HomeMy WebLinkAboutAppendix 4.17-3, Sanitary Sewer Technical StudyAppendix 4.17-3:
Sanitary Sewer Technical Study
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TECHNICAL MEMORANDUM
Date: January 29, 2021
Subject: Sanitary Sewer Technical Study
Southline Development Project
The following summarizes the sanitary sewer analysis performed by BKF Engineers at the planning level
for the Southline Development Project. This has been prepared for review by the Cities of South San
Francisco and San Bruno, and for use in the environmental documents for the Southline Development
Project.
Regulatory Setting
The City of South San Francisco owns and maintains all of its sanitary sewer systems and infrastructures
within the public rights-of-way. The collected wastewater is conveyed to the Water Quality Control
Plant (WQCP) located adjacent to San Francisco Bay on Colma Creek. This facility provides secondary
wastewater treatment for the cities of South San Francisco, San Bruno, and Colma. It also provides the
dechlorination treatment of chlorinated effluent for the cities of Burlingame, Millbrae, and the San
Francisco International Airport prior to discharging the treated wastewater into San Francisco Bay. The
average dry weather flow through the facility is 9 million gallons per day (MGD). Peak wet weather flows
can exceed 60 MGD. The State Water Resources Control Board (SWRCB) has adopted the Waste
Discharge Requirements (WDR) Order which requires the City of South San Francisco to develop and
implement a Sanitary Sewer Management Plan. Additionally, SSF is currently updating its Sanitary Sewer
Master Plan (SSMP) which identifies ongoing maintenance and system improvements necessary to
maintain the sewer system.
Existing Conditions
The Southline project site is located in Tributary Basin B-9 within the South San Francisco West Sanitary
Sewer System (SSMP, Figure 4.2). Existing parcels within the Project Site discharge wastewater to the
public sanitary sewer mains located in the adjacent public roadways. These public mains are part of the
Lowrie Trunk system, and drain to Lift Station #11 (FLS-11) on Shaw Road, which pumps the wastewater
to the WQCP (SSMP, Table 9.2 and Figure 9.1).
Based on the existing private sewer connections, the existing 26.5 AC site is divided into three
sewersheds which drain the wastewater from the parcels to the public sanitary sewer mains in each
street frontage. Exhibit A illustrates the three sewersheds, Areas A, B, and C, which drain to mains
located in Tanforan Ave., S. Maple Ave., and S. Linden Ave. respectively. Area A, located at the south
portion of the site area, drains to a 24” vitrified clay pipe (VCP) main in Tanforan Avenue. This line is
identified as LO-P14 in the SSMP. Area B, at the northwest portion of the site, drains to an 8” VCP main
located in South Maple Avenue. This main in turn drains to an 18” VCP main identified as LO-P12 and
LO-P13 in the SSMP. Area C, comprised of parcels located on the east, drains to an 8” VCP main in
Linden Avenue, which also drains north to LO-P13. Refer to Exhibit A for the existing site drainage map.
Existing sewer flows for the project site area were estimated using the sewer demand factors provided
by the SSMP and applying to the existing building areas. The SSMP identifies the project site land use
to be “Mixed Industrial” (SSMP, Figure 2.2). The SSMP modeled Mixed Industrial areas with an Average
Dry Weather Flow (ADWF) Factor of 1,600 GPD/Acre (SSMP, Table 3.5). The SSMP model assumed an
FAR of 1.0 for the site area, which results in a building demand factor of 0.0367 GPD/SF. Using this
demand factor, sewer demand for the existing area was calculated. Peaking factors used to determine
the Maximum Day Dry Weather Flow (MDDWF) and Maximum Day Wet Weather Flow (MDWWF) were
taken from the SSMP (SSMP, Table 5.2). Refer to Table A for the existing sewer flows to each drainage
basin.
The SSMP identified sanitary sewer infrastructure serving the project site to be in need of capacity
improvements as part of the capital improvement program (SSMP, Figure 9.1). LO-P14, located within
Tanforan Avenue, is an existing 24” pipe indicated to be upsized to a 27” main. The next two segments
downstream, LO-P15 and LO-P16 are also identified to be upsized to 30” and 33” pipes in the SSMP
recommended capital improvements (SSMP, Table 9.2). The sizing of these required improvements was
determined as part of the SSMP, and was not part of this study. The mains indicated to be upsized by
the SSMP are as follows:
Pipe ID Alignment Existing Size Proposed Size Length
LO-P12 Maple Ave. 18” 24” 150’
LO-P13 ROW 18” 24” 1,425’
LO-P14 Tanforan Ave. 24” 27” 700’
LO-P15 Tanforan Ave. 24” 30” 875’
LO-P16 Shaw Rd. 27” 33” 1,075’
Additionally, pump station LS-11 is indicated to be deficient and requiring a capacity upgrades as part
of the recommended capital improvements (SSMP, Table 9.2). These capital improvements were
recommended by the SSMP in anticipation of General Plan land uses, and are not the result of the
specific proposed improvements for the project site area.
