Septic Tank vs Sewage Treatment Plant

Yes, but since 1 January 2020, the General Binding Rules require that septic tanks discharge to a drainage field (soakaway) — not directly to a watercourse. If your site cannot accommodate a drainage field, you must install a sewage treatment plant instead, which can discharge to a watercourse with an environmental permit or under the General Binding Rules.

A septic tank produces partially treated effluent (typically 200-300 mg/l BOD) which must pass through a drainage field for final treatment in the soil. A sewage treatment plant produces much cleaner effluent (typically 20-40 mg/l BOD), clean enough to discharge to a ditch or watercourse. Treatment plants use aeration or rotating biological contactors to break down waste more effectively.

A septic tank costs £2,000-4,000 for the tank plus £2,000-5,000 for a drainage field, totalling £4,000-9,000 installed. A packaged sewage treatment plant costs £4,000-8,000 for the unit plus £1,000-3,000 for installation and discharge pipework, totalling £5,000-11,000. Treatment plants have higher running costs (electricity for the air blower) of £50-80/year.

Septic tanks typically need emptying every 12 months (cost £150-250 per empty). Sewage treatment plants usually need desludging every 6-12 months depending on the model and usage, plus an annual service of the mechanical/electrical components (£100-200/year). Some modern treatment plants have very long desludge intervals.

Septic tanks and treatment plants under 7,000 litres are usually permitted development for single dwellings. However, you must comply with the General Binding Rules (England) or equivalent regulations in Scotland and Wales. Building regulations approval is needed for any new installation, and you should consult the Environment Agency if discharging within 50m of a water source.