Part 7 of 8: Temperature
Another important growth pressure is temperature. The growth of microorganisms is a direct result of metabolic re-actions, most of which are catalyzed by enzymes. As with all chemical reactions, temperature has an effect on the rate of these reactions. As the temperature increases, so does the rate of reac tion, and the consequent rate of cell growth and reproduction. As a general rule, the rate of microbial growth doubles with every 10 °C (18F) increase in temperature (up to a temperature limit).
The growth rate slows down at low temperatures, although some microorganisms can exist even slightly below freezing. At certain high temperatures they die from the destruction of enzyme systems. This heat destruction of an enzyme is called denaturation. The enzyme becomes inoperative if some of its chemical bonds are broken or its three-dimensional structure is changed. Again, cells die because they are dependent upon enzymes for essential metabolic reactions.
Microorganisms are classified on the basis of the temperature range in which they have optimum growth. Most microorganisms do best in moderate temperatures, but some types can tolerate, even thrive in, extreme cold or hot.
The majority of ASB microorganisms thrive within a temperature range of 20-35 °C (68-95 °F). These organisms are called mesophyllic microorganisms. They die if the temperature reaches 40-45 °C (104-113 °F). Microorganisms that grow best at high tem peratures (55-65 °C or 13 1-149 °F) are called thermophilic. But even these, with heat-resis tant enzymes, die if the temperature rises above 65 °C (149 °F).
Most microorganisms grow very slowly, if at all, at ex tremely low temperatures. However, a few organisms can withstand temperatures at or slightly below freezing. These are called psychrophilic microorganisms. Remember that growth rate and removal of food from the environment are directly related. Thermophilic and mesophyllic microorganisms remove more organic food from the environment faster than psychrophilic mi croorganisms. This is important to keep in mind in the operation of an ASB when temperature changes from summer to winter.
In pulp and paper aerated stabilization basins, temperature is generally not a controlled parameter. The influent coming from the mill tends to be very hot, in the range of 40 – 50 °C (104 – 120 °F). In systems with mechanical aeration (surface), the aeration equipment also serves as cooling devices, which may result in the bulk water being overly cooled in winter months and adversely impacting BOD conversion. In systems with diffuse aeration, the opposite problem may occur as the compressed air tends to warm the water resulting in limited cooling in summer months and high temperature related inhibition.
EBS has used respirometry to evaluate both high and low temperature impacts on aerated stabilization basins. This information can be used to determine the cost benefit of capital improvements, such as spray coolers or operational strategies, such as bioaugmentation or increased nutrient addition.
This article is adapted from “Aerated Stabilization Basins in the Pulp and Paper Industry” by Paul Klopping and Mike Foster. Copyright 2003. Callan & Brooks. For information on purchasing a copy of this manual, contact EBS.