ground charcoal grains with rough, pitted surfaces that trap impurities. Alternatively, sewage may be strained through a screen made of tiny seashells called diatomaceous earth. The effluent may also be treated with chemicals that transform the dissolved organic material. Some chemical compounds, for example, combine with the nitrates in sewage to produce various salts. Such treatments are expensive, however, and are difficult to perform routinely.

History

The use of specially constructed sewers dates to the time of Babylon and ancient Greece, but only during the 19th and 20th centuries was the water-carriage sewage system adopted in the Western world. In these early systems, streams often served the dual purpose of sewage disposal and water supply, and hence there were frequent, disastrous epidemics of cholera, typhoid fever, and other water-borne diseases. The most effective methods of sewage treatment were not developed until the second quarter of the 20th century. Today, because of the greater amount of sewage from growing populations and industrial activity, there is an unprecedented quantity of legislation designed to control water pollution. As a result, scientists and engineers continually search for methods to further increase the levels of sewage treatment.

Competition in Communities

Competition is a characteristic of all communities. Plant roots in dry range lands compete for water. The trees of a rain forest compete for light. Crops compete for both of these as well as for nutrients. Competition is usually keen in areas where one type of community seems to overlap another. For example, a continuum between a shrub community and a marsh contains some aspects of both communities. Animals and plants trying to establish a foothold in such an overlap must cope with difficulties often non-existent in a stable community. Shrubs moving toward the marshy area must compete with other pioneer shrubs and reeds for light and nutrients.

Similarly, reeds attempting to invade the shrubby area must compete with shrubs and other reeds. This shows that competition may often be greatest among living things that have the same needs. For the same reason, competition may be extremely harsh within a species among wolves for meat or among cattle for grass, for example.

On the other hand, competition is sometimes modified through behavioural adjustments even cooperation among the members of a community. Shrubs are spaced widely on deserts. Birds nest in patterns that prevent overcrowding. Bees live together in a hive. Man can make similar adjustments, and unlike other species he can achieve cooperation by rational means. Yet human competition sometimes ends in wars, and wars frequently destroy the very things which the belligerents are striving to take away from one another.

Should it become necessary to control an undesirable species in a community, this can best be done by modifying the community. A rancher, for example, may discover that weedy annual plants are invading his native perennial pastures. His initial reaction might be to attack the weeds with chemical herbicides. This approach would be self-defeating since nature would provide the resultant bare soil with an unlimited supply of weed seed. To solve the problem ecologically, the rancher should modify the community by managing the degree and time of cattle grazing to permit normal growth of the native plant community, which would then crowd out the undesirable weeds.

Succession in Communities

A third major principle of ecology is that an orderly, predictable sequence of development takes place in any area. This sequence is called ecological succession. The successive changes produce increasingly mature communities from a barren or nearly barren start. Succession usually culminates in a climax, a fairly stable community in equilibrium with, and limited by, climate and soil.

At one time or another virtually all land surfaces have undergone basic climatic changes and been occupied by types of plants and animals which they may no longer be able to sustain. This, however, is not what is meant by ecological succession. It is known as biotic history, extends over the vast scale of geologic time, and is deduced from fossil remains. The future communities of an area cannot be predicted from its biotic history. Such prediction can be based only on a knowledge of ecological succession.

As soon as the first patches of soil are formed in barren areas, a series of events takes place that eventually terminates in the establishment of a climax community. This process is called primary succession. Because soil formation requires the slow weathering of rock, primary succession ordinarily spans hundreds of years. Once it begins, however, the sequence of events rarely alters. As soil formation proceeds, a succession of plants and animals appear. The last stage in this progression is the climax community.

Disclimax, an ecological community that occurs following a disturbance.

A disturbance at any point during primary succession or even at the climax can destroy the vegetation of a primary succession in whole or in part. The vegetation that follows a disturbance of this kind is called a disclimax. The disturbance can be caused by ploughing, logging, or overgrazing. When such a disturbance takes place, climate and soil are no longer the principal determinants of vegetation. The further natural growth of plants at the site of a disclimax, as contrasted with the raising of crops, is called secondary succession. This can be completed in a few years or, at most, in decades because soil has already been formed. After secondary succession restores a balance between eroded soil and vegetation, the further development of both again becomes dependent on primary succession. Wise landowners use secondary succession to restore overgrazed range lands, cut over timber lands, and abandoned crop lands They need only protect the land from further disturbances while secondary succession heals the scars of abuse.

