Epuration des eaux usées par lagunage à microphytes et à macrophytes en Afrique de l'Ouest et du Centre : état des lieux, performances épuratoires et critères de dimensionnement

Stabilization ponds and water lettuce-based systems in West and Central Africa - State of the art, removal performances and design criteria Despite several projects undertaken so far, sanitation remains a huge challenge in West and Central Africa. Big cities are growing without sustainable solution for sanitation planning. Sewage systems and wastewater treatment plants are underdeveloped or absent. Stabilization ponds have been introduced 30 years ago, but this low-cost technology remains marginal in wastewater treatment systems. The climate context is favourable to develop a set of low-cost wastewater treatment plants, particularly macrophyte-based systems. Pilot scale studies show the enormous potentialities of such sustainable technologies for water pollution control and treatment. None of the full scale applications works and this is due to the low economic level and the lack of political support. Another important reason is the lack of training and research. This situation requires the development of a collaborative network in which African researchers can share knowledge and promote sustainable wastewater treatment plants, as sanitation demand is growing exponentially in conjunction with the population growth in the cities. Stabilization ponds account for only 7% of the whole wastewater treatment technologies built in West and Central Africa, against 75% of activated sludge processes that in most cases failed. In spite of their bad quality, the effluents are widely reused in urban agriculture. Experimental data showed a maximum removal rate of 30% and 60% for COD and BOD5, respectively, and a maximum loading rate of 500 kg BOD/ha/d. Macrophytes-based systems are even more scarce (3%). The link of this technology with potential risks of malaria has slowed down its development. Nevertheless, data from studies in Niamey and Yaounde, and from rice culture irrigation zones showed that this correlation is not significant. Little research is done on wastewater treatment technologies for local application. This is one more reason why this study was based at EIER in Ouagadougou (Burkina Faso). The aim of this work was to determine the parameters for optimal removal performance and to elucidate design criteria of water lettuce-based wastewater treatment systems, possibly in combination with stabilization ponds to promote restrictive irrigation in market gardening. In secondary treatment, the maximum admissible loading rate was found to be 500 kg BOD5/ha/d (400 mg O2/l). Above this value, sludge accumulation at the pond surface caused by intensive methanogenic degradation of the sediments lead to plants die-off. COD and BOD5 removal rate was not correlated to redox state or dissolved oxygen concentrations. This indicated that the removal was mainly due to settlement and trapping of suspended solids in roots and sediments. Aerobic degradation remains low because the extent of oxygen release by plants cannot explain the yield obtained. COD and BOD5 remo