Choosing the Right Secondary Containment Engineering Control
Secondary containment — the system that catches liquid if primary containment (the drum, IBC, tank, or pipe) fails — is a mandatory engineering control under COSHH Regulation 7 for substances that present a significant risk through the liquid contact route, and under the Environmental Permitting (England and Wales) Regulations 2016 for any site at risk of water pollution. The question of whether to use a permanently constructed bunded floor or a portable/modular bund system is one of the most practically significant decisions in facility design. Both have genuine advantages and both have specific failure modes that are frequently missed.
Permanently Constructed Bunded Work Floors
A bunded work floor (also called a sump floor, contained floor, or liquid-tight floor) is a permanent engineering structure incorporating an impermeable floor surface with raised edges, a contained drainage system, and typically sumps or collection points. In the UK, they are designed to the requirements of CIRIA C736 (Containment Systems for the Prevention of Pollution, 2nd edition) — the definitive technical guidance for secondary containment in the UK.
Key Design Standards Under CIRIA C736
- Containment volume: Must hold 110% of the largest single container, or 25% of the total aggregate volume for multi-container stores, whichever is greater. For flammable liquid stores, EA PPG2 recommends 110% of the largest container regardless of total aggregate.
- Liquid tightness: The floor construction must be liquid-tight to the stored substance. Concrete without appropriate lining is rarely sufficient — sulphuric acid, sodium hydroxide, and many solvents will penetrate plain concrete within hours. Epoxy coating, polyurethane resin, or polymer-impregnated concrete must be specified for chemical resistance.
- Structural loading: Must withstand the static and dynamic loads of the equipment stored and moved across it, including forklift loadings for IBC handling.
- Drainage management: A bunded floor with a drain connected directly to the sewer provides no secondary containment. Drainage from contained areas must go to a collection sump (pumped out by a competent contractor) or through a controlled sampling point with isolation valve.
Portable and Modular Bund Systems
Portable bunds — including spill pallets, modular bund walls, drip trays, and collapsible bund decks — provide secondary containment without permanent construction. They are specified under the same CIRIA C736 volume principles and must be compatible with the stored substances.
Advantages of Portable Systems
- No planning permission, building works, or contractor requirement
- Immediately deployable — critical for leased premises or temporary operations
- Easily relocated as storage requirements change
- Inspectable and replaceable as discrete units
- Can be specified with UKCA/ATEX ratings for DSEAR zones
Direct Comparison
| Criterion | Bunded Work Floor | Portable Bund/Spill Pallet |
|---|---|---|
| Capital cost | High (£5,000–£50,000+) | Low–medium (£50–£3,000) |
| Installation | Permanent construction; 2–4 weeks | Immediate; no installation |
| Containment volume | Very high — suitable for tank farms | Limited — typically up to 4,400 litres for large IBC pallets |
| Chemical resistance | Specified by coating/lining selection; highly customisable | Limited by material (PE, steel, GRP); check per substance |
| DSEAR suitability | Engineered with appropriate fixtures and equipment ratings | ATEX/UKEX rated units available |
| Maintenance requirement | Floor integrity inspection; drain isolation testing; annual minimum | Visual inspection each use; full inspection 6-monthly |
| Regulatory recognition | CIRIA C736; EA guidance primary recommendation | Accepted where permanent construction not reasonably practicable |
| Failure mode | Coating delamination; drain not isolated; inadequate volume | Overloading; incompatible material; drain holes not blanked |
The 'Reasonably Practicable' Test
The EA and HSE both operate on the principle that the most robust engineering control should be used unless there is a justified reason it is not reasonably practicable. For permanent facilities handling significant quantities of hazardous liquids, a permanent bunded floor will generally be considered the reasonably practicable standard. Portable bunds are accepted where:
- Quantities are small (single drum quantities)
- The facility is leased and permanent modification is not permitted
- Storage is genuinely temporary (less than 90 days)
- The risk profile of the substance does not justify permanent construction costs
Documenting the reasoning for choosing portable over permanent containment in your COSHH assessment protects you if challenged by HSE or the EA during inspection.
Common Failure Points in Portable Bunds
- Drain holes unblanked: Many spill pallets include a drain hole for routine water drainage. If this is left open in chemical storage, it provides no containment. Drain plugs must be fitted and their presence verified in each inspection.
- Overloaded grating: IBCs exceeding the dynamic load rating of the pallet cause progressive grating failure. Always verify the dynamic load rating (forklift access) and static load rating separately.
- Incompatible material: Galvanised steel pallets used with alkalis; PE pallets with concentrated nitric acid; standard steel with hydrochloric acid. Material compatibility must be confirmed against a chemical resistance chart for the specific substance and concentration.
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