Changes in the community during secondary succession are rapid, because every living thing contributes to its alteration. For instance, the weeds that grow on a vacant lot produce shade and increase the soil’s ability to absorb and store water. They also attract insects and birds and enrich the soil when they die and decay. The bare ground of the vacant lot is the best possible place for the pioneer sun-loving weeds to grow. Later the weeds are replaced by tree seedlings if the lot is in a forest climate, by native grasses if it is in a grasslands climate. Such changes occur until plants and animals that can make maximum use of the soil and climate are established.

The Ecosystem

A fourth key principle of ecology asserts that a community and its environment the living and the non-living constitute an ecological system, or ecosystem. Every natural community draws vital materials from its surroundings and transfers materials to it. Raw materials and decay products are exchanged continuously. Thus, in an undisturbed area basic resources are sustained, never exhausted.

Ecosystems exist on many kinds of lands, in lakes, in streams, and in oceans. They are found wherever soil, air, and water support communities. The combined ecosystems of the Earth constitute the biosphere.

Ecosystems generally contain many kinds of life. A cornfield, for example, contains more than just corn. Also present are smaller plant species, insects, earthworms, and a host of soil microbes. Each of these organisms fills a specific niche each performs an essential function in the ecosystem.

The inhabitants of an ecosystem are classified as producers, consumers, and decomposers. Green plants of any kind, whether stately oaks or tiny algae, are producers because they make their own food through photosynthesis. Animals, including man, feed on plants or on other animals and are therefore classed as consumers. Organisms that cause decay bacteria and fungi are decomposers.

Food chain, sequences in which organisms within an ecosystem feed on one another.

The sequences in which the organisms within an ecosystem feed on one another are called food chains. Usually organisms of higher biological rank feed on those of lower rank. Ecologists group the members of any food chain into a pyramid of numbers. At the base of such a pyramid are the green plants, which are the most numerous organisms in the chain. The next level might contain first-order consumers, such as the sheep that eat the green plants. At the peak of the pyramid might be second-order consumers, such as the herdsmen who feed on the sheep. When the producers and consumers of an ecosystem die, their bodies are broken down by the decomposers into nutrients used by new plants for growth. In this manner, the food chain is perpetuated.

The biosphere seems capable of sustaining life even in the absence of consumers. Without consumers, the rate of plant growth would eventually strike a balance with the rate of decay caused by the decomposers. Hence, even if all herbivores, or plant-eaters, were absent from the biosphere, plant growth could be expected to stabilize at certain levels.

Through plant growth and decay, water and carbon, nitrogen, and other elements are circulated in endless cycles. The driving force behind these cycles is the sun. Solar energy becomes converted into food through the photosynthesis of green plants and into heat through the respiration of plants and animals.

APPLICATIONS OF ECOLOGY

Ecologists are often employed to solve serious environmental problems. Early in this century, for example, southern Ohio was ravaged by a terrible flood. The inhabitants of the area, determined to prevent a repetition of the disaster, constructed large earthen dams across the valleys north of Dayton to contain future flood waters Since the slopes of these dams consisted of gravel with an admixture of clay, they washed away easily. It was necessary to stabilize the steep slopes quickly with plant cover. Knowing which plants would grow best in such places, an ecologist recommended the scattering of alfalfa and clover seed, followed by brome grass and Japanese honeysuckle. His recommendations were followed, and dam slopes were soon covered with a fine cohesive turf. Many of the hills on neighbouring farms lacked such cover and were quickly eroded.

In the Dust Bowl region of Texas, sandy soil in dry areas blew into great dunes after the land was ploughed for wheat. Bulldozing these dunes was thought too expensive. However, an ecologist recommended that certain plants be raised near the shifting dunes. The plants in front of the dunes caught and held the soil, while those behind them kept the rear from blowing deeper. In a short time, wind had levelled off the high dune tops and vegetation had anchored the soil.

Ecology and Wildlife Conservation

Measures for the preservation of ducks and other migratory wild fowl are examples of ecological work with animals. When these birds grew scarce, state and federal agencies sought ways to protect them and help them reproduce. At first, laws were recommended that forbade shooting the birds in the spring when they were flying north to nest. Every female killed in the spring could mean one less brood returning in the fall. Further studies showed that many of the birds’ breeding places were being destroyed when the land was drained for other uses. Some of these sites were not well-suited for the sustained growth of crops; others, where marshes and potholes once released stored water slowly, now contributed to downstream floods. Draining thus had a doubly harmful effect. Ecologists captured the endangered birds and put aluminium bands on their legs to trace their breeding places and movements. In this way it was discovered that the problem was international. As a result, the United States began to work in close cooperation with Canada and Mexico for the protection of migratory birds.

