Industrial wastewater can have a very different temperature, depending on the process in progress, reaching peaks of 70-80 °C followed by wastewater at 20-25 °C. If these effluents are treated by biological oxidation, the bacteria are not able to withstand such abrupt changes in temperature. An accumulation and homogenization tank is able to limit and counter the effects of the temperature peaks, even if it is not able to ensure the optimal temperature. Anyway, if the temperature in the equalization tank is higher than 38°C, it is necessary to install cooling towers in order to reduce the inlet temperature of water
The mesophilic bacteria, which develop between 4 and 42°C, have an optimal metabolization rate between 24 and 38°C. Over 38°C, there is a significant decrease of their metabolism or of their reaction rate. The sludge flakes break down, creating a sludge that goes out of the clarifier, causing an high turbidity; the COD removal rate decreases and , thus, also the rate of oxygen transfer is reduced, halving its value in temperature of between 32 and 42 °C. So too high temperatures cause low efficiency of oxidation, high concentration of COD, BOD5 and suspended solids in outlet and an exponential increase of aeration costs.
With a cooling tower, instead, it is possible decrease the water temperature until 4 - 6 °C above of the temperature of humid bulb of the place where the cooling tower is installed. The heat is not recovered, but dispersed into the atmosphere. Usually the cooling tower is made in PRFV (reinforced plastic fibers), or other material resistant to temperature and corrosion. Inside, it has a bed of cross-flow filling bodies in plastic material. .
The characteristics of these filling bodies’ bed are very important, in order to avoid quick dirtying and clogging of the channels, which would otherwise lead to a consequent decrease of the available surface and, then, a strong reduction in the efficiency of the system.
After an operational point of view, the water to be treated enters the cooling tower from the top and is sprayed onto the filling bodies through the nozzles. Water crosses the bed from top to bottom, while the atmospheric air, sucked from one or more fans, crosses the bed in the other direction, opposing the fall of hot water, and cooling it.
Working with wastewater with high organic load, bacteria, microorganisms and algae can grow on filling bodies, reducing the efficiency of the tower. For this reason, it is necessary periodically treating it with chlorine to eliminate these microorganisms.
This technology is an integral part of the treatment process of a Biological plant.