Transport of bacteria and colloids in intermittent sand filtersMaria Auset1, Arturo A. Keller1, François Brissaud2, Valentina Lazarova3
1 Bren School of Environmental Science and Management, University of California, Santa Barbara. ; e-mail: [email protected] Maison des Sciences de l’Eau, Université Montpellier II, 34095 Montpellier, France.
3 Technical and Research Center, Ondeo Services, Le Pecq-sur-Seine, 78230, France.
FLUSH
Introduction
Intermittent filtration through porous media used for water and wastewater
treatment can achieve high pathogen and colloid removal efficiencies. To
predict the removal of bacteria, the effects of cyclic infiltration and draining
events (transient unsaturated flow) were investigated. The experiments were
conducted at two scales: pore and column.
Experimental setup
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.Bacteria
suspensionWastewater
solution
FlowmeterEpi-fluorescent microscopy
Tensiometer
Experimental conditions
• Sequential applications of wastewater
(ph 7.3, ionic strength 3 mM)
• 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.
ResultsPore scale: visualization
Conclusions
• Transport of bacteria and soluble tracer is influenced by variations in water velocity and moisture content.
• Advancement of the wetting front remobilized bacteria which were held in thin water films, attached to the air-water interface (AWI), or entrapped in stagnant pore water between gas bubbles. Remobilization leads to successive concentration peaks of bacteria.
• Bacterial detachment from the AWI is only observed during complete gas bubble dissolution or if bubble interface stress occurs during the dissolution process.
• Earlier breakthrough of bacteria compared to tracer takes place because of exclusion processes.
•Colloids are essentially irreversibly attached to the solid-water interface, which explains to some extent the high removal efficiency of microbes in the porous media.
• Transport of bacteria and dissolved tracer correlated with intermittent hydraulic flushes.
• Earlier breakthrough of bacteria compared to the dissolved tracer.
• Bacteria concentrations fluctuated up and down, with a gradual reduction.
• High microbial retention (99.972%).
• Both tracers exhibited persistent tailing (more than 72 hr).
• After the flush, the micromodel progressively dries back. As air moves in, spontaneous coalescence of the bubbles takes place as well as trapping of colloids within a thin film of water.
Flow direction
FLUSH
Flow direction
8h 45 secSw 66 %
7h 04 secSw 39 %
8 h 02 min Sw 74 %
8h 20 secSw 43 %
14 h 45 minSw 65%
15 h 10 minSw 60%
15 h 35 minSw 49%v
15 h 50 minSw 38%
16 h 25 sec Sw 45 %
16 h 35 sec Sw 53 %
16 h 02 min Sw 76 %
8 h 05 min Sw 81 %
8 h 10 min Sw 86 %
• As the infiltration front advances, air is pushed out carrying colloids attached to the AWI. Colloids trapped in stagnant water regions are remobilized. Colloids travel through the mobile water phase and accumulate irreversibly at the solid-water interface (SWI) and reversibly at the air-water interface (AWI).
Column scale: quantification
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
C
once
ntra
tion
0
50
100
150
200
250
Flow
[mL/
min
]
Conc IODE
BACTERIA
Effluent flow
Traced flush Traced-free flushes (every 4 hours)
IODIDE
*Sw= Water saturation, s=solid, a=air, w=water
0 Time
InputFlow
Tracer-free flushesPulse