Introduction to membrane aerated biofilm reactors
‘MABR’ stands for ‘membrane-aerated biofilm reactor’. An MABR is an alternative aerobic biological treatment process to MBRs (membrane bioreactors).
An MABR is an example of a ‘fixed film’ process - like a trickling filter - as opposed to a purely suspended growth process, i.e. one based on activated sludge (as is the case for the MBR).
In an MABR, the membrane (often a hollow fibre) is used as an alternative aerator, rather than for biomass separation. As such, it does not provide the highly-clarified effluent of a classical MBR: an additional membrane separation stage would be required for this.
The MABR allows almost all the oxygen introduced to the bioreactor via the membrane to be utilised by the micro-organisms since it is introduced in molecular (or ‘bubbleless’) form. Because a biofilm is formed on the membrane itself, the oxygen is delivered directly into the biomass. The usual mass transfer limitations of a conventional fine bubble diffuser aerator (FBDA), limiting the standard oxygen transfer efficiency (SOTE) to somewhere between 10 and 40%, therefore no longer apply.
The membranes used usually provide selective permeation of the oxygen in the air. This means that the feeding into the biofilm is oxygen-rich.
While the membrane provides greatly increased aeration efficiency, and commensurately reduced aeration energy costs, the process is subject to the additional cost incurred by the membrane, compared to a conventional FBDA, and the loss of mixing provided by the air bubbles introduced by the FBDA. Also, as with conventional HF membranes, the membranes can clog as a result of the biofilm growth on the membrane surface, requiring a process configuration and operation which permits clearing of the membrane channels. This can be achieved using the nitrogen-rich unpermeated gas stream to scour the membrane surface.
The MABR still has a mixed liquor of suspended particles, as with other fixed film processes such the moving bed bioreactor (MBBR), but at lower concentration than the MBR. Biological treatment is achieved both by the biofilm and by the suspended flocs. The nature of the process, and in particular the flows of oxygen and nitrogenous species across the membrane and biofilm layers, means that both nitrification and denitrification can be achieved in the same tank.
Although MABRs have only been commercialised since around 2015, the process was originally studied in the 1990s (Pankhania et al, 2004; Brindle et al, 1998,1999). The term ‘MABR’ was first introduced in the 1990s by Mike Semmens of the University of Minnesota, and was originally based on a pure oxygen-fed technology.
The original work was based on a pure oxygen-fed technology, which attracted the interest of gas suppliers like Air Products. For this mode of operation almost all of the oxygen introduced to the bioreactor via the membrane was found to be utilised by the microorganisms forming the biofilm at the membrane surface, the oxygen seeming to go straight into biomass (Brindle et al, 1998). More recent work at bench scale, based on air rather than pure oxygen, indicates a reduction of up to 85% in process aeration energy compared with conventional fine bubble diffusion (Castrillo et al, 2019).
This MABR process has only been commercially developed and implemented at full scale since around 2015, limiting the number of established reference sites.