CHAPTER 4 SYSTEM DESIGN CRITERIA

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1 INTRODUCTION CHAPTER 4 SYSTEM DESIGN CRITERIA Adequate design of the District s wastewater conveyance facilities requires the determination of the quantity of wastewater from contributing sources. The District s wastewater is predominantly domestic in origin, with lesser amounts contributed by commercial and industrial businesses and by public use facilities, such as schools, parks, hospitals, and municipal functions. Infiltration and inflow contributions result from groundwater and surface water entering the sewer system during periods of rainfall or as a result of high groundwater levels. Data from previous years are used to estimate unit quantities for critical parameters related to population, land use, and overall land area in the existing service area. These unit quantities are then applied to the projected future population, comprehensive land use, and the future land area that may be served to determine the design criteria for selecting and sizing various components required to serve the study area in future years. DEFINITION OF TERMS In this chapter wastewater characteristics for the existing service area are analyzed and projections made for future conditions. The terms and abbreviations used in the analysis are described as follows: WASTEWATER Wastewater is defined as wastes carried by water from residential, business, and public use facilities, together with quantities of groundwater and surface water which enter the sewer system through defective piping and direct surface water inlets. The total wastewater flow is quantitatively expressed in millions of gallons per day (mgd) or in gallons per minute (gpm). DOMESTIC WASTEWATER Single- and multi-family residences generate domestic wastewater. Domestic wastewater flow is generally expressed as a unit flow based on the average contribution from each person per day. The unit quantity is expressed in terms of gallons per capita per day (gpcd). Silver Lake Water & Sewer District 4-1 Wastewater Comprehensive Plan April 2011

2 INFILTRATION Infiltration is defined as groundwater, which enters a sewer system by means of defective pipes, pipe joints, or manhole walls. Infiltration quantities generally exhibit seasonal variation in response to groundwater levels. Storm events can typically trigger a rise in groundwater levels and an increase of infiltration flows. The highest infiltration flows are observed following significant storm events or following prolonged periods of precipitation. Since infiltration is related to the total amount of piping and appurtenances in the ground and not to any specified water use component, it is generally expressed in terms of the total land area being served or in terms of the lengths and diameters of sewer pipe. The unit quantity used in this study is gallons per acre per day (gpad). INFLOW Inflow is surface water that enters the sewer system from yard, roof, and footing drains, from cross-connections with storm drains, and through holes in manhole covers. Peak inflow can occur during heavy storm events when storm sewer systems are surcharged, resulting in hydraulic backups and local ponding. Inflow is expressed in terms of gallons per acre per day (gpad). AVERAGE DRY WEATHER FLOW (ADWF) Average dry weather flows consists of wastewater that flows during periods when the groundwater table is low and the precipitation is at its lowest of the year. The dry weather flow period normally occurs from June to September. The average dry weather flow is taken to be the average daily flow during the three lowest consecutive flow months of the year. PEAK DOMESTIC FLOW Peak domestic flow is wastewater from domestic sources during the peak hour of a day. Peak domestic flow is an estimate of the domestic wastewater produced by the simultaneous discharge to the sewer system by the entire population of a drainage basin. A peaking factor can be determined by taking the ratio of the peak hour flow to the average day flow. MAXIMUM MONTH FLOW Maximum month flow is the average daily flow during the highest flow month of the year. This flow is composed of the normal domestic flow with contributions to the sewer system from infiltration and inflow. Maximum Month Flow is used in the District s agreements for purchased capacity in wastewater treatment facilities. By agreement, the District is allowed to discharge a Maximum Month flow of 6.6 mgd to the Everett Water Pollution Control Facility (WPCF). This is expressed as the total volume of flow for the month divided by the number of days in that month. 4-2 Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

