The temperature of the wastewater entering a municipal wastewater treatment plant is usually not a concern. But for industrial plants the wastewater temperature can be of great concern such that, when too high, it can have an adverse impact on the microorganisms in the activated sludge system.
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 industrial wastewater systems cannot maintain their wastewater at or below this temperature. Nonetheless, the literature is 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.
Industrial operations can often make for difficult conditions in the wastewater treatment system. There can be large variations in flow, organic loading, pH, nutrient value (insufficient nitrogen and/or phosphate), and temperature. 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, contribute to poor solids settling in the secondary clarifiers.
Three pages of references, from a number of wastewater textbooks and research articles, describing the temperature ranges you can expect to encounter in an industrial biological wastewater treatment system are provided. 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 a biological reactor. Links to two research articles are given to provide additional material and further insight into the evaluation of the temperature ranges found in wastewater treatment.
Bacteria fall into one of four temperature classifications or ranges as follows: 1) psychrophilic, 2) mesophilic, 3) thermophilic, and 4) extreme thermophilic or hyperthermophilic. As you look through the referenced material that follows you’ll see there is both agreement and disagreement regarding these four classifications. It is agreed that the microbial populations in aerobic biological wastewater treatment systems fall into the Mesophilic temperature range. What is surprising 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, the upper limit is actually closer to 90°F (32.2°C). Regardless, when we are providing chemicals to wastewater treatment plants with biological systems operating at temperatures above 95°F, it should be recognized that we are dealing with a stressed biomass that will tend to be more dispersed. Stated another way, the floc-forming ability of the bacteria will be greatly diminished resulting in poor settling in the clarifier, as previously stated.
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 organisms will slow down, even die off, and another group will gain influence. 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 microbial populations 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.
Microorganism Optimization as a Function of Temperature