Variations in temperature affect all biological processes. There are three temperature regimes: the mesophilic over a temperature range of 4 to 39°C [39.2 to 102.2°F], the thermophilic which peaks at a temperature of 55°C [131°F], and the psychrophilic which operates at temperatures below 4°C [39.2°F]. For economic and geographical reasons, most aerobic biological treatment processes operate in the mesophilic range, which is shown in Figure 6.23. In the mesophilic range, the rate of the biological reaction will increase with temperature to a maximum value at 31°C [87.8°F] for most aerobic waste systems. A temperature above 39°C will result in a decreased rate for mesophilic organisms.
At temperatures above 96°F (35.5°C) there is deterioration in the biological floc. Protozoa have been observed to disappear at 104°F (40°C) and a dispersed floc with filaments to dominate at 110°F (43.3°C).
In the past, hot wastewaters such as those in the pulp and paper industry were pretreated through a cooling tower so that the aeration basin temperature did not exceed 35°C [95°F].
Figure 6.23: Effect of temperature on biological oxidation rate constant K.
Source: Eckenfelder, W. Wesley, Jr. Industrial Water Pollution Control. 3rd ed. Boston: McGraw-Hill, 2000. (See pages 240−245)
—Water Environment Federation
Seasonal variations in temperature can markedly influence the makeup of microbial communities. Just as with pH, each species is characterized by a minimum, optimum, and maximum temperature that will support growth. Psychrophiles grow within the range of 0 to 20°C (32 to 70°F), with an optimum of 10 to 15°C (50 to 60°F). Mesophiles, which comprise most of the species commonly found in wastewater treatment processes, grow within the range of 10 to 45°C (50 to 115°F), with an optimum of approximately 30 to 35°C (85 to 95°F). Thermophiles, found in compost piles and other high-temperature environments, grow within the range of 40 to 75°C (105 to 165°F), with an optimum growth rate at 55 to 65°C (130 to 150°F). A few species of heterotrophic bacteria, classified as extreme thermophiles, can grow at temperatures higher than 100°C (212°F). These organisms live in highly specialized environments, such as geothermal vents in the ocean floor, and have not been implicated in wastewater treatment processes.
Source: Water Environment Federation. Wastewater Biology: The Life Process. Alexandria, VA: Water Environment Federation, 1994. (See page 74)
—Metcalf & Eddy
Optimum temperatures for bacterial activity are in the range from 25 to 35°C [77 to 95°F]. Aerobic digestion and nitrification stops when the temperature rises to 50°C [122°F]. When the temperature drops to about 15°C [59°F], methane-producing bacteria become quite inactive, and at about 5°C [41°F], the autotrophic-nitrifying bacteria practically cease functioning. At 2°C [35.6°F], even the chemoheterotrophic bacteria acting on carbonaceous material become essentially dormant.
Source: Metcalf & Eddy. Wastewater Engineering: Treatment and Reuse. 4th ed. Boston: McGraw-Hill, 2003. (See page 55)
—Grady, et al.
Temperature affects the performance of activated sludge systems as a result of its impact on the rates of biological reactions. Procedures for estimating the magnitudes of its effects are presented in Section 3.9. Two additional factors must be considered: the maximum acceptable operating temperature and the factors that affect heat loss and gain by the process.
The maximum acceptable operating temperature for typical activated sludge systems is limited to about 35° to 40°C [95 to 104°F], which corresponds to the maximum temperature for the growth of mesophilic organisms. Even short-term temperature variations above this range must be avoided since thermal inactivation of mesophilic bacteria occurs quickly. Successful operation can also be obtained if temperatures are reliably maintained above about 45° to 50°C [113 to 122°F], since a thermophilic population will develop, provided that thermophilic bacteria exist with the capability to degrade the wastewater constituents. Unacceptable performance will result for temperatures between about 40° and 45°C due to the limited number of microorganisms that can grow within this range. These considerations are particularly important for the treatment of high temperature industrial wastewaters.
One of the factors that affect heat gains in biological processes is the production of heat as a result of biological oxidation. As discussed in Section 2.4.1, the growth of bacteria requires that a portion of the electron donor be oxidized to provide the energy needed for biomass synthesis. Energy is also needed for cell maintenance. This oxidation and subsequent use of the energy results in the conversion of that energy into heat. Although this may seem surprising at first, it is directly analogous to the release of energy that occurs when material is burned; the only difference is the oxidation mechanism. The amount of heat released in the biooxidation of carbonaceous and nitrogenous material is directly related to the oxygen utilized by the process. For each gram of oxygen used, 3.5 kcal of energy are released. Since 1 kcal is sufficient energy to raise the temperature of one liter of water 1°C, the impact of this heat release depends on the wastewater strength. For example, a typical domestic wastewater requires only one gram of oxygen for each 10 liters treated, therefore the temperature rise would be only 0.35°C, a negligible amount. On the other hand, it is not unusual for an industrial wastewater to require one gram of oxygen for each liter treated, in which case the temperature rise would be 3.5°C. This could be quite significant, particularly if the wastewater itself is warm.
Other heat gains and losses occur in biological systems. Heat inputs to the system include the heat of the influent wastewater, solar inputs, and mechanical inputs from the oxygen transfer and mixing equipment. Heat outputs include conduction and convection, evaporation, and atmospheric radiation.
Source: Grady, C.P. Jr., Glen T. Daigger, and Henry C. Lim. Biological Wastewater Treatment. 2nd ed. New York: Marcel Dekker, 1999. (See pages 407−408)