Inside-Out Filtration with PES membranes

As described in Liqui-Flux® Operating Modes, Ultrafiltration processes require regular backflushing to maintain their performance. This is the case in both Pressure Driven In-to-out (PDI) and Pressure Driven Out-to-in (PDO) formats.

The hydrodynamics in backwashing PDI-Membranes are much more effective than in PDO formats caused by the fact that concentrate is flushed out through a well defined (circular) flow channel in PDI, whereas "dead-zones" with very low flow velocity exist outside of the hollow fiber membranes at the bottom of PDO modules at backwash.

PDI Advantages

To provide a sufficient cleaning effect to the separation layer on the outside of a PDO membrane, air is injected to improve the removal of the inhibiting debris layer from the membrane (air scouring). This and also the fact that the feed flow is applied from the shell side of the housing require a membrane material that provides sufficient elasticity to overcome the mechanical impacts of the typical operation. PDO formats therefore largely use PVDF with its high elasticity as a membrane material but with some significant disadvantages such as a wider pore size distribution and lower permeability.

A PDI format as 3M's UltraPES does not require air scouring and by design the hydrodynamic conditions create a superior backwashing performance. It is therefore possible to use Polyethersulfone (PES) as base membrane material which provides excellent mechanical strength at high permeability and a narrow pore size distribution accompanied by the ability to generate a distinctive and functional layer structure.

As a consequence PDO formats made of PVDF serve mostly as microfiltration membranes with a lower retention and a higher transmembrane pressure to generate a sufficient filtrate flow. This generally leads to higher operating costs compared with PDI-formats.

The PDI format of 3M’s Liqui-Flux modules was chosen to provide durable, high performance by superior hydrodynamics and low operating costs. The latter is due to low transmembrane pressure enabled by PES as the membrane material.

Keyword PDI in-out (PES) PDO out-in (PVDF)
Physical cleaning regime Back flush in various modes; 3M P.E.T.™ Technology supports efficiency. To increase back flush efficiency air scouring required, no turbulent flow during BF
PH tolerance 1 - 13 1 - 11
Chlorine tolerance Sufficient High
Caustic resistance High Sufficient
Chemical cleaning regime CEB is very effective; usually only caustic soda (NaOH) is added to dissolve organic matter from the membrane surface.
If required an acidic cleaning step (e.g. HCI), can be executed to remove anorganic matter. Both effluents will neutralize each other and create neutral salt.
CEB is less effective caused by flow conditions. Moreoften CIO is required. Usually Chlorine is applied to release tenacious debris from the membrane surface. The use of chlorine is causing disinfection by-products such as THMs.
Membrane structure Asymmetric, leading to high flux rates at low TMP. Rather symmetric, higher hydrostatic resistance.
Membranes permeability High Moderate
Pore size and distribution True ultrafiltration membrane at very narrow distribution. Rather microfiltration membrane with broader pore size distribution.
Trans membrane pressure Frequently between 0,1 bar and 0,6 bar. Frequently between 0,5 bar and 1,5 bar.
Maximum turbidity and particle range Maximum particle size: 300 µm maximum turbidity: 100 NTU Maximum particle size: 500 µm maximum turbidity: 300 NTU
Mechanical membrane High tensile strength and burst pressures. Sufficient elongation at break; (not required since flexibility is not a relevant characteristic). Good tensile strength. High elongation at break properties; (required due to air-scouring and shell side feed), danger for permanent elongation.
OPEX Intended to be lower due to lower operating pressures. Suspected to be higher due to elevated operating pressures and need for compressed air.
Pump energy demand Lower Higher
Coagulant dosing PES membrane with an inline coagulant dosing proved to remove organic substances. PVDF does not tend to benefit a lot from coagulant dosing.