Generally, in the treatment of wastewater, the evaporation is used to increase the concentration of a solution, separating a part of the water in the form of water steam, or to reduce the solute in the crystalline form.
The technologies normally used are all quite expensive, both from the point of view of energy consumption and investment costs. Therefore, when it needs this type of system, it is desirable to minimize the quantity of effluent to be treated, supplying it with previously concentrated solutions.
A method certainly cheaper is the forced air evaporation, which simply accelerates the natural evaporation. This phenomenon, that is the passage of a body from the liquid to the gaseous state, with a consequent decrease of the liquid itself, involves only the surface of the liquid and occurs at any temperature. However, it is more enhanced under the following conditions
a) a) Low humidity;
b) b) High air speed;
c) c) High temperature.
Thanks to its internal energy (enthalpy), the air mass maintains in balance the dry bulb temperature with the wet bulb, through water evaporation, thus increasing the relative humidity and reducing the dry-bulb temperature.
The dry bulb temperature is the one measured by a common bulb thermometer, while the temperature of the wet bulb is that obtained with the bulb of the thermometer wrapped in a damp cloth. In case the air is not saturated, the dry bulb temperature is higher than the one measured with wet bulb. This means that, for a certain value of enthalpy (kcal or kJ per cubic meter of air), if the air is not yet steam saturated and there is the presence of liquid water, the air tends to absorb water to approach equilibrium conditions.
In order to accelerate the natural phenomenon, some special panels in HDPE with strongly alveolar structure are used, in the way to increase the surface of evaporation, resulting in approximately 120 m2 of surface evaporation in 1 m3 of volume. In this way, you are working with a film of water of the order of about 4 to 5 microns and a ? potential tending to zero, exponentially increasing the evaporative effect.
To further implement this phenomenon, fans artificially produce a forced ventilation.
The number of evaporative modules are calculated according to the quantity of water to be concentrated and the climatic conditions of the place. The sizing considers that, with a relative humidity of 40-45% and a temperature of about 30°C, each module is able to evaporate approximately 6-7 m3 per day, with energy costs comprised between 15 and 17 kWh/m3 of water to be evaporated.
In addition to being an economic solution, the system does not produce spray emissions of pollutants or gas (only air and moisture) and is practically free from any kind of maintenance, since the particular composition of the polyethylene net prevents the permanent adhesion of concentrated molecules, avoiding possible clogging phenomena.
A further advantage of using this technology comes from the reduction of CO2 emissions (benefits in terms of "carbon credits"). In fact, the production of CO2, using our air forced evaporators, is approximately 14.5 kg of CO2 for 1 m3 of evaporated water, against the 168/205 kg produced by the traditional systems of evaporation steam, i.e. approximately 12/18 times lower.
This technology is used as an integral part of the treatment process in a Zero Liquid Discharge Plant.