This chapter evaluates the issues of where to locate treatment
facilities to handle Spokane County’s wastewater, and how much capacity
to assign to existing or new treatment facilities. To address these
questions, five major concepts were developed and evaluated. For some
concepts, multiple “sub-alternatives” were considered.
Prior to presentation of the alternatives, information is presented
regarding the development of representative treatment and conveyance
components incorporated in the wastewater management options.
To meet future capacity needs and more stringent effluent quality
requirements, all of the alternatives include upgrades and expansions of
existing facilities and/or construction of new treatment plants. The
following sections describe the assumptions and methodology used to
determine treatment facility requirements and develop estimated costs.
Currently, Spokane County owns a capacity allocation in the Spokane
Advanced Wastewater Treatment Plant (SAWTP) of 10 mgd, based on “average
dry-weather flow”. The various alternatives involve treatment scenarios
that maintain, increase or decrease the County’s capacity allocation at
the SAWTP.
For each scenario, it has been assumed that the City of Spokane will
continue to use the SAWTP for wastewater treatment. It also has been
assumed that the level of treatment at the SAWTP will be improved to
meet future, more stringent effluent quality requirements. Based on
discussions with City staff and their consultant, the process
configuration shown in Figure 6‑1 has been used as the basis for
facility planning. This treatment train assumes continued use of
chemical phosphorus removal addition, addition of ballasted flocculation
and filtration, and conversion to UV disinfection.
Facility requirements and costs associated with the SAWTP are presented
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found., Error! Reference source
not found.. A summary of the findings is presented in the
following discussion.
Maintain Current Capacity Allocation
To maintain the current 10 mgd capacity allocation, the County must
increase its investment in the SAWTP. There are several reasons for
this:
·
Restoration of 44 mgd capacity. Following the
major expansion of the SAWTP in the late 1970s, the plant had a nominal
“dry weather” capacity of 44 mgd, of which 10 mgd was allocated to the
County and 34 mgd to the City. With the establishment of more stringent
ammonia-nitrogen limits in recent NPDES discharge permits, the
Figure 6‑1.
Anticipated Future Process for SAWTP
|
City’s consultant has estimated that the nominal capacity of the current
facility has been reduced to 40 mgd. Restoring the capacity to 44 mgd
requires increasing the aeration basin volume and implementing other
secondary treatment improvements.
·
Maintenance and safety improvements. A number of
aging equipment items such as clarifier mechanisms, pumps and digester
covers are deteriorating, and require replacement or upgrades. A
reserve account for equipment replacement has not been established as
part of the SAWTP operating fund, so these measures must be funded from
the capital program. Major system improvements also are needed to
improve odor control, upgrade solids processing facilities, convert
disinfection to ultraviolet (UV) radiation, and improve access for
maintenance. These improvements do not significantly increase the
capacity of the plant, but are needed to improve operation or to resolve
aesthetic, safety, or code compliance issues.
·
Improved level of treatment. To meet anticipated
future effluent quality requirements, it has been assumed that effluent
filtration will be required. Furthermore, the City’s consultant has
projected that ballasted flocculation may be needed as an intermediate
process between the secondary clarifiers and filters. Other process
modifications will include conversion of the activated sludge process to
a step-feed system providing partial denitrification.
The total projected capital cost of these improvements, including
contingencies and allowances is $100 million. The County’s
proportionate share (10/44th) of the costs would be $22.7 million. This
equates to a unit cost of $2.27 per gallon of dry weather capacity.
Increase Capacity Allocation
The City’s consultant developed preliminary estimates of the facility
requirements and capital cost to increase the dry weather capacity of
the SAWTP to 75 mgd. These projections were then adjusted to match a
flow rate of 65 mgd, which is the projected combined dry-weather flow
rate from the City and County in 2025 (see
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The total projected cost to take the plant from its current condition to
a 65 mgd capacity and improved level of treatment is $216 million.
Subtracting the cost to maintain a nominal capacity of 44 mgd with
improved treatment levels results in a cost of $116 million associated
with capacity expansion. For the additional 21 mgd of capacity
attained, the unit cost is $5.54 per gallon.
