3.1        Introduction

To plan for future wastewater facility needs, it is necessary to project the amount of wastewater that will be received and treated.  The served population, the magnitude of commercial and industrial activities, and the quantity of extraneous flow, such as sewer system infiltration and inflow, all influence wastewater quantity.  To define required treatment facilities, it also is necessary to define the projected wastewater strength for such constituents as organic content and nutrients (nitrogen and phosphorus).  These characteristics help define the type and size of facilities needed for liquid treatment and solids handling processes.  In this chapter, projected flows and loadings are developed for both 20-year and 50-year planning horizons.

3.2        Planning Horizons

Wastewater facilities are typically planned for the service area characteristics that will develop over a period of 20 years; however, consideration of a longer planning period is useful to assess the long-term feasibility of wastewater management strategies and to identify land requirements for future expansion of facilities.  For this facilities plan, three planning horizons will be used: 2020, 2025 and 2050.

Year 2020 coincides with the planning horizon used in Spokane County’s Growth Management Plan.  Use of this planning year is needed to ensure concurrency between the wastewater facilities plan and the Growth Management Plan.

Spokane County Utilities has requested use of a Year 2025 planning horizon when developing and comparing alternative wastewater management strategies.  The basis for this planning year is Utilities desire to have a “20-year solution” once the recommended plan is implemented.  It is estimated that implementation (planning, design, construction) will take about five years.  This approach is consistent with the Washington State Department of Ecology’s Criteria for Sewage Works Design [i], which require that the wasteload estimated for the 20-year horizon be considered when selecting a wastewater management approach. The 20-year period coincides with the anticipated life of major treatment process equipment.

Use of a Year 2050 planning horizon will allow a “long-term” view of wastewater management scenarios and allow planning for future infrastructure, programmatic and land acquisition activities.  

3.3        Population and Land Use Forecasts

Realistic projections of population growth and development of industrial and commercial properties provide the foundation for wastewater flow and loading projections.  Population and land use forecasts for 20-year and 50-year horizons were developed as part of the Year 2001 Comprehensive Wastewater Management Plan for Spokane County (CWMP) [ii].  Table 3‑1 and Table 3‑2 present the results of these projections and the subsequent discussion provides a summary of key assumptions used.  The reader is referred to the CWMP for a detailed discussion of the forecasting methodology.

Table 3‑1.  Population Projections

a                    Estimated population served by County sewers as of December 1999

b                    Growth projected to occur within the sewer service area

c                    Population within current sewer program area.  This population is assumed

                      to become sewered starting in 2000 and to be completely served by 2015

d                    Population outside the current sewer program area.  This population is assumed

                      to become sewered starting in 2005 and to be completely served by 2020.

e                    Sum of Columns a through d

Table 3‑2.  Projected Commercial and Industrial Development

a                           Estimated combined commercial/industrial acreage served by County sewers as of

                             December 1999. Calculated from existing Equivalent Residential Unit (ERU) data.

b                           From CWMP, based on Spokane County Long-Range Planning Department Information.

c                           From CWMP, based on Spokane County Long-Range Planning Department Information.

d                           Sum of columns a through c

3.3.1        Current Base Condition

As described in Chapter 2, the planning area for this study is the Interim Urban Growth Area (IUGA).  Major portions of the planning area have been sewered by the County as part of the Septic Tank Elimination Program (STEP).  In December 1999, the estimated residential and commercial/industrial developments connected to the County sewer were:

§         Residential population:  58,318

§         Commercial/industrial acreage:  1,265

This currently sewered area, and its resultant wastewater flow, represent the “Current Base Condition” from which future projections will be developed.

3.3.2        Existing Development to Be Served By Sewer Extensions

A key planning assumption is that during the next twenty years, sewers will be extended to serve the entire planning area.  In doing this, the County will provide service to existing residential, commercial and industrial development that are currently on septic tanks.  This affects two portions of the planning area:

§         Sewer Program Areas:  Areas within the current sewer program that become sewered starting in 2000 and are completely served by 2015.

§         Non-Program Areas:  Areas outside the current sewer program that become sewered starting in 2005 and are completely served by 2020.

For both areas, the CWMP provides estimates of (1) the amount of current, unsewered development, and (2) the schedule for connecting these existing developments to the collection system. 

