Septic Tanks and Cesspools: 2020 General Binding Rules, BS EN 12566 and Environment Agency Registration

Summary

Off-mains drainage serves approximately 5% of UK properties — around 1.3 million homes — mostly in rural areas without access to the public sewer network. Septic tanks and cesspools are fundamentally different in function: a septic tank is a treatment system that reduces biological oxygen demand before discharge; a cesspool is simply a sealed collection vessel requiring regular emptying. Both are subject to environmental and planning regulation.

The Environment Agency (EA) in England, Natural Resources Wales (NRW), SEPA (Scotland) and NIEA (Northern Ireland) regulate septic tank discharges under the Environmental Permitting Regulations 2016. The 2020 General Binding Rules changed the regulatory landscape significantly. Previously, many old septic tanks discharged legally to watercourses under deemed consents. After January 2020, direct discharge to any watercourse became non-compliant unless the property has a specific EA permit — which is rarely granted for septic tank effluent.

For plumbers, drainage engineers and builders working on rural properties, understanding the current rules is essential. Installing a non-compliant system, or failing to advise a client of the need to upgrade, creates liability exposure.

Key Facts

• General Binding Rules (GBRs) — Statutory Instrument 2016/1154, updated 2020; set conditions under which small sewage discharges (including septic tanks) can operate without a bespoke permit
• Direct discharge banned — From 1 January 2020, septic tanks discharging directly to a watercourse are non-compliant unless they have an existing bespoke permit
• Permitted discharge — Septic tank discharge to a drainage field (soakaway) complying with BS EN 12566-3 is permitted under GBRs
• Sewage treatment plant — A package sewage treatment plant discharging directly to watercourse is permitted under GBRs if it meets BS EN 12566-3 treatment standards
• Property sale obligation — Sellers must disclose drainage arrangements; since 2015, direct discharge to watercourse has been an issue at conveyancing
• Cesspool — Sealed collection tank; no treatment; cannot discharge anywhere; requires pumping every 6–8 weeks typically; minimum capacity 18,000 litres under BS EN 12566-4
• Septic tank capacity — BS EN 12566-1: minimum 2,700 litres for a 5-person household; larger for more occupants
• Drainage field — Perforated pipe in gravel trench distributes settled effluent into unsaturated soil; must be >10m from watercourse, >50m from borehole/spring
• Infiltration testing — VSA (Vp) soil percolation test required before drainage field design per BS EN 12566-3
• EA registration — Under GBRs, you must register a new septic tank or treatment plant with the EA before installing; no charge for standard registration
• Scotland — SEPA and The Water Environment (Controlled Activities) (Scotland) Regulations 2011; similar framework but different threshold sizes and registration

Quick Reference Table

Detailed Guidance

General Binding Rules 2020: What Changed

Before 2020, many septic tanks operated under a "deemed consent" — they predated the regulatory framework and their discharge was effectively grandfathered. The 2020 changes ended this:

Key changes:

1. All direct discharges of septic tank effluent to watercourses became non-compliant
2. Owners had until January 2020 to either obtain a bespoke permit (rare) or upgrade
3. Properties changing ownership from January 2015 onwards were specifically called out: direct discharge is a known issue that must be addressed at sale
4. The Environment Agency can enforce against non-compliant discharges; penalty: fixed penalty notice (£300) or criminal prosecution with fines up to £50,000

What still complies under GBRs:

• Septic tank discharging to a drainage field (soakaway) per BS EN 12566-3
• Package sewage treatment plant discharging to surface water if it meets the relevant GBR conditions
• Any system built and registered before January 2020 that has not changed and is maintaining compliance

What requires a bespoke permit:

• Discharging to a watercourse within 500m of a Site of Special Scientific Interest (SSSI)
• Systems treating more than 2,000 litres/day (approximately 10 persons at 200L/day)
• Any scenario where the GBRs say the standard conditions cannot be met

Septic Tank Design and Sizing

A septic tank is a multi-chamber vessel (typically two or three chambers) in which:

1. Heavy solids settle to the bottom (sludge)
2. Floating material (scum) collects at the top
3. Partially clarified liquid (effluent) exits through a T-pipe outlet to the drainage field

Sizing per BS EN 12566-1:

• Minimum working capacity: 2,700 litres for ≤5 persons
• Add 180 litres per additional person above 5
• Note: this is the minimum; larger is better for longer intervals between desludging

Two-chamber design:

• First chamber: 2/3 of total volume; primary settlement
• Second chamber: 1/3 of total volume; secondary settlement
• T-pipe inlet to prevent inlet flow disturbing settled sludge
• T-pipe outlet (or 3-way effluent filter) to prevent scum passing to drainage field

Modern GRP (glass-reinforced plastic) tanks:

• Pre-manufactured; delivered and lowered into excavation
• Typical sizes: 2,800L, 3,800L, 4,500L, 6,000L+
• Require concrete or sand surround (avoid direct fill against thin walls)
• Anti-buoyancy measures required in high groundwater areas (concrete collar or filled tank)

Drainage Field Design and Installation

The drainage field (soakaway) is the second and most critical element of a compliant septic tank system. It must be designed to match the soil percolation rate.

