Pond Liner for Agricultural Slurry Storage UK — EA Compliance & WCA 1991 Guide

Legal Requirements for Slurry Storage in the UK

The storage of livestock slurry is one of the most tightly regulated areas of agricultural environmental management in England. Slurry is classified as a controlled substance under the Water Resources Act 1991 (WCA 1991), and farmers are legally obliged to store it in a manner that prevents pollution of controlled waters. The Water Resources (Control of Pollution) (Silage, Slurry and Agricultural Fuel Oil) (England) Regulations 2010 — commonly known as the SSAFO Regulations — set out the specific construction and maintenance requirements for slurry stores.

SSAFO Regulations — Construction Requirements

Under the SSAFO Regulations, slurry stores must:

• Be constructed or provided with a liner to prevent leakage
• Have a minimum capacity of 4 months' storage (in Nitrate Vulnerable Zones — up to 6 months)
• Be constructed of materials capable of withstanding the loads imposed by the slurry and any machinery operating nearby
• Be inspected annually by the occupier
• Have a record kept of all inspections, maintenance, and any repairs

Liner Specification for Slurry Stores

Slurry contains ammonia, hydrogen sulphide, and organic acids at concentrations that will degrade unsuitable liner materials. The key chemical resistance requirements for slurry pond liners are:

• Resistance to ammonia solutions up to 5% concentration
• Resistance to organic acids (acetic acid, lactic acid) — typical pH 6–7.5
• Resistance to hydrogen sulphide gas at ambient concentrations

HDPE — The Recommended Specification

HDPE geomembrane is the material of choice for slurry lagoon lining. Key reasons:

• Excellent resistance to ammonia, organic acids, and hydrogen sulphide
• Weldable seams — eliminates the risk of joint failure under biological gas pressure
• GRI-GM13 compliant — provides documented performance baseline
• Minimum thickness 1.0mm for slurry applications (1.5mm recommended for large lagoons)
• Textured HDPE on slopes steeper than 1:3

Materials to Avoid

• PVC — plasticiser migration in slurry environment; limited chemical resistance
• EPDM — poor resistance to oils and some organic compounds present in slurry
• Butyl — can be degraded by concentrated organic acids

Environment Agency Inspection and Enforcement

The EA has enforcement powers under the WCA 1991 to serve works notices on farmers whose slurry stores present a pollution risk. Repeated or significant pollution incidents can result in criminal prosecution. The EA's guidance document "Storing silage, slurry and agricultural fuel oil: environmental protection" provides detailed practical guidance.

A slurry lagoon that is not lined, or that has a degraded liner, presents an unacceptable pollution risk and will trigger EA enforcement action. Annual inspections and liner condition assessments are essential.

SSAFO Regulations 2010 — Full Technical Requirements

The Water Resources (Control of Pollution) (Silage, Slurry and Agricultural Fuel Oil) (England) Regulations 2010 — commonly known as the SSAFO Regulations — set out specific construction standards for slurry stores. For lined lagoons, the key requirements are:

Construction Standards (Regulation 10 and Schedule 2)

• The lagoon must be designed to resist the loads imposed by the slurry and any plant operating nearby, with a safety margin.
• The liner must prevent leakage of slurry to ground or watercourses.
• The store must have a minimum freeboard of 750mm (space above the liquid level to the top of the bund) to allow for storm rainfall.
• All slurry stores must have a channel or drainage system that prevents yard runoff from entering the store and overflowing.

Minimum Capacity Requirements

Livestock Type	Outside NVZ	Inside NVZ
Dairy cows	4 months	5–6 months
Beef cattle	4 months	5–6 months
Pigs (intensive)	4 months	6 months
Poultry slurry	4 months	6 months

Nitrate Vulnerable Zones (NVZs) cover approximately 55% of England's agricultural land. Farmers in NVZs must also comply with the Farming Rules for Water (The Reduction and Prevention of Agricultural Diffuse Pollution (England) Regulations 2018), which include restrictions on slurry spreading periods.

Chemical Resistance of Liner Materials to Slurry

Slurry is a complex mixture requiring careful liner material selection. The key chemical stressors are:

Chemical	Typical Concentration	HDPE	EPDM	PVC	Butyl
Ammonia (NH₃)	0.5–2.5 g/L	Excellent	Good	Fair	Good
Hydrogen sulphide (H₂S)	10–50 mg/L	Excellent	Good	Fair	Good
Acetic acid	0.1–0.5 g/L	Excellent	Good	Good	Good
Methane (dissolved)	Variable	Excellent	Excellent	Good	Good
Oils/fats	Variable	Excellent	Poor	Poor	Poor
Biocides (disinfectants)	Variable	Good	Poor	Fair	Fair

HDPE is uniquely suited to slurry applications: it resists all the primary chemical stressors including the oils, biocides, and disinfectants that would degrade EPDM or PVC. For slurry lagoons used for pig slurry (higher fat content) or where regular disinfection is practised, HDPE is the only suitable liner material.

EA Inspection and Enforcement — What to Expect

The Environment Agency conducts compliance inspections of agricultural businesses, including slurry store inspections. Inspections focus on:

• Evidence of current capacity (slurry volume vs storage capacity calculation)
• Physical condition of the store — liner integrity, bund condition, covers and valves
• Inspection and maintenance records
• Proximity to watercourses and any evidence of seepage

Enforcement options available to the EA include: informal advice and guidance (most common first response); warning letters; improvement notices requiring specific works by a deadline; civil sanctions including monetary penalties; and criminal prosecution (typically reserved for serious or repeated pollution incidents).

A slurry store with a visibly failing liner — discoloration of surrounding soil, dead vegetation around the base of the bund, or seepage channels — is likely to result in a formal improvement notice with a tight compliance deadline and potentially a civil monetary penalty.