Model and methods for determining mixed liquor suspended solids of aerobic granular sludge reactors

Representative sampling and pipetting of aerobic granular sludge (AGS) is challenging, leading to large inaccuracies when measuring the mixed liquor suspended solids (MLSS) by current methods. This study aimed to develop alternative methods to better quantify the MLSS of AGS reactors. A model and equations derived from the physics of settled AGS beds were developed, resulting in a new method allowing estimating the suspended solids concentrations based on parameters of the sludge layer, including the buoyant granule density, bed volume, and particle segregation coefficient. The study highlighted the importance of accounting for particle stratification for improving the MLSS estimations. Based on measured and/or default values of the model parameters, the proposed equations allow for quickly determining the suspended solids in actual AGS reactors, or predicting the bed size and the sludge volume index in simulated granular systems. A second MLSS measurement method built on good-sampling practices was also evaluated, innovating by grinding the granules and calculating the MLSS and its volatile fraction from the total minus the dissolved solids. The latter was used as reference method providing accurate MLSS values against which the model-based MLSS method could be calibrated and evaluated. Both the model- and the grinding-based methods should allow for improved process monitoring and further progress on outstanding AGS research questions that require accurate MLSS data (e.g., actual sludge yield and resource recovery potential, actual sludge retention times, kinetic parameters of AGS vs activated sludge).