Sump Cleaning and Recharge: Machine Cleaners, Flushing and a Clean Restart

Sump cleaning and recharge is the drain, clean and refill of a machine's coolant circuit, done when a water-mix metalworking fluid can no longer be held in condition, for example when a dip slide reads 10^6 CFU/mL or more (UKLA/HSE Good Practice Guide v2.5, February 2025).

The procedure applies to the water-mix metalworking fluids in machine sumps and central systems: soluble oils, semi-synthetics and synthetics. German statutory accident insurance guidance (DGUV Regel 109-003) circulates a system cleaner at 2 to 3 % in the spent fluid for at least 8 hours before draining, and UK guidance rinses with the diluted fluid rather than plain water. Neat oil sumps follow a shorter procedure, covered in one section below.

A restart dip slide about one week after refill should read below 10^4 CFU/mL (HSE COSHH essentials MW3, published 03/21).

Standards, regulations and exposure limits are checked against their issuing bodies, product values against the named data sheet, and every figure carries its source and date; see our methodology and fact-checking.

Key figures

• Drain and clean: dip slide at 10^6 CFU/mL or more (UKLA/HSE Good Practice Guide v2.5, February 2025; HSE MW5, 03/21)
• System cleaner: 2 to 3 % in the circulating spent fluid for at least 8 hours, supplier instructions first (DGUV Regel 109-003)
• Rinse: the top-up dilution of the fluid with a pH check, not plain water (UKLA v2.5)
• Restart check: dip slide about one week after refill, below 10^4 CFU/mL (HSE MW3, 03/21)

What Is Sump Cleaning and Why Does a Coolant Sump Need It?#

Sump cleaning removes the spent fluid, chips, sludge and biofilm from the tank, lines and machine, and OSHA's best practices manual lists a system cleaned before recharge as part of a fluid management program. The manual is guidance, not a standard: OSHA has no metalworking fluid standard.

Cleaning is the reset step of coolant management once monitoring and correction have run out of room.

Drain, clean and recharge: what the job covers#

A full drain, clean and recharge covers the tank, the lines and the machine, starts with a cleaner added to the spent fluid and ends with an immediate refill and baseline measurements (DGUV Regel 109-003).

OSHA's self-assessment checklist asks whether clean-outs are scheduled and whether a written drain-clean-recharge procedure exists. It also asks whether systems are power washed and rinsed before recharge.

The drained fluid goes to disposal or treatment in the German sequence. Fluid that is reclaimed instead of discarded falls under metalworking fluid recycling, which sets its own quality gate before anything returns to a sump.

Why biofilm and residues survive a fluid change#

A fluid change alone leaves biofilm on surfaces in place: in one 2025 laboratory study (von Känel et al.), biocides and metalworking fluids blocked new biofilm but had largely negligible effect on established biofilms.

A 50-week study of one semi-synthetic system found substantial viable and non-viable bacterial counts in freshly recharged fluid (Microorganisms, 2024). A residue survey in Pathogens (2024) found fungi in more than 50 % of 417 residue samples against 5.6 % of 48,695 in-use fluid samples collected from 2014 to 2023. In the 2025 study, bacteria became detectable by plate count only after the pH drop, odor and performance loss had occurred.

Causes, odor and counts between cleanings are covered under bacteria, fungi and odor in coolant.

How Do You Know a Sump Needs Cleaning?#

A water-mix sump needs draining and cleaning when a dip slide reads 10^6 CFU/mL or more, when biofilm or fungal growth is visible, or when the emulsion turns unstable with heavy tramp oil (UKLA/HSE Good Practice Guide v2.5, February 2025; HSE MW5, 03/21).

The table compares the signs, their thresholds and the action each one calls for.

Dip slide counts and visible signs#

Dip slides are read against three UK action bands, and only the top band, 10^6 CFU/mL or more, calls for draining and cleaning (UKLA v2.5, Figure 12; HSE MW5). Below 10^4 CFU/mL the fluid is under control, and from 10^4 CFU/mL the cause is reviewed and corrected before the count rises.

How to take and read the slide is explained under dip slides for coolant. For very small sumps, UK guidance treats a count at the top band as a reason to dispose of the fluid rather than treat it.

Scheduled clean-outs and fluid life#

OSHA's self-assessment checklist asks whether clean-outs are scheduled and a written drain-clean-recharge procedure exists, and the UKLA/Pera disposal guide puts the service life of a UK water-mix fluid at 9 to 12 months.

For small sumps, UKLA v2.5 states that regular replacement of the fluid may be safer and more cost-effective than treatment, and that it reduces biocide overuse.

What Fouls a Coolant Sump?#

A coolant sump fouls from biofilm and fungal residues on its walls, tramp oil that destabilizes the emulsion, chips and fines that settle as sludge, and fluid that stands still in dead-legs or idle machines.

