D05_100_OVW_015_PlantOverview.htm Plant Overview Title: Durham Plant Overview Last Update: October 30, 2013 Purpose: To provide an updated layout of the Durham Advanced Wastewater Treatment Facility while describing the various unit processes systematically. Description: Introduction and Facility Background The Durham Advanced Wastewater Treatment Facility is located on 58.16 acres, west of Interstate 5, south of Highway 217. It is situated near the intersection of Durham Road and SW 85th Avenue, southeast of Tigard. The plant was initially constructed and placed into operation in 1976. The support facility, now known as the Operations and Maintenance Building, has an elevation benchmark stamped on the curb in the front parking lot. The elevation for this benchmark is 156.24'. This single treatment plant became a regional facility replacing multiple plants that were located throughout the drainage basin. This regional facility was a major element in a water quality improvement program done by Clean Water Services (formerly Unified Sewerage Agency) to improve the water quality in the Tualatin Basin. The treatment plant is currently receiving about 22.6 of dry weather flow. Collection System The Durham AWWTF receives raw sewage from an extensive collection system providing service to the cities of Durham, King City, Sherwood, Tigard, Tualatin, portions of Beaverton, Lake Oswego, and unincorporated Washington County. Septage Receiving Station and RV Station Durham has a Septage Receiving Station that discharges directly into the Influent Pump Station (IPS). The septage consists primarily of pumping from septic tanks and holding tanks, with a minor fraction from chemical toilets. All wastes received at the Septage Station are regulated under Clean Water Services' Septage Hauler licensing rules, and are subject to regular inspection and compliance sampling. Annual septage receiving represents less than 0.02% of the plant flow. The RV Station provides courtesy waste receiving at no charge for recreational vehicles, Mobile Food Kitchens, and Mobile Carpet Cleaners. The wastes received at the RV Station discharge directly to the Influent Pump Station (IPS). Treatment Plant The original plant employed activated sludge for removal of BOD 5, lime addition for phosphorous removal, and effluent filtration. Solids handling consisted of gravity thickening, centrifuge dewatering and incineration. Since the mid-1980 s, the plant has been modified or expanded through a number of construction projects to produce the current site plan features. New and innovative methods have been implemented, such as Enhanced Biological Nutrient Removal (EBNR). Incineration has given way to the beneficial reuse of biosolids by land application. A commercial fertilizer product is now being created from wastes.
Each of the plant's unit processes are briefly described in the following subsections. For convenience, the processes have been divided into liquids and solids processing streams. Liquids Processing The raw wastewater that is received at the Treatment Plant is pumped to the Headworks Facility, where fine screening, rag removal, grit removal and flow measurement occur. The wastewater then flows to four (4) Primary Clarifiers for initial settling. Primary effluent is distributed amongst four (4) Activated Sludge Trains that provide Enhanced Biological Phosphorus Removal, nitrification and partial denitrification. Following Secondary Clarification, the flow passes through Tertiary Clarifiers, Chlorine Contact Basins and Effluent Filters, in series. A portion of the liquid stream, extracted during the solids dewatering process, is sent to the Struvite Recovery Facility for commercial fertilizer product manufacturing. The final effluent is discharged to the Tualatin River from Outfall D001. Because of seasonal permit limitations for phosphorus and ammonia-nitrogen, select processes employ differing operational modes during the summer and winter seasons. Influent Pump Station and Headworks The Durham AWWTF receives influent flows from four (4) interceptors serving the Durham Basin: Upper Tualatin, Lower Tualatin, 85th Avenue, and Fanno Creek. The plant drain containing plant recycle flows also discharges into the Fanno Creek