Proposed Improvements
Construction of the Southline project will increase the developed building area, and, thus, proposes to
increase sanitary sewer flows to the public sewer system. The proposed development anticipated to be
phased. Phase 1 proposes to construct three commercial office buildings totaling approximately
700,000 square feet. Phase 1 development will be located within sewershed Area A on the south side of
the project site area, and will discharge wastewater to sewer infrastructure located in Tanforan Ave.
Future phases of development will construct as much as 2,100,000 SF of additional commercial office
space on the north and east side of the project site in sewershed Areas B and C. Refer to Exhibit B for
the proposed site drainage map.
Proposed sewer flows for the project were estimated using the sewer demand factors provided by the
SSMP and applying to the proposed building areas. The SSMP moldeled the planned project site land
use to be “Office” based on the 1999 SSF General Plan (Figure 2-1). The SSMP modeled Office
Commercial areas with an Average Dry Weather Flow (ADWF) Factor of 1,080 GPD/Acre (SSMP, Table
3.5). The SSMP model assumed an FAR of 1.0 for the planned site area, which results in a building
demand factor of 0.0248 GPD/SF. Using this demand factor, sewer demand for the proposed area was
calculated. Peaking factors used to determine the Maximum Day Dry Weather Flow (MDDWF) and
Maximum Day Wet Weather Flow (MDWWF) were taken from the SSMP (SSMP, Table 5.2). Refer to
Table B1 and B2 for the proposed sewer flows to each drainage basin. The Tech Office Campus scenario
proposes the greatest development area, and, therefore, greatest sewer demand. Based on these
predicted flows, Phase 1 of the proposed Tech Office project will generate an increased sewer demand
of approximately 13,000 GPD ADWF and 84,000 GPD MWDDF. The total project, including future phase,
will generate an increased sewer demand of approximately 57,000 GPD ADWF and 370,000 GPD
MDWWF. Table C summarizes the proposed increases to the public infrastructure and to each sewer
trunk in the downstream system.
Anticipated Mitigations
The SSMP originally modeled the planned project area assuming an FAR of 1.0. The General Plan allows
a maximum FAR of 2.5. The maximum proposed development of the Southline project is approximately
an FAR of 2.4. Based on this proposed FAR, the City had the SSMP model revised to evaluate the
anticipated demands for the maximum FAR allowed under the General Plan. The results of that
reevaluation indicate that the maximum proposed Southline sewer demands are still within the
anticipated SSMP levels for future demands for the project area. Based on the revised evaluation, pump
station LS-11 will have 48 GPM surplus capacity after development of the project site to the maximum
FAR of 2.5. Any development below FAR of 2.5 will result in additional surplus capacity for the pump
station LS-11.
The capital improvements identified in the SSMP for the sewer infrastructure serving the project will
need to be constructed prior to occupancy by any phase of the project being served by that
infrastructure. The City will enter a development agreement with the project proponent regarding the
construction and funding of the required improvements, as they are not currently funded under the
City’s scheduled capital improvements. Regarding treatment capacity, the WQCP has confirmed that
there is sufficient capacity available to provide wastewater treatment for 0.4 MGD, the proposed
MDWWF flows for the Southline Project. This also takes into account the anticipated contributions from
the City of San Bruno, according to references to the San Bruno SSMP within the SSF SSMP.
