By Année Tousseau, June 2018
One of the most essential components of Silicon Valley’s centralized recycled water network is the Advanced Water Purification Center operated by Santa Clara Valley Water District (SCVWD).
At SSV’s Navigating Bay Area Water event on May 31, 2018 in Palo Alto, Dr. Hossein Ashktorab of SCVWD gave attendees a look at the state-of-the-art purification plant and explained the philosophy behind wastewater reuse. The district also brought samples of the purified water for participants to taste.
SCVWD’s Water Purification Center is the largest advanced water purification plant in Northern California. It takes in wastewater that has already undergone a two-step treatment process at a neighboring wastewater treatment plant. This treated wastewater would normally be discharged back into the Bay, but the Water Purification Center treats it further. Currently, the purified water is used only for nonpotable purposes—for things like irrigation and cooling industrial facilities, but not for drinking. Every day, the plant produces up to 8 million gallons of purified water that is then mixed with existing recycled water and piped out to customers around the valley via the purple-pipe network.
The driving philosophy behind the Water Purification Center is the idea that wastewater can be cleaned and reused as a sustainable, drought-resistant water supply.
“There’s a lot of water in wastewater that’s available to us, and we can use it as a new water supply. It’s drought-proof and locally controlled—it’s already here,” Ashktorab said.
Safety and taste
Though the purified water is not yet used for drinking, rigorous testing shows that it exceeds federal and state drinking water standards. Over a 15-month period, 284 different constituents in the water were tested every three months, for a total of 4,000 water quality parameters collected and analyzed.
But how does it taste? At Navigating Bay Area Water, attendees clustered around the table where the district handed out samples. A few sips demonstrated what the test results prove—the purified water tastes clean, refreshing, and neutral, without any hint of minerals found in some tap water. If you didn’t know where it came from, you’d probably guess it was bottled water from the store.
Why purify water?
Purified water is a water source that’s hiding in plain sight. In 2016-2017, we discharged as much as 510 million gallons of treated wastewater into the Bay every day, according to the Bay Area Clean Water Agencies. But now that we have the technology to purify much of this water and make it useful again, this may be more wasteful than we originally thought.
Orange County, for example, has been using recycled water to augment its groundwater since 2008. The county’s water and sanitation districts operate the world’s largest water recycling system for indirect potable reuse, which means that the recycled water is ultimately used for drinking, but not directly. After undergoing a similar microfiltration, reverse osmosis, and UV light purification process as used at the Silicon Valley Advanced Water Purification Center, the purified water is transported to percolation basins, where it eventually seeps underground into aquifers that Orange County uses for drinking water.
Recycled water has advantages over other alternative water sources. For one, it requires less energy to purify wastewater than seawater. Saltwater has TDS concentrations of 35,000-40,000 mg/L; the treated wastewater that flows into the Silicon Valley Advanced Water Purification Center has TDS levels of only 800 mg/L. Desalination requires much more energy and effort to bring that number down to an acceptable, drinkable level. In addition, at the Water Purification Center, the reverse osmosis pumps require 190 pounds per square inch (psi) of pressure to do their job; desalination can require up to 5,000 psi of pressure.
Climate change and population growth are also putting pressure on our current water supplies. Silicon Valley imports up to 55% of its water. The cost of that water is rising, and its long-term reliability is in question, as climate change will likely reduce the Sierra Nevada snowpack. Meanwhile, the population of the Bay Area may increase by as much as 24% in the next 25 years.
Luckily, according to a 2014 report by the Pacific Institute, there is a “tremendous” opportunity to increase water recycling in California. The report estimates that some 1.2 million to 1.8 million acre-feet of water could be reused each year, in addition to the 670,000 acre-feet of municipal wastewater that’s already being reused.
Indirect and direct potable reuse
Right now in California, recycled water can only be used for nonpotable (nondrinking) purposes and for some kinds of indirect potable reuse—like what Orange County is doing with their groundwater replenishment system.
SCVWD has similar plans to use purified water to boost groundwater supplies. The potential first phase of a project would increase the Water Purification Center capacity by up to 24,000 acre-feet/per year by 2025 and build a conveyance pipeline to transport purified water to the Los Gatos Recharge Ponds.
However, the state is now developing regulations to permit direct potable reuse. The State Water Resources Control Board has prepared the Proposed Framework for Regulating Direct Potable Reuse, a document that reflects the board’s current thinking on how to regulate this new water resource. Public information workshops were held in April in northern and southern California, and the document and a fact sheet can be downloaded here.
If direct potable reuse is allowed, SCVWD could blend purified water with imported water before sending it through the district’s conventional water treatment plants. See the graphic below.
SCVWD offers free public tours of the Advanced Water Purification Center, which is located on Zanker Road in north San Jose. See http://purewater4u.org/ to sign up for a tour.