1. introduction - fortuna center...2018.04.05 fortuna design flows and loads tm final.docx 9 figure...

GHD

718 Third Street Eureka California 95501 USA T 707 443 8326 F 707 444 8330 W www.ghd.com

April 5, 2018

To: City of Fortuna Ref. No.: 11146364-30

From: GHD Inc. Tel: 707.443.8326

CC:

Subject: Revised Flow and Loads Analysis - Fortuna WWTP Upgrade

1. Introduction

The purpose of this memorandum is to document the development of the design flows and loads for the City

of Fortuna Wastewater Treatment Plant (Fortuna WWTP). The design flows and loads are based on the

analysis of the historical data from 2015 to 2017 and 2018 supplemental sampling wherever possible and

estimations were made when data are not available. The existing WWTP has a permitted annual average

flow of 1.9 million gallons per day (mgd); however, based on discussions with the City and population

projections an annual average day flow of 1.5 mgd will be the basis for the flow and loads development as

documented in this memorandum. This technical memorandum presents the analysis of the historical data

from 2015 to 2017, the 2018 supplemental sampling, as well as the design flows and loads for facility’s

planned upgrade.

2. Historical Flow

Historical flow from January 2015 through November 2017 are presented in Figure 1. The annual average

flow increased from 0.96 mgd in 2015 to 1.27 mgd in 2017. Data shows that there is a seasonal flow pattern

with the highest monthly average flows occurring in December through February of each year as the result of

precipitation. Figure 2 shows the historical precipitation (based on data from Rohnerville Airport in the City of

Fortuna). Data shows that the high seasonal influent flow is the direct result of rainfall.

Table 1 shows the annual average flow and annual precipitation from 2015 to 2017. The flow increase of

about 22% from 2015 to 2016 is proportional to the increased rainfall.

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2018.04.05 Fortuna Design Flows and Loads TM Final.docx 2

Figure 1 Historical Wastewater Flow


Figure 2 Historical Precipitation

Table 1 Annual Average Flow vs. Precipitation

Year

Annual Average

Flow, mgd

Annual

Precipitation(1), in

2015 0.96 28.6

2016 1.17 34.4

2017 1.27 33.2 Note: (1) Precipitation data from Rohnerville Airport in the City of Fortuna

Table 2 shows the historical annual average, maximum month and peak day flows and the corresponding

peaking factors. The average “maximum month to annual average (MM/AA)” flow peaking factor is 1.95. The

average “peak day to annual average (PD/AA)” flow peaking factor is 3.18.


Table 2 Historical Flows and Peaking Factors

Flows Annual Average (mgd)

Max Month (mgd)

Annual PD (mgd)

2015 0.956 1.89 3.56

2016 1.169 2.28 3.42

2017 1.267 2.46 3.68

Peaking Factors MM/AA PD/AA

2015 - 1.97 3.72

2016 - 1.95 2.93

2017 - 1.94 2.90

Average PF - 1.95 3.18

3. Historical Wastewater Characteristics

Historical influent and primary effluent flows and wastewater characteristics for Total Suspended Solids

(TSS) and Biochemical Oxygen Demand (BOD) from 2015 to 2017 were analyzed to develop the basis of

the design. The plant staff analyze influent BOD and TSS using a composite sampler once per week, and

analyze primary effluent BOD and TSS using a composite sampler once per month. The location of the

composite samplers is shown in the process flow diagram in Figure 3. In analyzing the historical data, the

outliers, defined as loads greater than the 95th percentile and less than the 5th percentile, were eliminated

from the data set. Figure 4 and 5 show the historical flow weighted monthly average BOD and TSS

concentrations. Data shows that the BOD and TSS concentration is lower during the wet season due to the

precipitation and dilution of the wastewater.

The plant does not measure Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP), and alkalinity as part of

the routine analysis. Therefore, a set of supplemental sampling was performed in January/February 2018 to

determine these parameters.


