iMBR vs sMBR

sMBR (MBR with side stream membrane)

Until immersed (or submerged) membranes were commercialized, MBR relied on crossflow filtrations using mostly tubular modules and some plate and frame modules. Since membrane system is placed separately outside of aeration tank, it is called side-stream membrane process. High liquid velocity on membrane surface such as 2-5 m/s is required to prevent membrane fouling effectively. The sMBR typically provides reliable performance with relatively easy maintenance due to the accessibility to the externally installed membrane system.

However, the cost of building crossflow membrane system is high due to the necessity of housings to hold membranes in them at the relatively high pressure (3-6 bar). In addition, energy costs to circulate liquid to be filtered is prohibitively expensive for municipal wastewater treatment since only <5% of the feed to the membrane system is recovered as permeate in one pass, which means 20+ times more liquid  than the permeate volume needs to be circulated through membrane at high pressure. The energy cost of crossflow filtration is discussed with an example here.

The high TMP (3-6 bar) used in crossflow membrane is required primarily to maintain high crossflow velocity on membrane surface, not to obtain high flux. While the high TMP does not help obtain high flux, it triggers cake layer collapse/compaction and reduces permeability of the cake layer as discussed here. As a result, the average flux of the cross flow membrane is as low as 50-100 LMH in MBR despite of the high TMP (3-6 bar). The combination of the high capital costs and the high operating costs for liquid circulation make crossflow membrane system prohibitively expensive for most of wastewater treatment applications especially for municipal wastewater.

iMBR (MBR with immersed membrane)

Immersed (or submerged) membrane filtration was invented to save capital and operational costs by directly placing membranes in mixed liquor without housings. Permeate is obtained using vacuum suction (Tajima, 1988;Yamamoto, 1989). Due to the modest suction pressure (or TMP) at 5-30 kPa, cake layer compaction is moderate and permeability maintains relatively high at 10-50 LMH. As compared in Table 1, permeability in iMBR is significantly higher than that in sMBR (100-500 LMH/bar vs 7-30 LMH/bar) in full-scale operating condition although the clean water permeabilities are in a similar range (500-2,000 LMH/bar) for both types.

In iMBR, air scouring of immersed membrane is required to mitigate membrane fouling. The energy required for the scouring process is discussed here, but the specific energy required per permeate volume is less than one tenth of that of crossflow side-stream filtration. As summarized in Table 1 below, iMBR needs approximately 1/40 of energy required for sMBR in the filtration system assuming both iMBR and sMBR operate optimally (0.1 kWh/m3 vs 4 kWh/m3). The predominance of iMBR is undisputable as over 99% of the total installed membrane surface area in Europe in the period of 2003-2005 was immersed membranes (Lesjean, 2008). No sign that indicates otherwise has ever been found throughout the regions to date.

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Fig. 1. Schematics of the iMBR with immersed membranes and sMBR with side-stream crossflow membranes.

Table 1. General comparison of iMBR and sMBR

ItemUnitiMBR

(immersed)

sMBR

(side stream)

Typical configurationHollow fiber (HF)

Flat sheet (FS)

Tubular (TB)

Plate&Frame (PF)

Mode of operationCrossflowCrossflow
Operating pressurekPa5 – 30 (vacuum)300 – 600
Long-term

Average Flux

LMH (m/d)15-35 (0.36-0.84)50-100 (1.2-2.4)
Permeability1)LMH/kPa0.5-50.07 – 0.3
Recycle ratiom3 feed/m3permeate25-75
Superficial velocitym/s0.2-0.35)2-6
m3 air/m3permeate7 – 306)
SED2)kWh/m3permeate0.1-0.54-12
Membrane cost3)$/m2<50>1,000
Capital costLowHigh
Operating costLowHigh
CleaningHardEasy
Odor/VOC emission potentialHighLow
Packing densityLowHigh
Market Share4)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.

 

© Seong Hoon Yoon