Raw sewage is the untreated byproduct of human activities, posing a substantial challenge to public health and environmental protection. This fluid waste stream is a complex mixture of organic matter, chemicals, and disease-causing agents. Understanding its nature is essential for appreciating the necessity of modern wastewater infrastructure and safe management practices.
Definition and Origin
Raw sewage, also called sanitary sewage or untreated wastewater, is the collection of water-carried waste flowing from a community. It is distinct from stormwater runoff, which is managed separately. Primary sources include residential, commercial, and institutional buildings, contributing waste from toilets, sinks, washing machines, and industrial drains.
This wastewater is categorized into two components: blackwater and greywater. Blackwater is the most hazardous part, referring specifically to toilet waste containing human excreta. Greywater comes from non-toilet sources like showers, baths, and laundry, carrying fewer pathogens but containing soaps and detergents. The entire untreated mixture flowing through a municipal sewer system is considered raw sewage.
Physical and Biological Makeup
The physical composition of raw sewage is overwhelmingly water, typically consisting of over 99% liquid. The small remaining fraction is responsible for the polluting potential, comprising suspended and dissolved solids. These solids include inorganic materials like grit and salts, alongside a significant load of organic matter.
The organic matter is a mixture of carbohydrates, fats, proteins, and urea. This material is highly biodegradable, and its concentration is measured by the Biochemical Oxygen Demand (BOD). Dissolved solids also contribute nutrients, specifically nitrogen and phosphorus, which are major pollutants. Total nitrogen concentrations in raw sewage can range from 35 to 60 milligrams per liter, with total phosphorus ranging from 4 to 15 milligrams per liter.
Pathogenic biological agents are the most significant health concern. These disease-causing microorganisms originate from human and animal excreta. They include bacteria (E. coli and Salmonella), viruses (Hepatitis A), protozoa (Giardia and Cryptosporidium), and helminths (parasitic worms).
Health and Ecological Hazards
The most immediate danger posed by raw sewage is the transmission of infectious diseases through direct contact, inhalation, or ingestion. Pathogens can contaminate drinking water supplies or recreational bodies of water, leading to outbreaks of waterborne illnesses. Conditions such as cholera, typhoid fever, and various gastrointestinal infections result from exposure.
Exposure can also cause skin infections like dermatitis and folliculitis, or lead to respiratory problems from inhaling gases released during organic matter decomposition. Hydrogen sulfide and ammonia are gases that give raw sewage its characteristic odor and can irritate the respiratory system.
Ecologically, the discharge of untreated wastewater causes two primary problems in aquatic environments. First, the high concentration of organic matter creates a significant biological oxygen demand (BOD). Microorganisms consume dissolved oxygen as they break down this organic load, rapidly depleting oxygen levels and leading to the death of fish and other aquatic life.
Second, the high nutrient content, specifically nitrogen and phosphorus, drives a process called eutrophication. These nutrients act as fertilizer for aquatic plants, causing excessive growth of algae and cyanobacteria. This results in dense algal blooms that block sunlight, further decreasing oxygen levels when the algae die and decompose, disrupting the entire ecosystem.