Scientists from the Technion - Israel Institute of Technology and Ben-Gurion University of the Negev have proven the efficiency of gray water purification systems and that people should be encouraged to use such water.
The project is one of the most comprehensive ever undertaken anywhere in the world to study the re-use of gray water - water that is generated in homes in laundry, dishwashing and bathing and can be recycled to irrigate gardens.
The study found that in most cases the quality of the water stored after treatment was very high. Among the categories checked were clarity, quantity of solids and concentrations of bacteria, which dropped after the addition of disinfectants, chlorine tablets and ultra-violet light.
An average household system supplies between 100 and 120 liters of water a day for irrigating the garden.
The findings have been presented to the Health Ministry, which, however, refuses to change its stance against the use of gray water in private homes. The ministry wants to see additional research on the health implications of recycling such water.
Use of gray water purifications systems are on the rise in Israel are are believed to be in use in 10,000 households. However, they are not approved by the Health Ministry, which is concerned about breakdowns in the system, and the ministry has issued no standards for use of such systems.
The new study, which was underway for nearly two years, was carried out by Prof. Amit Gross of Ben-Gurion's Zuckerberg Institute for Water Research, and Prof. Eran Friedler of the Faculty of Civil and Environmental Engineering at the Technion.
The study was funded by businessman Maccabi Carasso, who recently founded the Coalition for Gray Water Recycling in Israel, encompassing water experts and other scientists.
The study followed up 20 homes in various parts of Israel that used a gray water irrigation system developed in the Zuckerberg Institute. The system consists of two plastic tanks, one for purifying the water and the other for storing it. In case of a breakdown, the water flows to the central sewerage line instead of to the garden.
"We found that the system provides not only good-quality water, but the number of breakdowns is small and even when a breakdown occurs, the water does not reach the environment," Friedler said, adding that the soil itself was also checked and found not to have been harmed. "There were some cases in which water quality did not meet Health Ministry requirements and it is not at all certain that this was because of the gray water. It must be remembered that this can happen in any system," Friedler said.
The Health Ministry said in a statement that it was "in dialogue with the researchers," adding that they had held a meeting on the matter just last week. The Health Ministry said the research and the method have not proven that there is no risk in gray water irrigation, and that is why the ministry does not authorize such systems for private homes.
The ministry said it proposed that the researchers undertake a health-risk assessment compared to non-irrigation and irrigation with purified waste water throughout the country. "The researchers accepted the proposal and we will continue discussing the subject," the ministry said.
Friedler said the Health Ministry's concerns were "not unreasonable." However, he said they were being "a little strict in their quality demands" and that in a study of scientific literature no evidence of damage to health using gray water was revealed.
In recent years the Health Ministry has allowed the use of gray water recycling systems in public buildings, after demands were met for supervision and monitoring to prevent breakdowns. For example, some of the grounds of the sports center in the eastern coastal plain city of Shoham are irrigated with gray water.
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