CHAPTER 4 WASTEWATER CHARACTERISTICS WASTEWATER FLOWS

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1 CHAPTER 4 WASTEWATER CHARACTERISTICS Wastewater collection, treatment, and disposal facilities are designed to handle specific hydraulic and pollutant loads for 20 or more years after they are constructed. To properly size and equip such facilities, it is important to know what the service area population is likely to be by the end of the planning period, how much wastewater (and stormwater) will likely be diverted into the system from that population, and what quantities and kinds of pollutants are likely to be found in the wastewater throughout that period. This chapter presents an analysis of Sultan s current wastewater volume, flow rate, and constituents. From this data, combined with the population forecasts and land use information given in Chapter 2, hydraulic and pollutant load projections for the year 2029 were developed. These projections are presented in this chapter. WASTEWATER FLOWS In the design of wastewater facilities, average and peak flows are equally important. Major treatment process units are generally sized on the basis of maximum monthly flow, while the hydraulic capacity of pipes, pumps, channels, and inlet and outlet structures are generally sized on the basis of peak flows. The existing flows can be broken into three types: the domestic sewage produced by individual household and commercial customers, the groundwater infiltration flows caused by sewer lines that are not water tight and allow some groundwater seepage, and finally the rainfall-dependent inflow and infiltration (RDII) in the sewer system. Figure 4-1 summarizes this influent flow breakdown approach and provides a definition of the terms used in the rest of this chapter. Appendix A Confirmation of Flows and Determination of Loads provides additional information on this approach. Sultan WWTP influent flow recorded between January 2003 and February 2006 were used to prepare the flow projections presented in this chapter. RDII has been reduced since 2002 as a result of the I/I control project referred to as First Avenue Project. Therefore, the 2002 data are excluded from this analysis. Table 4-1 summarizes the existing flow data, based on the Discharge Monitoring Reports (DMRs) and on the flow meter records of the peak hour flow 1 for The peak hour flow may be undereported since the measured influent flow is a function of the Sultan Main Pump Station capacity. The influent wet well water level was observed to increase up to 4 to 5 feet above the crown of the influent gravity sewer during storm events. This higher wet well level operation mode could decrease the peak hour flow due to the following reasons: (1) the peak flow is attenuated by storing some of the flow in the sewer system, and (2) the amount of RDII might decrease when the sewer lines operate at a higher hydraulic grade line (i.e., full sewer lines do not infiltrate groundwater at as high a rate as empty sewer lines). A more accurate evaluation of peaking would require modeling of the sewer system and would require collecting data over at least one winter season.

2 4-2 Sultan Wastewater Treatment Plant Upgrade Engineering Report Table 4-1. Existing Sewer Flows to the Sultan WWTP (mgd) Average Percent of Design Capacity a Annual Average Flow Average Dry Weather Flow (Summer) Average Summer Dry Weather Flow (excluding summer rain events) Average Wet Weather Flow (Winter) Average Winter Dry Weather Flow (extended dry periods) Max Month Flow Peak Day Flow Peak Hour Flow b a. Current hydraulic design capacity is 0.72 mgd max month flow, and 2.16 mgd peak hour flow. The 1997 upgrade only used the max month flow and the peak hour flow as design flows. b. Peak hour flow was observed for the period and calculated for the 2003 and 2004 period using the peaking factor of 1.61 (peak hour to peak day). Domestic Sewage Domestic sewage production can be estimated from the dry weather influent flow to the WWTP, since there is assumed little to no stormwater or groundwater flow to dilute the domestic sewage flow. The dry weather periods used to estimate the existing domestic sewage production are July through September, excluding periods of heavy summer rain. The groundwater infiltration in the sewer system is estimated to be equivalent to the typical 3:00 am summer flow, as explained in the next subsection. The RDII during this period is estimated to be negligible since the summer rain events are excluded from the flow analysis. The dry weather flows recorded in 2003, 2004, and were million gallons per day (mgd), mgd and mgd, respectively, for an average value of mgd (Table 4-1). Deducting the mean summer groundwater infiltration of mgd (see next section) yields the mean domestic flow of mgd. The average domestic sewage production per capita is calculated by dividing the domestic flow by the served population. The served population is the sewered residential population 2 plus the commercial use expressed as population equivalent, or an equivalent total of 3,414 people (=3,315 + (38 x 2.6)). This yields a domestic sewage production of 59 gallons per day (gpd)/capita. Existing domestic sewage production was rounded up to 60 gpd/capita. This sewage production figure is low as typical domestic wastewater production ranges from 70 to 80 gpd/capita. To be 2 Source: February 2006 Amendment No.1 to the General Sewer Plan, July 2005.

