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Plant Overview
Permit Requirements
The City's wastewater treatment plant operates under National Pollutant Discharge Elimination System (NPDES) permit WA-002394-9, which authorizes discharge to the Columbia River. The plant effluent is discharged to the Columbia River through an outfall and multiport diffuser located at River Mile 466.6. Plant effluent must be oxidized, nitrified and clarified to meet the effluent BOD, ammonia, and suspended solids limits in the discharge permit. The treatment processes required for discharge to the Columbia include screening, grit removal, primary treatment, secondary treatment including nitrification, and disinfection.  
Screenings Removal
Two automatic, self-cleaning screens remove debris to protect downstream equipment from plugging. Screenings drop into washer compactors to remove organic debris, to reduce the water content and volume to be disposed. Washed and dewatered screenings are discharged into a dumpster for landfill disposal. The screened wastewater flows into the Influent Pumping Station.
Influent Pumping
Raw wastewater from the Screenings Building enters the Influent Pumping Station wet well via gravity. Four influent pumps lift the wastewater to the grit removal process.  Flows greater than the plant capacity are diverted to an equalization basin by one of the two influent pumps dedicated for flow transfer.  Stored water in the equalization basin are pumped back to the influent flow when flows return to normal.
Grit Removal
Two aerated grit removal basins remove sand and other coarse material to protect downstream equipment from abrasion and damage. Grit pumps deliver the grit slurry to hydrocyclones and classifiers. The clean, dry grit is discharged to a dumpster for landfill disposal.
Primary Clarification
Two primary clarifiers provide removal of settleable solids and floatables. Primary sludge and scum are pumped to the anaerobic digestion system for solids reduction and stabilization. The primary effluent flows by gravity to the biological treatment process.
Biological Treatment
The biological treatment facility utilizes the Modified Ludzack-Ettinger (MLE) process for nitrification/denitrification.

The biological treatment process consists of two multi-zone aeration basins, two secondary clarifiers, two return activated sludge pumps and three aeration blowers. The aeration basins provide oxidation of primary effluent BOD, and nitrification of ammonia to nitrate. Anoxic zones in the basins provide an environment for denitrification of nitrate to nitrogen gas.

The biological treatment basins are designed with four individual zones. The first zone is an anoxic zone for denitrification. One mixer is installed in each anoxic zone within each train to keep the MLSS in suspension and to promote consistent denitrification. The remaining three zones are oxic, or aerobic. These zones are used to oxidize BOD and to convert ammonia to nitrate.
 
Each train contains a Mixed Liquor (MLSS) recycle pump in the last oxic zone. This pump pumps MLSS containing a high concentration of nitrate back to the anoxic zone for denitrification.  Three centrifugal blowers are installed to provide aeration air for the biological process and for mixing. Fine bubble aeration diffusers are provided in the oxic zones for introducing aeration air into the basins.
Secondary Clarifiers/RAS Pumping
Mixed liquor flows from the aeration basins to the two circular secondary clarifiers. Mixed liquor solids settle in the secondary clarifier and the clarified effluent flows to the effluent disinfection system. Settled solids are removed by a rotating scraper on the bottom of the clarifier and flow to the return activated sludge (RAS) wet well. The two variable speed RAS pumps transfer the settled solids from the RAS wet well back to the aeration basin influent splitter box.
Effluent Disinfection
Secondary effluent is disinfected by ultraviolet (UV) light. The secondary effluent flows through UV channels where it is exposed to UV light. The UV disinfection system includes three parallel channels, each with capacity to treat 5.5 million gallons of plant effluent at the system design conditions. Each channel has a single bank of lamps, with 5 modules per bank and 16 lamps per module, for a total of 240 lamps in the three channels. Two of the UV channels are required for effluent disinfection at the plant peak design flow of 11 mgd. The third UV channel serves as a backup.  

A UV intensity monitoring system is provided for each channel. The intensity reading is used to automatically adjust the lamp output as required to meet the dosage requirements for disinfection.

A constant upstream level gate (hydraulically balanced and requiring power to operate) is provided downstream of each UV channel to maintain the level relatively constant over the range of operating flows.

The UV bulbs are cleaned in place by an automatic wiper system activated on a timed cycle to clean the lamps.  A portable cleaning tank has been provided in the event the lamps periodically need additional cleaning with a dilute acid solution. The automatic wiping system can be operated in the portable cleaning tank.
Waste Activated Sludge Thickening
Waste activated sludge (WAS) from the secondary treatment process is thickened with a gravity belt thickener (GBT). The WAS thickening system removes excess water from the WAS, thus reducing the volume sent to the anaerobic digestion process. The gravity belt thickener thickens the WAS from 0.7% total solids to approximately 5% total solids. WAS thickening improves digester operation by reducing the hydraulic loading, decreasing the heating demand, and increasing the sludge retention time. The WAS pump transfers the excess solids from the secondary clarifiers to the gravity belt thickener.
Anaerobic Digestion
Three anaerobic digesters provide solids stabilization. Anaerobic digestion is a series of biological processes in which bacteria break down biodegradable material in the absence of oxygen. The process is widely used to treat wastewater solids because it reduces the feed solids volume and mass and produces digester gas containing approximately 65% methane that is usable as a fuel. The digester gas is compressed for use as a fuel source to fire the hot water boiler. Excess digester gas is flared off.
Biosolids Dewatering
Biosolids from the anaerobic digestion process are dewatered with a belt filter press (BFP). The dewatering process removes excess water from digested biosolids, thus reducing the volume trucked offsite. The BFP thickens digested biosolids from about 4% total solids to approximately 20% total solids. Biosolids dewatering reduces operating costs by reducing the weight and volume of biosolids, resulting in fewer truck trips and lower hauling costs.
Plant Reference Drawings
Design Criteria
Site Plan 2012
Hydraulic Profile
Plant Schematic

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Last Updated: 6/3/2014 6:53:46 AM
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