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Exhaust air and wastewater: how to clean them correctly

The chemical industry, companies that build engines, hospitals and printing shops – they all discharge exhaust air and wastewater containing compounds that have a detrimental effect on the environment and/or human health. Researchers at the University of Stuttgart are investigating ways to counteract specific waste problems, including biological methods.

Prof. Dr. Karl-Heinrich Engesser is head of the Department of Biological Exhaust Air Purification at the Institute of Engineering, Water Quality and Solid Waste Management (ISWA) at the University of Stuttgart © private

Far from emitting unpleasant smells as would be expected, the pilot sewage plant set up by the University of Stuttgart is surprisingly pleasing, both to the nose and the eye. This could well be due to innovative methods used by the university's researchers to purify wastewater and exhaust air. Prof. Dr. Karl-Heinrich Engesser has no problem with the location of his office, which is right next to the pilot plant. Engesser has been working on the development of innovative methods to remove hazardous environmental factors from wastewater and exhaust air for 15 years. He is a biologist by training, but has gained in-depth experience in engineering and chemistry during his professional career. The combination of long experience and expert knowledge in both these fields enables him to carry out his work comprehensively and competently.

Many of the projects at the university are related to industrial application and Engesser is very much committed to contract research on behalf of industry as well as to cooperative projects involving industrial partners. His research is very much practically oriented: purifying exhaust air emitted by the chemical industry economically and in a way that conserves our natural resources, is one such project that combines ecology with economic interests.

Trouble shooting is the researchers’ bread and butter

On the large-scale, the washer step is carried out in the absorber tower (left, with ascending stage and manway openings for filling support medium). Untreated air enters the washer at the bottom and is guided in the reverse direction to the regenerated water phase. Since the relevant exhaust air components are highly water-soluble, the transfer of the components into the water phase is guaranteed. The water is steam-saturated and clean air leaves the absorber at the top. © University of Stuttgart/ISWA

Biowashers, where the exhaust air is purified in a two-tier process, are an example of the standard work carried out by the researchers. In a first step, the polluted air is passed through washing liquids, the simplest example being water. The harmful substances then move from one medium to the other (aqueous medium) where they are dissolved and subsequently biologically degraded. The purified air can then be discharged into the environment. "The faster this process is carried out, the more effective it is. Using a 400 PS engine enables 60,000 to 80,000 cubic metres of exhaust air to be turned over per hour," explains Engesser. Plants like this cause problems - in turn providing Engesser and his team with a contract research project - when they no longer function properly, because of age, lack of maintenance or as a result of switching to other production processes. "We are often called out when the plant has fallen ill," confirms Engesser, going on to explain that his engineers and chemists will then try to find out what the problems are in order to alleviate them. The second stage of the process followed by Engesser and his team involves biological steps: The polluted water is purified in a kind of bioreactor where it comes into contact with microorganisms that use the harmful substances as a food source, thus degrading them. Polluted wastewater can be purified directly. What remain are harmless metabolic products of the microorganisms and clean water that can then be put back into circulation.

"This type of regeneration is important for technical as well as environmental-political reasons; the plant is kept in good working order," said Engesser. Microbial degradation is usually based on oxidation reactions that can also be initiated by chemical or physical means. The choice of method is cost-dependent and also depends on whether suitable microorganisms are available for the substance or substance mixture that needs to be purified. "Normally, we use a mixture of naturally occurring microorganisms. However, special microorganisms need to be used to degrade substances that are more complex, for example chlorinated benzols generated as a by-product of the chemical industry. Burkholderia fungorum bacteria are used for degrading such substances," said Engesser.


Wastewater: the most problematic cases

The way that the actual degradation works is of little interest for industrial projects, but of huge interest for basic researchers such as Engesser. “The question as to whether fluorobenzols such as toluol are degraded by way of the ortho or the meta pathway is not important for practical applications. However, it is important for basic researchers like us who want to gain detailed insights into the biochemical degradation mechanisms,” said Engesser who is focusing on these issues in a project financed by the DFG (German Research Foundation).

Exhaust air treatment at a printing facility The figure depicts the principle of flexo printing for a specific colour. The printed paper is fed into a drying chamber for drying. The high VOC (volatile organic compounds) emissions occurring in the drying chamber and in the printing machine are removed through suction cleaning and used thermally in a block heat and power plant. The electricity is sold and the waste heat is used for heating the drying chambers. Diffuse emissions occur in the hall at these two sites. For occupational health reasons and in order to reduce the danger of explosions, the emissions have to be removed by suction from the production hall and the chemicals warehouse (left). This leads to a volume stream of 60,000 - 70,000 m³/h, which is then collected and fed into the biological treatment process. (© Prof. Engesser)

Engesser is also extremely interested in the substrate specificity of microorganisms. “Some bacteria strains are so specialised that they only degrade one isomer of a certain compound. Exhaust air or wastewater usually contain mixtures of different compounds, which are present in several isomeric forms. In such cases, the specificity of the microorganisms is a severe hindrance,” said Engesser.

Basic research is indispensable

Engesser’s team of researchers has been dealing with a very tough wastewater problem for quite some time, namely, the degradation products of certain peroxides. Peroxides are used as starters for syntheses in the chemical industry and are cracked chemically at high temperatures. The degradation leads to acid in a thick brine, which is a very hostile environment for microorganisms. Nevertheless, Engesser has discovered some suitable candidates that are able to degrade this “broth”. “We are working in collaboration with Spanish colleagues and hope to find halophile bacteria in Spanish salt mines that might be suitable for our purposes.”

Another issue that is becoming more and more important is the pollution of water with drugs, for example in hospital wastewater and also in normal household wastewater. However, the concentrations are too low to enable effective microbial degradation. The substances must be concentrated, a process that is much too expensive. Engesser: “I believe it is far cheaper to bind these substances to charcoal and subsequently burn them. But from the perspective of basic research, it is certainly interesting to find out whether there are any bacteria that are able to degrade such substances, and if there are, how they do it. It is just possible that some of them may be active at very low drug concentrations.”

Further information:

University of Stuttgart
Institute of Engineering, Water Quality and Solid Waste Management (ISWA)
Prof. Dr. Karl-Heinrich Engesser
Bandtäle 2
70569 Stuttgart
Tel.: +49 (0)711 685-63734
Fax: 49 (0)711685-63729
E-mail: karl-h.engesser(at)iswa.uni-stuttgart.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/exhaust-air-and-wastewater-how-to-clean-them-correctly