FM (Flow Meter; S (Composite Sampler);GBT (Gravity Belt Thickener); BFP (Belt Filter Press); AB (Aeration Basins); PS (Pump Station)

Figure 3 Process Flow Diagram and Location of Composite Samplers


Figure 4 Monthly Average Influent BOD Concentration

Figure 5 Monthly Average Influent TSS Concentration


Figures 6 and 7 show the historical influent BOD and TSS loads, respectively. The outliers, which were

defined as values greater than 95 percentile and less than 5 percentile, are shown in blue. Once the outliers

were eliminated, the annual average and monthly average BOD and TSS loads were determined. The

average loads and the average flows were then used to calculate the flow weighted average concentrations.

The monthly average flow weighted concentrations for BOD and TSS are shown in Figure 4 and 5, while the

annual average flow weighted concentrations are shown in Figure 6 and 7.

Figure 6 Historical Influent BOD Load


Figure 7 Historical Influent TSS Load

Table 3 shows the historical raw influent BOD and TSS load peaking factors. Based on the analysis of the

historical data, the maximum month BOD and TSS loads occur at the same time. As shown in Figure 8, the

maximum month flow and loads tend to occur during the high flows, i.e. the wet season. Therefore, as will be

discussed later, the design will assume the maximum month load will occur at the maximum month flow.

Based on the historical data, a maximum month to annual average load peaking factor of 1.4 will be used for

the design.

Table 3 Historical Load Peaking Factors

BOD Load Peaking Factor MM/AA

TSS Load Peaking Factor

MM/AA

2015 1.60 1.59

2016 1.32 1.35

2017 1.28 1.32

Average 1.40 1.42


Figure 8 Historical Dry vs. Wet Influent Loads

4. Historical Primary Clarifier Removals

Historical primary clarifier effluent TSS and BOD removal from 2015 to 2017 is shown in Figure 9. Removal

efficiencies were calculated assuming a 10% flow increase across the clarifiers due to the sidestream returns

from the gravity belt thickener and the belt filter press. On average, 71% TSS and 39% BOD has been

removed across the primary clarifiers at a three-year average flow of 1.12 mgd. The removal efficiencies do

not seem to be considerably impacted or correlated to the seasonal flow variations within the study period.

The hydraulic analysis of the existing primary clarifiers, displayed in Table 4, shows that at design average

flow of 1.5 mgd and peak hourly flow of 5.25 mgd, the primary clarifiers will be operating within the

recommended surface overflow rates (SOR) of 1,000 gpd/ft2 and 2,500 gpd/ft2 at average and peak flows,

respectively (Metcalf & Eddy, 4th edition, Table 5-20). The peak hourly SOR limit as per 10-State Standard is

2,000 gpd/ft2. The peak hourly flow of 5.25 mgd is based on a previous analysis (2016 Flow and Load TM),

where it was determined that influent flows over 5.25 mgd will be diverted to the pond.

However, since the historical removals of 71% for TSS and 39% for BOD is on the higher end of the typical

removals for municipal wastewater treatment plants (which is 50 to 70% for TSS and 25 to 40% for BOD),

and due to the shallow depth of the existing primary clarifiers (two of them 7 ft or less and one at 10 ft


sidewater depth), as well as the fact that the design average flow is about 34% higher than the current

average flow (1.5 mgd vs. 1.12 mgd), it was decided to derate the removals by about 10%, to 65% for TSS

and 35% for BOD.

Figure 9 Historical Primary Clarifier TSS and BOD Removal

Table 4 Existing Primary Clarifiers

Parameter

No. of Primary Clarifiers 2 1

Diameter, ft 35 30

Sidewater Depth, ft 7, 10 6.5

Surface Area, Each, ft2 962 707

Surface Area, Total, ft2 1,923 707

TOTAL Area, ft2 2,630

Average SOR, gpd/ ft2 (1) 570

Peak SOR, gpd/ ft2 (2) 1,996

Note:

Average SOR at 1.5 mgd

Peak SOR at 5.25 mgd


5. Historical Liquid Temperature

Figure 10 shows the historical temperature in the aeration basin (AB). The maximum and minimum month

liquid temperature over the data analysis period was 23 ͦC and 14 ͦC, respectively. The annual average

temperature was 19 ͦC.

