A view above sedimentation basins, where the water is clarified as heavy particles settle. Photo by Gabe Rivin
A view above the sedimentation basins at the Orange Water and Sewer Authority, where the water is clarified as heavy particles settle. Photo by Gabe Rivin

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By Gabe Rivin

The holiday season is in full swing, and many North Carolinians will soon hop on a plane for a much-needed vacation.

For some people, those planes will land in countries without guaranteed access to clean drinking water.

That’s something I’m thinking about. As I did last year, I’m traveling to Mexico over the holidays. The beach awaits. But if the trip is anything like last year’s, some moderate portion of my food expenses will go toward bottled water. No skimping with regular tap water – even at restaurants.

The reason? I’m trying not to relive last year’s “intestinal adventure.”

This seemingly small convenience – clean tap water – is by no means a worldwide norm. Which got us at N.C. Health News thinking: How do we get our drinking water? What coordinated dance of technology and people allows us to plunge our heads under kitchen taps and drink water with abandon?

See our related story: The Other Side of Thanksgiving.

Ken Loflin, the water supply and treatment manager for Orange Water and Sewer Authority at a drinking water plant in Carrboro, explained where the town’s drinking water starts and how it ultimately travels into kitchen sinks and shower heads.

Cane Creek Reservoir, a source of drinking water for Carrboro and Chapel Hill. Photo courtesy OWASA

Step 1: Get your raw water

Drinking water begins with a large source of untreated, raw water. Carrboro and Chapel Hill get most of this water from two sources: University Lake, a manmade reservoir that can store 450 million gallons of water, and Cane Creek Reservoir, with a capacity of 3 billion gallons.

In November and December of this year, the average demand for water topped 6 million gallons per day. But thanks to this year’s above-average rainfall, there are more than 500 days of water left for drinking.

Both reservoirs were originally tributaries of larger, running bodies of water, Loflin explained. But by damming these tributaries, Chapel Hill and Carrboro gained large and reliable sources of drinking water. These reservoirs continue to be replenished by rain.

Not all towns and cities get their raw water from reservoirs. Some, such as Eden, draw water straight from a major river.

Others use wells to draw water from aquifers. There are thousands of water systems in the state, many that service only a few dozen homes. Nonetheless, many North Carolinians – about 30 percent of households in the state – get drinking water from their own private wells, and are off municipal grids.

Step 2: Get pumped

OWASA pumps this raw water through pipes, which ultimately lead to the utility’s drinking water plant.

Raw water is pumped to drinking water treatment plants, which ensure the water in residents’ taps is clean. Photo credit: Gabe Rivin

The pumps are no small feat of engineering. Cane Creek Reservoir can pump up to 10.5 million gallons of water each day and University Lake can pump up to 18.5 million gallons a day.

Step 3: Flash mixing

That raw water arrives en masse from OWASA’s reservoirs and enters the drinking water plant. Its first stop is the flash mix.

In this covered chamber, OWASA adds several chemicals. One is ferric sulfate, which acts as a coagulant to clump together dirt and other particles. OWASA also adds activated carbon, which improves the water’s taste and smell.

The process lives up to its name. Powerful paddles stir up the water with the coagulant chemicals to even distribute them, but the process only takes about a minute.

Step 4: Mix it in the floc basin

The chemicals have been added. But the water is still murky and full of particles.

That’s where flocculation, or “floc,” comes in.

A view above sedimentation basins, where the water is clarified as heavy particles settle. Photo credit: Gabe Rivin

After the flash mix, water is transferred to a floc basin. You can think of it as a giant, gentle food processor.

In the bottom, the floc basin has a set of paddles that slowly rotate. This mixes the water and causes particles to stick together. As water flows from one basin to the next, each compartment has a different mixing speed, which decreases as water flows from the top of the basin to its bottom.

Eventually, this process makes the water clearer and the dirt and debris more concentrated.

Step 5: Sedimentation

The water is then transferred to sedimentation basins. These are rows of wide, rectangular pools. When the water enters these basins, it’s still somewhat murky. But after about seven hours, that’ll change.

The particles, which are now heavier and clinging together, begin to settle at the bottom of the basins. The water grows a notch clearer.

For about half its water, OWASA does use another technology for this step and step 4. The technology, known as a Superpulsator, takes advantage of the fact that sediment sinks. The Superpulsator uses vacuum pumping and gravity to concentrate the sediment into a sludge, which can be removed more quickly.

Step 6: Filtration

They’re bigger than Brita filters, but they accomplish a similar task. These are OWASA’s filters.

After the heaviest particles are taken care of, the water flows into a set of deep, square basins. These basins are lined at the bottom with anthracite, which is essentially activated charcoal, which binds up chemical impurities in the water. There’s also sand, which acts as a filter.

Gravity allows the water to pass through these filters, catching the remaining fine particles.

The water’s now clear.

Step 8: Preventing corrosion of pipes – and erosion of human teeth

OWASA adds fluoride to municipal water to prevent cavities in people’s teeth. Photo credit: Gabe Rivin

Once it’s been filtered, the water passes through a series of pipes in which OWASA adds several additional chemicals. One is a blend of polyphosphates and orthophosphates, which prevent corrosion in municipal and residential pipes.

Another additive is fluoride, used to prevent cavities in teeth, added to the concentration at 0.7 parts per million, the equivalent of about four drops in a bathtub completely filled with water. That’s right in the middle of the range of 0.5-1.0 parts per million recommended by the World Health Organization.

Step 9: Time to fend off bacteria

The water then travels to another chamber, where OWASA adds sodium hypochlorite. If the name of the chemical doesn’t ring a bell, take a look at a bottle of chlorine bleach. It’ll likely be listed as the main ingredient.

Liquid bleach at the supermarket contains several health warnings. But according to the Centers for Disease Control and Prevention, low levels of chlorine will disinfect water of harmful pathogens. And at these levels, chlorine does not threaten human health.

In this step, the water and sodium hypochlorite sit together in another basin for up to four hours, allowing the chemical to do its work.

Step 10: Into the grid it goes

Once the water is free of particles and pathogens, large pumps send it into the municipal grid. Photo credit: Gabe Rivin

Once the water has been disinfected of pathogens, it is ready to be pumped into the municipal grid. But before doing so, OWASA adds a small amount of additional sodium hypochlorite, which prevents bacterial buildup in pipes. OWASA also adds ammonia, which prevents the formation of disinfection byproducts such as trihalomethanes, potentially carcinogenic compounds. That happens for 11 months of the year, and in the twelfth month, the state requires using only bleach, which disinfects the entire system.

From here, the finished, clean water is pumped through pipes into the large municipal grid.

Some residential pipes are connected directly to OWASA’s 380 miles of distribution pipes. Others get their water from elevated storage tanks. These towers not only store and distribute water to residents, they provide water for firefighters.

Clean water isn’t very sexy. And as far as holiday gifts go, it’s certainly not as flashy as a new car. But for travelers who’ve spent their vacations on a toilet, debilitated by a stomachache, its importance is easy to remember.

Rose Hoban contributed reporting and writing to this story.

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Gabe Rivin

Gabe is our former environmental health reporter from 2014-2016. He is a former editor of The Cooperative Business Journal, and a former reporter for Inside Washington Publishers, where he covered federal...