—Jenkins, et al.
Aeration basin temperatures above 35 to 40°C can often cause dispersed growth of floc-forming and filamentous organisms (Norris et al., 2000; Parks et al., 2000). This is an increasing problem in many industrial wastewater treatment plants in which water conservation practices reduce effluent volume without reducing process heat losses, thereby increasing wastewater temperatures. A common observation is the occurrence of episodes of dispersed growth of single bacteria and dispersed filaments, high effluent turbidity, and loss of floc strength as the aeration basin temperature increases from below 35°C [95°F] to above this value. The dispersed growth and effluent turbidity often subside after a few days as a new thermotolerant floc-forming bacteria develop. Dispersed growth episodes occur also as the temperature decreases through this range, perhaps because the thermotolerant floc formers wash out of the system as they are replaced by mesophilic floc formers. For this reason, in activated sludge systems operated at high temperatures (>35°C), it is important to limit temperature variations as much as possible.
Source: Jenkins, David, Michael G. Richard, and Glen T. Daigger. Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems. 3rd ed. Boca Raton: CRC Press, 2004. (See page 65)
p. 34, Table 4.1: Temperatures >32°C (89.6°F) interrupt floc formation
With increasing wastewater temperature, bacterial activity increases. Increased production and accumulation of insoluble biological secretions such as lipids and oils accompany this increase in activity. These secretions are adsorbed or entrapped by the floc particles, resulting in a decreased settling rate of secondary solids. When air bubbles or gases become entrapped in these secretions, the settling rate of the secondary solids decreases more.
Figure 19.1 Impact of temperature upon the activated sludge process. Changes in wastewater temperature have a significant impact upon the activity of all organisms, floc particle structure, and the rate of floc formation.
Because increasing wastewater temperature and increased bacterial activity are critical factors that affect secondary solids settleability, a reduction in MLVSS concentration during warm wastewater temperature may be useful in preventing settleability problems and loss of solids. By reducing the MLVSS concentration, the amount of biological secretions that are produced and accumulated in floc particles is reduced.
If it is not possible to reduce the MLVSS concentration, alternate corrective measures are available to improve settleability. Bioaugmentation products that have bacteria with the enzymatic ability to degrade the biological lipids and oils that are produced during warm wastewater temperature may be added to the aeration tank. The addition of a metal salt or polymer to the secondary clarifier influent to add weight to floc particles or improve floc density may be used.
Source: Michael H. Gerardi. Settleability Problems and Loss of Solids in the Activated Sludge Process. Hoboken, NJ: John Wiley and Sons, 2002.
The adaptation of the microorganisms to abrupt temperature changes seems to be much slower at higher temperatures. For example, it was observed that several months would be needed for the acclimatization of the biomass to a change of 5°C in the temperature range of 30°C [86°F], while only 2 weeks were necessary for a similar adaptation in the range of 15°C [59°F].
Source: von Sperling, Marcos. Activated Sludge and Aerobic Biofilm Reactors. London: International Water Association, 2007. (See page 59)
Temperature affects all biological processes. Biological oxidation rates increase to a maximum at about 95°F (35°C) for most treatment systems. At temperatures greater than 95°F, treatment efficiency decreases by reducing bacterial floc formation. Temperatures in excess of 99°F (37°C) show a definite effect on biological systems. It is possible, however, in certain wastes, to operate efficiently at somewhat higher temperatures. Lower temperatures than 50°F (10°C) also affect performance of biological processes, especially nitrification efficiency.
The rate of biological activity is influenced by temperature because of the depth of penetration of oxygen into the floc or film. Oxygen penetration increases as temperature decreases, since oxygen is not used as quickly at floc surfaces and greater numbers of organisms per unit surface can react. Oxygen solubility also increases as temperature decreases.
Source: Nalco Company. The Nalco Water Handbook. 3rd ed. New York: McGraw-Hill, 2009. (See page 23.13)