Spokane County Wastewater Facilities Plan

Treatment Plant Location Alternatives

 

Introduction

During the fall of 2000, alternatives for providing wastewater treatment capacity for Spokane County were identified and screened.  The following concepts survived the screening process.

Ř      Alternative 1 – All Flow to the existing Spokane Advanced Wastewater Treatment Plant (SAWTP)

Ř      Alternative 2 – A New Mid-Valley Plant Combined with SAWTP and/or New North Spokane Plant

Ř      Alternative 3 – New In-City Plant Combined with SAWTP and/or New North Spokane Plant

Ř      Alternative 4 – Multiple Mid-Valley Plants Combined with SAWTP and/or New North Spokane Plant

 

For Alternatives 2, 3 and 4, a number of “sub-alternatives” were developed that represented differing allocations of treatment capacity amongst the multiple treatment facility locations.

 

The following discussion provides a brief description of each alternative and sub-alternative.  Facility requirements are based on the projected flow that will be generated from the County’s service area in the year 2025.

 

Accompanying the descriptions are simplified flow schematics.  These schematics:

Ř      Depict the amount of flow generated in each of the County’s major service areas:  North Spokane, North Valley and Spokane Valley. 

Ř      Present the number and size of treatment facilities.

Ř      Identify major facilities required to convey the wastewater.

 

Within the schematics, two flow values are usually shown.  The average flow represents the annual-average daily flow, expressed in millions of gallons per day.  The peak flow represents the maximum flow rate that must be conveyed or treated.  Peak flows usually occur following major, intense rainfalls.  The flow values represent County flows only.  Flows generated by the City of Spokane or other jurisdictions are not presented.

 

Alternative 1 – All Flow to Spokane Advanced Wastewater Treatment Plant (SAWTP)

Alternative 1 continues the current practice for conveyance, treatment and discharge of wastewater; however, it expands and improves those practices to meet anticipated permit requirements and to accommodate projected growth as shown in Figure 1.   In this approach, the SAWTP would be expanded to handle all flow from both the County and City through the year 2025.  The County’s average flow in the year 2025 is projected to be 22 mgd, whereas the combined City/County flow is estimated to be 65 mgd.  It is likely that this combined flow would exhaust the capacity of the current SAWTP site, requiring the construction of additional treatment facilities to handle the region’s growth beyond 2025. 

 

The County and City conveyance systems also would need to be expanded to handle the projected flows.  The major cost would be associated with conveying County flows from the Valley to the SAWTP.  Based on the current interlocal agreement between the City and County, the allowable peak flow through the City’s interceptor system is limited to 15.5 mgd.  This falls far short of the projected peak flow of 37 mgd from the Valley.   Providing the additional conveyance capacity is complicated by the fact that the City experiences multiple combined sewer overflows (CSOs) along the interceptor system that conveys Valley flows.  Given this significant restriction, it has been assumed that all Valley flows in excess of 15.5 mgd would be pumped through a new force main to the SAWTP.

 

In North Spokane, some segments of the City’s interceptor system need to be increased in size to handle the future County flows.

 

Alternative 2 – New Mid-Valley Plant Combined with SAWTP and/or New North Spokane Plant

Overview

In this concept, a new treatment plant would be constructed in the eastern part of the Spokane Valley service area.  This plant would serve part or all of the County flow generated in the Spokane Valley and could serve flow from the eastern portion of the City’s service area.  To handle all County flows, additional capacity would be provided at the SAWTP and/or at a new plant located in North Spokane.


Five sub-alternatives were identified that involve different allocations of flow to the three treatment plant sites.  These allocations are summarized in Table 1.

Table 1.  Allocation of Projected Average Flows from Spokane County in 2025 (mgd)

Alternative

SAWTP

Mid-Valley1

North Spokane

Total

2A

10

11.9

0

21.9

2B

10

7.0

4.9

21.9

2C

14.9

7.0

0

21.9

2D

4.9

17.0

0

21.9

2E

0

17.0

4.9

21.9

1.       Value shown is County flow only.  The Mid-Valley plant also could receive flow from the City of Spokane’s service area.

Sub-Alternative 2A – SAWTP at 10 mgd; No Plant in North Spokane

Through a 1980 interlocal agreement with the City of Spokane, the County purchased 10 mgd of capacity in the SAWTP.  In this concept, the County would retain this capacity allocation in the SAWTP to treat all flow from the North Spokane Service Area and a portion of the flow generated in the Spokane Valley.  Valley flow that could not be sent to the SAWTP would be treated at the new Mid-Valley Plant that would discharge to the Spokane River below Upriver Dam.  A schematic of Sub-Alternative 2A is shown in Figure 2.   A new pump station and force main would be required to convey flow from the Spokane Valley Interceptor to the new treatment plant.

Sub-Alternative 2B – SAWTP at 10 mgd with North Spokane Plant

This alternative retains the County’s 10 mgd capacity at SAWTP and builds a new plant in North Spokane to handle flows from that service area.  Compared to Sub-Alternative 2A, this would reduce the amount of County flow to be treated at a new Mid-Valley Plant. A schematic of Sub-Alternative 2B is shown in Figure 3.

