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How Filtration Works

Filtering water is not as simple as separating impurities from the water by running it through a filter. There are pretreatment steps before water sees the actual filtering process.

The first step is adding ozone gas for taste and odor control, disinfection, and as an aid to the next step: coagulation. Coagulation happens when chemicals added to the water are mixed with natural contaminants, causing the mixture to stick together.

The whole purpose of the plant is to remove particles like bacteria, dirt, river silt and other things that cloud the water. Some of the particles are so small they could pass directly through the filters, so we add chemicals to get them to attach to one another.

The water is mixed with paddles to create larger particles that become visible and filterable. That takes place in up to four “flocculation” basins, depending on how much water comes in from the river. Each basin can mix 42 million gallons of water per day, making the daily plant capacity 168 million gallons. Can’t picture that? It’s about what it would take to fill the Tacoma Dome.



The last step before the water reaches the filters is the sedimentation process. As the water moves through the sedimentation basins, particles drop down.

A feature that makes Water’s filtration plant unique is the option to skip the sedimentation process based on the quality of water coming into the plant. With that comes the option to reduce pretreatment chemicals and save on costs.



Finally! The water is ready to go through the filters. Eight concrete boxes totaling 13,440 square feet are filled with 50 inches of anthracite coal and 20 inches of sand for water to flow through, making it 50 to 200 times cleaner than the old process.

As the water exits the filtration system, it’s mixed with fluoride, sodium hydroxide for pH control and chlorine for disinfection.

After filtration, a 108-inch pipe carries the water to two holding tanks, or “clearwells,” that hold a combined 7.9 million gallons of water. The clearwells are filled and drained many times a day as water is supplied to the transmission system.


Controls & Solids

Filtration takes about an hour from start to finish. A crew of 17 operates and maintains the plant 24/7. Operators have their hands full, using 70 online instruments to monitor water quality indicators, and track over 5,000 distinct alarms.

What about all the material that’s filtered out? After going through various basins and pumps, the solid material goes through screw presses to remove any remaining water. Two large screws turn and squeeze out water that is returned to the filtration process. The resulting material – fill dirt – is sent through a chute into containers to be hauled away.

Tacoma is the first water plant of this capacity to use screw presses to remove water from solids.


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