gravel, diffuser plate, microbes?

  • Guest


    Hi. I\’m a design student from India working on natural, economic, effective ways of drinking water purification. I\’m particularly interested in the biosand filtration technique because of its simplicity and effectiveness, which is relevant in a country like India. I am trying to work on ways to make this a more popular method of filtration through design intervention. I\’m trying to understand the specifics of requirements in the design of the filter to maintain efficacy.
    My questions are:
    1. What is the significance of the diffuser plate? Since the flow rate of water is essentially to be maintained during the flow through the sand, what difference does it make what speed the water trickles onto the filter at?

    2. What do the particles of coarse and fine gravel do? Since most physical & biological contaminants have been filtered out by the sand, how does it help to have a coarser filter below that?

    3. What are the \”good microbes\” in the schmutzdecke that predate on the bacteria and coliform?

    4. Is it always necessary to maintain the height of the layers to 50 cms.? Is there a proportionate ratio that needs to be maintained between the height & width of the filter? If there is , can one then reduce the height of the filter if one reduces the width?

    I would appreciate greatly answers to these questions as soon as possible since this is a project I\’m currently working on. Thanks.

  • Guest


    Hi Rakhi,

    The diffuser plate is essential to break the force of the water being poured into the filter. Without the plate, the water would seriously disturb the top sand layer, and damage the important biological layer. Regarding flow rate, actually the strong point of the BSF is that it can be used intermittently. Hence, the flow rate can vary during startup, or as a result of partial blocking over time as the filter media gets plugged by dirt. In principle though: the slower the better, as this provides more time for particles to settle and gives more time for the biological layer to consume pathogens.

    Regarding particles of course and fine gravel at the bottom of the filter: these are only necessary to stop the sand media itself from blocking the outlet pipe. Water that percolates down trough the sand in all parts of the filter can easily find its way through the gravel to the outlet piple.

    Regarding your question on what good microbes are: that is not my field at all but I would imagine that these are numerous organisms of different types – as long as they are not pathogens.

    Finally, the height of the actual sand media (excluding the gravel layer) is best kept to around 50 cms as research has shown this to be an optimim minimum height to assure best filtration. Higher sand column is of course possible. The width of the filter can be altered without problems: this will only affect flow rate, e.g. the wider, the higher the flow rate.

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Guestgravel, diffuser plate, microbes?