Ultrafilter (UF) System Best Practices


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Ultrafiltration Best Practices


UF Permeate Rate is estimated using criteria below:

  •  17 l (4.5 gal) permeate used to rinse every 9.29 sm (100 SF) of ware
  •  Painted through-put x 4.5 gpm/100 SF = permeate requirement (for example lets say result is 84 lpm (22.2 gpm)
  •  7640 type UF Element is rated at 9.5 lpm (2.5 gpm) permeate, divide permeate requirement by 2.5 gpm/7640 type UF Element - round up to get the total number of 7640 type UF Elements [84 pm/ 9.5 lpm/UF Element = 9 x 7640 type UF Elements].
  •  7640 type UF Element needs 383 lpm (84 gpm) of paint flow at 50 psi, so 9 UF Elements x 84 gpm/UF Element = 3,447 lpm (756 gpm).
  •  Grey E-Coat is normally easier to UF than black E-Coat.

Glossary of Terms

  • Painted Through-Put: Maximum painted production rate (area/min)
  • 7640 Element: 7.6” diameter (nominal) x 40” long - Industrial standard 8” UF Element
  • 7940 Element: 7.9” diameter x 40” long, about 15% more UF membrane area, produces 10.8 lpm (2.85 gpm) of permeate and requires 424 lpm (112 gpm) of paint flow
  • MWCO: Molecular weight cutoff, used to describe a particle's size
  • Pre-filter: 25 micron bag or cartridge filter ahead of the UF system, for a #2 bag limit max flow rate to 395 lpm (105 gpm) paint
  • Feed & Bleed: Paint is circulated through UF Elements and a side feed stream is introduced [at 20 x permeate rate] while an equal amount is returned back to the paint tank
  • Single Pass: Paint travels only once through UF and returns
  • UF Membrane: Polymer UF membranes are cast on a non-woven substrate
  • Ultrafilter(UF): Particles > 0.001 micron and < 0.1 micron

UF Element Configurations

  • Tube: Original style that is ~ 25 mm (1”) diameter, many of these tubes are required even for small systems.
  • Hollow fiber: Smaller version of the tube type UF where many are bundled together to form a collection or module (not popular).
  • Spiral: Developed from flat sheet UF membrane, membrane is wound into a roll and permeate is collected at center. Ideal life is 1 to 2 years.
  • Plate & Frame: Uses flat sheet UF membrane to create a series of partitions, and permeate is collected at the corner of a partition.
  • Ceramic: UF Elements are used for high temperature oily water from the alkaline cleaner. Life can be as much as10 years or more.

Preventive Maintenance

Several PM activities performed during the year will pay benefits, reduce surprises, & lessen down-time:

  • Do not mix new and old UF Elements. Doing this disrupts hydraulic balance as new Elements take more flow than old ones.
  • Establish a monthly permeate flushing regimen and stick to it.
  • Perform 1st chemical clean of UF Elements once they are 80% of baseline. Consider using a formic acid cleaner. Ask your paint vendor for specific UF cleaner recommendations.
  • Each day, utilize the operator checklist attached to the UF machine.
  • Close off permeate valve if a leak occurs and no spare is available.
  • Change bag filters when you lose 3% flow or Delta P of 5 psi (.3 bar)
  • Train personnel on Lockout/Tagout procedures.


Newer generation paints are better for the environment but tend to promote the growth of bacteria. (Lead & solvents in older generation paints also acted as biocides):

  • Sources: Look for sources of poor quality DI or RO water that already contain high levels of biological growth and resolve the matter up-stream. Inspect the carbon filter and recharge it if needed. Add a UV lamp to the DI or RO holding tank as part of a recirculation loop.
  • Approved biocides: Use approved biocides in the rinse tank(s). Keep the side walls of the rinse tank(s) free and clear of bacterial growth.

Baseline Data

Every system will change as it is operated and ages. When trouble occurs, baseline data provides a valuable basis for comparison:

  • Paint flow: Use a magnetic paint flow meter to monitor (If using a VFD, the flow meter can control the pump to maintain flow)
  • 'Permeate flow: Record the flow rate after 4 hours of operation
  • Return pressure': Maintain at 15 psi (1 bar)
  • Permeate pressure: 60% of return manifold, max
  • Paint temperature: Decreasing temperature will suppress permeate and rising temperature will increase permeate rates.
  • Paint pH & %NV: Hold as steady as possible - Use automatic replenishment paint feeder with air operated piston pumps.


Create an Operator Checklist for all normal and abnormal operating conditions that are typical for the E-Coat machine:

  • Include a laminated valve tag ID drawing of all the valves & controls.
  • Permeate valve is the first one opened and the last one closed.
  • When starting paint flow, open the exit valve first.
  • When stopping paint flow, close the entry valve first.
  • Supply manifold valve should never be throttled, only throttle return.
  • Use electronic spreadsheets to log permeate rates and system data. These logs are required if you plan to submit a warranty claim.
  • Use a siphon breaker on the highest point of the paint piping to avoid damage to the fragile UF membrane when there is power outage and paint is siphoned from the UF Element Housings
  • If no permeate is available buffer DI/RO water to paint pH first
  • Use the Operator Checklist to train & orient new employees


  • Single bag vessels can be grouped together at much lower cost than multi-bag vessels to create a duplex filtration system for pre-filtration ahead of a spiral UF system.
  • Use a dedicated UF feed pump with a soft start or VFD.
  • Use 25 micron glazed (i.e. sintered) filter bags with plastic collars that do not allow any blow-by.
  • Keep paint velocity 8 - 12 ft/sec by using tapered manifolds.





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