Firewater containment at a lower-tier COMAH chemical blender
Alderney Chemicals blends solvents and acids. It stores 30,000 litres of flammable solvents and 10,000 litres of corrosive acids. The site is lower-tier COMAH and has an EMS aligned to ISO 14001 but not yet certified. The emergency plan relies on a firewater retention pond sized by a consultant for a two-hour fire; a more recent review says the credible worst case needs four hours. The pond has a manual penstock valve. The last exercise was 18 months ago, when response took 25 minutes and the valve was found seized. The pond overflows to a foul sewer if its capacity is exceeded. The sewer undertaker requires notification of any firewater discharge. The local authority off-site plan assumes the site will contain firewater. Capital is available for one major project this year. The environmental manager must decide whether to enlarge the pond, improve other controls, or both, and must set objectives for the next review.
1. Evaluate the adequacy of the emergency preparedness and response arrangements and prioritise the improvements.
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The arrangements are not adequate. A 25-minute response and a seized penstock mean the plan exists on paper but cannot deliver containment. The pond is sized for two hours when the credible worst case is four, so even a correct response could overflow to foul sewer. The last exercise was 18 months ago, which is too infrequent to maintain competence and equipment. The plan also depends on the local authority off-site plan assuming containment, so the gap is not only the company's.
Prioritise by consequence and feasibility. First, make the penstock operable: free it, maintain it, consider remote or automatic operation, and add a secondary isolation method. Second, test the response with a realistic exercise, including out-of-hours, and set a response standard. Third, provide secondary containment or reduce the firewater volume through foam, shutdown and drainage segregation. Fourth, agree notification with the sewer undertaker. Fifth, review the pond capacity and either enlarge it or document why other controls make the two-hour basis acceptable; if the four-hour case is credible, a two-hour pond is a known gap that should be in the action plan and risk register.
Record the decisions and residual risk. The regulator and local authority should be engaged where the off-site plan depends on containment. Do not rely on the sewer as a route; it transfers pollution and may breach the trade effluent consent.
2. Decide whether to invest in a larger pond or in other controls, and justify your decision using risk, legal duties and ALARP reasoning.
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Start from the legal and risk position. If the credible worst case requires four hours and the pond provides two, the site cannot demonstrate containment. A larger pond may be the most robust solution, but it is capital-intensive and may not be the only option. Compare: enlarging the pond; reducing firewater volume through foam, isolation, drainage segregation and firefighting strategy; improving response so the penstock closes early and firewater is contained; or a combination. Use ALARP reasoning: the risk of off-site pollution and the assumptions in the off-site plan matter, not just cost.
If the regulator or COMAH obligations require containment, a larger pond may be necessary. But a larger pond with a seized valve and slow response still fails, so the first spend should be on operability and response. A sensible package is: fix and automate the penstock, train and drill, add drainage isolation, and commission a study to size the pond against the credible scenario. If capital is limited to one major project, choose the option that closes the most risk: often a combination of smaller controls plus a right-sized pond, phased.
Document the decision, the residual risk, the assumptions about firewater volume, and the date for review. Inform the local authority and sewer undertaker if the off-site plan assumptions change. Avoid treating the pond as the only control or assuming the sewer can accept firewater.
3. Explain how to set measurable environmental objectives and targets for spill prevention and emergency readiness, with specific examples and indicators.
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Objectives should be specific and measurable, with a baseline and a date. Examples: reduce the risk of uncontrolled firewater discharge; ensure the penstock is operable and tested; improve response time; and verify containment capacity against the credible fire scenario. Targets could be: 100% of monthly penstock checks completed; response time under 10 minutes in the next two exercises, including one out-of-hours; all spill and firewater equipment inspected monthly; pond capacity verified by a competent engineer against a defined worst case by a set date; and 100% of reportable discharges notified within the required time.
Set indicators: number of drills, response time, equipment defects closed, volume of firewater retained versus released, number of sewer notifications, and near-miss reports. Assign owners and resources in an environmental management programme. Review progress at management review and after each exercise. If a target is missed, require an explanation and a revised action, not just a new date.
Make the objectives relevant to the significant aspects and legal duties: COMAH, trade effluent consent and the off-site emergency plan. Communicate them to operators and contractors. The test is whether the objective changes behaviour on the ground, for example a valve check that is actually done and recorded, and a drill that reveals and fixes problems.