Effect on physical water quality

Sand filters are able to improve the physical and aesthetic quality of raw water. Physical attributes are colour, pH, taste, odour, temperature and turbidity.

The effect of sand filters on the physical quality of raw water is very important. This is because for some users, these parameters will affect whether they will drink the water or not, even if it is known that there is no pathogenic health risk. In fact, for marketing household sand filters, these parameters can be used as an effective tool to encourage people to use bacteriologically safe water. 

Note that water hardness can affect turbidity removals. Research done by the Jenkins et al (2009 [ref 05]) of the University of California, Davis, found that turbidity removal was worse in sand filters receiving very soft raw water. When the hardness of this water was artificially raised using calcium carbonate, turbidity removals improved dramatically. This shows that the biofilm’s ability to attract and remove suspended particles is influenced by water hardness and could be a major factor in the field where filters don’t appear to improve turbidity for no particular reason.

Evidence of filter performace on physical water quality is detailed below.

Evidence from continually operated slow sand filters

• With slow sand filtration a filtrate quality is possible that has less than 1 NTU and a 75% removal of colour when compared to the raw water (WEDC, 1999 [ref 01]).
• Turbidity and colour removal efficiencies were found to definitely improve in slow sand filters due to adsorption processes. Removal efficiencies increase with increased so with increased sand bed depth. This shows that adsorption occurs throughout the filter column in purifying water. Consequently, a decrease in sand bed depth causes a reduction in total surface area of the sand grains and ultimately total adsorption capacity is reduced (Muhammad, et al, 1996 [ref 02])

Evidence from intermittently operated slow sand filters

• During an evaluation in 2003 by Fewster, Mol and Wiessent-Brandsma [ref 03], 82% of household sand filters that were sold in 1999 still produced water of an acceptable turbidity (less than 10 TU). The temperature of the water from the concrete filters was also noted by users to be cooler. 6% of the owners considered the taste of raw water to be good, compared to 51% for filtered water. There were no bad perceptions of filtered water, only one case of rainwater becoming tasteless where it had been sweet before. However, perceptions of colour changed dramatically. 35% of raw water was considered clear, compared to 92% of filtered water. Likewise, 41% of households perceived raw water as milky in contrast to only 6% for filtered water. Note that both the taste and the colour of filtered water were independent of high or low coliform counts - higher coliform counts occurred regardless of people's perception.
• 93% of the respondents from a study carried out by Hurd et al. (2001 [ref 04]) overwhelmingly liked the bio-sand filter, particularly citing the treated water's taste, high flow rate, cooling properties, as well as turbidity removal.
  

ref.01: Unpublished information supplied by WEDC, 1999.

ref.02: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D. (1996) Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online.

ref.03: Fewster, E.; Mol, A.; Wiessent-Brandsma, C. (2004) The Bio-Sand Filter. Long term sustainability: user habits and technical performance evaluated. Presentation given at the 2003 International Symposium on Household Technologies for Safe Water, 16-17 June 2004, Nairobi, Kenya. Available online.

ref.04: Hurd, J.; Tse, L.; Paynter, N.; Smith, M. (2001) Nepal Water Project. Massachusetts Institute of Technology, USA.

ref.05: Jenkins, M.W.; Tiwari, S.K.; Darby, J.; Nyakash, D.; Saenyi, W.; Langenbach, K. (2009). The BioSand Filter for Improved Drinking Water Quality in High Risk Communities in the Njoro Watershed, Kenya. Research Brief 09-06-SUMAWA, Global Livestock Collaborative Research Support Program. University of California, Davis, USA. Available here.