Effect of biosand filters on water quality

There are 3 types of water quality features House and Reed, 1997 [ref.01]Ref.01: House, S. J.; Reed, R. A., Emergency Water Sources: Guideleines for selection and treatment. Water, Engineering and Development Centre (WEDC), Loughborough, 1997. Available online.):

1. Physical (for example, colour, pH, taste, odour, temperature and turbidity)
2. Chemical (for example, arsenic, fluoride, chloride, conductivity, dissolved oxygen, iron, manganese, nitrate, nitrite, sulphates, pesticides and heavy metals)
3. Microbiological and biological (for example, bacteria, protozoa, viruses, helminths and higher organisms)

These water quality features can affect health in the following ways (House and Reed, 1997 [ref.01]Ref.01: House, S. J.; Reed, R. A., Emergency Water Sources: Guideleines for selection and treatment. Water, Engineering and Development Centre (WEDC), Loughborough, 1997. Available online.):

• Some can be directly harmful to health, such as microbiological and biological contaminants, fluoride, pesticides and heavy metals
• Colour, taste, turbidity and odour can make the water objectionable to consumers and cause them to use less objectionable water which may not necessarily be safer
• Other features, such as pH and turbidity, can reduce the effectiveness of treatment processes such as disinfection

The effect of slow sand filtration on water quality has been divided into 3 sections, each dealing with one of the main water quality features – biological & microbiological, chemical and physical. More information on each of these is available through the links on the left. In turn, each section has been sub-divided into 2 separate parts that deal with the effectiveness of continually operated versus intermittently operated filters. The reasons for this are as follows:

Biological treatment that takes place in a continually operated slow sand filter is well documented. By comparison, intermittent sand filtration is less well documented, mainly because until the early 1990’s, it was not really practised at all. In fact, there was a general consensus that intermittent filtration should be avoided. One of the reasons for this had been observed declines in bacteriological quality alongside declining oxygen levels (Paramasivam et al, 1980 [ref.02]Ref.02: Paramasivam, R.; Joshi, N.S.; Dhage, S.S. and Tajne, D.S. (1980). Effect of Intermittent Operation of Slow Sand Filters on Filtered Water Quality. Indian Journal of Environmental Health, Vol.22, No.2, pp.136-150.).  The processes involved in continually or intermittently operated filters are slightly different, and it makes sense to differentiate the studies done on both. This is especially important to clarify because the household level intermittent sand filter potentially has a huge applicability at a household level in developing countries, where water is only put in at certain times of day. Proof of the intrinsic effectiveness of intermittent sand filtration is therefore very important in order to launch household level water filters on a wider scale.

The table below, taken from Shaw (1999 [ref.03]Ref.03: Shaw, R. (Ed) (1999). Running Water: more technical briefs on health, water and sanitation. Intermediate Technology, London, UK, p.103.), shows that as a single process, slow sand filtration was ranked second most effective of all treatment processes after desalination/evaporation, given a range of pathogenic, chemical and aesthetic factors.

References:

Ref 01: House, S. J.; Reed, R. A., Emergency Water Sources: Guideleines for selection and treatment. Water, Engineering and Development Centre (WEDC), Loughborough, 1997. Available online.

Ref 02: Paramasivam, R.; Joshi, N.S.; Dhage, S.S. and Tajne, D.S. (1980). Effect of Intermittent Operation of Slow Sand Filters on Filtered Water Quality. Indian Journal of Environmental Health, Vol.22, No.2, pp.136-150.

Ref 03: Shaw, R. (Ed) (1999). Running Water: more technical briefs on health, water and sanitation. Intermediate Technology, London, UK, p.103.