Ecologists also investigated the food habits of birds. They recognized that if proper food was unavailable, the birds would disappear even if hunting was regulated. Experts examined the stomach contents of thousands of birds from many different areas. This work led to the finding that bird food consists mainly of plant materials that thrive under natural conditions. To ensure the availability of these materials, man had to cease altering many natural communities and to stop polluting them with his wastes.

WATER CONSERVATION

Polluted water in one part of the world affects water sources everywhere. This happens because water moves through what is called the hydro logic cycle.

In the hydro logic cycle

1) water moves into the air and clouds as it evaporates from oceans, lakes, and rivers.

2) the winds that blow around the Earth blow the water vapour around until enough builds up inside a cloud to cause rain to fall.

3) wherever that rain falls, sooner or later it will flow into another body of water.

In this way, a water molecule that left the ocean near California may fall as rain in India. Unfortunately, products that pollute the water can also be carried to other parts of the world along with the water molecules.

Water is an essential natural resource. Everyone uses it. It is needed in homes for drinking, cooking, and washing. Communities must have it for fire protection and recreational activities such as swimming, boating, and fishing. Industries use it to produce electricity and to perform a large number of manufacturing processes.

Watersheds and Their Importance

Watershed (or drainage basin), area of land, of any size, from which all precipitation flows to a single stream or set of streams.

A watershed is the area drained by a river or a stream in a region. Such an area slopes toward a common land trough. Some rain runs off, or drains, over the ground surface. Run-off water forms small streams, which flow into larger ones. These eventually join to form rivers.

A natural watershed conserves water. It has clear streams and an ample cover of trees, grasses, and other plants. Plants help contribute to form a part of the topsoil called humus. Humus consists of decaying leaves and wood, bacteria, dead insects, and other plant and animal remains. It provides some of the nutrients for new plant life. Together with a network of roots, it acts as a blotter that soaks up rain. Plants break the force of falling rain and scatter the drops over leaves and branches. Some of the water returns to the air by evaporation. Part of the water used by plants is passed through their leaves into the air again by transpiration. The rest of the water sinks into the earth through countless tiny channels. Some of the spaces in the soil through which water percolates are caused by natural features of the geology or soil itself. Others are made by plant roots and burrowing animals like earthworms, insects, and moles.

The level at which the earth is permanently saturated is known as the water table. This vast underground water supply fluctuates with the seasons and the amount of rainfall. During long, heavy rains the soil may not be able to soak up all the water. Some of it runs off the surface, but in a forested watershed it moves slowly. Deep snow that melts slowly allows water to soak into the soil gradually.

When all the trees in an area have been cut down or burned off due to poor forestry practices, or grasses and other plants have been stripped off by fire, overgrazing, or poor farming practices, the watershed suffers. The water from rainfall flows over the ground’s surface instead of being absorbed by the vegetation and organic materials that would be present on a natural forest floor.

When there are no leaves and branches or grasses to break the force of falling rain and the blotter of roots and humus is gone, mud closes the channels through which water sinks into the soil. If the land is level, the water stands in stagnant pools; if it slopes, the water runs downhill into the rivers. Streams in a mismanaged watershed become brown with silt, or suspended soil, because the racing water carries soil along with it.

A mismanaged watershed can result in destructive floods in the spring because heavy rains and melting snows overflow the riverbanks. In the summer, streams, springs, and wells can dry up because little or no water has sunk into the underground reservoirs.

Water Pollution

Water can be polluted by many things. One of these is the topsoil or silt that washes into streams and rivers. This silt washes into streams and rivers from land that has been badly managed.

When silt washes into streams and rivers, two harmful things may happen.

1) Silt that floats in the water limits the amount of air in the water. Fish need air to breathe. When silt limits the air in the water, the fish die.

2) As the movement of water slows down, silt drops to the bottom of the stream beds

There are ways of controlling erosion of silt from land into streams and reservoirs. Conservationists try to make sure that the right steps are taken to prevent the silting of streams.

The silting of streams is one kind of water pollution. A heavy load of silt kills fish indirectly by reducing the amount of oxygen in the water. Then, as the flowing water slows, silt is deposited on stream beds Reservoirs behind dams also fill with silt unless erosion is stopped in watersheds above.

The main problem with our waterways is that they have been used as a garbage can for every kind of human waste that you might imagine.