Figure 10 Historical Liquid Temperature

6. Supplemental Sampling

A set of supplemental sampling was performed in January/February of 2018 from the influent, primary

effluent, and the final effluent at the plant. Table 5 compares the historical BOD and TSS concentration with

the supplemental sampling results. The supplemental sampling results indirectly confirmed the BOD and

TSS concentrations derived from the historical data analysis. As expected, data shows that there seems to

be a correlation between precipitation and the incoming concentrations. As can be seen in Figure 11,

precipitation in January and February 2018 (the sampling Period) was significantly lower compared to the

same time period in 2015, 2016, and 2017; and consequently the average flow in January and February

2018 was lower than the average flow in January and February of the previous years, resulting in a higher

influent BOD and TSS concentrations during the sampling period.


Figure 11 Supplemental Sampling BOD and TSS Concentration vs. Precipitation

Figure 12 and 13 once again confirm that the influent BOD and TSS concentration from the supplemental

sampling period (January/February 2018) is higher relative to the same time period in the previous three

years (2015 to 2017). As explained before, the lower concentration is due to the fact that there has been less

precipitation during the 2018 sampling period compared to the same time period in the previous years.

Based on these observations, as well as the fact that the historical data analysis is based on three years of

historical data, it was found reasonable to use the historical influent BOD and TSS concentrations for the

design.

Table 5 compares the historical and supplemental sampling results for the primary effluent BOD and TSS

concentrations. The primary effluent BOD and TSS concentration between the historical data and

supplemental sampling found to be comparable. Again, due to the fact that the historical data analysis is

based on three years of data and that the historical primary effluent is more conservative than the

supplemental sampling data, it was decided to use the historical primary effluent BOD and TSS

concentration for the design.


Figure 12 Supplemental Sampling vs Historical Influent BOD Concentration

Figure 13 Supplemental Sampling vs Historical Influent TSS Concentration


Table 5 Supplemental Sampling vs Historical BOD and TSS Concentration

Parameter Historical Supplemental Sampling %Difference

Influent (1)

BOD, mg/L 270 315 117%

TSS, mg/L 295 433 147%

Primary Effluent

BOD, mg/L 160 149 93%

TSS, mg/L 94 82 88%

Note:

(1) Higher Influent BOD and TSS concentration for the supplemental sampling is due to a lower precipitation during the sampling period.

Since there is no historical data on the influent and primary effluent TKN and TP, the supplemental sampling

data set, after eliminating the outliers, was used to develop the design TKN and TP. Again, since

supplemental sampling was limited to 8 samples over a short period of time, the average measured raw

influent TKN concentration was increased from 38 to 45 mg/L (about 18%) to be more in line with typical raw

BOD/TKN ratios of 6 as per Metcalf & Eddy (Wastewater Engineering: Treatment and Disposal, 5th edition).

This was found necessary to prevent the design from potentially underestimating the aeration capacity. The

TP concentration of 6 mg/L from the sampling found to be reasonable to use as the basis of design. For

design, the primary effluent TKN and TP was increased from the supplemental sampling result of 29 mg/L

and 4 mg/L for TKN and TP to 36 mg/L and 4.5 mg/L, respectively.

Since there is no historical data available on alkalinity, the average raw sewage alkalinity concentration of

260 mg/L from the supplemental sampling was revised to 220 mg/L based on the BOD ratio from the

sampling value of 315 mg/L and the historical/design BOD value of 270 mg/L by a ratio of 270/315=0.857.

7. Design Annual Average Day Flow Determination

A review of the population projections was conducted to project future flow and loads. Figure 14 presents the

historical population data for the City of Fortuna as well as population projections from several sources.

Figure 14 shows significant variation in the projected population estimates. The City’s general plan growth

rate of 1.6% was used to project the future population.


Notes: DPEIR = Draft Programmatic Environmental Impact Report (DPEIR 2010); MSR = Municipal Service Report

Figure 14 City of Fortuna Population Data and Projections

The current average flow is 83 gallons per capita per day based on the 2013 City of Fortuna US Census

estimate of 11,788 people plus a Palmer Creek CSD population of 360 (156 connections and 2.3 people per

connection) for a total population of 12,148, and the current annual average day flow of 1.01 mgd,. The per

capita flow rate value was used to estimate the annual average day flow for the projected project population.

