The high rate algal pond (HRAP) is an efficient low cost wastewater system. It is well adapted to small communities with high summer loads.

Many studies have been carried out on HRAP in order to adapt them to the seasonal variations of climate but very few have been devoted to the diurnal behavior of the system which could be an important design parameter.

The work for this investigation was carried out at the ‘Centre de Lagunage de Recherche Pluridisciplinaire‘ in Mèze, France, community located on the Mediterranean

The results show that all variables, except COD, do have diurnal variations. During daylight, photosynthetic activity is at its peak between 13:00 GMT and 15:00 GMT with oxygen saturation rates of 350 %, high pH's (pH>9) and low total alkalinity. The highest removal rates of NH4-N and PO4-P are 94 % and 89 % respectively. During the night, photosynthesis stops and the dissolved oxygen concentrations drop to very low levels or even to zero. pH decreases (8.6) and total alkalinity increases as a consequence of the intense respiration of the algae and other organisms of the system.The various efficiencies decrease. Nutrient removal is linked to the variations of solar radiation and temperature which are considered as the main characteristics controlling algal activity. Algae contribute to the elimination of nutrients through two mechanisms: biological uptake and pH increase causing a volatilization of NH3 and a chemical precipitation of calcium phosphates. The decrease of the performances of a HRAP during the night is caused on the one hand by the interruption of photosynthesis combined with respiration, causing a decrease in pH and, on the other hand, to the continued inflow of sewage in the pond. The knowledge of the diurnal behavior of HRAP leads to a new concept of pond management. When HRAP pond effluent was withheld according to photosynthetic activity, this resulted in a more efficient removal of nutrients. Automatic systems tied to time or pH could control the release of effluent and easily implement such a management.

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