A metalcutting coolant that smells like rotten eggs on Monday morning is already in serious trouble. The hydrogen sulphide odour is produced by anaerobic bacteria that have been multiplying in the sump over the weekend, without agitation, in the absence of oxygen. By the time the smell is obvious, bacterial counts are in the billions per millilitre. Detecting the problem earlier — before odour, before skin irritation, before the coolant turns black — requires active measurement rather than waiting for symptoms. The dipslide is the standard tool for this, and it is both simple and cheap enough that there is no good reason not to use it.
Why Bacterial Contamination Is a Real Process Problem
Bacteria are not just a hygiene inconvenience. In metalcutting coolant, high bacterial counts correlate directly with coolant degradation, shortened sump life, accelerated corrosion, and tooling problems. The mechanism is this: bacteria consume the emulsifiers and biocide packages in the coolant chemistry, breaking the emulsion stability and removing the protective corrosion inhibitor layer from both the machined part and the machine tool surfaces. A coolant with a bacterial count above 106 cfu/ml is actively destroying its own formulation chemistry. The cost of replacing a sump’s worth of degraded coolant, plus the downtime for cleaning and recharging, is far higher than the cost of routine monitoring and early intervention.
The aerobic bacteria that drive this degradation include Pseudomonas aeruginosa and Pseudomonas oleovorans — both preferentially colonise the oil phase of metalworking emulsions and produce enzymes that break emulsifier bonds. They reproduce by binary fission under favourable conditions: one bacterium becomes two in 20–30 minutes. Starting from a single cell in ideal conditions, that doubling time produces over one billion bacteria in 12 hours. A contaminated machine top-up or a dirty chip conveyor provides the inoculum; the warm, nutrient-rich coolant provides the growth medium. The outcome is rapid colonisation.
Aerobic vs Anaerobic: Two Different Problems
Aerobic bacteria require oxygen and thrive in the aerated coolant during operation. They are responsible for most emulsion-breaking and corrosion-inhibitor consumption. However, they are also more susceptible to biocides and to the elevated pH of a well-maintained coolant (typically pH 8.5–9.5). Maintaining correct coolant concentration is the primary control against aerobic bacterial growth — a coolant at correct concentration has sufficient biocide reserve to suppress aerobic populations below the critical threshold of 105 cfu/ml.
Anaerobic bacteria are a different and more persistent problem. Desulfovibrio desulfurican — the species responsible for the Monday morning sulphide stench — does not need oxygen. It colonises the sediment layer at the bottom of the sump, under chip mats, in dead legs of the coolant circuit, and anywhere the coolant does not circulate. It uses sulphate as its terminal electron acceptor instead of oxygen, producing hydrogen sulphide (H₂S) as a metabolic waste product. H₂S is both the source of the rotten-egg smell and a corrosive agent in its own right — it reacts with iron to produce iron sulphide, which turns the coolant black and stains machined surfaces.
Anaerobic colonisation typically indicates inadequate sump cleaning, excessive chip and swarf accumulation, or tramp oil layers floating on the coolant surface that create oxygen-depleted zones beneath them. Biocide treatment helps, but anaerobic bacteria are generally more biocide-resistant than aerobic species. The primary intervention is physical — removing the source of the anaerobic zone by cleaning the sump, removing settled swarf, and skimming tramp oil.
Using Dipslides for Bacterial Measurement
A dipslide is a paddle-shaped plastic carrier with two agar surfaces — typically one for aerobic bacteria (tryptone soya agar, appearing yellow or orange) and one for yeasts and moulds (rose bengal agar, appearing pink). To use it: open the container, dip the slide into the coolant for a few seconds, drain, close the container, and incubate the sealed slide at 30–37°C for 48–72 hours. After incubation, colony density on the agar surface is compared to a printed reference card that maps colony density to cfu/ml ranges.
The result is not a precise count — it is a range (e.g., 103–104, 105–106, >107 cfu/ml) — but this is sufficient for operational decision-making. The cost per dipslide is low, the test requires no laboratory equipment, and results can be read by machine operators without microbiological training. Autoflo supplies dipslides for use with metalcutting coolant monitoring programmes.
Dipslide samples should be taken from the flowing coolant in the machine sump during operation, not from the surface of a static sump. The surface may show foam or visible contamination but does not represent the bulk coolant population. Draw from a return line or an agitated portion of the sump for a representative sample.
Intervention Thresholds
The commonly used thresholds for metalcutting coolant are as follows. Below 103 cfu/ml: acceptable — coolant is well-controlled and biocide reserve is sufficient. Between 103 and 105 cfu/ml: caution zone — check coolant concentration, top up if below target, check for tramp oil contamination. Above 105 cfu/ml: action required — dose with biocide immediately, check concentration, inspect sump for swarf accumulation and stagnant zones. Above 107 cfu/ml: the coolant is severely contaminated and biocide treatment alone is unlikely to rescue it — plan for a sump dump, clean, and recharge.
Fungus (mould) is also detectable via dipslide but is a separate colony type on the rose bengal agar. Fungal contamination is more common in neat oil or semi-synthetic coolants than in full emulsions. Visible fungal mats — solid masses of mycelium — form in filter beds, on chip conveyors, and in dead zones of the coolant system. These mats are a physical barrier to coolant flow and a reservoir for bacterial colonisation within the mat structure. Remove mats physically and treat the system with a fungicide-containing biocide.
Prevention Is Cheaper Than Treatment
Routine monitoring with dipslides on a weekly or biweekly cycle, combined with maintaining coolant concentration within the manufacturer’s recommended range, eliminates most bacterial contamination problems before they reach the sump-dump threshold. Weekly measurement takes less than five minutes of operator time. The 48-hour incubation period means results are available before the next routine check. Trending the results — rather than just checking against the threshold — reveals whether bacterial populations are stable, declining, or accelerating, which guides the intervention timing.
Tramp oil is the most common enabler of serious bacterial contamination. Oil from hydraulics, guideways, and spindle lubrication enters the coolant sump continuously. Skimming tramp oil weekly, using a belt skimmer or disc skimmer on active machines, removes the primary anaerobic habitat and the primary nutrient source for aerobic bacteria. Maintaining a clean, aerated, correctly concentrated coolant is the most reliable way to keep dipslide counts below the action threshold without repeated biocide interventions.
Autoflo supplies dipslides for metalcutting coolant monitoring. Contact us at info@autoflotechnology.com to discuss monitoring programmes for your machine shop.