Maintenance shutdown planning: stock, staff and standby doses

The plant goes down for five days in April. The sump does not. That single sentence is the whole planning problem, and every year some site discovers it on the Tuesday of the shutdown week when the level alarm goes off and the maintenance crew is still inside the tank.

Shutdowns are planned around equipment availability, and leachate is planned around weather. The two calendars rarely meet, which is why the leachate plan needs to be written by someone who has looked at both.

Forecast the inflow across the shutdown window

Start with the ten-year rainfall range for that week, not the forecast. Take the 80th percentile, add your base flow, and add whatever the site receives from neighbouring contributors. That is the volume you must be able to handle without the plant.

Then add 20 percent. A shutdown that runs two days over is common; a shutdown that runs under is rare. The 20 percent is not pessimism, it is the observed distribution of contractor overruns on leachate plants.

Stock and standby arrangements

Three options exist for the volume you cannot treat: store it, tanker it, or solidify it on a reduced basis. Most sites use a combination, and the mix should be decided in the planning meeting rather than improvised.

Storage is cheapest per cubic metre until the lagoon reaches its limit, at which point it becomes the most expensive option on the list. Tankering is predictable but needs booking, and hauliers do not hold capacity for a week on the chance you call. Solidification on a reduced basis works if you can keep one mixer and one operator available, which means the shutdown scope must explicitly exclude that equipment.

Staffing the gap

The maintenance crew and the operations crew are usually the same people on a small site. If the person who runs the mixer is also the person holding the spanner, the leachate plan has no operator. Name a standby operator in the plan, confirm they are not on leave, and confirm they have done a batch in the last month.

Contractors need the same treatment. A shutdown brings electricians, scaffolders and tank cleaners onto the pad, and every one of them is a person who might open a valve or walk through a bund. A fifteen-minute induction covering the drains, the emergency stop and the spill kit is worth more than a folder of method statements.

Shift patterns need the same scrutiny. A shutdown often runs twelve-hour days, and the operator who covers the pad in the evening is doing it after a full day on the tools. Fatigue is not a character flaw, and a dose error at 19:00 on day four of a shutdown is a predictable outcome rather than bad luck.

The April that went wrong

A site scheduled a five-day shutdown in April, a month chosen because rainfall is usually low. The previous year’s April had 8 mm. That year’s had 96 mm in four days, including 42 mm on the Wednesday.

The lagoon had been drawn down to 60 percent to give the contractors room, which removed the buffer. By Thursday the level was 200 mm below the emergency overflow. The site mobilised two tankers at short notice, paid a premium rate, and solidified 140 m³ with a borrowed operator who had not run a batch in eight months. The batch dose was wrong by 30 percent and two loads were rejected.

Three changes came out of it. The lagoon now stays at 85 percent through any shutdown. The standby operator list names two people and both run a practice batch the week before. And the shutdown plan now includes a trigger for calling the haulier, written as a level rather than a feeling.

If your next shutdown does not have a rainfall percentile in it, it does not have a volume plan. The leachate solidification guide sets out the reduced-rate operating mode that keeps a pad running with one mixer.