Transport of bacteria and colloids in intermittent sand filters

1
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_at_bren.ucsb.edu 2 Maison
des Sciences de lEau, 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, ssolid, aair, wwater
FLUSH
Flow direction
8h 45 sec Sw 66
8 h 02 min Sw 74
8h 20 sec Sw 43
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/d102.72) 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 49v
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
• Micromodel2 min injection/8 hr drainage
• Column5 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.