Section 4 Wastewater Flow Projections

Section 4 Wastewater Flow

Projections

April 2015

2014 Comprehensive Wastewater Management Plan 4-1 Section 4 – Wastewater Flow Projections

4.0 Wastewater Flow Projections Wastewater flow projections were developed to serve as a basis for use in planning and system analysis of the interceptor and collection system. Total wastewater flow is a function of the combined flows from residential, commercial, and industrial customers, as well as infiltration and inflow (I&I) and contributions from future development. This section describes the methodology used to develop flow projections for the Comprehensive Wastewater Management Plan (CWMP).

4.1 Methodology

Average daily wastewater flow projections are based on both billing records and future land use information used by Spokane County (County) in its long-range planning efforts. For the existing system the flow estimate was based on an inventory of connected equivalent residential units (ERUs). Future flow estimates are generated using the following flow factors for residential and commercial/industrial land uses, which are based on observations of typical historical wastewater flow volumes associated with the respective land uses in Spokane County.

ERU-based flow factor of 200 gpd/ERU

Residential Land Uses:

o Flow factor of 1,000 gallons per day (gpd) per acre (5 ERU per acre x 2.5 people per ERU x 80 gpd per person)

o I&I factor of 125 gpd per acre (5 ERU per acre x 2.5 people per ERU x 10 gpd per person)

Commercial/Industrial Land Uses:

o Flow factor of 2,000 gpd per acre

o I&I factor of 200 gpd per acre

Flow projections have been prepared for the existing system, the 6-year and 20-year planning horizons, and the buildout condition. For the purposes of this forecast, the existing (base) year is 2013, and the 6-year and 20-year planning horizons correspond to the years 2020 and 2034, respectively. The buildout horizon assumes all parcels within the UGA are fully developed, with the exception that 10.2% of all land within the UGA is assumed to be undevelopable, due to physical constraints like wetlands and steep slopes. The development of existing and future flow estimates are described in the sections below.

4.2 Existing Flow

This section describes the existing wastewater flows in the County’s sewer system, as determined by two methods: 1) estimation of flows based on a count of current ERUs; and, 2) evaluation of historical flow records. The two approaches were taken for comparative purposes, primarily to examine the accuracy of the County’s current ERU-based flow factor of 200 gpd/ERU.

April 2015


To estimate the current flows in each of the County’s three sewer interceptors1, County staff identified the number of parcels connected to the sewer system as of June 2012, along with each parcel’s associated ERU count2. At that time, there were approximately 35,660 connected parcels representing a total of 51,302 ERUs. ERUs are broken down by five property types: single family residential, duplex, multi-family, commercial and public. The distribution of ERUs within each interceptor system by property type is shown in Table 4-1.

Table 4-1. ERUs by Interceptor (June 2012)

Interceptor

ERUs by Property Type TOTAL ERUs

Single-Family Residential

Duplex Multi-Family Commercial Public

NVI 5,986 242 1,991 2,180 40 10,439

SVI 19,297 2,040 5,150 3,945 248 30,680

NSI 7,309 378 1,366 1,130 0 10,183

TOTAL 32,592 2,660 8,507 7,255 288 51,302

Interceptor flow data collected by the County from January 2012 through December 2013 is presented in Table 4-2. Flow from the NVI and SVI are directed toward the Spokane County Regional Water Reclamation Facility (SCRWRF). Both interceptors are monitored to record flow, and to track the portion of flow pumped to the SCRWRF and the portion diverted to the City of Spokane’s Riverside Park Water Reclamation Facility (RPWRF). The data indicate that the average daily flow in the NVI during this period was about 1.8 mgd, while the average daily flow in the SVI was about 5.1 mgd. Flow in the NSI system was measured at the Marion Hay Pump Station. Average daily flow in the NSI was about 1.7 mgd.

1 North Valley Interceptor (NVI), Spokane Valley Interceptor (SVI) and North Spokane Interceptor (NSI). 2 This effort was conducted in conjunction with development of hydraulic models used to analyze the sewer

collection and conveyance system. Section 5 provides additional detail regarding model development.

April 2015


Table 4-2. Historical Average Daily Flow for NVI, SVI (Measured at SCRWRF and PS Flumes) and NSI (Measured at Marion Hay Pump Station)

Year Month Total NVI

Flow (mgd)

Total SVI Flow (mgd)

Total Flow NVI + SVI

(mgd)

Flow Diverted to RPWRF

(mgd)

Flow Pumped to SCRWRF (mgd)

NSI Flow (mgd)

