Temperature

Temperature exerts two significant effects upon a bacterial population. First, it affects the rate of diffusion of substrates and nutrients into bacterial cells. Second, it affects the rate of enzymatic activity. With increasing temperature the rate of diffusion of substrates and nutrients into bacteria cells increases, and the rate of enzymatic activity increases. The rates for diffusion and enzymatic activity decrease during decreasing temperatures.

Therefore, with increasing bacterial activity during warm wastewater temperatures, an operator of a wastewater treatment plant can decrease solids (bacteria) inventory and still maintain acceptable treatment of wastewater. However, with decreasing bacterial activity during cold wastewater temperatures, an operator of a wastewater treatment plant may need to increase solids inventory in order to maintain acceptable treatment of wastewater.

The impact of temperature upon bacterial activity is significant. For every 10°C rise in temperature, enzymatic activity nearly doubles. However, once the optimum temperature for enzymatic activity and cellular growth has been exceeded, enzymes become denatured (damaged) and can no longer efficiently catalyze biochemical reactions.

The temperature range of growth for an organism is determined by acceptable enzymatic activity in that range. The temperature range consists of three critical values. These values are (1) the minimum temperature that permits cell division, (2) the maximum temperature that permits cell division, and (3) the optimum temperature that permits the most rapid cell division.

There are three groups of bacteria with respect to the minimum and maximum temperatures that they remain active. These groups are psychrophiles, mesophiles, and thermophiles (Figure 3.7). Psychrophiles are cold-loving organisms and grow best at temperatures from 12°C to 18°C. Their range of temperatures for growth is from -10°C to 30°C. Psychrophiles are able to grow at low temperatures, because

FIGURE 3.7 Temperature ranges of growth and generation rates for psychrophiles, mesophiles, and thermophiles.

FIGURE 3.7 Temperature ranges of growth and generation rates for psychrophiles, mesophiles, and thermophiles.

their cell membrane contains a large quantity of unsaturated fatty acids. These acids help to maintain membrane fluidity at cold temperatures.

Mesophiles consist of the largest group of organisms and grow best at temperatures from 25°C to 40°C. Their range of temperatures for growth is 20°C to 50°C. Mesophiles are common inhabitants of the gastrointestinal tract of humans (body temperature approximately 37°C) and enter wastewater treatment plants in large numbers in human feces. They are present in very large numbers in the activated sludge process and the mesophilic anaerobic digester.

Thermophiles are heat-loving organisms and grow best at temperatures from 50°C to 65°C. Their range of temperatures for growth is from 35 to 75°C. Thermophiles are common inhabitants of thermophilic anaerobic digesters and thermophilic composting operations. Thermophiles are able to grow at high temperatures, because their cell membrane contains a large quantity of saturated fatty acids. These acids help to maintain membrane fluidity at high temperatures.

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