Three waste stabilization ponds of a size ranging between 400 and 1500 eq.inhabitants, two of which are fed by long pipes, receiving in the head pond a load of 6 to 7 g BOD/m2.day, were monitored over 15 months with a sampling frequency of 1 to 3 weeks. The aim of the study was to determine the origin of malfunction phenomena causing at certain periods unpleasant smells, detrimental to the development of the ponding process. The behaviour of the head basins was analyzed by monitoring the trends of the following parameters: thermal gradient, active and degraded chlorophyll, sulfides, pH and EH profiles.

A thermal stratification was evidenced in the spring and summer at the 3 plants. The comparative analysis of the behaviour of the ponds revealed that the septicity of the raw effluent is not enough to explain the malfunctions. A set of factors act in synergy, upsetting the biological equilibrium when the organic load exceeds roughly 60 % of the nominal load. Treatment of the sulfides through the network was tested in one plant and its impact on the operation of the pond was assessed.

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