transport of bacteria and colloids in intermittent sand filters
DESCRIPTION
Transport of bacteria and colloids in intermittent sand filters. Maria Auset, Arturo A. Keller, Francois Brissaud and Valentina Lazarova. Bren School of Environmental Science and Management, University of California, Santa Barbara. University of Montpellier II, France. - PowerPoint PPT PresentationTRANSCRIPT
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Transport of bacteria and colloids in intermittent sand filters
Maria Auset, Arturo A. Keller, Francois Brissaud and Valentina Lazarova
229th ACS San Diego National Meeting
Bren School of Environmental Science and Management, University of California, Santa Barbara
University of Montpellier II, France
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Saturated Saturated ZoneZone
Vadose Vadose ZoneZone
Sewer lineSewer lineor Septic tankor Septic tank
Contaminated Water Supply ?
Contaminants Contaminants -VirusesViruses- BacteriaBacteria
Water
Air
Sand grain
Cycles of household water use → Transient unsaturated flow
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SolidSolid
WaterWater
SuspendedSuspended
AirAir
Attached Attached
Attached Attached Inactivated Inactivated
Inactivated Inactivated
Inactivated Inactivated
SuspendedSuspended
Attached Attached
V
Fate and Transport of Colloids
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Objective
Investigate the effects of cyclic infiltration and
draining events (transient unsaturated flow) on
microorganism transport, in order to help predict
removal of pathogenic bacteria in sand filters
and natural porous media.
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Pore scale PDMS hydrophilic micromodels of realistic pattern of pore network. Pore diameters from 20 to 100 μm.
Pore depth = 12 μm. Column scale 1.5 m sand (d60/d10=2.72) sequentially dosed with secondary effluent percolating in a single pass through the unsaturated porous medium.
Experimental setup
200 μm
1.5 m
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• Unsteady flow:
Sequential applications of wastewater
• Cycles: Micromodel:2 min injection/8 hr drainage Column:5 min infiltration/4 hr drainage
• One unique application of tracers: - Soluble salt, NaI. - Escherichia coli, - 5 μm latex particles, followed by tracer-free applications.
• Monitoring output tracer concentrations for 4 days.
0 Time
InputFlow
Flushes
Experimental conditions
0 Time
InputFlow
Tracer-free flushes
Traced flush
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Experimental Setup
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Water Content: 41%Direction of flow
Air
Water
Solid
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First Flush 12 sec after flush
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Water Content: 76%26 sec after flush
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Water Content: 78%38 sec after flush
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Water Content:78%51 sec after flush
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Water Content: 79%1 min 04 sec after flush
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Water Content: 80%1min 12 sec after flush
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Water Content: 82%1min 28 sec after flush
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Water Content: 83%1min 39 sec after flush
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Water Content: 83%1min 55 sec after flush
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Water Content: 84%10 min 09 sec after flush
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Water Content: 68%2 h 59 min after flush
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Water Content: 58%3 h 49 min after flush
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Water Content: 55%4 h 08 min after flush
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Water Content: 47%4 h 51 min after flush
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Water Content: 43%5 h 04 min after flush
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Second Flush 11 sec after flush
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13 sec after second flush
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Water Content: 77%23 sec after flush
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Water Content: 79%1 min after flush
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Water Content: 80%1 min 18 sec after flush
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Water Content: 82%1 min 35 sec after flush
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Water Content: 83%2 min after flush
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Water Content: 85%2 min 28 sec after flush
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Water Content: 87%5 min 49 sec after flush
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Key Findings
• Sorption onto AWI and SWI is “irreversible”.
• Colloids trapped in thin water films.• Colloids sorbed onto AWI can be
transported – Along with the moving air bubble– As colloidal clusters– As water is remobilized
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Unsaturated column setup
Bacteria suspension
Wastewater solution
Flowmeter
Epi-fluorescent microscopy
Pressure transducer
Pump
Data AcquisitionSystem
0 Time
InputFlow
Tracer-free flushes
Traced flush
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1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72
Time [h]
Nor
mal
ized
Con
cent
ratio
n
0
50
100
150
200
250
300
Flow
[mL/
min
]
IODIDE
BACTERIA
Effluent flow
Traced flush Traced-free flushes (every 4 hours)
Results
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 12 24 36 48 60 72 84 96Time [h]
Nor
mal
ized
Iod
ide
Mas
s
0.000%
0.005%
0.010%
0.015%
0.020%
0.025%
0.030%
Nor
mal
ized
B
acte
ria
Mas
s
IODIDE
BACTERIA
Column results
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Key findings
• Transport of bacteria and tracer is influenced by variations in water velocity and moisture content.
• Advancement of the wetting front remobilizes bacteria either attached to the AWI or entrapped in stagnant pore water between gas bubbles leading to successive concentration peaks of bacteria in the effluent.
• Microbial retention rate was high, 99.972 %.
• Retention is due to reversible bacteria entrapment in stagnant regions and sorption onto the AWI and irreversible attachment onto SWI.