MBR Products Explained

MBR schematic

A glossary and brief description of the major product types included on The MBR Site:

What are membrane bioreactors?

Membrane bioreactors (MBRs) are a water and wastewater treatment technology, combining a permselective membrane process (eg microfiltration/ultrafiltration) with a suspended growth bioreactor. All commercial MBR processes available today utililise the membrane as a filter, rejecting the solid materials which are developed by the biological process, thus resulting in a clarified and disinfected product effluent. The three main configurations of MBR used for water and wastewater treatment today are hollow fibre, flat sheet and multitube.

When used with domestic wastewater, MBR processes can provide a substantially clarified and disinfected effluent of high enough quality to be discharged to sensitive receiving bodies or to be reclaimed for such applications as urban irrigation, utilities or toilet flushing. Other advantages of MBRs over conventional processes include small footprint, and easy retrofit and upgrade of old wastewater treatment plants.

MBRs are increasingly becoming the technology of choice for water and wastewater applications where the above criteria apply, as is evident by the substantial increase in the membrane bioreactor market over recent years - MBRs are now implemented in more than 200 countries and global market growth rates of between 11.5% and 13% are regularly reported in market analysis reports, the MBR industry market value being estimated as worth $500 million by 2013.

Confidence in the process appears to be increasing as the number and size of reference installations grows, with plants over 100 MLD PDF now installed.

The technology continues to undergo development, although the market is still dominated a few established global players. However, the range of membrane products continues to expand at the rate of 3 to 5 a year as the market grows. Today there are approaching 60 MBR membrane module products available and the number of technology suppliers continues to expand.

For more information on membrane bioreactors, read our Features, Publications and MBR Resources pages or see The MBR Book by Simon Judd (Elsevier, November 2010).

Membranes - terms and materials

Terms for the individual components of a membrane technology are used inconsistently for membrane bioreactors. An “element” can be considered to be the smallest replaceable component of the membrane technology. The term “module” normally applies to a single element for a rectangular hollow fibre technology, but usually to a collection of elements in the case of flat sheets. The term “stack” can be considered to apply generally to a collection of elements, though the terms “rack”, “unit”, “cassette” and “skid” are also used. A collection of immersed stacks is usually called a “train”, and the train may include the biological tank.

Terminology for MBR membrane products and technologies:

Component Flat sheet (FS) Hollow Fibre (HF) rectangular panel Hollow Fibre (HF) cylindrical bundle; Multitube (MT)
Element Panel, element, sheet, cartridge Module, sub-unit Module
Collection of elements Module, cassette, unit Rack, cassette, cartridge, module Skid
Stack, frame  

Abbreviations for membrane polymeric materials are:

Abbreviation Material
PAN      Polyacrylonitrile
(HD)PE   (High density) polyethylene
PES     Polyethylsulphone
PS    Polysulphone
PTFE     Polytetrafluoroethylene
PVDF Polyvinylidine difluoride

Products glossary

Flat sheet membranes

Flat sheet (FS) membranes have planar configuration and are mainly rectangular, though other geometries exist for membrane modules designed to rotate.

The element may be referred to as a 'sheet', 'cartridge' or, most commonly, a 'panel'. Most of the rectangular panels are (semi-)rigid, though some of the thinner products are flexible and are mounted on a rigid frame.

Rigid panels have a plastic backing plate to which the membrane’s edge is welded on both sides. Water flows from outside to inside the panel and the permeate collected either from the permeate outlet tube(s) or side/central manifold(s). Sludge is air-lifted up through the membrane channels by air bubbles provided from a coarse-bubble aerator placed underneath the module.

Modules may be stacked to provide a double deck, with further stacking possible for a few products.

Flat sheet membranes are used almost exclusively for immersed MBRs, both for industrial and municipal applications, where they are sometimes favoured for smaller installations on the basis of their operational simplicity.

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Hollow fibre membranes

Hollow fibre (HF) membranes are normally vertically oriented with the aerators integrated either with the module or frame.

