Wastewater temperature is important for two reasons: (1) biological processes are temperature dependent and (2) chemical reactions and reaction rates, as well as aquatic life are all sensitive to even minor temperature fluctuations.
The temperature is very important in biological wastewater treatment systems because of its effects on microbial growth. While most microorganisms are able to exist over a broad temperature range, there is usually an optimum temperature at which each species grows best. In treatment plants, a slow adaptation occurs due to seasonal changes in temperature. However, sudden changes in temperature affect the microbial activity which might affect processes like flocculation due to changed surface properties of the microbial cells.
The best temperatures for wastewater treatment range from 25 to 35 °C. In general, biological treatment activity accelerates in warm temperatures and slows in cool temperatures but extreme hot or cold can stop treatment processes altogether.
Activated sludge liquors are less filamentous at higher temperatures and take longer to acclimatise to changes in temperature. After 40 °C, bacterial cells become reduced in size and number, thus growing in a dispersed phase and resulting in turbid effluents.
Temperature has a significant effect on the growth rate of Denitrifying bacteria .It may be estimated by using the following expression:
Solar radiation is a major removal mechanism in maturation ponds. The sun promotes the die-off of faecal bacteria in waste stabilization ponds by increasing the pond temperature, providing energy for algal photosynthesis that increases the pH above 9 and promotes photooxidative damage.
Furthermore, as temperatures rise, COD reduction efficiency decreases, and Suspended solids concentration increases, with a corresponding increase in SVI. Thus high temperatures have adverse effects on sludge settling. This is possibly due to the effects of higher temperatures on compounds forming the extracellular polymers that influence biofloculation. The extracellular polymer matrix consists of polysaccharides, proteins and lipids that exhibit structural sensitivity to temperature variation, so that high temperature reduces their viscosity and consequently the biofloculation and settling ability of the sludge.