Wastewater Treatment in the Walla Walla Area

By Ethan Aumann  

Walla Walla, Washington

Background Information

            Generally speaking, sewage is the wastewater from all of our local sources that is carried off in sewers or drains.  These sources include residences, industries, businesses, and small amounts of groundwater that infiltrate, or seep into, sewer system pipes.  For residents of Walla Walla, the materials rinsed down a drain or flushed down a toilet enter the sewage collection system.  The Street Division of Public Works is responsible for maintaining the two lift stations and approximately 150 miles of pipe throughout the city.  The Walla Walla Wastewater Treatment Plant is the ultimate destination for this sewage. 

            The treatment plant is located along Mill Creek, at 571 Hatch Street, just northwest of the Blue Mountain Mall.  It is classified as an advanced secondary treatment plant with activated sludge.  Operations Management International, Inc., is under contract with the City of Walla Walla to operate the plant.  It is staffed 365 days a year and is regulated by the following organizations: the Environmental Protection Agency, the Washington State Department of Ecology, the Washington State Department of Health, and the Walla Walla County Health Department

            Wastewater treatment is a multistage process to purify sewage before the water is reused or enters another reservoir.  While the exact steps may differ from site to site, the ultimate goal of any plant is to remove solids, nutrients, metals, organic material, and other pollutants.  The final solid product may also be recycled in the form of agricultural fertilizer.  The Walla Walla treatment plant processes approximately six to eight million gallons of wastewater everyday. 


The Wastewater Treatment Process

            The following outline summarizes the treatment process for effluent entering the Walla Walla Wastewater Treatment Plant.  This particular plant was chosen because it is the largest facility in the vicinity and it provides a variety of treatment methods for examination.  The process has been organized into five main sections: Preliminary, Primary, Secondary, Tertiary, and Solids Treatment.  Although water exiting the facility always meets or exceeds quality standards, it is important to stress that there is no single treatment sequence that is used 100 percent of the time.  The treatment plant is capable of running any one of at least four different secondary treatment configurations.  Plant operators may occasionally modify the treatment steps in order to optimize the process as a whole.  For clarity, some of the less significant elements have been left out of the following descriptions.


I. Preliminary Treatment                           The headworks or preliminary treatment of incoming effluent.

            Another name for preliminary treatment is the headworks.  The headworks is the first station in the wastewater treatment process.  This concrete structure filters out the larger pieces of incoming effluent using two screens, and a circular pit, to remove grit and gravel.  Wood, rocks, pieces of food, and other screened materials are collected and sent to a landfill.  This step is essential in protecting the equipment in the rest of the plant.  The percentage of material removed is reported to the Department of Ecology – typically, domestic sewage is over 99 percent water. 




II. Primary Treatment  

            The primary treatment step includes two large circular settling ponds, each 90 feet in diameter and capable of holding 480,000 gallons.  Technically speaking, the role of this step is to clean out the “floaters and sinkers.”  The organic solids drawn off the bottom and skimmed off the top are termed sludge.  Raw sludge is collected and pumped directly to the anaerobic digesters (see Section V, Part A).  Approximately 10,000 gallons of sludge are pumped every day.  The wastewater is pushed to the outside of the pond and over the outer lip by a large revolving spray bar.  These clarifiers are designed to remove approximately 60 percent of the total suspended solids (TSS) and 30 percent of the biochemical oxygen demand (BOD) from the plant influent.  Biochemical oxygen demand is a measure of the strength of the wastewater.  More specifically, it is the amount of oxygen required by organisms to break down sewage in a specific amount of time.


One of the two primary clarifier settling ponds.

III. Secondary Treatment

A) Trickling Filters   

            These filters look similar to the circular settling ponds of the primary treatment step, except they are filled with large slime-covered rocks.  There are three of these 145-foot diameter filters filled with four feet of rock media.  More than just slime, the grimy material covering the rocks is actually composed of treatment bacteria.  This step relies on the aerobic and biological removal of unwanted nutrients. The water from the primary treatment ponds is sprayed onto the tops of these rocks by large revolving spray bars.  The bars depend on water pressure to keep them turning – a slower version of a typical lawn sprinkler.  The biological agents covering the rocks help to treat the water as it trickles down.

B) Trickling Filter Clarifiers                 

A view of a trickling filter.

            This is an optional step identical to primary treatment.  There are three of these older clarifiers (one 90 feet in diameter and two 60 feet in diameter), but they are seldom all used.

C) Aeration Basins                                                                                                                

            Wastewater is then pumped into a series of anaerobic, anoxic, and aerobic basins.  This is the only step in the treatment process that requires pumping – the others are dependent upon gravity to move the water.  There are two of these basin complexes, each comprised of two anaerobic selector cells, two anoxic selector cells, and a large aeration ditch.  Each cell has a 300,000-gallon capacity, and the aeration ditch holds 
1.4 million gallons.  Although two complexes were constructed during the recent        Two of the anoxic and anaerobic selector basins
facility upgrade,  only one is currently used.  The basins effectively speed up the treatment processes of nature.  Each step in the process is intended to select a predominance of certain bacteria and allow others to break down.  This system stirs and suspends microorganisms in wastewater.  As these microorganisms absorb other materials from the wastewater, they grow in size and eventually settle out as sludge. 