Reduce Capacity Allocation
If the County were to reduce its 10 mgd capacity allocation in the
SAWTP, the unused allotment could be sold the City, or perhaps to
another agency. The true value of this capacity can be determined only
through negotiations; however, a brief analysis was conducted to provide
insight as to the potential value of this asset.
Sale to City
If the treatment capacity were sold to the City, two primary
considerations would impact the potential price:
·
Deferred cost of plant expansion. This is the avoided
cost associated with using existing County capacity rather than
expanding the SAWTP. As shown above, the unit capital cost would be
$5.54 per gallon.
·
Upgrade cost to address maintenance and compliance
issues. To make the County’s capacity of use to the City,
investments would be needed for maintenance, odor control, code
compliance and enhanced operation. The unit capital cost of these
investments is $2.27 per gallon.
The total potential value would equal the difference between the two
items. This results in a net unit value of $3.27 per gallon.
In addition to the value of the treatment facilities, reduction of
County flows to the City system would free up conveyance capacity in the
City’s interceptor system. This capacity has value to the City,
particularly with respect to CSO control. This issue is addressed later
in this chapter under the heading “Development of Conveyance
Requirements”.
Sale to Another Agency
If the County’s unused capacity allocation were sold to another agency,
the following considerations would impact the potential price:
·
Deferred cost of new plant construction. This is the
avoided cost of building new treatment capacity. The estimated
present-worth unit cost for this capacity is roughly $8.00 per gallon,
as discussed in the next section on “new treatment plants”.
·
Upgrade cost to address maintenance and compliance
issues. This cost is the same as described for “Sale to City” --
$2.27 per gallon.
The total potential value to a new agency would be the difference
between these two unit costs, or $5.73 per gallon.
Operating Cost of SAWTP
At the beginning of 2001, the City charged the County a unit cost for
operations and maintenance (O&M) of $1,171 per million gallons (MG).
The monthly charge is based on the sum of the readings for the North
Spokane, North Valley and Spokane Valley meter stations. Recently, the
City announced that the unit O&M charge will increase. At the time of
this report, the new rate had not been announced.
To facilitate an “apples to apples” comparison of the treatment
alternatives, each of the potential new treatment facilities was assumed
to have similar processes for treatment of the liquid and solids
streams. Facility requirements and cost estimates are based on the
process configuration shown in Figure 6‑2.
The liquid treatment scheme would be similar to that projected for the
future SAWTP, with the exception that biological phosphorus would be
provided, eliminating the need to feed alum to the primary clarifiers.
Sludge treatment would consist of gravity thickening for primary sludge,
flotation thickening for secondary and chemical sludge, anaerobic
digestion, sludge storage, and dewatering. The resulting Class B sludge
would be hauled by truck to agricultural fields for beneficial reuse.
It is anticipated that this treatment system would produce effluent
quality equal to or better than the upgraded SAWTP treatment process
shown in Figure 6-1. Effluent from this facility would be suitable for
discharge to surface water or may be used in a variety of reuse
applications such as urban or agricultural irrigation.
Once the preferred number, location and capacities of the treatment
plants have been determined, other effluent and biosolids management
options (see Chapters 5 and 8) may be incorporated into the preferred
wastewater management concept.
Figure 6‑2.
Anticipated Future Treatment Process for New Plant
|
Capital Costs for New Treatment Plants
Estimates of the unit capital cost for new treatment facilities were
developed in two ways:
·
Comparison with recent, similar plants. In this
approach, construction cost data were obtained for four treatment plants
built in the past decade that had similar characteristics to the
proposed concept for a new Spokane County plant. These plants had
design capacities ranging from 4 to 12 mgd. The cost data for each
plant were then adjusted to account for differences in the types of
treatment processes provided, the level of architectural treatment and
odor control implemented, and such allied costs as contingencies, sales
tax, design allowances, etc. Finally, the costs were adjusted to 2001
dollars and a $1.00 per gallon “safety factor” was added to account for
miscellaneous costs that may not have been accounted for in the cost
analysis.
Figure 6‑3.
Projected Unit Capital Cost for New Treatment Plants
|
·
Process-by-process estimates. HDR’s Envision Cost
Model was used to develop projected costs for each of the unit treatment
processes anticipated in the new plants. The unit process costs were
then totaled to produce an estimate for the complete plant. The cost
model is based on cost curves that HDR developed for the U.S.
