Waste management and sewage disposal

Wastewater consists of domestic and industrial effluent that is collected by a sewage system and conveyed to a central plant, where it is treated prior to release into the ground or, more usually, into a surface watercourse. For public health considerations, the proper disposal of wastewater is a critical parameter in environmental planning.

Background

The Minoan civilization on the island of Crete near Greece had one of the earliest known sewage collection systems in the world (c. 1600 BCE). Ancient Greece had hot and cold water plumbing systems. A large sewer known as the Cloaca Maxima was built during the sixth century BCE in ancient Rome to drain the Forum. The Romans also reused public bathing water to flush public toilets. London had a drainage system by the thirteenth century, but effluent could not be discharged into it until 1815. Sewers were constructed in Paris before the sixteenth century, but fewer than 5 percent of the homes were connected to the system by 1893. In general, the widespread introduction of sewage collection systems in densely populated areas did not occur until the mid-nineteenth century. For example, the first sewer that was carefully engineered was constructed in Hamburg, Germany, in 1848.

Wastewater disposal systems usually consist of a collection system of sewer pipes of varying diameters and materials, a treatment plant of varying size and level of treatment, and an outfall. The outfall may be to the ground or, more commonly, to a receiving watercourse such as a stream or (typically along a coast) the ocean. Older wastewater systems are generally combined—domestic, industrial, and stormwater runoff are conveyed in the same pipe to the treatment plant. Although cheaper to build initially, combined systems are less desirable, as most of the effluent must bypass the treatment plant during storms, when street runoff increases rapidly. Modern wastewater systems are designed to be separate, with different pipes for wastewater and storm runoff.

Wherever possible, sewage systems are designed to be below the depth of frost and at a slope that allows gravity drainage. In some low-lying locations and other areas with low relief, the effluent must be pumped, a process that adds expense.

Wastewater Characteristics

About 60 to 75 percent of the water supplied to a community will wind up as effluent or spent water, which must be treated and disposed of. The remaining water is used in industrial processes, lawn sprinkling, and other types of consumptive use. Domestic sewage contains varying proportions of human excrement, paper, soap, dirt, food waste, and other substances. Much of the waste substance is organic and can be used by organisms of decay (saprophytic microorganisms). Accordingly, domestic sewage is (putrescible) and capable of producing offensive odors. The composition of industrial waste varies from relatively clean rinse water to effluent that can contain corrosive, toxic, flammable, or even explosive materials. This is why communities usually insist on some form of pretreatment by industry before the effluent enters the treatment plant.

The organic material in sewage is decomposed by aerobic (oxygen-requiring) bacteria. However, the oxygen that is dissolved in water (DO) can be used up in the process of microbial decomposition. If too much organic waste enters the water body, the biochemical oxygen demand (BOD) can exhaust the DO in the water to the extent that the aquatic is damaged. Most species of fish die if the DO concentration falls below 4 milligrams per liter for periods of time. Some species, such as trout, are even more sensitive to DO levels and do best when DO is 8 milligrams per liter or higher.

The function of wastewater treatment plants is to produce a discharge that is free of odors, suspended solids, and objectionable bacteria. Coliform bacteria, which are common in the lower intestines of mammals, may not be pathogenic themselves but are taken as an indicator of contamination in the watercourse.

Treatment processes are often categorized as primary, secondary, or tertiary. The distinction among the three processes is somewhat arbitrary, but the main point is that higher levels of treatment result in a more purified discharge that becomes increasingly more expensive to attain. Primary treatment is mostly mechanical, as it involves the removal of floating and suspended solids by screening and sedimentation in settling basins. As an optional step, chemicals that flocculate or precipitate solids may be added as a means of speeding the process. This type of treatment can remove 40 to 90 percent of the suspended solids and 25 to 85 percent of the BOD. The final effluent may be chlorinated prior to release into a receiving watercourse.

Secondary treatment involves biological processing after the wastewater has been through primary treatment. One of the two forms of biological processing is by means of a trickling filter, in which wastewater is sprayed over crushed and allowed to flow in thin films over biologic growths that cover the stone. The organisms in the biologic growths, which include bacteria, fungi, and protozoa, decompose the dissolved organic materials in the wastewater. Some of the breakdown products in the wastewater, such as carbon dioxide, escape into the atmosphere; others, such as nitrate, which is a mobile ion, remain in solution. Still others are absorbed into the cells of the biologic growths. These growths eventually slough off and are carried to settling tanks by the flow of the wastewater. The other type of secondary treatment is known as the activated sludge process. In this procedure, flocs of bacteria, fungi, and protozoa are stirred into the wastewater with results that are about the same as trickling filters. Depending upon the efficiency of the plant and the nature of the incoming wastewater, both types of biological processes can remove 50 to 95 percent of the suspended solids and BOD. The efficiency of secondary treatment can be seriously lowered if the design capacity of the plant is overloaded with excessive effluent coming from storm runoff in combined sewers. This is one important reason that separate sewers, even though more expensive, are favored by public health officials. The biologic processes can also be severely affected by toxic industrial waste that can kill the “good” bacteria, which are crucial to the treatment process. Accordingly, many communities require pretreatment for industrial wastes.

Tertiary treatment is the most advanced form of waste treatment. It includes a number of practices such as the use of ozone, which is a strong oxidizing agent, to remove most of the remaining BOD, odor, and taste, and the addition of alum as a phosphate precipitator. A recent and innovative method of tertiary treatment is to spray chlorinated effluent on either croplands, wooded areas, or mine after it has been given secondary treatment. This method has several distinct advantages over the traditional direct discharge of the effluent into surface watercourses. First, biologic digestion in the soil removes almost all of the remaining BOD. Second, soil and plants are capable of absorbing large amounts of nitrogen and phosphorus during the growing season, which slows their release into the environment. Other benefits include increased crop and timber yields and recharge. The land area needed to handle treated wastewater by the spray irrigation method is approximately 6.4 square kilometers per 100,000 people.

Wastewater Disposal in Rural and Suburban Areas

In areas where population densities are less than about 1,000 people per square kilometer, the cost of a sewer system and treatment plant are difficult to justify. Septic systems are commonly used in residential areas for disposal of domestic wastewaters. Household effluent is piped to a buried septic tank, which acts as a small sedimentation basin and anaerobic (without oxygen) sludge digestion facility. The effluent exits from this tank into a disposal field where aerobic (with oxygen) biologic breakdown of dissolved and solid organic compounds occurs. In order to operate effectively, the soil must be of sufficient depth and permeability so that microbial decomposition can take place prior to the effluent reaching the water table. The Environmental Protection Agency estimates that 25 percent of the homes in the United States use some form of a septic disposal system.

Bibliography

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"Waste Management Isn't Just About TrashIt's About Resources, Too." ASCE, 2 May 2024, www.asce.org/publications-and-news/civil-engineering-source/civil-engineering-magazine/article/2024/05/waste-management-isnt-just-about-trash-its-about-resources-too. Accessed 29 Dec. 2024.