Interceptor. The current Influent Pump Station (IPS) has a total rate capacity of 180 MGD and a firm capacity of 140 MGD. The IPS has two Wetwells that employ a self- cleaning design capability. Each Wetwell may be run individually or both may be utilized at the same time. Each Wetwell is equipped with two (2) 25 MGD, 600 hp VFD-driven Influent Pumps and one (1) 40 MGD, 1000 hp VFD-driven Influent Pump. The IPS was placed into service in 2007 and replaces the original Raw Sewage Pump Station for increased capacity. The IPS discharges to either a 42" Force Main or a 54" Force Main. During high flows, both Force Mains may be used. All channels together handle a combined flow of 200 MGD; both channels are equipped with magnetic flow meters. The Force Mains convey wastewater to the plant's Headworks. Preliminary Treatment Preliminary treatment at the Headworks consists of rock removal, mechanical fine screening, rag removal, grit removal and flow measurement. These facilities are housed in an enclosed Headworks Facility equipped with odor control. Wastewater entering the Headworks passes through a Rising Well where rocks settle out and are removed annually. The flow is then divided to four (4) parallel channels, each containing a mechanical 3/8" fine screen and an Induced-Vortex Grit Removal System. Two (2) of these channels are rated at 40 MGD each; the remaining two (2) are rated at 60 MGD each. Alum and polymer may be fed at the Headworks Flumes to enhance primary treatment and provide for phosphorus removal in the primary process. Primary Treatment Primary treatment is provided in two (2) 120 foot-diameter Clarifier Basins that were constructed in
1976, and rated at 18.8 MGD each; and two (2) 130 foot-diameter Clarifier Basins constructed in 1991 and 1994, rated at 34 MGD each. In 1991 the height of the older Clarifiers was raised by one foot to permit a flow split between primary and secondary treatment. Primary sludge is withdrawn in dilute form (approximately 0.2% solids) and little or no sludge blanket is maintained in the Basins. All four (4) Primary Clarifiers are covered and ventilated to odor treatment systems. Primary effluent from Primary Clarifiers #1 and #2 is collected in an Excess Flow Diversion Structure. During peak flow events, flow can be diverted to the Surge Basins or downstream processes from this Structure. Lime is fed at the Excess Flow Diversion Structure during the nutrient removal season to provide alkalinity for Nitrification. The primary effluent then flows to the Primary Effluent Pump Station (PEPS) where it is combined with the effluent from Primary Clarifiers #3 and #4. The PEPS pumps and distributes primary effluent to the four (4) Activated Sludge Aeration Basins. Secondary Treatment Secondary treatment is provided in four (4) parallel trains containing Activated Sludge Aeration Basins, Secondary Clarifiers and RAS/WAS pumping. During the summer nutrient removal season, the activated sludge process is operated in an Enhanced Biological Nutrient Removal (EBNR) Mode. During the winter, the system is operated for CBOD5 removal using a step feed configuration. Each Activated Sludge Aeration Basin has a dedicated Secondary Clarifier where alum can be added for additional phosphorus removal. Secondary effluent is collected in the Secondary Effluent Splitter Box where it is conditioned with alum for final phosphorus polishing and split evenly between the three (3) Chemical Clarifiers. When operated in the EBNR Mode with Nitrification, each Activated Sludge Train has an estimated maximum-month capacity of 7.7 MGD, providing a total capacity of 31 MGD. During winter operation for CBOD5 removal only, the secondary treatment process has a total rated capacity of 80 MGD during maximum-week conditions. Primary effluent flow above 80 MGD is diverted around secondary treatment. Tertiary Treatment Three (3) Tertiary (chemical) Clarifiers provide parallel treatment of secondary effluent. The Chemical Clarifiers have a total design capacity of 75 MGD. Alum is added only during the summer season when phosphorus removal is required. The Chemical Clarifiers have a mechanical flocculation zone at the influent of each Clarifier. The flocculators are only operated when alum is being fed.
