Biological hot degreasing

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Under biological soak cleaning is the removal of oils and fats from degreasing and rinsing baths surface machined enterprises using thermophilic microorganisms . Through the biological treatment, the degreasing or rinsing baths should be continuously prepared on site in the companies, which results in a number of advantages for the companies. In the following, fats and oils are referred to synonymously as "fats".

Conventional aqueous alkaline hot degreasing

The process of hot degreasing

The aqueous alkaline hot degreasing process is used to prepare material surfaces prior to their further processing in order to remove existing grease from the workpieces . Figure 1 shows a possible arrangement of the cleaning baths for conventional hot degreasing, consisting of a degreasing bath, the so-called active bath, and a 3-stage rinsing bath cascade.

Figure 1: Typical arrangement of the cleaning baths

The workpiece is first immersed in an aqueous alkaline degreasing bath, in which coarse dirt particles and the adhering grease film are removed. After the degreasing bath, the workpiece goes through a cascade of rinsing baths in which the adhering cleaning solution is continuously diluted and rinsed off. This reduces the concentration of alkaline residues that are carried over into the subsequent baths. Fresh water ( deionized water ) is added to the rinsing baths to compensate for the loss through evaporation .

Mechanism of degreasing

In the degreasing bath, two interlocking mechanisms cause the workpiece surfaces to be degreased. On the one hand chemically, since the degreasing bath contains various types of washing-active substances that remove the grease in the film and ensure that it is emulsified . On the other hand, mechanical processes (such as introducing steam, circulating the basin contents or moving the workpiece) accelerate the degreasing process. Theoretically, degreasing would be completed successfully if the film on the metal surface consisted entirely of pure water. In practice, the state of absolute purity can never be achieved through dilution (cf. section “The process of hot degreasing”), the concentration of grease in the film decreases, but there is always residual contamination on the surface.

Chemistry of degreasing

According to Lutter, the degreasing bath generally consists of an aqueous sodium hydroxide solution (caustic soda, NaOH ) and a combination of interface or surface-active substances ( tensides ) and inorganic dirt carriers ( builders ), which are tailored to the degree of soiling and the material surface . The saponification of the fats and the setting of the desired pH value are carried out by adding sodium hydroxide solution (concentration is between 1 and 10%). The cleaning effect of the surfactants is based on wetting, detaching and emulsifying the fats and solids. The builder's job is to stabilize the fat droplets and solid particles in the washing liquid. In addition, they take on the task of softening water , alkanizing and dispersing insoluble dirt.

Physics of degreasing

The temperature range of a hot degreasing system in which alkaline cleaners are used is above 60 ° C. At these operating temperatures, both the flowability and the solubility of the oil increase, making the dirt more easily detached from the surface. In addition, at these high temperatures there is no formation of pathogenic germs, so that the addition of germicidal biocides can be dispensed with. The dwell time of the workpiece in the bath depends on the degree of soiling and is in the range of approx. 10 to 30 minutes.

Plant engineering

Oils or fats and other dirt particles accumulate in the cleaning solution, i. H. the cleaning solution "uses up" itself. A regeneration of the cleaning solutions is not usual in practice, which is why they have to be renewed after reaching the dirt-carrying capacity. Bath care would also be conceivable, in which the fats and solids introduced are continuously removed. One possibility of continuous bathroom maintenance can be seen in Figure 2 using an example of a selected application. Part of the contents of the rinsing bath enters a concentrate container and is conveyed through a downstream membrane module by means of a pressure booster pump . The permeate consists mainly of water and the membrane-permeable surfactants and is recycled in the rinsing bath and / or in the degreasing bath. The withdrawal of water leads to a concentration of the retentate , which is returned to the concentrate container. The concentrate is withdrawn and disposed of when required.

Figure 2: Bath maintenance in a conventional degreasing system

Biological hot degreasing

Microbial hot degreasing now offers a solution for improved on-site bathroom maintenance, in that the oils and fats are not only separated, but ideally converted by microorganisms into carbon dioxide (CO 2 ) and water (H 2 O). This results in a reduction in costs for input materials, energy requirements and disposal.