3 Wastewater Characteristics 4-3 conservative, an average day domestic sewage production of 70 gpd/capita will be used to estimate future sewage production in this Engineering Report. Plant data show that peaking of domestic sewage flow production is low. Over the last three years, the average flow during Thanksgiving and Christmas (two holidays when domestic sewage production is anticipated to be the highest) is mgd. When the average RDII and infiltration are deducted from this flow, the resulting domestic portion of the flow is mgd, which results in a 13 percent peaking factor for the peak day. Analysis of influent flow data shows that the peaking factor for the domestic sewage portion of the flow in the maximum month is negligible. Groundwater Infiltration The winter groundwater infiltration in the sewer system can be estimated by the difference between the base domestic sewage production and the average winter dry weather flow (see Figure 4-1 for definition). The mean average winter dry weather flow between 2003 and 2006 is mgd (Table 4-1). When the base domestic sewage production is deducted, the average groundwater infiltration in the sewer system is estimated to be 44,000 gpd. The City sewer system serves about 650 acres. The winter groundwater infiltration rate is therefore estimated to be 68 gpd/acre on average. The summer groundwater infiltration in the sewer system can be estimated from the 3:00 am flows to the plant during dry periods in the summer time. Data were collected at 15-minute intervals from the influent flow meter during the year The median 3:00 am flow between July and September 2005 was 20,000 gpd, which translates into a summer groundwater infiltration rate of about 30 gpd/acre. Rainfall-Dependent Inflow and Infiltration The RDII to the sewer system can be estimated by the wet weather flows to the plant less the sum of domestic sewage flow and groundwater infiltration flow. The sum of the latter two flows corresponds to the average winter dry weather flow. Note that the high flows recorded in January 2005 had to be excluded from the analysis because they occurred at the time of the First Avenue sewer reconstruction, and elements of the sewer system were open to direct inflow. As presented in Table 4-1, the average wet weather flow was mgd between 2003 and When the winter dry weather flow is deducted, the average RDII is estimated to be approximately 106,000 gpd (354,000 gpd less 248,000 gpd). Therefore, the average RDII for the 650-acre service area is about 160 gpd/acre. Maximum month flows were mgd both in December 2004 and March 2003, and mgd in January When the winter dry weather flow is deducted, the maximum month RDII is estimated to be approximately 200,000 gpd in December 2004 and March 2003, and 250,000 gpd in January For planning purposes, the highest RDII value of 250,000 gpd

4 4-4 Sultan Wastewater Treatment Plant Upgrade Engineering Report was divided by the 650-acre service area to produce a maximum month RDII of 385 gpd/acre. The peak day flows were 1.61 mgd on November 19, 2003, 1.54 mgd on December 11, 2004, and 1.50 mgd on January 30, When the winter dry weather flow is deducted, the peak day RDII is estimated to be 1.35 to 1.25 mgd. The December 10, 2004, storm appears to be the most important, with 5.8 inches of rainfall, and a four-day accumulated rainfall total of 10.2 inches. A storm on November 30, 2003, appears to have been a smaller storm; and the higher RDII value could be explained by weaker I/I control at that time. Using the December 11, 2004, flow, or 1.30 mgd divided by the 650-acre service area, the peak day RDII for the service area would be 2,000 gpd/acre. However, the RDII rate of 392 gpd/capita (1,300,000 gpd/3,315 people) is considered excessive, compared to EPA s criteria for non-excessive RDII of 275 gpd/capita during a storm event. Using EPA s criteria, a non excessive RDII parameter would correspond to approximately 1,300 gpd/acre. This latter value is proposed to be used for future developments flow projection in this Engineering Report. The higher value of 2,000 gpd/acre is proposed to be used for all existing developments. The summer RDII is relatively small because rain events are infrequent. Summer RDII is estimated by the difference between the average dry weather flow (0.227 mgd) and the summer dry weather flow (0.200 mgd), as defined on Figure 4-1. Summer RDII is therefore estimated to be mgd or 10 gpd/acre.