3.3.3        Future Population Growth and Development Through 2020

Projections of population growth and future development of commercial and industrial properties are based on the forecasts presented in the Spokane County Comprehensive Plan – Draft Plan 2000.  Key assumptions and observations are presented below.

Residential.  The residential population growth within the service area is projected to be 40,036 between 1999 and 2020. 

Commercial.  Projected commercial development is based the vacant commercial property identified in the Draft 2000 Plan.  A uniform rate of development has been assumed for the next 20 years.

Industrial.  Projected industrial development is based on the vacant industrial property identified in the Draft 2000 Plan.  It has been assumed that properties within “Tiers 1 and 2” will be developed between 2000 and 2020, and that properties within “Tier 3” will be developed between 2010 and 2020. 

In addition, an assumption was made that two “wet industries” would be located within the planning area between 2011 and 2015 – one in the North Valley service area and one in the Spokane Valley service area.

3.3.4        Future Population and Growth Beyond 2020

Beyond 2020, several simplifying assumptions were used to project population and land use development:

§         The mix of residential, commercial and industrial development in the years 2021 through 2050 will be the same as that in 2020 (i.e., all will increase at the same rate).

§         A straight-line annual growth rate (approximately 1.6 percent, based on the Year 2020 baseline) will occur for residential, commercial and industrial contributions.  This growth rate coincides with the population growth rate from 1999 to 2020.

§         No further allowances for wet industries.

3.4        Wastewater Flow Projections

This section summarizes the development of flow projections for the entire planning area.

3.4.1        Definition of Terms

As a preface to the discussion of flow projections, it is useful to define the key terms employed:

General Classifications of Wastewater by Source

Base Sanitary Flow includes water-carried wastes from residences, businesses, institutions, and industrial establishments.  It consists of the following components:

§         Domestic Wastewater is that wastewater which is principally derived from the sanitary conveniences of residences or produced by normal residential activities. 

§         Commercial Wastewater is that wastewater which is generated in predominantly business or commercial districts, including not only sanitary wastes but also wastes from the commercial activities themselves.  Typically, commercial wastewater might include wastes from restaurants, laundromats, and service stations.

§         Industrial Wastewater is generated from all activities conducted at industrial establishments.

Extraneous Flows consist of groundwater or stormwater that enters the collection system.  This overall classification of flows may be divided into the following components:

§         Infiltration results from the unintentional entry of water into the wastewater collection systems from the surrounding soil.  Common points of entry include broken pipes and defective joints in the pipe or in walls of manholes.  Infiltration may result from sewers being laid below the groundwater table or from saturation of the soil by rain or irrigation water.  Infiltration usually varies seasonally depending on precipitation or irrigation trends.

§         Direct Stormwater Inflow consists of rainwater that enters the collection system through roof and patio drain connections, catch-basin connections, and holes in the tops of manhole covers in flooded streets.  Occurrence of direct stormwater inflow is usually indicated when the wastewater flow increases immediately in response to precipitation or snowmelt. 

§         Interflow, or rapid infiltration, is the entry of infiltrated precipitation into building laterals and French drains.  It occurs as the recent precipitation percolates into the ground.  Interflow responds to precipitation on a delayed basis, and may take up to six hours to develop after precipitation starts and may extend for several days after precipitation ends.

Other Common Terms

Annual Average Flow is the total daily flow that occurs on average for a twelve-month period.  The period may be based on a calendar year (Jan – Dec) or a water year (Nov – Oct).

Maximum Month Flow is the highest 30-day average flow that occurs in a one-year period.

Maximum Week Flow is the highest 7-day average flow that occurs in a one-year period.

Maximum Day Flow is the highest 24-hour average flow that occurred in a one-year period.

Peak Hour Flow is the highest one-hour average flow that occurred in a one-year period.

Dry Weather Flow is the total wastewater flow during periods of little or no rainfall.  Rates of flow exhibit hourly, daily, and seasonal variations.  A certain amount of infiltration may be present.

Wet Weather Flow is the total wastewater flow during periods of moderate to heavy rainfall.  Storm water inflow may increase the wet weather flow to a rate many times greater than the dry weather flow and, unless provided for in sewerage design, can produce hydraulic overloads resulting in wastewater overflows to streets or watercourses.

