The water discharged from your homes and businesses enters a maze of underground pipes, which direct the flow to the Wastewater Treatment Plant. This is called the collection system. The City has about 13.7 miles of sewer lines.

The collection system works primarily as a gravity system in which the water flows down hill through the lines. There are three lift stations, which have to pump the wastewater from low points in the city to a higher level so it will flow to the Water Reclamation Facility (WRF).

Maintenance of the collections system consists of inspecting the lines visually and with video equipment, cleaning the lines to prevent build up of grease or other debris which could block the lines, removal of roots growing through joints or cracks in the pipes, and repair or replacement of degraded lines, manholes, or lift stations and lift station equipment.

PRETREATMENT

Pretreatment is done prior to discharge to the collection system. It prevents hazardous or other harmful chemicals from being discharged to the collection system and Water Reclamation Facility (WRF). Pretreatment can include grease traps, used by restaurants and other businesses for the removal of fats, oils and grease (FOG).

HISTORY OF THE PLANT

The City of La Center Wastewater Treatment Plant has been in the same location since 1967 when the first wastewater system was constructed. The plant has been upgraded about every 5 years during this time period to keep up with growth, technology and regulations.

The Wastewater Treatment plant was upgraded to a Water Reclamation Facility in 2004 when it was converted to a Sequencing Batch Reactor followed by a disk filter and Ultraviolet Disinfection.

In August 2006, the City of La Center assumed operations of the Water Reclamation Facility (WRF) and collection system from Clark Public Utility District who had owned and operated it since 1992. The plant is classified as a Class II treatment plant and operates under regulations from the Department of Ecology (DOE) and the Environmental Protection Agency (EPA).

The National Pollutant Discharge Elimination System (N.P.D.E.S.) permits the city to discharge up to 0.56 Million Gallons per Day (MGD) with a maximum day flow of 1.0 MGD. The plant is designed to treat an annual average flow of 0.33 MGD. Current average daily flows in the summer are 0.22 MGD and winter flows average 0.28 MGD with a maximum day flow of 0.68 MGD. The treatment plant serves approximately 2,510 residents in La Center as well as the four cardrooms and other commercial businesses located in the city.

In 2009, the City of La Center upgraded the facility and will dedicate it on April 29th, 2011. A new permit will be issued in 2011 to reclassify the upgraded facility. The new facility is a Membrane Bioreactor with a capacity of 1.5 MGD. It is designed to be expanded to treat over 6 MGD on less than an acre. The new facility is being designed in phases to be efficient and add capacity as needed.

Phase 1 added a new headworks building, screening system, emergency generator, electrical system, utility water system as well as pump, UV, and blower rooms all sized for 20 year flows based on the City’s Comprehensive Plan. Modifying the process basin, adding membrane basins, blowers, and pumps were included in Phase 1. The flow capacity of Phase 1 is 1.04 MGD average annual flow with a peak day flow of 3.0 MGD.

Phase 2 addresses additional solids handling that will be required in 5 to 10 years as the amount of flow and solids discharged to the plant increases.

Phase 3 would double the capacity of the plant by duplicating the Phase 1 upgrades for the Membrane Bioreactors (MBR) and should have capacity to treat the 2028 population. Flow capacity would be 1.65 to 2.08 MGD average annual flow and a peak day flow of 4.2 to 6.0 MGD.

PRELIMINARY TREATMENT

There are a number of steps to the treatment process. Treatment of the waste is done through both physical and biological means.

Preliminary treatment is the removal of large debris and inorganic matter like rags, sticks, coffee grounds, gravel etc. by physical means. This area is referred to as the headworks building. The flow or influent enters the plant through a gravity line. It flows into one of two Enivrocare Rotary Drum Screens. The 3 mm openings remove a significant amount of debris. The screenings are deposited in a conveyor where sprays wash the organic matter into the influent stream and the inorganic material is compacted and conveyed into a garbage can in the lower headworks area. The headworks building is fully contained and the odors are scrubbed through a Purafil odor scrubber that utilizes a media to remove odor causing compounds. This has significantly improved the aesthetic issues that surround a wastewater facility. The garbage cans are emptied daily into a dumpster. Waste Management Service empties the dumpster once a week and takes the debris to the Sanitary Transfer Station.

The wastewater flows through a channel into a parshall flume where the flow is measured and water quality samples are collected. The water contains suspended and colloidal solids (small particles that will not dissolve and that remain dispersed in a liquid). After the water is sampled it flows in a channel and combines with the mixed liquor that is exiting the membrane basins.

The treatment system is a called a Membrane Bioreactor (MBR). There are a number of processing steps that occur in the MBR. The system is divided into treatment stages; Anoxic, Aerated, and Membranes. The influent is combined with the return flows from the MBR tanks. This is referred to as mixed liquor (ML). The ML is high in solids and bacteria that will break down the compounds in the influent. The right conditions have to be met in each tank for the right process to occur.