5 Wastewater Characteristics 4-5 Summary of Existing Flows Table 4-2 summarizes the data used to project future plant flows. Figure 4-1 illustrates the flow breakdown approach and the use of the unit rates to estimate future flows. Table 4-2. Unit Rates for Future Sewer Flow Production Projections Flow Breakdown Domestic Sewage Production a Average Day Peak Day Groundwater Infiltration b Average Day Winter Average Day Summer Current Sewer Flow Unit Rates 60 gpd/capita 68 gpd/capita 68 gpd/acre 15 gpd/acre Projected Current Sewer Flow Unit Rates 70 gpd/capita 79 gpd/capita 68 gpd/acre 15 gpd/acre RDII c Average Dry Weather Day Average Wet Weather Day Average Day, Max Month Peak Day 10 gpd/acre 160 gpd/acre 385 gpd/acre 2,000 gpd/acre 10 gpd/acre 160 gpd/acre 385 gpd/acre 1,300 gpd/acre a. Domestic sewage production is based on existing summer dry weather average flow less summer infiltration, divided by the served population. See Figure 4-1. b. Groundwater infiltration is based on existing winter dry weather average flow less summer dry weather flow, divided by the existing service area, which is 650 acres. See Figure 4-1. c. RDII for each period is based on the average flow for that period less average winter dry weather flow, divided by the existing service area, which is 650 acres. Projection data peak day RDII is based on the EPA criteria for non-excessive RDII, since new developments can be constructed following stringent specifications for I/I control. See Figure 4-1. Wastewater Flow Projections The projected sewer flows are presented in Table 4-3, using the unit rates of sewage flow presented in Table 4-2. These projected flows are also based on the population growth projection presented in Chapter 2, and agreed to by Snohomish County and the City of Sultan.

6 Existing Flow Breakdown Domestic Sewage Annual Average = mgd Summer Infiltration = mgd Summer Average Day RDII = mgd Winter Average Day RDII = mgd Max Month Average Day RDII = mgd Peak Day RDII = 1.3 mgd Domestic Sewage Groundwater Infiltration Rain Dependent Infiltration and Inflow (RDII) Description of Flow Breakdown Domestic Sewage Annual Average = Average Summer Dry Weather Flow a Summer GW Infiltration Summer Infiltration = Summer 3 am median flow Winter Infiltration = AverageWinter Dry Weather Flow b Domestic SewageFlow Summer Average Day RDII= ADWF c Domestic Sewage Flow Summer Infiltration Winter Average Day RDII = AWWF d Domestic Sewage Flow Winter Infiltration Max Month Average Day RDII = MMF e Domestic Sewage Flow Winter Infiltration Peak Day RDII = PDF f Domestic Sewage Winter Infiltration ADWF c : AWWF d : MMF e : PDF f : Domestic Sewage: mgd Infiltration: mgd RDII: mgd Domestic Sewage: mgd Infiltration: mgd RDII: mgd Domestic Sewage: mgd Infiltration: mgd RDII: mgd Domestic Sewage: mgd Infiltration: mgd RDII: 1.3 mgd a. The Average Summer Dry Weather Flow is the average influent flow for July, August, and September, excluding rainy periods. b. The Average Winter Dry Weather flow corresponds to the average influent flow during periods when no precipitation was recorded for several consecutive days (see Appendix 1). This average only considers data between Nov 1 and April 1 (high water table conditions). It is estimated to be equal to the base domestic sewage production and the winter groundwater infiltration. c. The Average Dry Weather Flow (ADWF) is the average influent flow for July, August, and September. d. The Average Wet Weather Flow (AWWF) is the average influent flow between Nov 1 and April 1. e. The Max Month Flow (MMF) is the highest monthly average flow in a year. Note: The highest value of RDII recorded in 2005 was assumed. f. The Peak Day Flow (PDF) is the highest daily average flow in a year Definition of Terms and Description of Influent Figure Flow Breakdown 4-1

