Only recently, Love and Bott100 (2002) hypothesized how different microbial stress responses to oxidative chemicals lead to decreased BOD removals, increased colloidal suspended solids, and affected sludge bioflocculation. Love and Bott100 (2002) suggested that cis-trans isomerization of unsaturated fatty acids (i) decreases substrate active transport through the membrane and increases effluent BOD due to decreased membrane fluidity; (ii) changes the membrane surface chemistry, affecting bioflocculation; and (iii) alters the transport rate of extracellular hydrolytic enzymes, decreasing colloidal material biodegradation. Love and Bott100 (2002) also hypothesized that decreased COD removals are related to redirecting energy from growth to stress protein synthesis, but this hypothesis was discarded when the induction of the stress protein GroEL was not correlated to poor COD removals in sludge deflocculation experiments. Nevertheless, deflocculation due to electrophilic stressors has been correlated to K+ efflux from activated sludge flocs into bulk liquid. Potassium-ions efflux may increase local K+ concentrations inside flocs and increase the monovalent to divalent cation ratio, causing weak floc structure and defloccu-lation. Potassium-ions efflux is a possible stress response mechanism that directly relates to deflocculation.100

Only hypotheses have been put forward for explaining anaerobic granule disintegration. Under a thermophilic temperature shift from 55°C to 65°C, Uemura et al.63 (1995) explained anaerobic granule disintegration by digestion of EPS and microbial cells, and by the selective lysis of acidogens due to the temperature shift. Under a temperature shift from 38°C to 75°C in an UASB reactor, biomass granulation was hindered by the growth in suspension of active methanogens that were eventually washed out.67 Van Lier et al.67 (1992) explained granule disintegration due to a temperature increase from 38°C to 55°C in an UASB reactor by the absence of acidifying bacteria and their metabolites. The anaerobic granules at 55°C showed a spongy appearance both in the core or medulla and the surrounding cortex 4 months after increasing the temperature from 38°C to 55°C, and were observed to eventually wash out from the reactor. The formation of these spongy granules may have been favored by the VFA-based substrate (without sugars).67

The understanding of the mechanisms governing the flocculation of some industrially significant yeast systems is much more advanced than that of bacterial or microbial-community flocs in biotreatment. This derives from the longer history of brewing processes and also the homogeneity of these types of aggregates; yeast flocs are generally formed by yeast cells from the same flocculating species. An extensive and detailed review has been conducted by Calleja,1 who also reports the many reviews on the topic before 1981. Reports on deflocculation of aggregates from brewer's yeast date from 1896, and yeast deflocculation has been experimentally investigated under gravity sedimentation, sugar spikes, heating, low ionic strength, removal of Ca2+, and protein denaturation due to proteases, alkali, or urea. Flocculation of yeast is so well understood that the nature of the binding forces that keep yeast cells together are known, and the effect of physico-chemical parameters have been well characterized.1 For example, flocculating and non-flocculating yeast strains are known, yeast flocculation is known to depend on wall-wall interactions involving proteins and glycoproteins on the protein-manan layer, hydrogen bonds are the principal forces involved in yeast flocculation, and deflocculation is reversible as long as protein denaturation does not occur. The experience and knowledge accumulated through the study of yeast aggregates can likely find parallels in the study of bacterial and mixed community flocs in biotreatment.

Finally, one important aspect that must be remembered when studying defloc-culation is that there is a wide range of floc strength values within activated sludge or anaerobic granules, as work on activated sludge sonication suggests.107 Deflocculation may be related to the fraction of flocs with low floc strength.

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