Sewer Ventilation as a VI Mitigation Alternative- Case Studies

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Sewer Ventilation as a VI Mitigation Alternative- Case StudiesApril 10, 2018

CH2M is now Jacobs

Keri Hallberg, PE, JacobsLoren Lund, PhD, JacobsJessica High, PE, JacobsQuinten Bingham, JacobsDavid Cleland, PG, NAVFAC Mid-AtlanticMark Roginske, Air Force Civil Engineer Center

Presentation Outline

• Introduction

• Overview of the Sewer Preferential Pathway

• Ventilation Objectives

• Case Study – Site A

• Case Study – Site B

• Conclusions

2

Atypical Preferential Pathways (APPs)

Definition: A conduit into building (e.g., sewer line) that intercepts VOC source area and provides little resistance to vapor flow

Modified from Johnson 2015

Example APP

Frequency and Type of APPs (Literature Review*)

1

4

11

2 Faulty wax seal and ordamaged sewer pipes

Utility conduits

Gravity sanitary sewer

Gravity storm sewer

(all industrial)

*Mix of residential and commercial

Sewer Atypical Preferential Pathways

• Gravity sewers – large headspaces (facilitates vapor flow)

• Most sewers leak both in and out

• Sewers receive flow from smaller pipe networks

• Larger receiving pipes can be over 20-ft below ground

www.kingcounty.gov/services/environment/wastewater/ii/what.aspx)

5

VOCs in Sewer Gas

• VOCs enter sewer gas through 2 primary mechanisms⎯ Contaminated groundwater (may include

NAPL) and/or soil gas enters sewer

⎯ Direct discharge of water containing VOCs to sewer system

Adapted from McHugh and Beckley AEHS 2017

https://www.fvrobertson.com/sewer‐video‐inspection/6

Ventilation Objectives

• Prevent VOCs in sewer gas from entering occupied space by:- Intercepting vapors between source

area and target buildings

- Preventing vapor build-up in sewer headspace

- Reversing flow of vapors away from target buildings

Source AreaBuilding

Sewer Manhole

7

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Site A

Site A Background

• Upgradient of source area• Source area

⎯ PCE ~600 µg/L and TCE ~300 µg/L⎯ Residual soil NAPL

• Sewer connects source area to building• TCE periodically detected in indoor air

⎯ Above regulatory targets⎯ IA concentrations did not correlate

with SG concentrations• Additional investigation to determine source

⎯ Uncapped pipe in mechanical room⎯ Dry or damaged p-traps⎯ HAPSITE confirmed PCE and TCE

inside plumbing

Source Area

Building

Sanitary Sewer

Sewer gas

Sewer gas blocked by water (or seal)

Phase I Sewer Ventilation Pilot Study

• Conducted to assess whether ventilation of the sewer line can:⎯ Reduce PCE and TCE concentrations within manholes

⎯ Reverse the flow of vapor to potential entry points inside Building

• Test blower was selected to match the flow rate of the blower tentatively specified (240 cfm)

• Conduct confirmation sampling at manhole locations MH-1, MH-2, and MH-3, the mechanical room plumbing, and within sink plumbing

MH-3MH‐1

MH‐2

MH‐3

Phase 1 Results

370.0

20.0

40.0

60.0

80.0

‐2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0HOURS

MH‐ 1PCE

TCE

cis‐1,2‐DCE

0.0

20.0

40.0

60.0

80.0

‐2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0HOURS

MH‐2PCE

TCE

cis‐1,2‐DCE

0.0

20.0

40.0

60.0

80.0

‐2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0

UG/M

3

HOURS

MH‐3

PCE

TCE

cis‐1,2‐DCE

Building

Source Area Little influence at MH‐1, but significant reductions at

MH‐2 and MH‐3

Phase 1 Results

37

0.0

5.0

10.0

15.0

‐2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0HOURS

Wall Plumbing

PCE

TCE

0.0

20.0

40.0

60.0

‐2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0HOURS

Restroom Sink Plumbing

PCE

TCE

cis‐1,2‐DCE

Significant reductions in

building plumbing

Sewer Venting System Design

• 4” ventilation pipe from the sewer

• Connected to skid mounted blower• 240 cfm, 25” H2O

• Blower exhaust vented above roofline

• Vapor monitoring port installed (MH-1)