2012 January 1.7 5.1 6.8 1.8 5.0 1.7

February 2.1 5.0 7.1 2.1 5.0 1.7

March 1.7 5.0 6.7 1.2 5.5 1.8

April 1.7 5.4 7.1 1.2 5.9 1.7

May 1.9 5.1 7.0 0.9 6.1 1.6

June 1.7 5.1 6.8 0.7 6.1 1.5

July 1.8 5.0 6.8 0.1 6.7 1.5

August 1.8 4.9 6.7 0.0 6.7 1.6

September 1.8 4.9 6.7 0.0 6.7 1.8

October 1.8 5.0 6.8 0.0 6.8 2.1

November 1.8 4.9 6.7 0.0 6.7 1.8

December 1.8 5.4 7.2 2.8 4.4 1.7

2013 January 1.8 5.1 6.9 0.0 6.9 1.8

February 1.8 5.1 6.9 0.0 6.9 1.8

March 1.8 5.2 7.0 1.6 5.4 1.7

April 1.8 5.1 6.9 0.7 6.2 1.7

May 1.8 5.0 6.8 0.1 6.7 1.7

June 1.9 5.2 7.1 0.6 6.5 1.7

July 1.9 5.2 7.1 0.8 6.3 1.6

August 1.9 5.1 7.0 0.0 7.0 1.9

September 1.9 5.2 7.1 0.3 6.8 2.2

October 1.9 5.2 7.1 0.3 6.8 1.8

November 1.9 5.1 7.0 0.3 6.7 1.7

December 2.0 5.3 7.3 0.1 7.2 1.8

AVERAGE 1.8 5.1 6.9 0.7 6.3 1.7

Data collected by County staff

Using the ERU count shown in Table 4-1, the existing average daily flow for each interceptor was estimated by using a flow factor of 200 gpd per ERU, which assumes 2.5 people per ERU and wastewater generation of 80 gallons per capita per day (gpcd). The resulting calculated average daily flows for each interceptor system are compared with the observed average daily flow in Table 4-3.

Table 4-3. Average Daily Flow Comparison, Existing

Interceptor Total ERUs Calculated Average

Flow (mgd)(a) Observed Average

Flow (mgd)(b) NVI 10,439 2.1 1.8 SVI 30,680 6.1 5.1 NSI 10,183 2.0 1.7

TOTAL 51,302 10.2 8.6 (a) Calculated using a flow factor of 200 gpd/ERU, applied to a June 2012 count of ERUs (b) Observed average flow based on data collected by County staff from 2012-2013.

As seen in Table 4-3, the observed values are slightly lower than those calculated from the connected ERU estimates. During the model calibration effort (described in Section 5) a flow

April 2015


factor slightly lower than the planning-level value of 200 gpd per ERU was derived for all three interceptor systems. Using the lower flow factor, calculated existing system flows based on the existing ERU count more closely matched the observed flows.

4.3 Flow Projections

The projection for future flow consists of several components, including flow contributions from existing connected areas and infill growth, the Septic Tank Elimination Program (STEP), and individually identified future growth areas. Each component is further described in the subsections below.

4.3.1 Growth Projections for the Existing System

The projected increase in wastewater flows generated within the County’s service area is based in part on estimated county-wide population growth. In 2010, the county-wide population was estimated at 470,300. The County’s Long-Range Planning Program projects the population to reach 612,226 by 20313. This is equal to a compound annual growth rate of approximately 1.3% over the 21-year period. This growth rate has been applied to the existing ERU count to project ERUs for future years in the areas of the system that are currently served. This growth represents infill that is expected to happen in these areas. The resulting ERU projections are shown in Table 4-4. The 6-year and 20-year planning horizon dates are highlighted in bold in the table.

The corresponding residential population for each planning year was determined using the existing ratio of residential ERUs to total ERUs. Data from Table 4-1 results in the following ratios for each interceptor system. Residential ERUs consist of residential, duplex and multi-family property types, and commercial ERUs consisted of commercial and public property types.

NVI System: 79% residential; 21% commercial

SVI System: 86% residential; 14% commercial

NSI System: 89% residential; 11% commercial

Based on the above ratios and the population factor of 2.5 people per ERU, an estimated residential population served was determined and is also shown in Table 4-4.

3 Board of County Commissioner's Findings of Fact and Decision. (2013, July 18). No. 13-0689. Spokane County.

April 2015


Table 4-4. ERU Projections and Residential Population (Existing System)

Planning Year

Projected ERUs(a) Residential Population Served(b)

NVI SVI NSI Total ERUs

Total Population

Served NVI SVI NSI

2012 10,439 30,680 10,183 51,302 20,620 65,960 22,660 109,240 2013 10,439 30,680 10,183 51,302 20,620 65,960 22,660 109,240 2014 10,571 31,068 10,312 51,951 20,880 66,800 22,940 110,620 2015 10,705 31,460 10,442 52,607 21,140 67,640 23,230 112,010 2016 10,840 31,858 10,574 53,272 21,410 68,490 23,530 113,430 2017 10,977 32,261 10,708 53,946 21,680 69,360 23,830 114,870 2018 11,116 32,668 10,843 54,627 21,950 70,240 24,130 116,320 2019 11,256 33,081 10,980 55,317 22,230 71,120 24,430 117,780 2020 11,398 33,499 11,119 56,016 22,510 72,020 24,740 119,270 2025 12,137 35,670 11,839 59,646 23,970 76,690 26,340 127,000 2030 12,923 37,982 12,607 63,512 25,520 81,660 28,050 135,230 2033 13,420 39,440 13,091 65,951 26,500 84,800 29,130 140,430 2034 13,589 39,939 13,256 66,784 26,840 85,870 29,490 142,200

(a) The ERU estimate for 2012 is held constant through 2013, since the 2012 ERU count was based on mid-year data and because the County experienced minimal growth in 2013. Beginning in 2014, ERUs are projected to increase at approximately 1.3% per year.

