Septic Tanks and Sewage Treatment Plants

What This Requirement Covers

Where a public foul sewer is not available, Approved Document H2 provides guidance on alternative foul water drainage systems. The hierarchy of preference is: connection to public sewer, then package sewage treatment plant, then septic tank with drainage field, and lastly cesspool (holding tank).

Key Requirements

When Off-Mains Drainage Is Acceptable

1. Off-mains drainage is only acceptable where it is not reasonably practicable to connect to a public sewer
2. The building owner must demonstrate that connection to a public sewer is not feasible (typically where the nearest sewer is more than 30 metres away or where the terrain makes connection impractical)

Septic Tanks

1. Septic tanks provide primary treatment (settlement of solids); the effluent must be further treated by a drainage field (soakaway) or reed bed
2. Design to BS EN 12566-1 (Small wastewater treatment systems - prefabricated septic tanks)
3. Minimum capacity: Based on the number of bedrooms (P = number of persons = 2 x number of bedrooms)
4. Drainage fields must be designed to BS 6297 and sized based on percolation test results
5. Septic tanks must not discharge to a watercourse; the effluent must be treated through a drainage field

Package Sewage Treatment Plants

1. Package treatment plants provide secondary treatment (biological treatment) and can discharge to a watercourse or drainage field
2. Design to BS EN 12566-3 (Small wastewater treatment systems - packaged and/or site assembled domestic wastewater treatment plants)
3. Discharge to a watercourse requires an environmental permit from the Environment Agency (or a general binding rules exemption for small discharges)
4. Treatment plants produce higher-quality effluent than septic tanks and are preferred where drainage field conditions are poor

Cesspools

1. A cesspool is a sealed holding tank with no discharge; contents must be emptied regularly by a licensed waste carrier
2. Cesspools are the least preferred option due to ongoing emptying costs (typically GBP 200-400 per empty)
3. Minimum capacity: 18,000 litres (approximately 6 weeks' storage for a 4-bedroom house)
4. Only acceptable where a treatment plant or septic tank cannot be used

Environment Agency Requirements

1. All discharges to the ground or watercourse require either an environmental permit or compliance with general binding rules
2. General binding rules apply to domestic discharges of less than 2,000 litres per day (approximately up to 5 bedrooms) provided certain conditions are met
3. Larger discharges (more than 2,000 litres per day) require a bespoke environmental permit

Practical Compliance Tips

1. Check with the local sewerage company first to confirm that public sewer connection is not feasible
2. Carry out a percolation test (to BS 6297) to determine whether the ground is suitable for a drainage field
3. Position the treatment plant or septic tank at least 7 metres from any habitable building and at least 10 metres from any watercourse
4. Ensure the system is accessible for desludging (tanker access required)
5. Register with the Environment Agency under general binding rules or apply for an environmental permit before installing the system
6. Maintain the system in accordance with the manufacturer's instructions; failure to maintain treatment plants leads to permit breaches and pollution
7. Keep maintenance records, permit documentation, and installation certificates for the building file