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.
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.
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.
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).
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.
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.”