5.1        Introduction

This chapter describes the infrastructure used to convey and treat wastewater generated in the County’s wastewater service area.  The discussion of the collection system is limited to a brief overview of major facilities since the County’s sewer construction program is extensively discussed in the Year 2001 Comprehensive Wastewater Management Plan for Spokane County (CWMP) [i].  At the end of the chapter, key County ordinances addressing wastewater management are summarized.

5.2        Regional Integration of Wastewater facilities

The County's wastewater management facilities are integrated into a regional network of conveyance and treatment facilities serving the greater Spokane area.  A simplified schematic of the regional network is shown in Figure 5‑1.  Major regional facilities are shown in Drawing 5-1.  Regional Conveyance and Treatment Facilities.

Figure 5‑1.  Simplified Schematic of Regional Collection and Treatment Facilities for Municipal Wastewater

Key intergovernmental arrangements are summarized below:

§         The Town of Millwood operates a wastewater collection system that discharges to Spokane County’s interceptor system [ii].

§         Liberty Lake Sewer and Water District No. 1 (District) and Spokane County entered into an agreement that allows the District to send “excess” flow to the Spokane County interceptor system on an interim basis should the capacity of the District’s treatment plant be exceeded, and if capacity is available in the County system.  To date, no District flows have been sent to the County system [iii].

§         Spokane County purchased capacity in the Spokane Advanced Wastewater Treatment Plant (SAWTP) and in the City of Spokane’s interceptor system.  The County’s wastewater is discharged to the City’s interceptor system at three locations [iv].

§         A small portion of Spokane County wastewater is treated at five small package plants or community septic tanks.

§         The City of Spokane provides wastewater treatment for the City of Airway Heights and Fairchild Air Force base on a contract basis.

5.3        Wastewater Collection Systems

The discussion of wastewater collection facilities is divided between the Spokane County and City of Spokane systems.

5.3.1        Spokane County System

Overview

Over the last two decades, Spokane County has implemented an extensive sewer construction program to eliminate on-site wastewater disposal facilities located over the Spokane-Rathdrum Aquifer.  Through the end of 1999, the County had installed 270 miles of sewer and connected 54,000 people to the system.  This sewer extension program will be continued through 2015 to provide service to all of the County’s designated wastewater planning area (see Chapter 2).  The history of the sewer construction program and the driving forces behind it are described in the CWMP.

The Spokane County collection system is divided into two distinct systems: one serving the Spokane Valley and one serving North Spokane.  The locations of the sewer basins and major conveyance facilities serving each area are shown in Drawing 5-1.  Spokane County’s sewer system is less than 20 years old and in good physical condition.

Hydraulic Analysis

To evaluate the capacity of the existing interceptor sewer system and to establish a tool for developing future conveyance alternatives, a rudimentary hydraulic model of the sewer network was established.  The program Hydra, developed by Pizer Inc., was used for this purpose.  Hydra is a commercially available program that uses an integrated visual display to present system information and to report hydraulic capacity (see example in Figure 5‑2).  Older, DOS-based versions of the Hydra model are used for preliminary design of County conveyance facilities.

Figure 5‑2.  Example Output from Hydraulic Model

Model development was based on information gained from the County’s maps and construction record drawings.  Drawing 5-2.  Existing Wastewater Conveyance Facilities provides a skeletal depiction of the conveyance facilities included in the model.

Spokane Valley System

The Spokane Valley service area lies to the east of the City of Spokane.  The topography slopes westward, paralleling the Spokane River drainage.  Except for a few small pumping stations serving low-lying areas and river crossings, the Spokane Valley is served primarily by gravity sewers.  Two main interceptors exist: the Spokane Valley Interceptor (SVI), which serves the area south of Interstate 90; and the North Valley Interceptor (NVI), which serves the area north of Interstate 90.  Metering stations are provided at the locations where the SVI and NVI connect to the City of Spokane’s collection system.  Table 5‑1 identifies the diameter and length of the major interceptors in the Spokane Valley system.  It also presents the results of capacity calculations using the hydraulic model.  These calculations were developed based on a depth of flow to sewer diameter ratio of 0.9. 