(b) Residential population served estimated based on the following: Residential population =Number of ERU x 2.5 people/ERU x Residential Factor - NVI Residential Factor = 0.79 - SVI Residential Factor = 0.86 - NSI Residential Factor = 0.89

4.3.2 STEP and Other Additional Near-Term Connections

The STEP is nearing completion. Whereas all of the construction was completed in 2013, a remaining 3,000 existing ERUs are expected to be connected in 2014 and 2015. For planning purposes it is assumed that STEP connections are evenly split in the remaining time frame, and that the majority of connections occur in the SVI system.

In addition to the STEP flows, there are 300 existing ERUs that are not within the STEP but will be required to connect in the next few years through County enforcement actions. Like the STEP ERUs, these properties (almost all residential) are currently on septic systems but have live service extended to them. These ERUs will be required to connect over the next few years and will contribute flow to the collection system over and above the growth accounted for in Section 4.3.1. The flow contribution from these areas has been added to year 2016 flows.

Table 4-5 shows the ERU percentages and distribution of flow to the different interceptor systems for these additional flows.

April 2015


Table 4-5. ERU and Flow Projections for Remaining STEP and Other Additional Connections

Interceptor % of

Connections ERUs

2014 Flow (gpd)(a)

2015 Flow (gpd)(a)

2016 Flow (gpd)(a)

NVI 3 99 9,000 18,000 19,800

SVI 96 3,168 288,000 576,000 633,600

NSI 1 33 3,000 6,000 6,600

TOTAL 100 3,300 300,000 600,000 660,000 (a) Flow estimated using a factor of 200 gpd/ERU and assuming 1,500 ERUs in 2014 and 2015, and 300 ERUs in

2016.

4.3.3 Future Growth Areas

For each interceptor system, specific growth areas within the Urban Growth Area (UGA) that are not currently connected to the County’s sewer system have been identified for inclusion in the analysis of the future system. These areas, listed in Table 4-6 and shown in Figure 4-1 and Figure 4-2, contain both developed properties that will transition from onsite systems and areas of new development. These areas are further discussed in Section 5.3. The buildout flow for each of these areas is estimated based on land use designations and the flow factors indicated above. This represents a high-end estimate over the planning period. Flows from each of these areas are expected to enter the system gradually over time. To estimate the amount of flow entering each interceptor by the interim planning horizons (6-year and 20-year windows), a percentage of the buildout flow is defined based on an estimate of area expected to connect to the system. For the 20-year planning horizon, two separate estimates were made. The high estimate assumes that 30% of the developable land does not develop in the 20-year planning horizon due to market factors, consistent with the County’s land quantity analysis for the UGA. The low estimate assumes a slower rate of development for each area. These estimates are shown in detail in Table 4-6. Years when flows from these areas are assumed to begin entering the system are estimated based on the current understanding of planned County infrastructure extensions. Flows are then assumed to gradually increase over time to the estimated flow targets defined for the 6-year and 20-year horizons. All flows presented in this section represent the high estimate, assuming full parcel development.

The following is a description of the basis for flow projections over time for each area:

NVI System

Flora Road (total area = 1,308 acres)

o 6-year horizon:

Estimated start year for flow contributions: 2016

Includes parcels in the west portion of the future growth area (about 290 acres)

o 20-year horizon:


Includes parcels in the east portion of the future growth area (about 470 acres)

April 2015


Kaiser Aluminum

o Kaiser’s facility located in Spokane Valley currently has its own wastewater treatment facility and does not discharge into the County’s system. However, for the purpose of this analysis, it is assumed that this large property may redevelop with future land uses discharging wastewater to the County’s system. This flow has been added to the buildout timeframe.

SVI System

Barker Road (total area = 260 acres)

o 6-year horizon:


Includes parcels in the east portion of the future growth area (170 acres)

o 20-year horizon:


Includes parcels in the west portion of the future growth area (about 90 acres)

NSI System

Mead/Mt. Spokane (total area = 1,324 acres)

o 6-year horizon:


Includes parcels in the northwest portion of the future growth area and parcels adjacent to the US Highway 2 corridor (about 340 acres)

o 20-year horizon:


Includes Peone Pines and all parcels outside the US Highway 2 corridor and the northeast portion of the future growth area (about 850 acres)

U.S. Highway 2 Extension – North (total area = 333 acres)

o 6-year horizon:


Includes parcels in the north portion of the future growth area (about 90 acres)

o 20-year horizon:


Includes parcels in the northeast portion of the future growth area (about 160 acres)

April 2015


U.S. Highway2 Extension- South (total area = 2,803 acres)

o 6-year horizon: No flow contributions

o 20-year horizon:


Includes entire future growth area (about 700 acres)

Table 4-6. Flow Estimates and Timing for Future Growth Areas

Location

Estimated Buildout Average Flow (gpd) (a) Planning Horizon % of Buildout Flow

Residential Commercial Total 6-Year 20-Year (Low)

20-Year (High)(c) Buildout

NVI System Flora Road 100,600 1,594,100 1,694,700 20% 60% 70% 100% Bigelow Gulch 51,200 27,400 78,600 0 100% 100% 100% Kaiser & WSDOT 0 919,200 919,200 0 0 0 100%

