Regional Wastewater Flow Forecast Model (RWFFM)
Blue Plains Service Area (BPSA) Flow Projections Update
BPSA Inputs & Flow Management Assumptions, & COG’s Cooperative Forecasting Schedule
(Updated, 6/28/12)
BPSA INPUTS
Overview
In 2000 Metcalf and Eddy (M&E) were retained by MWCOG to update the base year wastewater
flows and input parameters to the RWFFM for the BPSA. The project was completed in 2001,
with the BPTC (Blue Plains Technical Committee) accepting the M&E recommended base year
flows and input parameters.
In 2010, DC-WASA contracted Black and Veatch to update 2000 Base Year Flow, Unit Flow
Factors (UFFs) and I/I (inflow/infiltration) assumptions for each jurisdiction. In May 2010, the
Operating Agency Work Group agreed with the recommendation to use 2009 Base Year Flow,
UFFs and I/I. At its May 26th, 2010 Special Session, the BPRC concurred with the overall
findings of the OAWG – that Blue Plains’ 370 MGD capacity could meet the BP Users’ needs
until and beyond Year 2040.
Base Year Flow- 2009
The Base Year 2009 flow for each jurisdiction was selected as a conservative Base Year in
consideration of rainfall and groundwater impacts to the wastewater flows. Base Year 2009
flows are actual flows for the Calendar Year 2009, not calculated; and reflect flow
measurements and methodologies that have been investigated and verified by the BPSA
parties.
Table ES-1 – Base Year 2009 Actual Flows*
Jurisdiction
Flow (MGD)
Fairfax County
29.201
WSSC
125.966
Loudoun Water
11.913
District**
127.730
Vienna
0.827
Dulles
0.846
Navy
0.050
National Park Service
0.013
*Flows correspond to Table ES-1 on p. 5 of the Final Report (August 2010).
** District Flow does not include estimated 15 MGD Captured Stormwater Flow in Base Year.
RWFFM Input Parameters
Black and Veatch updated M&E’s flow factors based on flow contributors communications,
recent wastewater flow experience, current trending of gallons per capita per day water
demand, COG demographic data and current flow contributors design guidance and standards.
The incremental I/I rate of 44% of incremental sanitary flow remained the same. These flow
factors were reviewed and approved by the BPRC members.
Table ES-2 – 2009 Recommended Flow Factors*
Category
Fairfax
WSSC Loudoun District
Dulles
County
Water
VA
Household (gpd/unit)
150
180
185
170
NA
Employment
30
28
25
25
48
(gpd/employee)
Vienna
VA
170
21-50
RWFFM - BPSA Flow Projections
BPSA Inputs & Flow Management Assumptions, & COG’s Cooperative Forecasting Schedule (Updated, 6/23/11)
Page 2 of 3
* P. 5 of the Final Report
COOPERATIVE FORECAST - FORMAL APPROVAL SCHEDULE
Background
The MWCOG’s Cooperative Forecast is a multi-stage, ‘top-down/bottom up” process and
employs 1) a regional econometric model and 2) local jurisdictional forecasts of employment,
population, and households for the metropolitan Washington region. The demographical data
derived from the MWCOG Cooperative Forecast is the driver behind the RWFFM, which is the
modeling tool used to project wastewater flow in the BPSA.
The Cooperative Forecast Model projects employment, population, and household data for the
metro region based on national economic trends and local demographic factors. Concurrently,
local jurisdictions, while taking into account the preliminary regional projections, develop their
own projections based on pipeline development, market conditions, planned transportation
improvements, and adopted land use plans and zoning. Both projections are reviewed by
COG’s Cooperative Forecasting and Data Subcommittee and must be within three percent of
each other for the new set of Cooperative Forecasts to be reconciled. Each of these forecasts
is identified as a ‘Round’. Significant changes to the projections are done periodically and are
designated as say Round 7.0, with generally annual updates being designated as Round 7.2.
There were twenty three Cooperative Forecasts (Rounds and Subrounds) adopted since 1976.
Regional Wastewater Flow Forecast Model (RWFFM) Update
Wastewater projections are based on demographic information (employment and households)
from the Cooperative Forecast Model – which is linked to the appropriate flow factors to derive
wastewater flows. Population data is not used in this process.
The intermediate series of demographic projections are aggregated into Transportation Analysis
Zones (TAZs), which form the basis for allocating employment and household demographic
information to the appropriate sewersheds and sub-sewersheds in the BPSA.
COG staff expects that Round 8.1 Forecast will be adopted concurrently by the COG Board and
the TPB in July 2012. Highlights for Round 8.1 – will establish new base year 2010 & integrate
Census 2010 data. Note: Fairfax County (as well as Charles County) is expected to include the
Census 2010 data as well as updated (population and households) land use assumptions for
Tysons Corner in the next round of Cooperative Forecasts – Round 8.2.
BPSA FLOW PROJECTIONS
As part of developing the BPSA flow projections, flows are generated using the latest COG
Cooperative Forecast demographic projections and agreed upon base year flows, flow factors,
and I/I values. The results are the ‘Unadjusted’ BPSA flows that reflect just the impact of
demographic changes. Then various flow management assumptions are incorporated and
‘Adjusted’ BPSA flows are generated.
At its June 23, 2011 meeting, the BPRC members agreed to continue to use the ‘BPSA Flow
Projections – Round 7.2A (vs. Round 8.0) for BPRC long-term planning purposes. COG staff
plans to provide the updated ‘BPSA Flow Projections – Round 8.1’ for consideration by
the Blue Plains IMA Regional Committee at their September 2012 meeting.
COG Staff Contact: Lana Sindler 202-962-3347/ lsindler@mwcog.org
RWFFM - BPSA Flow Projections
BPSA Inputs & Flow Management Assumptions, & COG’s Cooperative Forecasting Schedule (Updated, 6/23/11)
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