An in-depth study of the serious problems caused by the proliferation of various microorganisms in the emulsions affecting the efficiency of the fluids and the working environment
Micro-organisms – or microbes – can be defined as living organisms visible only thanks to a microscope and their world is extremely varied: they are divided into unicellular organisms, which are less evolved and therefore more simple on a structural level, and multicellular. Bacteria and Fungi (intended as micromycetes) hold a main relevance into the world of Metalworking.
Bacteria
Bacteria are a large group of single-celled organisms, a few micrometers long (a micrometer equals one million of a meter). Thousands are the known species on Earth, able to colonize any habitat, and this means that bacteria contribute to the majority of the biomass of the planet.
For their survival, like all living species, bacteria depend on energy sources, such as light or chemicals, and on the availability of nutrients, mainly carbon, necessary for their metabolism.
However, three parameters are key for their optimal proliferation. The pH level, temperature and above all oxygen, which represents a key discriminant for the identification of two large categories: aerobic bacteria, which require oxygen for their metabolism, and anaerobic bacteria, which instead do not.
For example anaerobic bacteria include sulfur-reducing bacteria (SRB) which generate the typical smell of rotten eggs and can cause dark stains on industrial machines and machined parts. Other types of very common in coolants anaerobic bacteria are echerichia coli, which is normally found in the intestine, bacillus cereus, widespread in the soil, and clostridium sp, ubiquitous and capable of generating spores.
Among the aerobic ones it’s due to mention pseudomonas oleovorans, which it’s the most common in coolants, staphylococcus sp, potentially pathogenic, and finally achromobacter sp, very frequent in waters.
Bacteria growth and proliferation
In ideal conditions bacteria grow in number duplicating themselves every 10-20 minutes, following an exponential growth until reaching the so-called plateau, that is the maximum sustainable population density. Beyond this limit, there is a slow decline (starvation) in the number of units. Thus bacteria, seeing their proliferation at risk, activate a series of mechanisms.
One of these mechanisms is the production of spores, in the case of spore-generating bacteria. The formation of resistant spores allows many bacterial species to resist extreme environmental conditions while waiting to reach a better one.
Biofilm formation
Another strategy that many species of bacteria can implement to continue to proliferate, while protecting themselves from the external environment, are biofilms: biofilms are created when a small number of bacteria attach themselves to a substrate like the internal side of a machine or a pipe, managing to resist undisturbed, and begins to aggregate. The first colonizing individuals start to produce particular cell mediators, or specific molecules used as a language of communication, which promote the release of large quantities of polysaccharide-based polymers. This progressively allows other micro organisms to adhere, forming a very resistant complex structure over time.
This film creates a micro habitat in which different species of micro organisms often coexist, which united together create a highly stable concretion that is difficult to penetrate.
The cooperation of the different bacterial species within a biofilm determines the expression of peculiar features by the species themselves composing the biofilm and this leads to the greater resistance of the individuals against biocides and detergents.
In the next articles the reasons why biofilms are a serious problem in Metalworking operations will be analyzed: they must be physically removed by emptying the systems and opening the machines and, moreover, most of the bactericides used for the treatment of emulsions find it difficult to penetrate these biofilms.
