What is a Pollutant

A pollutant, in the context of drainage, wastewater and environmental engineering, refers to any material or substance that is washed into sewers or drainage systems and has the potential to cause environmental harm when discharged into receiving waters such as rivers, lakes, groundwater or coastal areas. Pollutants can originate from domestic, commercial or industrial sources and are often introduced into the system via surface water runoff, misconnections, spills, or improper disposal.

Once a pollutant enters the drainage system, it may pass through treatment works or, in the case of combined sewer overflows or storm events, be discharged directly into the environment. Understanding the nature, sources, pathways and impacts of pollutants is essential for managing water quality and protecting aquatic ecosystems.

Common types of pollutants in drainage systems

Pollutants can be classified in various ways, including by their source, chemical nature or environmental effect. Some of the most common types encountered in sewer and stormwater systems include:

  • Organic matter
    Such as food waste, faecal matter, leaves and plant debris. When discharged into water, these materials decompose and consume oxygen, which can lead to fish kills and ecosystem stress.

  • Nutrients
    Primarily nitrogen and phosphorus from fertilisers, detergents and human waste. In high concentrations, they can cause eutrophication, algal blooms and oxygen depletion.

  • Pathogens
    Bacteria, viruses and parasites from sewage and animal waste. These pose health risks to humans and wildlife, especially where untreated discharges occur.

  • Oils and hydrocarbons
    From vehicles, industrial processes and spills. These substances float on water, reduce oxygen transfer and are toxic to aquatic life.

  • Heavy metals
    Such as lead, zinc, copper and mercury, typically from industrial sites, old infrastructure or urban runoff. They accumulate in sediments and organisms, causing long-term environmental damage.

  • Plastics and microplastics
    Washed into drains from litter, cleaning products and synthetic fabrics. They persist in the environment and enter food chains.

  • Sediments and silt
    Often carried by stormwater from construction sites, roads and eroded land. Sediment can smother habitats and transport attached pollutants.

  • Chemical pollutants
    Including cleaning agents, solvents, pesticides and industrial waste. These can be highly toxic even at low concentrations and are difficult to remove once in watercourses.

Sources of pollutants

Pollutants enter drainage systems from both point sources and diffuse sources:

  • Point sources
    Include identifiable inputs such as industrial discharges, combined sewer overflows, and illegal connections.

  • Diffuse sources
    Arise from widespread activities like agricultural runoff, road traffic, and urban surface water drainage.

Other significant contributors include:

  • Domestic sinks, toilets and washing machines

  • Vehicle washing areas

  • Industrial estates and factories

  • Food establishments

  • Construction sites

  • Misconnected plumbing systems

Even seemingly minor actions, like pouring cooking oil down a drain or washing a car on the street, can introduce pollutants that accumulate and cause downstream harm.

Impact on receiving waters

Pollutants discharged into receiving waters can have a range of negative environmental effects:

  • Oxygen depletion
    Organic pollutants are broken down by bacteria that consume oxygen, reducing levels available for fish and other aquatic life.

  • Toxicity to organisms
    Chemical substances and heavy metals may be directly harmful to aquatic species, impairing growth, reproduction or survival.

  • Eutrophication
    Excess nutrients stimulate rapid plant and algae growth, which disrupts natural ecosystems and leads to decay and further oxygen loss.

  • Bioaccumulation
    Pollutants like mercury or persistent organic compounds build up in the tissues of organisms and magnify up the food chain.

  • Habitat degradation
    Sediment and litter can smother habitats, clog fish gills, and reduce light penetration in water.

  • Aesthetic and recreational loss
    Visible pollution such as scum, debris, or odour can make waterbodies unpleasant and unsafe for public use.

Long-term pollution can also affect groundwater quality, leading to restrictions on water use and expensive remediation efforts.

Monitoring and regulation

To manage pollutants, most countries enforce strict water quality standards and monitor discharges into the environment. In the UK, this is overseen by the Environment Agency and guided by:

  • The Water Framework Directive

  • Environmental Permitting Regulations

  • Urban Wastewater Treatment Regulations

  • General Binding Rules for septic tanks and small treatment plants

Wastewater treatment plants are designed to remove many pollutants, but not all. During extreme rainfall, combined sewer overflows may release untreated water containing pollutants directly into rivers or seas.

As a result, sustainable urban drainage and pollution prevention at source are increasingly emphasised.

Pollution prevention in drainage systems

Reducing pollutant loads in drainage systems starts with controlling what enters them. Common strategies include:

  • Source control
    Educating households and businesses not to pour fats, oils or chemicals down drains, and promoting correct waste disposal.

  • Sustainable Drainage Systems (SuDS)
    Features such as swales, permeable paving, green roofs and retention ponds help filter and reduce pollutants in stormwater before it reaches sewers.

  • Oil and silt traps
    Installed in car parks, garages or forecourts to intercept pollutants before they enter the system.

  • Regular maintenance
    Gully cleaning, sewer jetting and catchpit emptying remove accumulated debris and pollutants.

  • Pollution incident response
    Spill kits and emergency protocols help contain and mitigate accidental releases.

  • Monitoring and enforcement
    Routine inspections, sampling and enforcement help identify sources and ensure compliance with pollution control laws.

The role of individuals and property owners

Every person connected to a drainage system has a role in reducing pollution. Common actions include:

  • Avoiding flushing wipes, nappies or sanitary products

  • Never pouring oil, paint or chemicals down sinks or gullies

  • Using phosphate-free detergents

  • Disposing of garden and food waste responsibly

  • Maintaining private drainage systems to avoid leakage

By keeping pollutants out of the sewer in the first place, households help protect rivers, wildlife and water resources.

Conclusion

Pollutants are materials carried through drainage systems that can cause environmental damage when released into the environment. Their sources are varied and widespread, but their effects are often concentrated and harmful. From nutrients and oils to heavy metals and plastics, these substances can degrade water quality, harm ecosystems and pose risks to human health.

Through effective monitoring, regulation, infrastructure design and public awareness, the impacts of pollutants can be reduced. Understanding what pollutants are and how they behave is essential for building cleaner, safer and more sustainable drainage systems.