3.4.2        Current Base Flow

Except for a minor amount of flow discharged to satellite treatment facilities, Spokane County wastewater is discharged to the City conveyance system at three points: (1) North Valley Interceptor, (2) Spokane Valley Interceptor, and (3) North Spokane Interceptor.  Monitoring of average daily wastewater is performed at these locations. 

A summary of the average daily flow for the three monitoring points for the last six years is presented in Figure 3‑1.  Nearly two-thirds of the wastewater is generated in the area tributary to the Spokane Valley Interceptor.  Wastewater flow at each monitoring point has been increasing annually as more homes and businesses are connected to the sewer system.   As of December 1999, average daily wastewater flow discharged to the City of Spokane was measured to be 5.68 million gallons per day (mgd).

Figure 3‑1.  Spokane County Wastewater Flow Monitoring

A review of the flow records for the three interceptors resulted in the following observations:

§         There is no significant seasonal variation in flow rates. 

§         On an annual basis, the volume of extraneous stormwater and groundwater flows entering the collection system is quite low (less than five percent of the total flow).

§         The estimated average unit flow rate in the three interceptors is approximately 76 gallons per capita per day (gpcd).  This value falls in the lower range of typical domestic wastewater generation rates (without infiltration and inflow) and is lower than the 100 gpcd planning value recommended in the Washington State Department of Ecology’s Criteria for Sewage Works Design.

3.4.3        Projected Average Annual Flow

Projections of average annual flow rates were developed as part of the CWMP.  The results of this analysis are presented in Table 3‑3 for the full Spokane County planning area. The subsequent discussion provides a summary of key assumptions used. 

Table 3‑3.  Projected Average Flow

1.        NOTES:                                          

a        Base Flow: Average daily flow for 1999, including current infiltration, is based on meter readings at three City/County connections and reflects flows from customers existing at that time.

b        Sanitary wastewater flow increase due to study area growth identified in Table 3-1, at 90 gpcd.

c        Sanitary flow from the new commercial acreage identified in Table 3-2, at 1,000 gpad.

d        Sanitary flow from the new industrial acreage identified in Table 3-2, at 1,000 gpad.

e        Sanitary wastewater from sewer program population identified in Table 3-1, at 90 gpcd.

f         Sanitary wastewater from non-program population identified in Table 3-1, at 90 gpcd.

g        Sum of Columns a through f

h        Infiltration from existing sewer system.  Increases to 10 gpcd by 2020.

i         Infiltration from new development.  Increases to 10 gpcd by 2020.

j         Sum of columns d and e.

k        Total Wastewater = Sum of sanitary wastewater and infiltration (Columns g and j)

Future Residential Contributions

Additional domestic wastewater will be contributed by two sources: 1) existing development to be served by sewer extensions and 2) population growth within the planning area.  In both cases, the projected sanitary flow rate (excluding infiltration and inflow) was determined by multiplying the population values in Table 3-1 by 90 gpcd.  The 90 gpcd value falls between the estimated current rate 76 gpcd) and Ecology’s recommended planning value (100 gpcd).  It was considered to represent a reasonable, if slightly conservative estimate of future conditions.

Future Commercial Contributions

Future commercial contributions were estimated by multiplying the projected developed acreage in Table 3-2 by a unit flow rate of 1,000 gallons per acre per day (gpad), a typical planning value for mixed commercial development.

Future Industrial Contributions

Future industrial flow contributions are projected to be generated by 1) typical industrial development and 2) wet industries.  Flow from typical industrial development was estimated by multiplying the projected developed acreage in Table 3-2 by 1,000 gpad.  To project future flows from wet industries, it was assumed that two industries would be developed, each with a wastewater flow rate of 0.5 mgd.  They would be located in the North Valley and Spokane Valley service areas.

Extraneous Flows

As described earlier, it appears that current flow contributions from infiltration and inflow (I/I) are quite low.  In projecting future conditions, it was assumed that aging of the collection system would gradually allow a modest amount of extraneous flow to enter the system.  The estimated future infiltration rates were calculated as follows:

§         Base Flow.  No infiltration contribution was assumed for the Year 1999 Base Flow.  In the future, the rate of infiltration contribution was assumed to be 2 gpcd for Year 2000 flows and to increase by 2 gpcd every 5 years, until 2020, at which point it is held constant at 10 gpcd through 2050.

