Walla Walla wastewater goes down the drain, and then?

Walla Walla's wastewater treatment process takes about a day for any given flush, resulting in clean water to put back in Mill Creek.



In front of a near 1,000,000 gallon skimmer tank full of microscopic sewage consuming animal life, CH2MHill Operations Supervisor Paul Olson (far right) talks about the filtering process at the City of Walla Walla Waste Water Treatment Plant. Thursday, May 13, 2010


Paul Olson holds a ball of rich composting material derived from what used to be sewage at the City of Walla Walla Sewage Treatment Plant. Olson said the compost is perfect for local farmers but may still make you sick without washing your hands after handling it. Thursday, May 13, 2010


CH2MHill Operations Supervisor Paul Olson pulls a long UV light tube from a full bank of tubes while talking about how hundreds of UV lights are used to help clean and clarify waste water at the City of Walla Walla Waste Water Treatment Plant. Thursday, May 13, 2010

WALLA WALLA -- Do you ever wonder what happens to the water in the toilet after you flush it? We often do not think about the people and the processes that work day in, day out to make the essential service of wastewater treatment possible.

The completion of various construction projects in 2009 has kept the Walla Walla Wastewater Treatment Plant up-to-date with technological advances, a characteristic the plant has boasted since its founding in 1927.

In 2000, the Walla Walla City Council approved a contract for CH2M Hill OMI to begin managing its wastewater facilities. According to Project Manager William Breshears, the privatization has made the plant more efficient. Among the changes, the plant went from running two daily 10-hour shifts to one 8-hour shift.

"We're very cost effective ... in the way we operate," Breshears said. "We have access to a lot of specialists outside of our facility that the city wouldn't have because we're part of a larger entity with people that specialize."

Paul Olson, the plant supervisor, has worked at the plant for 25 years, and he knows it inside and out. If something goes wrong, he is immediately alerted, even if he is at home. He and the 12 other plant employees test the water throughout the system multiple times a day to ensure it is meeting the city's -- and their own -- standards of cleanliness.

Wastewater enters the treatment plant at the "headworks," where it undergoes a screening process in which inorganic materials are removed. Water then proceeds to the primary clarifiers, large tanks that filter up to 60 percent of the waste through the physical process of sedimentation.

According to Olson, some newer wastewater treatment plants skip this first step and move straight into the biological, secondary treatment, but because secondary treatment is far more expensive, the Walla Walla plant is able to save money and conserve energy by continuing the use of primary treatment.

"We try to save electricity where we can," Olson said. "But we don't do anything that will make the water quality less."

Until 1999, the plant used three trickling filters for secondary treatment, in which water is sprayed over large tanks of rocks, on which microbes grow and consume wastes in the water. Last year, construction was completed on an activated sludge system, and the plant now uses only one trickling filter.

In activated sludge treatment, the water is aerated in large tanks, and bacteria consume solid and dissolved waste in the water. Activated sludge tanks are more expensive to maintain than trickling filters, but they are a more effective means of waste removal and function better through different weather conditions.

After secondary treatment, the water moves to the 1-million-gallon secondary clarifier tanks, the components of which Olson likes to call "vacuum cleaners" because of their ability to remove solids and microbes.

The water is filtered, and then sent through ultra violet treatment -- a process started at the Walla Walla plant in 2005 to replace the sole use of chlorine, which in large quantities can upset ecosystems. (Chlorine is still used in lesser quantities to kill additional pathogens and bacteria.) Water comes in contact with UV lights that sterilize bacteria.

The entire process of water treatment takes approximately one day. Currently, the plant treats 5.5 million gallons of water a day, though it is designed to treat up to 9.6 million gallons.

From Dec. 1 to May 1 of every year, the treated water goes directly into Mill Creek. During the remaining months, the clean water is used for irrigation.

The solids removed through the various processes are treated in a heated anaerobic digester for a minimum of 15 days, a process during which pathogens are removed. Methane gas released from the breakdown of the solids is recycled into the system and used to power the generators that heat the digester. The treated solids from the process are organically rich and are recycled as safe and effective fertilizer for nearby wheat fields, officials say.

Annually, the treatment plant produces approximately 500 dry tons of these valuable solids.

The plant has standby generators, which keep it running in the case of a power failure. And if something is wrong with the water, it can be pumped into a constructed lagoon, keeping it from being dispersed to the irrigation system. The lagoon can hold three to four days worth of water.

More than $30 million has been spent on construction at the plant in the last decade. Recent upgrades have ensured that the water put back into Mill Creek and sprayed onto the fields is clean.


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