3 PEAK WET WEATHER FLOW (PWWF) The peak wet weather flow (PWWF) is the highest wet weather flow during the day. This flow includes contributions to the sewer system from infiltration and inflow and from peak domestic discharge. PWWF is used in sizing and evaluating the hydraulic capacity of conveyance and pumping components of a wastewater collection system. By agreement, the District is allowed to discharge a Peak Hourly Flow of 11,500 gpm to the Everett South End Interceptor. WASTEWATER FLOW DESIGN CRITERIA In order to properly design the District s sanitary sewer facilities, it is necessary to determine the unit quantities for each of the major components that constitute the volume of sewage flows. The two unit quantities to be identified are domestic wastewater and infiltration and inflow. The domestic wastewater unit is generally expressed as gallons per capita per day (gpcd). Estimates of sewage flows for the study area and individual drainage basins require the application of the unit flow design criteria to the size of the contributing area, the comprehensive land use for the contributing area, and the projected population to be served within the contributing area. AVERAGE DOMESTIC FLOW Winter water production records are used to identify the average domestic flow value. To negate the effect of irrigation water use, which is water that generally does not enter the sewer system, water production records are utilized during the winter period to estimate the amount of water that is generally discharged to the sewer system. The District s records for November 2006 through December 2008 are used to determine per capita winter water production. For each month, the water produced in mgd, the number of customers served, and the estimated population served are used to calculate per capita water usage. The calculated values are presented in Table 4-1. The results indicate the per capita average winter water production is 64.6 gpcd. Not all of the winter water produced reaches the sewer collection system as wastewater, due to water distribution system leakage estimated to be about 5 to 9 percent, as reported in the District s Water Use Efficiency Plan. Also, not all the water delivered to the consumer reaches the collection system, because some of this water is used for outdoor activities such as irrigation, car washing, etc. An average domestic flow of 60 gpcd is used in the calculation of existing and future sewage flows in the District. The same unit flow was used in the District s 2003 Wastewater Comprehensive Plan and was based on average winter water consumption flows. Silver Lake Water & Sewer District 4-3 Wastewater Comprehensive Plan April 2011

4 TABLE 4-1 Winter Water Production Year Month Production (MGD) (1) Water Connections (2) Population (3) Per Capita Production (GPCD) (4) 2006 Dec ,962 42, Jan ,000 42, Feb ,038 43, Nov ,380 43, Dec ,419 44, Jan ,435 44, Feb ,452 44, Nov ,602 44, Dec ,619 44, Jan ,624 44, Feb ,629 44, Nov ,675 44, Dec ,680 44, Average ,424 44, (1) Based on production measured at the Master Meters, data taken from 2010 Comprehensive Water System Plan, Table (2) Includes residential and commercial connections, unsewered connections, and does not distinguish between AWWD and City of Everett basins. (3) Population based on 2.86 persons per connection, including commercial accounts. (4) Winter production divided by population equals per capita production. FLOW MONITORING In order to develop current I/I rates, City of Everett flow monitoring data has been used. The City of Everett installed flow monitoring equipment in its 30-inch and 36-inch interceptors, in an effort to determine peak wet weather flows from the District to Everett. The City of Everett began monitoring flows at two locations on August 27, In order to calculate flows from the District sent to Everett, flow data from a flow monitor located at the 27 th Avenue SE and approximately 116 th Street SE intersection determined flows from the Everett sewer system entering the District s service area. These flows are subtracted from the flow data measured at a second Everett flow monitor location, at 35 th Avenue SE and approximately 94 th Street SE, to determine the total flows leaving the District to Everett. Flow monitoring data was obtained for the months of June 2009 through January 2010, by accessing Everett s online flow monitoring database. Dry-Weather Flow Monitoring Based on the flow data obtained, it appears that the month of September 2009 was the driest month. A typical, dry-weather weekday for the District is depicted in Figure 4-1, for Wednesday, September 23, Total flow on that day, measured at the 4-4 Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

5 35 th Avenue/94 th Street meter was 2.11 million gallons and the total flow, measured at the 27 th Avenue/116 th Street meter was 0.71 million gallons. The difference between these two is the daily total for the District, or approximately 1.40 million gallons. The part of the day wherein the flows are the lowest was chosen to be from 3:00 a.m. to 5:00 a.m. This time period, generally represents the base infiltration. Dry-weather flows are used to determine the domestic peaking factor and confirm the domestic per capita flows. Table 4-2 provides a summary of the dry-weather flow, including basin size, estimated population served, estimated base I/I rate, and peaking factor. FIGURE DIURNAL CURVE SILVER LAKE WATER & SEWER DISTRICT FLOWS ONLY SEPTEMBER 23, ,500 2,000 GPM 1,500 1,000 DIURNAL CURVE :00 AM 1:00 AM 2:00 AM 3:00 AM 4:00 AM 5:00 AM 6:00 AM 7:00 AM 8:00 AM 9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM 3:00 PM 4:00 PM 5:00 PM 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM 12:00 AM TIME Silver Lake Water & Sewer District 4-5 Wastewater Comprehensive Plan April 2011

