What Is Sewage Composition? Types And Treatment

Sewage is the used water and waste that flows from homes, businesses, and industries through pipes to a treatment facility. Its composition is roughly 99.9% water, with the remaining 0.1% made up of organic matter, inorganic solids, microorganisms, and chemicals. Treatment is the multi-stage process that removes these contaminants so the water can be safely returned to the environment.

What Is Sewage Composition? Types And Treatment

Sewage composition varies from place to place and hour to hour, but the core components remain consistent. Understanding what is in sewage matters because each component requires a different removal strategy during treatment.

The main categories of sewage constituents are physical, chemical, and biological. Physical components include solids of all sizes, from large debris to fine suspended particles. Chemical components include nutrients like nitrogen and phosphorus, along with oils, greases, and household chemicals. Biological components include bacteria, viruses, and parasites, some of which can cause disease.

What Are The Main Types Of Sewage?

Sewage is generally classified by its source. The three main types are domestic sewage, industrial sewage, and stormwater runoff.

Domestic sewage comes from homes and residential buildings. It includes water from toilets, sinks, showers, bathtubs, and washing machines. This type is also called sanitary sewage. It contains human waste, food scraps, soap, detergents, and paper products. Domestic sewage is relatively consistent in composition and is highly treatable with standard biological processes.

Industrial sewage comes from manufacturing plants, factories, and commercial facilities. Its composition varies dramatically depending on the industry. Food processing plants may discharge organic waste, while metal plating facilities may discharge heavy metals. Chemical plants can release solvents and synthetic compounds. Because of this variability, industrial sewage often requires pre-treatment before it enters the municipal system.

Stormwater runoff collects rain and melted snow as it flows over streets, parking lots, roofs, and lawns. It picks up oil, tire particles, animal waste, fertilizers, pesticides, and litter. In many older cities, stormwater and sewage share the same pipes, creating a combined sewer system that can overflow during heavy rain events.

A fourth category, commercial sewage, comes from businesses like restaurants, hotels, and office buildings. It is often grouped with domestic sewage, but restaurants contribute significantly more fats, oils, and grease. These substances can clog pipes and complicate treatment if not managed at the source.

What Exactly Is In Raw Sewage?

Raw sewage is the untreated wastewater that arrives at a treatment plant. Its precise makeup changes continuously, but the following components are consistently present.

Physical Components

Suspended solids are particles that remain floating in the water. These include fecal matter, food particles, paper fibers, and soil. Total suspended solids (TSS) is a standard measure used to quantify this material. Some solids dissolve completely into the water, while others settle out when the water slows down.

Organic Matter

Organic matter is material derived from living things. In sewage, this includes carbohydrates, proteins, and fats from human waste and food. Biodegradable organic matter is the primary food source for the bacteria used in biological treatment. The biochemical oxygen demand (BOD) measures how much oxygen bacteria will consume while breaking down this organic material. High BOD means the sewage will deplete oxygen in the receiving water if not treated.

Nutrients

Nitrogen and phosphorus are the two nutrients of greatest concern in sewage. They come from human waste, urine, food, and detergents. In natural water bodies, these nutrients fuel excessive algae growth, a condition called eutrophication. When algae die and decompose, they consume oxygen, which can kill fish and other aquatic life.

Microorganisms

Sewage contains enormous numbers of microorganisms. Most are harmless bacteria that occur naturally in the human gut. However, pathogenic organisms can also be present. These include bacteria like E. coli and Salmonella, viruses such as norovirus and hepatitis A, and parasites like Giardia and Cryptosporidium. Treatment aims to reduce these pathogens to safe levels.

Chemicals And Emerging Contaminants

Household chemicals like bleach, ammonia, and drain cleaners appear in low concentrations. Pharmaceuticals, including antibiotics and hormones, pass through the body and enter sewage. Personal care products, microplastics, and PFAS (per- and polyfluoroalkyl substances) are also present. Many of these compounds are not fully removed by conventional treatment, and researchers are still studying their environmental and health effects.

How Does Sewage Treatment Work?

Sewage treatment happens in stages. Each stage removes a different class of contaminants. The full sequence is often described as primary, secondary, and tertiary treatment.