SVI System

Barker Road 208,400 7,700 216,100 45% 65% 70% 100% SE Valley 11,600 6,200 17,800 0 100% 100% 100% Ruddell / Tupper 33,900 0 33,900 0 100% 100% 100%

NSI System Peone Pines 39,700 0 39,700 0 100% 100% 100% Mead / Mt Spokane 709,400 677,100 1,386,500 25% 65% 70% 100% North Metro 183,300 62,500 245,800 0 0 0 100% Havana-Lyons(b) 3,100 0 3,100 0 100% 100% 100% Other Infill Areas 115,900 0 115,900 0 0 70% 100% U.S. Hwy 2 Extension (North)

91,400 248,800 340,200 25% 50% 70% 100%

U.S. Hwy 2 Extension (South)

402,000 1,176,200 1,578,200 0 25% 70% 100%

(a) Average flow estimate is based on land use area and flow factors described in Section 4.1. (b)The area within Havana-Lyons bounded by Yale Street, the west boundary, of the Humane Society, Lyons

Avenue, and Weile Avenue drain to the NSI System. The Humane Society drains south to the City of Spokane System.

(c) For those areas not expected to fully develop in the 20-year planning horizon, the 20-Year (High) distribution of flows assumed 30% of developable land would not develop within the 20-year planning horizon. This is consistent with assumptions stated in Urban Growth Area Update Land Quantity Analysis for Unincorporated Spokane County dated September 8, 2010. The exceptions are the Kaiser Aluminum and WSDOT area in the NVI system, and the North Metro area in the NSI system. The Kaiser Aluminum and WSDOT sites currently have a private treatment facility and the North Metro area is currently outside the Urban Growth Boundary. Both are not expected to discharge into the collection system within the 20-year horizon.

In addition to the future growth areas, it is assumed that additional “wet” industries may be served in the future by the County’s system, contributing a potentially significant amount of flow. To accommodate this, an additional amount of 1.0 mgd (0.5 mgd allocated to each of the NVI and SVI systems) is built into the flow projection in 2034.

April 2015


4.3.4 Buildout Flow Estimation

Buildout flows for the interceptor and collection system have been estimated by applying the flow factors described in Section 4.1 to parcel land areas according to land use designations. The calculation was performed assuming 100 percent of each parcel area is developable, which results in a conservative (i.e., high) estimate for buildout flows. The flow factors have been applied only to the parcels not already connected to the collection system. For parcels already connected, the existing ERU-generated flows were maintained. In addition, a factor for I&I was applied to each parcel within the UGA. Table 4-7 summarizes the estimated buildout flow for the three interceptor systems. These values include the flow estimated for the future growth areas described in Table 4-6.

Table 4-7 Buildout Flow

Interceptor Flow (mgd) (a) I&I Flow (mgd)

(b) Total Average

Flow (mgd)

NVI 15.0 1.0 16.0

SVI 12.2 1.9 14.1

NSI 7.4 0.7 8.1

TOTAL 34.6 3.6 38.2 (a) Buildout flow determined by applying the following flow factors to newly connected parcels

within the service area and adding the additional flow to the flow generated from the existing ERUs: - Residential = 1,000 gallons per acre per day - Commercial/Industrial = 2,000 gallons per acre per day

(b) I&I flow determined by applying the following I&I factors to each parcel in the service area: - Residential = 125 gallons per acre per day - Commercial/Industrial = 200 gallons per acre per day

4.3.5 Flow Projection Summary

Projections for each flow component are summarized in Table 4-8 through Table 4-10, for the NVI, SVI, and NSI systems, respectively. The 6-year and 20-year planning horizon dates are highlighted in bold. Flow estimates for the remaining STEP and future growth areas have been included in the appropriate planning year. In addition, projected buildout I&I contributions have been distributed starting in 2015 and continuing through the 20-year horizon. At the 20-year horizon the fraction of I&I was assumed to be equal to the fraction of the total buildout projection. An overall summary of the high projected flows for each interceptor is provided in Table 4-11.

April 2015


Table 4-8. NVI Flow Projections

Planning Year

High Projected Average Daily Flow (mgd)

Existing System(b)

STEP and Other Flow

Flow from Future Areas

Flow Subtotal

I/I(c) Total Flow

2012(a) 1.76 0.00 0.00 1.76 0.00 1.76 2013(a) 1.89 0.00 0.00 1.89 0.00 1.89 2014 1.91 0.01 0.00 1.92 0.01 1.93 2015 1.92 0.02 0.00 1.94 0.03 1.97 2016 1.94 0.02 0.08 2.04 0.04 2.08 2017 1.96 0.02 0.17 2.15 0.05 2.20 2018 1.98 0.02 0.25 2.25 0.06 2.31 2019 1.99 0.02 0.34 2.35 0.08 2.43

2020(d) 2.01 0.02 0.40 2.43 0.09 2.52 2025 2.11 0.02 0.70 2.83 0.15 2.98 2030 2.21 0.02 1.00 3.23 0.22 3.45

2034(d) 2.46 0.02 1.76 4.24 0.27 4.51 Buildout 11.82 0.02 3.19 15.03 0.95 15.98

(a) Measured flow reported for the NVI system. (b) Existing system flow assumes a compound annual growth in ERUs in the existing system at 1.3% per year, with

flow for all future ERUs estimated using a flow factor of 200 gpd/ERU. (c) Constant I&I contributions was assumed from 2015 through 2034. For the NVI system a value of 0.26 mgd (0.95

mgd x 4.05 mgd / 15.03 mgd) was evenly spread over 20 years. (d) 2020 and 2034 projections include the proportion of flow from the additional areas as outlined in Table 4-6. In

the case of the 2034 projection, the 20-year (High) projection was used.