Biological foundations

The microbial degradation of hydrocarbons has been known for a long time and is the subject of research and development in a wide variety of areas (e.g. oil degradation in tanker accidents, soil remediation in contaminated sites , biological degreasing in industrial plants). It is also known that the breakdown of hydrocarbons is not limited to just a few genera, but applies to a large number of microorganisms. Depending on the different habitats , there are a number of different types of microorganisms (with regard to habitats e.g. halophilic , psychrophilic , thermophilic, etc.) that are able to break down hydrocarbons. The degradation can take place through bacteria , yeast or fungi .

There are two groups of microorganisms:

  1. Methylotrophic organisms that only break down methane (e.g. Methylomonas , Methylococcus , Methylosinus )
  2. Microorganisms that metabolize all other hydrocarbons (except methane), which, however, represent the much larger number of species (e.g. Mycobacterium , Flavobacterium , Nocardia , Candida lipolytica , Candida tropicalis )

The breakdown of hydrocarbons is based on synergistic interactions between many different microorganisms. A microbial community is therefore required (see biocenosis ) to break down the various hydrocarbons that are present.

Uptake and breakdown of hydrocarbons

The breakdown of the hydrocarbons takes place intracellularly, so the fats have to get into the cell . The problem here is that fats i. d. Usually hydrophobic (insoluble in water). Since the microorganisms live in the aqueous phase, this makes it difficult for the hydrocarbons to be absorbed into the cell. However, there are also so-called oil-positive microorganisms whose cell wall surface is hydrophobic and can therefore "store themselves directly in the oil phase".

The hydrocarbons can be transported into the cell in two different ways:

  • Diffusion through the cell wall through direct contact with oil droplets: The cell wall has high diffusion coefficients for hydrophobic, non-polar molecules (e.g. benzene, methane) and small, uncharged, polar molecules (e.g. water, glycerine, carbon dioxide) .
  • With the help of extracellular substances: The microorganisms form surface-active substances (biosurfactants) which are excreted into the environment. With these biosurfactants, fat or oil-water emulsions are formed, which can then be absorbed into the cell through pores in the cell membrane .

The hydrocarbons are broken down through several oxidation stages, in the best case the end products are only CO 2 and H 2 O. The oxidation proceeds from the alkane molecule via alcohol , aldehyde to fatty acid . The fatty acids are then catabolized by the β-oxidation . The decomposition takes place aerobically with high oxygen demand in an aqueous environment. For the microorganisms to grow, nutrients such as phosphate, oxygen, nitrogen, sulfur and trace elements such as potassium, calcium, magnesium and iron must be present. In the absence of oxygen, there is no noticeable breakdown of hydrocarbons.

Chances of microbial degradation of oils and fats for hot degreasing

If degreasing baths are operated hot (bath temperatures> 60 ° C), it makes sense to use thermophilic microorganisms for fat breakdown whose metabolic optimum is in the range of 50 to 80 ° C. For fat reduction in degreasing baths z. B. Organisms of the genera Bacillus and Thermus are used; The following table shows the metabolizable compounds of the respective genus.

Thermophilic microorganisms Metabolic compounds
Bacillus spp. carbohydrates
Strength
Methanol
Alcohols
Short chain fatty acids
Alkanes
Aromatics
Halogenated aromatics
Thermus spp. carbohydrates
Short chain fatty acids
Aromatics

Origin of such thermophilic microorganisms can be, for. B. the hot springs "Las Trinjeras" in Venezuela and the "Blue Lagoon" (hot spring) in Iceland, as work at the Institute for Biological Process Engineering at the Mannheim University of Applied Sciences shows.

Technical implementation

As in the section "System technology", even if the contents of the rinsing bath are reprocessed, there are disposal costs due to the accumulation of fat and dirt particles. This is where the process improvement through microbial support comes in.

Figure 3 shows the modified process based on the principle of biological hot degreasing.

Figure 3: Technical principle of biological hot degreasing

Instead of the concentrate container, the rinsing water, which contains fats and cleaning agents, is now continuously fed to a bioreactor in which the fat-degrading, thermophilic microorganisms are located. In addition to the substances that do not penetrate the membrane, the retentate also contains the biomass , which is returned to the bioreactor. The permeate also contains the biosurfactants (see section “Biological Basics”) that the microorganisms have formed to absorb hydrocarbons. Returned to the degreasing bath, these biosurfactants can also support the degreasing of the workpiece surface.

