Chemical storage
The 110% rule: sizing bunds and secondary containment correctly
A bund that can hold the largest container, plus a margin, is what keeps a leaking drum or tank from becoming a reportable pollution incident.
Secondary containment (a bund, a spill pallet or a containment tray) catches whatever escapes from the primary container. It is one of the cheapest and most effective chemical controls, but only if it is the right size and is kept in good condition.
The sizing rule. Good practice, and the standard in many operator procedures, is that bund capacity must be the greater of:
- 110% of the largest container in the bund; or
- 25% of the total volume stored in it.
Bund volume ≥ the larger of 110% × the largest container, or 25% × the total volume stored
Worked examples
| What is stored | 110% of largest | 25% of total | Minimum bund |
|---|---|---|---|
| One 1,000 L IBC | 1,100 L | 250 L | 1,100 L |
| Four 205 L drums (820 L total) | 226 L | 205 L | 226 L |
| Twenty 205 L drums (4,100 L total) | 226 L | 1,025 L | 1,025 L |
| One 10,000 L diesel tank | 11,000 L | 2,500 L | 11,000 L |
The extra 10% allows for rainwater, firefighting water and the surge when a container fails suddenly. The 25% rule protects stores with many small containers, where several could leak at once, for example in a fire.
Design and upkeep points
- Make the bund floor and walls impermeable and resistant to the stored chemical.
- Do not put a drain valve in the bund. If one exists, keep it locked shut and log every release.
- Remove rainwater regularly, after checking it is not contaminated, so the full capacity stays available.
- Do not share a bund between incompatible chemicals.
- Keep pipework and valves inside the bunded area.
- Inspect for cracks, corrosion and standing liquid, and record each check.
Why it matters legally. A spill that stays inside a bund is a housekeeping matter. A spill that escapes to soil, drains or a watercourse can become pollution under s. 78 of the National Environment Act Cap. 181. That brings duties to notify, clean up and restore.
What to do
- Calculate the required capacity for every bund and spill pallet, and compare it with the actual capacity.
- Mark the maximum permitted storage on each bunded area.
- Add bund condition and rainwater checks to weekly store inspections.
How INRC can help
Our environmental audits check bund capacity against what is actually stored, and inspect bund condition, drainage and rainwater management. For new facilities, we build containment requirements into our ESIAs from the start. Contact us to discuss your site.
This article is general guidance, not legal advice. Check current legislation and regulations for your specific project.