Biofilm and fungi sit on surfaces and in residues, as the survey figures above show: fungi in more than 50 % of 417 residue samples. A biocide does not restore a fluid whose degraded components, metabolites and dead biomass remain in the tank.

Tramp oil is held below 2 % by volume, checked at least weekly (HSE MW5, 03/21; UKLA v2.5). Chip and fines removal between cleanings is the job of CNC coolant filtration.

Stagnation is the fourth cause. HSE MW5 advises avoiding bottlenecks, dead-ends and dead-legs and keeping fluid circulating. After a weekend or shutdown without circulation, strict anaerobes reduce sulfate compounds to hydrogen sulfide (DGUV publication on microbes in metalworking fluids), which is why an idle machine smells on Monday.

How to Clean a Coolant Sump Step by Step#

A coolant sump is cleaned in ten steps, from circulating a system cleaner at 2 to 3 % for at least 8 hours to a dip slide check about one week after refill.

The sequence follows German statutory accident insurance guidance (DGUV Regel 109-003), completed with the UK rinse rule (UKLA v2.5) and the UK restart check (HSE MW3, 03/21).

1. Remove deposits from the tank and chip conveyors.
2. Add the system cleaner at 2 to 3 % to the circulating spent fluid and circulate for at least 8 hours, following the cleaner supplier.
3. Drain the sump and circuit.
4. Clean the tank, lines and machine mechanically; heavy soiling takes brushing, steam, high-pressure water or pulse flushing.
5. Rinse with the top-up dilution of the metalworking fluid and check the pH to confirm the cleaner is gone.
6. Drain, spray out and vacuum the residue.
7. Send the spent fluid, rinse and sludge to disposal or treatment.
8. Refill immediately, adding concentrate to water.
9. Measure concentration, pH and nitrite as the baseline.
10. Check a dip slide about one week later; continue normal monitoring below 10^4 CFU/mL.

Machine and system cleaners: dose and circulation time#

A system cleaner is a liquid cleaner for water-mixed fluid circuits that contains surfactants, emulsifiers, corrosion inhibitors, defoamers and possibly biocides, and German guidance adds it at 2 to 3 % to the circulating spent fluid for at least 8 hours before draining, following the cleaner supplier's instructions.

The 2 to 3 % is stated in the spent fluid, and the guidance gives no weight or volume basis. Shock dosing releases dead biomass that can block filters and lines, and endotoxin is released with it (DGUV publication on microbes in metalworking fluids). In the US, the OSHA manual advises that biocidal products used in fluids and as sump-side additives be EPA-registered under FIFRA for metalworking fluid use.

Mechanical cleaning: chips, sludge and heavy soiling#

After draining, the tank, lines and machine are cleaned mechanically, and heavy soiling takes brushing, a steam jet, high-pressure water or pulse flushing (DGUV Regel 109-003). Swarf is removed with brushes and shovels, not by hand, and gloves are worn only with the machinery off (HSE MW5, 03/21).

Sump and swarf vacuums are the equipment class for lifting sludge and fines out of a drained tank. Dry-ice blasting can lower the pH of water-mixed fluid, the CO2 workplace limit applies and it is very loud (DGUV Regel 109-003).

Rinsing: diluted fluid, not plain water#

UK guidance rinses a cleaned sump with the top-up dilution of the metalworking fluid and checks the pH to confirm the cleaner is gone, and it advises against plain water because of corrosion (UKLA/HSE Good Practice Guide v2.5, February 2025). German guidance likewise rinses with diluted fluid, then drains, sprays out and vacuums.

The rinse matters because residual surfactants cause heavy foaming (DGUV Regel 109-003). Leftover cleaner is one of the causes listed under CNC coolant foaming.

Recharge and restart checks#

Refill immediately after the rinse, adding concentrate to water rather than water to concentrate, then record concentration, pH and nitrite as the baseline (DGUV Regel 109-003). Adding concentrate to water avoids a water-in-oil cream (DGUV; UKLA v2.5, section 4.1).

A dip slide about one week after refill confirms the restart: monitoring continues as normal if the count is below 10^4 CFU/mL (HSE MW3, 03/21). A biocide after refill is optional and follows the supplier's advice. HSE MW5 requires fluid check records to be kept for at least 5 years in the UK, so the baseline belongs in that log.

The refill strength and how to check it are on coolant concentration.

Cleaning a neat oil sump#

A neat oil sump is drained with the system and, if contaminated, flushed with a compatible neat flushing oil circulated for the minimum time with its ancillaries (HSE MW4, 03/21). Seals are repaired and the sump is refilled; respiratory protection is not normally needed.

