Improving MBR Flux: Ultrafiltration With a Twist—Helix Membrane Technology

About the Helix flux-enhancing technology

The Helix technology has been developed to harness the energy contained in the crossflow and, at low crossflow velocity, enhance the shear stress to boost the flux. Helix membranes (Fig. 1) feature a helically-winding ridge on the inside of the membrane. The ridge is made from the same material as the surrounding membrane, takes part in the filtration and is backwashable. There are no sharp edges or pits where rejected material can accumulate and cause blockage, as opposed to the net-like spacers used in spiral-wound modules which promote turbulence but do so by partially filling the channel [1].

Figure 1 The Helix versus the standard tubular membrane.

Two mechanisms are at work in the Helix membranes:
1)    the induction of secondary flows – Dean vortices – at the ridge structure (Fig. 2a) [2]
2)    forced helical flow of the feed stream along the membrane and increased wall crossflow velocity (Fig. 2b).

Figure 2 a) Induction of secondary flows at the ridges. b) Fluid forced to flow in a helical manner.

Both of these mechanisms, which possibly work synergistically, have the effect of increasing the shear stress at the membrane wall and so promoting the flux at a low crossflow velocity.

A visual demonstration of the Helix function is provided in Figure 3. Two pieces of 8-mm membrane, 300-mm in length, have been challenged with the same flow rate of 300 l·h-1 of clean water, equivalent to a 1.65 m.s-1 CFV. The outflow from the Helix membrane is clearly more turbulent and shows a helical twist in comparison to the standard.

Figure 3 Helix vs standard membrane flow.

To show their applicability in real streams, the Helix membranes have been employed at both pilot-scale and full-scale, and under aerobic and anaerobic conditions, at Ootmarsum and Hooge Maey (aerobic operation) and at a brewery (anaerobic operation), all with encouraging results.

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• Membrane Bioreactor Systems
• Membranes
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• Membrane Technology