ch2mbr

Membrane Separation Process

MBR membranes can be set up in two process configurations: immersed (iMBRs) and side-stream (sMBRs)

1. Immersed Membrane Bioreactors (iMBRs)

The immersed configuration is also sometimes referred to as ‘submerged’, which obviously leads to confusion
when abbreviated, or ‘internal’ - as opposed to external - for a side-stream membrane.

Further process configurations using immersed membranes, based on the way in which the membrane
is used, include ‘diffusive’ and ‘extractive’. However, neither of these have reached the stage
of commercialization; all current commercial MBRs employ a membrane solely for retaining
the sludge in the bioreactor whilst producing a high-quality permeate product.

2. Side-stream Membrane Bioreactors (sMBRs)

The first membrane bioreactors developed in the late 1960s were based on the side-stream configuration.
For this configuration the membrane system is placed outside the biological treatment process tank.

The sludge is then pumped through the membranes in ‘crossflow’ mode, normally at an elevated pressure (2-3 bar),
and the permeate forced through the membrane under the combined action of the pressure
and the crossflow. The unpermeated stream is then returned to the bio-tank.

For the more recent immersed configuration, introduced in the early 1990s, the membrane
is placed in the bio-tank and scoured with air introduced by an aerator placed beneath or within the membrane module.

As with almost all membrane separation processes, key to sustaining the flow of water through
the membrane is the generation of ‘shear’. This relates to the flow of fluid over the membrane surface,
which for side-stream configurations is sustained by the flow of sludge and for immersed systems by the action of the air bubbles.

General Comparison of iMBR and sMBR

Item	Unit	iMBR (immersed)	sMBR (side stream)
Typical configuration		Hollow fiber (HF) Flat sheet (FS)	Tubular (TB) Plate&Frame (PF)
Mode of operation		Crossflow	Crossflow
Operating pressure	kPa	5 ? 30 (vacuum)	300 ? 600
Long-term Average Flux	LMH (m/d)	15-35 (0.36-0.84)	50-100 (1.2-2.4)
Permeability	LMH/kPa	0.5-5	0.07 ? 0.3
Recycle ratio	m3 feed/m3permeate	-	25-75
Superficial velocity	m/s	0.2-0.35	2-6
	m3 air/m3permeate	7 ? 30/td>	-
SED	kWh/m3permeate	0.1-0.5	4-12
Membrane cost	$/m	<50	>1,000
Capital cost		Low	High
Operating cost		Low	High
Cleaning	-	Hard	Easy
Odor/VOC emission potential	-	High	Low
Packing density		Low	High
Market Share	-	99%	1%

1)   Permeability in operating condition.
2)   Specific energy demand including energy for permeate suction, but excluding biological aeration.
3)   Including module/frame/housing, if applicable, but not including installation costs.
4)   Membrane surface area based in municipal and industrial (Lesjean, 2008)
5)   When gas superficial velocity is 0.02-0.04 m/s in FS. From Fig. 6 in Yamanoi, 2010.