Sewage Engineering

Summary

Sewage engineering is the discipline of civil engineering that addresses the separation, decontamination, and disposal of waste, especially human excrement, from water supplies. Domestic sewage contains solid and dissolved materials, such as fecal matter, urine, toilet paper, vomitus, food waste, soaps, and other cleaning agents. Separation methods include filtration, adsorption, and precipitation. Decontamination methods include biocides, chemical neutralization, and phagocytosis. Disposal involves collecting, handling, and destroying the final waste products in a manner that is safe for humans and the environment. These processes aim to provide improved water quality for drinking, cooking, and bathing.

Definition and Basic Principles

Sewage is more than 99 percent water. Sewage engineering addresses the remaining 5 percent, which contains various materials and requires various separation and disposal methods. Wastewater contains nonpathogenic bacteria from the human digestion system and pathogenic microbes (bacteria, viruses, and parasites). It also contains insoluble organic material (such as fecal matter, hair, food, vomitus, and paper) and soluble organic material (such as urine, sugars, proteins, and prescription medications). Similarly, it contains both insoluble inorganic matter (sand, metal particles) and soluble inorganic matter (such as ammonia, salt used to deice roads, salt from ocean water, and sulfur compounds). Wastewater also contains gases (carbon dioxide, hydrogen sulfide, methane), oily emulsions (hair dye, paint, glue, salad dressing), and toxins (pesticides, insecticides, herbicides). Finally, sewage contains large pieces of garbage and even dead animals.

The process of sewage treatment occurs in three phases. In the first stage, the largest solids are removed by mechanical means and sedimentation. The captured material is checked for toxicity before it is transported to landfills. In the second stage, the remaining solids are treated to become nonpathogenic; ironically, nonpathogenic bacteria are introduced to digest the organic matter. This material is often sold to farmers, nurseries, and garden centers as an organic soil enhancement. In the third stage, the remaining solids are treated to become nontoxic. This is a costly and difficult step, but some effort must be made to remove or neutralize minerals, such as nitrates and phosphates. If these compounds enter a lake, they can cause eutrophication, a chain reaction of algae overgrowth, rotting, and oxygen depletion that destroys the aquatic ecosystem.

Background and History

Archaeological excavations have found that civilizations in Mesopotamia and Pakistan earlier than 2500 Before the Common Era had sophisticated sewage drainage systems using clay pipes. Until the Roman Empire emerged, Minoans on Crete had the most complex wastewater systems in that part of the world, including primitive toilets that flushed. The Roman Empire engineered a complex underground aqueduct system with covered stormwater and household sewage drains. As the empire expanded, so did this technology. However, with the fall of the Roman Empire, the world reverted to unsanitary conditions. Personal hygiene was not a priority, the cleanliness of homes and towns was similarly neglected, and water supplies were not protected from contamination. Plagues in the Middle Ages wiped out entire towns. Finally, in the mid-1800s, after several cholera epidemics in Paris, the people there began constructing a sewer system. England, Germany, and the United States (US) soon followed France's lead in developing networks of sewers, plumbing with pipes, and rudimentary toilets to create more sanitary living conditions. These facilities have continued to evolve as people's understanding of sanitation, public health, and sewage treatment has advanced.

How It Works

Wastewater is 99.94 percent water by weight. In homes, it goes down the sink, toilet, bathtub, shower, dishwasher, and washing machine to either a septic tank or municipal sewage treatment facility. Wastewater also comes from other sources, such as schools, businesses, hospitals, commercial laundries, food-processing plants, and car washes. Urban runoff from rainstorms and melting snow also goes down street drains to the treatment plant. This runoff carries pollution from roads, parking lots, and rooftops that would harm natural waterways; therefore, this water gets treated as well.

In a sewage treatment plant, large solids are retained by screens in preliminary treatment and then broken down by tumbling for further processing or allowed to settle by gravity for disposal as sludge. Material that floats is skimmed off the surface for disposal. Chemicals are added that cause small particles to form larger aggregates that can be more easily removed by straining and sedimentation. These processes, called primary treatment, remove about 60 percent of the solids. The remaining wastewater is subjected to further treatment.