April 2015


Table 4-9. SVI Flow Projections

Planning Year


Existing System(b)

STEP and Other Flow


Flow Subtotal

I/I(c) Total Flow

2012(a) 5.08 0.00 0.00 5.08 0.00 5.08 2013(a) 5.15 0.00 0.00 5.15 0.00 5.15 2014 5.20 0.24 0.00 5.49 0.06 5.55 2015 5.25 0.48 0.00 5.83 0.12 5.95 2016 5.30 0.63 0.02 6.09 0.18 6.27 2017 5.35 0.63 0.05 6.17 0.24 6.41 2018 5.40 0.63 0.07 6.24 0.30 6.54 2019 5.45 0.63 0.10 6.32 0.36 6.68

2020(d) 5.51 0.63 0.10 6.38 0.42 6.80 2025 5.78 0.63 0.12 6.67 0.71 7.38 2030 6.07 0.63 0.14 6.98 1.01 7.99

2034(d) 6.82 0.63 0.70 8.30 1.21 9.51 Buildout 10.70 0.63 0.77 12.24 1.89 14.13

(a) Measured flow reported for the SVI system. (b) Existing system flow assumes a compound annual growth in ERUs in the existing system at 1.3% per year,

with flow for all future ERUs estimated using a flow factor of 200 gpd/ERU. (c) Constant I&I contributions was assumed from 2015 through 2034. For the SVI system a value 1.19 mgd

(1.89 mgd x 7.73 mgd / 12.24 mgd) was evenly spread over 20 years. (d) 2020 and 2034 projections include the proportion of flow from the additional areas as outlined in Table 4-6.

In the case of 2034 projection, the 20-year (High) projection was used.

April 2015


Table 4-10. NSI Flow Projections

Planning Year


Existing System(b)

STEP and Other Flow


Flow Subtotal

I/I(c) Total Flow

2012(a) 1.60 0.00 0.00 1.60 0.00 1.60 2013(a) 1.60 0.00 0.00 1.60 0.00 1.60 2014 1.62 0.002 0.00 1.62 0.02 1.64 2015 1.63 0.005 0.00 1.64 0.04 1.68 2016 1.65 0.007 0.02 1.68 0.06 1.74 2017 1.67 0.007 0.04 1.72 0.08 1.80 2018 1.68 0.007 0.24 1.93 0.11 2.04 2019 1.70 0.007 0.43 2.14 0.13 2.27

2020(d) 1.72 0.007 0.57 2.30 0.15 2.45 2025 1.81 0.007 1.27 3.09 0.25 3.34 2030 1.91 0.007 1.97 3.89 0.36 4.25

2034(d) 2.15 0.007 2.44 4.60 0.42 5.02 Buildout 3.69 0.007 3.71 7.41 0.71 8.12

(a) Measured flow reported for the NSI system. (b) Existing system flow assumes a compound annual growth in ERUs in the existing system at 1.3% per year,

with flow for all future ERUs estimated using a flow factor of 200 gpd/ERU. (c) Constant I&I contributions was assumed from 2015 through 2034. For the NSI system a value of 0.41 mgd

(0.71 mgd x 4.34 mgd / 7.41 mgd) was evenly spread over 20 years. (d) 2020 and 2034 projections include the proportion of flow from the additional areas as outlined in Table 4-6.

In the case of the 2034 projection the 20-year (High) projection was used.

The above tables reflect a conservative approach to the flow projections for the interceptor and collection system, generating a high-end of the potential future flow range. Several assumptions were used to develop these values. To evaluate the sensitivity of the calculations and establish a low-end projected flow range, adjustments were made to the following key assumptions:

Growth Rate. The high end estimate assumes a growth rate in the presently served areas of 1.3% over the 20-year planning period. For the low-end of the projection range, a growth rate of 0.5% is used to represent slower general population growth in the area.

Developable Land. The high end of the forecast flow range assumes that all land within the County’s service area is developable, with an exception of 10.2%, which represents unbuildable lands (e.g., those with steep slopes, wetlands, or hazardous geotechnical conditions). For the low end of the forecast, a market factor was added, to further discount the amount of available land to reflect other, market-based factors (i.e., although physically able to be developed, some land may be hard to market for uses other than its current undeveloped state). Therefore, for planning purposes, it is assumed that an additional 30% of all future lands to be served will not be developed at build-out and are thus removed from the low-end of the flow projections.

Flow Factor. The high-end of the forecast assumes future ERUs generate 200 gpd of wastewater. However, as noted during hydraulic model calibration, currently connected ERUs are generating flows lower than this. Thus, to define the lower end of the flow projections, all future ERUs are converted to flow using a factor of 120 gpd per ERU, a

April 2015


40% reduction in the flow factor. In addition to reducing the ERU flow factor, the flow factor for future residential and commercial/industrial land use was also reduced by 40%. The residential future flow factor was reduced to 600 gpd per acre (from 1,000 gpd per acre), and the commercial/industrial future flow factor was reduced to 1,200 gpd per acre (from 2,000 gpd per acre).

