ATTACH 3_EngineeringReport_BridgeportSewer_09.24.2020.pdf

Bridgeport Sewer System Replacement Project Engineering Report

Sept. 24, 2020

Project Location: The Bridgeport Administrative Site Sewer System is located approximately 4 miles northwest of Bridgeport, California. The system serves all site facilities on the east side of US 395. The project is located on the Bridgeport Ranger District of the Humboldt‐Toiyabe National Forest, Mono County, California, in Section 23, T5N, R24E, approximately 38.2768º N by 119.2879º W. The project may be accessed from US Highway 395 which divides the administrative site. A Warehouse facility on the west side of the highway is served by a separate on‐site wastewater disposal system, and will not be included in this project.

Existing Site Conditions: The existing Bridgeport Administrative Site has a package sewage treatment plant and lift station that is nearly 50 years old, has exceeded its life‐cycle, and is in need of replacement. Additionally, numerous changes on the site have resulted in a collection system that is not able to be maintained properly, and a planned drinking water treatment system will tax the current treatment system with additional influent.

The original sewer system and treatment system was installed in 1973 to replace numerous individual septic systems and cesspools that did not meet treatment standards. The sewer system serves 3 single family homes, 3 seasonal bunkhouses, and 5 RV pads. No visitor facilities are located on the site. The existing collection system is a mix of asbestos cement pipe from the original system and PVC and ABS installed more recently. The original treatment works includes a metal wet well (6’ diameter, 8’ deep), and compartmentalized fiberglass treatment tank (9’ square, 8’ deep), housed in a metal building with control systems. There are no septic tanks in use, but rather the treatment system is fully relied upon to break down effluent before disposal in an absorption field. Effluent is pumped to an absorption field located on the west side of the highway through a 2” PVC pressure sewer line.

New Design ‐ General: The new sewer system will split flows to two septic systems identified as System #1 and System #2 throughout the remainder of this report.

System #1 will serve 2 homes, 2 bunkhouses, and 5 RV pads with a septic tank/lift station combination, and pumped to a conventional absorption field with leaching chambers. Not having 100% confidence that the drinking water treatment effluent will ultimately be sent to System #2, System #1 has been oversized to also accommodate the treatment inflow. The septic tanks and lift station will be located near the existing sewage treatment facility. A connection will be maintained to the existing absorption field for backup use only. Valves will control flow between the new and existing fields. The primary septic tank will be a single chamber 3000 gallon tank. This will be followed by a 4000 gallon single chamber tank constructed to retain 1500 gallons for secondary treatment. The remaining 2500 gallons capacity will be used for emergency storage. Effluent from the secondary tank will pass to the new wet well – a 60” concrete manhole, 11’ deep. The wet well will be set and sized to provide

ATTACHMENT #3-1240LR21R0007

800 gallons per dose to the field, and to provide additional emergency storage. The combined emergency storage will be approximately 4000 gallons – about 24 hours storage when the site is at full occupancy.

System #2 will serve the remainder of the site (one bunkhouse, one single family home, and the future drinking water treatment system). Sewage will flow by gravity to a conventional septic system with leaching chambers.

New Design – System #1:

Average Daily Flows: The 2016 California Plumbing Code (CPC), Appendix H was used as the basis for design. Residential system sizing is based on residence size, while non‐residential system sizing is based on flow rates. Here we have mixed uses, therefore engineering judgment has been used to determine residential flow rates. The estimated daily sewage flow for the facilities served by System #1 are:

Building Type bedrooms # units GPD/unit Est. GPD

Barracks #2 Duplex 2 2 500 1000

Barracks #3 Duplex 2 2 500 1000

House B Single Family 2 1 500 500

House C Single Family 3 1 700 700

RV Pads x 5 Units ‐‐ 5 100 500

Water treatment Actual ‐‐ 1 200 200

Total 3900

Pipeline Sizing: Though flows are low enough to manage all facilities with 4” and 6” pipelines, some lines are oversized to 6” and 8” diameter due to grade requirements. Where pipelines had to be laid flatter than 2%, the size was increased to 6” diameter. And where lines had to be laid flatter than 1%, the size was increased to 8” diameter. The minimum grade 8” lines were laid is 0.5%, which is the flattest grade allowed by the CPC.

