Effects of mixing regimes and inert solid suspension on sewage sludge anaerobic digestion performance through experimental study and CFD modelling

• Main Author: Nogueira Filho, Jose Carlos
• Format: Dissertation
• Language: English
• Published: University of Birmingham 2019
• Subjects: TA Engineering (General). Civil engineering (General); TD Environmental technology. Sanitary engineering

Anaerobic digestion is a widespread technology that is used to treat sewage sludge from wastewater. Mixing has been proven to be a key element for a well-performed digester but it is a power-intensive operation whose quantitative contributions to the system's energy-efficient balance in different designs is not well understood. In order to cut costs, operators try to keep mixing to minimum levels where biogas production is not adversely affected. However, sewage sludge inherently contains inert dense particles, such as grit, that cannot be biodegraded and tend to accumulate at the bottom of the digester. For the first time, the impact of inert particle suspensions on biogas production was evaluated at a range of mixing speeds and particle loadings in mechanically-mixed, laboratory-scale digesters. Five digesters were operated for a total period of five months at different speeds and particle loadings whilst digestion stability, biogas production and treatment quality were monitored. Digesters were fed with real sewage sludge samples six days a week. Computational fluid dynamics models were used to assess the liquid-solid flow characteristics. It has been shown that inert particles in suspension at highly-intensive mixing detrimentally affects anaerobic digestion performance and biogas yields. Low-speed mixing does not suspend inert particles but the digesters are not affected by their presence. Short-term, highly-intensive intermittent mixing, along with continuous low intensive mixing, resulted in particle suspension, stable digestion and biogas production; this being the best energy-efficient mixing regime. Computational fluid dynamics has successfully simulated sedimentation behaviour as well as particle solid distribution and cloud height within the liquid column. The liquid flow patterns change with increased particle loadings.