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Wet-Weather Freakonomics: A Cost-Benefit Calculus for a Sanitary

Sewer Backup Reduction Program

Presented by

Vinnie Bergl, P.E.

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The Past

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Knee of the Curve ca. 1984

• Many I/I sources still

“low-hanging fruit”

• Treatment capacity

more feasible

• Relief capacity? Justtell us where to send it!

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Knee of the Curve Present Day

• I/I sources are higher

up in the trees

• Treatment capacity a

tougher sell

• Relief capacity?There’s nowhere to

send it!

   +   i   n    f    l   a   t   i   o   n 

+storage

???

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I/I Source Distribution ca. 1984

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I/I Source Distribution Present Day

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The Situation Today

• Rare storms increasingly less rare

•

Single performance criteria elusive

• Growing emphasis on basement backups

• Infinite possibilities, finite resources

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 A Metric for 2014

• How can we compare the costs of:

 – I/I reduction

 – capacity improvements

 – private backup protection

 – and many combinations thereof

marginal cost per potential backupprevented

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Marginal Cost per Potential BackupPrevented

 A. Establish level of protection (LOP) throughout systemB. Determine number of properties within each LOP

C. Annualize potential basement backups (PBBs)

D. Compare to existing conditions for reduction in PBBs

E. Price each alternativeF. Divide reduction in PBBs by cost for marginal cost

A × B = C

D1 = C0  – C1 ; D2 = C0  – C2 ; etc.

F = E ÷ D 

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Marginal Cost per Potential Backup Prevented

A × B = C

D1 = C0  – C1 ; D2 = C0  – C2 ; etc.

F = D ÷ E 

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Level of Protection Analysis

partially full (plenty of freeboard)

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Level of Protection Analysis

surcharged (danger)

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Level of Protection Analysis

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Level of Protection Analysis

1-year storm

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Level of Protection Analysis

2-year storm

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Level of Protection Analysis

5-year storm

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Level of Protection Analysis

10-year storm

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Level of Protection Analysis

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Level of Protection Analysis

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Level of Protection Analysis

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Level of Protection Analysis

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Level of Protection Analysis

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Level of Protection Analysis

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Level of Protection Analysis

(special methodology for the mayor’s block)

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Basin-Wide Level of Protection

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Basin-Wide Level of Protection

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Case Study 1: City of Elmhurst SouthwestWet-Weather Control Facility

• Model run for existing conditions

•

Less than 1-year LOP in many locations

• Level 1 flow reduction % from I/I source

data

• Model used to assess improvement

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I/I Source Quantification

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Level 1 Flow Reduction

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Existing Conditions

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 After Level 1 Flow Reduction

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Development of Alternatives

• Level 1 flow reduction short of desired LOP

• Target LOP of 10 to 25 years

•  Additional measures considered

 –Option A: Level 2 flow reduction

 –Option B: Relief and storage

 –PLUS targeted overhead sewers

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Option A: Level 2 Flow Reduction

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Option A: Level 2 Flow Reduction

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Foundation Drain Removal

Excavation aroundfoundation

Overhead sewerrecommended

BMPs for overlanddischarge

Up to $15,000 per

home

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Option A: Level 2 Flow Reduction

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Option A: Level 2 Flow Reduction

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Option A: Level 2 Flow Reduction

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Option B: Relief and Storage

•

Targeted two of lowestLOP areas – upstream of Saylor and

Jackson Lift Station – vicinity of FM discharge

•

Concept included: – upsizing the Saylor and

Jackson lift station – upsizing the tributary sewer – wet-weather force main – wet-weather storage

• Both 10- and 25-year LOPalternatives modeled

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Option B: Relief and Storage

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Option B: Relief and Storage

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 Analysis of Alternatives

*Each alternative included targeted overhead

sewers

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 Analysis of Alternatives

*Each alternative included targeted overhead

sewers

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Option X: Overhead Sewers for All!

•

Some ideas sound great on paper…

• Major coordination challenge

• High variability in costs

• Slow, plodding process

• Still wouldn’t get rid of SSOs!

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Cost-Benefit Analysis

blue = marginal reduction in potential backups

green = marginal cost

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Epilogue

• Capacity improvements under construction – 24” influent sewer

 – 18” wet-weather force main

 –2.0 million gallon storage tank at WRF

• I/I reduction plan continues to evolve

• Incentives for private sector remain crucial

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Case Study 2: Village of Wilmette HarmsRoad Region Wet-Weather Storage Plan

• Continuation of 2012 Village-wide modeling

•

2012 model calibrated from flow monitoringperiod with little rain

• Estimated 3.2 MG storage for 10-year LOP

• Model recalibrated in 2013 with 6-month monitoring program

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Case Study 2: Village of Wilmette HarmsRoad Region Wet-Weather Storage Plan

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1. Larger sample to refine RTK parameters

2. Outfall conditions!

What changed?

36” Wilmette sewerMWRD

Harms

Road

Interceptor

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2013 Metering

 April 18

~16 hrs. reverse flow

June 26

~7 hrs. reverse flow

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2013 Metering

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• Need for storage more evident

• How much is optimal?

Development of Alternatives

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 Analysis of Alternatives

Backups Per Annum by Alternative

Storm

Event

 Average #

of Events

Per Year

Existing

Condition

1.5

MG

3.0

MG

4.5

MG

5.5

MG

8.0

MG

11.0

MG

9.0

MG

(P)

2-Month 6 0 0 0 0 0 0 0 0

6-Month 2 116 0 0 0 0 0 0 0

1-Year 1 209 40 38 17 7 7 7 72-Year 0.5 473 441 97 48 48 29 29 29

5-Year 0.2 91 141 284 229 54 49 45 49

10-Year 0.1 4 5 3 43 118 92 92 87

25-Year 0.04 0 1 0 0 5 10 6 7

50-Year 0.02 0 0 0 0 0 4 2 6

100-Year 0.01 0 0 0 0 0 0 2 0Total 893 628 422 337 232 191 184 185

Reduction n/a 265 471 556 661 702 709 708

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Storage Volume Optimization

5.5 MG

(optimal)

3.2 MG

(previous)

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Storage Volume Optimization

ExistingCondition 3.0 MG 4.5 MG 5.5 MG

9.0 MG(P)

Existing 471 556 661 708

3.0 MG $14,642 85 190 2374.5 MG $17,791 $35,211 104 151

5.5 MG $16,643 $21,615 $10,528 47

9.0 MG (P) $25,427 $46,901 $53,476 $148,968

blue = marginal reduction in potential backups

green = marginal cost

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Epilogue• 5.5 MG storage facility under construction

•  Village addressing I/I sources to further increase LOP

• Discussions with MWRD and neighboring communitiesfor regional solutions

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The Future

• Improvements to RDII hydrology and I/Isource quantification

• More complete SSO and backup data

•

Integrated storm / sanitary modeling

• Cost optimization engines

…the future, Vinnie?

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The Future