Domestic wastewater treatment by infiltration-percolation is a process that is becoming common in France. The aim of this study is to find the depth of biologically active substrate in an infiltration basin by determining the depth of the media colonised by the biomass and by studying oxygen renewal mechanisms. The study using sand columns has allowed simultaneous comparison, on the same profile, of biomass content (ATP), gaseous composition (chromatography) and the variations of the effluent quality (carbon, nitrogen, phosphorus). Down to a depth of 30 cm, removal rates achieved in terms of conventional treatment parameters are very high (COD and SS > 90%, NH4+ ≈ 95%, total phosphorus ≈ 50%). Beyond a depth of 15 cm, the biomass content (expressed in ATP) is ten times less than at the surface, and virtually ceases to develop.
Monitoring of O2 levels points to the need for drying periods in order to ensure natural ventilation of the basins. The primary settling stage must be effective in order to avoid any risk of clogging which would prevent the air from being renewed by diffusion.The length of the drying period must be almost double that of the flooding period to allow the media to recover as much of its treatment capacity as possible.
This study pinpointed the depth of the biologically active substrate at arround 30 cm. The data obtained from this trial project point to the following design criteria: 1.5 m2/p.e. spread over three basins, and a drying period twice as long as the flooding period. The sand depth will depend on the plant's overall water quality objectives: around 0.50 m for the removal of carbonaceous pollution and nitrification of Kjeldahl nitrogen, a greater depth for disinfection purposes.
Domestic wastewater treatment by infiltration basins is a process that is becoming common in France. In the absence of accurate operating data, we felt it would be useful to conduct a case study at an existing plant. The Saint Symphorien de Lay plant, serving a population-equivalent of 500, was chosen because the basins are watertight and drained.
The study revealed 85% TSS, COD and Total Phosphorus removal. Nitrogen yields varied over the flooding period, with a high production of nitrates at the beginning of the period. The rate of decontamination (1 to 2 Log unit) was lower than expected.
The infiltration surface should increase from 1 to 1.5 m2/p.e. and spread over three basins. Research is now focusing on finding the biologically active depth of an infiltration basin.
S u m m a r y : I d e n t if ic a t io n o f t w o s ib l in g s p e c ie s A a n d B o f
AEDES DETRITUS COMPLEX o n ATLANTIC COASTLINE IN FRANCE.The electrophoretic analysis of five natural populations of Aedes detritus has shown that the two sibling species A and 8 of the complex, described in 1 9 7 7 in an other geographical area by Pasteur et al., cohabit on atlantic coastline in France.
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