Table 5‑1.  Spokane Valley Interceptor Capacity

            a              See Drawing 5-2 for location of interceptor sections.

North Spokane System

The North Spokane service area is located along the Little Spokane River, north of the Spokane City limits.  The low point of this service area is situated at an approximate elevation of 1,600 feet, about 600 feet below the nearest connection point to the City of Spokane sewer system.  The topography of the sewer basin requires significant pumping to convey wastewater to the regional treatment facility.  In order to minimize force main pressures and hydraulic transients, and to allow competitive bidding of the pumping equipment, the allowable lift in each pumping station was limited to 200 feet.  Consequently, three major pumping stations are required to transfer wastewater from the area along the Little Spokane River to the City system. 

The capacity and features of the major pumping stations within the North Spokane service area are presented in Table 5-2 [v]^,[vi]^,[vii]^,[viii]^,[ix].  The Marion Hay Pumping Station receives nearly all the wastewater flow from North Spokane.

Several of the interceptors tributary to the North Spokane pumping stations are also fairly large.  The interceptor leading to the Whitworth Pumping Station is 42-inch diameter, and the interceptor leading to the Marion Hay Pumping Station is 36-inch diameter.  The sizes for the other influent sewers are identified in Table 5‑2.

Table 5‑2.  North Spokane Pumping Station Capacities

Note:  Data collected from County’s pumping station operation and maintenance manuals

5.3.2        City of Spokane System

The City of Spokane collection system consists of 290 miles of separated sanitary sewer, 130 miles of storm sewer and 400 miles of combined sewer.  A simplified schematic of the City’s major interceptors is shown in Figure 5‑3, along with the City’s estimate of capacity for the major segments [x].  This figure also shows where County wastewater enters the City system.

In the 1980s, the City implemented a major sewer separation program that reduced the annual volume of CSO discharge by approximately 85 percent.  Despite these measures, CSOs continue to occur at a frequency that exceeds State requirements.  Currently, the system has 30 CSO regulating structures and 24 potential CSO discharge points.  In 1994, the State approved a City plan to further eliminate CSOs [xi], but the City delayed implementation of the improvements.  A new implementation schedule was approved by the State in March 1999, giving the City until 2017 to meet the State’s CSO requirements [xii].  However, the City is now conducting a further multi-year study to revisit its approach to CSO control.  It is anticipated that the recommendations of the new study may substantially change the 1999 implementation plan.  Consequently, the future configuration and capacity of the City collection system remains unclear and may not be defined for an additional two to three years.

Through a 1980 interlocal agreement, Spokane County purchased capacity in the City collection system to convey 10 mgd of County wastewater to the SAWTP.  The original agreement provided “dry-weather” capacity to the County, requiring the County to implement flow equalization or other flow control measures during wet-weather periods when flows from the City’s combined system exceed the hydraulic capacity of the interceptor system.  In 1983, the City and County amended the interlocal agreement to provide the County with 10 mgd of “wet-weather” conveyance capacity for service to the Spokane Valley.  In exchange, the County agreed to pay for sewer separation projects in the Trent and Mallon basins and installation of a sewer to bypass the Hartson collection system.

The City’s ability to convey future County flows, beyond the current capacity allocation, is impacted by hydraulic limitations in several interceptor sections and the City’s need to maximize conveyance and treatment of flows generated in its combined system.  According to the City’s Wastewater Facilities Plan, the interceptors that appear to have the most significant capacity constraints in the near term are the South River and North River Interceptors.  In addition, the North River-Bridge Interceptor is nearing its capacity during peak flows ^x.  Finally, there may be hydraulic limitations in the City pipelines that convey County flow from North Spokane to the SAWTP, although this seems to a less significant constraint than the other conveyance bottlenecks. 

If the County implements a wastewater management alternative that involves sending wastewater flows in excess of its current capacity allowance, improvements to the City collection system will be needed.  Unfortunately, the nature, cost and implementation time frame for these improvements may not be identified until the City completes its on-going CSO planning effort.  Consequently, this information may not be available in sufficient time for use in the County’s facility planning effort.  To allow development of alternatives, the County and its consultants will:

§         Meet with the City to gain an understanding of the City’s conveyance system, identify the size and capacity of City sewers that convey County flows, and discuss known bottlenecks or limitations within the conveyance facilities that carry County flows.

