Sewage treatment and disposal

Definition: Processes applied to the domestic and industrial effluent collected by a sewer system

The proper treatment and disposal of wastewater is a critical element in environmental planning because improper disposal or inadequate treatment can result in the contamination of groundwater and of drinking-water supplies.

The Minoan civilization on the island of Crete near Greece had one of the earliest known sewers in the world (c. 1600 b.c.e.). A large sewer known as the Cloaca Maxima was built during the sixth century b.c.e.in ancient Rome to drain the Forum. The Romans also reused public bathing water to flush public toilets. London, England, had a drainage system by the thirteenth century, but effluent could not be discharged into it until 1815. Sewers were constructed in Paris, France, before the sixteenth century, but less than 5 percent of the homes were connected by 1893. In general, the widespread introduction of sewers in densely populated areas did not occur until the mid-nineteenth century.

A typical wastewater disposal system consists of a network of pipes, a treatment plant, and an outfall to the ground or, more commonly, to a stream or the ocean. Older wastewater systems are generally combined; that is, domestic, industrial, and stormwater runoff are conveyed in the same pipes to a treatment plant. Although initially cheaper to build, combined systems are less desirable than separated systems because most of the effluent must bypass the treatment plant during storms, when street runoff rapidly increases. Newer wastewater systems are designed so that separate pipes handle wastewater and storm runoff.

About 60 to 75 percent of the water supplied to a community will wind up as effluent that must be treated and disposed. 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 is decayed by bacteria. Accordingly, domestic sewage is biodegradable 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. Therefore, communities usually require pretreatment of industrial effluent.

The organic material in sewage is decomposed by aerobic (oxygen-requiring) bacteria. However, the dissolved oxygen (DO) in water 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, thereby damaging the aquatic ecosystem. Indeed, most species of fish die in water in which the DO falls below 4 milligrams per liter for extended periods of time.

The function of a wastewater treatment plant is to produce a discharge that is free of odors, suspended solids, and objectionable bacteria. The processes of wastewater treatment are categorized as primary, secondary, or tertiary. Primary treatment is mostly mechanical, as it involves the removal of floating and suspended solids through screening and sedimentation in settling basins. This form of treatment can remove 40 to 90 percent of the suspended solids and 25 to 85 percent of the BOD.

Secondary treatment involves biological processing in addition to mechanical treatment. One form of biological processing is a trickling filter, where wastewater is sprayed over crushed stone 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. These growths eventually slough off and are carried to settling tanks by the wastewater flow. Another type of secondary treatment is the activated sludge process. In this procedure, flocs of bacteria, fungi, and protozoa are stirred in the wastewater with results that are about the same as those achieved with trickling filters. Depending on 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 stormwater runoff in combined sewers. This is one important reason public health officials favor separate sewers, even though they are more expensive. The biologic processes can also be severely affected by toxic industrial waste, which can kill the “good” bacteria that are crucial in the treatment process. Accordingly, many communities require pretreatment of industrial wastes.

Tertiary treatment is the most advanced method and consequently the most expensive. It includes several procedures such as the use of ozone, which is a strong oxidizing agent, to remove most of the remaining BOD, odor, and taste; adding alum to remove phosphate; and denitrification. The final effluent from any treatment level is usually chlorinated prior to release.

In areas where population densities are lower than about 1,000 people per square kilometer (2,600 per square mile), the costs of a sewer system and treatment plant are difficult to justify. Accordingly, septic systems are commonly used for wastewater disposal in low-density residential areas. In such a system, 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 biologic breakdown of dissolved and solid organic compounds occurs. In order for a septic system to operate effectively, the soil must be of sufficient depth and permeability so that microbial decomposition can take place before the effluent reaches the water table. The Environmental Protection Agency estimated that about 25 percent of the homes in the United States used septic systems as of 2014.

Bibliography

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