Backwash water may be heavily contaminated with pathogenic micro-organisms and harmful chemicals. Backwash water should never be reused or recycled without adequate treatment.
Only granular (sand) filters require backwashing. Regular backwashing of filters in accordance with the manufacturer's specification is essential for proper cleaning, maintenance and operation of pool filters. Swimming pool backwash wastewater consists of all of the pollutants that are filtered from a pool. Most pollutants are introduced into the pool from bathers. Backwash water may be heavily polluted with a wide range of pathogenic micro-organisms and chemicals.
Usually the wastewater generated from backwashing is discharged to the sewerage system in accordance with a trade waste agreement with the local sewerage authority. As water is scarce in many areas of Australia, treated backwash water is reused for cleaning and irrigation purposes and, if highly treated, for recycling into the pool and for toilet flushing.
Under no circumstances should backwash wastewater be directly discharged to the environment or to the stormwater system. The wastewater is extremely harmful to the environment and promotes weed growth in natural bushland areas.The backwash wastewater may be suitable for reuse on parks and gardens or for dust suppression on road works if properly assessed and pre-treated. Untreated and un-disinfected backwash wastewater must never come into direct contact with people.
The reuse of backwash wastewater must be fully assessed and a water reuse plan developed. A health risk assessment should be performed using Environmental Health Risk Assessment - Guidelines for assessing human health risks from environmental hazards (Updated 2012) .
Issues to consider in the health risk assessment include:
To conserve water, recycling of treated backwash water to top-up swimming pools and spa pools is supported provided the backwash is treated to an acceptable standard and controls are in place to protect public health.
Recycling swimming pool backwash water involves treatment to a suitable standard to allow recycling into the pool. As pool water will be accidentally swallowed, the quality of recycled backwash water should meet the Australian Drinking Water Guidelines (Updated 2012)  and controls need to be in place to protect against system failures. Any deviations from these guidelines should be supported by a health risk assessment.
The National Water Quality Management Strategy, Australian Guidelines for Water Recycling: Managing Health and Environmental Risk (Phase 1)  should be used as a guide. Environmental Health Risk Assessment - Guidelines for assessing human health risks from environmental hazards (Updated 2012)  also includes useful information.
Appendix C of the NSW Ministry of Health, Public Swimming Pools and Spa Pools Advisory Document December 2012 provides an example of some of the components to consider for recycling of swimming pool backwash water based on Australian Guidelines for Water Recycling: Managing Health and Environmental Risk (Phase 1) .
Reverse osmosis is presently the best available technology for the treatment of backwash water for recycling. Pre-treatment using ultra-fine filtration and granular activated carbon may be necessary to prolong the life of the reverse osmosis membrane.
Reverse osmosis has been shown to remove the majority of dissolved contaminants (more than 99.5% dissolved salt and up to 97% of most dissolved organics), and 99.99% of micro-organisms. However because of the high set up, operational and maintenance costs, the cost of recycling backwash may outweigh any benefits. Reverse osmosis should be installed with supporting treatments to greatly enhance its efficiency. The advice of a consultant in designing a recycling plant is essential.
(Developed for use with Australian Guidelines for Water Recycling: Managing Health and Environmental Risk (Phase 1) (2006)
For further information in NSW call 1300 066 055 to talk to your local Public Health Unit.