Inflow and Infiltration. The high-end of the forecast assumes 125 gpd per acre and 200 gpd per acre for I&I contributed by residential and commercial lands, respectively. For the lower projected flow range, these I&I factors were reduced by 50% to 62.5 gpd per acre for residential and 100 gpd per acre for commercial land uses. In addition, the rate at which I&I appears in the system was reduced to half the rate of appearance that was used in the high-end estimate (i.e., it is assumed to take longer for existing pipes to develop conditions that lead to increased I&I).

Industrial Flows. The high-end of the forecast assumes future “wet” industrial reserve flow for both the NVI and SVI systems. For the lower projected flow range this reserve was removed. In addition, industrial flows from the Kaiser Aluminum-Trentwood property are assumed to continue to be treated onsite and flow from these parcels is not discharged to the County’s collection system.

Both the low- and high-ends of the flow projection range are summarized in Table 4-11. Exhibit 4-1 illustrates the range in the flow projections over the 20-year planning horizon for the entire County system.

Table 4-11. Flow Projection Summary

Planning Year

Low Projected Average Daily Flow (mgd) (a) High Projected Average Daily Flow (mgd)

NVI Flow

SVI Flow

NSI Flow

Total Flow NVI Flow

SVI Flow

NSI Flow

Total Flow

2013 1.9 5.1 1.6 8.6 1.9 5.1 1.6 8.6 2020 2.1 6.0 1.8 9.9 2.5 6.6 2.3 11.4 2034 2.6 6.6 2.4 11.6 4.5 9.4 5.0 18.9

Buildout 8.3 9.1 4.4 21.8 16.0 14.1 8.1 38.2 (a) Low projections estimated assuming:

Growth rate for ERUs in the currently served areas is 0.5% (as opposed to 1.3% in the high projection). 30% of future served land is not developed (in addition to the 10.2% non-developable land assumed in the

high projection), due to market factors. 40% reduction in flow factor, as compared with the high projection. No industrial reserve for the NVI and SVI systems. I&I flow factor is reduced by 50%, and rate of appearance of I&I in the system is half of what was assumed in

the high estimate. Wastewater from Kaiser property in the NVI system remains on private treatment system (i.e., no flow

contribution from this property is added at buildout).

April 2015


Exhibit 4-1. Projected Total System Average Daily Flow

4.4 Evaluation of Wastewater Treatment Facility Capacity

While this CWMP is focused primarily on analyzing the ability of the County’s collection and conveyance system to accommodate current and projected future wastewater flows, as discussed in detail in Section 5, a summary evaluation has also been prepared to compare the flow projections presented in Section 4.3 with current treatment facility capacities. This is not a detailed analysis of treatment capacity, which would involve consideration of various flow metrics beyond average daily flow (e.g., maximum month flow, peak hour flow) compared against the capacities associated with specific unit processes within the treatment facilities used by the County, and projection of the ability of the treatment facilities to process future loadings and meet discharge permit requirements. Such detailed analyses are conducted by the County in its facility planning efforts related specifically to its treatment facilities. Rather, the intent of this summary hydraulic evaluation is to compare projected average daily flow with the average day capacity of the facilities that treat and discharge the County’s wastewater, in order to understand generally when currently available treatment facility capacities may expect to be exceeded.

For the purpose of this analysis, the average daily flow capacity of the SCRWRF is assumed to be 8.0 mgd, its design capacity. The County’s current allocation of capacity at the RPWRF is 10.0 mgd, as outlined in the 1980 Interlocal Agreement between the County and the City of

April 2015


Spokane (City), and the several amendments thereto. For the purposes of this analysis, the average daily flow capacity available to the County at the RPWRF is conservatively assumed to be 8.0 mgd. This assumption acknowledges that the County’s wastewater flows to the RPWRF have become more concentrated over time, due in large part to water conservation measures. As a result, the County’s share of the RPWRF capacity on a “loadings” basis would be reached at a flow of approximately 8 mgd, assuming current concentrations. The County’s future allocation at the RPWRF, in terms of both flow volumes and loadings, is the subject of current and future negotiations between the County and the City, as the City moves forward with the planning and implementation of major improvements to the RPWRF treatment processes, such as tertiary membranes.

Table 4-12 summarizes the comparison of the average daily flow capacities of the SCRWRF and RPWRF (County allocation) with current and projected average daily flow in the County’s system. This analysis is done considering both the low-end and high-end of the flow projection range presented in Section 4.3. While it is appropriate to consider the high-end of the flow projections when planning for collection and conveyance system infrastructure, the analysis of treatment capacity is focused on the low-end of the flow projections, as those projections for the next 20 years are more in alignment with the rate of flow increases the County has experienced for many years.

Table 4-12 shows the total flows for the NVI and SVI compared with the SCRWRF capacity, assuming that as much of the flow from these two interceptors as possible will be treated at the County’s facility. Any NVI/SVI flows in excess of the current SCRWRF capacity are assumed to be diverted to the RPWRF, where they are treated along with flows from the NSI.