Environmental Protection Agency.
Based on these analyses, the unit capital costs shown in Figure 6‑3
were used in the alternative comparisons. These costs demonstrate the
economy of scale associated with new plants in the capacity range of 3
to 12 mgd.
Operating Cost for New Treatment Plants
Recent benchmarking studies of competively-operated wastewater treatment
facilities indicate that a 10 to 20 mgd plant providing nutrient
removal, effluent filtration and Class B biosolids production would be
expected to have an operating cost of $900 to $1,000/MG of wastewater
treated. For smaller plants, the operating cost would increase
slightly. For this study, operating costs of $1,000 to $1,200/MG were
used, depending on the size of the plant. Unit operating costs were
assumed to increase at a rate of 3 percent per year.
All of the alternatives involve new or expanded conveyance facilities to
route projected flows to the treatment plants. In this evaluation, the
analysis for conveyance facilities is limited to the following areas:
·
Major interceptor and/or pumping facilities needed to
convey flow from the North Valley and Spokane Valley Interceptors to new
or existing treatment facilities.
·
Interceptor improvements needed to convey North Spokane
flow to the SAWTP.
·
Interceptor improvements needed to reroute North Spokane
flow to a new treatment facility.
Collection system requirements upstream of these facilities were
considered common to all alternatives and are addressed in the Year
2001 Comprehensive Wastewater Management Plan for Spokane County.
To allow fair comparison of the alternatives, cost estimates for
conveyance facilities were based on the providing sufficient capacity to
handle projected peak flows through the year 2025. When these
facilities are actually implemented, design would be based on a longer
planning horizon (50 years).
Sizing of gravity systems was based on accommodating peak flows with a
water depth in the pipeline of 70% of the pipe diameter. Cost estimates
were based on data from the County’s sewering program, updated to 2001
dollars.
Cost estimates for pumping stations and force mains were based on HDR’s
cost data for similar facilities constructed n the Northwest. All costs
were updated to 2001 dollars.
If the County were to reduce its capacity allocation in the City’s
conveyance system, the unused conveyance capacity potentially could be
sold the City. The true value of this capacity would need to be
determined through negotiations between the City and County. In this
analysis, a nominal amount was included for this transaction.
For the baseline comparison of the
alternatives, no costs have been included for conveyance of treated
effluent to reuse applications.
During the fall of 2000, workshops were held to brainstorm and screen
alternatives for providing wastewater treatment capacity for Spokane
County. The following concepts survived the screening process.
·
Alternative 1 – All Flow to Spokane Advanced Wastewater
Treatment Plant (SAWTP)
·
Alternative 2 –New Mid-Valley Plant Combined with SAWTP
and/or New North Spokane Plant
·
Alternative 3 – New In-City Plant Combined with SAWTP
and/or New North Spokane Plant
·
Alternative 4 – Multiple Mid-Valley Plants Combined with
SAWTP and/or New North Spokane Plant
·
Alternative 5 – No Action
Figure 6‑4.
General Locations of Potential Sites for New Wastewater
Treatment Facilities
|
Generalized locations for potential new treatment facilities are shown
in Figure 6‑4.
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 sections of this chapter provide brief descriptions 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:
·
Depict the amount of flow generated in each of the
County’s major service areas: North Spokane, North Valley and Spokane
Valley.
·
Present the number and size of treatment facilities.
·
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 (mgd). 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 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 (see Figure 6‑5). 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.
Figure 6‑5.
Alternative 1 – All
Wastewater Routed to SAWTP (Year 2025 County Flows)
|
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. This would require one or two new pumping stations and
approximately 8 miles of force main. Much of the force main would need
to be routed through densely developed portions of the City.
In North Spokane, some segments of the City’s interceptor system would
need to be increased in size to handle the future County flows.
In this concept, a new treatment plant would be constructed in the
western 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 6‑1.
Table
6‑1.
Allocation of Projected Average Flows from Spokane County in 2025
|
Alternative 1 |
SAWTP, mgd |
Mid-Valley2, mgd |
North Spokane, mgd |
Total, mgd |
|
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. Values shown are
County flow only.
