The evaporation technique is characterized by transforming liquid effluent into two flows, one of high quality water and the other comprising a concentrated waste. The water obtained is of sufficiently high quality to be re-used, whereas the waste can be concentrated, even reaching almost total dryness. Waste management costs decrease markedly when concentrating the waste to this extent.
To evaporate the water without significantly increasing energy costs, this process is conducted under vacuum rather than at atmospheric pressure. This allows energy savings. As the pressure is decreased, the temperature at which water boils also decreases. For example, although water boils at 100 °C at sea level (pressure of 760 mm Hg), at the summit of Aneto (3404 m above sea level, with an atmospheric pressure of around 500 mm Hg) it boils at 88 °C and at the summit of Everest (8848 m above sea level, with an atmospheric pressure of around 225 mm Hg) it boils at 68 °C As such, if the pressure inside the evaporator is substantially reduced, water boils at close to room temperature: when working at an absolute pressure of 40 mm Hg, water evaporates at 34 °C. In practice, as the liquid that boils is not pure water, the boiling temperature is slightly higher.
Wastewater evaporators are a competitive and efficient solution for treating wastewater for which acceptable results cannot be achieved using more conventional methods (physicochemical and biological treatments). This typically occurs when the effluent contains a very high concentration of salts, non-biodegradable compounds, substances that are toxic to microorganisms, metals, etc. Such effluents are produced industrially by general services: boiler purging, ion exchange resin regeneration effluents, reverse osmosis rejection processes, process water treatment sludges, cooling tower purges, etc., as well as specific effluents from the food industry (brine treatments), the electroplating industry (depleted baths, wash and surface treatment waters), the chemical, pharmaceutical and cosmetics industries (tank and reactor washing waters, etc.), the paint manufacturing industry (reactor washing), the car and metal industries in general (oil-based emulsions, degreasers, cutting fluids, penetrating fluids), the graphical arts industry (ink treatment and concentration and roller washing waters), waste management companies (landfill leachates, high conductivity waters, etc.), hospital waste, etc. In addition to its use during effluent treatment, evaporation is also widely used in the food industry to concentrate many types of heat-sensitive substances (to concentrate fruit juices, produce condensed milk, remove alcohol to obtain alcohol-free beer, etc.).
The equipment required to carry out the vacuum evaporation process can be classified into three main groups depending on the procedure used to heat the effluent to the process temperature:
- Heat pump-based vacuum evaporators: heat is passed to the liquid to be evaporated, via a heat exchanger, upon compression of a refrigerant gas. Subsequently, a condenser that cools the liquid evaporated, using a thermostatic valve, causes the refrigerant gas, which circulates in a closed loop, to expand again. As the equipment operates under vacuum, it is possible to evaporate at temperatures of around 40°C, thus meaning that no other heat or cold source is required and making this an economically attractive process.
- Mechanical vapor compression vacuum evaporators: the distillate is compressed mechanically to increase its temperature and obtain superheated steam, which, by way of a heat exchanger, releases its energy to heat the liquid to be evaporated whilst the vapor itself condenses. This does away with the need for energy to heat the liquid to be evaporated and a refrigeration source for condensation.
- Multiple effect vacuum evaporators: these comprise various evaporators connected sequentially. The first uses hot water or fresh steam to heat the liquid to be evaporated. The distillate generated in the first evaporator acts as a heating agent in the second evaporator, and so on, with the vapor generated in the second evaporator being used to heat the liquid in the third evaporator. This is a very competitive option when the flow to be treated is high as significant heat savings with respect to a single-effect evaporator are possible.
The advantages of wastewater evaporators for the treatment of wastewater and liquid effluents are wide ranging and important.
The first of these is that this is an efficient technique for the treatment of waters that are difficult to treat using conventional techniques, which often do not provide optimal results. Vacuum evaporation is an effective and competitive technique in such cases. It should also be noted that the distilled water obtained is of very high quality and can be re-used within the same process, thereby allowing a zero waste policy to be implemented. In addition, the concentrated waste, which undergoes a significant weight reduction, means that waste management costs are markedly lower. Moreover, there is generally no need for chemical reagents, except on specific occasions when an anti-foam agent must be added. Similarly, the equipment required is compact, practical and instrumentalized, thus meaning that operational monitoring is simple and allowing effluent flows of up to 20 m3/h to be treated in a single evaporator. Finally, it should also be noted that as the effluent does not need to be heated to high temperatures, as the water boils at 35-40°C (depending on the operating pressure) when working under vacuum, the evaporator’s energy requirements need not be high quality power supplies and excess energy from other processes will be of use in the majority of cases.
In summary, evaporation is a novel, efficient and competitive technology that provides very good results as regards treating those effluents that prove complicated to treat using other techniques. This technique often allows the implementation of zero waste policies, with all their inherently positive environmental repercussions. In addition, as a result of the lower quantity of waste generated and the production of a high quality water flow, the initial investment is recovered relatively quickly. Furthermore, this is even faster if excess energy from another process can be used.