Facts on MBR-Technology

The technology of membrane separation of activated sludge, commonly referred to as “membrane bioreactor” (MBR), is the combination of activated sludge treatment together with a separation of the biological sludge by micro- or ultra-filtration membranes with pore size of typically 10 nm to 0.5µm to produce the particle-free effluent. The latter step replaces the final clarifiers used in conventional activated sludge treatment which achieve solid separation by gravity only. The physical barrier imposed by the membrane system provides complete disinfection of the treated effluent. It also enables operation at higher sludge concentrations (typically up to 20 g/L instead of max. 6 g/L with conventional systems), and therefore permits to reduce the required footprint and/or sludge production. However, for municipal applications, the MBR technology is usually related to a higher total life cost, due to the high energy cost. In addition, the perceived risk related to the fouling and the replacement costs of the membrane remains an important limiting factor to its broad application.

Short history of MBR filtration systems
After initial development started in the late 60s, the MBR technology for wastewater treatment experienced rapid development from the early 1990’s onwards. The first systems commercialised in the 70’s and 80’s were based on what have come to be known as sidestream configurations, i.e. the membrane separation step was employed in an external sludge recirculation loop, mainly with in-to-out flow through organic or ceramic tubular membranes. Due to the high energy demand, these technologies targeted only small and niche market applications such as treatment of ship-board sewage, landfill leachate or industrial effluents.

In the early 90’s, the Japanese Government launched an ambitious 6-year R&D project which led to a major technological and industrial breakthrough of the MBR process: the conception of submerged membrane modules, working with low negative pressure (out-to-in permeate suction) and membrane aeration to reduce fouling. This paved the way towards a significant reduction of capital and operation costs, due to the reduction and simplification of equipment and the abandonment of the energy demanding sludge recirculation loop.

Nowadays, two types of technologies of submerged membrane modules are predominant on the MBR market. Both feature out-to-in permeate filtration and comprise the flat-sheet (or plate & frame) membrane module and the hollow fibre membrane module. Novel and alternative MBR filtration systems have recently appeared in the market and we can expect that the most innovative products will raise commercial interest in the coming years.

The commercialisation of the submerged MBR system precipitated rapid and extensive market penetration. The first pilot-scale European submerged MBR plant for municipal wastewater was built in 1996 (in Kingston Seymour, UK), soon followed by the construction of the full-scale Porlock WWTP (UK, commissioned in 1998, 3,800 p.e.), Büchel and Rödingen WWTPs (Germany, 1999, resp. 1,000 and 3,000 p.e.), and Perthes-en-Gâtinais WWTP (France, 1999, 4,500 p.e.). A few years later only, in 2004, the largest MBR plant worldwide was commissioned to serve a population of 80,000 p.e. (in Kaarst, Germany). The size of installations has thus grown from few thousand to hundreds of thousands population equivalent in only a few years.

Industrial and municipal MBR in Europe (from 2006 onwards: prevision)
Source: Lesjean and Huisjes, 2007

By 2006, around 100 municipal full-scale plants with a capacity >500 p.e. were in operation in Europe, and around 300 large industrial plants with a capacity >20 m3/d. The development and successful commercialisation of the technology in the past few years has led to significant decrease in capital and operating costs. However, the delineation between municipal and industrial systems shows that the technology remains especially competitive for industrial applications. In the municipal sector, it is now considered that for a green field and for a given treatment quality the capital cost of MBR plants is comparable to a conventional scheme. However, the energy costs remain 30 to 50% higher. Should this discrepancy be reduced in the coming years, the MBR technology would become a State-of-the-Art process for the municipal sector.

The four projects within the “MBR-Network” cluster will participate in the development and optimisation of the MBR technology, with the aim of rendering this high-tech process a common and competitive treatment alternative for future municipal applications. Ultimately, the results of these projects will accelerate its technological acceptance and broaden its field of application in Europe.

Text written by:
B. Lesjean, Berlin Centre of Competence for Water (boris.lesjean(at)kompetenz-wasser.de),
and S. Judd, Cranfield University (S.J.Judd(at)Cranfield.ac.uk).