Abstract
To date, mixing design practice in anaerobic digestion has focussed on biogas production, but no adequate consideration has been given to energy efficiency. A coherent, comprehensive and generalized strategy based on computational fluid dynamics (CFD) modelling is proposed to improve mixing efficiency of a full-scale, unconfined gas-mixed digester for wastewater treatment. The model consists of an Euler-Lagrange (EL) model where biogas bubbles are modelled as the Eulerian dispersed phase, and non-Newtonian sludge as the Lagrangian continuous phase. Robustness tests show that mixing predictions are independent of bubble size. The CFD strategy comprises the assessment of different mixing geometries and a range of input gas flow rates. Quantitative results show that simple retrofitting measures are able to achieve a significant improvement in the degree of mixing with reduced mixing times, and consequently recommendations for best mixing geometry and gas flow rate are given. A generalization to a generic digester is discussed in a form that is readily usable by professionals and consultants.
Highlights
The wastewater industry is expected to face unprecedented challenges over future decades, with worldwide demand for clean water increasing by 50% and food demand by 30% by 2030 (WWAP )
A multiphase model for gas mixing in anaerobic digestion was introduced in Dapelo et al ( ) and later applied in Dapelo & Bridgeman ( a, b)
Validation was performed through comparison of simulated flow patterns (Dapelo et al ) with experimental measurements performed through particle image velocimetry (PIV) (Dapelo et al ) and positron emission particle tracking (PEPT) (Sindall et al )
Summary
The wastewater industry is expected to face unprecedented challenges over future decades, with worldwide demand for clean water increasing by 50% and food demand by 30% by 2030 (WWAP ). The role of anaerobic digestion of sludge from wastewater treatment is relevant here as, whilst methane-rich biogas is produced, mixing is necessary for the digestion process and is responsible for 17–73% of digester energy consumption (Owen ). Recent experimental data (Kress et al ) show that it is possible to reduce input mixing power without compromising nutrient distribution and process performance. Computational fluid dynamics (CFD) has established itself as a powerful tool to investigate mixing in anaerobic digestion.
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