According to Christine McInnis, Dow Microbial Control, Spring House, Pennsylvania, U.S., Biocides added to the metalworking fluid concentrate or added tank side can remove or prevent the formation of biofilms. However, selecting the biocide can be tricky because the type of fluid influences the effectiveness and solubility of the biocide.

Problems with Biofilms

In a presentation at the STLE Annual Meeting in Detroit, Michigan, U.S., last May, McInnis said, Biofilms cause surface fouling that increases the microbial load on the biocides. They are also a source of reinoculation after a microbial attack seems to be under control.

New bacteria can be introduced in splash zones, swarf and system dead spots, effectively reducing the performance of biocides in the system. So, even though you think youve solved your microbial problem, you can continually get more and more bacteria in the system, she added.

Biofilms can also block filters, weirs and screens, reducing fluid flow, and they can induce corrosion. Microbially induced corrosion can occur when a galvanic cell develops in the biofilm, McInnis said. Corrosion leads to deep pitting in piping and on any metal surface that the biofilm is on. This can be particularly problematic because while corrosion does not appear to be widespread, the pitting can create deep holes that can cause leaks.

Biofilms produce visible slime on the fluid surface, and they pose a potential health risks to workers from mycobacteria in the system and atmosphere. Finally, McInnis said, the presence of biofilms can reduce the performance of biocides, compared to systems where bacteria are dispersed in the bulk fluid.

How Biofilms Form

Biofilms form in a series of steps as shown in the figure on page 24. First, the bacteria attach to the surface of piping or the reservoir. At the beginning, the attachment is reversible, said McInnis, so the bacteria can readily attach to and detach from the surfaces.

Eventually, the bacteria excrete extracellular polymeric substance, which is a sticky, tenacious substance that helps them adhere to the surface and agglomerate into distinct blobs. Biofilm EPS, which is also called slime (although not everything described as slime is a biofilm), is a polymeric conglomeration generally composed of extracellular DNA, proteins and polysaccharides.

After bacteria have adhered to the surface, they start to grow into colonies and eventually form a biofilm with secondary structures, McInnis said. Finally, the biofilm creates vesicles that can erupt to disperse the bacteria to another part of the system. And biofilm can be created throughout the entire system.

As the film matures, it turns patchy and forms a distinct structure with channels that allow access for nutrients to feed the bacteria. McInnis described the channels as being similar to the capillaries in the body. Just as capillaries carry oxygen to the cells, the channels feed oxygen dissolved in the water to the biofilm to allow aerobic activity. In fact, biofilms are mostly water, she said.

Biofilms can exist in the piping that carries metalworking fluid to the cutting face and back to the sump. Its growth is promoted by scale and corrosion, which provide a great place for the film to attach, McInnis noted. Scale and corrosion create a lot of nooks and crannies that provide a microenvironment to protect the biofilm from the shear flow moving through the piping.

The system also contains a lot of particulates. And the metal fines or swarf are good places for biofilm to grow. One of the best places to see biofilm is in the swarf, McInnis said. The sump area where all the metal fines collect provides a great area for microbes to grow.

Controlling Biofilms

Biofilms are harder to kill than bacteria in the bulk fluid for a variety of reasons. One reason is that the EPS, besides allowing the bacteria to stick to the surface, also helps the cells stick to one another, McInnis said. It is very proteinaceous and is difficult for the biocide to penetrate.

The EPS also covers the biofilm, and in penetrating through it, biocides may react with biofilm components and dead cells. Also metabolic byproducts, such as sulfides, can deactivate biocides. As a result, the biocide is often depleted before it can effectively begin killing the bacteria, McInnis concluded.

In addition, biofilms may have an inherent safety mechanism called quorum sensing that involves cell-to-cell signaling when a threat is detected. The cells communicate via chemicals called homoserine lactones, a class of signaling molecules that enables the bacteria to coordinate group-based behavior based on population density. Changes to the biofilm EPS and outer membrane components activate repair systems to restore the biofilm.

