In Situ Rheology of the Biofilm from a Nitrifying Bioreactor Relevant for RAS – a Proof of Principle Study

Abstract

Nitrifying bioreactors are a vital component of recirculating aquaculture systems (RAS) to maintain the required water quality. Bioreactors contain suspended plastic carriers on which nitrifying microorganisms grow. Understanding the rheology of biofilms that form on these carriers can provide useful information that can be correlated with other analyses to monitor the development and stability of the microbial communities, both over the lifetime of the bioreactor and in response to environmental changes, such as the salinity changes typical of salmonid aquaculture. The standard carriers have an open patterned geometry for biofilm formation and good fluid flow through but is not convenient for in situ rheological measurements. In this study, we included carriers in the same material (HDPE) and with the same external dimensions but with a 20mm diameter flat surface area suitable for placement within the rheometer. The carriers were placed in stainless steel “tea-ball” cages in two lab-scale bioreactors with different configurations for five weeks and then retrieved. After securing the carrier within the rheometer (Kinexus Ultra+ with a 20mm roughened parallel plate upper geometry) the upper plate was lowered to a normal force of 0.05N.  Oscillatory measurements (frequency sweep and amplitude sweep) and creep compliance measurements confirmed the biofilm on the carrier surface as a solid dominant viscoelastic solid at low strain.  This is comparable with the reported behaviour of microbial biofilms.