Biological degreasing rinsing bath

Biological degreasing rinsing baths are used to improve the galvanizing result in hot-dip galvanizing plants. This is achieved through the microbial breakdown of fats and oils.

State of the art

Figure 1: Process management in a hot-dip galvanizing plant

Fats and oils are often found on the surface of workpieces as temporary protection against corrosion . These must be removed before the deposition of coatings in order to ensure good adhesive properties. The fats and oils are removed from the workpiece by a multi-stage degreasing system, as shown in Figure 1.

In hot-dip galvanizing plants, the workpieces are degreased in a degreasing bath with acidic or alkaline cleaning agents as active components. The degreasing bath also contains surfactants that emulsify the fats and oils that have been introduced.

A rinsing bath follows the degreasing step. This is used to dilute cleaning components that are brought in via the workpieces. By continuously immersing the workpieces from the degreasing bath into the subsequent rinsing bath, fats, oils and inorganic machining residues are enriched. After a certain period of time, the rinsing bath's fat absorption capacity is exhausted and a grease film forms on the surface of the water.

The organic compounds can lead to relubrication when the workpiece is excavated from a certain concentration. This situation is comparable to dishes that are taken from already heavily soiled washing-up water and are therefore still covered by a film of grease.

Since the quality of the galvanizing depends on the quality of the degreasing, it is important to avoid relubricating the workpiece. This can be achieved with a degreasing rinsing bath in which grease and oil-degrading microorganisms have been introduced. One then speaks of a biological degreasing rinsing bath.

Microbiological aspects

In nature, hydrocarbons occur in a multitude of compounds: not only in the form of crude oil, natural gas or coal, but also, for example, in the form of vegetable and animal fats, waxes and oils. Also, metabolic intermediates and -endprodukte many microorganisms represent aliphatic or aromatic represents.

A large number of microorganisms are capable of breaking down these groups of substances. These include, for example, gram-positive species such as Arthrobacter, Bacillus, Nocardia, gram-negative species such as Flavobacterium, Enterobacter, Escherichia, Pseudomonas, but also fungi such as Aspergillus or algae, such as Chlorella.

In investigations of biological degreasing rinsing baths, microorganisms of the genera Flavobacterium, Arthrobacter and Bacillus were isolated, the percentage distribution of which can be seen in Figure 2.

Figure 2: Biocenosis predominating in the degreasing rinsing bath

The decomposition of fats and oils ( lipids ) takes place in several steps. The breakdown of hydrocarbons takes place in the cell. Water-soluble hydrocarbons are absorbed directly through the cell membrane, water-insoluble ones are attached to the fat-loving cell wall. Here can biosurfactants which are synthesized by the microorganisms, the lipid-water mixture to emulsify . This enlarges the phase interface and thus increases the degradation efficiency. The actual lipid breakdown takes place intracellularly.

The majority of organisms build molecular oxygen into fats and oils through an enzymatically catalyzed oxidation reaction in order to be able to introduce them into their metabolism. For this reason, aliphatic hydrocarbons, such as solid paraffins and aromatic hydrocarbon compounds, are most effectively degraded in the presence of oxygen.

The breakdown pathway for lipids depends on the molecular structure and the chain length. In the following, the breakdown of longer-chain alkanes and aromatics is considered, since these are used in the form of waxes, oils and fats as corrosion protection agents on workpieces.

Breakdown of aliphatic hydrocarbons

Long-chain hydrocarbons (aliphatic alkanes) are first broken down by catalysis of the alkanes into fatty acids. This requires three enzymatic reaction steps in which oxygen is involved.

{\ displaystyle \ mathrm {R {-} CH_ {2} {-} CH_ {2} {-} CH_ {3} {\ xrightarrow {(1)}} R {-} CH_ {2} {-} CH_ { 2} {-} CH_ {2} OH {\ xrightarrow {(2)}} R {-} CH_ {2} {-} CH_ {2} {-} CHO {\ xrightarrow {(3)}} R {- } CH_ {2} {-} CH_ {2} {-} COOH}}

(1) The enzyme monooxygenase (alkane oxygenase) catalyzes alkanes to alcohols.

(2) The alcohols formed are oxidized to aldehydes via alcohol dehydrogenase.

(3) The enzyme aldehyde dehydrogenase then oxidizes the aldehydes to fatty acids.

The resulting fatty acids are then chemically activated and broken down into acetyl-coenzyme A units via β-oxidation . Acetyl-coenzyme A is introduced directly into the citric acid cycle and converted into carbon dioxide and energy. If necessary, the cell can also use the fatty acids directly as cellular building blocks.

Breakdown of aromatic hydrocarbons

The breakdown of aromatic hydrocarbons also takes place via the formation of fatty acids. For this purpose, the aromatics are first hydroxylated and further oxidized to fatty acids during the ring cleavage. The resulting fatty acids are then broken down in the same way as described under “Breakdown of aliphatic hydrocarbons”.

