Wastewater and sewage enter aquatic systems from sources ranging from surface runoff and septic systems to wastewater treatment facilities and storm drain outfalls. Every year about 3.5 million Americans get sick from recreational activities like swimming and boating because the water is contaminated. Many don't connect their illness with the water they touched. The impact of water pollution on aquatic ecosystems extends far beyond human illness, however.
What Is Sewage?
Sewage can be defined as the waste liquids and solids usually carried away by sewers. According to the "International Journal of Environmental Research and Public Health," wastewater can be "defined as any storm water runoff, as well as industrial, domestic or commercial sewage or any combination thereof carried by water."
The four main types of wastewater are domestic, industrial, agricultural and urban. Domestic wastewater consists of black water containing human and animal fecal matter as well as gray water from household activities like bathing, washing, cooking and gardening. Industrial wastewater consists of industrial waste like pulp, paper, petrochemical runoff, chemicals, salts and acids. Agricultural wastewater comes from agricultural activities, contaminated groundwater and farming techniques, especially related to fertilizers and pesticides. Urban wastewater is defined as a combination of domestic and industrial wastewaters combined with sewage infiltration and rain water.
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Sewage and Wastewater Disposal
Wastewater treatment has three phases. The first phase or primary treatment places the wastewater in holding ponds. Solid waste settles at the bottom, and low-density materials like fats and oils float to the top. These materials can then be removed. The second phase or secondary treatment removes dissolved and suspended biological material. Most secondary treatment systems use aerobic bacteria to consume organic material in the wastewater. Tertiary or third-phase treatment further cleans wastewater that will eventually be released into sensitive environments. Tertiary treatment can be accomplished by several methods, depending on the remaining contaminants. Sand filtration removes particulate matter. Phosphates may be removed using bacteria called polyphosphate accumulate organisms. Nitrifying bacteria can be used to remove nitrogen. A method called lagooning stores the water in a lagoon where plants, bacteria, algae and zooplankton consume the remaining contaminants through natural processes.
The solid waste called sludge removed during primary treatment receives secondary treatment as well. The sludge may be treated with bacteria. Sometimes the bacteria generate enough methane to be used as fuel. Or, the sludge can be incinerated. Another method to treat the sludge starts by condensing the sludge, heating it to disinfect it and then finally using the treated sludge as fertilizer.
Despite the Clean Water Act of 1972 requiring secondary treatment of wastewater, some U.S. municipalities filed and received exemptions. Around the world, an estimated 2.5 billion people lack improved sanitation facilities. Increasing populations, aging infrastructure and natural disasters also impact the effectiveness of wastewater treatment systems.
Wastewater in Aquatic Environments
Domestic wastewater contains pollutants ranging from biologic hazards and microplastic particles to soaps and fats. Agricultural wastewater contains biologic hazards, salts, pesticides and fertilizers. Urban wastewater includes domestic and industrial wastewater but also contains runoff from storm drains. Storm drains carry pollutants from yards and parks (dirt, pet waste, pesticides, herbicides and fertilizers) as well as from streets and parking lots (oil, gasoline, dirt and trash). Industrial wastewater contains a wide range of chemicals that include petrochemicals and other chemicals, acids, radioactive materials and salts. Recent findings show that a variety of drugs also contaminate wastewaters.
The University of Michigan notes that in a 2018 report, the U.S. Environmental Protection Agency (EPA) stated that "53% of river and stream miles, 71% of lake acres, 79% of estuarine square miles, and 98% of Great Lakes shoreline miles that have been assessed are classified as impaired (unacceptable for at least one designated use)."
Biologic Hazards in Aquatic Environments
Biologic hazards found in wastewater include bacteria, fungi, parasites and viruses. Bacteria and bacterial diseases range from E. coli, typhoid fever, salmonella, cholera and shigellosis. Fungi include aspergillus. Parasites include cryptosporidium, giardia and roundworms. Viruses like hepatitis A can also be found in wastewater. Health problems caused by sewage pollution impact an estimated 3.5 million Americans each year. An estimated 50 percent of the wastewater entering the Mediterranean is untreated sewage. Biologic waste from farms, houses, parks and beaches cause health problems that impact more than humans.
