Tertiary lagoons: A review of removal mechanisms and performance

Tertiary lagoons, or maturation ponds, have generally been viewed as an eff ective and low-cost method of removing pathogens from wastewater. Their low operation and maintenance costs have made them a popular choice for wastew ater treatment, particularly in developing countries since there is little need for specialised skills to run the systems. This paper provides a criti cal review of the literature on the removal mechanisms operating in tertiar y lagoons, and their overall performance. Numerous physical and chemical pa rameters have been suggested as factors involved in bacterial removal, and particular attention is paid in this paper to the relationship between pH,d issolved oxygen concentration and light. Very little information is availab le 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 discuss ed. 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. (C) 1998 Elsevier Science Ltd. All rights reserved.