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Environmental Advisors and Engineers, Inc. 1
WASTEWATER FLOW FORECASTING USING
POPULATION AND LAND USE
Seminar for Engineers and Technicians
by Alan W. Mitchell, P.E.
3
Why Forecast Wastewater Flows?
Predict and adapt to the future. Identify infrastructure capacity failure. Develop alternatives to the existing
infrastructure.
5
The Basic Principal
Processes that operated in the past will continue to operate similarly in the future.
We attempt to determine past trends that we hope can be extended into the future.
6
How Are Wastewater Flows Forecast?
Flow monitoring/WWTP flow records.
Water use records. Population projections. Land use projections. Peaking Factor /
Design Curves.
7
Flow monitoring/WWTP Flow Records
Various mathematical extrapolation methods are used.
The advantages include: – Simpler, faster method than population or land use
projection. – More data available.
The disadvantages include: – May not be valid to apply data to other locations. – Wet weather effects randomize the data and are not easily
removed.
8
Flow monitoring/WWTP Flow Records
y = -0.5031x + 1085.1 R² = 0.0378
y = 0.5927x - 1176.3 R² = 0.5366
y = 0.3273x - 641.21 R² = 0.1563
y = 0.9768x - 1850.7 R² = 0.0553
0
20
40
60
80
100
120
140
2000 2005 2010 2015 2020
AD
F in
MG
D
Year
Annual Average Daily Flow (ADF) in MGD
Green River ADF
Jackson ADF
Montgomery ADF
Total ADF
Linear (Green River ADF)
Linear (Jackson ADF)
Linear (Montgomery ADF)
Linear (Total ADF)
Example Extrapolation Using “Best Fit” Line Through a Scatter of Points
9
Population Projections
Various mathematical extrapolation methods are used to obtain future population.
Historical water use or monitoring data provide per capita wastewater generation factors.
Base wastewater generation is the per capita rate multiplied by the population.
Infiltration/Inflow should be estimated and added. Peak flow must be estimated from historical diurnal
(24-hour) wastewater flow patterns.
10
Census Population for Kansas City 1980 - 2000
Depicts the population change from 1980 to 2000.
Greens and yellows depict positive growth.
Reds and orange colors depict negative growth.
Also shown are WWTPs and their service areas.
11
Census Population for Kansas City 1980 - 2000
0
100,000
200,000
300,000
400,000
500,000
600,000
700,000
800,000
1980 1990 2000
Popu
latio
n
Census Year
Census Population by WWTP Service Areas
Montgomery Plant
Linda Creek Plant
Clifftop Branch Plant
Yellow Creek Plant
Green River Plant
Jackson Plant
none
12
Population Projections by Extrapolation
y = 3452x - 6482002 R² = 1
y = 13077x - 25414537 R² = 1
0100,000200,000300,000400,000500,000600,000700,000800,000900,000
1,000,000
1980 1990 2000 2010 2020
Popu
latio
n
Year
Population Projections Total and WWTP Service Areas Best Fit
through 3
P
Extrapolation through
Last 2 Points
13
Wastewater Forecasts Based on Population Projections
Advantages: – Data is readily available for free. – Planning departments provide population
projections routinely. Disadvantages:
– Population projections do not accurately predict commercial and industrial wastewater generation.
14
Land use projections
Requires planning services to create future land use scenarios.
Historical water use or monitoring data provide per acre wastewater generation factors.
15
Wastewater Forecasts Based on Land Use Projections
Advantages: – Land use classes can account for any type of
generator and non-generator of wastewater. – Lends itself readily to digital GIS analysis.
Disadvantages: – Correlation of land area with wastewater
generation rates is not strong. – Predicting land use is more art than science. – Historical data is often hard to get.
16
Synchronize Data
Population, land use and wastewater generation rates are continuously changing.
As closely as possible, select billing data, land use and population from the same time period.
18
Example Forecasting Using Actual Data
Demonstrate GIS techniques for forecasting wastewater using billing data, population and land use.
Derive unit rates of generation by exploiting the spatial relationships between billing data, population and land use.
19
Process Description
Identify the Study Area. Find the per capita water use. Multiply the projected population by the per
capita water use to estimate base flow. Use typical factors to account for
Infiltration/Inflow (I/I). Use peaking factors to find the typical peak
flow.
20
Study Area: Brush Cr. and Blue River Watersheds
Define the Study Area before attempting any data collection!
Obtain current land use, water use, and wastewater flows specific to the Study Area.
21
Data Collected
Population - Missouri Spatial Data Information Service (MSDIS) provided 1980, 1990, and 2000 Census data.
Land Use - Derived from City DMS Parcels GIS data. Annual and monthly wastewater Flows 2000 - 2008-
Provided by City staff. Water Meter Data – Obtained from the City.
24
Study Area Population Projections
y = -1049.95x + 2200941.83 R² = 0.99
80,000
90,000
100,000
110,000
120,000
130,000
1975 1985 1995 2005 2015 2025
Popu
latio
n
Year
Population Summary
25
Study Area Population Projections
Year 1980 1990 2000 2001 2002 2010 2015
Actual 121,566 112,491 100,567
Linear 122,041 111,541 101,042 99,992 98,942 90,542 85,293
Logarithmic 122,048 111,524 101,052 100,008 100,008 90,632 85,442
26
City Water Use Data
Spreadsheet format exported from database Meter number Address of meter location Time period – November through January Metered water volume – 100 cubic feet units Use class: Commercial, Free water, Large
Commercial, Residential
27
Water Use Summary
Service Class Year 2001 Water
Use, MGD Year 2002 Water
Use, MGD
Commercial 20.52 22.14
Free Water 0.52 0.49
Large Commercial 1.52 1.26
Residential 11.21 10.50
Grand Total 33.76 34.40
28
Forecasting Water Use Classes
Water is used by: – Residents. – Employees of commercial establishments. – Employees of factories and repair shops. – The City utilities
Each uses and disposes of water differently. Each has a different water use trend. We will assume that all these groups generally
follow the population trend.