§         Where sufficient information is available and the City’s future conveyance plans are reasonably clear, make engineering judgments about the size, length, timing and cost of improvements to City-owned collection facilities needed to convey projected increases in County flows.

§         Where insufficient information is available or the City’s conveyance strategy has not been developed, define facility requirements and costs for “worst-case” conveyance scenarios, such as building new pipelines from the County/City service boundary to the Spokane plant.

5.4        Wastewater Treatment Facilities

Nearly all of the wastewater generated in the Spokane County service area is conveyed to the Spokane Advanced Wastewater Treatment Plant (SAWTP) for treatment and disposal.  A small portion of the wastewater is treated at five satellite wastewater treatment facilities that are owned and operated by the County.  This section reviews the size, type and capabilities of the wastewater treatment facilities, beginning with the small County-owned systems.

5.4.1        Spokane County Facilities

Description of County Facilities

Each treatment facility consists of either an extended-aeration package wastewater treatment plant or a community septic tank.  Effluent disposal is by either a community drainfield or infiltration pond.  A facilities summary is presented in Table 5‑3.  The location of each facility is shown on Drawing 5-1.

Table 5‑3.  Spokane County Wastewater Treatment Facilities

1.         PSSA = Priority Sewer Service Area; UGA = Urban Growth Area

Hangman Valley Treatment Plant

The Hangman Valley Treatment Plant has a capacity of 86,000 gallons per day, which is considerably greater than the other County treatment facilities.  This plant facility is located south of the Spokane urban area outside the Priority Sewer Service Area (PSSA), and sewer extension to this area is not anticipated in the next twenty years.  The treatment facility, which discharges to a rapid infiltration system, has a wastewater discharge permit issued by the Washington State Department of Ecology [xiii].  This facility is planned to remain in service for the foreseeable future.

Peone Pines Treatment Plant

The Peone Pines Treatment Plant has a capacity of 30,000 gallons per day.  This facility is located north of the North Spokane urban area, and just outside of the Priority Sewer Service Area.  Centralized sewer service will likely not be extended to this location until it is included in the County’s Urban Growth Area boundary.  This is not expected to occur within the next 20-years.

Other County Treatment Facilities

The County’s goal is to eliminate the other three satellite treatment facilities within the 20-year planning period.  The Green Hollow facility will be removed from service when the proposed Waikiki Pumping Station is constructed in North Spokane.  The Heritage Estates and Monte Del Ray facilities are located within the PSSA and are expected to be removed from service during the next 20 years when the County’s sewers are extended to serve the entire designated planning area. 

Package treatment plants serve Peone Pines and Monte Del Ray, which are on the fringe of the planning area.  Sewer extension to these areas may not occur until the latter portion of the 20-year planning period.  Consequently, the useful life of the treatment equipment should be considered.  The Peone Pines facility is nearly 20 years old and may be reaching the end of its useful life.  However, the Monte Del Ray plant is less than 10 years of age and may be adaptable to other uses.

5.4.2        Spokane Advanced Wastewater Treatment Facility

Introduction

The Spokane Advanced Wastewater Treatment Plant (SAWTP) is located on a 28-acre site in northwest Spokane along the north bank of the Spokane River (see Figure 5-1).  The site is bounded by Riverside State Park.  The plant provides wastewater treatment for the City of Spokane, Spokane County, the Town of Millwood, the City of Airway Heights and Fairchild Airforce Base.

Expansion History

The City of Spokane completed construction of a combined stormwater/wastewater collection system and a primary treatment plant in 1958, and expanded the capacity of the treatment plant in 1962.

In 1977, the plant was upgraded to provide secondary treatment and seasonal phosphorus removal.  As part of this expansion, the dry weather capacity of the plant was increased to 40 mgd.  In 1979, the rated capacity of the plant was increased to 44 mgd based on engineering evaluations and review of performance records.

