| Temperature transients in biological treatment processes are common and related to poor performance from system instability. In this research, the effects of mesophilic-thermophilic temperature transients on activated sludge were studied in continuous sequencing batch reactors (SBRs) and batch experiments. Long-term (6 months) continuous experiments were conducted to identify the effects of controlled temperature shifts within 30° to 50°C on activated sludge biodegradation capacity, bioflocculation characteristics, microbiology, and solids discharges. Sludge deflocculation was characterised and quantified under temperature shifts from 30° to 45°C in batch experiments (<9 h) with municipal activated sludge. Different operating strategies to promote stronger sludge flocs and minimise deflocculation were tested in a second set of continuous experiments in the SBRs.; Overall, temperature shifts from 30–35° to 45°C were shown to increase the effluent suspended solids (ESS) levels above 25–100 mg/L, to increase turbidity, and to decrease the soluble chemical oxygen demand (SCOD) removals approximately 20%. Temperature transients (within 35° to 50°C) and periodic oscillations (from 31.5° to 40°C, 6-day period, 30 days) were associated with poor sludge settleability [sludge volume index (SVI) > 100 mL/g MLSS; zone settling velocity (ZSV) < 1 cm/min], more negatively charged sludge (up to −0.35 ± 0.03 meq/g MLSS), and filament proliferation. Sludge deflocculation was demonstrated to explain the increased levels of ESS, SCOD and turbidity in the effluent, and to occur via floc fragmentation and solubilisation of extracellular polymeric substances [proteins, carbohydrates, humic substances, and deoxyribonucleic acid (DNA)]. The sludge flocs became structurally weaker or less stable due to the temperature upshifts.; Although the manipulation of the sludge physico-chemical properties by sludge magnesium enrichment and a high sludge age of 33 days increased floc stability, these operating strategies and spikes of an easily degradable substrate (methanol) were unsuccessful in deterring sludge deflocculation from occurring completely. Sludge deflocculation mechanisms remain unclear, but some of them appear related to microbial physiological stress responses involving increased respiration rates, reduced substrate removal capacities, and more negatively charged sludge flocs. |
