Sewage water, also known as wastewater, is any water that has been used in homes, businesses, and industries and subsequently discharged. This used water contains a complex mixture of dissolved and suspended waste products that have negatively affected its original quality. Effectively managing sewage water is a foundational practice for modern society, as it directly impacts public health and the surrounding environment. The process of collecting and treating this fluid is a continuous operation designed to mitigate the risks posed by its contents.
Origins and Classifications
Wastewater originates from three main sources: domestic, commercial, and industrial use. Domestic sources, which come from residential properties, are differentiated into two categories based on their contamination level.
One category is blackwater, the highly contaminated wastewater from toilets, kitchen sinks, and dishwashers. This water contains human waste, food particles, and grease, making it rich in pathogens and organic matter that requires extensive treatment. The second category is graywater, which is relatively less contaminated and comes from non-toilet sources like showers, bathtubs, bathroom sinks, and laundry machines. Graywater contains soaps, detergents, hair, and minor organic residues, but carries a lower concentration of harmful pathogens than blackwater.
Commercial and industrial wastewater is highly diverse in its composition depending on the activity. Wastewater from manufacturing facilities might contain substances such as process chemicals, heavy metals, or extremely high or low pH levels. These industrial effluents often necessitate specialized pre-treatment before they can be safely combined with domestic sewage for municipal processing.
Physical, Chemical, and Biological Content
The contents of sewage are broadly classified into physical, chemical, and biological components, though solids typically make up less than 0.1% of the total volume.
Physical Content
The physical content includes suspended solids that can be settled out, such as organic matter, grit, and debris like rags and plastics. Sewage water is typically warmer than the local water supply due to household and industrial processes, which affects the rate of biological activity. Fresh sewage exhibits a light brownish-gray color that darkens over time as oxygen is depleted and decomposition begins.
Chemical Content
Chemical content is characterized by both organic and inorganic compounds. Nutrients like nitrogen and phosphorus are prominent, primarily from human waste and detergents. Nitrogen is found as organic nitrogen, ammonia, nitrites, and nitrates, while phosphorus is present as phosphates. Sewage also carries dissolved organic compounds, household chemicals, and trace quantities of heavy metals, such as copper, lead, and cadmium. Increasingly, sewage contains micro-pollutants like pharmaceuticals, personal care products, and endocrine-disrupting chemicals.
Biological Content
The biological content consists of a vast array of microorganisms. This includes non-pathogenic bacteria, fungi, and protozoa that break down organic material. Sewage also contains pathogens, which are disease-causing organisms originating from human and animal feces. These pathogens include bacteria like E. coli and Salmonella, viruses such as Hepatitis A, and protozoa.
Health and Environmental Hazards
The release of untreated sewage poses significant risks to public health and the environment, stemming directly from the chemical and biological contaminants present in the wastewater.
Public Health Risks
One immediate danger is the public health risk associated with waterborne diseases. Pathogens in raw sewage can contaminate drinking water sources and recreational waters, leading to the spread of serious illnesses like cholera, typhoid fever, dysentery, and hepatitis. Direct contact with contaminated water can also cause skin infections and gastrointestinal problems.
Aquatic Ecosystem Damage
Untreated sewage causes profound aquatic ecosystem damage due to nutrient pollution. The high concentration of nitrogen and phosphorus acts as a fertilizer in natural water bodies, triggering excessive growth of algae, a process known as eutrophication. When these algal blooms die, their decomposition consumes dissolved oxygen in the water, leading to oxygen depletion and the creation of “dead zones.” This excessive consumption of oxygen is measured as Biochemical Oxygen Demand (BOD).
Chemical Contamination
The introduction of chemical contamination impacts wildlife and the water supply. Heavy metals and persistent organic pollutants can bioaccumulate in the tissues of aquatic organisms. Trace amounts of pharmaceuticals, such as antibiotics and hormones, can interfere with the reproductive cycles of fish and other aquatic species, contributing to the development of antibiotic-resistant bacteria.
The Basic Journey to Treatment
Once water is used, it begins its journey through a system of underground pipes and channels known as the sewer network. This infrastructure collects sewage from all sources and transports it to a centralized facility. Pumping stations are sometimes necessary to move the wastewater against gravity toward the treatment plant.
The sewage must undergo multiple stages of treatment to remove contaminants before being returned to the environment. The first phase, known as primary treatment, uses gravity to physically separate large, settleable solids and floating materials. The subsequent secondary treatment is a biological process that uses beneficial microorganisms to consume and break down the remaining dissolved organic matter.
Finally, the water moves to tertiary treatment, which involves advanced filtration and disinfection using methods like chlorine or ultraviolet light. This final stage removes any remaining pollutants, including pathogens, ensuring the treated water is clean enough to be safely discharged back into rivers, lakes, or oceans.