THE USE OF ATP AS A MEASURE FOR CONTROL
OF MICROBIAL CONTAMINATION IN METALWORKING FLUIDS
Shin Joh Kang, Research and Development Manager
Lucy B. Pugh, Project Engineer
McNamee, Porter and Seeley
Ann Arbor, Michigan 48104
BACKGROUND
Microbial contamination of metalworking fluids has been a long-standing problem in the machining industry. The conditions maintained in these coolants and lubricants are ideal for microbial
growth, since the fluids are circulated which keeps them aerobic and they contribute high COD concentrations, providing a good substrate. Although the extent of microbial growth is a function of
the coolant type and system conditions, uncontrolled microbial contamination can increase exponentially. This, in turn, degrades the coolant, decreases its useful life, and in extreme cases can cause
health problems for workers exposed to the contaminated fluids. The need for monitoring and timely
control of these systems is apparent.
Three major types of fluids are used in machining plants [1]. In the past, petroleum-based soluble
oils were used for most applications. More recently developed are synthetic fluids which do not contain any petroleum oils, and semisynthetic fluids which are partially composed of oils. Synthetic and
semisynthetic fluids normally contribute more COD than the soluble oils, but due to the nature of
these fluids, sustain lower levels of microbial contamination.
In large machining plants where monitoring and control programs are in effect, individual "warning levels" of microbial contamination are established for each system based on previous experience.
As contamination approaches or reaches a predetermined level, fresh coolant and/or biocides and
fungicides are added and the system is retested to determine the effectiveness of the control methods.
Some plants do not have system monitoring programs, and simply dose with biocides/fungicides
based on past experience. In other plants without monitoring or control programs, systems are emptied to the drain and refilled with fresh coolant as problems with coolant performance are encountered.
The methods used for monitoring of coolant systems at large machining facilities include pH and
viability tests. The pH of coolant systems is high and is normally maintained in the range of 7.5 to
9.0. A pH depression is indicative of excessive microbial contamination.
Viability tests include the Standard Plate Count, and similar, more simplified tests. One such
simplified test, B-F Indicator [2], consists of a small plastic paddle coated with a nutrient medium
on each side. The paddle is dipped into the sample and then incubated. The incubated paddle is
compared with a chart which pictorally shows order of magnitude levels. Another test, Petrifilm [3],
manufactured by the 3M Company consists of a sheet of nutrient media covered by a sheet of transparent, gas-permeable film. The sample is pipetted on the media, covered with the film, and incubated. The number of colonies can be counted from the grid imprinted on the transparent sheet. The
major drawback of these viability tests of course, is that two or more days of incubation are required
before results are known, by which time the contamination in the coolant system may have changed
significantly or at worst be completely out of control.
The ATP (adenosine triphosphate) assay has been shown to be an accurate measure of microbial
activity in wastewater processes [4], drinking water [5], food products [6], urine [7], and other applications. ATP is present in all living cells, and degrades rapidly following the cell's death. It reacts
with the enzyme, luciferin-luciferase, to produce light which can be easily measured and quantified.
The measurement of ATP can be usually made within minutes, and can be directly correlated with
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