What is a Discharging

In the context of wastewater treatment and on-site sewage systems, discharging refers to the final stage of the treatment process in which treated effluent exits a septic tank, package treatment plant, or similar system and is released either into the ground or into a designated watercourse. This controlled release of effluent is essential for the proper functioning of decentralised wastewater systems and must meet strict environmental and regulatory standards to prevent pollution and protect public health.

The two primary forms of discharge are:

  1. Discharge to ground, via a drainage field (also known as an infiltration field or leach field). 
  2. Discharge to surface water, such as rivers, streams, ditches, or lakes. 

The discharge process is critical in ensuring that the wastewater, once treated to a suitable standard, is safely and sustainably reintroduced into the environment.

Overview of the Treatment and Discharge Process

A typical on-site treatment system, whether a septic tank or a package sewage treatment plant, follows a multi-stage process:

  • Primary treatment: Solids settle in the tank and are separated from liquids. 
  • Secondary treatment (in treatment plants): Biological processes further break down organic matter in the liquid phase. 
  • Tertiary treatment (optional): Additional filtration or disinfection may be used to improve effluent quality. 

Following these steps, the treated liquid must be discharged in a way that:

  • Does not pollute groundwater or surface water 
  • Complies with environmental legislation 
  • Protects human and ecological health 
  • Minimises odour, visual, or nuisance impacts 

The chosen discharge method depends on site conditions, soil type, water table, environmental sensitivity, and regulatory permissions.

Discharge to Ground via a Drainage Field

A drainage field, or infiltration field, is the most common method for discharging treated wastewater into the ground. It consists of a network of perforated pipes laid in trenches filled with gravel or similar media, which allow the effluent to percolate into the subsoil.

Key features of a drainage field:

  • The effluent receives final treatment as it passes through unsaturated soil, where microbial activity further reduces pathogens and pollutants. 
  • The area must be at a suitable distance from watercourses, buildings, and boreholes. 
  • It must be designed according to British Standard BS 6297 and determined using a percolation test. 

Discharging to ground is only appropriate if the site has sufficient permeability and if the effluent is of a high enough standard to prevent contamination of groundwater.

Advantages of discharging to ground:

  • Provides natural polishing of effluent 
  • No direct contact with surface waters 
  • Typically requires no ongoing discharge permits if conditions are met 

Limitations:

  • Unsuitable in areas with high groundwater, impermeable soils, or risk to drinking water supplies 
  • Drainage fields can clog if the system is poorly maintained 
  • Requires adequate land area 

Discharge to Surface Water

In cases where ground conditions are unsuitable, or where a high-standard treatment plant is used, it may be permissible to discharge to a watercourse, such as a stream, ditch, or river.

Surface water discharge requires that the effluent meets higher quality standards, often including parameters such as:

  • Biochemical Oxygen Demand (BOD) 
  • Suspended solids 
  • Ammonia concentration 

In the UK, discharge to surface water requires an environmental permit from the Environment Agency (EA) or the relevant regulator in devolved nations. Some discharges may qualify for an exemption under the General Binding Rules, provided specific conditions are met.

Conditions for discharging to surface water without a permit (England):

  • Discharge volume must be less than 5 cubic metres per day 
  • The receiving watercourse must be flowing and not seasonally dry 
  • The discharge must not cause pollution 
  • The treatment plant must be maintained to manufacturer’s standards 

Discharges exceeding these limits, or into sensitive locations, require a bespoke permit, which involves water quality assessments and site inspections.

Regulatory Context and General Binding Rules

In the UK, the General Binding Rules (GBRs) were introduced by the government to simplify the regulation of small domestic discharges and reduce the administrative burden for homeowners and small businesses, while still protecting the environment.

The GBRs stipulate that:

  • Discharges must not cause pollution 
  • Systems must be regularly maintained and emptied 
  • Discharge points must be a set distance from buildings, watercourses, and drinking water sources 
  • New discharges to ground or surface water must not occur in designated sensitive areas unless authorised 

Failure to comply with these rules can result in enforcement action, fines, or a requirement to upgrade or replace the treatment system.

The Environment Agency and Natural Resources Wales, among others, are responsible for enforcing the GBRs and issuing permits for systems that fall outside their scope.

Design and Siting Considerations

Correct discharge design is essential to ensure system performance and environmental compliance. The following factors must be considered when planning a discharge route:

  • Soil infiltration capacity: Established through percolation testing 
  • Groundwater level: High water tables can cause system failure or pollution 
  • Proximity to buildings and watercourses: Defined setback distances must be respected 
  • Flow rates: Calculated based on number of occupants or site usage 
  • Effluent quality: Must meet standards for the chosen discharge route 
  • Access for maintenance: Ensuring ongoing system performance 

In many cases, a Drainage and Wastewater Management Plan (DWMP) or similar report will be required during the planning process, especially for new developments.

Monitoring, Maintenance, and Environmental Protection

Discharging treated effluent, even to approved standards, requires ongoing vigilance to prevent pollution and system degradation. Poorly maintained systems can quickly fail, leading to contamination, odours, and fines.

Good practice includes:

  • Regular emptying of septic tanks or treatment plant sludge (typically every 12 months) 
  • Visual inspections of discharge points for signs of blockage or overflow 
  • Periodic water quality testing if required by permit 
  • Keeping maintenance records and receipts 
  • Replacing or upgrading systems that do not meet current standards 

In sensitive areas, regulators may require additional monitoring, such as installing sample chambers or flow meters at the discharge point.

Environmental Impact and Best Practice

While discharging treated effluent is a necessary and accepted part of decentralised wastewater management, it carries environmental responsibilities. Poor discharges can contribute to:

  • Nutrient pollution in rivers and lakes 
  • Algal blooms and oxygen depletion 
  • Pathogen transmission in recreational waters 
  • Groundwater contamination 

To minimise these risks, the following best practices should be followed:

  • Install systems designed for the expected load and site conditions 
  • Use secondary or tertiary treatment if discharging to surface water 
  • Regularly inspect and maintain systems 
  • Educate property owners about their obligations 
  • Upgrade systems that are no longer fit for purpose 

Many modern package treatment plants now include built-in alarms, telemetry, and smart monitoring to ensure that discharges remain within safe and legal limits.

Conclusion

Discharging is the final and essential step in any private wastewater treatment system. Whether effluent is discharged to ground through a drainage field or to surface water, it must be done in a way that is safe, sustainable, and legally compliant.

Understanding the discharge process is vital for engineers, homeowners, developers, and regulators alike. It involves more than just pipework—it requires careful site assessment, system design, regulatory awareness, and long-term maintenance. With environmental standards tightening and public awareness increasing, the correct handling of discharges has never been more important.

By adhering to good design principles, maintaining systems regularly, and following the General Binding Rules or permit conditions, discharges from septic tanks and treatment plants can be managed safely, protecting both people and the environment.