6 TABLE 4-2 Dry-Weather Flow Monitoring Basin Information District Flow to Everett Flow Monitoring Period September 2009 Basin Area 4,411 Acres Sewered Dwelling Units 11,939 Sewered Population 34,145 ADWF 1,086 gpm Base I/I (1) 306 gpm Base I/I (1) 100 gpad Peak Domestic Flow (2) 2,202 gpm Domestic Peaking Factor (3) 2.0 (1) Base I/I calculated as average dry weather flow between 3:00 a.m. to 5:00 a.m., averaged over September, (2) Peak hourly dry weather flow for September 5, (3) Peak Domestic Flow divided by ADWF, September 5, 2009 see Figure 4-2. DOMESTIC FLOW PEAKING FACTOR A peaking factor is used to convert average domestic flow to peak domestic flow. As the average domestic flow of a drainage basin increases, the peaking factor decreases. A peaking factor of 2.0, per Figure 4-2, will be used for all future flow estimations. This peaking factor is the highest calculated for all days in the month of September 2009, and therefore is conservative. The average peaking factor for the month of September 2009 is Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

7 FIGURE PEAKING FACTOR SEPTEMBER 5, ,500 HOURLY PEAK = 2,202 2,000 1,500 GPM DAILY AVERAGE = 1,086 1, PEAKING FACTOR = :00 AM 1:00 AM 2:00 AM 3:00 AM 4:00 AM 5:00 AM 6:00 AM 7:00 AM 8:00 AM 9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM 3:00 PM 4:00 PM 5:00 PM 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM 12:00 AM TIME INFILTRATION AND INFLOW The amount of infiltration and inflow (I/I) can be estimated on an annual average, maximum month, and maximum day basis by subtracting the dry weather flow from the annual average, maximum month, and maximum day flows. Also, dry weather and peak hour I/I can be calculated from observations of flow data. Based on the Agreement with the City of Everett regarding discharge to the South End Interceptor, only Peak Wet Weather Flow (PWWF) and Maximum Month Flow are calculated for the District service area draining to the interceptor, which includes the developed portions of the CVWD agreement areas. For this Plan, infiltration and inflow is expressed in units of gallons per acre per day (GPAD). The developed sewer service area currently draining to the interceptor is comprised of approximately 4,411 acres, and includes portions of the CVWD Agreement areas. Areas designated for recreation and open space and unsewered areas are excluded from the total acreage to estimate the developed sewer service area. As areas develop in accordance with the population projections set forth in Chapter 2, the total acreage for the service area will increase over time. At the time of Buildout, the total developed sewer service area is approximately 6,717 acres. Based on the flow monitoring data obtained for the Everett interceptor, the District s current peak hourly I/I, in the Everett service area, is approximately 1,134 gpad (see Figure 4.3). The highest average hourly peak flow measured for the District was 4,461 gpm at 11:20 p.m. on January 7, The average hourly flow at 11:20 p.m. on Silver Lake Water & Sewer District 4-7 Wastewater Comprehensive Plan April 2011