TABLE A: EXISTING SANITARY SEWER FLOWSSOUTHLINE PROJECT - SEWER STUDYA30 TANFORANOFFICE6,3000.03672313241,50340 TANFORANWAREHOUSE42,0000.03671,5412,15810,019337 S. MAPLEWAREHOUSE4,5000.03671652311,073339 S. MAPLE WAREHOUSE9,5000.03673494882,26650 TANFORANWAREHOUSE34,0000.03671,2481,7478,11154 TANFORANWAREHOUSE26,0000.03679541,3366,202122,3004,4886,28429,175B315 S. MAPLEWAREHOUSE19,0000.03676979764,532319 S. MAPLEWAREHOUSE20,0000.03677341,0284,771325 S. MAPLEWAREHOUSE55,0000.03672,0192,82613,12094,0003,4504,83022,424C160 S. LINDENINDUSTRIAL60,0000.03672,2023,08314,313180 S. LINDENWAREHOUSE7,5000.03672753851,789240 DOLLAR INDUSTRIAL60,0000.03672,2023,08314,313127,5004,6796,55130,415343,80012,61717,66482,013Notes:1Sewersheds represent parcels draining to identified sewer trunk lines located in the respective public roadways. See Exhibit A.2Trunk line designations are identified in the SSF Sanitary Sewer Master Plan (SSMP). 34Average Dry Weather Flow (ADWF) peaking factor (PF) from the SSMP Table 5.2.5Maximum Day Dry Weather Flow (MDDWF) pearking factor (PF) from the SSMP Table 5.2.6Maximum Day Wet Weather Flow (MDWWF) peaking factor (PF) from the SSMP Table 5.2.LO-P12 LO-P13LO-P13LO-P14 LO-P15 LO-P16SEWERSHED SUBTOTAL:SEWERSHED SUBTOTAL:EXISTING SANITARY SEWERSSMP Figure 2.2 indicates the existing project area to be "Mixed Industrial" land use. From SSMP Table 3.5, the demand factor for Mixed Industrial land use is 1,600 GPD/AC, or 0.0367 GPD/SF. SEWERSHED SUBTOTAL:PROJECT TOTALS:MDDWF5 (PF=1.40) (GPD)MDWWF6 (PF=6.50) (GPD)(Tanforan Ave.)(Maple Ave.)EXISTING SEWERSHED DESIGNATION1(S. Linden Ave.)ADDRESSTOTAL AREA (GSF)SSMP PIPE SYSTEM2USE TYPESSMP DEMAND FACTOR3ADWF4 (PF=1.0) (GPD)12/17/2020BKF Engineers#20180624
TABLE B1: PROPOSED SANITARY SEWER FLOWS (TECH OFFICE)SOUTHLINE PROJECT - SEWER STUDYA 1317,495 0.02487,87411,02351,180288,200 0.02482,1873,06214,2187295220 0.02487,32110,25047,589SEWERSHED SUBTOTAL:700,91517,38324,336112,987B 3576,900 0.024814,30720,03092,9964455,560 0.024811,29815,81773,4365342,265 0.02488,48811,88355,173SEWERSHED SUBTOTAL:1,374,72534,09347,730221,606C (S. Linden Ave.)SEWERSHED SUBTOTAL:724,360 17,964 25,150 116,767PROJECT TOTALS:2,800,00069,44097,216451,360Notes:12Trunk line designations are identified in the SSF Sanitary Sewer Master Plan (SSMP) Figure 7.5. 34Average Dry Weather Flow (ADWF) peaking factor (PF) from the SSMP Table 5.2.5Maximum Day Dry Weather Flow (MDDWF) pearking factor (PF) from the SSMP Table 5.2.6Maximum Day Wet Weather Flow (MDWWF) peaking factor (PF) from the SSMP Table 5.2.7See Table A for Existing Sewer Flows for the project parcels.8Abbreviations: GSF (Gross Square Foot); GPD (Gallons Per Day)SSMP Figure 2.3 indicates the General Plan land use for the project area to be "Office" land use. From SSMP Table 3.5, the demand factor for Office Commercial land use is 1,080 GPD/AC (FAR = 1.0), or 0.0248 GPD/SF. PHASE 1PROJECT PHASINGFUTURE PHASEPROPOSED SANITARY SEWER - TECH OFFICE CAMPUS (Tanforan Ave.)(Maple Ave.)0.0248PROPOSED SEWERSHED DESIGNATION1SSMP PIPE SYSTEM2PROPOSED BUILDINGTOTAL AREA (GSF)724,360SSMP DEMAND FACTOR3Proposed Sewersheds represent buildings draining to the same sewer trunk lines as identified in the Existing condition. See Exhibit B.6 17,964 25,150 116,767LO-P13LO-P12 LO-P13LO-P14 LO-P15 LO-P16ADWF4 (PF=1.0) (GPD)MDDWF5 (PF=1.40) (GPD)MDWWF6 (PF=6.50) (GPD)12/14/2020BKF Engineers#180624