§         Existing Development to be to be Served by Sewer Extensions.  The rate of infiltration contribution was assumed to be 2 gpcd through 2005 and to increase by 2 gpcd every 5 years, until 2020, at which point it is held constant at 10 gpcd through 2050.

§         Residential Growth.  The infiltration projection assumes 2 gpcd through 2005, increasing by 2 gpcd every 5 years until 2020, after which it is 10 gpcd of total infiltration.  The rate of infiltration is held constant at 10 gpcd through 2050, but increases in proportion to population increases within the planning area between 2020 and 2050.

§         Industrial and Commercial Development.  No infiltration allowance was provided for these flow contributions, assuming that overall system infiltration is adequately addressed through the assumptions made for residential contributions.

Total Wastewater Flow

The total wastewater flow is equal to the sanitary wastewater input plus infiltration.  Inflow is not a significant component if the average annual flow.  For new residential connections, the total wastewater quantity is equal to 90 gpcd of sanitary flow plus 10 gpcd of infiltration, for a total amount of 100 gpcd.  Infiltration is projected to develop over a period of 20 years.

3.4.4        Peak Flow Projections

Development of annual average flow projections provides a baseline for future planning; however, design of facilities and assessment of permit compliance are typically based on more extreme flow events such as maximum-month, maximum-day or peak instantaneous conditions.  This section describes the development of peak flow projections for the Spokane County system.

Review of Interceptor Flow Records

To gain an understanding of flow variation in the Spokane system, a review was conducted of flow meter records for the major interceptors.  Unfortunately, electronic data were available only for the Spokane Valley Interceptor (SVI).  This record provided flow data in five-minute increments.  For the North Valley Interceptor (NVI), hard copies of continuous circular charts were available. For the North Spokane Interceptor, the flow meter is located downstream of the main pumping station serving this area, and reflects the on/off operation of the constant speed pumps.   Consequently, this flow record was not found useful for evaluating the relationship between rainfall and wastewater flow.

Determination of Peaking Factors for Dry-Weather, Non-Storm Conditions

The NVI and SVI information for the last three years was reviewed to identify maximum month, maximum week, maximum day, and instantaneous peak flow values.   From the data, peaking factors for the different conditions were calculated.  The term “peaking factor” refers to the ratio of the flow event observed to the average annual flow for the year of record.

Peak events were compared with precipitation events, and the estimated wastewater contribution due to precipitation was subtracted from the flow.  As a result, the calculated peaking factors were based on dry-weather, non-storm conditions.  A summary of the observed peaking factors for the SVI data is presented in Table 3‑4.  This table also presents the values that were selected for use in projecting peak flow events.  These peaking factors are representative of peak flow conditions at the downstream end of the service area.  Higher peaking factors can be expected upstream in the collection system, where the contributing sewer basin is smaller and localized impacts to flow rates are not dampened by a large base flow rate.

Table 3‑4.  Analysis of Dry Weather Non-Storm Peaking Factors for Spokane Valley Interceptor 

Determination Infiltration and Inflow Contributions

The SVI and NVI data were reviewed to determine whether any evidence of infiltration, interflow, or inflow exists.  As described earlier, annual contributions of I&I appear quite low, as evidenced by the low per capita wastewater generation rate (80 gpcd) and the lack of seasonal variation in flow rates.  This may be attributed to the relatively new collection system infrastructure and the fact that the most common source of prolonged infiltration – rising  groundwater due to precipitation or irrigation – is not present in the Spokane County service area.

Continuous flow data following precipitation events were reviewed to ascertain whether short-duration interflow or inflow events occur.  Precipitation events between late spring and early fall were selected to eliminate snowfall events that would not generate an immediate flow response (i.e., flow response would not occur until snowmelt).  

