Analysis of Retention Time Distribution (RTD) curves in an anaerobic digester with confined-gas mixing using a compartment model

Abstract

Stimulus-response experiments were conducted in a continuous vessel with a single, centrally mounted draft-tube. The obtained retention time distribution curves were then analysed using conventional methods. These resulted in contradictory interpretation of the experimental data. To overcome this problem, a compartment model was developed to represent the mixing behaviour of this type of system. This mixing model consists of an ideally mixed compartment around the uptake tube and two cascades of tanks-in-series describing the circulation flow. The adjustable parameters of the model are the volume of the ideally mixed compartment, the circulation time and the number of tanks in each circulation loop. These parameters were estimated by fitting simulations to experimental mixing curves. The model consistently described the improvement of mixing with volumetric power input. Furthermore, the correlations found between power input and model parameters allowed the prediction of independently measured pulse-response curves rather well. The proposed compartment model brings the prospect of coupling microbial kinetics to reactor hydraulics to quantify the impact of mixing on the performance of anaerobic sludge digesters.