Treatment of the domestic wastewater by High Rate Algal Ponds (HRAP) shaked by airlift under semi-arid climate
Arid and semi-arid climate areas such as Morocco are affected by water shortages, especially shortages in the water intended for irrigation. In this particular climatic context, the reuse of treated wastewater may partly contribute to solving this problem. Currently, a number of studies have shown the effectiveness of extensive treatment systems and their adaptability to Moroccan socio-economic conditions. The high rate algal pond (HRAP) is an efficient alternative to natural lagoons. The objective of this study was (i) to determine the various hydraulic parameters of a HRAP equipped with an air-lift agitation system, and (ii) to study its operation and to evaluate its performance (physico-chemical and bacteriological) under the semi-arid climatic conditions of Marrakech. The hydrodynamic study showed that the flow rates calculated for the pilot HRAP equipped with the air-lift met the flow range rates recommended for this installation and those of other agitation systems often used, such as the paddlewheel. The physico-chemical study related to the monitoring of organic load parameters (Chemical Oxygen Demand, COD; Biochemical Oxygen Demand, BOD5) and nutrients (nitrogen and phosphorus), was done in accordance to standard methods. For BOD5, the system eliminated approximately 44%, whereas total COD remained high. In the HRAP system, the transformation of the incoming organic matter into algal biomass was expected, and explains the weak performances of the HRAP with respect to total COD. In contrast to the results for the organic load, the pond system showed a remarkable efficiency for the elimination of nutrients (77% for PO4 and 90% for NH4).