Bacteria and other organisms in freshwater use oxygen to metabolize the sewage they accompany. While breaking down the sewage, these micro-organisms can cause hypoxic (oxygen-depleted) dead zones. These dead zones lack oxygen that fish and other native organisms need to survive. Shellfish infected with sewage-related bacteria sicken people around the world. In marine environments, human gut bacteria can infect coral and cause coral bleaching disease. When coral lose their natural bacteria and algae, they die, resulting in zones where the coral ecosystem, from bacteria to fish populations, dies.
Drugs ranging from hormones (which affect reproductive development in fish and amphibians) to legal and illegal amphetamines to antidepressants have entered aquatic ecosystems. Some of the drugs pass into the sewage system in the urine and feces of users while some of the drugs have been flushed down the drain. One controlled study of the effects of amphetamines on aquatic organisms showed accelerated insect reproduction, decreased algae populations and changes in diatom and microbe diversity.
Nutrient Hazards in Aquatic Environments
Nutrient-rich materials from fertilizers, especially nitrogen and phosphorus, and waste material cause eutrophication in both fresh and marine ecosystems. Algal blooms from the excess of nutrients decreases light transmission in the water, impacting plants and plankton while reducing the amount of oxygen in the water. As the algae dies, decomposer bacteria consume even more of the dissolved oxygen. In extreme cases, the loss of oxygen results in large dead zones. Runoff of fertilizer and nutrient-rich material from the midwestern United States has caused a 7,728-square-mile oxygen-depleted dead zone in the Gulf of Mexico.
Industrial Wastes in Aquatic Environments
Industrial wastes often pass through the same sewer treatment facilities as domestic wastes. Industrial waste often contains a variety of chemicals and may also contain heavy metals like lead, mercury, cadmium and arsenic. Not all of these chemicals are completely removed in sewage treatment plants, so the chemicals are released into rivers, lakes and marine waters. In addition, some waste may be released or spilled into aquatic ecosystems without any treatment. The effects of sewage pollution on marine life impact organisms throughout the food chain.
Heavy metals build up in fish tissues as the fish consume plankton, algae and smaller prey containing the metals. This process is called biomagnification. As other animals, including humans, eat these fish, the heavy metals can reach sufficient concentrations to poison the consumer. These heavy metals may accumulate in toxic amounts for fish as well.
Control of releases of industrial sewage like petroleum products, radioactive waste and persistent organic pollutants has improved, with oily wastes reduced by 90 percent between the 1980s and 2006. These pollutants caused immediate and long-term effects on ecosystems by poisoning or smothering plankton, plants and animals.
Air Pollution and Aquatic Ecosystems
Industrial soot and smoke also impact aquatic ecosystems. For example, sulfur dioxide combined with water vapor forms sulfuric acid or acid rain. Acid rain and runoff decrease aquatic pH, which interferes with the ability of fish to absorb oxygen, salts and nutrients. A low pH also interferes with calcium absorption. Improper calcium balance for many fish means their eggs do not develop properly, becoming too brittle or weak. Calcium deficiency also causes weak spines and bones in fish and weak exoskeletons for crayfish. Acid rain also leaches aluminum from soils, interfering with reproduction in crustaceans and fish. Furthermore, when pH drops below 6, insects like mayflies and stoneflies can't survive, impacting the food chain.
Litter in Aquatic Ecosystems
Urban sewage includes litter washed into storm drains and eventually into waterways. An estimated 70 percent of this litter ends up on the seabed, about 15 percent lands on beaches and about 15 percent is floating in the ocean. Most of the litter, 70 percent, is plastics with metal and glass making up the majority of the remaining 30 percent. Studies show that more than 1,200 aquatic species interact with the litter by eating it, living in or on it, or getting tangled up in it. Much of the plastic is in the form of microplastics, tiny pieces from the breakdown of larger plastics. Animals as diverse as mammals, fish, crustaceans and others are impacted by this litter.