The element is most commonly referred to as a 'module'. The fibres are usually provided with some slack, to allow them to move laterally in the flow of air bubbles, which also air lift the sludge through the fibre bundle.

Water flows from outside to inside the individual fibres and is collected from the fibre ends potted either at one or both ends and fed into a collection chamber or manifold. For some products the fibres are reinforced with a braided core to which the membrane is bonded.

Hollow fibre membranes are used almost exclusively for immersed MBRs, both for industrial and municipal applications, where they are often favoured for larger installations on the basis of their lower membrane aeration energy demand.

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Multitube membranes

Multitube (MT) membranes are currently the only MBR membrane products which are standardised, and - for MBR technologies - are only used for sidestream configurations.

A module comprises a bundle of tubes within a standard-sized cylindrical casing – most often 8” (200 mm) in diameter – and the water flows from inside to outside the membrane tubes.

For pumped systems, the modules are placed in series to provide a high conversion, with one individual stream comprising a number of horizontal “loops” (normally 2-4 in all) in a serpentine arrangement.

For air-lift sidestream systems, the modules are discrete, vertically-oriented and generally longer than those used for the pumped type.

The pumped systems are often favoured for treating small effluent flows from industrial installations on the basis of their robustness and operational flexibility and control. The air-lift type competes with FS and HF immersed systems.

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Degritters

Equipment placed downstream of the coarse screen at the inlet works to remove coarse, dense particles (primarily grit) by settlement. They can either be configured as channels or cyclones. At large plants the grit is normally washed and reused as a building material, usually for roads.

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Screens

Equipment placed at the inlet works (coarse screens) and downstream of the degritter and/or upstream of the biological and membrane tanks (fine screen) for removing extraneous rag-like material by sieving. Coarse screens at the inlet works are normally rated as 6 mm, and the apertures are normally slits for this duty. Fine screens for protecting the membrane can be rated between 0.8 and 3mm, depending on the membrane type, and can have slit, square or circular apertures.

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Buffer tanks

A tank placed upstream of the biological and membrane tanks to provide buffering, or equalisation, of the flow and thus reduce hydraulic surges.

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Macerators

Equipment used for chopping and grinding bulky and fibrous material in a wide variety of applications in wastewater treatment systems, as well as for mixing and grinding various liquid/solid deposits. They are sometimes used for treating screens to reduce the coarseness of the solids and allow them to be passed through the screen for treatment along with the main flow.

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Blowers/compressors

Equipment used for providing air to the aerators. Wastewater treatment normally employs blowers, which operate at lower pressure and have a lower energy demand than compressors.

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Aerators

Equipment for distributing air, placed in the bottom of the biotank and/or underneath the membrane modules for immersed and sidestream air-lift MBRs. Biotank aerators employ fine-bubble aerators for encouraging oxygen dissolution. Membrane tanks normally have coarse-bubble aerators, with much larger apertures, to produce bubbles large enough to provide vigorous membrane scouring and/or agitation.

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Chemical dosing

Chemical dosing is the application of chemicals for such applications as pH correction, cleaning (of the membranes and aerators, in the case of an MBR), and disinfection. A complete chemical dosing system will normally comprise chemicals storage tanks, dosing pumps and a system for measuring, monitoring and controlling the dose rate.

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Pumps

An apparatus or device for forcing the movement of liquid through equipment.

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Sanitisation

Sanitisation applies to any process which makes the water and/or related equipment sanitary, as by cleaning or disinfecting to substantially remove pathogenic or problem micro-organisms in treated water. This is often taken as including biofilms formed on the inner surfaces of pumps, pipework and fittings, as well as in membrane permeate channels.

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UV irradiation

A process, as applied in water and wastewater treatment, which provides disinfection through transmitting ultraviolet irradiation through the water and inactivating micro-organisms. It is sometimes used downstream of an MBR or membrane process to provide additional protection against pathogenic bacteria.

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Sludge/residuals management

Residuals are those materials forming the waste products from water and wastewater treatment, which for MBRs primarily comprises sludge discharged from the membrane and/or biotank.

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