Increased viscosity and the presence of extracellular polymeric substances (EPS) and soluble microbial products (SMP) are important factors that may negatively affect wastewater treatment processes, e.g. foaming, inefficient mixing or poor dewatering. Many industries, including the pulp and paper industry, are running their production processes at shifting conditions, leading to large variations in wastewater composition to downstream treatments. The aim of this study was to investigate how changes in organic loading rate (OLR) and hydraulic retention time (HRT) affect the viscosity and production of EPS and SMP during anaerobic digestion of pulp and paper mill sludge. Two lab-scale continuous stirred tank reactors (CSTRs) were operated for 800 days at 37⁰C. The OLR was increased and the HRT was decreased in steps. Reactor fluid was sampled once a month for rheological characterization and analysis of EPS and SMP.

Our results demonstrated a clear positive correlation between viscosity and the production of EPS and SMP. OLR, magnesium and potassium were important for EPS and SMP formation and the protein fraction of SMP was negatively correlated to HRT and sludge retention time. The production of EPS and SMP was important in foam formation and sludge bulking, either directly through their surface-active properties, or indirectly by increasing the viscosity. Sludge bulking was avoided by more frequent mixing. In conclusion, rheological measurements and estimates of EPS and SMP contents could prove valuable tools to avoid the severe consequences of sludge bulking and foaming in full-scale applications.