2. The Mid-Valley plant also could receive flow from the City of
Spokane’s service area.
In this concept, the County would retain its current 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 a new
12-mgd Mid-Valley Plant that would discharge to the Spokane River below
the Upriver Dam. A schematic of Sub-Alternative 2A is shown in
Figure 6-6.
For initial planning purposes, it has been assumed that the new
Mid-Valley plant would be located in the vicinity of the North Valley
interceptor, but that an influent pumping station would be needed to
lift the North Valley flow into the treatment works. To convey flow
from the Spokane Valley Interceptor to the new treatment plant, a new
pump station and force main would be required.
Figure 6‑6.
Alternative 2A – 12 mgd Mid-Valley Plant, SAWTP at 10 mgd, No
North Spokane Plant (Year 2025 County Flows)
|
Figure 6‑7
Schematic of Current City of Spokane Interceptor System
|
With this approach, the amount of flow that must be conveyed from the
Spokane Valley to the SAWTP would be substantially reduced. No County
flow would be routed to the portion of City Interceptor I-05 located
upstream of the convergence with the Southeast Spokane Interceptor (see
Figure 6-7). This would reduce flows in a section of interceptor
with multiple CSO locations. Also, the total County flow from the
Spokane Valley would be reduced to an average rate of 5 mgd and a peak
rate of 10 mgd, which is well below the County’s current capacity
allocation in the City’s interceptor system (15.5 mgd). This negates
the need for the County to expand capacity in this section of the City’s
system, and helps alleviate the potential for CSOs in the central
portions of the City’s conveyance system.
To convey future North Spokane flows to SAWTP, some segments of the
City’s interceptor system must be replaced, or parallel sewers must be
installed.
This alternative retains the County’s 10 mgd capacity at SAWTP and
builds a new 5-mgd plant in North Spokane to handle flows from that
service area. To handle all 22 mgd of the County’s flow in 2025, the
required size of the new Mid-Valley Plant would be 7 mgd. A schematic of
Sub-Alternative 2B is shown in Figure 6‑8.
The North Spokane Plant would require an outfall to the Little Spokane
River. Conveyance of wastewater to the new plant would require a major
reconfiguration of the conveyance system serving North Spokane.
Currently, wastewater flows to multiple lift stations that pump the flow
southward over a ridge to the SAWTP. Reversing the direction of flow
and converting the collection system to gravity flow would involve
installation of a number of interceptors and abandonment of some current
pumping stations.
Figure 6‑8.
Alternative 2B – 7 mgd Mid-Valley Plant, SAWTP at 10 mgd, 5 mgd
North Spokane Plant (Year 2025 County
Flows)
|
The new Mid-Valley plant would receive flow primarily from the North
Spokane Interceptor. Similar to Alternative 2A, this would eliminate
County flows from entering the upper reaches of the City’s CSO-prone
I-05 interceptor (see Figure 6-7).
A difficulty with Alternative 2B is the conveyance of flows from the
Spokane Valley. In 2025, the average flow generated in this portion of
the County’s service area is projected to be 10 mgd, which matches the
County’s capacity allocation at SAWTP. However, the projected peak flow
is 20 mgd, which exceeds the County’s capacity in the City interceptor
system. Given the City’s CSO issues, the County may not be able to
obtain additional conveyance capacity in this section of the City’s
system, or it may prove to be expensive. For planning purposes, it has
been assumed that the County would install a pumping station along the
Spokane Valley Interceptor that would send all flow above 15.5 mgd to
the new Mid-Valley Plant. This would require that the Mid-Valley plant
be designed to withstand high peak flows. The Spokane Valley pumping
station would operate only intermittently during peak flow conditions.
In this approach, the County would increase the amount of flow treated
at the SAWTP to 15 mgd, with 5 mgd originating in North Spokane and 10
mgd being sent from the Spokane Valley service area. 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 6‑9 presents a schematic of
this concept.
The Mid-valley plant would be identical to that presented for
Sub-Alternative 2B. Similarly, the conveyance facilities required to
handle North Valley and Spokane Valley flows would be identical to
Sub-Alternative 2B.