The results of the low-end projection analysis summarized in Table 4-12 indicate that existing treatment facility capacities, in total, are sufficient to meet projected flows beyond the 20-year planning horizon. However, additional treatment capacity will likely be needed to support long-term buildout needs of the County’s service area.

The County will continue to monitor increases in the flows that are influent to the treatment facilities it utilizes, and when flows reach 85% of the design and/or contractual capacities for three consecutive months, facility planning would begin (if it had not already been started) to determine the optimal method to address treatment facility needs for future flow increases. The capital improvement program (CIP) outlined in Section 6 includes funding for improvements and future capacity upgrades at the SCRWRF and RPWRF. The CIP also considers funds to support other means of wastewater management, such as the County’s water reclamation and reuse program. In recent years the County has supported and funded a water conservation program to reduce potable water usage and resulting wastewater flows. In the future, a program to proactively manage I/I may also be considered, if I/I becomes significant and cost-effective to reduce.

April 2015


Table 4-12. Treatment Plant Capacity Evaluation (Average Daily Flow Basis)

Average Daily Wastewater Flow (mgd)

Base Year 2013

6-Year 2020

20-Year 2034

Buildout

Low Flow Projection SCRWRF Capacity (a) 8.0 8.0 8.0 8.0 NVI+SVI Flow (b) 7.0 8.1 9.2 17.4 SCRWRF Capacity Surplus/(Deficiency) (c) 1.0 (0.1) (1.2) (9.4) RPWRF Capacity (d) 8.0 8.0 8.0 8.0 NSI Flow (b) + SCRWRF Capacity “Deficiency” (c)

1.6 1.9 3.6 13.8

RPWRF Surplus/(Deficiency) (e) 6.4 6.1 4.4 (5.8)

High Flow Projection SCRWRF Capacity (a) 8.0 8.0 8.0 8.0 NVI+SVI Flow (b) 7.0 9.1 13.9 30.1 SCRWRF Capacity Surplus/(Deficiency) (c) 1.0 (1.1) (5.9) (22.1) RPWRF Capacity (d) 8.0 8.0 8.0 8.0 NSI Flow (b) + SCRWRF Capacity “Deficiency” (c)

1.6 3.4 10.9 30.2

RPWRF Surplus/(Deficiency) (e) 6.4 4.6 (2.9) (22.2) (a) Average daily flow capacity of the SCRWRF. (b) See Table 4-11 for low and high flow projections by interceptor. (c) This analysis assumes that the capacity of the SCRWRF is fully utilized to manage NVI and SVI flows,

with any flows exceeding the current capacity of the SCRWRF being diverted to the RPWRF. (d) Average daily flow capacity of the RPWRF allocated to the County. (e) Deficiencies noted here represent deficiencies in the total treatment plant capacity currently available to

the County, considering both the SCRWRF and the RPWRF.

Figures

4-1 NVI & SVI Future Growth Areas

4-2 NSI Future Growth Areas

Flora Rd

Barker RdWSDOT

SE ValleyRuddel / Tupper

£¤395

UV27

§̈¦90

L ib e r ty L a k e

S h el le y L ak e

E a sto n

M iss io n

Towner

B a sc e tt

a

P a sa d en a P a r k

Tw in v iew

C o r b inIn la n d

Wolfcre s t

Sp ok a ne In d u str ia l P ar k

W es t F a r m s

Oppor tu n i ty Te r rac e

Walnu t

W . S p ra g u e

S. P on d eros a

A r g o n n e

H illc r es t

F a ir A c re s

W . P on d e ros a

Glenvi

ew

Ve rac re s t

Ha rr ing ton

Bella V

ista

M id ilo

m e

E d ge r ton

M icaview

W el le sle y M a n o r

Sk y v ie w

Valley

V is ta

Ea rly D aw n

S ip p le

Tr en tw o o d

G r an d v iew A c re s

E . Sp r a g ue B a sin AM ille r P a r k

Park s

R oa d

N or th

K ok om

o

O ld O r

ch ard

W o lf lan d

G o od S am C tr

W es t Va lle y

Va lle y v iew

C h ro n ic le

Ve r a d a le

So uth G r e en A cr e s

E lec tr ic

R R

C h es ter f ie ld

E . Sp r a g ue E

G r ee n H a v e n

So uth K o k om o

H u tc h in son

R iverw a lk

E . Sp r a g ue B a sin B

W o o d la w n /B e v e r ly H il ls

E . Sp r a g ue B a sin D

R ock w e ll

H i llv ie w E st

E u c lid

R o tc h fo r d A c re s

E . Sp r a g ue B a sin C

M oun ta in V iew

Sp a ld in g s

So uth B la k e

Chest

er Hil ls

Everg

re en P

t

Su tte rs

N o r th P in es

C ro n k

C a r n a h a nC le m e n t

A p ple Va lley

Su m m e r fie ld

H i llv ie w A cr e s

U -H ig h

U p riv e r Te rr a c e

U n iv e r si ty P la c e

N . P o nd e ro sa

W ea the r w o o dW h ite B irc h

M ir a b e a u

O r ch a r d A v e nu e

U p riv e r D r iv e

Ve r a Te rr a c e

N o r th S u ll iv a nVe r a d a le H e ig h ts

J o h n sto nE m p ire H e ig h ts

Sp o k a ne Va l le y M a l l

C a st le/A lo h a

C h es ter Te r r a c e

O w e n s

P in e c ro ft /M a n sf ie ld

Sh e r w o o d F o re st /M ic a P a r k

H e a th e r P a r k R id g e m o nt E sta te s

M o n te D e l R e y /H ig h la n d E st .

