Recherches expérimentales sur la membrane biologique des filtres à sable submergé

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Summary

The film is a thin pervious layer which forms on the sand from silt particles and micro-organisms deposited by the raw water.

There are various opinions about the purifying value of this film: the film is harmful (Puech), the film is non-essential (Pennink), the film together with the sand exercise a purifying function (Kemna), the film is the one real filtering agent (Frankland).

It is desirable to control the algae without damaging unduly the film upon which the efficiency of purification depends.

The destruction or deterioration of the film in waterworks releases Bacteria.

In our laboratory, we made a small filter with a wire gauze spread over the sand. When the film has formed, the sand is removed from below. The water then percolates only through the film. A collar disposes of any water which may have trickled round the sides. The film itself retains as much as 75 % of E. coli (indol). The effluent is always negative to neutral-red (Rochaix reaction).