143 - Revealing Internal Details of Anaerobic Digester Mixing with CFD

Anaerobic digesters are commonly used in water resource recovery to convert the organic materials in the end process solids to biosolids and biogas. The digestion process requires complete mixing to maintain an optimal contact between the active biomass in the digester and incoming feed. Different types of mixing systems with a wide range of power inputs are available in the industry. Selecting the proper mixing system for a certain digester system is challenging since oversized mixing systems lead to high construction and operating costs, while undersized systems lead to subpar performance. Understanding the mixing in a more fundamental way can not only reduce the costs in designing the right-size mixing systems but also reduce energy consumption while maximizing biogas production and organics destruction.
Mixing efficiency can be evaluated post construction through tracer testing, temperature profile measurement, and solids distribution measurement. Those approaches can help understand the mixing of existing facilities but cannot help in the design stage. Computational fluid dynamics (CFD) is a promising tool to help designers to understand the mixing of anaerobic digesters prior to construction. CFD allows to “see” inside the tank, visualize the mixing, identify dead spots or areas of over-mixing, and provide an easy platform to customize and compare different technologies or designs. Therefore, it is very helpful in selecting, designing, and optimizing the mixing system. Development of CFD models for different types of mixing systems and comparison of model outputs to actual field-measured data could refine the CFD-aid design tool and then extend to use for digester system design and optimization. This talk will focus on the development of CFD modeling approaches for evaluating digester mixing. A recent new installation in South San Francisco, California, used different mixing technologies and included detailed startup testing, providing ideal data for developing computational fluid dynamics (CFD) modeling approaches to study internal mixing details. In addition, different types of fluid (i.e., Newtonian, and non-Newtonian) were considered in the CFD simulation. Key physics and Model results will be presented with a comparison with the field measurements. This study demonstrated the capability of estimating the time needed to fully mix the digester tank under any operation condition and with any type of fluids, showing that CFD could be a valuable tool for energy consumption optimization.