29
Study Area Per Capita Wastewater Forecast
YEAR POPULATION PER CAPITA WATER USE
gallons per day
WASTEWATER FLOWS
MGD
2001 99,992 33.76
2002 98,942 34.40
2010 90,542
2015 85,293
338
348 343
343
31.06
29.26
30
Accounting for I/I
Rigorous accounting for Infiltration/Inflow (I/I) requires flow monitoring studies, rainfall data collection and analysis.
Combined sewers behave more like storm sewers during wet weather.
Peak flows in combined sewer areas are limited by the capacity of the sanitary sewers.
31
Average Precipitation
Average MGD
0
20
40
60
80
100
120
140
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
JAN
.
FEB
.
MA
R.
AP
R.
MA
Y
JUN
.
JUL.
AU
G.
SE
P.
OC
T.
NO
V.
DE
C.
Month
Average Monthly Wastewater Flow
Inch
es o
f Rai
nfal
l
Milli
ons
of G
allo
ns P
er D
ay
9-Year Average Monthly Wastewater Generation Rates
Roughly equal to Average Daily
I/I
32
Peaking Factor
Peaking Factor Curve:
5/((population/1000)0.107). 2008 Population = 92,642 (estimated from
population extrapolation curve). Peaking Factor = 3.08 Average Day Flow = 31.48 MGD Estimated Peak Day Flow = 96.97 MGD
33
Forecasting Using Land Use
Classify land use consistent with water use classes.
Summarize the areas by land use/water use class.
Summarize water use by class. Dividing water use summary
by land use summary results in water use per unit area.
34
Land Use Related to Water Use by Service Class
Service Class Area, acres
Year 2001 Water Use,
MGD
Year 2001 Water Use, gallons per
acre
Year 2002 Water Use,
MGD
Year 2002 Water Use, gallons per
acre
Average Water Use, gallons per
acre
R.O.W. 19
Commercial 1,691 20.52 12,135 22.14 13,092 12,614
Large Commercial 134 1.52 11,317 1.26 9,439 10,378
Open Space 467
Residential 7,229 11.21 1,550 10.50 1,453 1,501
Vacant 616
Grand Total 10,156 33.76 3,324 34.40 3,387 3,356
36
Beware of Blindly Extrapolating Measured Data!
-5.00
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
2000 2005 2010 2015
Mill
ions
of G
allo
ns p
er D
ay (M
GD
)
Year
Extrapolated Metered Water by Use Class
CommercialFree WaterLarge CommercialResidentialGrand Total
Commercial has exceeded
the Total
Large Commercial
has gone negative
37
Land Use Projection Assumptions
Loss of Residential Area is Proportional to Population Decrease – 79.2 acres per year.
Three-quarters of the Loss is Permanent – turns into Open Space or Vacant.
Quarter of the Loss becomes Commercial. Large Commercial is assumed constant.
38
Projected Land Use
Service Class
2005 Land Use Area,
acres
2010 Land Use Area,
acres
2015 Land Use Area,
acres ROW 19 19 19 Commercial 1,691 1,790 1,889 Large Commercial 134 134 134 Open Space 467 566 665 Residential 7,229 6,833 6,437 Vacant 616 814 1,012 Grand Total 10,156 10,156 10,156
39
Water Use Forecast Using Land Use Data
Service Class
Average Water Use, gallons per
acre
2010 Land Use Area,
acres
2010 Water
Use, MGD
2015 Land Use Area,
acres
2015 Water
Use, MGD
ROW 19 19
Commercial 12,614 1,790 22.58 1,889 23.83
Large Commercial 10,378 134 1.39 134 1.39
Open Space 566 665
Residential 1,501 6,833 10.26 6,437 9.66
Vacant 814 1,012
Grand Total 3,356 10,156 34.23 10,156 34.88
40
Converting Water Use to Wastewater Flows
Not all Metered Water is delivered to the sewer. – Irrigation. – Exterior washing. – Sold as part of product.
The most rigorous forecasts account for “return water” and analyze flows for infiltration and inflow.
Losses often balance with infiltration and inflow in residential and commercial dominated areas.
This demonstration assumes Water Use = Wastewater Flow.
41
How Well Did We Do?
0
10
20
30
40
50
2000 2002 2004 2006 2008 2010 2012 2014 2016
Mill
ions
of G
allo
ns p
er D
ay
Year
Forecast Methods Compared
Total ADFWastewaterForecast byExtrapolation
Total ADFWastewaterForecast by LandUse
Total ADFWastewaterForecast byPopulation
42
Why the Differences?
The Extrapolation assumed that water usage for two years was sufficient to establish the overall trend.
The Land Use method respected differing trends between land use classes – some were upward, others downward.
The Population method assumed that all growth was proportional to the population trend, which was downward.
43
Process Review
Use GIS to establish the relationships among water use, land use, and population.
Summarize flows by population distribution and/or land use areas.
Calculate unit flow rates per person and/or per unit land area.
Extrapolate population and/or land areas into the future.
Multiply unit flow rates by the extrapolated future population and/or land areas.