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 co nstructed wetlands in the remediation of a variety of wastewaters. Despite the widely recognised importance of sanitary indicator bacteria as fundamen tal measures of water quality, surprisingly little work has been conducted on the behaviour of enteric bacteria, and other microorganisms, in macrophy te systems. This paper reports the findings of an investigation of faecal c oliform (FC) and faecal streptococci (FS) removal in four small, parallel T ypha-dominated, surface flow reed beds, constituting the tertiary phase of treatment at the Crow Edge sewage treatment works near Holmfirth in Yorkshi re. Samples were collected at the inflow and outflow points of each bed, tw ice-weekly for 10 weeks between June and September 1998. On average, an ord er of magnitude reduction in concentration was observed between inflow and outflow wastewater for both indicator bacteria, giving mean bed removal eff iciency values of approximately 85-94%. However, FC concentration in the fi nal effluent was still several thousand CFUs/100ml on most sampling occasio ns. Moreover, bacterial removal efficiency was strongly, negatively correla ted with the flow rate of effluent through the system and, at times of high flow, beds were often relatively inefficient in removing both indicator or ganisms. Overall, the relative performance of the beds was similar, but som e significant differences were found. Reasons for these findings are discus sed. (C) 2000 Elsevier Science Ltd. All rights reserved.