Vacuum distillation for the concentration of aqueous solutions and wastewater

Industrial production companies such as in the metal or printing industry, and also in the chemical and pharmaceutical industry, frequently produce highly polluted wastewater, which may not be discharged into the communal sewage system. Some of the contaminations (heavy metals, cyanide salts, solvents, complex chemical compounds, etc.) are very complex and difficult to break down.

For many companies this is a problem. Even if treatment were possible, especially at small firms, where often only limited quantities of wastewater are produced, a conventional water treatment plant mostly cannot be operated economically and/ or efficiently by the firm itself.

Therefore in many cases the waste water has to be disposed of externally as special waste. The cost of external disposal amounts to up to € 700/m³ plus the transport costs of up to € 500/m³, depending on the branch of industry and/or the type of wastewater. Typical of such kinds of wastewater is that there is a pollutant contamination in a high degree of dilution with, in some cases, 90 % or more water. Since the disposal costs are calculated on the basis of the volume of  waste, the percentage of water is of decisive importance. Also, as a result of the disposal of the waste material, the company loses both the water and valuable substances such as organic solvents without any possibility of recovering them.

The distillation is a common process for material separation and here offers a way of solving the problem. The principle of this thermal separation process is to remove the water and other volatile components by means of evaporation and condensation – at the same time retaining the residual wastewater contents. Generally, the necessary evaporation temperature depends on the pressure, whereby the temperature is significantly reduced as the pressure drops, i.e. in a vacuum.

Building on this, the Fraunhofer IGB has developed, in cooperation with Maschinenbau Lohse GmbH, an inexpensive and modular vacuum distillation process which has already been implemented in a first prototype.

The system is based on simple technologies and therefore involves only low investment and operating costs and requires very little maintenance. An innovative plant design was developed, especially to the advantage of small and medium-sized companies. The system brings about an evaporation of the wastewater direct in the customer‘s disposal container, thus minimizing the expenditure for decanting and cleaning. In addition, deposits or incrustations in the plants and the resulting stoppages are avoided.

As a result of the reduced boiling temperature by vacuum, heat flows from a temperature of approximately 40–50 °C can be used for this process. In particular, it enables the use of low-temperature waste heat or solar heat as a source of energy. Moreover, the energy set free again in the condensation of the vapor can be used, for example, to preheat the wastewater or various process flows in the production plant. The water removed in this process can in many cases be re-used in production or for rinsing processes. Organic solvents can in principle also be separated and re-used in this process. Even wastewater contents of varying volatility can be separated and recovered individually by condensing the vapor in several steps at different temperatures.

The system described here aims, in particular, at flexible use in small and medium-sized companies, which up until now have not been able to treat their wastewater at all or only partially in their own plants and thus are dependent on providers for disposal services. In such cases, the quantities of wastewater, for which disposal has to be paid, can be reduced significantly. In addition, an optimized use of raw materials and of process water and/or heat flows often becomes possible. The branches of industry aimed at are, for example, the paint and dye, printing or textile industries, as well as the metal-producing, metal-working, and electroplating industries.