The objective of the present study vas to ascertain the mechanisms involved in nitrogen and phosphorus removal from wastewater and to determine the most effective treatment conditions for removing these nutrients by high rate ponding in a Mediterranean climate. Tests carried out in Mèze (France) showed that variations of climatic factors (light and temperature), can be compensated by changes in detention times. The following strategy was adopted: the pond was operated with long detention times (8 days) in winter, which is characterized by low light intensity and low temperatures, and with short detention times (4 days) in summer, when the climatic conditions are more favorable. The results obtained show that the rates of HH4-N and PO4-P removal were high and stable (94 and 71%, respectively) during the study period February to September, 1988. The phenomena responsible for nutrient elimination in the high rate pond are the following: photosynthetic assimilation, volatilization of ammonia and precipitation of calcium phosphate due to the high pH obtained by intense algal photosynthetic activity. Wastewater treatment by high rate ponding appears to be an attractive technique for preventing eutrophication in receiving water.

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