7 4-6 Sultan Wastewater Treatment Plant Upgrade Engineering Report Table 4-3. Flow Projections for the Sultan WWTP Existing (2005) Population served (persons) 3,315 3,960 4,990 9,017 12,540 20,000 Commercial ERUs (ERU) a Service Area (acres) ,000 1,492 2,500 Domestic Sewage Flow Infiltration RDII b Average Day (mgd) Peak Day (mgd) Summer (mgd) Winter (mgd) Avg Dry Weather Day (mgd) Avg Wet Weather Day (mgd) Avg Day, max Month (mgd) Peak day (mgd) Average Dry Weather Flow (mgd) c Average Wet Weather Flow (mgd) d Avg Day, Max Month Flow (mgd) e Peak Day Flow (mgd) f Peak Hour Flow (mgd) g Peaking Factor ADWF/PHF 1:11 1:10 1:8 1:6 1:6 1:6 Percent of Average Day, Max Month Existing Design Capacity (0.72 mgd) h Percent of Pk Hr Existing Design Capacity (2.16mgd) h 69% 79% 93% 117% 127% a. One commercial ERU is assumed equivalent to 2.6 persons. b. RDII is based on higher unit values for the existing system than for the new developments. New developments RDII is based on EPA s non-excessive RDII criteria, as presented in Table 4-2. c. ADWF is estimated to be the base domestic sewage production plus summer infiltration. d. AWWF is estimated to be the base domestic sewage production plus winter infiltration and RDII. e. The Max Month Flow is estimated to be the sum of the max month domestic sewage, plus the winter infiltration, plus the max month RDII. f. The Peak Day Flow is estimated to be the sum of the peak day domestic sewage, plus the winter infiltration, plus the peak day RDII. Note that it is higher than the peak day on records, because it combines a storm event condition with a peak day domestic sewage production (worst case scenario). g. The Peak Hour Flow is estimated to be 1.61 times the Peak Day flow, based on the peaking factor observed on Jan 30, This peaking factor is within the normal range. h. Source: Gray&Osborn As Built Drawings, 1999.

8 Wastewater Characteristics 4-7 SEPTAGE As noted in Chapter 2, 350 homes within the Sultan sewer service area are not currently connected to the public sewer system. These houses provide residences for an estimated 910 people, and are being served by on-site treatment systems. The septage from these facilities is currently hauled to other treatment plants and therefore does not affect the Sultan WWTP. All new residences are required to obtain a sewer connection, and it is anticipated that by 2029, all the houses currently using on-site treatment systems would be connected to the public sewer system. WASTEWATER COMPOSITION AND LOADING Two wastewater components of primary concern to wastewater treatment plant designers are biochemical oxygen demand (BOD) and total suspended solids (TSS). The current discharge permit under the National Pollutant Discharge Elimination System (NPDES) for the City of Sultan provides specific effluent limits of 1,205 ppd and 964 ppd, for BOD and TSS respectively. Other constituents that can affect plant design and be of possible concern for future NPDES permits include the nutrients nitrogen and phosphorus. The projected BOD and TSS loadings for the City of Sultan and the existing wastewater composition are presented below. Biochemical Oxygen Demand Current and Projected Loadings BOD is a measure of the oxygen that the wastewater constituents require in order to be decomposed via biological reactions to stable compounds that can be safely discharged to receiving waters. Oxygen-demanding substances consist of soluble and insoluble organic matter, which, as a result of bacterial decomposition, causes the removal of dissolved oxygen from the wastewater. BOD loading, expressed in terms of pounds per day (ppd), is of primary importance to treatment facility designers. BOD concentration is ordinarily expressed in terms of mg/l. The average monthly NPDES effluent limit for BOD is 30mg/L and 1,205 ppd. Influent BOD measurements taken at the Sultan WWTP from January 2003 through January 2006 reveal that average monthly BOD loads ranged from 534 ppd to 400 ppd. The existing BOD load to the Sultan WWTP corresponds to approximately 41 percent of the existing plant s design capacity. On a peak day, BOD loads at the plant was recorded as high as 1,071 ppd. Influent BOD concentrations have ranged from 45 mg/l to 286 mg/l. The annual average, the maximum month, and the peak day BOD load averaged 0.13 ppd/capita, 0.14 ppd/capita, and 0.22 ppd/capita, respectively. The Orange Book recommends a value of 0.20 ppd/capita peak day load for new developments. This recommendation is slightly lower than the measured data for Sultan WWTP. Because human waste is a major source of BOD, future population increases are anticipated to produce BOD increases at a rate per capita that is similar to that produced by the existing population. Table 4-4 presents the rates used to project future BOD loading to the WWTP.

