What are sludge granules? Sludge granules are at the core of UASB and EGSB technology. A sludge granule is an aggregate of microorganisms forming during wastewater treatment in an environment with a constant upflow hydraulic regime.  In the absence of any support matrix, the flow conditions creates a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Eventually the aggregates form into dense compact biofilms referred to as "granules" (see Figure 1 below). Due to their large particle size (generally ranging from 0.5 to 2 mm in diameter) , the granules resist washout from the reactor, permitting high hydraulic loads. Additionally, the biofilms are compact allowing for high concentrations of active microorganisms and thus high organic space loadings in UASB and EGSB reactors. One gram of granular sludge organic matter (dry weight) can catalyze the conversion of 0.5 to 1  g of COD per day to methane. In layman terms that means on a daily basis granular sludge can process its own body weight of wastewater substrate.

Granulation Process: The process of granular sludge formation is one of the most interesting and enigmatic questions when attempting to understand the fundamentals of anaerobic granular sludge technology. This topic has fueled many PhD research projects. There are many theories, ranging from extracellular polysaccharide slime to calcium as key players in the initial aggregation process. However, the most promising theory is the "spaghetti" theory (proposed by Dr. W. Wiegant) in which filamentous microorganisms become entangled in one another analogous to the formation of fungal pellets  as shown in Figure 2 below. In support of theory is the fact that the methanogens known as Methanosaete, which are better adapted for low substrate concentrations (a condition desired for wastewater treatment), happen to be filamentous microorganisms. The initial pellets ("spaghetti balls") of Methanosaete can serve as a surface of attachment or support matrix for other microorganisms involved in the anaerobic degradation process. For the attachment of diverse microorganisms to the pellet, perhaps slime layers and calcium may play an important role.

Look Inside a Granule: Each granule is an enormous "metropolis of microbes" containing billions of individual cells and perhaps thousands to millions of different species. Follow the link in  Figure 3 below to a slide show which takes you inside a granule to take a closer look at the microorganisms inside.

Settling Properties: According to Stoke's law, sedimentation rates are a function of particles size squared. Due to their large particle sizes, anaerobic sludge granules have exceptional settling properties. The rapid settling velocities permits the application of high hydraulic loads to UASB and EGSB reactors without having to be concerned about wash-out of biologically active sludge particles (responsible for the bioconvresions).  Because high hydraulic loads are tolerated, UASB and EGSB systems can handle wastewater streams with relatively low concentrations of substrate, even as low as a few hundred milligrams COD per liter (previously considered impossible for anaerobic treatment). As is illustrated in Figure 4,  granular sludge settles extremely rapidly and is completely clarified within a few minutes. By comparison dispersed sludge (like that from an anaerobic digester at a municipal treatment plant) has not even begun to clarify in the same time scale. Flocculent sludge, also clarifies rapidly but not as fast and as granular sludge.

v = 2r2g(d-D)/9N

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