Shear and solid-liquid separation behaviour of anaerobic digested sludge across a broad range of solids concentrations

Abstract

Due to the non-homogeneous and multiphase nature of anaerobic lagoon constituents, CFD modelling for process optimisation requires continuous functions for shear and solid-liquid separation properties across a large range of solids concentrations. Unfortunately, measurement of existing material properties of anaerobic sludges is limited to only shear or solid-liquid separation, or to a limited solids concentration. In this work, the shear properties of an anaerobic sludge were measured from 0.4 to 12.5 vol%, which corresponds to the solids concentrations seen in lagoons. The sludge showed Newtonian behaviour at 0.4 vol% and Herschel-Bulkley yield stress fluid behaviour for higher concentrations ranging from 0.5 to 12 vol%. We compared multiple approaches to determine relationships between the model fitting parameters of consistency, k, flow index, n, and shear yield stress, τy with solids volume fraction ϕ.The solid-liquid separation properties were measured from sedimentation and filtration experiments to obtain compressibility and permeability properties across all the above-mentioned concentrations, enabling development of hindered velocity sedimentation curves. Comparison to full-scale anaerobic digestate identified that the pilot lagoon sludge had faster sedimentation at a given solids concentration in comparison to the digestate. This is the first study on simultaneous rheological characterisation and solid-liquid separation behaviour of an anaerobic sludge across a wide range of concentrations, thus enabling CFD modelling of the hydrodynamics and performance of anaerobic lagoons.