Detachment of Solids and Nitrifiers in IFAS Systems

Integrated Fixed Film Activated Sludge (IFAS®) systems utilize polyethylene biofilm carriers to achieve nitrification in municipal wastewater treatment plants (WWTPs). The movement of solids to and from the biofilm carrier has the potential to impact the biofilm thickness, the retention time of the micro-organisms and hence the nitrification capacity of the biofilm. Despite the potential importance of detachment to biofilm processes, detachment phenomena are not well understood. In this study biofilm detachment from free-floating biofilm carriers that were established in a demonstration-scale IFAS installation in Mississauga, Ontario was investigated. A methodology for assessment of detachment from polyethylene biofilm carrier systems was devised, evaluated and refined during this study. In batch testing with carriers extracted from the full scale plant, suspended solids and oxygen uptake rates were analyzed on a daily basis to assess the rate of change in solids and biological activity on the carrier and in the bulk liquid. The testing investigated several aspects of detachment and the general techniques could be useful in future studies. In the absence of substrate, superficial air velocity within the range tested had no apparent effect on the decline in carrier nitrification capacities. These findings suggest that the biofilm carriers tested may be resistant to nitrifier washout and offer an operationally robust process despite stressed conditions such as an absence of feed and high airflows. The rate of detachment of solids and nitrifiers did not appear to be significantly impacted by the short-term growth conditions. The measured solids detachment rates were found to be described by a second order function of biofilm attached growth total solids (AGTS) with a detachment coefficient of 0.006 ± 0.0008 (g˙m˙d)-1. Future detachment modeling and investigative efforts should focus on biofilm morphology and composition as opposed to air flow.