In the next step, the wastewater is mixed with bacteria, which digest the organic material, and oxygen, which activates the bacteria. The bacteria may be free in solution or fixed to a medium matrix. The bacteria are collected for further digestion, and the undigested matter is collected as sludge. The remaining liquid is then treated to remove the fine particles and odors by chlorination, filtration, and reverse osmosis. These processes, called secondary treatment, remove about 90 percent of the remaining solids. The remaining water is subjected to further treatment.

In tertiary treatment, phosphorus is removed by binding it with aluminum-based compounds; these aggregates can then be filtered out. Soluble nitrogen is converted to nitrogen gas through a series of steps, which can safely be discharged into the air. Removing these two elements prevents eutrophication when clean water is discharged into local waterways. Any remaining microorganisms may be killed by ultraviolet radiation or ozone disinfection. The water is now safe for humans and the environment.

Sludge is untreated solid waste; biosolids have undergone waste treatment. Sludge is concentrated wastewater, but it is still only about 6 percent solids. To drive off the water, it may undergo conditioning, which is treatment with chemicals or heat, or it may undergo thickening, which is treatment with different chemicals or gravity. Stabilization involves subjecting the biosolids to anaerobic digestion, which produces methane gas. Biosolids may also be chemically digested with lime, which reduces odors. Finally, dewatering steps, such as filtering, pressing, and centrifugation, leave the solids with the consistency of dry dirt. These solids can be used for soil conditioning (bulking up soil to prevent erosion and retain more water), incinerated for energy, or disposed of in landfills. They may also be mixed with wood chips and leaves to create a natural compost product for parks and golf courses.

Applications and Products

Sewage Holding Tanks. Underground sewage holding tanks are necessary for places without public sewer or on-site treatment facility access. Such places include campgrounds, amusement parks, and commercial ventures. Each tank is built to hold from 4,000 to 10,000 gallons of sewage. These tanks were originally made of concrete but were vulnerable to acid erosion from human waste. Contemporary tanks are made of polyethylene or fiberglass, which are acid-resistant, and they are cast in one piece to be completely watertight. These tanks require periodic emptying and cleaning.

Package Sewage Treatment Plants. For places without access to a public sewer, such as a small housing development or remote commercial site, where septic tanks do not meet existing regulations, a small on-site package sewage treatment plant is appropriate. Depending on its size, one of these units may accommodate the needs of one household to as many as a community of 375 people. The unit employs sedimentation and aerobic bacterial digestion. The quality of the treated wastewater is such that it may be safely discharged into a nearby waterway or the ground with sufficient soil percolation. The unit requires periodic emptying and cleaning.

Onboard Sewage Treatment Systems. Sewage treatment systems for workboats are becoming smaller, more efficient, and less objectionable to maintain. Such systems are practical for workboats, such as fishing and Coast Guard vessels, fireboats, and harbor tugs. These onboard sewage treatment systems typically comprise a tank packed with a medium on which bacteria are enmeshed. This medium has a high-surface-area-to-volume ratio for greater efficiency. The medium becomes immersed in sewage, and the bacteria digest it. Because the bacteria are fixed to the medium, the system can be flushed without manual cleaning.

Wastewater Lagoon Systems. Small rural communities that cannot afford to build or support a mechanical wastewater treatment plant often choose the less expensive, less labor-intensive option of wastewater lagoons. A product developed by Wastewater Compliance Systems in Utah increased the efficiency of wastewater lagoons with modest expense. Bio-Domes, formerly called Poo-Gloos, are igloo-shaped plastic domes that work in clusters of at least two dozen sitting on the bottom of the lagoon. Each dome is a set of nested domes with plastic packing between the layers to provide a large surface area on which bacteria are grown. Sewage is bubbled up through the bottom of the dome and out through a hole in the top. As it passes by the bacteria, they digest it in optimal dark, aerobic conditions.

Careers and Course Work

Students interested in sewage engineering typically pursue a Bachelor of Science degree in civil, chemical, or mechanical engineering, followed by a Master of Science degree in sewage or waste-management engineering. Some schools also have a concentration program within a graduate environmental engineering degree. Doctorate programs are also available. To specialize in sewage engineering, graduate students take classes in geographic information systems, hydrology, public health, urban water management and drainage, wastewater treatment processes, solid waste management, and water transport and distribution. Elective courses should include writing and public speaking, logic and problem-solving, computer modeling, societal governance, and regulatory compliance.

