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This work concerned the application of anaerobic ponds for the primary treatment of urban wastewater in a Mediterranean climate. It was carried out on anaerobic ponds at large scale in Mèze (France). The anaerobic ponds constitute a good primary treatment with the removal of 55% of SS and 30% of BOD5, with a small surface area. The accumulation rate of sludge was only 0.017 m3/capita.year, due to their intensive anaerobic degradation. The anaerobic digestion reached equilibrium after one year of operation. The accumulation of sludge then showed seasonal variations with a substantial accumulation in winter and the digestion of the stock in summer. This change can be related to the influence of the temperature on methanogenesis. The production of biogas (83% CH4) was measured by gas collectors especially developed for this study and was also strongly dependent on temperature. The mass balance of carbon showed that 74% of the removed organic carbon was converted into CH4, 13% into dissolved inorganic carbon and 15% was stored in sludge. However, the anaerobic ponds presented a risk of creating odor nuisances with the emission of H2S.

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.

Due to its high sanitary efficiency, treatment of wastewater by Stabilization Ponds (SP) is proposed for sensitive coastal areas (proximity to bathing areas, shellfish farming lagoons). This process, suitable for small communities with high summer populations, requires a large surface area. The objective of this study is to compare High Rate Algal Pond (HRAP) purifying efficiency with that of SP. We have experimented with a HRAP process, which by generating algal growth reduces the required surface area; our studies suggest this to be by a factor of 5. This process is particularly interesting for nutrient removal, especially nitrogen, and could be useful in coastal areas susceptible to eutrophication. Its sanitary performance is comparable to that of SP. Environmental factors and pond depth play a more important role in HRAP efficiency than retention time of water in the basins.

A network of 63 stations was used on four occasions (