The removal of pathogens in surface-flow constructed wetlands and its implications for water reuse

Microbiological quality represents the biggest concern to the reuse of treated wastewater. This paper reports and discusses the results of an international survey on the removal of indicators of microbiological contamination in surface-flow constructed wetlands. Constructed wetlands consistently provide a reduction of 90-99% (1-2 log-removal) in the concentration of indicators such as coliform bacteria and faecal streptococci. This removal is found in wetlands treating water from different types of pretreatment (primary sedimentation, activated sludge, trickling filter, maturation ponds). On the other hand, when the influent is of high microbiological quality, wetlands act as sources of pathogenic contamination. The final water quality, however, is still compatible with medium to no-contact recreational activities and other final water uses. High variability in the effluent quality and seasonality might limit the opportunities for reuse. The role of constructed wetlands in different treatment schemes and the remaining open questions concerning removal mechanisms and reference pathogens are discussed.