In a bid to rapidly expand recycled water use in the northern parts of the city, Mountain View City Council members voted unanimously Monday night to join a regional partnership to construct a $20 million water purification facility. Once built, the city is expected to ramp up recycled water use from 400,000 gallons per day to 1 million.

The three-way deal between the Santa Clara Valley Water District and the cities of Mountain View and Palo Alto tackles a long-standing problem that has rendered much of the North County's existing recycled water supply unusable. Water treated at Palo Alto's Regional Water Quality Control Plant is too salty for irrigating certain types of landscaping, particularly sensitive species like redwood trees.

In order to use more of the recycled water, city officials say the salinity -- measured in total dissolved solids present in the water -- must be reduced to 600 parts per million (ppm). Despite multiple improvements designed to meet that goal from 2013 to 2016, it never got below 700 ppm, and the recent drought conditions only made things worse. At the last check, the plant's recycled water was measured at 835 ppm.

What that means is that most of the treated water is simply getting dumped. Mountain View is contractually allowed to use 3 million gallons of recycled water per day, but it only averages about 400,000 gallons. About 95% of the treated water is discharged into the San Francisco Bay.

Under the terms of the agreement, which council members swiftly approved at the Nov. 18 meeting with little discussion, the water district will spearhead the design and construction of a new advanced water purification facility -- in a yet-to-be-determined location in Palo Alto -- aimed at getting around the salt problem.

The facility will be able to "highly treat" more than 1 million gallons of water per day and bring salinity down to a mere 25 to 50 ppm, which will be diluted with the regular recycled water and be good enough for toilet flushing, industrial cooling towers and irrigating landscaping at parks, schools and golf courses.

Pat Showalter, a former council member and an engineer who previously worked for the water district, hailed the plan as way to steel the city against future droughts by cutting down on the finite amount of drinking water that's currently being used for irrigation in lieu of recycled water.

"In trying to figure out the best way to plan for climate resilience and water supplies, one of the ways is to come up with drought-proof supplies," she said. "And that's really what this is. We're taking our waste water and we're purifying it so much that we can use it in our recycled water supply."

Building the new purification facility is also a big deal for Mountain View as it prepares to rapidly redevelop North Bayshore, which will be an ideal place to use the new source of cleaner recycled water, Showalter said. The area was recently rezoned to allow up to 9,850 housing units and millions of square feet of additional offices, and is already outfitted with recycled water pipes that can circulate water in areas ranging from Shoreline Park to the north all the way down to La Avenida Street near Highway 101.

"We're not using nearly as much recycled water as we have now, but with that expected development we'll be much closer to it," Showalter said.

In addition to North Bayshore, the city will be providing recycled water to Google's Bay View development east of Stevens Creek Trail, a 1.1 million-square-foot office campus currently under construction. Starting in 2021, the project is expected to use 250,000 gallons of recycled water per day.

The city is also planning to expand the recycled water system into the East Whisman area of Mountain View, extending the network south of Highway 101 to support future development in the area. The council signed off on plans earlier this month to rezone the area for 5,000 additional homes and 2 million square feet of offices. In the case of both North Bayshore and East Whisman, all new construction is required to have dual plumbing to support recycled water use.

The Palo Alto City Council voted to approve the plan at its Nov. 18 council meeting, and the Santa Clara Valley Water District's Board of Directors is scheduled to consider final approval of the deal at a Dec. 10 meeting.

The advanced water treatment facility is expected to cost $20 million, of which the water district will pay $16 million, while Mountain View will pay $3 million and Palo Alto will pay $1 million. City officials are already looking at ways to defray the costs with grant money. In exchange, the city is agreeing to sell its treated wastewater, or effluent, for the water district to use at the purification facility or for "other beneficial uses" elsewhere in the county. Mountain View city staff said this shouldn't be a problem, as long-term projections show the city has adequate water supplies to grow through the year 2045 without access to the effluent water.

Though construction on the advanced water treatment plant is slated to begin in June 2021, the agreement gives the water district a lengthy 13 years to complete construction of the facility. If the facility is not constructed within that time frame, the water district has an extra 10 years to identify "alternative beneficial uses" for regional wastewater, according to a city staff report. The full term of the agreement is 76 years.

A 2016 analysis found that recycled water accounts for less than 5 percent of the city's total water consumption. The vast majority of the city's daily water is instead provided by the San Francisco Public Utility Commission (SFPUC), which draws water from the Tuolomne River. The new water purification plant is expected to reduce the city's reliance on SFPUC water, an important step toward preparing for future droughts and water supply shortages caused by unforeseen circumstances like an earthquake or regional power outage.