Time reactionphase (h)
Time reaction phase (h)
Time reaction phase (h)

-■—Two days before T shift (Day 32) -e>- One day after T shift (Day 35) -a- Three days after T shift (Day 37)

FIGURE 17.2 Typical SOUR profiles during the reaction phase of an operating cycle of an SBR before and after a temperature shift from 30°C to45°C. (a) SOURs without feed addition; (b) SOURs with 10% dilution with feed (SOUR10%); (c) SOURs with 50% dilution with feed (SOUR50%). The 30°C to 45°C temperature shift took place on Day 34.

lower than the SOUR before the shift. In the control reactor, at a constant temperature of 30°C, the standard SOUR, the SOUR10%, and the SOUR50% remained constant during the experiments. This was consistent with a healthy sludge under limited substrate conditions with a high substrate removal capacity in the control reactor. The control reactor's SOUR10% and the SOUR50% were greater than the standard SOUR, indicating that the sludge biomass was under starvation and had a higher substrate removal capacity.

The SOUR profiles during the reaction phase indicate substrate utilization at the beginning (< 3 h) and starvation at the end (hours 3 to 6 of the reaction phase before the temperature shift; Figure 17.2). The substrate uptake profiles during the reaction phase of the SBRs prior to the temperature shifts were consistent with the standard SOUR profiles. During the reaction phase before the shift, the SOURs10% and SOURs50% were higher than the standard SOURs consistent with higher oxygen requirements associated with increased feed. Higher SOURs10% and SOURs50% than SOURs describe a healthy sludge able to utilize substrate to which it is acclimated. At the end of the reaction phase, the SOURs10% tended to decrease slightly (Figure 17.2b); however, the SOURs50% increased significantly (Figure 17.2c), indicating a high substrate removal capacity by starving sludge. Starving conditions are inherent to SBR operation13 and microbial internal decay or endogenous respiration is associated with these conditions.35

The set of three SOURs measured during the reaction phase of the SBRs under the temperature transient showed the inhibition of the sludge substrate removal capacity and increased respiration rates during the endogenous respiration phase. The temperature stress caused the SOUR to decrease at the beginning of the reaction phase and to increase at the end of the reaction phase, relative to the control (Figure 17.2a). This suggests that increased respiration rates are a stress response of the microorganisms to the shift from 30°C to 45°C. These increased respiration rates are not indicative of higher substrate uptake rates, as demonstrated by the reduced SOUR10% and SOUR50%. The increased respiration rates relative to the control occurred only at the end of the reaction phase when part of the available substrate had been utilized, and the sludge is not at its highest substrate removal capacity. This is in agreement with the higher standard SOUR measured after the temperature shift in the transient reactor compared to the standard SOURs in the control in batch experiments (Figure 17.1a).

The activated sludge floc responses and how they could relate to sludge defloc-culation and impaired treatment performance are presented in Figure 17.3. The link between increased maintenance respiration rates, more negative sludge surface charge, and sublethal metabolic inhibition, and sludge deflocculation and treatment performance can only be hypothesized, and remains a research area in need of further investigation.

Increased maintenance respiration rates can be explained by increased microbial maintenance requirements as a sublethal-inhibition stress response to the temperature shifts. High maintenance requirements and intense metabolism (high biochemical activity) have been associated with thermophilic organisms.110-112 It is probable that microorganisms under a heat shock stress may induce an active metabolism to be able to repair damage by heat.111 Misfolded protein detection is considered to be a mechanism ofheat shockresponse initiating protein folding, degradation, and resynthesis,113 processes which are known to be maintenance functions.114 Variations in microbial energy maintenance requirements occur via energy loss mechanisms; energy spilling is a mechanism considered to protect cells from toxicity resulting from sudden sugar

Mesophilic-thermophilic temperature shift

Sludge response

Increased maintenance respiration rates

Decreased sludge charge

Metabolic inhibition .

Release of Mn and Fe

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