Transport of bacteria and colloids in intermittent sand filters

1. Tensiometer Bacteria suspension Wastewater solution Epi-fluorescent microscopy Flowmeter IODIDE Pulse Tracer-free flushes Input Flow 0 Time Transport of bacteria and colloids in intermittent sand filters Maria 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.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. Results Pore scale: visualization 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. *Sw= Water saturation, s=solid, a=air, w=water FLUSH Flow direction 8h 45 sec Sw 66 % 8h 20 sec Sw 43 % 8 h 02 min Sw 74 % 8 h 05 min Sw 81 % 8 h 10 min Sw 86 % 7h 04 sec Sw 39 % • 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). FLUSH • 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. Flow direction 14 h 45 min Sw 65% 15 h 10 min Sw 60% 15 h 35 min Sw 49%v 15 h 50 min Sw 38% 16 h 25 sec Sw 45 % 16 h 35 sec Sw 53 % 16 h 02 min Sw 76 % • 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. Column scale: quantification • 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). • 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. • 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.