Soil percolation test (Vp test):

1. Dig 3 trial holes to distribution pipe level (typically 500–600mm deep)
2. Fill with water and allow to drain over 24 hours (saturation)
3. Refill to 300mm above invert level
4. Measure time in seconds for water level to fall 150mm (from 300mm to 150mm above invert)
5. Repeat twice; take the mean
6. Vp = mean time in seconds / 150 (expressed as time for water to fall 1mm)

Drainage field area calculation (BS EN 12566-3 method):

• A = P × Vp × 0.25
• Where: A = area in m², P = daily flow (persons × 150 litres), Vp = percolation value
• Example: 4 persons, Vp = 20s/mm: A = (4 × 150) × 20 × 0.25 = 3,000m² — [verify calculation method against current EA guidance as this can vary]

Distribution pipe layout:

• 100mm perforated drain laid in gravel-filled trench
• Trench width: 300–900mm; gravel to 50mm above pipe
• Maximum run length of any distribution pipe: 30m before returning to manifold
• Gradients: level or slight fall (1:200 maximum) to ensure even distribution

Exclusion zones:

• 10m minimum from any watercourse
• 50m minimum from any borehole, well, or spring used for drinking water
• Not within 15m of a building
• Not in waterlogged, heavily compacted, or clay soil (Vp > 100s/mm = unsuitable)

Cesspools: Where They Are Still Used

A cesspool (cesspit) is a sealed underground tank with no outlet. It collects all wastewater from the property and must be pumped out regularly. It is not a treatment system.

Where cesspools are appropriate:

• Sites where soil is unsuitable for a drainage field (clay, high groundwater)
• Sites where the watercourse proximity rules prevent drainage field siting
• Sites where environmental sensitivity makes any discharge inappropriate
• Existing properties where installation is impractical

Sizing (BS EN 12566-4):

• Minimum: 18,000 litres
• Recommended: 45 days storage capacity; for a 4-person household at 150L/person/day: 45 × 4 × 150 = 27,000 litres minimum

Pumping frequency: Depends on tank size and occupancy. A 18,000L tank for 4 persons (600L/day) fills in 30 days — monthly pumping. A 27,000L tank fills in 45 days. Most waste contractors offer scheduled service.

Planning implications:

• New cesspools are rarely permitted by planning authorities because they are unsustainable (require tanker emptying)
• Planning conditions for new rural developments typically require septic tank + drainage field or package treatment plant
• Replacement of existing cesspool with cesspool: no change of use; planning notification only
• Replacement with septic tank: improvement; more likely to receive planning permission

EA Registration Process

Under the GBRs, before installing a new septic tank or package treatment plant, you must register with the Environment Agency (England):

1. Complete EA's online registration for a Small Sewage Discharge [verify current process via Gov.uk]
2. Provide: property address, type of system, capacity, destination of effluent
3. No fee for standard registration
4. Registration must be completed before installation starts
5. EA may object within a specific period; if no objection, proceed

For replacement of an existing system: registration required for new system if the destination of discharge changes.

In Wales: NRW handles registrations; process similar but check NRW website. In Scotland: SEPA; Water Environment (Controlled Activities) (Scotland) Regulations 2011; different threshold sizes.

Frequently Asked Questions

Our septic tank discharges to a ditch — is this legal since January 2020?

No, unless you have a bespoke permit from the Environment Agency (which is rarely issued for septic tank effluent). From January 2020, direct discharge from a septic tank to any watercourse including ditches is non-compliant under the General Binding Rules. You need to either install a drainage field (soakaway) to divert the discharge, or replace the septic tank with a full package sewage treatment plant (which can discharge to a watercourse under GBR conditions if it meets the treatment standard).

Can I connect a septic tank to a public sewer if one becomes available?

Yes, and in most cases the local water company will request or require connection when a public sewer is brought within a certain distance of the property (typically 30m under Water Industry Act 1991). Connection is usually the most cost-effective long-term option, as it removes ongoing maintenance costs and compliance risk. Costs include connection charge and plumber's work; the water company may contribute if they have constructed the new sewer.

How often should a septic tank be emptied?

A properly designed septic tank with a drainage field requires annual desludging for a typical household. The sludge level should be checked annually; if sludge occupies more than 1/3 of the first chamber volume, desludge required. If the drainage field is working correctly, the septic tank does not "fill up" — it maintains a constant liquid level with effluent draining into the field.

What signs indicate a drainage field is failing?

• Wet, smelly ground over or near the drainage field (surfacing effluent)
• Slow drainage from all fixtures in the property
• Sewage smell inside the property (drainage field backing up)
• Gurgling noises from drains
• Lush, green grass over the drainage field (not always a problem — nitrogen from effluent feeds grass)

A failing drainage field is often caused by: bio-mat clogging (excess solids entering field from undersized or poorly maintained septic tank), root intrusion, soil compaction from vehicle traffic, or saturated ground.

Regulations & Standards

• Environmental Permitting (England and Wales) Regulations 2016 (SI 2016/1154) — statutory framework for septic tank discharges; includes 2020 General Binding Rules

• BS EN 12566-1:2016 — Small wastewater treatment systems: prefabricated septic tanks

• BS EN 12566-3:2016+A2 — Prefabricated and/or site assembled domestic wastewater treatment plants (drainage field design)

• BS EN 12566-4:2007 — Septic tanks assembled on site from prefabricated kits

• Water Industry Act 1991 — powers of water companies to require sewer connection

• Building Regulations Approved Document H — Drainage and waste disposal: drainage field design guidance

• Environment Agency: General Binding Rules for Small Sewage Discharges — Current GBR guidance for England

• Environment Agency: Septic Tanks and Treatment Plants — Permitting and registration

• BS EN 12566 Series — BSI publications on small wastewater treatment systems

• suds design — Surface water drainage design for sites where septic tank drainage field is not appropriate

• part g sanitation — Building Regulations Part G sanitation and wholesome water requirements