Disinfection Disinfection is achieved using sodium hypochlorite. Three (3) Chlorine Contact Basins (CCBs) are located ahead of the Effluent Filters to allow gravity discharge from the plant. The hypochlorite feed is flow-paced and added to the Tertiary Clarifier effluent. Low pressure air is provided to maintain any solids in suspension, to augment effluent dissolved oxygen levels and provide mixing for better disinfection. Chlorine also is provided at the Excess Flow Diversion Structure for flow to the Surge Basins. This assures that any flow discharged from the Surge Basin is disinfected. Sodium bisulfite is then added to dechlorinate the disinfected Surge Basin effluent as per permit requirement. Effluent Filtration and Dechlorination Thirteen (13) Mixed-Media Filters are used to filter effluent following disinfection. During the summer, three (3) Filters can be dedicated to the production of reuse water, with filtered effluent entering a separate Reuse Clearwell. Filter backwash is directed to the Filter Backwash Basin, which drains back to the Fanno Creek Interceptor. Based on design criteria of 3 GPM/square foot and two (2) Filters out of service for backwash, the filtration capacity is 45 MGD. During the winter season when the loading rates can be higher, the capacity is approximately 60 MGD on an average day basis. During peak flows, the capacity is 100 MGD for short-term events, but 85 MGD for sustained events. Filtered effluent enters a Clearwell which may be aerated using Coarse-Bubble Diffusers, though this process is rarely needed to meet the dissolved oxygen limits. Sodium bisulfite solution is flowpaced and fed to the filtered effluent as it leaves the Clearwell. Following dechlorination, the effluent is discharged to the Tualatin River through the Plant Outfall and Diffuser. Surge Basins All flow through the plant receives a minimum of primary clarification, disinfection and dechlorination. When flows exceed 80 MGD (the capacity of the Biological Process), the excess flow is diverted to the Surge Basins. From there, the flow may be: Returned to the plant for full treatment by way of the Excess Flow Diversion Structure. Returned to the plant for filtration prior to dechlorination and discharge. Allowed to discharge to the Wet Weather Outfall. Drained back to the Fanno Creek Interceptor.
Solids Processing The solids processing at the Durham AWWTF is done to produce a Class B Biosolids for land application. Primary sludge is typically withdrawn from the Pprimary Clarifiers in a dilute form and gravity thickened. Waste Activated Sludge (WAS) and chemical sludge are combined and stored in an aerated WAS Storage Tank prior to centrifugal thickening. Thickened primary sludge and thickened WAS/chemical sludge are combined and pumped to a Blend Tank where they are mixed and pumped to Anaerobic Digestion for stabilization. Primary scum is pumped directly into the digester feed line. Secondary scum from sides 3 and 4 are combined with the WAS Sludge Flow. Secondary scum from sides 1 and 2 is sent to the Plant Drain and recycled by way of the Influent Pumping Station. Two (2) Primary Digesters provide an average HRT of 27.77 days based on a feed flow of 93,600 gpd. Stabilized sludge flows to a Storage Tank from which sludge is withdrawn for Centrifuge Dewatering. The dewatered sludge is then conveyed to two (2) Live Bottom Hoppers for storage and truck Load Out. The dewatered cake is hauled by truck to agricultural land for beneficial reuse land application. Struvite Recovery Facility The Struvite Recovery Facility is the location where a commercial product called Crystal Green is manufactured. The process is patented by our partner, Ostara Nutrient Recovery Technologies, Inc. Crystal Green is a commercial fertilizer product made from the recovery of phosphorus and nitrogen extracted as waste from our treatment process. If these wastes were not removed, they would create struvite, with the chemical name magnesium-ammonium-phosphate. Struvite deposits can damage instruments, piping, valving and increase maintenance costs. At the Durham Facility, phosphorus and ammonia are removed using both biological and chemical methods. Waste Activated Sludge (WAS) from the biological process is thickened, anaerobically digested, and transported off-site for farm land application. During anaerobic digestion, large portions of the previously removed phosphorus (P) and nitrogen (N) are re-released into the digested sludge which is then dewatered by Centrifuge. N and P enriched centrate is returned to the Aeration Basins, which reduces the treatment plant's capacity and results in struvite fouling of instruments and piplines. Ostara's Nutrient Recovery Process alleviates these problems by using proprietary technology to remove nutrients from the sludge centrate and recycles them into a high value, environmentally-safe commercial fertilizer, under the product name Crystal Green. Magnesium chloride is provided as feedstock which is stored in two 5000 gallon tanks in the Solids Recovery Building. There is a 750 gallon Day Tank in the basement of the Struvite Recovery Facility. Magnesium, a key constituent of struvite, is the limiting constituent inside a Digester. In order to generate more struvite, the Reactors must be supplied with additional magnesium as magnesium chloride, diluted with water. Centrate, another feedstock, is stored in two 250,000 gallon Storage Tanks, located under the