In order to prevent an excessive increase in the biomass in the bioreactor, one of the components necessary for cell reproduction is kept to a minimum. (see Liebig's law of the minimum )

Advantages of biological hot degreasing

Microbially assisted hot degreasing can have the following advantages:

  • Since the microorganisms break down the fats, there is no need to dispose of concentrates containing fats.
  • Due to the microbial degradation of the fats, an operating temperature below 60 ° C is possible, so that energy costs can be saved.
  • As a result of a consistently low proportion of fat phases in the rinsing baths (through the use of microorganisms), the service life of the rinsing bath and, depending on the cycle, also that of the degreasing bath, increase significantly.
  • Due to their ability to form and remove biosurfactants, the addition of surfactants to the degreasing bath can be reduced, as part of the treated rinse water is directed into the degreasing bath.

References and literature

  1. ↑ a b H.-J. Warnecke, K. Mertz: Lexicon of surface technology. Verlag Moderne Industrie, Landsberg / Lech 1989, ISBN 3-478-43200-6 .
  2. ↑ a b c d e f g M. Wolzenburg: Evaluation of production-integrated environmental protection using biotechnology in the field of metal and plastics processing. Institute for Biological Process Engineering, HS Mannheim, 2006.
  3. ↑ a b c G. Hofmann, J. Spindler: Process of surface technology. Fachbuchverlag Leipzig in Carl Hanser Verlag, 2004, ISBN 3-446-22228-6 .
  4. ↑ a b E. Lutter: The degreasing - basics, theory and practice. 2nd Edition. Eugen G. Leuze Verlag, Saulgau 1990, ISBN 3-87480-055-5 .
  5. ^ PW Atkins: Physical Chemistry. 4th completely revised edition. WILEY-VCH, Weinheim 2007, ISBN 978-3-527-31828-5 .
  6. ↑ PM Kunz, G. Frietsch: Microbicidal substances in biological sewage treatment plants - immissions and process stability. Springer Verlag, Heidelberg-Berlin 1986, ISBN 3-540-16426-X .
  7. ↑ a b c P.M. Kunz: environmental bio-process engineering. Vieweg-Verlag, Braunschweig / Wiesbaden 1992, ISBN 3-528-06451-X .
  8. ↑ a b c d e f g H. Schlegel: Allgemeine Mikrobiologie. Thieme-Verlag, 7th revised edition, Stuttgart / New York, 1992, ISBN 3-13-444607-3 .
  9. ^ R. Watkinson, P. Morgan: Physiology of aliphatic hydrocarbondegrading microorganisms. In: Biodegration. No. 1, 1990, pp. 79-92.
  10. ↑ a b c d e f P.M. Kunz, K. Dickbertel: Final report KVS project: Production of biosurfactants during the hot degreasing of surfaces using thermophilic microorganism communities from hot springs in Iceland and Venezuela. Institute for Biological Process Engineering, HS Mannheim 2008.
  11. ↑ F. Schauer, R. Sietman: Petroleum-degrading microorganisms. In: BIOspectrum. 5/2010, Spektrum Akademischer Verlag, Heidelberg 2010.
  12. ↑ R. Müller-Hurtig, F. Wagner: Microbial degradation of aliphatic hydrocarbons under environmentally relevant aspects. In: Biotechnology Yearbook. Volume 3, Hanser-Verlag, Munich 1990.
  13. ↑ T. Lapara, J. Alleman: Thermophilic aerobic biological wastewater treatment. In: Water Resources. Vol. 33, No. 4, 1999.
  14. ↑ K. Lutchmiah: Determination of degradation rates of thermophilic microorganisms from hot springs in Iceland and Venezuela. Institute for Biological Process Engineering, HS Mannheim 2006.
  15. ↑ PM Kunz, J. Benra, M. Kugel: Production of biosurfactants in hot degreasing using thermophilic microorganisms. Final report to the Alfred Kärcher Foundation, Institute for Biological Process Engineering, HS Mannheim 2010.