Figure 6‑9.
Alternative 2C – 7 mgd Mid-Valley Plant, SAWTP at 15 mgd, No
North Spokane Plant (Year 2025 County Flows)
|
To convey future North Spokane flows to SAWTP, some segments of the
City’s interceptor system must be replaced, or parallel sewers must be
installed.
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 6‑10 shows this arrangement.
Facility requirements for conveyance of North Spokane flows would be the
same as described for Sub-Alternative 2A.
Figure 6‑10.
Alternative 2D – 17 mgd Mid-Valley Plant, SAWTP at 5 mgd, No
North Spokane Plant (Year 2025 County Flows)
|
To convey Spokane Valley flows to the new Mid-Valley plant, a 20 mgd
pumping station and force main would be needed.
This alternative completely removes all County wastewater from City
interceptors I-05, I-04 and I-02, all of which have significant capacity
limitations or CSOs (see Figure 6-7).
Implementation of Alternative 2D would allow the County to sell to the
City its conveyance capacity between the Valley and SAWTP.
Figure 6‑11.
Alternative 2E – 17 mgd Mid-Valley Plant, No Flow to SAWTP, 5
mgd North Spokane Plant (Year 2025 County
Flows)
|
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 (see Figure 6‑11).
In this alternative, the County would sell treatment and conveyance
capacity back to the City.
This alternative is similar to Number 2, 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 two and possibly three locations.
The division of flows for the Alternative 3 “sub-alternatives is
presented in Table 6‑2.
Table
6‑2.
Allocation of Projected Average Flows from Spokane County in 2025
|
Alternative 1 |
SAWTP, mgd |
In-City2, mgd |
North Spokane, mgd |
Total, mgd |
|
3A |
10 |
11.9 |
0 |
21.9 |
|
3B |
10 |
7.0 |
4.9 |
21.9 |
|
3C |
14.9 |
7.0 |
0 |
21.9 |
|
3D |
4.9 |
17.0 |
0 |
21.9 |
|
3E |
0 |
17.0 |
4.9 |
21.9 |
1. Values
shown are County flow only.
2. In-City plant would also receive flow from the City of
Spokane’s service area.
Figure 6‑12.
Figure 6-12. Alternative 3A – 12 mgd In-City Plant, SAWTP at 10
mgd, No North Spokane Plant (Year 2025 County Flows)
|
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. Given 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 3A is shown in
Figure 6‑12.
Figure 6‑13.
Alternative 3B – 7 mgd In-City Plant, SAWTP at 10 mgd, 5 mgd
North Spokane Plant (Year 2025 County Flows)
|
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 3A, this would reduce the amount of
County flow to be treated at a new In-City Plant. A schematic of
Sub-Alternative 3B is shown in Figure 6‑13.
Figure 6‑14.
Alternative 3C – 7 mgd In-City Plant, SAWTP at 15 mgd, No North
Spokane Plant (Year 2025 County Flows)
|
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 6‑14 presents a schematic
of this concept.
Figure 6‑15.
Alternative 3D – 17 mgd In-City Plant, SAWTP at 5 mgd, No North
Spokane Plant (Year 2025 County Flows)
|
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 6‑15 shows this arrangement.
Implementation of Alternative 3D would allow the County to sell to the
City its conveyance capacity between the Valley and SAWTP.
Figure 6‑16.
Alternative 3E – 17 mgd In-City Plant, No Flow to SAWTP, 5 mgd
North Spokane Plant (Year 2025 County Flows)
|
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 (see Figure 6‑16).
In this alternative, the County would sell treatment and conveyance
capacity back to the City.
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.
Figure 6‑17.
Figure 6-17. Alternative 4A – Two Mid-Valley Plants (7 mgd & 5
mgd), SAWTP at 10 mgd, No North Spokane Plant
(Year 2025 County Flows)
|
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
6‑17.
Figure 6‑18.
Alternative 4B – Two Mid-Valley Plants (4 mgd & 3 mgd), SAWTP at
10 mgd, 5 mgd North Spokane Plant (Year
2025 County Flows)
|
In Sub-Alternative 4B 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 6‑18
presents this concept.
