Environment and Sewage Disposal Essay ⋆ Essays on Controversial Topics ⋆ EssayEmpire

The history of civilization, especially urban civilization, is the history of water—of the fundamental need to supply clean, fresh, disease-free drinking water while simultaneously disposing of vast quantities of disease-ridden, foul human and animal waste. From at least the time of the Phoenicians up to the present day, the growth of the world’s major metropolitan centers—Rome, Paris, London, New York, Beijing— has been defined by how well they solved that dilemma of water supply and waste disposal.

The challenge of water supply and disposal is directly related to population density. In prehistoric time, and in many impoverished areas even today, “night fertilization” of open field and cropland is not uncommon and may not pose an unwarranted risk of disease to the population or a heavy burden to the environment. Under rural farm conditions, both the supply of potable water and the safe disposal of waste are only minor difficulties. More often than not, nature itself, without the need of human intervention, provides the mechanisms by which water is filtered and cleansed of contaminants. Simple precautions such as keeping the outhouse far from the water well are adequate to ensure human health wherever population density is low.

Increasing population density demands increasingly complex treatment solutions. There are at least three major environmental issues that have to be addressed when disposing of human and animal waste: (1) reducing organic and inorganic solids, (2) reducing the demand for oxygen in wastewater that is a consequence of its dissolved and suspended load of organic matter, and (3) minimizing the concentration of nutrients and chemicals that always characterize waste materials.

As population density increases above rural, farm levels, some sort of engineered solution to the waste disposal problem must be found. Cesspools and septic systems have long provided such solutions in many suburban areas of the world. A cesspool is little more than an underground tank with many holes in it. Household waste enters one (settling) chamber of the tank from which overflow is allowed to escape into the soil surrounding the tank. Natural or introduced microbes and enzymes within the cesspool tank decompose the organic matter in the waste stream while the tank itself provides storage for any indigestible materials. Soil organisms and vegetation around and above the tank provide further filtering of the wastewater and take up some, if not all, of the chemical burden. Cesspools have the drawback of becoming clogged over time and require periodic cleaning and flushing to work efficiently. Septic systems are a slight improvement over cesspools—the overflow wastewater stream from a septic tank exits through a gridded system of perforated pipe that is laid out in soil at shallow depth. The slow flow of water through the septic-field grid enhances the decomposition of organic matter and the absorption of nutrients and chemicals by soil organisms and vegetation. Septic systems require less maintenance but demand large areas of lawn or open field not often available in urban centers.

Both cesspools and septic systems cannot remove 100 percent of the burden of organic matter, nutrients, and chemicals from typical household waste. The increasing use of detergents, soaps, and other chemicals in modern households has placed an additional and growing burden on the ability of house-specific waste disposal systems. At some point on the population growth curve of urban systems, cesspools and septic systems are no longer a viable alternative and some other approach has to be taken to sewage treatment. One of the most spectacular examples of this transition in modern times is provided by Nassau and Suffolk Counties on Long Island, New York. What was once rural farmland was rapidly suburbanized following World War II. Initially the waste disposal of homes was served by cesspools and septic systems— one or the other depending primarily on lot size. Over time, however, these house-specific systems dispensed an ever-increasing load of nutrients into the groundwater system of the island—a drinking water supply on which the suburban community was totally dependent. The concentration of one particular nutrient, dissolved inorganic nitrate, was the trigger that led to a complete reengineering of the two counties’ waste disposal systems—a construction project of immense size that is still under way. High levels of nitrate had led to “blue-baby syndrome,” a condition under which newborns have difficulty breathing. The solution was to place the homes in every community in the two counties on sewerage systems connected to central processing plants. Nearly every street and highway had to be excavated, every household lawn dug up, and sewer lines installed. In addition, a place had to be found for the huge and not-always-welcome treatment plants.