How to Clean a Sump Safely#

Sump cleaning avoids high-pressure hoses where practicable, and where a hose or jet creates mist, HSE MW3 (published 03/21) calls for respiratory protection with an assigned protection factor of at least 20, such as an FFP3 mask or a powered TH2 respirator.

The table compares the main hazards of the job and the control each source names.

German guidance asks for an FFP2 respirator where aerosols form during heavy-soiling cleaning. HSE's Legionella page adds that jet-washing a sump disturbs biofilm and needs a risk assessment.

Large enclosed sumps and underfloor tanks may be confined spaces that need a permit to work (HSE L101). Hydrogen sulfide can build up in a sump that stood idle; the UK limits are in the table.

Mist, skin and lung effects are summarized under metalworking fluid health hazards.

Fines of titanium, magnesium and aluminum carry an ignition risk during clean-out (UKLA v2.5, section 7.2.2).

How to Keep a Recharged Sump Clean Longer#

A recharged sump stays clean longer when appearance, odor, foam and biofilm are checked daily, concentration, pH, dip slide, tramp oil and temperature weekly, and the fluid keeps circulating at or below 30 °C (86 °F) (HSE MW5, 03/21; UKLA v2.5, February 2025).

• Daily checks: appearance, odor, foam and biofilm.
• Weekly checks: concentration, pH, dip slide, tramp oil and sump temperature.
• Tramp oil: below 2 % by volume.
• Sump temperature: at or below 30 °C (86 °F).
• Top-ups: a premix below the target concentration, not water or concentrate added directly in most circumstances.
• Circulation: no dead-legs; flush unavoidable dead-legs, for example weekly.
• Idle machines: replace the fluid rather than dose biocide again and again.
• Records: every check kept for at least 5 years in the UK.

HSE MW5 also says not to add biocide until concentration, pH, tramp oil, metal contamination, temperature, agitation and flow are corrected. The 2 % limit and removal methods are on tramp oil in coolant.

When to Call a Sump Cleaning Provider#

Call a sump cleaning provider when many machines or a central system need draining at once, when sumps are large enclosed or underfloor tanks that may be confined spaces, or when the spent fluid and sludge need compliant disposal.

Provider categories for this work include sump cleaning and recharge, laboratory testing and fluid audit, and disposal and haul-away of spent fluid.

Audits, recycling and recharge are scoped under coolant management services.

What Happens to the Drained Coolant and Sludge?#

A drain, clean and recharge leaves spent fluid, rinse fluid and sludge, and German guidance sends all of them to disposal or treatment rather than back into the machine (DGUV Regel 109-003).

Can Drained Coolant Go Back Into the Machine?#

Only with evidence of acceptable quality: UK guidance does not return recovered fluid to the sump without it (UKLA/HSE Good Practice Guide v2.5, section 7.3).

Is Drained Coolant Hazardous Waste?#

In the EU, yes: spent machining emulsions and solutions free of halogens carry the absolute hazardous entry 12 01 09 of the List of Waste (Decision 2000/532/EC as amended by Decision 2014/955/EU, applying from 2015-06-01).* The entry for emulsions with halogens is 12 01 08*. US used-oil rules and treatment routes are on disposal of waste metalworking fluids.

Sump Cleaning FAQ#

These five answers cover cleaners, circulation time, rinsing, biocide and small sumps.

What is a coolant sump cleaner?#

A coolant sump cleaner is a liquid system cleaner for water-mixed fluid circuits, added to the spent fluid before draining (DGUV Regel 109-003). It contains surfactants, emulsifiers, corrosion inhibitors, defoamers and possibly biocides, and is dosed at 2 to 3 % in the spent fluid.

How long should a system cleaner circulate before draining?#

German guidance circulates the cleaner for at least 8 hours at 2 to 3 % in the spent fluid, and the supplier's instructions come first (DGUV Regel 109-003). The guidance records no weight or volume basis for the 2 to 3 %.

Can you rinse a coolant sump with plain water?#

Not under UK guidance: rinse with the top-up dilution of the metalworking fluid and check the pH, because plain water risks corrosion (UKLA v2.5). Rinsing carefully also matters because residual surfactants cause heavy foaming.

Should you add biocide after a recharge?#

Only on the supplier's advice, and only after concentration, pH, tramp oil, metal contamination, temperature, agitation and flow are corrected (DGUV Regel 109-003; HSE MW5). Actives, dosing and approval status are on biocides for metalworking fluids.

Is it cheaper to replace coolant in a small sump than to treat it?#

According to UKLA v2.5, regular replacement of the fluid may be safer and more cost-effective than treatment in a small sump, and it reduces biocide overuse. For rarely used machines without circulation, German guidance replaces the fluid instead of repeated biocide dosing.