AbstractThe Bertrix wastewater treatment plant was designed and built for experimental purposes, especially for comparing aerated lagoons and stabilization ponds in a temperate climate. This plant was designed for a capacity of 7500 inhab. eq, and aerated lagoons were dimensioned to eliminate 50% of the organic load. The remaining load has to be degraded in the series of stabilization ponds.

In this paper we shall present the plant in more detail and the results of a 3-year study, i.e., 79 rounds of samples for each of the five ponds under study, placing emphasis on the performances of aerated lagoons and stabilization ponds. Principal components analysis (of inflow and outflow) of aerated lagoons and stabilization ponds will be presented and commented on. The most important factors are the hydraulic loading and the concentrations. Seasonal variations appear only in the basins' temperatures. The fates of nitrogen compounds are quite different from those of organic compounds, confirming that a high efficiency of nitrogen removal is difficult to achieve, especially for short residence times (less than eight days).

A few other conclusions of our study are given below:

The ponds' hydrodynamics has been studied and a mathematical model is now available If there is no stratification in the ponds, a thermal model can be proposed where the mean absolute difference is 0.7°C±0.2.

In the system under study, the contribution of algal biomass to the system is very small.

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