Abstract
Tertiary lagoons, or maturation ponds, have generally been viewed as an effective and low-cost method of removing pathogens from wastewater. Their low operation and maintenance costs have made them a popular choice for wastewater treatment, particularly in developing countries since there is little need for specialised skills to run the systems. This paper provides a critical review of the literature on the removal mechanisms operating in tertiary lagoons, and their overall performance. Numerous physical and chemical parameters have been suggested as factors involved in bacterial removal, and particular attention is paid in this paper to the relationship between pH, dissolved oxygen concentration and light. Very little information is available on the mechanisms of removal for intestinal parasites or viruses, or for BOD, COD or heavy metals. The two proposed mechanisms for nitrogen removal (ammonia volatilisation and sedimentation of organic nitrogen) are discussed. It has become obvious from the wide variations in performance reported in the literature that the design criteria currently in use for lagoons are not adequate. It is thought that by gaining a better understanding of the removal mechanisms operating in the lagoons, the design of new lagoons can be improved.