Ecological Models

From microscopic examination and activity measurements, Dubourgier et al. (1987) suggested that granulation mechanism starts by the covering of filamentous Methanothrix by colonies of cocci or rods (acidogenic bacteria), forming microflocs of 10-50 |xm. Subsequently, Methanothrix filaments, due to its filamentous morphology and surface properties, might establish bridges between several microflocs forming larger granules of size greater than 200 |xm. Further development of acidogenic and syn-trophic bacteria favors the granules growth. The authors support the idea that Methanothrix plays a vital role in enhancing granule strength by forming a network that stabilizes the overall structure. The role of extracellular polymers and cell walls are also emphasized.

Morgan et al. (1991a,b) suggested that granules are developed from a precursor that consists of a small aggregate of Methanothrix and other bacteria. Growth of the Methanothrix filaments form distinctive bundles separated by a surrounding matrix in which other methanogenic and non-methanogenic bacteria are embedded. As the bundles increase in size, the surrounding matrix is excluded leading to a region towards the center of the granule, which consists exclusively of compact filaments of Methanothrix and where discrete bundles are not distinguishable. Thus, the authors support previous suggestions on the importance of Methanothrix and bacterial polymers in the growth of the granules.

From the research developed in 1980s, de Zeeuw (1988) explains the formation of three types of granules developed in laboratory UASB reactor start-up experiments using digested sludge as inoculum and VFA as substrate. Methanothrix and Methanosarcina seem to be of predominant significance for granule formation. The characteristics of the formed granules were described as follows:

(A) Compact spherical granules mainly composed of rod-shaped bacteria resembling Methanothrix soehngenii in short chains or single cells (rod-granules).

(B) More or less spherical granules mainly consisting of loosely intertwined filamentous bacteria attached to an inert particle (filamentous granules). The prevailing bacteria resembled Methanothrix soehn-genii.

(C) Compact spherical granules composed predominantly of Methano-sarcina-type bacteria (Fig. 1.3).

The development of each type of granular sludge was explained on the basis of seed sludge selection and sludge bed erosion and expansion, and the consequent differences in selection pressure and mean sludge

Fig. 1.3. Aggregate of Methanosarcina present at the bottom of a UASB reactor.

residence time. Methanosarcina granules develop due to the capacity of this genus to produce clumps of bacteria independently of the selection pressure. The clumps can reach macroscopic dimensions and show cavities, which can be inhabited by other species (Bochem et al., 1982). However, this kind of granules were just found in experiments where the concentration of acetate as a sole substrate was maintained above 1000gCOD/m3, which means that Methanosarcina was able to outcompete Methanothrix (de Zeeuw, 1984).

At the low loading rates (low selection pressure) applied during the initial phase of a UASB reactor start-up, Methanothrix filaments will grow in and on small flocs present in the seed sludge leading to the formation of a "bulking" anaerobic sludge.

When a high selection pressure is applied, Methanothrix, that has a high affinity to attach to all kind of surfaces (van den Berg and Kennedy, 1981), attach onto carrier materials originating from the seed sludge or from the wastewater itself forming filamentous granules (type B).

More compact Methanothrix granules (rod granules, type A) are thought to be formed by the colonization of the central cavities of Methanosarcina clumps by Methanothrix bacteria, which have a higher acetate affinity, eventually leading to a loss of the outer layer of Methanothrix. Another explanation for these rod-type granules can be the filling of the filamentous granules with more bacteria, leading to a more compact Methanothrix granule.

The development of A or B type granules is related to the mean cell residence time maintained in the start-up process. When the mean cell residence time is too short, the opportunity to form compact granules consisting almost exclusively of biomass is slim. This means that large conglomerates of bacteria can only be formed through attachment to inert carriers, which must be heavy enough to be retained in the reactor (type B). Compact bacterial granules (type A) would only be formed if the mean cell residence time is sufficiently long.

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