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Schematic of complete immersed MBR process, showing screen, blowers and pumpsCredit: Judd Water and Wastewater Consultants
Feat lynxasm1 fig 4Credit: Héctor Rey
The membrane tank fittings: air lines (above) and permeate streams (below)

Designing an MBR

The components of a membrane bioreactor system largely comprise the pretreatment, biological and membrane separation steps. The biological and membrane steps can take different configurations depending on various factors, including the treatment goal, the feedwater characteristics, available land area, and limits on energy consumption.

Fundamentals of MBR design

The design of a membrane bioreactor system largely relates to the configuration of the membrane separation process and biotreatment process stages, as well as the membrane configuration (its geometry and the direction of the permeate flow).

Fundamentals of MBR design
Image of a Toray module with bubbles
Credit: Toray

Flux, TMP, permeability and shear

The key design parameter of a membrane process is the permeate flux, or flow rate per unit membrane area. This relates to the pressure applied across the membrane (transmembrane pressure, TMP). The permeability is the ratio of the flux to TMP, promoted by the shear generated from fluid flowing parallel to the membrane surface.

Immersed vs sidestream MBRs − flux and shear

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Schematic of dead-end and crossflow modes of membrane process operation
Credit: Judd Water and Wastewater Consultants Ltd

Dead end vs crossflow

Membrane filtration can either be 'dead-end' or 'crossflow'. If there is no retentate stream then operation is termed dead-end or ‘full-flow’. If retentate continuously flows from the module outlet then the operation is termed crossflow, since this means that retentate is flowing across the membrane surface.

MBR operation & maintenance – dead end vs crossflow
Credit: Image abstraction

Process and membrane aeration

Aeration is needed both for the process and the membrane tanks for an immersed membrane bioreactor (iMBR), and normally only for the process tank in sMBRs. Aeration accounts for the major part of the energy consumed by an iMBR installation.

Process and membrane aeration

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Images of membrane modules of PCI (HF), Berghof (MT) and Ecologix (FS), with MBR plant (Shanghai Megavision)
Credit: PCI Membranes, Berghof, Ecologix, Shanghai MegaVision

Choosing the process and membrane configuration

The selection of any wastewater treatment technology is generally informed by the treatment efficacy − the potential of the technology to achieve the required treated water purity − and cost. MBRs are often considered more costly than conventional processes but can achieve higher levels of treated water purity.

MBRs − choosing the process and membrane configuration
Grit classifier from San Luis Obispo site
Credit: Judd Water and Wastewater Consultants

Pre-treatment: screening and degritting

Screens are used for removing extraneous rag-like material by sieving. They are essential for suppressing the clogging of membrane channels with gross solids in the influent.

Degritters are intended to selectively remove solids, which may otherwise impair downstream processes through abrasion or accumulation.

Pretreatment: Screening and degritting
Rotary bar screen
Credit: Ecologix

Ancillary equipment

Two key pieces of additional equipment required for an MBR which are not needed in classical activated sludge treatment comprise membrane process operation control and fine screening (generally sub-3mm). Automated control of the membrane cleaning cycles is a key requirement of an MBR process technology.

Ancillary equipment for membrane bioreactors
Containerised MBR plant, Sperta
Credit: Sperta Environmental Technology

Containerised and packaged plants

Containerised, or packaged, wastewater treatment plants are systems where all the wastewater equipment is preinstalled within a single transportable container or skid. This includes the instrumentation and control components as well as the unit processes.

Containerised/packaged plants