Tank Size: Per 2016 CPC, Appendix H, tank size is based on home size in Table 201.1 (1), or by occupancy type found in Table 201.1 (2). Because this system will serve such a large number of facilities of varying types, summing together the many equivalent individual tank volumes would generate an overly conservative combined septic tank volume. Therefore the estimated GPD for each unit was used to generate a total estimated GPD which was then adjusted by the factor found in Table 201.1 (2) notes. The factor for such a volume is 0.75 + 1125.

Building Type Bedrooms # units Equiv. GPD Adjusted GPD (x 0.75 + 1125)

Barracks #2 Duplex 2 ea. 2 1000

Barracks #3 Duplex 2 ea. 2 1000

House B Single Family 2 1 500

House C Single Family 3 1 700

RV Pads RV w/ hkups ‐‐ 5 500

Water treatment Actual ‐‐ 1 200

Combined Tank

3900 4050

Therefore, the next larger standard size 5000 gallon tank would be an appropriate selection.

But an alternative method of installing in series two single compartment tanks has been selected. The tanks utilized are a 3000 gallon primary tank and a 1500 gallon secondary tank.

The secondary tank will be increased in size to accommodate emergency storage for the pump system as described later. However, the tank outlet will be set to an elevation to retain a constant volume of 1500 gallons so as to meet the intent of the CPC, Section 501.4, which states, “The secondary compartment of a septic tank shall have a capacity ….. not exceeding 1/3 of the total capacity of such tank.“

Field Size: Per 2016 CPC, Appendix H, field size is based on tank size and soil type using Tables

201.1 (3) and (4). Soil type is sandy loam with percolation rates varying between 18 and 25 minutes/in. A percolation rate of 25 min/in is used for this system. The field size was calculated as follows:

Area / 100 gal tank capacity

A 40 sf from Table 201.1 (3)

Leaching chamber multiplier

B 0.7 per Section H301.1 (5)

Adjusted area / 100 gal cap.

AxB=C 40 x 0.7 = 28 sf

Tank capacity D 4500 gal from tank sizing above

Field size DxC/100=E 4500 x 28/100 = 1260 sf

# of rows F 5 rows

Row width G 3 ft

Perc width FxG=H 5 x 3 ft = 15 ft

Field length E/H=G 1260 sf / 15 ft = 84 ft

Round up 88 ft next highest 4 ft chamber length

Therefore, the field will be made up of 5 rows of chambers, 88’ each.

Chambers will be laid with 3’ separation between chamber trenches. Actual perc area will be 1,320 sf, and actual field size will be 88 ft by 27 ft.

Pump Design:

The lift station will have two pumps working in a duplex configuration. The septic tank system is placed immediately upstream of the wet well, so effluent pumps will be used. The new pumps will be the Orenco PF300512 High Head Effluent Pumps (see the pump curve at right for performance), or an approved equal.

The flow rate to the new field will be 34 GPM at 44 feet dynamic head. Should the need arise to divert flow to the old absorption field, the same pump will work with a flow rate of 30 GPM at 59 feet dynamic head. The new pump will be a single phase, 50 GPM, 240 volt, ½ horsepower effluent pump.

Electrical Controls:

The electrical components of the lift station will contain the following parts:

‐ Control Panel: Model: DAX‐2‐RO‐ETM‐HT‐SA‐PRL, as manufactured by Orenco Systems, Inc.

‐ External Splice Box: Model: SBEX1‐4, as manufactured by Orenco Systems, Inc.

‐ Liquid Level Alarm Panel (Battery Powered): Model: AMSENTII‐W, as manufactured by

Orenco Systems, Inc.

‐ Floats Assembly: MF##**‐66FS, as manufactured by Orenco Systems, Inc. (##** = number and type floats coordinated with manufacturer supplying pumps and controls.)

o The float elevations, based on a wet well floor elevation of 6443.06’, will be the following:

High Level Alarm (both pumps on): 6446.30’ Pump On: 6446.13’ Pump Off: 6444.77’ Redundant Off: 6444.60’

The primary source to power the lift station will be from utility‐provided electrical service, but the control panel will have the ability to connect to a portable generator as a secondary or emergency power source.

Emergency Storage: Refer to the average daily flows above. The CPC, Appendix H requires secondary systems to be sized for total flow for 24 hours. Because the site has a history of unreliable electrical service, and the staffing level does not always allow for prompt response to an alarm, we also desire to meet or exceed a target of 24 hours emergency storage when the site is at full capacity – approximately 4000 gallons. The storage will be obtained by utilizing 2500 gallons capacity in the secondary septic tank above the tank outlet, and setting the wet well elevation to acquire the remaining 1500 gallons emergency storage. The final design provides an actual emergency storage of 4293 gallons.

