What Are the Causes of Sewage?

Sewage Has More Sources Than Most People Assume

Sewage, also called domestic or municipal wastewater, is the used water discharged from residences and from commercial, institutional, and industrial facilities within a community. It is typically collected and transported through a sewer system to a treatment plant.

Most conversations about sewage focus only on what people flush or pour down a drain. But a meaningful share of what actually reaches a treatment plant never came from a toilet or sink at all.

Understanding every genuine cause of sewage, not just the obvious ones, is essential for designing a treatment system that can actually handle what it will receive.

The Primary Sources of Sewage

Domestic and Residential Sources

Domestic wastewater, generated from households, is the most familiar source of sewage. It comprises water from toilets (blackwater), along with sinks, showers, dishwashers, and washing machines (greywater).

It carries organic matter, nutrients, pathogens, personal care products, and, from laundry specifically, detergents and microfibres shed from fabric. High organic content and fats, oils, and grease from kitchen sources can clog sewer lines and contribute excessive nutrient loads to receiving water bodies if inadequately treated.

Commercial Sources

Commercial wastewater comes from restaurants, hotels, hospitals, schools, offices, and retail establishments serving the local community. It typically blends characteristics of domestic sewage with additional grease, food scraps, and cleaning chemicals specific to commercial operations.

Restaurant kitchens and hotel laundry facilities are particularly significant contributors of concentrated organic and chemical load compared to a typical household.

Industrial Sources

Industrial wastewater, generated by manufacturing, mining, food processing, and similar activities, can differ dramatically from domestic sewage in both volume and composition. It can carry heavy metals, toxic chemicals, solvents, and oils depending on the specific industrial process involved.

Because of this difference, industrial wastewater is frequently collected and pretreated separately before being combined with the municipal sewer system. Many facilities treat it entirely on-site through a dedicated effluent treatment plant rather than relying on municipal infrastructure designed around domestic sewage characteristics.

Beyond Direct Sources: Infiltration and Inflow

A cause of sewage volume that most non-technical discussions overlook entirely is Infiltration and Inflow (I&I). This is water from sources other than actual wastewater generation that enters the sanitary sewer system and dilutes it.

This dilution reduces treatment efficiency and, in severe cases, pushes sewage volumes beyond a plant's design capacity.

What Causes Infiltration

Infiltration is groundwater entering sanitary sewer pipes through cracks, loose joints, or deteriorated pipe sections. It is typically caused by age-related wear, poor original construction, root intrusion, or ground movement over time.

The amount of infiltration a system experiences depends heavily on how the sewer's depth compares to the local groundwater table. It tends to worsen progressively as the sewer network ages, making it a genuine long-term maintenance concern rather than a one-time issue to fix and forget.

What Causes Inflow

Inflow is relatively clean water from sources like roof drains, sump pumps, and yard drains entering sanitary sewers through improper or illegal connections. This is water the sanitary sewer system was never designed to carry.

The scale of this problem is easy to underestimate. A single sump pump can contribute over 7,000 gallons of water to a sanitary sewer system in a 24-hour period, roughly equivalent to the average daily wastewater flow from 26 homes combined.

An eight-inch sanitary sewer pipe, sized to comfortably handle domestic wastewater flow from up to 200 homes, can be pushed toward overload by as few as eight actively discharging sump pumps or six homes with downspouts improperly connected to the sanitary line.

Combined Sewers and Stormwater: A Special Case

Older urban areas, particularly those developed before the mid-20th century, often used combined sewers, a single pipe system carrying both sewage and stormwater runoff together to the treatment plant or disposal point. During normal dry weather, this works reasonably well.

During rain events, though, the combined flow can exceed both sewer capacity and treatment plant capacity. This can result in a sanitary sewer overflow, where untreated or partially treated sewage discharges directly into the environment before reaching the treatment facility.

Newer urban development generally separates sanitary sewers from stormwater drainage specifically to avoid this problem, since precipitation-driven flow variation otherwise creates the wide swings in volume that make consistent, efficient treatment considerably harder to maintain.

Many Indian cities still operate substantial stretches of combined or partially combined sewer infrastructure inherited from decades-old urban planning. Retrofitting full separation across an entire city is rarely financially or practically feasible in the short term.

In these situations, adequate equalization capacity at the treatment plant itself becomes a pragmatic middle path. This buffer storage absorbs a monsoon-driven flow spike before it overwhelms the biological treatment stage, sitting between full sewer separation and simply accepting recurring overflow events during the rainy season.

Why Understanding These Causes Matters for Treatment Design

A treatment plant designed only around expected domestic, commercial, and industrial sewage volume, without accounting for realistic infiltration and inflow contribution, is effectively undersized before it is even commissioned. Municipalities and facility owners planning new sewage treatment plant capacity should factor in a genuine I&I allowance based on the age and condition of the connected sewer network.

A well-designed treatment plant fed by a deteriorating, leak-prone sewer network will still struggle with capacity and dilution problems, regardless of how well the treatment process itself is engineered.

This distinction also affects how a plant's biological treatment stage performs day to day. Sewage diluted by infiltration or inflow arrives at a lower concentration than the design engineer originally calculated.

Biological processes are tuned to a specific expected organic loading, so wildly variable dilution from a rainfall event or a newly connected sump pump can disrupt that balance just as surely as a sudden spike in industrial discharge would. Municipalities that invest in sewer line rehabilitation, alongside enforcement against illegal stormwater connections, often see a more stable and cost-effective treatment plant as a result.

Industry Applications / Use Cases

Why Choose Trity Enviro

Trity Environ Solutions is an experienced sewage treatment plant manufacturer and effluent treatment plant manufacturer in India. Our engineering team designs treatment capacity around your actual sewage sources and realistic infiltration allowance, not just theoretical population or production figures.

As a trusted STP and ETP manufacturer and supplier, we help municipalities and industrial clients avoid the common mistake of undersizing a plant because sewer network condition wasn't properly factored into the original design. Every installation is backed by pan-India Annual Maintenance Contract and operation and maintenance support.

We are ISO 9001:2015 certified, QCI approved, and deliver CPCB-compliant engineering nationwide.