1 Briefing for IMA Participants on Results of Flow Studies October 31, 2006 District of Columbia Water and Sewer Authority.

1

Briefing for IMA Participants on Results of Flow Studies

October 31, 2006

District of ColumbiaWater and Sewer Authority

2

Bases for Analyses

3-yr runs (average 1988, 1989 1990)

Average flow = COG Round 6.3

Base flows vary with amount of rain in year

Suburban wet weather relationships = new regression

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3. Identify what Suburbs would need to do if not connected to DC CSO system

1. Compare CSO with and without suburbs

Three Evaluation Methods

2. Annual Volumes Handled

Suburban Flows Routed Around D.C. (C3-5)

Storage

Service Area

DC

Suburban WWTP: 2.0/1.38 x DWF

With Suburbs (C3-B1)• Suburbs up to IMA

transmission limit• System with pump stations

rehabilitated, inflatable dams in place (2008)• Potomac @ 460 mgd

With out Suburbs• No suburban flow• BPWWTP derated to DC

share• Potomac PS @ 460 mgd• Potomac PS @ DC share =

228.8 mgd

Calculate: Wet & Dry Weather

Flow generated by Suburbs/DC Treated volumes &

CSO

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Method 1: Compare CSO with and Without Suburbs

Blue Plains Capacity (mgd)CSO Overflow Volume

(mg/avg yr)

Tunnel Size to Achieve LTCP Performance

No. ScenarioSuburban

Flows1st 4 hrs

After 4 hrs

During Tunnel

Pump Out Ana Pot RC Total

Outfall 001 Volume

(mg/avg yr)Ana Pot RC Total

1

C3-No Suburbs Pot @ 460 mgd

No suburban flow 652 554 229 1,219 430 35 1,685 2,532 132 32 5 169

2

C3-No Suburbs Pot @ 228.8 mgd

No suburban flow 652 554 229 1,233 459 35 1,727 2,414 132 35 5 172

3 C3-B1Up to IMA Trans. Limit 1076 847 450 1,271 553 36 1,859 1,294 132 46 5 183

% Diff. between 1 & 3 9.4% -95.7% 7.6%

% Diff. between 2 & 3 7.1% -86.5% 6.0%

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Method 2: Annual Volumes Treated

Average Annual Flow Rate in avg

year (mgd)

ParameterC3-B1 (with Suburbs)

Flow Inputs

Suburban dry weather flow

Suburban wet weather flow

Total suburban flow

191.3

6.6

197.9

DC dry weather flow

DC wet weather flow

Total DC flow

151

21

172

DC + Suburbs dry weather flow

DC + Suburbs wet weather flow

Total DC + Suburbs flow

342

28

370

Flow Outputs

Complete Treatment (002)

Excess Flow Treatment (001)

CSO Overflow

Total Flow

361.4

3.5

5.1

370

% of Flow Handled by System

Parameter Suburbs DC

Dry Weather Flow 191.3/342=56% 44%

Wet Weather Flow 6.6/28=24% 76%

Total Flow 197.9/370=53% 47 %

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Calculations Requested by Suburbs: Blue Plains Treatment Volumes Suburban calculation

methods: Wet Wet Weather Flow

(>511 mgd) Area 1 - Flow receiving full

treatment in the first 4 hours, when plant capacity is 740 mgd

Area 2 - Flow receiving full treatment after the first 4 hours, when plant capacity is 511 mgd

Area 3 - Flow sent to excess flow treatment (336 mgd, or a total of 1,076 then 847 mgd)

Dry Weather Flow Area 4 – flow <511 mgd

Suburbs calculated volumes for 1 event & requested that volumes be calculated for entire 3 years