MEMBRANE BIOREACTOR

This next step is the biggest change from conventional activated sludge treatment. The ML is pumped into a splitter box which allows the ML to flow into the membrane cells. These cells contain Submerged Membrane Units (SMU) containing Kubota flat plate membranes. There are 10 SMU’s in each tank with 200 plates in each SMU. Currently there are 4000 plates in the facility. As the ML flows through the tank, water is permeated through the membranes on the plates and is collected in a header in the pipe gallery between the four MBR basins. The depth of the water in the basins allows this to be done by gravity but pumps are supplied for backup in case the flows are too high or the river level is high enough to impeded the flow. The membrane is a physical barrier that allows water through but keeps the bacteria, and solids in the MBR tank. The ML flows by gravity out of the membrane tanks and combines with the influent and flows into the anoxic tanks completing the cycle. To prevent solids from accumulating on the surface of the membranes and to keep the basins aerobic coarse air scrubbing is forced through a diffuser and up through the SMU plates.

We need to remove a portion to keep the organisms to the amount of solids entering the system in balance so we pump them to the solids storage basin for further processing.

Disinfection and Discharge – Permeate is collected in a series of pipes and then flows through one of three ultraviolet vessels. The ultraviolet lights disrupt the DNA of any remaining bacteria and prevent them from growing, effectively disinfecting the effluent. After disinfection the effluent is sampled.

After disinfection a portion of the flow is put into a storage tank for reuse in the facility and grounds irrigation.

The rest flows by gravity through a pipe and discharged through a diffuser into the East Fork of the Lewis River adjacent to the Treatment Plant.

SOLIDS PROCESSING

The waste solids removed from the process basins is pumped into the solids storage basin and aerated with a mechanical mixer. Recent upgrades replaced a belt filter press with a Prime Solutions rotary fan press.

The system injects polymer into the solids which makes the solids stick together and allows for the water to be removed. After most of the water is removed the solids pass through a compaction zone where additional water is pressed out. The solids are then processed through a dryer where the final product is over 90% solids. The solids from the MBR process contain slightly over 1 % solids before the press and achieves 15 % solids after they are processed. The reduction in the water content of the solids sent to the dryer has cut the time a cost of drying the solids in half. The heat in the dryer kills the bacteria and allows the solids to be classified as a Class A exceptional quality biosolid. The solids contain nitrogen and phosphorus that is available for plant growth.

The solids from the city are taken to the Lewis River Tree Farm where they are beneficially reused as a soil amendment to grow trees for reforestation.

LABORATORY ANALYSIS

The city operates an accredited laboratory onsite. We run the analysis required by our permit as well as process control testing as necessary. Weekly approximately 82 samples are collected and 260 analyses are completed. Advanced testing is done twice yearly at an outside laboratory.

Sampling is the most critical part of water testing. If the sample does not represent the system being tested or if it is contaminated by outside sources the test is worthless. No matter how accurately the test is done, it will be meaningless if it does not represent the source from which it is taken.

This graph is the Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD) that is in the effluent in parts per million parts of water. Since the MBR was installed the effluent results have been at or near the detection level of the tests.

SCADA COMPUTER SYSTEM

Supervisory Control and Data Acquisition (SCADA) is a computer system that allows the interface between the operator and the system that controls the equipment. It allows for the operator to change setpoints and to turn equipment on and off from the computer system. Since it is on the computer it allows for the operator to log in from a remote location with the laptop and monitor the plant and make changes as needed on a 24 hour basis. Data acquisition enables the operator to gather evaluate and trend information gathered by the instruments like flow meters and level sensors through out the plant.

Data is gathered and can be looked at visually in graph format. It is used to make sure the treatment process is working properly and efficiently. This information is also used to prepare the monthly and annual reports required by the Department of Ecology.

ELECTRICAL SYSTEM

Phase 1 construction included modifying the existing filter building into a main electrical building where all of the main electrical gear is housed in a clean, dry, temperature controlled environment. All conduits for the 20 year build out had to be installed for the wires to be pulled through since they are buried in the ground and encased in concrete. The electrical equipment upgrade included replacement of the main transformer that serves the plant this was installed by Clark Pubic Utilities and the electrical contractor. The main service and Motor Control Centers for all three phases were upgraded. Motor starters and variable frequency drives for motors that need to operate at different speeds were installed for Phase 1.

Backup Generation – In order to ensure the uninterrupted processing of the wastewater received at the plant a 1 megawatt generator and an automatic transfer switch was installed. When there is a power outage the automatic transfer switch starts up the generator and transfers the load onto the generator. When power is restored it switches back to the incoming power. This provides a virtually seamless transition for the plant operations. The emergency generator system should be able to provide for the plant up to the 20 year build out of the facility.

DESIGN DRAWING LINKS

Click on the links below to view selected design drawings of the new La Center Wastewater Treatment Plant.