Summary

In this dissertation several principles of full-scale AGS were explored. In chapter Effluent suspended solids the main processes contributing to elevated effluent suspended solids in the full-scale aerobic granular sludge process were explored. The two most important processes were (1) rising of sludge due to degasification of nitrogen gas (produced by denitrification) and (2) wash-out of particles that intrinsically do not settle such as certain fats and foams. A mathematical model was made to describe the process of degasification of nitrogen gas during the feeding phase in an AGS reactor. The process of rising sludge due to degasification could be limited by stripping out the nitrogen gas before starting the settling phase in the process cycle. The wash-out of scum particles could be reduced by introducing a vertical scum baffle in front of the effluent weir, similar to weirs in traditional clarifiers. The Prototype Nereda^® Utrecht was operated with a nitrogen stripping phase and scum baffles for 9 months at an average biomass concentration of 10 g L^-1 and an average granulation grade of 84 %. In this period the influent suspended solids concentration was 230 ± 118 mg L^-1, while the concentration of effluent suspended solids was 7.8 ± 3.8 mg L^-1.

In chapter Settling behaviour the settling behaviour of AGS is discussed. The settling behaviour of AGS in full-scale reactors is different from the settling of flocculent 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. For this, the Patwardhan and Tien model was used. This model is an adaption of the Richardson and Zaki model, allowing for multiple classes of particles. To create the granular settling model, 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 behaviour of granular sludge beds with uniform granular sludge particles. The obtained parameters were integrated in a model containing multiple granule classes, which was then 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.

Chapter Nitrous oxide emission presents insights on nitrous oxide emissions from AGS. The nitrous oxides emission was measured over 7 months in the full-scale aerobic granular sludge plant in Dinxperlo, the Netherlands. Nitrous oxide concentrations were measured in the bulk liquid and the off-gas of the Nereda^® reactor. Combined with the batch wise operation of the reactor, this gave a high information density and a better insight into nitrous oxide emission in general. The average emission factor was 0.33 % based on the total nitrogen concentration in the influent. The yearly average emission factor was estimated to be between 0.25 % and 0.30 % of the nitrogen load. The average emission factor is comparable to continuous activated sludge plants, and it is low compared to other sequencing batch systems. The variability in the emission factor increased when the reactor temperature was below 14 °C, showing higher emission factors during the winter period. A change in the process control in the winter period reduced the variability, reducing the emission factors to a level comparable to the summer period. Different process control might be necessary at high and low temperatures to obtain a consistently low nitrous oxide emission. Rainy weather conditions lowered the emission factor, both in the rainy weather batches and the subsequent dry weather flow batches. This was attributed to the first flush from the sewer at the start of rainy weather conditions, resulting in a temporarily increased sludge loading.

In chapter On the mechanisms a mathematical framework is presented to describe aerobic granulation based on 6 main mechanisms: microbial selection, selective wasting, maximizing transport of substrate into the biofilm, selective feeding, substrate type and breakage. A numerical model was developed using four main components; a 1D convection/dispersion model to describe the flow dynamics in a reactor, a reaction/diffusion model describing the essential conversions for granule growth, a setting model to track granules during settling and feeding, and a population model containing up to 100.000 clusters of granules to model the stochastic behaviour of the granulation process. With this approach the model can explain the dynamics of the granulation process observed in practice. This includes the presence of a lag phase and a granulation phase. Selective feeding was identified as an important mechanism that was not yet reported in literature. When aerobic granules are grown from activated sludge flocs, a lag phase occurs, in which few granules are formed, followed by a granulation phase in which granules rapidly appear. The ratio of granule forming to non-granule forming substrate together with the feast/famine ratio determine if the transition from the lag phase to the granulation phase is successful. The efficiency of selective wasting and selective feeding both determine the rate of this transition. Breakup of large granules into smaller well settling particles was shown to be an important source of new granules. The granulation process was found to be the combined result of all 6 mechanisms and if conditions for one are not optimal, other mechanisms can, to some extent, compensate. This model provides a theoretical framework to analyse the different relevant mechanisms for aerobic granular sludge formation and can form the basis for a comprehensive model that includes detailed nutrient removal aspects.

This dissertation is finalized in chapter Outlook with an outlook on future developments in AGS technology.