The Control of Substances Hazardous to Health Regulations 2002 (COSHH) require employers to assess not only routine exposure to hazardous substances but also the consequences of foreseeable accidents. Regulation 6 specifically requires the assessment to consider the "worst-case foreseeable accident" — and for sites that store chemicals, this means calculating the volume of secondary containment required if storage fails completely.
Many COSHH assessments address this requirement superficially, with a vague statement that "adequate bunding is in place." This is not sufficient. The assessment must demonstrate that the bund has been sized correctly and that integrity has been verified. Here is how to do it properly.
The Legal Basis
COSHH Regulation 6 requires a "suitable and sufficient" assessment of the risks created by hazardous substances. HSE guidance (EH40, and the COSHH Approved Code of Practice) makes clear that for liquid chemicals, this includes assessing spillage scenarios. The Environment Agency's Pollution Prevention Guidelines (PPG2, PPG26) provide the engineering methodology for sizing containment.
Step-by-Step: Calculating Your Worst-Case Spill Volume
Step 1 — Identify the Largest Single Container on Site
Survey your chemical storage areas and identify the single largest container of hazardous liquid. This might be a 1,000-litre IBC, a 205-litre drum, a bulk tank, or a day-use vessel. Use the maximum fill volume, not the nominal capacity.
Step 2 — Assume Complete Failure
The worst-case scenario assumes the entire contents of the largest single container are released instantaneously. Do not average — assume total loss.
Step 3 — Add 10% for Firefighting Water and Foam
In a fire or emergency, responders may apply water or foam suppression agents. This additional liquid volume must be accommodated within the bund. Add 10% to the container volume to account for this.
Formula: Minimum Bund Volume = Largest Container Volume × 1.1
However, where multiple containers are stored together, you must also apply the 25% rule:
Alternative formula: Minimum Bund Volume = 25% of Total Storage Volume
Use whichever figure is greater.
Step 4 — Check the Bund Can Contain This Volume
Measure the internal dimensions of the bund (length × width × height to the overflow point). Calculate the available volume. Subtract the displacement volume of any equipment or containers sitting inside the bund — forklift pallets, pump stations, and frames all reduce effective capacity.
Step 5 — Check Bund Integrity
A correctly sized bund that leaks is useless. Inspect for:
- Cracks in concrete (particularly around pipe penetrations)
- Failed joints or sealant
- Drain valves — must be closed and locked when not actively dewatering
- Adequate freeboard (at least 75mm above calculated worst-case fill level)
Step 6 — Document in the COSHH Assessment
Record the calculation, the bund dimensions, the effective capacity, and the inspection findings. Date and sign the assessment. Schedule an annual bund integrity check and note it in your maintenance programme.
Worked Examples
Example A: Single 205-litre drum store. Worst case = 205 L. Add 10% = 225.5 L. Minimum bund volume = 226 L.
Example B: Four × 205-litre drums (total 820 L). Largest single container = 205 L × 1.1 = 225.5 L. 25% of total = 205 L. Use greater figure: 226 L minimum.
Example C: One 1,000-litre IBC and four 205-litre drums (total 1,820 L). Largest single container = 1,000 L × 1.1 = 1,100 L. 25% of total = 455 L. Use greater figure: 1,100 L minimum.
Bund Volume Calculator — Container Size vs Minimum Bund Volume
| Largest Container (L) | × 1.1 Rule (L) | Total Storage (4 containers) | 25% Rule (L) | Minimum Bund Volume |
|---|---|---|---|---|
| 25 L (jerry can) | 27.5 L | 100 L | 25 L | 28 L |
| 60 L (small drum) | 66 L | 240 L | 60 L | 66 L |
| 205 L (standard drum) | 225.5 L | 820 L | 205 L | 226 L |
| 500 L (half IBC) | 550 L | 2,000 L | 500 L | 550 L |
| 1,000 L (IBC) | 1,100 L | 4,000 L | 1,000 L | 1,100 L |
| 2,500 L (bulk tank) | 2,750 L | 10,000 L | 2,500 L | 2,750 L |
| 10,000 L (large tank) | 11,000 L | 40,000 L | 10,000 L | 11,000 L |
Practical Tips
- Recheck bund volumes every time you add new storage — an additional IBC can push you outside compliance
- Account for rainwater accumulation in open bunds — a flooded bund has no residual capacity
- For mixed chemical stores, check compatibility — some spills react with others and could compromise containment integrity
- Photograph and record bund conditions at each inspection as evidence of compliance
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