USING DIURNAL-VARIATIONS IN A HIGH-RATE ALGAL POND FOR MANAGEMENT PATTERN

The high rate algal pond (HRAP) is an efficient low cost wastewater sy stem. 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 t he seasonal variations of climate but very few have been devoted to th e diurnal behavior of the system which could be an important design pa rameter. The work for this investigation was carried out at the ''Cent re de Lagunage de Recherche Pluridisciplinaire'' in Meze, France, comm unity located on the Mediterranean The results show that all variables , except COD, do have diurnal variations. During daylight, photosynthe tic activity is at its peak between 13:00 GMT and 15:00 GMT with oxyge n 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 % respe ctively. During the night, photosynthesis stops and the dissolved oxyg en concentrations drop to very low levels or even to zero. pH decrease s (8.6) and total alkalinity increases as a consequence of the intense respiration of the algae and other organisms of the system.The variou s efficiencies decrease. Nutrient removal is linked to the variations of solar radiation and temperature which are considered as the main ch aracteristics controlling algal activity. Algae contribute to the elim ination 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 duri ng the night is caused on the one hand by the interruption of photosyn thesis combined with respiration, causing a decrease in pH and, on the other hand, to the continued inflow of sewage in the pond. The knowle dge of the diurnal behavior of HRAP leads to a new concept of pond man agement. When HRAP pond effluent was withheld according to photosynthe tic activity, this resulted in a more efficient removal of nutrients. Automatic systems tied to time or pH could control the release of effl uent and easily implement such a management.