9 4-8 Sultan Wastewater Treatment Plant Upgrade Engineering Report Table 4-4. BOD Load Projections for the Sultan WWTP Loading per capita (ppdc) Existing (2005) Population served (pers) 3,315 3,960 4,990 9,017 12,540 20,000 Commercial ERUs (ERU) a Annual Average BOD (ppd) ,234 1,698 2,750 ADWF BOD Concentration (mg/l) b AWWF BOD Concentration (mg/l) b Max Month BOD (ppd) ,329 1,828 2,961 Max Month BOD Concentration (mg/l) b Peak Day BOD (ppd) ,144 2,088 2,873 4,653 Peak Day BOD Concentration (mg/l) b Percent of Peak Month Design Capacity (1,205 ppd) c 40% 48% 60% a. One commercial ERU is assumed equivalent to 2.6 persons. b. Values in italics are concentrations derived from the load value in ppd divided by the flow value. c. Source: Gray&Osborn As Built Drawings, Total Suspended Solids Current and Projected Loadings Total suspended solids include particulate matter and insoluble substances suspended in the wastewater. The concentration of these materials can be determined by filtering a sample of known volume, then drying and weighing the residue. Effluent TSS limits, expressed in terms of mg/l and pounds per day, are established in the City s NPDES permit. The average monthly effluent concentration limit for TSS is 30 mg/l and 964 ppd. Influent TSS measurements taken at the Sultan WWTP from January 2003 through January 2006 reveal that monthly TSS loads ranged from 548 ppd to 234 ppd. The existing TSS load to the Sultan WWTP corresponds to approximately 54 percent of the existing plant s design capacity. On a peak day, TSS load at the plant was recorded as high as 1,153 ppd. Influent TSS concentrations have ranged from 52 mg/l to 352 mg/l. The annual average, the maximum month and the peak day BOD load averaged 0.12 ppdc, 0.15 ppdc, and 0.28 ppdc, respectively. The Orange Book recommends a value of 0.20 ppdc peak day load for new developments. This recommendation is slightly lower than the measured data at the Sultan WWTP. TSS load is also anticipated to increases at a rate per capita that is similar to that produced by the existing population. Table 4-5 presents the rates used to project future TSS loading to the Sultan WWTP in this Engineering Report.

10 Wastewater Characteristics 4-9 Table 4-5. TSS Load Projections for the Sultan WWTP Loading per capita (ppdc) Existing (2005) Population served (pers) 3,315 3,960 4,990 9,017 12,540 20,000 Commercial ERUs (ERU) a Annual Average TSS (ppd) ,139 1,567 2,538 ADWF TSS Concentration (mg/l) b AWWF TSS Concentration (mg/l) b Max Month TSS (ppd) ,424 1,959 3,173 Max Month TSS Concentration (mg/l) b Peak Day TSS (ppd) ,152 1,455 2,658 3,657 5,923 Peak Day TSS Concentration (mg/l) b Percent of Peak Month Design Capacity (964 ppd) c 53% 64% 81% a. One commercial ERU is assumed equivalent to 2.6 persons. b. Values in italics are concentrations derived from the load value in ppd divided by the flow value. c. Source: Gray&Osborn As Built Drawings, Current Wastewater Composition Because the NPDES permit only requires removal of BOD and TSS, historical plant measurements of other typical wastewater constituents were not routinely performed. From February 26 to March 12, 2006, an in-depth wastewater characterization was performed on the influent as a basis for existing and future process modeling. The results of this wastewater characterization are summarized in Table 4-6 and represent the average daily wastewater composition for the sampling period. The composition of the Sultan wastewater is typical of a low- to medium-strength wastewater. For the characterization period, the per capita BOD and TSS loadings closely match the historical ( ) loadings measured for the average annual flow conditions. This indicates that the measurements taken during the February-March 2006 characterization period accurately reflect average wastewater loading conditions at the Sultan WWTP.

11 4-10 Sultan Wastewater Treatment Plant Upgrade Engineering Report Table 4-6. Average Daily Wastewater Composition During the Sampling Period (February 26, 2006, to March 12, 2006) Constituent Concentration (mg/l) Loading per capita (ppdc) Total BOD a Soluble BOD a Total Suspended Solids a Volatile Suspended Solids a Total Chemical Oxygen Demand a,b Soluble Chemical Oxygen Demand a,b Total Kjeldahl Nitrogen a,c Soluble Total Kjeldahl Nitrogen a,c Ammonia d Total Phosphorus e Phosphate e Alkalinity f Notes: a. The designation total vs. soluble for the various constituents relates to whether the sample was filtered (soluble) or not (total) prior to analysis. b. Chemical oxygen demand is a measure of the total oxidizable material within the wastewater sample and relates to the biodegradable and reactive material in the wastewater. c. Kjeldahl nitrogen is a measure of the organic and ammonia nitrogen in the samples. This provides a measure of the nitrogen available for growth of bacteria necessary for wastewater treatment and, along with ammonia, is important if ammonia and/or nitrogen removal is required by the NPDES permit. d. Ammonia is important in nitrification and nitrogen removal. Nitrification of ammonia to nitrate will increase the oxygen demand of the wastewater. e. Phosphorous (and phosphate), like nitrogen, is an important nutrient for bacterial growth. f. Alkalinity is important in that it provides a measure of the wastewater s ability to maintain a constant liquid ph and resist ph swings that may occur during treatment processes like nitrification.