Sewage treatment plants have been built in major cities since the 19th century. Basically sewerage systems consist of a network of collection pipes from an urban environment directed by gravity to the central facility typically located at the lowest elevation available—either on the coastline or next to a river. Initially the treatment facilities consisted of little more than settling tanks in which “floatables” and sediment were accumulated and later disposed of as sludge. This “primary” treatment did little for overall concentration of organic matter, nutrients, and chemicals in the water. The outflow of these plants was dumped directly into the ocean, stream, or river to be dealt with by natural decomposition. In some urban and suburban environments, sewer lines were merged with storm drains from streets in what is known as “combined sewer overflow” systems.

During a rainstorm the huge quantities of runoff, rapidly developed in urban environments, mixed with sewage and were discharged directly to streams and rivers without any treatment. U.S. cities are still dealing with the legacy of these combined sewer overflow systems today.

Few natural aquatic ecosystems are capable of dealing with the heavy demands for decomposition placed on them by the influx of large amounts of primary-treated sewage. As decomposition proceeds within the natural system, the demand for oxygen to fuel the microbial decomposition process exceeds the capacity for the absorption of oxygen through the water’s surface or the generation of oxygen via photosynthesis by plants. An “oxygen sag” develops downstream from the wastewater treatment plant outflow. Depending on several factors—the load from the treatment plant, the discharge of the stream or river, and tidal flushing of bays and estuaries—the depletion of oxygen in water can reach critical levels, typically set at below 4 mg/L of dissolved oxygen. At those low levels, fish, and the insect life on which they depend, die. Other pollution-tolerant organisms replace those fish and “higher-order” insects. Eventually all oxygen-dependent life can be extinguished and the aquatic system can descend into anoxia, capable of supporting only anaerobic organisms. Those anaerobic organisms, more often than not, produce foul-smelling hydrogen sulfides among other noxious chemicals.

In the United States and Europe this sad state of affairs for most urban streams and rivers was reached sometime during the 1960s and 1970s and led directly to the passage of water-quality legislation such as the Clean Water Act in the United States. Existing sewage treatment plants were upgraded, and new plants were built nationwide. This upgrading consisted chiefly of the addition of “secondary” treatment at each facility. Secondary treatment is designed to reduce the dissolved and suspended organic matter in the waste stream. This is accomplished in a variety of ways. The simplest way is to aerate the wastewater in huge tanks, either by mixing or by direct aeration with bubblers. Some treatment plants employ anaerobic digestion, that is, enclosed tanks from which oxygen is excluded. Inside these tanks bacteria reduce organic matter to methane, which can fuel the treatment process itself. In addition, anaerobic digesters, as they are called, emit far less odor than open-tank oxygen-decomposition facilities. They are more expensive to maintain, however, and can require periodic injections of microbes if, for example, antibiotics enter the tanks from the sewer lines.

Secondary treatment is effective at reducing the level of organic matter in wastewater plant discharge, but it has little effect on the nutrient and chemical load of the plant discharge. In fact, secondary treatment may even increase the levels of nutrients such as nitrates and concentrate and elevate existing loads of phosphate in the plant discharge. This discharge of nutrients can promote the rapid growth of algae and macrophytes in streams and rivers. If those plants grow excessively and if conditions (such as a drought) in the aquatic environment lead to the wholesale death of those plants, the stage may be set for a calamitous decline in dissolved oxygen concentrations. Such oxygen-depletion events have caused repeated occurrences of huge die-offs of fish in modern times.

Thus, today, we are faced with the problem of going beyond secondary treatment of sewage to tertiary treatment—the removal of nutrients—an expensive process that will involve rebuilding nearly all of the thousands of treatment facilities throughout the world.

Bibliography:

Arundel, John. 1999. Sewage and Industrial Effluent Treatment. 2nd ed. Boston: Blackwell.
Dunne, Thomas and Luna B. Leopold. 1978. Water in Environmental Planning. San Francisco: W. H. Freeman.
Phelps, Earle B. 1944. Stream Sanitation. London: Wiley. United Nations World Water Assessment Programme. 2006.
“Water: A Shared Responsibility: The United Nations World Water Development.” Report 2. New York: Berghahn Books.

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