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

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

SolidSolid

WaterWater

SuspendedSuspended

AirAir

Attached Attached

Attached Attached Inactivated Inactivated

Inactivated Inactivated

Inactivated Inactivated

SuspendedSuspended

Attached Attached

V

Fate and Transport of Colloids

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.

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

• 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

Experimental Setup

Water Content: 41%Direction of flow

Air

Water

Solid

First Flush 12 sec after flush

Water Content: 76%26 sec after flush

Water Content: 78%38 sec after flush

Water Content:78%51 sec after flush

Water Content: 79%1 min 04 sec after flush

Water Content: 80%1min 12 sec after flush

Water Content: 82%1min 28 sec after flush

Water Content: 83%1min 39 sec after flush

Water Content: 83%1min 55 sec after flush

Water Content: 84%10 min 09 sec after flush

Water Content: 68%2 h 59 min after flush

Water Content: 58%3 h 49 min after flush

Water Content: 55%4 h 08 min after flush

Water Content: 47%4 h 51 min after flush

Water Content: 43%5 h 04 min after flush

Second Flush 11 sec after flush

13 sec after second flush

Water Content: 77%23 sec after flush

Water Content: 79%1 min after flush

Water Content: 80%1 min 18 sec after flush

Water Content: 82%1 min 35 sec after flush

Water Content: 83%2 min after flush

Water Content: 85%2 min 28 sec after flush

Water Content: 87%5 min 49 sec after flush

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

Unsaturated column setup

Bacteria suspension

Wastewater solution

Flowmeter

Epi-fluorescent microscopy

Pressure transducer

Pump

Data AcquisitionSystem

0 Time

InputFlow

Tracer-free flushes

Traced flush

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

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

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.