The aim of the article is to explain the mechanisms responsible for eliminating pathogenic bacteria through “Geopurification”. Bacteriological tracing at the “Geopurification” plant of Orléans la Source showed that percolation through a 1.50 m thick filter bed of building sand resulted in a reduction by 2 to 4 logarithmic units of the health-related bacterial colonies doping the effluent: Eschericha coli O126B16, Serratia marcescens, Salmonella typhirium and Pseudomonas aeruginosa. Laboratory experiments carried out on “microcosms” revealed the kinetics of bacterial elimination and showed that the stress due to competition with an autochthonous flora making up the biofilm was responsible for more than 90% of the observed reduction of health-related bacteria. These conclusions enable recommendations to be put forward for optimizing the reduction in health-related bacteria so as, for example, to protect the littoral environment against effluents of domestic wastewater.

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