S lo a n s A d d

L a zy F o x E sta te s

P o n d e ro s a R id g e

E a stw o o d

Sprague Av

Broadway Av

Sulliv

an R

d

Argo

nne R

d

32nd Av

Trent Av

Wellesley Av

Marke

t St

Dishm

an-M

ica R

d

Barke

r Rd

16th Av

29th Av

Bigelow Gulch Rd

Everg

reen R

d

Unive

rsity

Rd

Appleway Bv

Mission Av

Francis Av

Starr

Rd

Park

Rd

Ray S

t

Mulla

n Rd

Palou

se H

y

Appleway AvFanc

her R

d

Forker Rd

Thor

StGr

eene

StHa

ven S

tRay

PlFre

ya St

Haven Pl

Thor Pl

Country Vista Dr

Trent Rd

Marke

t Pl

Bigelow Gulch Rd

Wellesley Av

16th Av

Sprague Av

Freya

St

S p o k a n e

L ib e r tyL a k e

NVI & SVIFuture

Growth Areas

M illw o o d

S p o k a n eVa lle y

Sp ok an e R iv er

0 1 20.5 Miles

Map Produced April 2015 by Spokane County Utilities GIS

F ig u re 4 -1Future Growth Areas

Future Growth Area in Urban Reserve

Spokane County Sewered Basins

Municipalities

Municipal Boundaries

Proposed Urban Growth Area Boundary

o

§̈¦90

Trent Av

UV290

P ain ted H il ls

S p ok an e R iv e r

Other infill area

US Hwy Extension - South

North Metro

US Hwy Extension - North

Havana-Lyons

UV206

UV290

UV291

£¤395

£¤2

£¤395

C oz z a

F a ir w o o d

W a ik ik i

C o ll e g e

W h itw o r th

F o re s t H il ls

Hastin

gs

L inw o o d

C o u n try H o m es

Town &

C ou n t

ry

P in e R iv e r

W es tw oo dC a m e lo t

P in e r y

C o ll e g e H o m es

C ity G a r d en

S p o k a n e T er r a c e

W o o d w a y

W a n d er m e r e

P ip e r G le n

N o r th v ie w E st

P e o n e P in es

E d g e r to n

H e r ita g e E st

G r a n g e r Te r r a c e

S . W a n d e r m e re

W h itw o r th T e rr a c e

P a r k s R o a d

Marke

t St

Francis Av

Ash S

t

Wall S

t

Divisi

on St

Neva

da St

Freya

St

Wellesley Av

Maple

St

Monr

oe St

Perry

St

A St

Mill R

dDa

rtfor

d Dr

Upriver Dr

Belt S

t

Rowan Av

Austi

n Rd

Post

St

Waiki

ki Rd

Boone Av

Northwest Bl

Five M

ile R

d

Trent Av

Peone Rd

Albert

a St

Cres

tline S

t

Lincoln Rd

Strong Rd

Little Spokane Dr

Empire Av

Hatch

Rd

G St

Farwell Rd

Stoneman Rd

Ruby

St

Colbert Rd

Hastings Rd

Ceda

r Rd

Wandermere Rd

Shady Slope Rd

Hawthorne Rd

Illinois Av

Addis

on St

Bridgeport Av

Indian Trail Rd

Euclid Av

Maxwell Av

Hami

lton S

t

Whitworth Dr

Thier

man R

d

Queen Av

Coun

try Ho

mes B

v

Frederick Av

Pettet Dr

Midway Rd

Day-Mt Spokane Rd

Regina Dr

North Foothills Dr

Gree

ne St

Cozza Dr

Have

n St

Newpo

rt Hy

Asse

mbly

St

Rutter ParkwayWa

shing

ton StDownriver Dr

Lidge

rwoo

d St

Holland Rd

Rutter Av

Glencrest Dr

Fanc

her R

d

Napa

St

Mission AvFort George Wright Dr

Summit Bl

Buckeye Av

Driscoll Bl

Indiana Av

Magnesium Rd

Sharp AvFreya Wy

Garland Av

Montgomery Av

Price Av

Dako

ta St

Haven Pl

Perry

Pl

Trent Rd

Adam

s St

Cres

tline S

t

Upriver Dr

Rowan Av Rowan Av

Mission Av

Driscoll Bl

Belt S

tBe

lt St

Mission Av

NSI Future

Growth Areas

S p o k a n e

L it tl e S p o k a n e R iv er

0 1 20.5 Miles

Map Produced April 2015 by Spokane County Utilities GIS

F ig u re 4 -2Future Growth Areas

Future Growth Area in Urban Reserve

Spokane County Sewered Basins

Municipalities

Municipal Boundaries

Proposed Urban Growth Area Boundary

o

WandermereLake

G len e d e n

Jesse's

BluffMead / Mt. Spokane

£¤2

Section 4 Wastewater Flow Projections

Documents

Transcript of Section 4 Wastewater Flow Projections