What is a Sewage water

Sewage water, also referred to as foul water, is wastewater that originates from domestic and commercial activities. It includes water from toilets, baths, showers, sinks, washing machines, dishwashers and kitchens. This type of water contains a mixture of organic and inorganic substances, detergents, fats, oils, human waste and potentially pathogenic organisms. Because of its content, sewage water cannot be released directly into the environment without treatment.

In the context of drainage systems and wastewater management, sewage water is distinct from surface water or clean rainwater. It is conveyed separately or jointly through sewerage systems to treatment works, where it undergoes a series of processes to remove contaminants and ensure it can be safely discharged into watercourses or used for non-potable purposes.

Sources of Sewage Water

Sewage water primarily arises from the following sources within residential and commercial premises:

  • Toilets: The main contributor, containing human waste, urine and toilet paper.

  • Bathrooms: Water from showers, bathtubs and hand basins, often containing soap, hair and skin particles.

  • Kitchens: Water from dishwashing, food preparation and sink use, typically high in grease, oils, detergents and organic waste.

  • Utility rooms: Wastewater from washing machines and other appliances, carrying fibres, detergents and micro-pollutants.

  • Commercial premises: Restaurants, laundries, salons and healthcare facilities contribute additional waste with different chemical or biological characteristics.

Industrial processes are generally not included in the definition of sewage water unless they are mixed with domestic flows and meet the definition of trade effluent.

Composition and Characteristics

Sewage water is complex in its composition, though it typically contains over 99% water by volume. The remaining fraction includes:

  • Suspended solids: Organic and inorganic particles such as faecal matter, food scraps and grit.

  • Biodegradable organic matter: Proteins, fats and carbohydrates that bacteria can break down.

  • Nutrients: Nitrogen (ammonia and nitrates) and phosphorus compounds, often from urine, detergents and food residues.

  • Pathogens: Bacteria (e.g. E. coli), viruses, protozoa and parasites capable of causing disease.

  • Detergents and chemicals: From household cleaning products and personal care items.

  • Oils and greases: Particularly from kitchens, contributing to blockages and fatbergs in the sewer system.

  • Micropollutants: Including microplastics and pharmaceutical residues.

These components make sewage water a potential hazard to both the environment and public health if not adequately managed and treated.

How Sewage Water Is Collected and Conveyed

In developed regions such as the UK, sewage water is collected via a network of drains and sewers. Every property connected to the mains sewer system is fitted with internal and external drainage pipes that carry foul water to the public sewer.

The systems used include:

  • Gravity sewers: Most common, where pipes are laid with a slight downward gradient to allow natural flow.

  • Pumped systems: Used where gravity drainage is not possible due to topography.

  • Combined sewers: Older systems that carry both foul and surface water in one pipe.

  • Separate systems: Modern installations use distinct pipework for foul and surface water to prevent overloading during storms.

Sewage water is conveyed to a treatment works where it undergoes staged processing before being discharged or reused.

Sewage Water vs. Greywater

A useful distinction within household wastewater is the classification between greywater and blackwater:

  • Greywater: Comes from baths, sinks, washing machines and showers. It contains fewer pathogens and is sometimes suitable for reuse in toilet flushing or irrigation after basic treatment.

  • Blackwater: Contains faecal matter and urine, primarily from toilets. This is more heavily contaminated and always requires full treatment before discharge.

Although both types contribute to the total volume of sewage water, their differing characteristics influence how they are managed and treated.

Treatment of Sewage Water

Before it can be safely returned to the environment, sewage water must be treated to remove solids, organic matter, nutrients and pathogens. Treatment typically takes place in sewage treatment works and involves the following stages:

1. Preliminary Treatment

  • Screening removes large debris such as rags, plastics and sanitary products.

  • Grit chambers remove sand and gravel to protect equipment.

2. Primary Treatment

  • Settling tanks allow solids to settle as sludge.

  • Floating materials are skimmed off the surface.

3. Secondary Treatment

  • Biological processes use bacteria to break down organic matter.

  • Common methods include activated sludge systems and trickling filters.

4. Tertiary Treatment (optional)

  • Additional treatment removes nutrients, suspended solids and pathogens.

  • Technologies include filtration, UV disinfection and membrane systems.

The result is a clear effluent that meets the discharge standards set by regulatory authorities. Sludge from the process is separately treated and either recycled or safely disposed of.

Risks Associated with Untreated Sewage Water

If not properly managed, sewage water poses serious environmental and public health risks:

  • Water pollution: Raw sewage entering rivers or lakes reduces oxygen levels, kills aquatic life and encourages algal blooms.

  • Disease transmission: Pathogens in untreated sewage can cause infections such as cholera, typhoid, hepatitis A, and gastroenteritis.

  • Odour and air pollution: Decomposing organic matter produces unpleasant smells and gases like hydrogen sulphide.

  • Soil contamination: Infiltration or improper disposal can lead to the accumulation of toxic substances in the soil.

  • Blockages and overflows: Improper waste disposal, such as fats or wipes, leads to sewer blockages and backups.

In urban areas, combined sewer overflows (CSOs) are a particular concern during heavy rainfall, as they discharge untreated sewage water directly into water bodies to prevent flooding.

Regulations and Standards in the UK

Management and discharge of sewage water in the UK are regulated by several legal frameworks and overseen by the Environment Agency and Ofwat. Key regulations include:

  • The Water Industry Act 1991

  • The Environmental Permitting (England and Wales) Regulations

  • The Urban Waste Water Treatment Regulations 1994

  • The Building Regulations (Part H): Cover drainage and waste disposal in new buildings.

Effluent quality from treatment plants must meet prescribed standards for biochemical oxygen demand (BOD), suspended solids and ammonia. Discharges to ground or water require a permit or must comply with the General Binding Rules.

Sewage Water in Rural and Off-grid Areas

Where connection to mains drainage is not available, sewage water must be treated on-site using:

  • Septic tanks: Provide basic settlement and allow effluent to soak into the ground via a drainage field.

  • Sewage treatment plants: More advanced systems using aeration and bacterial processes for higher-quality effluent.

  • Cesspools: Sealed tanks that store all sewage water and require regular emptying. Used only when no other option is viable.

Each system must be correctly designed, installed and maintained to ensure compliance with environmental rules.

Sustainable Management and Future Developments

With increasing demand on water resources and growing concern for environmental sustainability, there is a shift towards more sustainable sewage water management. Strategies include:

  • Water-efficient appliances: Reducing sewage volume by using low-flow toilets and showers.

  • Greywater recycling: Collecting and treating greywater separately for reuse in irrigation or flushing.

  • Nutrient recovery: Technologies to extract phosphorus and nitrogen from sewage for use in fertilisers.

  • Decentralised systems: Small, localised treatment units that reduce reliance on centralised infrastructure.

  • Digital monitoring: Sensors and smart systems to detect blockages, monitor flow and optimise treatment.

These innovations support a circular approach, where sewage water is not just waste, but a resource for recovery and reuse.

Conclusion

Sewage water is a fundamental output of daily human activity that must be carefully managed to prevent pollution and protect public health. Comprising wastewater from toilets, kitchens, bathrooms and other domestic sources, it carries a range of contaminants that make treatment essential before discharge. Through collection, conveyance and modern treatment technologies, sewage water can be transformed into safe effluent and valuable by-products. As population and environmental pressures increase, effective and sustainable sewage water management will remain central to sanitation and water stewardship across the UK and beyond.