The MBR Book: Principles and Applications of Membrane

58 The MBR Book
commonly referred to as membrane aeration bioreactors (MABRs) (Brindle et al.,
1999). They present an attractive option for very high organic loading rates (OLRs) when
oxygen is likely to be limiting, whilst retaining the advantages of a fixed film process
(i.e. no requirement for downstream sedimentation and high OLRs). MABRs present
an alternative to more classical high-gas transfer processes for oxygenation using
pure oxygen such as a Venturi device. However, whereas these devices provide high
levels of oxygenation (i.e. high OTRs, Equation (2.18)), it is not necessarily the case
that they also provide high levels of utilisation by the biomass (oxygen utilisation
efficiency (OUE)). MABRs, on the other hand, have been shown to provide 100%
OUEs (Ahmed and Semmens, 1992b; Pankhania et al., 1994, 1999) and organic
removal rates of 0.002–0.005 kg m �2 d �1 from analogue effluents (Brindle et al.,
1998; Suzuki et al., 1993; Yamagiwa et al., 1994) and OLRs of almost 10 kg m �3 d �1
(Pankania et al., 1994) – around five times that of conventional MBRs. This means
much less membrane area is required to achieve organic removal, but removal efficiencies
also tend to be lower.
Extractive MBRs allow the biodegradable contaminant to be treated ex situ. This
becomes advantageous when the wastewater requiring biotreatment is particularly
onerous to micro-organisms which might otherwise be capable of degrading the
organic materials of concern. Examples include certain industrial effluents having
high concentrations of inorganic material, high acidity or alkalinity, or high levels of
toxic materials. Extraction of priority pollutants specifically using a permselective
membrane, such as a silicone rubber membrane used to extract selectively chlorinated
aromatic compounds from effluents of low pH or of high ionic strength (Livingston,
1993b; Livingston, 1994), allows them to be treated under more benign conditions
than those prevailing in situ.
Whilst the diffusive and extractive configurations offer specific advantages over
biomass separation MBRs, they are also subject to one major disadvantage. Neither
process presents a barrier between the treated and untreated stream. This means
that little or no rejection of micro-organisms takes place and, in the case of diffusive
systems, there is a risk of sloughing off of biomass into the product stream in the same
way as in the case of a TF. On the other hand, both configurations offer promise for
the particular case of nitrate removal.
2.3.3 Denitrification
The three alternative membrane process modes can all be employed for the removal
of nitrate from potable water supplies. Denitrification is the biochemical reduction of
nitrate (Equation (2.30)). This process is conventionally configured as a packed bed
in which denitrification is achieved by the biofilm formed on the packing material.
Full-scale schemes for potable duty based on this technology can nonetheless encounter
problems of (a) sloughed biomass and (b) residual organic carbon (OC) arising in the
treated product.
Biological anoxic denitrification is extensively employed in wastewater treatment
but various configurations have been trialled for drinking water denitrification
(Matěj°u et al., 1992; Soares, 2000). Its full-scale application has been limited, however,
because of poor retention of both the microbial biomass and the electron donor – an