The MBR Book: Principles and Applications of Membrane

iomass. There are also issues with nitrite (NO 2 � ) arising in the effluent due to incomplete
nitrification, and this becomes a serious limitation, particularly in European
Union (EU) countries which are subject to more rigorous limits on nitrite levels than
the US (0.03 vs. 1 mg/L as NO 2 � -N). If the membrane is not used for direct filtration,
as is the case for diffusive and extractive denitrification MBRs, further downstream
processing is required for colour, taste and turbidity improvements and for disinfection.
There are additionally health and safety concerns to address with the hydrogenotrophic
systems. Notwithstanding the elegance of the diffusive and extractive systems, it is
possible that the conventional biomass rejection configuration may hold the most
promise since, as a barrier system, it also achieves disinfection. However, with only
one full-scale plant in existence, there is clearly further development required in
this area.
2.3.4 Elements of an immersed biomass-rejection MBR
MBRs employing immersed membranes to reject biomass represent the most widely
employed of all MBR configurations, since they incur the lowest specific energy
demand and therefore become the most economically viable for large-scale applications.
There are essentially five key elements of the iMBR process which are key to its
design and operation (Fig. 2.25). These are:
1. The membrane, its design and the sustaining of permeability,
2. Feedwater, its characteristics and its pretreatment,
3. Aeration of both membrane and the bulk biomass,
4. Sludge withdrawal and residence time,
5. Bioactivity and nature of the biomass.
Fundamentals 63
These elements are obviously largely inter-related (Fig. 2.26), in particular the latter
three which obviously relate to operation. The rate at which sludge is withdrawn
controls the residence time (i.e. the SRT) which then determines the concentration of
the biomass (or, strictly speaking, the mixed liquor). The MLSS concentration then
impacts both on the biological properties, that is, the bioactivity and microbial speciation
(Section 2.2.3), and also on the physical properties such as the viscosity
and oxygen transfer (Section 2.2.5). The feedwater chemistry provides the biggest
impact on MBR operation, in that the membrane fouling propensity of the mixed
Feed
(screens)
Air
(energy)
Out
(membrane fouling)
Bioreactor
(activity � nature)
Sludge waste
(quantity and quality)
Figure 2.25 Elements of an MBR