1. Transport of bacteria and colloids in intermittent sand filters Maria Auset 1, Arturo A. Keller 1, François Brissaud 2, Valentina Lazarova 3 1 Bren School of Environmental Science and Management, University of California, Santa Barbara. ; e-mail: mauset@bren.ucsb.edumauset@bren.ucsb.edu 2 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 (d 60 /d 10 =2.72) sequentially dosed with secondary effluent percolating in a single pass through the unsaturated porous medium. Bacteria suspension Wastewater solution Flowmeter Epi-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. Results Pore 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 sec S w 66 % 7h 04 sec S w 39 % 8 h 02 min S w 74 % 8h 20 sec S w 43 % 14 h 45 min S w 65% 15 h 10 min S w 60% 15 h 35 min S w 49%v 15 h 50 min S w 38% 16 h 25 sec S w 45 % 16 h 35 sec S w 53 % 16 h 02 min S w 76 % 8 h 05 min S w 81 % 8 h 10 min S w 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 IODIDE *S w = Water saturation, s=solid, a=air, w=water 0Time Input Flow Tracer-free flushes Pulse