8 the diurnal curve from September 23, 2009 is 987 gpm. The difference between the two data points, or 3,474 gpm (1,134 gpad) is the peak hourly I/I. FIGURE PEAK HOURLY I/I RATE 5,000 4,500 4,000 3,500 JANUARY 7, ,461 3,000 GPM 2,500 2,000 MAX DELTA = 3,474 GPM 1,500 1, SEPTEMBER 23, :00 AM 1:00 AM 2:00 AM 3:00 AM 4:00 AM 5:00 AM 6:00 AM 7:00 AM 8:00 AM 9:00 AM 10:00 AM 11:00 AM 12:00 PM 1:00 PM 2:00 PM 3:00 PM 4:00 PM 5:00 PM 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM 12:00 AM TIME Generally, I/I rates can be expected to increase over time, as the system ages and deteriorates. This number will be used for the existing flow projections, in the Everett basin, but future I/I rates will be reduced to meet the District s standard of 1,000 gpad for peak hourly I/I, under the assumptions that the District will successfully undertake an Infiltration and Inflow reduction program. Flow projections for the Alderwood drainage basins will utilize 1,000 gpad for peak hourly, since accurate flow data was not obtained. Infiltration and Inflow Analysis using EPA Criteria Another analysis of infiltration and inflow was performed to compare estimates of per capita I/I to EPA criteria. These infiltration and inflow rates are summarized in Table 4-3. The U.S. EPA manual entitled I/I Analysis and Project Certification provides recommended guidelines for determining if infiltration and/or inflow is excessive. 1. To determine if excessive infiltration is occurring, a threshold value of 120 gallons per capita per day (gpcd) is used. This infiltration value is based on an average daily flow over a seven to fourteen day non-rainfall period during seasonal high ground water conditions. 4-8 Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

9 2. To determine if excessive inflow is present in a collection system, the U.S. EPA uses a threshold value of 275 gpcd. If the average daily flow (excluding major commercial and industrial flows greater than 50,000 gpd each) during periods of significant rainfall exceeds 275 gpcd, the amount of inflow is considered excessive. TABLE 4-3 Per Capita Infiltration and Inflow Based on EPA Criteria Parameter EPA Criteria for Excessive I/I (gpcd) Estimated District I/I Value (gpcd) EPA Excessive Infiltration Criteria EPA Excessive Inflow Criteria Infiltration District precipitation records show a 12-day period, January 13 through 24, during which only two hundredths of an inch of rainfall was measured. This would be a period of relatively high groundwater due to a total rainfall of over four and a half inches from the middle of December leading up to January 13 th. The average daily flow recorded during this period is 3.82 million gallons per day. The highest daily flow was 4.15 million gallons. The intent of the EPA criteria is to only include domestic flows. Since most of the District s customers are residential, no subtraction of commercial or industrial flows is necessary. Based on the lowest daily flow measured on September 23, 2009 of 1.40 million gallons (975 gpm), the contribution from infiltration would be the difference between 3.82 mgd and 1.40 mg, or 2.42 mgd. With a total population of sewer users in 2008 of 34,145, and an average daily infiltration flow of 2.42 million gallons, the peak infiltration is estimated at 71 gpcd. Because this value is less than the EPA guideline of 120 gpcd, the District is not considered to have excessive infiltration by EPA criteria. Inflow The maximum day flow over the period of October 1, 2008 to February 1, 2010 was 4.84 mgd (recorded on January 11, 2009). The intent of the EPA criteria is to only include domestic flows. Since most of the District s customers are residential, no subtraction of commercial or industrial flows is necessary. With a total population of sewer users in 2008 of 34,145, and a maximum day flow of 4.84 million gpd for this day, the highest possible maximum inflow is estimated at 142 gpcd. Because this value is less than the EPA guideline of 275 gpcd, the District is not considered to have excessive inflow by EPA criteria. Silver Lake Water & Sewer District 4-9 Wastewater Comprehensive Plan April 2011

10 I/I Summary In general, I/I for the District s sewer collection system can be considered a moderate problem. Based on EPA criteria, I/I is not considered excessive. I/I values typically cover a range of 20 to 3,000 gpad (Wastewater Engineering Treatment Disposal and Reuse, Metcalf & Eddy, Inc., 3 rd Edition). For the District s flows, I/I values fall at the low end of this range. MAXIMUM MONTH FLOW The maximum winter month flow is used for the calculation of the District s share in the Everett Water Pollution Control Facility (WPCF). The calculations include the average domestic flow with contributions from peak infiltration and inflow. Utilizing flow data from February 2009, the total monthly flow was 4.07 mgd, or 2,827 gpm. If the daily average flow of 1,086 gpm is subtracted, and divided by the basin area (4,411 acres), a maximum monthly I/I value of 568 gpad is calculated. This number will be used for the existing flow projections, but future I/I rates will be reduced to meet the District s standards of 500 gpad for average I/I to predict Maximum Month flows, under the assumptions that the District will successfully undertake an Infiltration and Inflow reduction program. Flow projections for the Alderwood drainage basins will utilize 500 gpad for maximum month I/I, since accurate flow data was not obtained. POTENTIAL FUTURE SERVICE AREAS Included in the study area are those areas lying outside the District s current boundaries and the Urban Growth Area (UGA) that could most likely be served by gravity into the District. These areas include portions of the Cross Valley Water District (CVWD) for which an agreement has not been made for sewer service and an area yet to be claimed by any service provider for future sewer service the Little Bear Creek area. Table 4-4 shows the size, estimated population, and peak hourly flows should these areas be served by the District in the future. These areas of potential future service are shown in Figure Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