Calculations Requested by Suburbs: Blue Plains Treatment Volumes

Volumes in million gallons

Vol. w/Full Treatment

PF=2.0

Vol. w/Full Treatment PF=1.38

Excess Flow

Volume

Total Wet Weather Volume

Vol. Receiving

Full Treatment

Vol. treated when

cap<511/218

Total Volume Treated

No. ScenarioSuburban

Flows Area 1 Area 2 Area 3 Area 1+2+3 Area 1+2 Area 4 (1)Area

1+2+3+4

1

C3-No Suburbs Pot @ 460 mgd

No suburban flow 553 1,372 598 2,523 1,925 1,616 4,138

2 C3-B1Up to IMA Trans. Limit 758 1,778 332 2,867 2,536 5,856 8,723

% Diff. between 1 & 2 37% 30% -45% 14% 32% 262% 111%

1

C3-No Suburbs Pot @ 228.8 mgd

No suburban flow 17,518 11,463 7,085 36,066 28,982 72,082 108,148

2 C3-B1Up to IMA Trans. Limit 28,455 23,016 3,839 55,311 51,471 180,163 235,474

% Diff. between 1 & 2 62% 101% -46% 53% 78% 150% 118%

3-Ye

ar

Run

s

Cal

cs p

erfo

rmed

by

Subu

rbs

base

d on

1

stor

m

7(1) Model does not run between storm events. This dry weather volume is that which occurs immediately before and after storm events

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Calculations Requested by Suburbs: Graphs

Total Vol. vs Wet Weather Vol.

127,326108,148

36,06619,245

0

40,000

80,000

120,000

160,000

Total Flow (Area1+2+3+4)

Wet WeatherFlow (Area

1+2+3)

Mil.

Gal

.

DC Only Suburbs

Total Vol. vs Wet Weather Vol Percent of Total.

65%54%

46%35%

0%

20%

40%

60%

80%

Total Flow (Area1+2+3+4)

Wet Weather Flow(Area 1+2+3)

Mil.

Gal

.

DC Only Suburbs

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Calculations Requested by Suburbs: Graphs

Total Volume with Full Treatment (3 yrs)

51,471

28,982 28,455

17,51823,016

11,463

0

10,000

20,000

30,000

40,000

50,000

60,000

DC Flow/CapacityOnly

DC & SuburbanFlow/Capacity

Mil

. G

al.

Total

1st 4 hrs

After 4 hrs

Total Volume Treated Through Excess Flow (3-yrs)

7,085

3,839

0

2,000

4,000

6,000

8,000

DC Flow/CapacityOnly

DC & SuburbanFlow/Capacity

Mil.

Gal

.

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Method 3: Suburban Flows Routed Around DC

Scenario Suburban Flows

SSO Volume for 2-yr 24-hr Design Storm

(mg)

C3-B5Flows > 2.0/1.38 x IMA allocation go to storage 82

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Summary

Total Annual Overflow Volume Storage Volume Required

Annual Wet Weather Volume Handled by System

Suburban Flows mg % Diff with C3-B1 mg % Diff with C3-B1 mgd % of Total

Method 1

C3-B1 (with Suburbs) 1,727 - 172 - - -

C3-No Suburbs, Pot @ 460 1,685 9.4% 169 7.6% - -

C3-No Suburbs, Pot @ 228.8 1,859 7.1% 183 6.0% - -

Method 2 - Annual Wet Weather Volume Handled

Suburbs WW Flow 6.6 24 %

DC WW Flow 21

Suburbs Storage Volume Req’d

(mg)Year 2001 cost @

$10/gal

Method 3 - Suburban Flows Routed Around DC 82 $820 M

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Technical Findings Suburban flow peaks exceed

treatment allocations (>2.0/1.38 x annual average)

Suburbs contribute to excess flow

Suburbs contribute to CSOs

Suburbs get SSO relief connected to CSO system Governed by CSO Policy where

overflows are allowed in average year, not SSO Policy where no overflows are allowed ever

It’s less expensive for DC and suburbs to be in the system together than to be apart D.C. uses suburbs treatment

capacity during CSO events and tunnel dewatering

Suburbs use D.C. treatment capacity after D.C. storm surge passes due to lag

System is shared, timing of flows works to benefit of all

Separate systems for D.C. and suburbs would be larger and more expensive