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Sidestream tubular MBR membranes: a summary of commercial products

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Updated
Berghof Membranes skid, single loop
Credit: Berghof Membranes

Simon Judd

In this final part of three features exploring the MBR membrane market, Simon Judd summarises characteristics of some of the commercially-available tubular membrane modules used in sidestream MBRs, and compares the key characteristics of this process configuration with immersed membranes.

Simon is a Director of Judd Water & Wastewater Consultants, and co-owner of The MBR Site and SludgeProcessing.com.

1. Process configuration and history

Multi-tube (MT) modules are used for sidestream MBRs (sMBRs), called 'out of basin' MBRs in the US. In this MBR process configuration the membrane is external to the bioreactor and a pump used to circulate the mixed liquor through the membrane channels. This configuration pre-dates the now more widespread immersed MBR, which employs flat sheet (FS) and hollow fibre (HF) membranes. A brief explanation of the MBR process configurations is provided schematically below.

Membrane bioreactor configurations

Membrane bioreactor configurations Source: Judd Water & Wastewater Consultants / YouTube

Sidestream MBRs were originally developed in the late-1960s/early-1970s by Dorr Oliver in the US – some 20 years-or-so before the immersed membrane systems were first commercialised. The Dorr Oliver Membrane Sewage Treatment (MST) system employed a flat sheet membrane, as did the sMBR technology developed by Rhone Poulenc and installed at a number of buildings in Japan in the 1990s. But at around this time sMBRs fitted with MT modules were already being implemented in Europe by Wehrle Environmental for landfill leachate treatment.

Currently almost all sMBRs implemented for wastewater treatment employ MT membrane modules (Fig. 1). This MBR membrane configuration is currently the only one which can be considered as standardised in terms of the overall module dimensions, commonly supplied in a cylindrical casing 8” (200 mm) in diameter and 3 m in length. The modules comprise a bundle of tessellated membrane tubes. The mixed liquor is conveyed along the length of the inside of these tubes with the permeate flowing from inside ('lumen side') to outside ('shell side').

Fig. 1(a) Berghof MT membrane module Credit: Berghof Membranes
Fig. 1(a) Berghof MT membrane module
Berghof multitube membrane module, 8 mm ID tubesCredit: Berghof Membranes
Fig. 1(b) RisingSun MT membrane module Credit: RisingSun
Fig. 1(b) RisingSun MT membrane module
RisingSun multitube membrane moduleCredit: RisingSun

2. Module orientation

There are two module orientations for sMBRs: horizontal and vertical. The classical 'pumped' sMBRs employ horizontal modules. To provide a high conversion for a single passage of sludge pumped along the membrane surface, the modules are mounted as a series of loops in a serpentine arrangement within the membrane skid (Fig. 2). The total retentate path length, including the semi-circular connectors, therefore exceeds 20m.

Fig. 2(a) Single-loop MT sMBR skid Credit: Berghof Membranes
Fig. 2(a) Single-loop MT sMBR skid
Berghof membrane skid, single loopCredit: Berghof Membranes
Fig. 2(b) Dual-loop MT sMBR skid Credit: Berghof Membranes
Fig. 2(b) Dual-loop MT sMBR skid
Berghof skid, double loopCredit: Berghof Membranes

For sMBRs employing vertically-oriented modules the mixed liquor is air-lifted through the membrane channels by introducing large air bubbles into the mixed liquor entering the modules. The air forms 'slugs' which fill the membrane channel (Fig. 3). This flow regime, termed 'slug flow' where the mean free bubble diameter is larger than the tube diameter, is known to generate higher fluxes than those attained from smaller bubbles ('bubbly flow'). For these air-lift systems the modules in the array are discrete (Fig. 4), rather than interconnected in a serpentine arrangement as with the pumped systems.

Fig. 3 Bubbly vs slug flow, illustrated schematically (Morgado et al, 2016) and using image capture (Xue et al, 2022) Credit: Morgado et al (2016) and Xue et al (2022)
Fig. 3 Bubbly vs slug flow, illustrated schematically (Morgado et al, 2016) and using image capture (Xue et al, 2022)
Slug flow, schematic and image capture representationCredit: Morgado et al (2016) and Xue et al (2022)
Fig. 4 Airlift sMBR membrane skid Credit: Pentair
Fig. 4 Airlift sMBR membrane skid
Air-lift MBR, PentairCredit: Pentair

3. Commercial providers

3.1 MT membrane modules

Selected MT membrane suppliers and the product specifications are listed below. The pore dimension dp is rated either as the size in nm or the molecular weight cut-off (MWCO) in kDa. All the providers listed offer module products in the standard size of 200 mm diameter and 3 m in length, and based on a PVDF membrane material of 30 nm pore size. There are a number of other MT membrane module suppliers whose products are not necessarily targeted at sMBR applications.

