Physical/mechanical filtration

The sand in a sand filter bed acts in a mechanical way to strain out solid particles in the raw water. While this may seem obvious, observation of the performance of rapid sand filter beds has revealed that such beds are capable of removing particles that are much smaller than the smallest pore space between the sand grains. For example, a typical sand bed using 0.5-1.0 mm sand will have pore spaces of around 0.1mm, yet will remove particles of less than 0.01 mm and can even trap bacteria with sizes of 0.001 mm. Note that since this is observed in rapid sand filter beds, it is not a result of biological action which is known to be the main factor for bacteria removal in slow sand filters. Rather it illustrates the importance of various transport and attachment mechanisms, which naturally are also at work in slow sand filters. These mechanisms are outlined below (WEDC, 1999 [ref.01]Ref.01: Unpublished information supplied by WEDC, 1999.):

Transport mechanisms

For fine particles to be trapped in a sand bed, they must first come under the influence of transport mechanisms, which move them out of the flow streamlines into lower velocity areas near the bed grains. These transport mechanisms include:

• Interception
  where streamlines pass close enough to bed grains so that particles can come into contact with bed grains
• Straining
  (like a sieve) where particles too large to fit between grains are strained out
• Diffusion
  random Brownian movements can bring colloidal particles into the vicinity of a bed grain
• Sedimentation: gravitational forces can move particles across streamlines into quiescent areas on upward-facing surfaces of bed grains
• Hydrodynamic
  particles in a velocity gradient often develop a rotation which produces lateral forces capable of moving them across streamlines.

These processes serve to provide the opportunity for particles to approach areas in the bed where entrapment would be possible. However, for removal to occur, the particles must then become attached to bed grains so that passage through the bed is stopped.

Attachment mechanisms

Attachment of particles to bed grains takes place under physico-chemical and molecular forces, which are similar to those that occur in coagulation. Bridging between particles and the influence of particle charge on electrokinetic forces are therefore mainly responsible for the development of attachment between particles and sand grains.

The physical and electrochemical nature of particles may change as flow penetrates through a bed, since the changing velocity gradients within a bed bear considerable similarities to those used for flocculation. The importance of these forces is illustrated in rapid sand filters, where the adjustment of pH and addition of coagulants to the raw water flow entering a filter bed can significantly improve the removal of fine colloidal material.

Adsorption of pathogens is also improved by lower pH raw water. Gerba et al (1988 [ref.02]Ref.02: Gerba, C.P.; Hou, K.; Sobsey, M.D. (1988) Microbial removal and inactivation from water by filters containing magnesium peroxide. J. Environ. Sci. Health. 23, pp.41-58.) documented that low pH favors virus adsorption whereas high pH favors virus elution. Lukasik et al (1999 [ref.03]Ref.03: Lukasik, J.; Cheng, Y-F.; Lu, F.; Tamplin, M.; Farrah, S.R. (1999) Removal of microorganisms from water by columns containing sand coated with ferric and aluminum hydroxides. Wat. Res. Vol. 33, No. 3, pp.769-777.) also found the same trend for ordinary sand columns where a pH of 5 favoured adsorption of E. coli and MS2 coliphage compared to water with a pH of 7. However, this adsorption was found to be affected by modified media – the sand columns with metal-coated media had removal efficiencies that were best at pH 7 for both E. coli and MS2, while removing MS-2 equally well over the pH range tested. In contrast to ordinary sand, the adsorption of E. coli to modified sand was found to be lower at pH 5 than pH 7.

The surface area available in a sand filter is huge. One cubic metre of filter sand with a porosity p and a grain diameter d has a gross surface area of (6/d) (1 -p) square metres. So for example in 1 m3 of sand having a porosity of 38% and an average diameter of 0.25 mm, the gross surface area of the grains amounts to 15,000 m2. Even after making full allowance for the surfaces not facing upward, in contact with other grains, or exposed to scour, the area of deposition below each square metre of filter surface will easily reach the value of 1000 m2 (Huisman and Wood, 1974 [ref.04]Ref.04: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.28. Available from WHO). Sand size therefore plays an important role in removal efficiency.

References:

Ref 01: Unpublished information supplied by WEDC, 1999.

Ref 02: Gerba, C.P.; Hou, K.; Sobsey, M.D. (1988) Microbial removal and inactivation from water by filters containing magnesium peroxide. J. Environ. Sci. Health. 23, pp.41-58.

Ref 03: Lukasik, J.; Cheng, Y-F.; Lu, F.; Tamplin, M.; Farrah, S.R. (1999) Removal of microorganisms from water by columns containing sand coated with ferric and aluminum hydroxides. Wat. Res. Vol. 33, No. 3, pp.769-777.

Ref 04: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.28. Available from WHO