Wastewater Temperature in the Bioreactor
Operators know that when the temperature of the wastewater entering the bioreactor is too high, it can have an adverse impact on the microorganisms in the activated sludge system. How high a temperature is too high? There are many opinions on this subject so I thought I would combine my own experience with a thorough review of the literature as detailed in the sections that follow.
Recommended Wastewater Temperature
My personal recommendation is that the maximum temperature of the wastewater entering a biological reactor should be < 95°F (35°C). It is to be understood that many wastewater treatment systems cannot maintain their wastewater at or below this temperature. Nonetheless, the literature seems to be consistent in setting 95°F as the upper limit, beyond which the operation of the biological system and solids settling in the clarifiers will begin to suffer.
There are many factors that can create difficult conditions in a wastewater treatment system. These factors can range from large variations in flow, to excessive organic loading, swings in pH, nutrient deficiencies (insufficient nitrogen and/or phosphate), and excessive temperatures (low or high). The biological systems themselves are often smaller than they should be because they were originally designed to treat smaller volumes of wastewater and/or smaller concentrations of organic compounds. In addition, biological systems are frequently undersized in terms of oxygen generation capability, resulting in chronically low dissolved oxygen levels. All of these conditions, individually or in combination, add stress to the microorganisms in the bioreactor, creating less than optimal oxidation of organics, poor settling of solids in the secondary clarifiers, and the potential for odor generation and migration of odors to residents which can lead to complaints.
Literature Review Conducted
I have an extensive wastewater library and I methodically went through every textbook looking for information on temperature. The results of that literature review are summarized below. The references are not listed in any particular order. You are encouraged to look through the information to draw your own conclusion as to what the maximum recommended temperature should be in your biological reactor.
Bacteria Temperature Ranges
Bacteria fall into one of four temperature classifications or ranges as follows: 1) psychrophilic, 2) mesophilic, 3) thermophilic, and 4) extreme thermophilic or hyperthermophilic. The graphic and table provided below detail these temperature ranges. As you look through the referenced material that follows you will see there is both agreement and disagreement regarding these four classifications. What is agreed is that microorganisms in aerobic biological wastewater treatment systems fall into the Mesophilic temperature range. What surprised me is the disagreement about the actual temperature ranges that define each classification (hyperthermophiles excluded).
Fortunately, there is broader agreement about the upper temperature limit that establishes the boundary for optimal conditions in a biological wastewater treatment system. This boundary occurs at 95°F (35°C). In my personal experience evaluating and auditing wastewater systems servicing “warm” industries such as refinery, chemical, steel, and paper, and municipal plants in warm climates, the upper limit for optimal treatment conditions is actually closer to 90°F (32.2°C).
Diversity of Bacterial Populations
A biological reactor does not contain a single, identical, bacterial population. There are numerous groups of microorganisms with dominance among any group constantly shifting in response, or adaptation, to the constantly changing composition and quality of the wastewater. As the temperature changes in the wastewater, one group of microorganisms will slow down, even die off, and another group will gain influence and become dominant. This diversity and adaptability in the microbial population continues up to a wastewater temperature of 95°F. As the temperature rises above 95°F the combined ability of the various microorganisms is diminished, as shown in the graph below, and optimal conditions in the biological system are lost. The result is reduced treatment capacity which will be reflected in higher effluent organic and total suspended solids values as measured in the secondary clarifier overflow.
Literature Review of Temperature Effects
in Biological Wastewater Treatment
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)
—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)