11 ³ SCALE 1" = 3,000' SR TH 35TH AVENUE SE I-5 100TH STREET SE 132ND STREET SE SE AT T LE H IL L R O AD LEGEND SILVER LAKE WATER & SEWER DISTRICT BOUNDARY AREA EAST OF CATHCART SOUTH CVWD AREA LITTLE BEAR CREEK STUDY AREA 164TH STREET SE 35TH AVENUE SE SILVER LAKE WATER & SEWER DISTRICT 180TH STREET SE SR 9 WASTEWATER COMPREHENSIVE PLAN FIGURE 4-4 AREAS OF POTENTIAL FUTURE SERVICE M:\SLWD\08564 WW Comp Plan\GIS\FIGURES\FIGURE 4-4.mxd

12 TABLE 4-4 Areas of Potential Future Service (1) Area Description Acreage Estimated Population Peak Wet Weather Flow (gpm) Little Bear Creek (2) 402 5,700 2,850 South CVWD Area (3) 614 8,800 4,400 Area East of Cathcart (4) 166 2,400 1,200 Total 1,182 16,900 8,450 (1) Areas already within the CVWD Agreement area are not included in this table. See Figure 4-4. (2) Area lies east of the Thomas Lake and 164 th Street SE basins, in an area yet to be claimed by any service provider. (3) All CVWD non-agreement areas lying south of the District boundary, between 55 th Avenue and SR 9, and north of 164 th Street. (4) Area lies between CVWD Agreement area and SR 9, north of District boundary. SUMMARY OF WASTEWATER FLOW DESIGN CRITERIA The wastewater flow design criteria for the District are summarized in Table 4-5. The table contains calculated values for the total flow generated in the District based on developed land area and population projections. Table 4-5 illustrates the projected flows in the District assuming the Cathcart Landfill is developed per the current Snohomish County proposal. The CVWD Interlocal Agreement areas, lying outside the District s boundary are included. Silver Lake Water & Sewer District 4-11 Wastewater Comprehensive Plan April 2011

13 TABLE 4-5 Summary of Wastewater Design Criteria for Entire Service Area (1) Design Criteria (Buildout) Sewered Population 41,298 59,731 76,221 Per Capita Domestic Flow 60 gpcd 60 gpcd 60 gpcd Average Domestic Flow 1,721 gpm (2.48 mgd) 2,489 gpm (3.58 mgd) 3,176 gpm (4.57 mgd) Peaking Factor (2) Peak Domestic Flow (3) 3,542 gpm 5,078 gpm 6,452 gpm Sewered Area 5,134 acres 6,101 acres 7,077 acres Peak Hourly I/I Rate 1,134 gpad 1,000 gpad 1,000 gpad Peak Hourly I/I Flow (4) 5.82 mgd 6.10 mgd 7.08 mgd Peak Wet Weather Flow (5) 7,584 gpm 9,314 gpm 11,366 gpm Maximum Month I/I Rate 568 gpad 500 gpad 500 gpad Maximum Month I/I Flow 2.92 mgd 3.05 mgd 3.54 mgd Maximum Month Flow (6) 5.39 mgd 6.63 mgd 8.11 mgd (1) Includes entire existing service area, including current proposal for Cathcart development and contract service areas lying outside the District s boundary (CVWD), and AWWD Agreement Areas, but excluding those areas shown on Figure 4-4. (2) Peaking Factor based on Flow Meter data. (3) Includes 100 gpm peak flow from Cathcart Landfill Treatment Facility. (4) I/I is applied to the developed service area, including public rights-of-way, but excluding site sensitive areas and open space. (5) Peak Wet Weather Flow is sum of Peak Domestic Flow and Peak Hourly I/I Flow. (6) Maximum Month Flow is sum of Average Domestic Flow and Maximum Month I/I Flow. The wastewater flow design criteria for the Everett Agreement Area are summarized in Table 4-6. The table contains calculated values for the total flow generated in the District and discharged to Everett based on developed land area and population projections. Table 4-6 illustrates the projected flows in the District assuming the Cathcart Landfill is developed per the current Snohomish County proposal, and the areas draining to Alderwood Water and Wastewater District, including the 180 th Street Lift Station, are subtracted from the total flows to the Everett system. The CVWD ILA areas, lying outside the District s boundary are included. Table 4-7 illustrates the projected flows for the Alderwood Water and Wastewater District s Agreement Area Silver Lake Water & Sewer District April 2011 Wastewater Comprehensive Plan

