What is a Septic tank

A septic tank is an underground watertight chamber designed to collect and partially treat domestic sewage and wastewater where no connection to a mains sewer is available. It is typically used in rural or isolated areas where full sewerage infrastructure is not feasible. A septic tank works by separating solids from liquids and allowing natural biological processes to break down waste in an anaerobic environment.

The tank receives wastewater from household plumbing systems including toilets, sinks, showers, and washing machines. Over time, heavier solids settle at the bottom forming sludge, while lighter materials like grease and oil float to the surface forming scum. The remaining liquid effluent exits the tank and is typically discharged into a drainage field or soakaway for further treatment in the soil.

Structure and Materials

Traditional septic tanks are made from a variety of durable, watertight materials designed to resist corrosion and structural failure over long periods. Common materials include:

• Reinforced concrete

• Glass-reinforced plastic (GRP)

• High-density polyethylene (HDPE)

• Brick or masonry (in older systems)

Most modern septic tanks are prefabricated, consisting of one or two chambers. Older designs may have a single chamber, but two- or three-chamber models are now preferred as they improve settlement and allow better effluent quality.

Key components include:

• Inlet pipe: Brings wastewater from the property into the tank.

• Baffles or tees: Slow the flow and direct solids to settle.

• Scum board or partition: Prevents floating solids from entering the outlet.

• Outlet pipe: Transfers partially treated effluent to the drainage field.

• Access cover or manhole: Allows for inspection and periodic emptying.

How a Septic Tank Works

The function of a septic tank relies primarily on passive separation and natural biological activity. The treatment process occurs in several steps:

1. Influx: Wastewater enters the tank via the inlet pipe. The flow is slowed to reduce turbulence and encourage solids to separate.

2. Primary treatment: Heavier solids settle at the base forming a sludge layer. Lighter materials such as fats, oils and grease rise to the top forming a scum layer. Anaerobic bacteria present in the tank begin to digest the organic matter in the sludge and scum.

3. Separation zone: Between the sludge and scum is a relatively clear liquid layer containing suspended solids and dissolved organic matter.

4. Effluent discharge: The liquid fraction exits the tank through the outlet pipe, typically fitted with a dip pipe or baffle to prevent scum from escaping. The effluent then enters a soakaway or drainage field for further filtration through the soil.

A well-functioning septic tank does not produce odour or visible pollution and requires minimal maintenance aside from regular desludging.

Septic Tank vs. Sewage Treatment Plant

While both septic tanks and sewage treatment plants serve properties without mains drainage, they differ significantly in function and discharge quality.

Septic tanks are simpler but less effective in pollutant removal. They are best suited for discharging to ground, not directly to surface water.

Siting and Installation Considerations

Proper siting and installation of a septic tank are essential for long-term performance and environmental compliance. Factors to consider include:

• Soil conditions: The drainage field must be in free-draining soil. Clay or waterlogged areas are unsuitable.

• Groundwater protection: The tank should be installed above the water table to avoid contamination.

• Distance from buildings and watercourses: Regulations typically require minimum separation distances, such as 7 m from a dwelling and 10 m from a watercourse.

• Vehicle loading: Tanks must be located away from areas where vehicles may drive, unless specifically reinforced.

• Ventilation: Adequate ventilation is needed to manage gases produced by anaerobic digestion.

The Environment Agency provides specific guidance on siting and discharge under the General Binding Rules.

Maintenance and Desludging

Although septic tanks are passive systems, they require regular maintenance to function correctly and avoid environmental harm. The most important maintenance task is desludging.

• Desludging frequency: Typically every 12 months for domestic systems, though frequency depends on tank size and occupancy.

• Inspection: Periodic checks of inlet and outlet pipes, baffles, and access covers should be performed to ensure integrity.

• Emptying: Only licensed waste carriers should be used to remove sludge. The waste must be disposed of at an approved facility.

• Record keeping: Property owners should retain records of emptying dates and service activities for compliance purposes.

Failure to maintain the system can lead to blockages, odours, surface pooling, and pollution of nearby land or watercourses.

Environmental and Legal Requirements

In the UK, the operation of a septic tank is regulated by environmental legislation. Discharges to ground and surface water are controlled under the Environmental Permitting Regulations and the General Binding Rules administered by the Environment Agency.

Key compliance points include:

• Septic tanks must only discharge to ground via a drainage field, not directly to ditches or streams (unless exempted).

• The drainage field must be designed and constructed in accordance with British Standard BS 6297.

• The tank must be maintained and desludged regularly.

• Owners are responsible for ensuring the system does not cause pollution or nuisance.

Since 2020, discharges to surface water from septic tanks are generally prohibited unless the tank has been replaced or upgraded to a sewage treatment plant.

Common Problems and Faults

Despite their simplicity, septic tanks can suffer from a number of problems:

1. Blockages: Caused by non-degradable items (e.g. wipes, sanitary products) or excessive sludge build-up.

2. Scum or sludge carryover: Occurs if the tank is not desludged, allowing solids to enter the drainage field.

3. Drainage field failure: Often due to compacted soil, root ingress, or poor siting. Results in surface pooling or foul smells.

4. Structural damage: Tank walls or baffles may crack, leading to leaks or ineffective settlement.

5. Toxic inputs: Chemicals such as disinfectants, antibiotics or paints can kill beneficial bacteria and impair digestion.

Regular inspection and responsible usage significantly reduce the risk of these issues.

Best Practices for Septic Tank Owners

To ensure optimal performance and system longevity, users should follow basic care and maintenance practices:

• Conserve water to reduce tank loading.

• Avoid flushing non-biodegradable items.

• Use septic-safe cleaning products.

• Pump out the tank regularly based on size and usage.

• Keep records of maintenance and desludging.

• Monitor for signs of failure such as slow drainage or smells.

Properly used and maintained, a septic tank can last for decades with minimal disruption or expense.

Conclusion

A septic tank is a reliable and low-maintenance solution for properties without access to mains sewerage. It provides essential primary treatment by separating solids and allowing partial breakdown of organic matter. When properly installed, maintained, and used in accordance with regulations, a septic tank protects human health and the environment. For rural homes, small developments and remote sites, the septic tank remains a widely used and trusted component of wastewater infrastructure in the UK.