The use of desalinated water has significantly improved the quality of effluent (treated wastewater), leading to a dramatic increase – from 32 percent to 80 percent – in the proportion of effluent that is suitable for all types of irrigation over the last decade. That is one finding in a report by Israel’s Water Authority on water quality in the country’s reservoirs of wastewater from 2006 to 2016. The main improvement was a decline in salinity.
The surveys of over 200 wastewater reservoirs were carried out by Israel Nature and Parks Authority employees between April and October, when the greatest use of treated wastewater for irrigation occurs.
Among other variables, concentrations of salt and nitrogen were measured. High salinity in water used for irrigation can damage crops directly as well as indirectly, by degrading soil quality. It can also increase the salinity of groundwater, potentially rendering it unpotable. For that reason, effluent may only be freely used for irrigation if the concentration of chloride (that is, salt) is below 250 milligrams per liter.
While in 2006 only 32 percent of the wastewater that was tested was found suitable for unlimited use in irrigation, a decade later that figure had increased to 80 percent. That, as a result of the increase in desalination during the period. In 2016, Israel used nearly 600 million cubic meters of desalinated water, up from 100 million in 2006. The disadvantage of using desalinated water for drinking is that it lacks essential minerals such as magnesium.
While salinity levels in effluent have dropped due to the increased use of desalinated water, excessive nitrogen levels are still a problem. The survey found that nearly half of the treated wastewater sources tested had nitrogen concentrations too high to allow for their unrestricted use in irrigation. The waste treatment plants are not yet capable of properly treating excessive nitrogen levels, a problem that should be ameliorated as the facilities are upgraded, within a few years.
Due to the prolonged water shortage in the region, wastewater treatment for purposes of irrigation is critical for continued agricultural development and reduction in the contamination of groundwater and rivers. Israel holds the world record for reusing domestic wastewater, 86 percent of which is treated and recycled for irrigation. In second place is Spain, which treats and recycles just 20 percent of its domestic wastewater.
Advanced treatment technologies remove most organic contaminants from the wastewater, but not salts. Salts are added to wastewater in the course of some household and industrial uses, in the form of foods and cleaning products. In addition, Lake Kinneret, once an important source of water for irrigation throughout Israel, has relatively high salinity.
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According to the new report, in the past two decades 100 effluent reservoirs were established in Israel, allowing for the addition of 200 million cubic meters of treated wastewater every year (equal to the output of two large water desalination plants). The area of cultivated land irrigated with effluent also increased during this period.