Since 1977, the following significant upgrades to the plant have been completed:

§         Replacement of original vacuum filter presses with belt filter presses – 1982

§         Installation of energy recovery systems consisting of steam generation and cogeneration facilities – 1990

§         Installation of dechlorination facilities

§         Upgrading the plant controls to a modern SCADA system – 1994

§         Replacement of surface aerators with fine bubble aeration systems – 1997

Currently, the solids handling system is being modified through a phased implementation program, as discussed later in this section.

Overview of Current Treatment Scheme

Because of seasonal limitations on phosphorus and ammonia-nitrogen, differing operational modes are used in the summer and winter seasons.  Special operational modes also are used during peak flow events resulting from stormwater entering combined sewers.  The following sections provide a brief overview of the liquid and solids treatment systems employed.  A site plan of existing treatment facilities is shown in Drawing 5-3.  SAWTP Site Plan and a summary of existing equipment, including number of components, size and design criteria, is presented at the end of the chapter in Table 5-4.

Liquid Treatment – Summer Permit Season.  During the summer (low flow) permit season, the SAWTP must provide at least 85 percent removal of phosphorus.  The minimum permit season for phosphorus removal extends from June 1 through October 15.  This permit season may be initiated earlier or extended later, based on flows in the Spokane River.  The SAWTP has year-round limits on ammonia-nitrogen; however, these become much more stringent during the summer permit season, which extends from July 1 through October 31.

The liquid treatment scheme for the SAWTP is shown in Drawing 5-4.  SAWTP Liquid Stream Schematic.  Wastewater flow enters the plant by gravity and is split amongst three channels, each containing a mechanically cleaned bar screen and a Parshall flume.  Following screening and flow measurement, the flow is recombined and split between two aerated grit basins for removal of sand and heavy inert material.  A septage dump located in the pretreatment area accepts domestic waste from collection trucks.  This material is introduced to the wastewater flow upstream of the mechanical bar screens.

Downstream of grit removal, an adjustable Venturi meter is used to regulate flow to primary and secondary treatment facilities. The Venturi is normally set at 1.75 times the average daily flow to handle peak hour flows.  The maximum flow allowed to primary and secondary treatment is 77 mgd.  Excess flow during large rainfall events is diverted to the combined sewer overflow (CSO) primary clarifiers to prevent overloading of the main processes.

Downstream of the Venturi meter, the wastewater passes through Preaeration basins enroute to primary clarifiers.  Both sets of facilities were part of the original primary plant built in 1958.

Following primary treatment, the wastewater enters an activated sludge treatment process for oxidation of carbonaceous BOD and nitrification.  This system consists of four aeration basins, four secondary clarifiers and pumping systems for returning and wasting the activated sludge biomass.  Prior to the aeration basins, alum is fed to the wastewater flow to chemically precipitate phosphorus, which is subsequently removed from the liquid stream in the secondary clarifiers.

A conventional chlorination process is used to disinfect the wastewater, with sulfur dioxide added to the final effluent for dechlorination.  The plant discharge structure consists of a Parshall flume followed by a chute that conveys the effluent to the north side of the river.  No subsurface outfall is provided.

Liquid Treatment – Winter Permit Season.  During the winter permit season, the major operational change is the elimination of alum feed.  This process change alters the nature of the mixed liquor in the activated sludge process and significantly reduces the amount of secondary sludge produced.

Liquid Treatment – Combined Sewer Overflow Event.  During peak flow events that exceed the capacity of the secondary treatment processes, excess flow is diverted to CSO clarifiers following preliminary treatment (screening and grit removal).  During small CSO events, the diverted wastewater may be stored in the CSO clarifiers and returned to the plant for full treatment following the peak flow event.  During more substantial CSO events, the diverted flow receives primary treatment and is combined with effluent from the secondary/tertiary process prior to disinfection and dechlorination.  Following the CSO event, the contents of the CSO clarifiers are returned to the plant for full treatment.

In 1997 through 1999, the annual volume of flow treated through the CSO clarifiers ranged from 22 to 222 MG, which represented 0.15 to 1.34 percent of the total volume of flow treated at the SAWTP during the year.  The number of occasions the CSO clarifiers were used ranged from 9 to 27 times per year.

