Theory of filtration and its limitations

Filtration

Filtration is the process of passing water through a bed of granular material (usually sand) to remove suspended, colloidal and microbial impurities that sedimentation could not remove.

Theory (mechanisms) of filtration

1. Mechanical straining: particles larger than the pore spaces between sand grains (0.1 to 0.2 mm) are stopped at the top; this effect builds up the schmutzdecke in a slow sand filter, which then strains finer particles.
2. Sedimentation: the pores act like tiny settling basins; fine particles settle on the sand grains' upper surfaces.
3. Adsorption / surface attraction: the sand grain and the sticky film on it attract the oppositely charged colloids and bacteria (electrostatic, van der Waals forces).
4. Biological action: in a slow sand filter the zoogleal film feeds on organic matter and bacteria, oxidising them to harmless compounds, and converting ammonia to nitrates.
5. Electrolytic / chemical action: the ion exchange between sand grains and impurities, and the chemical bonding with coagulant flocs (important for rapid filters).
6. Flocculation in the bed: fine particles collide in the pore passages and combine into bigger ones which are then trapped.

  water in
  ~~~~~~~~~~~~~~~~~
  ..  schmutzdecke     <- straining, biological film
  ::::::::::::::::::
  :::: sand grains ::: <- adsorption, sedimentation
  ::::::::::::::::::
  ooooo gravel ooooooo
  clean water out

Limitations

• Filtration cannot remove dissolved salts, hardness, dissolved gases or most tastes and odours.
• Filters need a clean influent: highly turbid water clogs the bed quickly (a slow sand filter needs turbidity below about 30 NTU; a rapid filter needs coagulation first).
• Head loss builds up with time, so the bed needs regular cleaning (scraping or backwashing), which adds to the maintenance work.
• Viruses and cysts are only partly removed; disinfection is still required.
• Efficiency depends on filtration rate, grain size and temperature; at high rates there is break-through of floc.
• The slow filter needs a large land area, and the rapid filter needs skilled operation, with possible mud balls, cracking and air binding.