Sub-Alternative 2C – Increased Flow to SAWTP

In this approach, the County would increase the amount of flow treated at the SAWTP to 15 mgd.  This would make maximize use of the capacity that the County has purchased in the City’s interceptor system.  A new 7 mgd plant would be built in the Mid-Valley to handle the remaining flows generated in the County’s service area. Figure 4 presents a schematic of this concept.

Sub-Alternative 2D – North Spokane Only to SAWTP

In Sub-Alternative 2D, only the flow from North Spokane would be sent to the SAWTP.  All flow from the Valley would be routed to a new, 17-mgd Mid-Valley treatment plant.  Figure 5 shows this arrangement.

Sub-Alternative 2E – No Flow to SAWTP

This approach sends all County flows to new treatment plants, eliminating treatment of County flow at the SAWTP.  It requires building a 5 mgd plant to serve North Spokane and a 17 mgd plant to serve the Spokane Valley as shown in Figure 6.

Alternative 3 – New In-City Plant Combined with SAWTP and/or New North Spokane Plant

This alternative is similar to number 3, except a new In-City Plant constructed in the south-central part of the City of Spokane would replace the Mid-Valley Plant.  This plant would serve part or all of the County flow generated in the Spokane Valley and would likely serve flow from the southern portion of the City’s service area.  Again, treatment of County flows would be provided at least two and possibly three locations.  The division of flows for the Alternative 3 sub-alternatives is presented in Table 2.

Table 2.  Allocation of Projected Average Flows from Spokane County in 2025 (mgd)

Alternative

SAWTP

In-City1

North Spokane

Total

2A

10

11.9

0

21.9

2B

10

7.0

4.9

21.9

2C

14.9

7.0

0

21.9

2D

4.9

17.0

0

21.9

2E

0

17.0

4.9

21.9

2.       Value shown is County flow only.  In-City plant would also receive flow from the City of Spokane’s service area.

Sub-Alternative 3A – SAWTP at 10 mgd; No Plant in North Spokane

In this concept, the County would retain its 10 mgd capacity allocation in the SAWTP to treat all flow from the North Spokane Service Area and a portion of the flow generated in the Spokane Valley.  Valley flow that could not be sent to the SAWTP would be treated at the new In-City Plant.  Give the limited conveyance capacity owned by the County and the occurrence of CSOs along the City’s interceptors, it has been assumed that all County flow sent to the In-City plant must be pumped through a new force main.  A schematic of Sub-Alternative 2A is shown in Figure 7.

Sub-Alternative 3B – SAWTP at 10 mgd with North Spokane Plant

This alternative retains the County’s 10 mgd capacity at SAWTP and builds a new plant in North Spokane to handle flows from that service area.  Compared to Sub-Alternative 2A, this would reduce the amount of County flow to be treated at a new In-City Plant. A schematic of Sub-Alternative 2B is shown in Figure 8.

 

Sub-Alternative 3C – Increased Flow to SAWTP

In this approach, the County would increase the amount of flow treated at the SAWTP to 15 mgd.  This would make maximize use of the capacity that the County has purchased in the City’s interceptor system.  A new 7 mgd In-City Plant would be built to handle the remaining flows generated in the County’s service area. Figure 9 presents a schematic of this concept.

Sub-Alternative 3D – North Spokane Only to SAWTP

In Sub-Alternative 3D, only the flow from North Spokane would be sent to the SAWTP.  All flow from the Valley would be routed to a new, 17-mgd In-City treatment plant.  Figure 10 shows this arrangement.

Sub-Alternative 3E – No Flow to SAWTP

This approach sends all County flows to new treatment plants, eliminating treatment of County flow at the SAWTP.  It requires building a 5 mgd plant to serve North Spokane and a 17 mgd plant to serve the Spokane Valley as shown in Figure 11.

Alternative 4 – Multiple Mid-Valley Plants Combined with SAWTP and/or New North Spokane Plant

This alternative builds on the concept presented in Alternative 2, but rather than constructing a single wastewater plant in the Spokane Valley, two smaller plants would be built.  This would place treatment facilities close to 1) where the wastewater is generated and 2) where the treated effluent could potentially be sent to reuse applications (irrigation, industrial, etc.).  Two sub-alternatives considered the potential of building an additional plant in North Spokane. 

Sub-Alternative 4A – Two Mid-Valley Plants; No Plant in North Spokane

In this approach, a 7 mgd plant would be built to serve all wastewater generated in the North Valley service area.  A second 5-mgd plant would be built to serve a portion of the flow from the Spokane Valley service area.  The remaining flow from Spokane Valley and all flow from North Spokane would be sent to the SAWTP.  A schematic is shown in Figure 12.

Sub-Alternative 4B – Two Mid-Valley Plants with North Spokane Plant

In this approach the capacity of the treatment plants along the North Valley and Spokane Valley interceptors would be reduced to 4 and 3 mgd, respectively, with all remaining flow from the Valley (10 mgd) being sent to the SAWTP.  A new 5 mgd plant would be built to serve North Spokane.  Figure 13 presents this concept.