Removal of enteric bacteria in a surface flow constructed wetland in Yorkshire, England

Recent years have witnessed a growing interest in the use of natural and constructed wetlands in the remediation of a variety of wastewaters. Despite the widely recognised importance of sanitary indicator bacteria as fundamental measures of water quality, surprisingly little work has been conducted on the behaviour of enteric bacteria, and other microorganisms, in macrophyte systems. This paper reports the findings of an investigation of faecal coliform (FC) and faecal streptococci (FS) removal in four small, parallel Typha-dominated, surface flow reed beds, constituting the tertiary phase of treatment at the Crow Edge sewage treatment works near Holmfirth in Yorkshire. Samples were collected at the inflow and outflow points of each bed, twice-weekly for 10 weeks between June and September 1998. On average, an order of magnitude reduction in concentration was observed between inflow and outflow wastewater for both indicator bacteria, giving mean bed removal efficiency values of approximately 85–94%. However, FC concentration in the final effluent was still several thousand CFUs/100 ml on most sampling occasions. Moreover, bacterial removal efficiency was strongly, negatively correlated with the flow rate of effluent through the system and, at times of high flow, beds were often relatively inefficient in removing both indicator organisms. Overall, the relative performance of the beds was similar, but some significant differences were found. Reasons for these findings are discussed.