Under this alternative, Spokane County would take no action to improve
or expand wastewater conveyance and treatment facilities. Consequently,
it would fail to provide adequate capacity to complete the septic tank
elimination program, and for projected growth within the service area,
and may fail to meet permit requirements for discharge to the Spokane
River. No action may result in fines being levied against Spokane
County. Most likely, it would also result in a building moratorium for
the County and may result in the imposition of judicial control over the
County’s wastewater management program.
Projected capital costs were developed for each alternative and
subalternative. These are summarized in
Error! Reference source not found..
It should be noted that the capital costs are presented as if all of the
infrastructure components needed to accommodate Year 2025 flows were
built today (2001). To simplify the analysis, no consideration was
given to the optimal phasing of the individual alternatives.
The costs in Error! Reference
source not found. are divided into the following categories:
·
Treatment Capacity Construction. These line items
reflect the cost to:
·
Upgrade and maintain the 10 mgd of capacity that the
County owns in the SAWTP. All alternatives include this $22.7 million
line item. If part of the County’s current capacity in the SAWTP is to
be sold to the City, that cost adjustment is addressed under “Treatment
Capacity Sale”.
·
Build 12 mgd of additional capacity at the SAWTP and/or
new treatment facility locations. These costs include the treatment
plants and outfalls to the receiving streams.
·
Treatment Capacity Sale. For those alternatives
that involve reducing the County’s flow to SAWTP below 10 mgd, this
value reflects the estimated “fair market value” for sale of the unused
capacity to the City. To attain this value, the County must first
participate in upgrading the current SAWTP plant to restore capacity,
provide maintenance and safety improvements and improve the level of
treatment.
·
Land. Costs for land are included for new
treatment plant sites. The cost is based on 2 acres per mgd of
capacity. The unit price for land varies with the location of the
proposed treatment plant, as described in Chapter 3.
·
Treatment Plant Siting/Permitting. For each new
treatment site, an estimated cost of $1 million was allowed for siting,
permitting and environmental documentation.
·
Conveyance Construction. These line items reflect
the cost of major pumping stations, force mains and interceptors needed
to convey the sewage to the treatment plants
·
Sale of Conveyance Capacity. This line item
identifies the potential value that may be received from sale of unused
conveyance capacity to the City. It applies to those alternatives that
divert flow away from the City’s interceptor system.
As described earlier in this chapter, the current unit operating charge
at the SAWTP is $1,171/MG. Benchmarking surveys of
competively-operated treatment plants similar to those anticipated for
Spokane County indicate a unit operating cost of $900 to $1,200/MG may
be anticipated, with the higher cost associated with small treatment
facilities (3 to 6 mgd).
To assess the impact of operating costs on the alternatives comparison,
a present value calculation was made of the projected cost to treat
Spokane County flows from 2007 through 2025. During this period, the
unit operating costs were assumed to increase by 3 percent each year,
and the present value calculation was based on a 6.375 percent discount
rate. The analysis found that each $100/MG increase in the unit
operating cost translates to a present value increase of $10 million.
Following development of the 14 subalternatives, a workshop was held to
reduce the number of final alternatives to a manageable number for
detailed comparison. Primarily, this involved selecting the best
subalternative for Alternatives 2, 3 and 4. In conducting this
screening, the following conclusions were reached:
·
For Alternatives, 2, 3 and 4, it is cost effective to
maintain the County’s current 10 mgd of capacity at SAWTP. This
conclusion eliminated alternatives that sold all or part of the County’s
capacity in SAWTP (Subalternatives 2D, 2E, 3D and 3E.)
·
Siting a new treatment plant in North Spokane proved
unattractive for the following reasons: 1) the North Spokane conveyance
system would require major realignment to accommodate this change, 2)
there are few suitable sites for a new treatment plant in North Spokane,
3) from an overall “cost of conveyance” perspective, it is best to send
all North Spokane flow to SAWTP and to reduce the flow from the Spokane
Valley to SAWTP, and 4) it was recognized that siting a new plant in
North Spokane and permitting a new discharge to the Little Spokane River
would be controversial, resulting in potential implementation delays.
Consequently, Subalternatives 2B, 3B and 4B were eliminated.
