Last Wednesday members of the water group of GreenTown Los Altos toured the Silicon Valley Advanced Water Purification Center, which processes waste water to levels of drinking water. Currently the processed water is used for landscaping and air conditioning, but plans are afoot to inject the treated water into sources of Silicon Valley’s drinking water. From the Center’s website,
The $72 million state-of-the-art facility receives secondary-treated effluent from the neighboring San José-Santa Clara Regional Wastewater Facility and purifies it to a very high quality using proven purification processes—microfiltration, reverse osmosis and ultraviolet disinfection.
The result is 8 million gallons a day of highly purified water that is expected to match California drinking water quality standards.
Shown in bottom right corner of this view from Google Earth, the Silicon Valley Advanced Water Purification Center is across the street from the San José-Santa Clara Regional Wastewater Facility, at the south end of the San Francisco Bay. The Wastewater Facility “cleans Silicon Valley’s wastewater to very high national standards”. The Wastewater Facility treats an average of 110 million gallons of wastewater a day, with most of the treated wastewater flowing through the dark green channel to the Bay.
Completed last year at a cost of $72M, the Purification Center is elevated ten feet above the surrounding area to protect it from flooding due to events such as storms and sea level rise.
Three intake pumps take water from the Treatment Plant (shown in the background of the pump photo) and force the water through strainers to screen out larger particles. Chloramine, a disinfectant used to treat drinking water, is added.
In the first step of the advanced water purification process, “water is pumped through tubes filled with tiny membranes. Each membrane is made up of hollow fibers, perforated with holes 1/300th the width of a human hair! Solids, bacteria, protozoa, and some viruses are removed from the water as it is drawn through the tubes.” They backwash the fibers periodically as the membrane in fibers get clogged.
The second step is reverse osmosis, where microfiltered water “is forced under high pressure through membranes with holes so small that a water molecule is almost the only substance that can pass through. As a result, constituents such as salts, viruses, and most contaminants of emerging concern (e.g. pharmaceuticals, personal care products and pesticides) cannot pass through the membranes and are left behind.”
The final step is ultraviolet light, where “the water is sent through ultraviolet light to break down any remaining trace organic compounds. Ultraviolet light is a powerful disinfection process”. Water is piped through two sets of ultraviolet light.
short-wavelength UV (UVC) considered “germicidal UV”. At certain wavelengths, UV is mutagenic to bacteria, viruses and other microorganisms. Particularly at wavelengths around 250nm–260nm, UV breaks molecular bonds within microorganismal DNA, producing thymine dimers that can kill or disable the organisms. It is a process similar to the effect of longer wavelengths (UVB) producing sunburn in humans. Microorganisms have less protection from UV and cannot survive prolonged exposure to it.
There is a pilot project to enhance the UV treatment.
Process control is required to ensure that the above water process is executed. A flat panel shows the overview for the microfiltration process.
Overall, I was very impressed with the tour and process. The Center manager, a civil engineer, led the tour. She’s an excellent presenter and very knowledgeable. I like that the three steps (filter, filter, and light) are relatively passive, with less chance of process control failure.
But process failure is always a concern. I asked about a microfiltration failure, where a fiber breaks and lets unfiltered water pass. The manager said that they monitor the water flow daily. If a leak is detected, they can plug the fiber. They can plug up to 15% of the fibers and remain within spec. They’ve had (detected) no fiber failures so far!
Beyond potential process failure, a second concern is that this advanced water purification process is advanced and new — world-wide, only about a dozen cities use it. Wastewater can contain all kinds of stuff, including carcinogens from medications and industrial waste. Is there long-term experience and medical studies to know that this advanced process takes care of everything in the water, so that we understand all the carcinogens and how to neutralize them? Until this is resolved, I’d feel safer if we take a conservative approach and let the treated water percolate through the ground to our aquifer, rather than injecting the water into the aquifer through wells.