Solids Processing.  Solids are generated by preliminary treatment (screenings and grit), the primary and CSO clarifiers (primary sludge), and the secondary clarifiers (waste activated and chemical sludge).  The operating mode for solids processing is the same during the summer and winter seasons, with the exceptions that chemical sludge is produced only during the summer and additional sludge storage may be required in the winter due to limitations on the City’s ability to land-apply biosolids.

Screenings are transported to the City’s Waste-to-Energy Facility for disposal; and grit is hauled to the North Side Landfill.

The solids handling facilities for primary and secondary sludge are undergoing modification.  Currently, primary sludge is thickened using gravity thickeners and secondary sludge is thickened using dissolved air flotation thickeners (DAFTs).  The two thickened sludge streams are then combined and fed to anaerobic digesters for stabilization.  Two of the digesters are operated in series to treat the sludge while a third digester is reserved for seasonal storage when weather conditions prevent hauling biosolids to offsite land application sites.  Digested sludge is dewatered using belt filter presses that produce a cake with a solids content of 18 to 20 percent.  Dewatering is practiced 5 days per week for 6 to 15 hours per day.  To equalize ammonia-nitrogen loadings to the liquid treatment processes, the filtrate from the dewatering operation is stored in a spare gravity thickener and metered back to the main wastewater flow stream.

In 1999, the City approved a phased approach to upgrade the solids processing systems. Key elements of this program include:

§         Co-thickening of primary and secondary sludge using gravity belt thickeners and discontinuing use of the gravity thickeners and DAFTs.

§         Installation of new belt filter presses and new storage facilities for dewatered biosolids.

§         Incorporation of recuperative thickening of anaerobically digesting solids to increase solids storage capacity.

§         Installation of odor control facilities for the solids processing area.

§         Enhanced facilities for side-stream equalization of dewatering filtrate.

The program is scheduled to be implemented in three phases, with all work completed by 2004.  The first facilities to be installed will be the gravity belt thickeners which are projected to be operational in late 2001.

Dewatered biosolids are beneficially used through land application to agricultural fields in Spokane and Lincoln Counties.  In 1999, 7,515 dry tons of biosolids were applied to 2,603 acres.

Effluent Quality Requirements

On March 30, 2000 the Washington Department of Ecology issued a new NPDES discharge permit for the SAWTP, which supersedes the last permit issued in 1992.  A summary of effluent quality requirements is presented in ..  Key changes to permit requirements are summarized below:

§         A revision in the allowable mass discharge for ammonia-nitrogen during both low-flow and high-flow permit seasons.

§         Inclusion of a concentration limit for ammonia-nitrogen in addition to a mass limit.

§         Elimination of an ammonia-nitrogen limit when the Spokane River’s 7-day average flow exceeds 5,000 cfs.

§         A revision in the allowable mass discharge for chlorine residual during both low-flow and high-flow permit seasons.

§         Inclusion of a concentration limit for chlorine residual in addition to a mass limit.

§         Addition of concentration limits for cadmium, lead and zinc during both the low-flow and high-flow seasons.

§         Deletion of limits for mercury and silver.

§         Lowering of the maximum allowable effluent pH value.

Treatment Performance

The historical performance of the SAWTP with respect to permit compliance has been reviewed in several recent documents ^x,xii,[xiv].  Generally, the compliance history has been very good.

Ecology reviewed compliance history with the 1992 permit for the period July 1, 1996 through July 30, 1998.  They identified two occurrences of elevated chorine residual during CSO events; however, the compliance schedule for the plant contained a companion Administrative Order allowing exemptions from the daily maximum chlorine limit if it is caused by excessive treated CSO flow being recombined with the normally treated plant effluent.  To improve control of the disinfection process, CSOs are now combined with the main plant effluent prior to disinfection and dechlorination.  This has reduced the mass of chlorine discharged to the river during high flow events.

