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www.ma-rs.org Montgomery Associates
Resource Solutions, LLC
The Impact of First Flush Removal on Rainwater Quality and Rainwater Harvesting Systems’
Reliability in Rural Rwanda
Kelly C. Doyle – Montgomery Associates: Resource
Solutions
Dr. Peter Shanahan – Massachusetts Institute of
Technology
May 17, 2010
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Outline• Project context
• Intro to first flush (FF)
• Fieldwork
• Storage-Reliability-Yield (SRY) simulation
• Effects of FF on SRY
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Project Details• Project Location
– Bisate Village, Rwanda
• Project Partner– Dian Fossey Gorilla
Fund International
African Conservation Foundation
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Project Context
Problem:• 95-99% of the population
infected with worms• Inadequate water supply
(volume)• Contaminated water
supply
Approach:• Rainwater harvesting
(water supply)• First flush diversion
(treatment)
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Average Monthly Rainfall Distribution
71
89
135
172
148
38
17
50
124
164
151
84
0
20
40
60
80
100
120
140
160
180
200
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Ra
infa
ll D
ep
th (
mm
)
R
ainf
all D
epth
(m
m)
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Rainwater Tanks 2007
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The Clinic- 41.35 m3
Trackers’ House-42.5 m3
Primary School-40 m3
People Served:- 20,000 people
served- 100 patients per
day- 15 patient beds
People Served:- 1,700 students
People Served:- 50 trackers
Rainwater Tanks 2008
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The first portion of the total discharged storm volume which contains the main proportion of the pollutant load during a storm event
(Bertrand-Krajewski et al. 1998)
What is the First Flush?
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Constant Volume Diverter
Storage tank
Gutters
Diversion pipe
Removable cap
Tank inlet
Courtesy of Matt Stevenson
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Research Questions
• How much first flush to
divert?
• How is reliability affected?
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The GIZMOGraduated Inflow-collector for Zero Mixing
with Overflow
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New Clinic Roof (CL)
First setup: 0.66 mm rain
Second setup: 1.34 mm rain
Roof material: new iron sheetsLocation: close proximity to
road
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Rusty Roof House (HS)
Roof material: old, rusty iron sheets
Location: far from vehicle road
Captures first 1.45 mm of rain
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Potato House (PT)
Roof material: old clay tilesLocation: moderate distance
from vehicle road
Captures first 1.61 mm of rain
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Water Quality• Membrane filtration
– Total Coliform– E. coli
• Turbidity• pH• Conductivity
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y = 0.93e-4.25x
R2 = 0.79
y = 1.55e-1.70x
R2 = 0.98
y = 1.31e-1.75x
R2 = 0.93
y = 1.35e-1.73x
R2 = 0.93
y = 1.37e-1.36x
R2 = 1.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Runoff (mm)
cu
mu
lati
ve
rela
tiv
e r
ed
uct
ion
(1s
t al
iqu
ot=
1)
S1
S3
S4
S5
S6
** Note: Storm S1 had a different array setup than subsequent storms
Storm Trendline Event Eq'n
Turbidity Reduction Trends-CL
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Turbidity Reduction Trends-HS
y = 1.39e-1.26x
R2 = 0.88
y = 0.58e-1.18x
R2 = 0.36
y = 1.27e-1.52x
R2 = 0.95
y = 1.77e-2.02x
R2 = 0.90
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Runoff (mm)
cu
mu
lati
ve
rela
tive
re
du
cti
on
(1s
t al
iqu
ot=
1)
S1
S3
S5
S6
** Note: Storm S4 was not included for clarity. See discussion regarding outliers.
Storm Trendline Event Eq'n
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Turbidity Reduction Trends-PT
y = 1.30e-1.04x
R2 = 0.99
y = 0.99e-0.79x
R2 = 0.82
y = 1.17e-0.63x
R2 = 0.95
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Runoff (mm)
cum
ula
tive
rel
ativ
e re
du
ctio
n (
1st
aliq
uo
t=1)
S3
S4
S6
Storm TrendlineEvent Eq'n
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Previous Work
• Martinson and Thomas 2005– Uganda– – k range 0.65 – 2.2
• Doyle 2008– Rwanda– k range 0.63 – 2.0
(without outliers)
krteNN 0
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0
20
40
60
80
100
120
0.0 0.5 1.0 1.5 2.0Runoff (mm)
Raw
Tu
rbid
ity
(NT
U)
CLHSPT
** Note: The turbidity value at CL-S1 was removed to make the graph more readable; the value was 893 NTU.
Sample Location
Recommendation to divert 1st mm of runoff:
Turbidity reduction
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Storage-Reliability-Yield (SRY)Simulation Model
n
dq f1
q = time-based reliability
df = # days demand is not met
n = total days in study
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SRY Simulation Results
15% losses, no FF diversion
15% losses, 1 mm diversion after 3 consecutive days each with < 1mm rain
X X
S
tora
ge (
m)
S
tora
ge (
m)
Yield Yield
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Effect of FF Diversion on Reliabilityin Bisate
Reliability (%)
ScenarioHealth Clinic
Primary School
Trackers’ House
15% losses, no diversion 92% 31% 89%
15% losses, 1 mm diversion after three consecutive days each with <1 mm rain
91% 30% 87%
Reduction in reliability from diversion scheme
1.5% 1.0% 1.7%
Additional days not meeting demand with first flush diversion
6 days 4 days 7 days
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Conclusions• 1 mm diversion of runoff• Small reduction in reliability (<2%)• 50% reduction in turbidity• Reduced coliform and E. coli
Rainwater from the gutters
Diverted first flush
Removal cap to empty pipe
with chainSoak-away
pit
Cleaner water to main tank
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Murakoze Cyane!
• Pete Shanahan• John Peter
Nshimyimana• Bernard Isaacson• DFGFI• MIT PSC• Montgomery
Associates
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Questions?
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