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

2 Bases for Analyses  3-yr runs (average 1988, )  Average flow = COG Round 6.3  Base flows vary with amount of rain in year  Suburban wet weather relationships = new regression

3 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) 460 mgd With out Suburbs No suburban flow BPWWTP derated to DC share Potomac 460 mgd Potomac DC share = mgd Calculate:  Wet & Dry Weather Flow generated by  Suburbs/DC  Treated volumes & CSO

4 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.Scenario Suburban Flows 1 st 4 hrs After 4 hrs During Tunnel Pump OutAnaPotRCTotal Outfall 001 Volume (mg/avg yr) An aPotRCTotal 1 C3-No Suburbs 460 mgd No suburban flow , ,6852, C3-No Suburbs mgd No suburban flow , ,7272, C3-B1 Up to IMA Trans. Limit , ,8591, % Diff. between 1 & 39.4%-95.7%7.6% % Diff. between 2 & 37.1%-86.5%6.0%

5 Method 2: Annual Volumes Treated Average Annual Flow Rate in avg year (mgd) Parameter C3-B1 (with Suburbs) Flow Inputs Suburban dry weather flow Suburban wet weather flow Total suburban flow DC dry weather flow DC wet weather flow Total DC flow DC + Suburbs dry weather flow DC + Suburbs wet weather flow Total DC + Suburbs flow Flow Outputs Complete Treatment (002) Excess Flow Treatment (001) CSO Overflow Total Flow % of Flow Handled by System ParameterSuburbsDC Dry Weather Flow191.3/342=56%44% Wet Weather Flow6.6/28=24%76% Total Flow197.9/370=53%47 %

6 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.Scenario Suburban FlowsArea 1Area 2Area 3Area 1+2+3Area 1+2Area 4 (1) Area C3-No Suburbs 460 mgd No suburban flow5531, ,5231,9251,6164,138 2C3-B1 Up to IMA Trans. Limit7581, ,8672,5365,8568,723 % Diff. between 1 & 237%30%-45%14%32%262%111% 1 C3-No Suburbs mgd No suburban flow17,51811,4637,08536,06628,98272,082108,148 2C3-B1 Up to IMA Trans. Limit28,45523,0163,83955,31151,471180,163235,474 % Diff. between 1 & 262%101%-46%53%78%150%118% 3-Year Runs Calcs performed bySuburbs based on 1 storm 7 (1) Model does not run between storm events. This dry weather volume is that which occurs immediately before and after storm events

8 Calculations Requested by Suburbs: Graphs

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10 Method 3: Suburban Flows Routed Around DC ScenarioSuburban Flows SSO Volume for 2-yr 24-hr Design Storm (mg) C3-B5Flows > 2.0/1.38 x IMA allocation go to storage82

11 Summary Total Annual Overflow VolumeStorage Volume Required Annual Wet Weather Volume Handled by System Suburban Flowsmg% Diff with C3-B1mg% Diff with C3-B1mgd% of Total Method 1 C3-B1 (with Suburbs)1, C3-No Suburbs, 4601,6859.4%1697.6%-- C3-No Suburbs, ,8597.1%1836.0%-- Method 2 - Annual Wet Weather Volume Handled Suburbs WW Flow6.624 % DC WW Flow21 Suburbs Storage Volume Req’d (mg) Year 2001 $10/gal Method 3 - Suburban Flows Routed Around DC82$820 M

12 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