From River to Tap HOW WE GET OUR DRINKING WATER
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n Sacramento, we live on the banks of two beautiful rivers. Maybe you’ve gone swimming in one of them, or boating or fishing. But have you ever drunk the water? Don’t be too quick to respond. The answer is almost certainly yes. The American and Sacramento rivers provide about 80 percent of the tap water to the city of Sacramento. (Residents of the county also consume surface water from the rivers, but the exact mix of groundwater versus surface water varies depending on where you live.) The water, of course, is not piped directly from the river to your home. Despite a current fad for “raw water” among a fringe of believers, drinking straight from the river would put you at risk of waterborne infectious diseases. River water is treated first to make it clean, clear and safe. The city’s Department of Utilities runs two surface-water treatment
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plants. The Sacramento River plant, located near Richards Boulevard, began operating back in 1924, and a major rehabilitation was finished in 2017. The American River water treatment plant, named E.A. Fairbairn, went live in 1964. This conventional water treatment facility is notable for the large and stylish intake building that’s clearly visible on the river near Sac State. Brett Ewart, a senior engineer for the city, gave me a tour of the Fairbairn plant. As I admired a kingfisher along the riverbank, Ewart explained why the intake structure is so big. “The intake can house up to eight pumps that lift the water up from the river. Flow through the treatment plant then works by gravity,” he said. “At maximum capacity, we can process 200 million gallons of water a day here.” If all that water entered through a single intake of modest size, the strong sucking pull would be dangerous. Instead, the intake was designed with a large surface area for the river water to flow into.
This keeps the velocity of the water low and protects the river’s fish—in particular, the American River’s fragile populations of migratory salmon and steelhead. The intake pumps themselves are covered by fish screens fine enough to exclude juvenile fish and allow them to swim away on their journey to the ocean. Raw water then flows downhill to the first phase of the treatment process. In a giant settling tank or “grit basin,” the water is allowed to stand rather still to allow larger particles of debris and sand to drop out and settle on the bottom. Next, the water flows through inlet meter pipes that measure how much water has been extracted from the river. Samples are taken continuously to monitor the ever-changing turbidity and composition of the river. Most samples are analyzed at the on-site laboratory. At this point, the water is still full of suspended particulates. In the next treatment phase, those tiny particles are encouraged to clump and fall out of solution by adding coagulants to the water. The silt and clay that make up a lot of the suspended material generally carry a negative charge, so the particles repel each other. By adding a positively charged coagulant such as alum (aluminum sulfate), the charge repulsion is overcome and the particles can clump together. This happens with slow, gentle mixing as the water passes through a serpentine path of flocculation tanks. Once the suspended material in the river water has flocculated into heavier clusters, the clusters (and the added coagulants) are allowed to settle out of the water in a wide, calm sedimentation basin. I could easily see
how the water progressed from cloudy to clear as I walked from one end of a pathway above this basin to the other end. Still moving with the flow of gravity, the visibly clear water now enters the filtration stage. Ewart pointed out that with groundwater, this process happens naturally. At the plant, water sinks down through a bed of anthracite and sand layers in a gradation of sizes that get finer as you go down. Now the turbidity is gone and many microbial contaminants have also been removed because they cling to particles in the water. To kill any lingering hazardous microorganisms (bacteria, viruses), the water is chlorinated for a period of time. Finally, fluoride is added for dental health in the community, and the water’s pH is balanced with lime in order to protect pipes and waterdelivery infrastructure from too much acidity. A final set of pumps pressurizes the water to drive it directly to the taps of half a million people, or to one of 11 giant freshwater storage tanks such as the Alhambra Reservoir. You’ve seen this tank while driving on the Cap City Freeway, with its blue-lit artwork representing the grid of city streets and the river flowing in. Sacramento’s drinking water meets or exceeds government standards. Wondering what that means, exactly? Consumers get an annual waterquality report on this data sent to them every June, or they can view the reports at the city’s utility website (cityofsacramento.org/utilities). Coming April 1, expanded Science in the Neighborhood columns will be a book! Visit amyrogers.com for details. n