Table 6 includes the planning population number for the WWTP. The projected population for Palmer Creek

CSD is 510 people. Based on a per capita flow of 83 gpcd, this is roughly equal to the maximum contracted

flow in the City’s sewer agreement of 42,120 gallons per average dry month. Based on the projected annual

average flow shown in Table 6, the design annual average flow is 1.5 mgd, based on rounding up the

calculated annual average day flow of 1.43 MGD.


Table 6 Population and Capacity Projections

Basis of Design Population Annual Average Day Flow (mgd)

2013 City of Fortuna US Census estimate plus 2013 Palmer Creek population (CSD)

12,148 1.01(1)

2035 WWTF Design Capacity 17,225(2) 1.43(3)

Notes:

1. The historical annual average influent flow from Jan 2011 to Dec 2013 was 1.01 MGD, which corresponds to a per capita flow of 83 gallons per capita per day (gpcd).

2. The population was projected based on the Fortuna General Plan Growth Rate of 1.6% for both the City and Palmer Creek from the 2013 population estimate.

3. Projections for future flow rates assume no change from the historical per capita demand of 83 gpcd.

8. Design Wastewater Characteristics

The historical average influent BOD and TSS concentrations were confirmed through supplemental

sampling. However, since the plant routinely does not measure the average influent TKN, TP and alkalinity,

these parameters were established based on the 2018 supplemental sampling. Table 7 provides the design

influent and primary effluent flows, while Table 8 summarizes the design raw and primary effluent

wastewater characteristics. Table 9 presents the design loads. The design primary clarifiers’ removal

efficiencies are shown in Table 10. As described earlier, since maximum loads are more likely to occur

during the wet season, the design maximum month concentrations are determined at the maximum month

flow.

Table 7 Design Flows

Design Flows Influent, mgd Primary Effluent, mgd

Annual Average 1.50 1.65(3)

Maximum Month (1) 2.95 3.25(3)

Peak Day(1) 4.80 4.80

Peak Hourly (2) 5.25 5.25

Note:

1. Rounded Flow 2. Flows above 5.25 mgd will be diverted to the pond 3. Primary effluent includes 10% recycles from GBT Filtrate and equalized BFP filtrate


Table 8 Design Raw and Primary Effluent Wastewater Characteristics

Raw Wastewater Primary Effluent (1)

Annual Average Max Month Annual Average Max Month

Flow, mgd 1.50 2.95 1.65 3.25

BOD, mg/L 270 192 160 114

TSS, mg/L 295 210 94 67

TKN, mg/L 45 32 36 26

TP, mg/L 6.0 4.3 4.5 3.2

Alkalinity, mg/L 220 157 200 142

Note:

1. Primary effluent concentrations are based on removals presented in Table 5.

Table 9 Design Raw Wastewater Loads

Raw Wastewater Primary Effluent

Annual Average Max Month Annual

Average

Max Month

Flow, mgd 1.50 2.95 1.65 3.25

BOD, ppd 3,378 4,729 2,196 3,074

TSS, ppd 3,690 5,167 1,292 1,808

TKN, ppd 563 788 500 701

TP, ppd 75 105 63 88

Alkalinity, ppd 2,752 3,853 2,752 3,853

Table 10 Design Primary Clarifier Removals

Primary Clarifier Removals (1) Value

%TSS 65

%BOD 35

Note: 1. %TSS and %BOD removal is derated by 10% from the historical removals. The historical BOD and TSS

removal was calculated assuming 10% flow increase across the clarifiers due to sidestream returns.

The design temperatures are shown in Table 11. The annual average wastewater temperature is 19 ͦC. The

maximum and minimum monthly average wastewater temperature is 23 ͦC and 14 ͦC, respectively.

Table 11 Design Temperatures

Parameter Value

Annual Average Temperature, ͦC 19

Maximum Month Temperature, ͦC 23

Minimum Month Temperature, ͦC 14

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