Abstract
The settling behavior of aerobic granular sludge (AGS) in full-scale reactors is different from the settling of normal activated sludge. Current activated sludge models lack the features to describe the segregation of granules based on size during the settling process. This segregation plays an important role in the granulation process and therefore a better understanding of the settling is essential. The goal of this study was to model and evaluate the segregation of different granule sizes during settling and feeding in full-scale aerobic granular sludge reactors. Hereto the Patwardhan and Tien model was used. This model is an implementation of the Richardson and Zaki model, allowing for multiple classes of particles. To create the granular settling model, the most relevant parameters were identified using aerobic granular sludge from different full-scale Nereda® reactors. The settling properties of individual granules were measured as was the bulk behavior of granular sludge beds with uniform granular sludge particles. The obtained parameters were combined in a model containing multiple granule classes, which then was validated for granular sludge settling in a full-scale Nereda® reactor. In practice a hydraulic selection pressure is used to select for granular sludge. Under the same hydraulic selection pressure the model predicted that different stable granular size distributions can occur. This indicates that granular size distribution control would need a different mechanism then the hydraulic selection pressure alone. This model can be used to better understand and optimize operational parameters of AGS reactors that depend on granular sludge size, like biological nutrient removal. Furthermore insights from this model can also be used in the development of continuously fed AGS systems.
Highlights
In biological wastewater treatment, the liquid/solid separation through gravity induced settling is an important step in producing clean effluent (Henze et al, 2008)
This study showed that the terminal settling velocity of aerobic granular sludge can be well described by a model based on Newtonian drag
A multi-dispersed settling model was made describing the settling and fluidization of aerobic granular sludge. Validation of this model with measurements in a full-scale AGS reactor showed a good resemblance between model and measurements
Summary
The liquid/solid separation through gravity induced settling is an important step in producing clean effluent (Henze et al, 2008). For the design of secondary clarifiers, a good understanding of the settling behavior of activated sludge is necessary and both design guidelines (Henze et al, 2008) and dynamic models (Takacs et al, 1991; Vesilind, 1968) are readily available. These guidelines generally use the sludge volume index or the zone settling velocity as input parameters for describing the settling behavior of activated sludge. These empirical relations can be used to dynamically describe the settling of sludge in secondary clarifiers, thereby ensuring proper design and operation of the conventional activated sludge plants
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