Conception et performance d'un champ d'épandage de grandes dimensions

A large disposal field (31 × 69 m) was designed in agreement with legal recommendations. A full scale test with clear water revealed that the field exfiltration rate was much lower than anticipated. This paper describes the design, the instrumentation performed with eleven piezometers, the ground-water conditions as modified by the disposal field, how the true exfiltration rates have been determined after due consideration of natural precipitations and evaporation losses, and also the method used to improve the performance. The value of the infiltration rate into the soil, initially established by percolation tests, was confirmed by permeability tests performed in the piezometers. However, the true exfiltration rate of this disposal field was only 8% of the rate predicted by the conventional, legal design. It is established that this exfiltration rate cannot be derived by simple transposition of the results of percolation tests as presently done by by-laws. Alternatively, more rigorous methods are described. For a good design, it is necessary to perform hydrogeologic studies much more detailed than those presently required by by-laws. Finally, for improving the design and service life of such disposal fields, several suggestions are made to avoid water mounds and to reduce the risk of clogging.