Ventilation Pipe

MH‐1 Sample Port

Phase 2 Field Activities

• Ventilation system and sample port installation

Blower Enclosure and Stack

Blower System

Manhole Sample Port

Blower Connection to Manhole

System Connection

System Connection

Piping into manhole

Phase 2 Field Activities• Baseline sampling, startup, and post-startup

performance monitoring ⎯ Highest sewer line concentrations observed at MH-1

(located closest to source area)

⎯ COCs not detected in indoor air above screening levels during baseline or after startup

⎯ Post-startup monitoring conducted 36 hours after startup

MH‐1

Office Plumbing

Wall PlumbingWall Plumbing

Blower Exhaust

IA SamplingIA Sampling

Phase 2 Performance Monitoring- PCE

0

1

10

100

1,000

PCE Co

ncen

tration (µg/m

3 )

Manhole 1 Blower ExhaustManhole 2 Office Restroom ‐ Indoor AirManhole 3 PCE VI IASL (8.34 µg/m3)

System Startup

PCE VI IASL ‐ 8.34 µg/m3

Unfilled symbols indicate non‐detect values

PCEconcentrations below IASL by

MH‐2

VI = Vapor IntrusionIASL = Indoor Air Screening Level

Phase 2 Performance Monitoring – TCE

0.0

0.1

1.0

10.0

100.0

TCE Co

ncen

tration (µg/m

3 )

Manhole 1 Blower ExhaustManhole 2 Office Restroom ‐ Indoor AirManhole 3 TCE VI IASL (0.42 µg/m3)System Startup

*TCE not detected above detection limits in Indoor Air:‐ HAPSITE = 0.54 µg/m3; analytical laboratory = 0.11 µg/m3

System Startup

TCE VI IASL ‐ 0.42 µg/m3

Unfilled symbols indicate non‐detect values VI = Vapor IntrusionIASL = Indoor Air Screening Level

TCEconcentrations below detection limits by MH‐2

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Site B

Site B Background

Manhole D‐7

Manhole 1

Manhole 2Effluent Discharge Location

Manhole E1‐2

Residence 2

• TCE in sewer gas related to remediation system effluents discharging to Manhole 2, and flowing toward Manhole 1⎯ Effluent water

concentrations 10-50 µg/L

• TCE detected in the indoor air of nearby residences above mitigation action level

Residence 1

19

Pre-Design Testing Activities

• Baseline sewer gas samples collected (3 manholes)

• FanTech® FKD 8XL inline-duct fan connected to existing manhole passive vent pipe

• Post-Startup Sampling⎯ 25 to 55 minutes

⎯ 67 to 87 minutes

⎯ 98 to 122 minutes

20

Manhole 1 Sampling

Inline –Duct Fan

Pre-Design Testing Results

• 50% reduction in TCEconcentration within 25 to 55 minutes

• Up to 95% reduction after 98 minutes at manholes nearer residences

• Reduction maintained over 2-hour period

21

5.3

2.42.0

1.1

62.724.5

16.2 21.9

244.1

23.0

13.5 13.1

1.0

10.0

100.0

1000.0

0 25 67 98

TCECo

ncentration (ppb

v)

Minutes

Manhole E1‐2

Manhole 1

Manhole D‐7

Ventilation System Design

• Vent fan installed on concrete pad within tamper-resistant enclosure

• Plastec PLAS25XS4P blower⎯ Initial Flowrate 770 cfm

22

Performance Monitoring

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2002 2004 2006 2008 2010 2012 2014 2016 2018 2020

TCECo

ncentration (ppb

v)

Date

Indoor Air Concentration ‐ Residences

Residence 2

Residence 1

TCE Mitigation Action Level‐ 0.39 ppbv

Vent Fan Installed at Manhole 2

Vent Fan Installed at Manhole 1

23

Conclusions

24

• Sewer ventilation is effective in mitigating VI through sewers by:⎯ Intercepting vapors between the source area and the building:

• Site A: concentrations of PCE and TCE in sewer manholes and in building plumbing reduced up to 99-percent

• IA concentrations < IASLs

⎯ Preventing accumulation of vapors:• Site B: TCE concentrations in the sewer line were reduced by 79-percent to

95-percent

• IA concentrations reduced to concentrations < mitigation action level

Sewer Ventilation as a VI Mitigation Alternative- Case Studies

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