How to clean a swimming pool after a flood

CycloneSlider Following the weekend’s devastating floods and storms on the east coast of Australia, we asked Bill Mansfield, Education Officer at BioGuard, to run through the process for pool service technicians and pool owners to clean affected swimming pools.

He starts by saying you should realise that a pool that “looks” dirty might not be much of a health problem – but a pool that looks clean but has been contaminated by sewage can be a serious health risk.

To make sure your pool is healthy after a flooding with potential contaminants, you must maintain a very high free chlorine level for approximately half a day with the filter and pump operating. Check your local state and council regulation for the precise measurements of how high you should dose your pool.

Mansfield offers some simple instructions for cleaning a flooded pool.

  • Remove all solid wastes (leaves, branches, rubbish);
  • Check equipment, there is no value in adding chemicals if the mechanics are not working (filter and pump).

In the worst case scenario – if there has been a danger of sewage contamination – the pool will need to be thoroughly disinfected.

The safest way to be sure of killing the pathogens caused from serious contamination is to increase the dosages of chlorine for a longer than normal period of time. It is called the CT value or ‘concentration time’, concentration of free chlorine (mg/L) x time (minutes).

The State you are in will determine the CT value you should achieve for your pool. On average the cleanup requires a CT value above 10,000.

The calculations for CT values work like this:

  • 20mg/L (free chlorine) for 12 hours
  • 20 x (12 hours = 720 minutes)
  • 20 x 720 = CT value of 14,400

The required CT values vary by State. Mansfield offers the following information regarding State regulations for faecal cleanup.

  • NSW: CT value above 15,300;
  • VIC: CT value of approximately 10,000;
  • QLD: Shock dose the pool overnight (no specific regulations on CT values);
  • SA: Super disinfect the contaminated area.

After you have sanitised your pool again, you can begin the cleanup and rebalancing. Even if you drain the pool, there may be bacteria that survive in the pipes and filter, so draining is not the solution if you are unsure if your pool is contaminated.

Draining might also be impractical for a number of reasons including the danger of the pool popping out of the ground because of hydrostatic pressure.

Mansfield offers the following cleanup method:

  1. Check mechanical equipment for damage;
  2. Remove large debris;
  3. Floc pool;
  4. Vacuum to waste;
  5. Leave pump/filter on till you have balanced the pool;
  6. Test for metals and treat accordingly;
  7. Test for Chlorine Demand and treat accordingly;
  8. Use algaecide;
  9. Manage chlorine residual with optimiser;
  10. Use clarifier;
  11. Manage water balance;
  12. Use clarifier.

Follow label instructions for application of all products.


Caution needs to be taken when draining pools at this time as the flood water has made ground conditions around the foundations of the pool very wet. Pools should not be drained until ground conditions have dried to at least the  depth of the pool as the entire shell of the pool (fibreglass or concrete) can “pop” out of its original position with the change in hydrostatic pressures.

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