The process of evaporation by ponds has been used for quite some time in wastewater treatment. The idea consists of depositing wastewater in large open ponds allowing water to evaporate through solar radiation and wind, leaving a pond of concentrated residual waste for treatment.
Despite their simplicity, evaporation ponds can be very useful in obtaining zero liquid discharge in salt rejection and other effluents of mineral composition since no effluent is discharged directly into the natural environment.
As previously mentioned, evaporation ponds are artifical ponds with very large surface areas that can contain potentially hazardous waste. Their purpose is to reduce the water contents of different solutions by «natural» evaporation. Thanks to such treatment, the volume of waste requiring treatment is lowered, thus achieving a reduction in costs while obtaining an increase in the concentration of materials (or products) that have commercial use.
Traditionally, evaporation ponds have been used for treatment of vegetable wastewater from olive oil in rural areas. While ponds occupy large areas, their costs remain reasonable due their rural location, although there also exist applications in landfill leachates as well as in the treatment of wastewater from mining processes.
Nonetheless, evaporation ponds may also present some problems, especially those related to odor generation when they are in close proximity to nearby towns and the ponds are storing water with high organic content. In these situations certain technologies can be employed for odor abatement (scrubbers, washing towers, regenerative thermal oxidation or activated carbon filters), however their costs should be taken into account.
It is therefore important to analyze the problems in each case and choose the combination of technologies that will prove most efficient, both from an environmental perspective, as well as the economic.
On the other hand, it is common that during the rainy season evaporation ponds can fill up much more than what they evaporate. The rectification of this problem requires proper pond design in addition to the help of a water mist system (forced evaporation), that facilitates an increase in the evaporation rate to 20 times more than that of natural evaporation.