Preliminary Treatment

The first step removes large objects that could damage equipment. Sewage passes through bar screens that catch rags, wipes, plastic, and other debris. Grit chambers then slow the water down so sand, gravel, and eggshells can settle out. This step protects pumps and prevents abrasive damage downstream.

Primary Treatment

Primary treatment is a physical settling process. Sewage flows into large tanks where it remains for several hours. Heavier solids sink to the bottom, forming a sludge layer. Lighter materials like oils and grease float to the surface and are skimmed off. Primary treatment typically removes about 50-60% of suspended solids and about 30-40% of BOD.

Secondary Treatment

Secondary treatment is the biological heart of the process. Bacteria are used to consume the dissolved and suspended organic matter that primary treatment missed. Two main approaches exist.

The activated sludge process is the most common. Air is pumped into aeration tanks, encouraging bacteria to multiply and break down organic waste. The bacteria form clumps called flocs, which settle out in a final clarifier. Some of the settled bacteria are recycled back into the aeration tank to maintain the population.

Trickling filters are an alternative. Sewage is sprayed over a bed of rocks or plastic media coated with bacteria. As the water trickles down, the bacteria consume the organic matter. Trickling filters use less energy than activated sludge but require more land area.

Secondary treatment typically removes 85-95% of BOD and suspended solids. It also removes a significant portion of pathogens, though not all.

Tertiary Treatment

Tertiary treatment polishes the water further. This stage targets nutrients and remaining contaminants. Filtration through sand or membrane systems removes fine particles. Disinfection with chlorine, ultraviolet light, or ozone kills remaining pathogens. Nutrient removal processes can strip nitrogen and phosphorus to very low levels.

Not all treatment plants include tertiary treatment. Its use depends on local regulations and the sensitivity of the receiving water body. Plants discharging into large rivers or oceans may only need secondary treatment, while plants discharging into small streams or drinking water reservoirs typically need full tertiary treatment.

What Happens To The Sludge?

Sludge is the solid material removed during treatment. It contains the settled solids from primary treatment and the excess bacteria from secondary treatment. Sludge is roughly 97-99% water when first collected and requires further processing.

Thickening removes some water through gravity or mechanical means. Digestion then stabilizes the organic material. Anaerobic digestion uses bacteria in an oxygen-free environment to break down volatile solids. This process reduces the sludge volume, destroys many pathogens, and produces methane gas that many plants capture and use for energy.

After digestion, the sludge is dewatered using centrifuges or belt presses. The resulting material, now called biosolids, can be applied to agricultural land as fertilizer if it meets safety standards for heavy metals and pathogens. Alternatively, biosolids are sent to landfills or incinerated.

Why Does Sewage Treatment Matter?

Untreated sewage poses serious risks to both human health and the environment. Pathogens in raw sewage can cause cholera, typhoid, dysentery, and hepatitis A. Discharging sewage into water bodies also depletes dissolved oxygen as bacteria decompose the organic matter, suffocating fish and aquatic invertebrates.

Nutrient pollution from untreated or inadequately treated sewage drives harmful algal blooms. Some of these blooms produce toxins that contaminate drinking water supplies and force beach closures. Excess nitrogen and phosphorus can also create dead zones, like the seasonal hypoxic zone in the Gulf of Mexico.

Modern sewage treatment has dramatically reduced these problems in developed countries. Waterborne disease outbreaks from sewage are now rare in the United States, and many rivers that once ran foul now support healthy fisheries. But aging infrastructure, population growth, and emerging contaminants like PFAS continue to challenge treatment systems.

What Happens When Sewage Treatment Fails?

Sewage overflows and treatment failures still occur. Heavy rain can overwhelm combined sewer systems, causing untreated sewage to discharge directly into waterways. Aging pipes crack and leak, allowing sewage to enter groundwater. Power failures at treatment plants can force bypasses of partially treated water.

These events trigger public health warnings. Beaches close, shellfish harvesting stops, and people are advised to avoid contact with affected water. The risks are real but generally short-lived. Most systems recover once the cause of the overflow is resolved.

Homeowners with septic systems face similar risks. A failing septic system can contaminate nearby wells and surface water with pathogens and nitrates. Regular maintenance, including pumping every 3-5 years, prevents most failures.