wet harrowing / cleaning the filter

  • Guest


    Working for a Belgian society working in waste water treatments, I worked a little bit on sand filtration and I found articles on your site. Some weeks ago I also corresponded with mister Patani who advised me to contact you.
    Next week I have to give 2 hours of courses about slow sand filtration in disinfection to African people who come from different country, to follow a formation on water treatment. I would like to talk to them about the project you already developed in some African countries, if you permit it…? In this context I wanted to ask you some precisions:

    1) What kind of sand do you furnish with the filter?

    W2) hen the owner scrap the sand (the schmutzdecke), what does he do with it? Is it washed or thrown away? In both case who has to do this? Is it the owner of the filter or someone else? And where is it stocked?

    3) Is the clean water stocked in a tank? Or do they generally use it directly?

    May I write you again if I have other question? Thank you.

  • Adriaan


    You are not the first to ask these questions and we will be updating our website to include information on use and maintenance of the filter. Basically, there are several ways to restore flow rate after clogging up. The first is the traditional slow sand filter method, which consists of removing the top 5 cm of sand, washing this with clean water before putting it back (or else replacing the dirty sand with clean): the drawback of this method is the destruction of the bio-film (schmutzdecke): thus bacteriological water quality will be less for some days immediately following cleaning.
    An alternative method is called wet harrowing. This has proved more easy and effective is to block (with a clean cork or so) the outlet spout of the filter. A bucket of water is then added and by hand the top sand is stirred up: this releases mud and filtered-out material into the water. Most, but not all of this muddy water is then scooped out. Finally, the cork is removed from the spout and filtration continues. The advantage of this method is that the sand it not washed totally clean and some sediment containing biological activity will settle immediately back on top of the sand: this restores the bio-film very rapidly. Few people in Africa take trouble to boil their water following filter maintenance, so this is important.
    Maintenance of the filter is carried out by the owner himself: the procedure is very simple. When to do it: depends on ‘convenience’. The more clogged up a filter is, the lower the flow rate (which gives more time for the bio-film to be effective): so in one way the slower the better. In practice, most people clean their filter when it takes too long for the water to come out of the filter to be convenient: no set time schedule is necessary.
    How is clean water used: this depends on the traditional practices, but we advocate the use of a jerry-can or narrow-spout pitcher instead of buckets, to reduce the risk of contamination after filtration. It would be good to stress the importance of including a sanitation and strong hygiene promotion component in any sand filter project: it has been well researched that the provision of clean water only has only a minimal impact on the reduction of diarrhoea and other water borne disease: a good project integrates sanitation and hygiene behaviour.
    There are by the way other good methods available for household (or point-of-use) water supply, which you might want to look up on the internet. One is colloidal silver impregnated ceramic filter, promoted by Potters for Peace; the second is for instance the Safe Water initiative which is a chlorine solution and promoted by USAID, PSI and the CDC especially.

    Feel free to ask me more if you need more information,

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