Groundwater Issues: At the time of the design, groundwater elevation was at 6443’ at the lowest point in the system where the septic tanks and wet well are located. The groundwater is being monitored for seasonal fluctuation to determine the likely maximum elevation that will be present under the site. It is not anticipated any of the sewer lines will be below groundwater. The following elevations are determined to be the maximum groundwater elevations that must be attained at each tank before measures will be required to prevent flotation. Calculations assume empty tanks which would only occur after maintenance pumping.

Tank Elevation at which flotation may occur

Primary Septic Tank 6453.1

Secondary Septic Tank 6452.3

Wet Well 6451.0

The ground surface elevation is at 6451’ within 50 feet to the southeast of the wet well.

Therefore it is highly unlikely that the groundwater will rise to a level that would result in flotation of any of the tanks in this system. Additionally, it is expected that the wet well will never be completely pumped out since all pump maintenance can be performed from the surface, and there will be approximately 1.5’ of water below the redundant pump off setting at all times. Therefore, no flotation prevention measures are planned for this system.

New Design – System #2:

Average Daily Flows: The 2016 California Plumbing Code (CPC), Appendix H was used as the basis for design. Per the CPC, residential system sizing is based on residence size, while non‐ residential system sizing is based on flow rates. Here we have mixed uses, therefore engineering judgment has been used to determine equivalent residential flow rates. The estimated daily sewage flow for the facilities served by System #2 are:

Building Type # rooms Est. GPD

Barracks #1 Single Family 3 700

House A Single Family 3 700

Water treatment Actual ‐‐ 200

Total 1600

Pipeline Sizing: Though flows are low enough to manage all facilities with 4” pipelines, some are 6” diameter due to grade requirements. Where pipelines had to be laid flatter than 2%, the size was increased to 6” diameter.

Tank Size: Per 2016 CPC, Appendix H, residential tank size is based on Table 201.1 (1), which indicates a 3‐bedroom home requires a 1000 gallon tank. Also, commercial uses are based on estimated gpd by occupancy type found in Table 201.1 (2). The water treatment share was calculated using known average flow multiplied by the appropriate factor (1.5) obtained from the table notes. Tank size was determined by summing the residential and commercial volumes using their respective design code tables. See the following table:

Building Type # rooms or gpd

Base tank gallons

+ Added rooms x 150 gal

Multi factor

= Tank size

Residential (per CPC Table 201.1(1))

Barracks #1 Single Family

3 1000 0 0 ‐‐ 1000

House A Single Family

3 1000 0 0 ‐‐ 1000

Commercial (per CPC Table 201.1(2))

Water treatment Actual 200 ‐‐ ‐‐ ‐‐ 1.5 300

Combined Tank 2300

Therefore, a standard size 2500 gallon tank was selected for use.

As a check, an alternative method was used whereby the actual estimated gallons was used for all three facilities served and multiply the total by the factor found in Table 201.1 (2) notes. The factor for such a volume is 0.75 + 1125.

Building Type # rooms

Est. GPD Adjusted (x 0.75 + 1125)

Barracks #1 Single Family 3 700

House A Single Family 3 700

Water treatment Actual

Combined Tank

1600 2325

Therefore, a standard size 2500 gallon tank again is an appropriate selection.

Field Size: Per 2016 CPC, Appendix H, field size is based on tank size and soil type using Tables

201.1 (3) and (4). Soil type is sandy loam with percolation rates varying between 18 and 25 minutes/in. A percolation rate of 25 min/in is used for this system. The field size was calculated as follows:

Area / 100 gal tank capacity

A 40 sf from Table 201.1 (3)

Leaching chamber multiplier

B 0.7 per Section H301.1 (5)

Adjusted area / 100 gal cap.

AxB=C 40 x 0.7 = 28 sf

Tank capacity D 2325gal from tank sizing above

Field size DxC/100=E 2325 x 28/100 = 651 sf

# of rows F 4 rows

Row width G 3 ft

Perc width FxG=H 4 x 3 ft = 12 ft

Field length E/H=G 651 sf / 12 ft = 54.25 ft

Round up 56 ft next highest 4 ft chamber length

Therefore, the field will be made up of 4 rows of chambers, 56’ each.

Chambers will be laid with 3’ separation between chamber trench. Actual perc area will be 672 sf, and actual field size will be 56 ft by 21 ft.