Wastewater treatment by evaporation and thin-film evaporators

Evaporators are employed in wastewater treatment, including waster in medical waste treatment facilities. Types include vapor compression evaporators, spray film evaporators, forced circulation evaporators, single effect, multiple-effect evaporators, and thin-film evaporators. Single effect evaporators are widely used in small-scale wastewater treatment. Industrial wastewater streams can be concentrated with multiple-effect evaporators. A system of multiple-effect evaporation helps to use a heat source economically through successive heating effects.

The design principle of a thin-film evaporator is two-part. The first is the heat transfer system, which functions on indirect heat transfer. The other is mechanical agitation. The efficiency of thin-film evaporators is relatively low at low temperatures and low residence times. The working principle of the evaporation process is applying heat to a liquid to evaporate the most volatile components and leave the least volatile components behind. The “tops” which evaporate off go to a condenser where much of the volatile stream is liquified and then recovered in a separate container.

There are two cases for evaporator equipment selection and specification. One is the specialized evaporator, fabricated by an equipment fabricating company based on customer specifications. However, suppliers also provide standard evaporators in stock sizes. Supporting equipment for the evaporation process include a pump, condenser, rotor, heating medium, preheater, compressor, collection tank, and mist eliminators. Pumps take the liquid wastewater from its source to the heating vessel. The vaporized contents are cooled or compressed to a liquid state with a heat exchanger or compressors. Mists eliminators remove entrained liquid from the overhead.

The most common and cheap heat sources for thin-film evaporators are very hot water or steam. Other heat sources include solar power, air, natural gas, and thermal oxidizers. Natural gas systems tend to have low evaporation efficiency and high cost. Solar heating has limited application because it requires a large amount of land. The construction materials for thin-film evaporators can be stainless steel, another ferrous alloy, borosilicate glass, or glass-lined steel. Thin-film evaporators operate with liquid in them. For this reason, the materials of construction should be corrosion resistant. They should also be temperature resistant.