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FIGURE 17.3 Sludge floc responses, sludge deflocculation, and treatment performance effects of a temperature shift from 30°C to 35°C to 45°C, as identified in research.109

metabolism (as from glucose pulse doses).114 The actual mechanisms of this extra energy spilling remain unclear and similar microbial energetic mechanisms could be triggered by heat stress.

The generation of negative sludge charge due to the temperature shift108,109 can have different causes. The adsorption of solubilized EPS or lysed soluble microbial products bearing negative charges (e.g., sulfate-, hydroxyl-, and carboxyl-containing polysaccharides, polypeptides, and humic substances) onto remaining sludge flocs could have rendered the sludge more negatively charged. EPS biopolymers with anionic functional groups tend to decrease the sludge surface charge, as experimentally shown by Mikkelsen et al.115 (1996) using an anionic detergent. This adsorption process is suspected to be collateral to a major cause driving the more negatively charged sludge. The decrease in sludge surface charge is not an immediate response and it takes from 1-2 to 8 days to manifest itself.108

The decrease in sludge surface charge may arise from bacterial physiological stress responses. The degree of unsaturation of fatty acids in the cytoplasmic membrane is thought to control the cellular stress response to temperature changes.116 Heat shocks could increase cytoplasmic membrane flexibility, which could trigger microorganisms to increase the levels of saturated fatty acids in the membrane phos-pholipids to decrease membrane fluidity and cope with increasing membrane fluidity. Saturation of phospholipids could be achieved by negatively charged groups that render the cell surface more negatively charged. An increase in saturated fatty acids in the cytoplasmic membrane, as a primary heat shock sensor, has been correlated to the heat shock induction of messenger RNA (mRNA) transcripts in yeasts.117 Gaughran110 (1947) reported experiments showing the occurrence of more saturated fatty acids in bacteria (Bacillus subtilis) and moulds (Aspergillus niger) at higher

Weakened floc structure/ deflocculation

EPS solubilization

Floc fragmentation

Microbial community change

Filament abundance

EPS solubilization

Floc fragmentation

Microbial community change

Filament abundance

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