The City reviewed plant performance for the period 1997 through 1999, comparing effluent quality to the requirements of the newly issued (March 2000) NPDES permit.  Key observations are summarized below:

§         BOD.  Mass discharge limits were met for the entire period.  Concentration limits and monthly percent removal requirements were met for the entire period with the exception of February 1999, when violations of the weekly concentration and percent removal requirements occurred.  The violations coincided with an upset in the activated sludge process that is believed to have been caused by introduction of a toxic material to the wastewater flow.

§         Suspended Solids.  Mass discharge limits and percent removal requirements were met for the entire period.  Concentration limits were violated only in February 1999 for the weekly limit.  This episode corresponded with the process upset described above for BOD.

§         Fecal Coliform.  The SAWTP met monthly limits for the entire period, but experienced three violations of the weekly limit.

§         Ammonia-Nitrogen.  Ammonia-nitrogen limits were met once the new aeration system was placed in operation in August 1997.

§         Phosphorus.  The 85-percent removal requirement was met during each of the low-flow permit seasons.

§         Metals.  Limits for cadmium, lead and zinc are based on historical performance of the SAWTP plus a 10 percent compliance buffer.  The SAWTP has complied with these limits.

Process Capacity

Following the re-rating of plant capacity in 1979, the SAWTP has generally been viewed as having an average dry-weather capacity of 44 mgd and a maximum secondary treatment capacity of 77 mgd.  The latter value is based on a 1.75 peaking factor historically used by the City.  The preliminary treatment facilities have a design capacity of 146 mgd and the CSO treatment systems has an estimated capacity of 70 mgd.

With the issuance of the new NPDES discharge permit and considering strategies for handling excess flows during CSO events, the City is in the process of reevaluating the capacity of the individual unit processes.  A report summarizing their findings is expected in late 2000.  Preliminary results indicate that near-term capacity bottlenecks will occur in the solids handling and activated sludge processes.  The solids handling limitations will be addressed through the phased improvement program described earlier.  The activated sludge limitation will likely require an additional aeration basin and secondary clarifier, or conversion of a CSO clarifier to this purpose.  It is anticipated that the City report will define the new capacity of the SAWTP once the near-term improvements are implemented. 

Currently, the plant is operating at approximately 88 percent of its nominal capacity rating.  In 1999, the average annual flow to the SAWTP was 39.5 mgd and the average dry season flow (May 1 through October 31) was 38.9 mgd.  Typical diurnal flows varied from 20 mgd during the early morning hours to 45 to 50 mgd during the afternoon.  During this period, the plant was generally able to meet the new permit conditions, as described above.

Process Condition

The SAWTP is generally in sound physical condition; however, some process equipment have been in operation for 20 to 40 years and are nearing the end of their useful lives.  The condition issues associated with solids handling facilities are largely being addressed through the phased improvement program described earlier.  Key elements of the liquid treatment system that have been identified as needing replacement are 1) the primary clarifier mechanisms, 2) the secondary clarifier mechanisms, 3) the return activated sludge pumps, 4) the waste activated sludge pumps, and 5) components of the plant-wide HVAC and plant water systems.  In addition, fire code requirements may necessitate improvements to the disinfection and dechlorination systems. (City of Spokane, 2000)

Staffing

Ecology has classified the SAWTP as a Class 3 treatment plant that impacts the certification requirements for the operating staff.  The plant is staffed 24-hours per day, seven-days per week by a minimum crew size of three people (one Class 3 and two Class 2 operators) working 8-hour shifts.  The crews are supported by additional administration, laboratory and management personnel. (Ecology, 2000)

5.5        Spokane County Wastewater Management Ordinances

The County has established an ordinance for controlling sewer use.  The ordinance establishes a fee arrangement for capital facilities, and disallows the discharge of stormwater to the sanitary sewer system.

In accordance with the interlocal agreement between the City and County, the County has revised the sewer use ordinance to amend the industrial wastewater pretreatment program.  The County sewer use ordinance also establishes industrial pretreatment limits to prevent the discharge of toxic compounds that could cause interference with wastewater treatment or conveyance, or cause effluent or biosolids contamination.  The ordinance sets charges for wastewater strength in excess of domestic wastewater.

Through an agreement with the Town of Millwood, The Town is required to implement similar pretreatment requirements.