Raw, untreated sewage contains:

• garbage from individuals and businesses

• waste products from industry

• run-off from sewers

Raw sewage is unhealthy and can cause outbreaks of disease. It also severely pollutes the environment.

Raw sewage can be treated in special ways to make it less harmful to the environment. For example, poisonous metals and objects that take a long time to break down can be removed from the waste so it can break down faster.

Other kinds of water pollution have created other problems. Many waterways are used as dumps for household and industrial wastes. Some communities dump untreated sewage and garbage into the nearest streams. Industries contaminate the waterways when they discharge acids, chemicals, greases, oils, and organic matter into them. Such materials foul drinking water and endanger public health. They destroy commercial fisheries. They also make waterways unusable for recreational purposes. Leaks and spills from offshore oil wells and wrecked or damaged oil tankers have caused the widespread destruction of marine life.

A food chain is made up of plants and animals linked together like a chain. Each creature depends on the other creatures in the chain for food.

It takes many creatures at the bottom of a food chain to feed just one animal at the top of the chain.

If one link becomes weak, it affects all others in the chain. Here’s an example:

For a period of time humans sprayed a chemical pesticide called DDT on plants to kill bugs.

The DDT washed off the plants and into rivers, lakes, and streams. Fish ate the poison. Many fish died, but many others survived with traces of the poison in their bodies.

Some animals ate the poisoned fish. Still others ate poisoned insects. Finally, other creatures ate the animals that had eaten the poisoned fish and insects.

Since higher animals in the chain eat a large amount of the lower animals, each link was getting more and more DDT.

While large doses of DDT can kill, smaller doses do damage as well. For example, DDT causes the shells of bird eggs to be too thin. Many kinds of birds, such as the American bald eagle, poisoned by DDT, were unable to hatch young, and their numbers became smaller and smaller.

When conservationists and others saw the harm caused by DDT to various creatures such as the bald eagle, they reasoned that other creatures, including humans, were being harmed by the pesticide. Although the spraying of DDT was stopped by passing a law in the United States, it continues in other parts of the world.

The large-scale use of organic insecticides, herbicides, toxic metals, and pesticides, particularly DDT, has polluted streams and destroyed wildlife. Some pesticides tend to concentrate in the tissues of plants and animals in nature’s food chains. Thus organisms at the ends of these chains, including humans, may take in harmful amounts of pesticides deposited in their food supply.

SOIL CONSERVATION

Whenever land is stripped of its plant cover, soil is inevitably lost by erosion, the so-called silent thief. A single rainstorm can wash away centuries-old accumulations of soil from neglected or badly managed fields. Topsoil is an extremely valuable natural resource. Under this thin blanket of rich dirt and humus, in which plants grow best, is a less fertile material called subsoil. If the surface layer of topsoil is blown or washed away, the remaining subsoil cannot support plant life. The submarginal farms must eventually be abandoned.

Types of Soil Erosion

More than 700 million acres (283 million hectares) of agricultural land in the United States are subject to erosion. Some 230 million acres (93 million hectares) of crop land require constant supervision to control erosion caused by wind and water.

Dust storms are the evidence of wind erosion. Soil unprotected by plant cover simply blows away. During the 1930s millions of acres of farmlands were badly damaged by wind. Many fields lost from 2 to 12 inches (5 to 30 centimetres) of vital topsoil during this period. As a result, the entire southern Great Plains area was called the Dust Bowl.

One of the several kinds of water erosion is sheet erosion the wasting away of level land in thin layers. The deterioration may go on for years without being noticed, though the land yields successively smaller crops. A patch of subsoil showing through on some slight rise of ground may be the first sign that the land is nearly finished as a food producer.

Splash erosion is the washing away of soil by the direct battering of rain. Small channels dug in the soil by run-off are called rill erosion. The little rills run together, form a network of larger rills, and then develop into gullies. When gully erosion occurs, the land can become a desert.

Conservationists also recognize that livestock can overgraze a plot of land until severe soil erosion occurs. About 4 million acres (1.6 million hectares) of topsoil are lost every year through erosion, and about 85 percent of this is the result of overgrazing by livestock.

Although major losses of productive agricultural lands occurred in the first half of the 20th century due to erosion, a major concern today is the loss of natural habitats as a result of commercial development. Large tracts of productive land an estimated 1 million acres (400,000 hectares) each year are lost through road building, suburban housing and industrial site developments, and airport expansion. New dams often flood some of the most productive agricultural land and natural forest habitats.