14 TABLE 4-6 Summary of Wastewater Design Criteria for Everett Agreement Area (1) Design Criteria (Buildout) Sewered Population 34,145 45,422 60,197 Per Capita Domestic Flow 60 gpcd 60 gpcd 60 gpcd Average Domestic Flow 1,423 gpm (2.05 mgd) 1,893 gpm (2.73 mgd) 2,508 gpm (3.61 mgd) Peaking Factor (2) Peak Domestic Flow (3) 2,945 gpm 3,885 gpm 5,116 gpm Sewered Area 4,399 acres 5,069 acres 5,945 acres Peak Hourly I/I Rate 1,134 gpad 1,000 gpad 1,000 gpad Peak Hourly I/I Flow (4) 4.99 mgd 5.07 mgd 5.94 mgd Peak Wet Weather Flow (5) 6,409 gpm 7,405 gpm 9,245 gpm Maximum Month I/I Rate 568 gpad 500 gpad 500 gpad Maximum Month I/I Flow 2.51 mgd 2.53 mgd 2.97 mgd Maximum Month Flow (6) 4.55 mgd 5.26 mgd 6.58 mgd (1) Includes current proposal for Cathcart development and contract service areas lying outside the District s boundary (CVWD). (2) Peaking Factor based on Flow Meter data (3) Includes 100 gpm peak flow from Cathcart Landfill Treatment Facility. (4) I/I is applied to the developed service area, including public rights-of-way, but excluding site sensitive areas and open space. (5) Peak Wet Weather Flow is sum of Peak Domestic Flow and Peak Hourly I/I Flow. (6) Maximum Month Flow is sum of Average Domestic Flow and Maximum Month I/I Flow. TABLE 4-7 Summary of Wastewater Design Criteria for AWWD Agreement Area Design Criteria (Buildout) Sewered Population 7,153 14,309 16,025 Per Capita Domestic Flow 60 gpcd 60 gpcd 60 gpcd Average Domestic Flow 298 gpm (0.43 mgd) 596 gpm (0.86 mgd) 668 gpm (0.96 mgd) Peaking Factor (1) Peak Domestic Flow 596 gpm 1,192 gpm 1,335 gpm Sewered Area 735 acres 1,032 acres 1,132 acres Peak Hourly I/I Rate 1,000 gpad 1,000 gpad 1,000 gpad Peak Hourly I/I Flow (2) 0.74 mgd 1.03 mgd 1.13 mgd Peak Wet Weather Flow (3) 1,107 gpm 1,909 gpm 2,122 gpm Maximum Month I/I Rate (2) 500 gpad 500 gpad 500 gpad Maximum Month I/I Flow 0.37 mgd 0.52 mgd 0.57 mgd Maximum Month Flow (4) 0.80 mgd 1.37 mgd 1.53 mgd (1) Peaking Factor based on Flow Meter data (2) I/I is applied to the developed service area, including public rights-of-way, but excluding site sensitive areas and open space. For Alderwood Agreement area, I/I rates are set to District standards for existing and future scenarios. (3) Peak Wet Weather Flow is sum of Peak Domestic Flow and Peak Hourly I/I Flow. (4) Maximum Month Flow is sum of Average Domestic Flow and Maximum Month I/I Flow. Silver Lake Water & Sewer District 4-13 Wastewater Comprehensive Plan April 2011