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: mauset@bren.ucsb.edu2 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