Table 1. Examples of MT module suppliers and specifications
Supplier Co. Model ID Material dp or MWCO ID, mm L, m D, mm Area, m2

PVDF Polyvinylidene fluoride; MWCO molecular weight cut-off in kDa

BerghofGerMO 83G I8 VPVDF30 nm8320727.2
BerghofGerMO 84G I8 VPVDF30 nm8421036.7
BerghofGerMO 103G I8 VPVDF30 nm8326040
BerghofGerMO 104G I8 VPVDF30 nm8426053.4
PCI MembranesUKA8PVDF20−450 kDa83, 420027, 36
PentairUSCompact 27, 33PVDF30 nm8, 5.2420027, 33
RisingSunChTG-30nm-8830PVDF30 nm8320027.2

3.2 sMBR technologies

In the sMBR space there are providers of systems that are not necessarily specific to a membrane product. This is because the MT membrane modules are of a standard dimension, such that one MT membrane module product can be substituted for another without demanding changes to the housing, pipework, or other infrastructure component. This presents an advantage of pumped sMBRs over the immersed configuration, where some modifications are often required if one product is to be substituted for another.

MT sMBR technologies generally have three modes or designs:

  • a) conventional pumped (horizontal membrane modules),
  • b) low-energy pumped (horizontal membrane modules), and
  • c) air-lift or air scour (vertical membrane modules).

The conventional pumped configuration is employed for both aerobic and anaerobic systems (AnsMBRs), and is often favoured for treating small effluent flows from industrial installations. This configuration incurs the highest membrane separation specific energy consumption (SEC in kWh per m3 permeate) of all the MBR technologies, but also incurs a low footprint because of the high flux attained (in excess of 150 LMH for the most treatable effluents, such as those from vegetable processing or beverage production).

The low-energy pumped system is based on the same configuration but employs a lower crossflow velocity (CFV) and a commensurately lower operating flux. This increases the required membrane area but also reduces the SEC.

The air-lift configuration (A-LsMBR) permits lower-energy operation for filtration, whilst also demanding the pumping of both the air and the sludge to maintain operation of the membrane. This technology, fitted with vertically-aligned modules, competes with FS and HF immersed systems in the municipal wastewater treatment space. The energy efficiency is comparable but the membrane system costs are normally higher.

Examples of suppliers and their technologies are given in Table 2. In the case of Berghof the company have now developed software (B-SMART) which can automatically adjust the CFV to the optimum value to minimise the SEC.

Table 2. sMBR suppliers and technologies
Supplier Pumped Low-energy pumped Air-lift

*Now replaced by the B-SMART® intelligent software

Aquabio/FreudenbergAMBRTMAMBR LETM-
BerghofBioflow*Biopulse*Bioair DS
DynatecHiRateTM-DynaliftTM
WEHRLEBIOMEMBRAT®BIOMEMBRAT-LE

4. Sidestream vs immersed: pros and cons

The vast majority of MBRs installed worldwide are configured with immersed membranes, on the basis of their lower energy consumption and overall cost. However, sMBRs offer specific advantages relating to their robustness and adaptability (Table 3). These facets mean that they tend to be selected for small flows of challenging industrial effluents, where the energy penalty may be counterbalanced by the operational flexibility and control.

Table 3. Immersed vs pumped MBR configuration
Immersed Pumped sidestream
Non-standardised modulesStandardised ~200 mm-diameter MT modules
Generally lower energy (<0.5 kWh/m3) for membrane componentGenerally higher energy (>0.5 kWh/m3) for membrane component
Lower fluxes/higher membrane area requirementHigher fluxes/lower membrane area requirement
Lower specific membrane area cost (cost per unit area)Higher specific membrane area cost (cost per unit area)
Fouling/clogging controlled by air scourFouling/clogging controlled by crossflow liquid scour
Control of shear limited by random path of air bubbles across membrane surfacesPrecise control of shear possible if variable-frequency drive (VFD) motors for pumps are fitted
Small chemical risk to biomass from clean-in-place chemical cleaningIn-situ chemical cleaning of membranes possible without any chemical risk to biomass
Downtime for maintenance generally higher, particularly for membrane module replacementDowntime for maintenance generally lower, particularly for membrane module replacement
Not generally desirable or possible to bring individual modules off- and on-lineLoops can be readily brought on- and off-line according to hydraulic loading
Limited operational flexibility with reference to fluxSwitching between high-flux/high-energy and low-flux/low-energy possible if VFD motors fitted

MT sMBR technologies generally have three modes or designs:

  • a) conventional pumped (horizontal membrane modules),
  • b) low-energy pumped (horizontal membrane modules), and
  • c) air-lift or air scour (vertical membrane modules).