·
Subalternatives 2C and 3C were considered unattractive
relative to other remaining alternatives because they required
constructing new treatment plants plus constructing new conveyance
capacity between the Spokane Valley and SAWTP. The remaining
alternatives eliminated one or the other of these requirements.
Based on this screening exercise, the selected finalists were as
follows:
·
Alternative 1 – All Flow to SAWTP
·
Alternative 2A – 12 mgd Mid-Valley Plant, SAWTP at 10 mgd,
No North Spokane Plant
·
Alternative 3A – 12 mgd In-City Plant, SAWTP at 10 mgd, No
North Spokane Plant
·
Alternative 4A – Two Mid-Valley Plants (7 mgd and 5 mgd),
SAWTP at 10 mgd, No North Spokane Plant
·
Alternative 5 – No Action
The final treatment location alternatives were compared relative to one
another using the evaluation criteria developed for this planning
effort. A summary of the comparison is presented in
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Key findings are presented in the following sections, with emphasis
placed on factors that strongly differentiate the alternatives.
Comparison of the treatment and conveyances alternatives with the
evaluation criteria is summarized in Figure 6‑19
This criterion addresses the alternative’s ability to meet the County’s
near-term and long-term requirements for wastewater treatment capacity.
The near-term requirement relates to the ability to implement the
solution before 2007, when County flows are projected to reach the
capacity allocation in the SAWTP. The long-term requirement relates to
the alternative’s ability to provide a long-term (50-year) solution.
Alternative 1 rated poorly against this criterion because 1) it does not
offer a long-term capacity solution and 2) it may be difficult to
implement by 2007. In 2025, the projected wastewater flows from the
City and County will total 65 mgd. If all of this flow is treated at
SAWTP, the estimated maximum capacity of the site will be reached and
conveyance systems feeding the plant will be at or above current
capacity. Consequently, additional treatment plants at new locations
would likely be needed to handle flows beyond 2025. Implementing
Alternative 1 would require negotiation of a new interlocal agreement
with the City and construction of major conveyance facilities through
urban areas.
Alternative 3A rated only slightly higher than Alternative 1. While
this alternative provides an effective long-term capacity solution, its
ability to be implemented by 2007 is strongly dependent on development
of the City’s strategy for CSO control. At this time, the City is
developing planning tools and gathering data to develop a CSO control
plan. Identification of recommended control measures, including the
potential role of new treatment plants within the CSO management
strategy, is not expected for 2 to 3 years. Delaying key implementation
decisions by this amount of time would make it very difficult to site
and build a new treatment plant by 2007.
Figure 6‑19.
Comparison of Treatment Location Alternatives with Evaluation Criteria
Alternatives 2A and 4A rated high against the capacity criterion. Both
alternatives provide considerable flexibility as long-term capacity
solution, including the ability to accept wastewater from portions of
the City’s service area. With respect to implementation, the
alternatives are not dependent on completion of the City’s CSO plan.
Alterative 4A rated somewhat lower than Alternative 2A because two
facilities would need to be sited and built, which may require
additional implementation time.
Alternative 5 (No Action) received the lowest rating because it fails to
meet the County’s near-term and long-term capacity needs.
This considers the operational complexity of the alternative. Since the
alternatives will likely use similar treatment technologies, the number
of plants and major pumping facilities that must be maintained primarily
affects the criterion. Fewer facilities result in a higher rating.
Consequently, Alternative 1 rated the highest and 4A rated the lowest.
This criterion applies to the extent and complexity of sewers and pump
stations that must be implemented. Alternative 1 has the greatest
conveyance requirements, including routing of a major force main though
urban areas. Similarly, Alternative 3A requires construction of a new
force main through the downtown area, although the pipeline length is
shorter than for Alternative 1. Alternatives 2A and 4A also include
pump stations and force mains, but these facilities are smaller than
those needed for Alternatives 1 and 3A, and would be located in areas
where construction would be less difficult and disruptive. Alternative
5 (No Action) would build no conveyance facilities.
Here, consideration is given to challenges associated with acquiring
property and permits. Generally, siting an In-City plant was considered
more challenging than siting a Mid-Valley plant. Also, siting multiple
new facilities resulted in a lower rating.
