The MBR Book: Principles and Applications of Membrane

Fundamentals 61
Moreover, the potential for membrane fouling from both organic materials and
hardness (Oldani et al., 1992) has not yet been explored. The relative expense of the
ion-exchange membrane (Crespo et al., 2004) is likely further to constrain development
of this process for the duty of drinking water denitrification.
2.3.3.3 Diffusive MBRs
As already stated, the use of hydrogen (H 2) as the electron donor combined with
either carbon dioxide or bicarbonate as the carbon source (Mansell and Schroeder,
2002) obviates the organic carbon contamination issue, since hydrogen gas does
remain dissolved in the water. Such autotrophic (or “hydrogenotrophic”) denitrification
can be considered both inexpensive and non-toxic (Haugen et al., 2002), as well as
producing a relatively low biomass yield (Lee and Rittmann, 2002). As with MABRs,
diffusive H 2 MBRs typically employ microporous HF membranes or silicon tubes (Ho
et al., 2001) to deliver gas directly to biomass (in this case a denitrifying biofilm)
attached to the shell-side of the membrane (Fig. 2.24b) providing up to 100% gas
transfer (Mo et al., 2005). Although around 40% slower than heterotrophic denitrification
according to some batch measurements (Ergas and Reuss, 2001), high nitrate
removal rates have none-the-less been reported in hydrogenotrophic MBRs (Haugen
et al., 2002; Ho et al., 2001). These are apparently attained through high H 2 gas mass
transfer rates sustained by limiting fouling. Fouling is mainly manifested as thick
and dense biofilms (Roggy et al., 2002) and, possibly, scalants (Ergas and Reuss, 2001;
Lee and Rittmann, 2002) at the membrane surface, though precipitation of mineral
deposits appears not to adversely affect H 2 transfer in all studies (Lee and Rittmann,
2003; Roggy et al., 2002).
The disadvantage of gas transfer MBRs is, as with the extractive processes, that
the membrane is not used for filtration, but it also does not retain the biomass. As
such, product water is prone to “sloughed” biomass and other organic matter in the
same way as any fixed film biological process. Also, regulation of the gas flux
through the membrane due to the partial pressure drop along the membrane fibre
length (Ahmed and Semmens, 1992) can produce uneven biofilm growth. In addition,
doubts about the safety of dosing water with hydrogen remain, notwithstanding
the apparent near quantitative retention of hydrogen by the biomass. Finally, the
poor adaptability of the autotrophic bacteria under drinking water denitrification
conditions demonstrated in several studies by long acclimatisation periods of 40 and
70 days (Ergas and Reuss, 2001; Ho et al., 2001) casts doubt on the suitability of this
configuration for full-scale operation.
2.3.3.4 Biomass rejection MBR
In the conventional configuration the membrane is actually used to filter the water,
and both nitrate and the electron donor enter the developed biofilm in the same
direction (Fig. 2.24c). Both heterotrophic and autotrophic systems have been investigated
using acetate (Barrieros et al., 1998), ethanol (Chang et al., 1993; Delanghe
et al., 1994; Urbain et al., 1996) and elemental sulphur (Kimura et al., 2002) as electron
donors, though studies based on an iMBR specifically for drinking water duties
appear to be limited to Kimura et al. (2002). These authors used rotating disc
modules to “control” fouling, thus avoiding aeration and maintaining an anoxic