Dewatering Centrifuges. The Centrate Return Pumps charge a header to a constant pressure. This constant pressure allows the Reactor Feed Valves to regulate the amount of centrate feed going into each Reactor. There are three Fluidized-Bed Reactors at the Struvite Recovery Facility. These Reactors allow the struvite to come out of solution and form small "prills", concentric layered crystalline beads similar to a pearl in an oyster. Product is periodically harvested by means of a washing or elutriation leg through a Dewatering Screen. The product is dried in an electrically heated Fluidized-Bed Dryer. After drying, the product flows into a Vibrating Classifier where it is sorted and selected by mm size. Four Product Storage Silos are located in the basement. Each Silo stores the classified and sized product until it is bagged.the product is bagged, weighed, and labeled, also in the basement. Labeled bags are lifted out to the ground floor, and then moved by forklift to be stored in the shipping/receiving area. Non-potable water (NPW) is added to the Reactor effluent to unsaturate the solution with respect to struvite, to prevent scale formation in the effluent lines. NPW enters the plant in a 2" line, and is distributed for use as needed. Thickeners/Fermenters The Durham AWWTF has three (3) Gravity Thickeners to thicken dilute primary sludge. The northeast Gravity Thickener is configured such that it may be used as a backup Waste Activated Sludge (WAS) Storage Tank if needed. Modifications to the original design were implemented to promote fermentation and elutriation in the Thickeners to generate volatile fatty acids (VFAs) for the Enhanced Biological Phosphorus Removal Process. Thickened primary sludge is pumped to the Digester Feed Tank where it is blended with thickened WAS sludge prior to being pumped to Anaerobic Digesters for stabilization. Scum is removed from the Thickeners and pumped to the Digester Feed Tank. Each Thickener has a rated loading capacity of about 47,000 lb/day. WAS and chemical sludge are blended and stored in an aerated WAS Storage Tank. These solids are sent to one or two of three (3) Centrifuges for thickening and then to Anaerobic Digestion for stabilization. These Centrifuges have a capacity of approximately 200 GPM each without polymer, and 300 GPM each with polymer. Digesters There are two Anaerobic Digester Complexes. Thickened primary sludge, thickened WAS/chemical sludge, secondary scum, and scum from the Thickeners/Fermenters are blended and sent to the Primary Anaerobic Digesters in Digester Complex No. 2 for stabilization. Primary scum is pumped directly into the digester feed line. These Digesters have a total volume of 2.6 MG. The Digesters operate in parallel and are fed nearly continuously using Timer-Controlled Valves which rotate the feed to each Digester. Each Digester is mixed using a two-blade, slow-speed Turbine Mixer.
Sludge is recirculated through Heat Exchangers, and a Cogeneration System uses digester gas to produce electricity. Recovered heat from the Generator is used to maintain heat in the Digesters and warm buildings. As new sludge is added to the digesters, the digesters overflow into a sludge sump where it is pumped to the Digested Sludge Holding Tanks at Complex No. 1. Digester Complex No. 1 has four tanks of which three are Digester Sludge Holding Tanks. Stabilized sludge from Digester Complex No. 2 is pumped to the holding tanks prior to dewatering. The fourth tank is currently unused, but the roof of the tank will hold the Duosphere digestor gas storage facility. Biosolids Dewatering Sludge dewatering is achieved using Centrifuges. The Durham AWWTF is equipped with two (2) Dewatering Centrifuges. Either liquid or dry polymer can be used to assist dewatering. Biosolids cake is conveyed by Screw Conveyors to one of two Storage Hoppers. The Storage Hoppers are located in the Solids Loading Room and are equipped with Live-Bottom Screw Conveyors to load trucks for transport. The Hoppers are equipped with Load Cells to meter the amount of solids loaded to each truck. The stabilized solids are land applied on local or Eastern Oregon agricultural land as a Class B product. Do you notice something wrong or would like something added to this document? Use the Feedback system. Last Updated: 10/30/2013 13:21:53 This document and its content is the property of Clean Water Services. Copyright 2013 All Rights Reserved