Alternative 1 received the highest rating since no new treatment plants
must be sited. However, construction of the pipelines would require a
significant number of right-of-ways and permits. Alternative 2A had the
next best rating since only one new plant site in the Spokane Valley
would be required. Alternative 3A rated low because of the need to site
a new plant near downtown Spokane. Alternative 4A rated low because two
new plants must be sited and permitted. Alternative 5 has no siting or
permitting requirements.
This factor relates to the County’s ability to implement the alternative
on its own, without relying on actions by other jurisdictions. This
criterion is significant given the need to implement additional capacity
by 2007.
Alternatives 2A and 4A rated highest because the County could implement
these solutions without the need for interlocal agreements with other
jurisdictions. Alternatives 1 and 3A would require interlocal
agreements with the City. With respect to Alternative 1, the City could
elect to deny the County additional capacity at the SAWTP.
Alternative 5 (No Action) would result in the County’s failure to meet
its requirements as a utility provider and could result in the
wastewater being taken over by the courts or another entity.
This criterion addresses anticipated impacts to the receiving streams
and the potential for compliance with discharge permits. Generally,
alternatives that spread the location of effluent discharge along
receiving waters are rated higher because they lessen localized
in-stream impacts. Alternatives with multiple plants were rated lower
because they result in more permits to comply with.
Alternative 1 received the lowest rating because it provides a single
point of discharge for all City and County flows. This concentrated
loading increases the potential for toxicity or temperature impacts and
makes limited use of the river’s assimilative capacity for organic and
nutrient loadings.
Alternatives 2A, 3A and 4A spread the effluent discharge to multiple
locations along the river, lessening localized water quality impacts.
Alternatives 2A and 4A achieve a greater separation of discharge
locations than Alternative 3A and are considered better from a receiving
water perspective. Alternative 4A was de-rated somewhat because
addition of another plant increases the number of permit conditions that
must be complied with.
Alternative 5 (No Action) would result in severe compliance violations.
This factor considers the proximity of the treatment plants to potential
reuse opportunities. Alternative 4A, which was designed to maximize
reuse potential, rated the highest. Alternatives 2A and 3A also offer
significant reuse potential, with Alternative 2A being rated slightly
higher because of he industrial reuse potential associated with Inland
Empire Paper. Reuse opportunities for Alternative 1 appear more limited
and would require long pipelines to reach locations with significant
water demand. Alternative 5 provides no reuse opportunities.
This criterion addresses the environmental and community impact of
building and operating wastewater facilities. Alternatives 2A, 3A and
4A were considered similar with respect to this criterion since all
involve construction and operation of new treatment plants, pump
stations and pipelines. Alternative 1 rated somewhat higher because no
new treatment plants would be built. Alternative 5 would have no direct
construction impacts, but may result in a building moratorium that would
have a dramatic impact on the community.
This factor compares total life cycle costs, including capital and
operating expenses. This comparison is driven primarily by the relative
capital costs of the alternatives, which was summarized in
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Alternatives 1 and 2A were found to have the lowest capital costs with
Alternatives 3A and 4A having capital costs that were 10 percent
higher. Operating costs are impacted by the size of the plant (economy
of scale), treatment technology, and the age of the facilities
(maintenance requirements). It is anticipated that the new treatment
plants planned for Alternatives 2A and 3A would have unit operating
costs that are somewhat lower than the SAWTP. Alternative 4A, with two
smaller plants would have higher operating costs than Alternatives 2A
and 3A. Alternative 5 would not have capital costs, but the potential
economic impact to the community would be very negative.
This factor considers the risk associated with the economic assumptions
used in the analysis. Alternatives that relied on other jurisdictions
buying conveyance or treatment capacity from the County were rated
lower. Also, those alternatives that required other jurisdictions to
participate jointly in the financing of projects rated lower.
Alternatives 1 and 3A rated lowest because the County would be dependent
on the City to jointly finance the treatment plant expansion projects.
The cost analyses for Alternatives 3A and 4A assume that the City would
be willing to buy back conveyance capacity between the Spokane Valley
and SAWTP; however, the allocated value for this action is small ($2
million) and has little impact on the economic viability or cost
comparisons for these alternatives.