TABLE B2: PROPOSED SANITARY SEWER FLOWS (LIFE SCIENCE)SOUTHLINE PROJECT - SEWER STUDYA 1317,495 0.02487,87411,02351,180288,200 0.02482,1873,06214,2187295220 0.02487,32110,25047,589SEWERSHED SUBTOTAL:700,91517,38324,336112,987B 3447,335 0.024811,09415,53172,110(Maple Ave.)4411,000 0.024810,19314,27066,253SEWERSHED SUBTOTAL:858,33521,28729,801138,364C (S. Linden Ave.)SEWERSHED SUBTOTAL:465,800 11,552 16,173 75,087PROJECT TOTALS:2,025,05050,22170,310326,438Notes:12Trunk line designations are identified in the SSF Sanitary Sewer Master Plan (SSMP) Figure 7.5. 34Average Dry Weather Flow (ADWF) peaking factor (PF) from the SSMP Table 5.2.5Maximum Day Dry Weather Flow (MDDWF) pearking factor (PF) from the SSMP Table 5.2.6Maximum Day Wet Weather Flow (MDWWF) peaking factor (PF) from the SSMP Table 5.2.7See Table A for Existing Sewer Flows for the project parcels.8Abbreviations: GSF (Gross Square Foot); GPD (Gallons Per Day)SSMP Figure 2.3 indicates the General Plan land use for the project area to be "Office" land use. From SSMP Table 3.5, the demand factor for Office Commercial land use is 1,080 GPD/AC (FAR = 1.0), or 0.0248 GPD/SF. PHASE 1LO-P14 LO-P15 LO-P16 (Tanforan Ave.)FUTURE PHASELO-P12 LO-P13LO-P136 465,800 0.0248 11,552 16,173 75,087Proposed Sewersheds represent buildings draining to the same sewer trunk lines as identified in the Existing condition. See Exhibit B.PROPOSED SANITARY SEWER - LIFE SCIENCE CAMPUSPROJECT PHASINGPROPOSED SEWERSHED DESIGNATION1SSMP PIPE SYSTEM2PROPOSED BUILDINGTOTAL AREA (GSF)SSMP DEMAND FACTOR3ADWF4 (PF=1.0) (GPD)MDDWF5 (PF=1.40) (GPD)MDWWF6 (PF=6.50) (GPD)12/14/2020BKF Engineers#180624
TABLE C: PIPE INFRASTRUCTURE SUMMARYSOUTHLINE PROJECT - SEWER STUDYA ADWF1.04,48817,38312,8940.020MDDWF1.46,28424,33618,0520.028MDWWF6.529,175112,98783,8130.130B ADWF1.03,45034,09330,6430.047MDDWF1.44,83047,73042,9010.066MDWWF6.522,424221,606199,1820.308C ADWF1.0 4,67917,96413,2850.021(S. Linden Ave.)MDDWF1.46,55125,15018,5990.029MDWWF6.530,415116,76786,3510.134PHASE 1ALL PHASESPHASE 1ALL PHASESFLOWGPDMGDLO-P12042,9010199,182ADWF56,8230.06LO-P13061,4990285,533MDDWF79,5520.08LO-P1418,05218,05283,81383,813MDWWF369,3460.37LO-P1518,05218,05283,81383,813LO-P1618,05218,05283,81383,813Notes:1Trunk line designations are identified in the SSF Sanitary Sewer Master Plan (SSMP) Figure 7.5. 2ADWF (Average Dry Weather Flow); MDDWF (Max. Day Dry Weather Flow); MDWWF (Max. Day Wet Weather Flow)3Peaking factors are from SSMP Table 5.2.4See Table A for Existing Sewer Flows for the project parcels.5Proposed flows are from the Tech Office Campus scenario as it generates the greatest demand. See Tables B1 and B2.6Abbreviations:GPD (Gallons per Day); CFS (Cubic Feet per Second); MGD (Million Gallons per Day) (Tanforan Ave.)LO-P14 LO-P15 LO-P16PROJECT PHASINGPHASE 1SEWERSHED DESIGNATIONSSMP PIPE SYSTEM1FLOW CONDITION2PEAKING FACTOR3EXISTING FLOW4 (GPD)PROPOSED FLOW5 (GPD)NET INCREASE (GPD)6NET INCREASE (CFS)6PIPE INFRASTRUCTURE IMPACTS - TECH OFFICE CAMPUS5TOTAL PROJECT NET INCREASEMDDWF Net Increase (GPD)Pipe IDMDWWF Net Increase (GPD)LO-P13(Maple Ave.)LO-P12 LO-P13FUTURE PHASE12/14/2020BKF Engineers#180624