Moderate precipitation events did not appear to stimulate increased wastewater flow.  However, inflow responses in the Spokane Valley data were observed for heavy summer thunderstorms that occurred on June 18, 1998 and August 14, 1999.  During both events, a rainfall rate of 0.25 inches of rain in an hour (measured at Geiger Field, approximately 10 miles from the flow monitoring station) caused a short-term flow rate increase of about 0.9 million gallons per day.  This response is shown in Figure 3‑2 and Figure 3‑3. The response to precipitation was triggered only by significant precipitation, and the response was nearly immediate.  The flow decreased as soon as precipitation ceased.  No evidence of interflow (rapid infiltration) was identified; consequently, the flow contribution appears to be solely from inflow.

Using a standard runoff equation, a runoff coefficient of 1, the storm intensity (0.25 inches per hour) and the resulting instantaneous flow rate increase (0.9 mgd), the approximate connected impervious surface was calculated (about 6 acres). 

The short-duration inflow events resulted in a relatively small volume of wastewater when viewed in terms of the total wastewater generated during a one-week or one-month period.  However, inflow contributions are important when defining maximum day and peak hour flow rates.  To assess these impacts, the results from the June 18, 1998 and August 14, 1999 storm events were extrapolated to reflect 1-in-10-year storm events. The ten-year recurrence is commonly used in guidelines for control of sanitary sewer overflows, and was selected as an appropriate planning basis for wastewater facilities. Department of Ecology guidelines do not offer criteria for stormwater recurrence interval selection.  For the maximum-day contribution, a 24-hour storm intensity of 1.6 inches per day was used, whereas, a storm intensity of 0.5 inches per hour was used for the peak hour event.

Figure 3‑2.  Spokane Valley Interceptor Flow, Summer 1998

 Figure 3‑3.  Spokane Valley Interceptor Flow, Summer 1999

Summary of Peak Flow Projections

Table 3‑5 presents a summary of the estimated current and future peak flows for the entire Spokane County service area.  The methodology used to calculate the values is presented in the table notes.

Table 3‑5.  Projected Peak Wastewater Flow

Notes:                                                                                                                                                                                

a           Average sanitary flow from Column g of Table 3-3.                                                                                                                                                                                                             

b           Infiltration from Column I of Table 3-3.                                                                                                                                                                                                                

c           Average wastewater flow =  Average sanitary flow plus infiltration                                                                                                                                                                                                     

d           Maximum month average flow = (average sanitary flow x maximum month peaking factor) + infiltration                                                                                                                                                                                                     

e           Maximum week average flow = (average sanitary flow x maximum week peaking factor) + infiltration                                                                                                                                                                                                       

f           Existing impervious surface for the Spokane Valley Interceptor tributary area (5.8 acres) calculated from storm

             events of June 19, 1998 and August 11, 1999.  In both cases, one-quarter inch of rain increased peak flow by 0.9

             mgd.  Calculations assume a 1.0 runoff coefficient.  Existing impervious surface for SVI divided by 5/8 to

             estimate impervious surfaces for all three service areas.  Impervious surface assumed to increase by 10 percent

             every five years                                                                                                                                                                                                                 

g           From NOAA Precipitation Atlas of the Western United States                                                                                                                                                                                                                 

h           Impervious surface x peak hourly precipitation                                                                                                                                                                                                 

i            Maximum day average flow = (average sanitary flow x maximum day peaking factor) + infiltration + peak daily

             stormwater inflow                                                                                                                                                                                                              

j            From Department of Commerce Rainfall Frequency Atlas of the United States                                                                                                                                                                                                         

k           Impervious surface x peak hourly precipitation                                                                                                                                                                                                 

l            Peak instantaneous flow = (average sanitary flow x maximum instantaneous peaking factor) + infiltration + peak

             hourly stormwater inflow                                                                                                                                                                                                                

m          Effective peaking factor obtained by dividing peak instantaneous flow by average daily flow                                                                                                                                                                                                   

3.5        Wastewater Loading Projections

Wastewater strength and unit wastewater loading rates may vary from one community to another because of differences in the physical and economic environments.  For this reason, it is generally desirable to use historical sampling data from the specific community as the basis for determining planning projections for wastewater loadings.  Only limited wastewater quality data exist for the Spokane County system.  City of Spokane data is only partially representative of conditions in the County’s service area, because of the differences in the collection system between the City and County.  To develop a reasonable projection of future wastewater loadings, an assessment of County and City data was supplemented by a review of other local wastewater utilities with generally similar service area characteristics.