Sewage engineers typically work for public health departments and government regulatory agencies. They may be hired by large cities to oversee urban infrastructure expansion and improvement. Some become professors and conduct academic research. Some operate sewage treatment plants and pollution-control facilities. Some work in agriculture, specializing in composting and organic soil amendment. Others work as consultants to manufacturers, corporations, and private homeowners. Others may work in remote areas in the US and abroad, where basic sanitation needs are not yet met. A professional engineering license is generally required for employment. This license distinguishes one's education and proficiency from a draftsman, machinist, mechanic, technician, plumber, or surveyor. Engineering is considered to be a safety-related practice, and licensure holds an engineer to legal liability.

Social Context and Future Prospects

Sewage engineering continued to evolve in the twenty-first century. Engineers continued to seek efficient and cost-effective ways to remove pharmaceutical compounds, including natural and synthetic hormones, from wastewater and sludge. These compounds are naturally excreted, but they are also typically flushed down toilets or rinsed down sinks as a convenient means of household disposal. Hormones that get into streams, rivers, and lakes have adverse effects on fish and subsequently on the animals and humans who eat them. By the end of the second decade of the twenty-first century, some plants were using granular activated carbon as a method. However, it was still considered prohibitively expensive. Water-treatment plants that use chemical oxidative processes to remove estrogens and other medications generate disinfection by-products in the water supply that pose potential risks to human health. Some communities organized collections of unused and unwanted over-the-counter and prescription medications for disposal by authorized incineration.

Researchers have begun studies in sewage epidemiology, a new field in which information about the health status of a population may be gathered by studying its wastewater contents in real-time. For example, a study on the consumption of illegal drugs in Brussels, Belgium, performed by daily sampling of the sewage coming into the wastewater treatment plant, verified a significantly higher weekend use of cocaine and amphetamines, no daily variation in heroin and methadone use, and negligible consumption of methamphetamine, trends that were consistent with public health reports. A similar study was conducted in Barcelona, Spain, to evaluate drug abuse in prison systems. Although this methodology does not identify individual offenders, it provides needs assessment data for a given population without subjects' active participation. After the coronavirus disease 2019 (COVID-19) pandemic was declared in early 2020, the value of sewage epidemiology in supplementing data regarding the transmission and prevalence of an infectious virus, especially at the community level, was demonstrated. Innovations continued to be made in sewage engineering. Scientists experimented with using membranes for filtration and electric currents to remove contaminants. Artificial intelligence and data analytics became valuable tools for analyzing wastewater data, and an increased focus was put on environmentally friendly wastewater systems in response to global climate change. 

Bibliography

Bagchi, Amalendu. Design of Landfills and Integrated Solid Waste Management. 3rd ed. Hoboken: Wiley, 2004.

Davis, Mackenzie L. Water and Wastewater Engineering: Design Principles and Practice. New York: McGraw, 2010.

Hammer, Mark J., and Mark J. Hammer, Jr. Water and Wastewater Technology. 7th ed. Upper Saddle River: Prentice, 2011.

"How US Sewage Plants Can Remove Medicines from Wastewater." ScienceDaily, 8 Jan. 2020, www.sciencedaily.com/releases/2020/01/200108160330.htm. Accessed 3 June 2024.

Larsen, David A., and Krista R. Wigginton. "Tracking COVID-19 with Wastewater." Nature Biotechnology, vol. 38, 2020, pp. 1151–53, doi:10.1038/s41587-020-0690-1. Accessed 29 Jan. 2021.

Lunani, Mahesh. “The Rise of AI in Water and Wastewater Management: Ensuring a Sustainable Future.” National Association of Clean Water Agencies, 16 Nov. 2023, www.nacwa.org/news-publications/news-detail/2023/11/16/the-rise-of-ai-in-water-and-wastewater-management-ensuring-a-sustainable-future. Accessed 3 June 2024.

Russell, David L. Practical Wastewater Treatment. Hoboken: Wiley, 2006.

Tchobanoglous, George, Franklin L. Burton, and H. David Stensel. Wastewater Engineering: Treatment and Reuse. 4th ed. New York: McGraw, 2003.

Zhang, Shubo, et al. "Artificial Intelligence in Wastewater Treatment: A Data-driven Analysis of Status and Trends." Chemosphere, vol. 336, 2023, p. 139163, doi.org/10.1016/j.chemosphere.2023.139163. Accessed 3 Jun. 2024.