Regular articleDesign, calibration and validation of a large lab-scale system for measuring viscosity in fermenting substrate from agricultural anaerobic digesters

- Laboratory-scale rheometer for fibrous biogas sludge was developed.
- High variance in viscosity values from a single sludge sample observed.
- Apparent viscosity and shear-thinning behaviour are strongly time dependent.

European biogas producers are increasingly using lignocellulosic substrates, such as grass. These break down slowly, causing high viscosity in the anaerobic digester slurry, leading to high mixing costs and problems including motor standstill. Unfortunately, viscosity in fibrous slurries is difficult to measure; therefore this work aimed to understand fermenting substrate rheology and to develop a measurement method. A mixing rheometer with a curved impeller was developed to measure the viscosity of fibrous slurries under anaerobic conditions. Using this, the apparent viscosity of fibrous anaerobic-digester content was shown decrease over time (from 5.5-8.5 Pa s to 3.9-6.0 Pa s after 2 h, γ˙ = 22 s−1). It was confirmed that fibrous anaerobic slurries are shear-thinning fluids that can be described by the power-law, Casson or Bingham models. It was also shown that the shear-thinning behaviour is less pronounced over time. Using two rheoreactors in parallel demonstrated the very large variability in viscosity measurements from a single fibrous sludge, highlighting the need for replicate measurements. A rheoreactor design for measuring the viscosity of fibrous anaerobic sludge and a measurement procedure to take into account the sample variability, as well as changes in viscosity over time and at different shear rates is proposed. This could be used to characterise rheology in many biogas plants, as well as to evaluate the effect of viscosity-reducing treatments.