The conventional pumped configuration is employed for both aerobic and anaerobic systems (AnsMBRs), and is often favoured for treating small effluent flows from industrial installations. This configuration incurs the highest membrane separation specific energy consumption (SEC in kWh per m3 permeate) of all the MBR technologies, but also incurs a low footprint because of the high flux attained (in excess of 150 LMH for the most treatable effluents, such as those from vegetable processing or beverage production).

The low-energy pumped system is based on the same configuration but employs a lower crossflow velocity (CFV) and a commensurately lower operating flux. This increases the required membrane area but also reduces the SEC.

The air-lift configuration (A-LsMBR) permits lower-energy operation for filtration, whilst also demanding the pumping of both the air and the sludge to maintain operation of the membrane. This technology, fitted with vertically-aligned modules, competes with FS and HF immersed systems in the municipal wastewater treatment space. The energy efficiency is comparable but the membrane system costs are normally higher.

Examples of suppliers and their technologies are given in Table 2. In the case of Berghof the company has now developed software (B-SMART) which can automatically adjust the CFV to the optimum value to minimise the SEC.

Some of the most significant advantages of sMBRs arise from the location and discreteness of the membrane modules. Membrane modules positioned external to the tank are more readily accessed than those submerged in a tank, making it easier to extract and/or replace individual modules. Also, all sMBR technologies allow energy management through the adjustment of the CFV or the shutting down of some of the modules during periods of low flow and readjusting/restarting when normal or high flow commences. This can be done without any risk of impairment of the mixed liquor quality.

The relative footprint of each configuration and module can be estimated from the packing densities of the modules and the module spacing. Packing densities for the Flat Sheet and Hollow Fibre membrane modules have already been determined. For these configurations the membrane area per unit floor area (FA) is dependent on the degree of stacking of the modules, with FA increasing with stacking.

Table 4. Summary of module attributes and equivalent surface overflow rate (SOR) values
Configuration Flux, m/d FA mod, m2/m2 (a) FA tank or skid, m2/m2 (b) SOR equiv., m/d

abased on mean values from commercial FS and HF modules; bassuming representative spacing or arrangement of modules in tank or on skid; cbased on mean data from FS modules; dbased on mean data from HF modules; ebased on operational parameter and product dimension values provided by Berghof

FS, rigid panel0.5 (c)221 (c)5528
FS, composite0.5 (c)310 (c)7437
HF0.5 (d)427 (d)17185
MT pumped2.6 (e)45 (e)65 (e)170
MT air-lift1.2866162194

According to the assumptions made in Table 4, where the flux and footprint figures for the FS and HF membrane configurations are based on averaged values at the maximum overall module height, the SOR for the conventional pumped sMBR technology is 2−6 times that of the immersed configuration. Since the MLSS employed also tends to be higher (12−15 g/L cf. 8−10 g/L for an immersed HF technology), leading to smaller biological tanks, the overall footprint for the sidestream configuration is commensurately lower. Against this, both the SEC and the specific membrane cost (in $/m2) for a pumped sMBR is higher than for the iMBR.

5. Other sMBR membrane module materials

Polymeric MT membrane products have been employed for industrial effluent, based on pumped sidestream MBR technology since the early 1980s. There are also a growing number of ceramic MT and multi-channel (MC) products though it appears these are not yet widely established as sMBRs. An example of a ceramic membrane product which has been implemented for sMBRs in the past is the Likuid Nanotek technology.

6. Summary

The sMBR technology is of historical significance, since it represents the MBR configuration first implemented. sMBR technologies were originally based on flat sheet membranes, but the MT configuration is now the most widely used in sMBRs. While considered viable primarily for industrial effluents, the air-lift mode finds use in municipal wastewater treatment. For both pumped and air-lift modes, the footprint incurred is significantly less than that for the immersed configurations. Whilst incurring a higher energy consumption than iMBRs, sMBRs offer a degree of simplicity and robustness associated with the accessibility of the membrane and the pumping of the mixed liquor through the cylindrical membrane channels.

References

Morgado, A. O., Miranda, J. M., Araújo, J. D. P., & Campos, J. B. L. M. (2016). Review on vertical gas–liquid slug flow. International Journal of Multiphase Flow, 85, 348-368.

Xue, Y., Stewart, C., Kelly, D., Campbell, D., & Gormley, M. (2022). Two-phase annular flow in vertical pipes: A critical review of current research techniques and progress. Water 14(21) 14213496

Acknowledgements

Thanks to all suppliers contributing graphics to this article.

A request: if your products are included in our MBR Membrane Products directory, please take a moment to check they are up to date and notify us of any changes required.

About this page

'Sidestream tubular MBR membranes: a summary of commercial products' was written by Simon Judd

This page was last updated on 12 April 2023

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