What is a Sewage

Sewage refers to the waterborne waste material that is transported through sewers or drains. It typically comprises domestic wastewater from toilets, baths, sinks, washing machines and dishwashers, as well as trade effluent from industrial or commercial premises. In some systems, it may also include surface water such as rainwater from roofs and paved areas. The nature and composition of sewage vary depending on the source, but it generally contains organic matter, nutrients, pathogens, suspended solids, and chemicals.

Sewage is not simply dirty water. It is a complex mixture of contaminants that, if left untreated, can cause significant environmental and public health hazards. Effective collection, conveyance and treatment of sewage are essential components of modern sanitation infrastructure and environmental protection.

Types of Sewage

Sewage can be classified into several categories based on its origin:

1. Foul Sewage

This is wastewater from domestic and commercial sources that includes human waste (urine and faeces), food residues, detergents, fats and greases. It is highly biodegradable but also potentially pathogenic, requiring full biological treatment before discharge.

2. Surface Water Sewage

Also called stormwater or runoff, this includes rainwater collected from roofs, roads, and other impermeable surfaces. While it may seem clean, surface water often contains silt, hydrocarbons, litter, pesticides, and other pollutants that accumulate in the urban environment.

3. Combined Sewage

In some older drainage networks, foul and surface water are conveyed in a single combined sewer. This system is efficient in terms of infrastructure but can lead to overflows and pollution during heavy rainfall when the combined flow exceeds treatment capacity.

4. Trade Effluent

This type of sewage originates from industrial processes and can include oils, solvents, metals, and chemical compounds. Trade effluent is usually subject to regulation and requires pre-treatment before being discharged into the public sewer system.

Components and Characteristics of Sewage

Sewage is composed of approximately 99.9% water and 0.1% solids, yet it carries a wide range of pollutants and contaminants. Key characteristics include:

  • Suspended solids: Particulate matter that may settle out if left undisturbed.

  • Biochemical oxygen demand (BOD): A measure of the organic material in sewage that consumes oxygen during decomposition.

  • Nutrients: Mainly nitrogen and phosphorus, which contribute to eutrophication in water bodies.

  • Pathogens: Bacteria, viruses, and protozoa that pose a risk to human and animal health.

  • Toxic compounds: Including cleaning agents, pharmaceuticals, microplastics and industrial chemicals.

The composition of sewage varies depending on population density, lifestyle, industrial activity and seasonality.

How Sewage Is Conveyed

Sewage is transported from its source to treatment facilities via a network of underground sewers and drains. There are two main types of sewer systems:

  • Separate sewer systems: These use two distinct pipelines – one for foul sewage and one for surface water. This design reduces the risk of overloading treatment works during storms and is standard in modern developments.

  • Combined sewer systems: These use a single pipe to transport both foul and surface water. While economical, they require overflow relief mechanisms such as combined sewer overflows (CSOs) to prevent flooding, which can lead to pollution of watercourses.

The flow in sewers is usually gravity-driven, although pumping stations are used in low-lying areas to move sewage to higher ground or treatment plants.

Treatment of Sewage

The treatment of sewage is essential before it can be safely released into the environment. Treatment generally occurs in several stages:

1. Preliminary Treatment

Removes large debris such as rags, plastics, grit and grease using screens, grit chambers and grease traps. This protects downstream equipment from damage and blockages.

2. Primary Treatment

Involves sedimentation in settlement tanks, where heavier solids settle to form sludge and lighter materials form a scum layer. This step reduces suspended solids and organic load.

3. Secondary Treatment

Biological treatment using bacteria to break down organic matter. Common methods include:

  • Activated sludge processes

  • Trickling filters

  • Rotating biological contactors

This stage significantly reduces BOD, ammonia and pathogens.

4. Tertiary Treatment

Advanced processes to polish the effluent and remove nutrients, pathogens and residual solids. Methods may include:

  • Sand filtration

  • UV disinfection

  • Membrane filtration

  • Phosphorus and nitrogen removal

The final effluent should meet discharge consent limits set by regulatory authorities such as the Environment Agency.

Sludge Management

The solids removed during sewage treatment form sludge, which must be properly managed. Sludge is treated through processes such as:

  • Anaerobic digestion to produce biogas and stabilise solids

  • Thermal drying or composting

  • Dewatering using centrifuges or belt presses

The treated sludge may be used as fertiliser, incinerated, or sent to landfill depending on its quality and composition.

Environmental and Public Health Impacts

Untreated or poorly treated sewage can have severe environmental consequences:

  • Water pollution: Discharge of sewage into rivers, lakes or coastal waters can lead to oxygen depletion, fish kills and algal blooms.

  • Soil contamination: Improper disposal of sludge or leakage from sewers can degrade soil quality.

  • Air pollution: Hydrogen sulphide and other gases from decaying sewage cause odours and respiratory problems.

  • Public health risks: Pathogens in sewage can spread diseases such as cholera, hepatitis, and gastroenteritis.

Effective sewage management is therefore not just an engineering task but a public health necessity.

Sewage Infrastructure in the UK

In the United Kingdom, the provision and maintenance of sewage infrastructure is largely the responsibility of regional water and sewerage companies. These organisations operate under the regulation of bodies such as:

  • The Environment Agency (EA)

  • Ofwat (The Water Services Regulation Authority)

  • Public Health England

Key legislation governing sewage management includes:

  • The Water Industry Act 1991

  • The Environmental Permitting Regulations

  • The Urban Waste Water Treatment Directive

New developments must adhere to Building Regulations, including requirements for connection to the public sewer or the installation of suitable on-site treatment systems.

Sustainable Approaches to Sewage Management

With growing population pressures and climate change, the sewage sector is increasingly focused on sustainability. Strategies include:

  • Sustainable Urban Drainage Systems (SuDS): Use of green roofs, permeable paving, swales and retention ponds to manage surface water locally.

  • Water efficiency measures: Reducing water use in homes and industry lowers the volume of sewage generated.

  • Decentralised treatment: Small-scale systems such as package plants or constructed wetlands for rural and remote areas.

  • Resource recovery: Extracting energy, fertilisers and water from sewage through innovative technologies.

The concept of the circular economy is gaining traction in the sector, aiming to treat sewage not as waste, but as a resource.

Common Challenges in Sewage Management

The management of sewage systems is complex and faces various challenges:

  1. Aging infrastructure: Many sewer networks in the UK are over 100 years old and prone to collapse or leakage.

  2. Infiltration and inflow: Groundwater or rainwater entering sewers increases treatment volumes unnecessarily.

  3. Fatbergs: Accumulations of fats, oils and non-biodegradable items like wet wipes block sewers and cause overflows.

  4. Climate change: More frequent and intense storms increase the load on combined sewers, raising pollution risks.

  5. Urbanisation: Rapid development can outpace sewer capacity if infrastructure is not upgraded.

Ongoing investment, smart monitoring and public education are needed to address these issues effectively.

Conclusion

Sewage is a vital part of the urban water cycle that carries waste from homes, businesses and streets to treatment facilities. Though it consists mostly of water, it contains a range of contaminants that pose risks to health and the environment if not properly managed. Through effective collection, transport, treatment and regulation, sewage can be safely returned to the environment or even used as a source of energy and nutrients. Understanding sewage and the infrastructure behind it is key to building resilient and sustainable communities.