Transport of bacteria and colloids in intermittent sand filters Maria Auset, Arturo A. Keller, Francois Brissaud and Valentina Lazarova 229 th ACS San Diego National Meeting Bren School of Environmental Science and Management, University of California, Santa Barbara University of Montpellier II, France

SaturatedZone VadoseZone Sewer line or Septic tank Contaminated Water Supply ? Contaminants -Viruses - Bacteria Water Air Sand grain Cycles of household water use → Transient unsaturated flow

Solid Water SuspendedAir Attached Attached Inactivated Inactivated Inactivated Suspended 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 (d 60 /d 10 =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 Input Flow Flushes Experimental conditions 0 Time Input Flow 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 Acquisition System 0 Time Input Flow Tracer-free flushes Traced flush

Results

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, %. Retention is due to reversible bacteria entrapment in stagnant regions and sorption onto the AWI and irreversible attachment onto SWI.