Finally, biofilms also contain a small population of so-called per-sister cells, said McInnis. Persister cells are dormant cells that exist in a non-growing or extremely slow-growing state that makes them less susceptible to biocides. As a result, they can survive antimicrobial chemicals that kill the majority of their genetically identical siblings, leaving a source of bacteria for the biofilm to regrow.

Monitoring the Fluid

A number of methods can be used to monitor bacteria in metalworking fluids, including dip slides, Petri Film testing and adenosine triphosphate (ATP) monitors. ATP is an enzyme present in all living cells, and an ATP monitor can detect the amount of organic matter in a fluid as an indication of the amount of bacteria present. This method works for both bulk fluid and biofilm monitoring, McInnis said. It provides fast results and detects non-culturable organisms.

To test the effectiveness of biocides to destroy biofilms, Dow researchers grew biofilm on glass cover slips (squares) with in six well microtiter plates. Bacteria were grown for 24 hours at 30 degrees C, and the biofilm was placed into a new set of six-well plates containing soluble oil or synthetic metalworking fluid. The samples were dosed with biocides and incubated for another 24 hours.

After incubating, the cover slips (glass squares) were placed in phosphate buffer and agitated to remove and disperse the biofilm. Each biocide and dose level was replicated a minimum of 4 times, and the Most Probable Number method was used to enumerate the living organisms in the biofilm.

The table on page 28 lists the biocides tested. We used biocides that can be put into a concentrate and added to a metalworking fluid, either in a metalworking formulation or as tank side additions, McInnis explained.

The tests showed that the undosed synthetic fluid contained 10 million microorganisms, and most biocides were effective against the biofilm, McInnis said, reducing the population to acceptable levels. BCO, DMO, morpholine, NaOPP, triazine, CTAC and MIT were able to effectively control biofilm. BIT and PolyQuat were less effective than the others.

She noted that even the most effective biocides never kill all the organisms. You will never get to a sterile condition, so be sure to use biocide continuously, she said.

In general, higher doses of biocide were needed to control biofilm in the soluble oil, McInnis said. Some biocides, such as NaOPP, BIT and Poly-Quat, were ineffective at controlling the biofilm. In fact, PolyQuat had no effect on the biofilm. Morpholine, DMO, triazine and CTAC reduced biofilm growth significantly. However, all except morpholine had to be used at the maximum allowable EPA level to be effective, said McInnis.

Keeping it Clean

McInnis concluded by outlining some steps to take to control the effects of biofilms on metalworking fluid systems. First, monitor biofilm growth periodically on surfaces, screens or metal coupons to track microbial buildup. Also, be sure to clean out or add biocide to dead spots in the system to reduce the potential for reinoculation.

She explained that routine application of biocides helps maintain low levels of microorganisms in the bulk fluid, thereby reducing the overall level of contamination. When heavy biofilm accumulation appears, shock the system with biocides, McInnis said.

Biodispersants, enzymes or surfactants can be added to assist biocides in reducing biofilm deposits. The chemicals can be useful in removing biofilm by solubilizing the EPS that helps the bacteria stick to surfaces and holds them together; McInnis said. Just be sure to use a low foaming product or add a defoamer or else youll have foam everywhere.

She suggested changing biocides periodically if a given product does not seem to be working. Also, the microbial composition of the biofilm may differ compared to that in the bulk fluid, so both must be sampled to determine the most effective biocide.

Another thing to remember is that low microbial counts in the bulk fluid are good, but counts in biofilms may still be high. Also, microbial counts in the bulk fluid may spike after biocide shock dosing because cells are released into the bulk fluid. This is actually a good situation because the biofilms have been loosened up, McInnis said.

Finally, biofilms grow back quickly, so be sure to keep the system adequately controlled with biocide. Simply put, effective monitoring is the best way to avoid fluid upsets, McInnis concluded.