3.5.1        Constituents of Concern

From the perspectives of treatment plant design and effluent quality permit compliance, the primary constituents of interest are biochemical oxygen demand, suspended solids, nitrogen, phosphorus and metals.

Biochemical Oxygen Demand

Biochemical oxygen demand refers to the quantity of oxygen required by microorganisms to effect oxidation of the organic matter present in wastewater to carbon dioxide and water.  Usually referred to as BOD, this characteristic defines the strength of a wastewater in terms of its potential impact on dissolved oxygen levels in a receiving stream.  BOD strength often determines the type and degree of treatment that must be provided to produce a required effluent quality.  BOD is traditionally expressed as the amount of oxygen utilized when biological oxidation of organic matter is allowed to proceed for 5 days at 20 degrees C (BOD₅).

Suspended Solids

Suspended material transported in wastewater is referred to as suspended solids.  The quantity of suspended material removed during treatment varies with the type and degree of treatment and has an important bearing on the sizing of many mechanical and process units.  The term total suspended solids is often used to refer to the fact than solids include both volatile (generally organic) and fixed portions. 

Nitrogen

Nitrogen is of concern in treated wastewater since it can 1) increase the oxygen demand in a stream, 2) act as a plant nutrient, stimulating the growth of undesirable species, and 3) as unionized ammonia, it can be toxic to fish.  The term Total Kjeldahl Nitrogen represents  the sum of the organic nitrogen and ammonia present in the wastewater.  It does not include oxidized nitrogen, such as nitrates and nitrites.  However, since oxidized nitrogen is present in only minute quantities in raw wastewater, Total Kjeldahl Nitrogen generally represents the entire nitrogen concentration of an untreated wastewater.  The term “Kjeldahl” refers to the laboratory distillation process necessary to convert organic nitrogen to measurable ammonia.

Total Phosphorus

Like nitrogen, phosphorus is a plant nutrient.  It is usually present in raw wastewater as orthophosphate (PO₄), polyphosphate, and organic particular forms.

Metals

Metals may exhibit aquatic toxicity at low concentrations, particularly in receiving waters with low hardness levels.  In the Spokane River, current instream concentrations of some metals exceed water quality criteria because of historical mining activities in the upper reaches of the watershed.

3.5.2        Projected Average Wastewater Characteristics

BOD, Suspended Solids and Nutrient Data - Spokane County Interceptors

Useful water quality data for the County’s interceptor system is limited to quarterly sampling conducted by the City along the North Valley and Spokane Valley interceptors.  Figure 3‑4 and Figure 3‑5 present the sampling results at each location for BOD, suspended solids and total phosphorus.  No data have been gathered on ammonia or total nitrogen concentrations.  Currently, the County is conducting more extensive sampling to supplement this database.

Figure 3‑4.  North Valley Interceptor Wastewater Characteristics

Figure 3‑5.  Spokane Valley Interceptor Wastewater Characteristics

A summary of the quarterly monitoring results is presented in Table 3‑6.  The data appear to show that the North Valley wastewater has the greater concentration of total suspended solids and total phosphorus, but the Spokane Valley wastewater has a higher concentration of BOD.   This observation may reflect the greater number of industrial customers in the North Valley area.

Table 3‑6.  Interceptor Sampling, Summary of Quarterly Sampling 1996 to 1999

BOD, Suspended Solids and Nutrient Data – Other Local Communities

Since limited information on Spokane County wastewater quality is available, information from surrounding communities was obtained to characterize regional wastewater trends.  Data were obtained from Liberty Lake Water and Sewer District #1 [iii], City of Cheney [iv], City of Spokane (which includes Spokane County contribution) [v], City of Coeur d’Alene [vi], City of Deer Park [vii], City of Post Falls [viii], and Hangman Valley (Spokane County Utilities) [ix].  Information on regional wastewater characteristics is summarized in Table 3‑7.  To provide a point of reference for the regional data, Table 3‑7 also presents literature values for per capita loading rates and wastewater concentrations.  These values are typically used if local data are insufficient or considered unreliable.

Table 3‑7.  Wastewater Loading Summary for Nearby Communities