Description of the biological degreasing rinsing bath

Figure 3: Alkaline degreasing with a downstream biological rinsing bath

The corrosion protection of the workpiece is removed from the surface by immersing it in the degreasing bath with the help of acidic or basic cleaning agents. The workpiece is then transferred to the downstream biological degreasing rinsing bath. Both process steps are shown in Figure 3.

By choosing suitable conditions within the rinsing bath, fat- and oil-degrading microorganisms can settle over time. In order to achieve faster colonization of the degreasing rinsing bath, sewage sludge or samples from an existing biological degreasing rinsing bath can be introduced.

The biological degreasing rinsing bath works on the principle of a bioreactor. This is operated at a process temperature of 42 ° C in order to prevent the multiplication of pathogenic germs. In addition, higher temperatures lead to a decrease in the viscosity of the oils and fats, accelerate chemical reactions and thus intensify the process.

Figure 4: Biological degreasing rinsing bath in a hot-dip galvanizing plant

The biological degreasing rinsing bath is adjusted to a pH value of 8.5, which the microorganisms can tolerate, using a phosphoric acid solution. The set pH also prevents an increase in pathogens in the biological degreasing rinsing bath. At the same time, the phosphoric acid serves as a source of phosphorus for the microorganisms.

The metabolism of the cells is optimized by a specially composed nutrient solution so that little biomass is produced. There is therefore only very little organic sludge that is discharged together with the inorganic solids that have been introduced. For this purpose, the sludge is sedimented by a lamella separator, drawn off and dewatered from time to time in a chamber filter press. The filter cake is then disposed of.

The microorganisms are supplied with oxygen by blowing in air through a compressor, as shown in Figure 3. Figure 4 shows a biological degreasing rinsing bath in a hot-dip galvanizing plant.

Advantages of the biological degreasing rinse

First and foremost, the process offers a decisive advantage in terms of reducing galvanizing defects caused by grease residues on workpieces. This is mainly due to the additional degreasing performance of the microorganisms within the degreasing rinsing bath. The treatment efficiency of the downstream pickling bath is thus increased. In the degreasing rinsing bath itself, oily sludge is avoided and the quality of the rinsing water is maintained over a long period of time.

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q Peter Kunz: Project report. Avoidance of residues through a biological degreasing rinsing bath in a hot-dip galvanizing plant on behalf of ABAG-itm GmbH. 1996.
↑ ^a ^b ^c ^d Peter Kunz: Environmental bioprocess engineering. Vieweg Verlag Braunschweig, Wiesbaden 1992.
↑ ^a ^b ^c Reinhart Schweisfurth: Applied microbiology of hydrocarbons in industry and the environment. Expert-Verlag, Ehningen 1988.
↑ ^a ^b ^c ^d Hans G. Schlegel: General microbiology. 7th edition, Georg Thieme Verlag, Stuttgart 1992.
↑ ^a ^b ^c ^d Maaß, Peter; Peißker, Peter: Manual hot-dip galvanizing. Wiley-VCH 3rd edition, 2008.
^ ^A ^b Donald Voet, Judith G. Voet, Charlotte W. Pratt: Textbook of Biochemistry. Wiley-VCH, Weinheim 2002.
^ Peter Kunz: Umweltbioverfahrenstechnik, lecture notes. Mannheim University of Applied Sciences, status: winter semester 2011.

[Projektbericht-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q Peter Kunz: Project report. Avoidance of residues through a biological degreasing rinsing bath in a hot-dip galvanizing plant on behalf of ABAG-itm GmbH. 1996.

[Umweltbioverfahrenstechnik-2] Peter Kunz: Environmental bioprocess engineering. Vieweg Verlag Braunschweig, Wiesbaden 1992.

[Angewandte_Mikrobiologie_der_Kohlenwasserstoffe-3] Reinhart Schweisfurth: Applied microbiology of hydrocarbons in industry and the environment. Expert-Verlag, Ehningen 1988.

[Allgemeine_Mikrobiologie-4] Hans G. Schlegel: General microbiology. 7th edition, Georg Thieme Verlag, Stuttgart 1992.

[Handbuch_Feuerverzinken-5] Maaß, Peter; Peißker, Peter: Manual hot-dip galvanizing. Wiley-VCH 3rd edition, 2008.

[Lehrbuch_der_Biochemie-6] A ^b Donald Voet, Judith G. Voet, Charlotte W. Pratt: Textbook of Biochemistry. Wiley-VCH, Weinheim 2002.

[Umweltbioverfahrenstechnik,_Vorlesungsskriptum._Hochschule_Mannheim-7] Peter Kunz: Umweltbioverfahrenstechnik, lecture notes. Mannheim University of Applied Sciences, status: winter semester 2011.