            Anaerobic basins contain very little or no oxygen and use Return Activated Sludge (RAS) to select particular bacteria.  They are intended to allow acclimated biological organisms to take up food (BOD) while releasing stored energy in the form of phosphate compounds.  
The organisms hold on to the food until it can be oxidized in the aeration ditch.  In the anoxic basins, organisms that have just absorbed incoming food (BOD) in the anaerobic basins are mixed with mixed liquor from the aeration ditch.  Nitrates in the mixed liquor are converted to nitrogen gas and water (denitrification) in the process of converting BOD to energy.  The anoxic selector basins contain some free oxygen, but too much will limit the effectiveness of the nitrates.  A number of low-power propellers work to keep the materials in suspension without adding oxygen to the mixture.

         The aerobic basins retain the biological organisms in an aerobic environment, allowing them to remove BOD and oxidize ammonia (nitrify).  Two 125 horsepower aerators add oxygen, while a pair of low-power banana shaped blades are used to keep the velocity of circling water at a certain level.  A jet engine-shaped device pumps the nitrogen rich mixed liquor back to the anoxic basins.



                        D) Activated Sludge Clarifiers                                                                                                            

            After leaving the aeration basins, wastewater progresses to one of three 100-foot activated sludge clarifiers.  These clarifiers look like the primary treatment clarifiers, but hold 960,000 gallons each.  The wastewater is detained, and sludge is allowed to settle out.  Suction is used to collect the sludge, which is then either sent back to the primary clarifiers or used in the aeration basins.  This type of sludge is termed return activated.  Excess sludge, known as Waste Activated Sludge (WAS), is sent directly to the digesters.




Wastewater trickles over the outer lip of the activated sludge clarifiers.

IV. Tertiary Treatment

A) Sand Filters

            Wastewater from the activated sludge clarifiers can then proceed to four gravity-flow sand filters.  The water filters through approximately 20 inches of sand media before exiting through an underdrain system.  Bacteria or other particles are caught up in the sand during the filtration in the form of colloidal particles.  Chlorine is added before wastewater enters the sand filters to help keep any residual oils or grease from building up and clogging the sand.  The chlorine also acts as a disinfectant.  Sodium hypochlorite, the chlorine disinfectant, is generated on-site in the solids building.           
These sand filters were originally designed to work with the trickling filters, before the activated sludge treatment process was added.  It is uncertain whether the sand filters will remain optional or be fully incorporated into the treatment system. 

B) Chlorine Contact Basin                                                   Chlorine contact basins, just before the water's exit from the treatment facility.

            The chlorine that was injected earlier in the treatment process is given ample time to react before the water is dechlorinated in the contact basin.  Sodium bisulfite is used as the dechlorination agent.  It is injected into the water just prior to the water exiting the basin.  

            Treated wastewater is then either discharged into Mill Creek, from December 1 through May 1, or sent to the Blalock and Gose irrigation districts the other seven months of the year.  This is in compliance with a 1927 court decree.  


V. Solids Treatment

A) Anaerobic Digesters         

            Sludge sent to the anaerobic digesters has a detention time of approximately 60 days.  During this time period, acid and methane-forming bacteria slowly eat away at the sludge.  Organic materials are broken down, and the overall volume of material is reduced.  The bacteria are extremely susceptible to changes in pH and temperature; therefore, these values must be constantly monitored.  Some of the methane formed in the digesters is used to run boilers that keep the temperature inside the digesters between 95 and 100°F.  A constant supply of methane is held inside a storage bubble to account for daily fluctuations in methane production.  Excess methane is flared off.

B) The Solids Building

            Solids from the digesters then proceed to the solids building, where the substance is dewatered.  The incoming material is composed of around five percent solids.  Polymers are added and the solution is run through a large-belt filter press.  The polymers help the solid material stick together.  A series of filters and large revolving cylinders gradually squeeze liquid out.  The water is recycled back into the wastewater treatment process.  The final solid product is around 15 to 20 percent solids.  It is dark brown and organic in composition.  This product is used as a fertilizer for local agriculture.   

           Bacteria break down sludge in these large anaerobic digesters.


Recent Renovation

            The Walla Walla Wastewater Treatment Plant completed an 18-month construction project in the summer of 2000.  The upgrade was intended to improve effluent quality, remove ammonia, and increase the daily treatment capacity.  Apollo, Inc., was the contractor in charge of the $16 million upgrade.  The plant was able to remain in full operation while the work was being done.  The construction project included the following elements:



            The Cities of College Place, Washington, and Milton-Freewater, Oregon each operate their own wastewater treatment plant, despite their close proximity to Walla Walla.  The treatment process at each of these plants is likely similar, but on a smaller scale than what is currently found at the facility for the City of Walla Walla.     



Flow Chart originally created in Environmental Studies 222 by Robert J. Carson, Ph.D., February 29, 2001.  Transferred into computer format by Ethan Aumann, October 2001.

Manci, Karen.  “Wastewater Treatment Principles and Regulations.”  Website accessed October, 2001.  <http://ohioline.osu.edu/aex-fact/0768.html>

Photographs taken at the Walla Walla Wastewater Treatment Plant by Ethan Aumann, October 18, 2001.  Ariel photograph courtesy of OMI, Inc. 

Process Control Strategy for the Walla Walla Wastewater Treatment Plant, courtesy of Paul Olsen, OMI, Inc.

Tour of the Walla Walla Wastewater Treatment Plant with Environmental Studies 222, February 27, 2001.  Tour with Paul Olson, Supervisor, October 18, 2001. 

City of Walla Walla Public Works Homepage, accessed October, 2001.  <http://www.ci.walla-walla.wa.us/Public_Works/Wastewater/>

“A Visit to a Wastewater Treatment Plant: Primary Treatment of Wastewater.” July 5, 2001.  Website accessed October, 2001.  <http://wwwga.usgs.gov/edu/wwvisit.html>