1. 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

3. 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

4. Solid Water SuspendedAir Attached Attached Inactivated Inactivated Inactivated Suspended Attached V Fate and Transport of Colloids

5. 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.

6. 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

7. 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

8. Experimental Setup

9. Water Content: 41% Direction of flow Air Water Solid

10. First Flush 12 sec after flush

11. Water Content: 76% 26 sec after flush

12. Water Content: 78% 38 sec after flush

13. Water Content:78% 51 sec after flush

14. Water Content: 79% 1 min 04 sec after flush

15. Water Content: 80% 1min 12 sec after flush

16. Water Content: 82% 1min 28 sec after flush

17. Water Content: 83% 1min 39 sec after flush

18. Water Content: 83% 1min 55 sec after flush

19. Water Content: 84% 10 min 09 sec after flush

20. Water Content: 68% 2 h 59 min after flush

21. Water Content: 58% 3 h 49 min after flush

22. Water Content: 55% 4 h 08 min after flush

23. Water Content: 47% 4 h 51 min after flush

24. Water Content: 43% 5 h 04 min after flush

25. Second Flush 11 sec after flush

26. 13 sec after second flush

27. Water Content: 77% 23 sec after flush

28. Water Content: 79% 1 min after flush

29. Water Content: 80% 1 min 18 sec after flush

30. Water Content: 82% 1 min 35 sec after flush

31. Water Content: 83% 2 min after flush

32. Water Content: 85% 2 min 28 sec after flush

33. Water Content: 87% 5 min 49 sec after flush

34. 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

35. 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

36. Results

37. Column results

38. 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.