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submerged hollow-fibre membrane filtration and a sequencing aerobic sludge blanket
reactor (SASBR) leads to a novel aerobic granular sludge membrane bioreactor (AGMBR)
[43]. The much better filtration characteristics of AGMBR mixed liquor are due to the low
compressibility of its biomass, which is dominated by aerobic granular sludge. However, it
has been reported that irreversible fouling is more severe than in conventional membrane
bioreactors, which has been attributed to the colloids and solutes generated [44]. A hybrid
process incorporating membrane distillation in a submergedmembrane bioreactor operated
at elevated temperature has been developed. Since organic particles are rejected by an
'evaporation' mechanism, the retention time of nonvolatile soluble and small organics in
the submerged membrane distillation bioreactor (MDBR) is independent of the hydraulic
retention time (mainly water and volatiles) [45]. A high permeate quality can be obtained in
the one-step compact process. Subject to the availability of waste heat/solar thermal energy
and a cooling system to exchange heat with a cooler natural resource, the MDBR may offer
a low primary energy process with a very high permeate quality and a stable flux, at a
practical value in a small-footprint configuration. An innovative osmotic membrane
bioreactor (OMBR) has been developed, based on forward osmosis (FO) driven by an
osmotic pressure difference. For the OMBR to be both technically and economically viable,
the performance of the FOmembranes must be sufficiently high and membrane fouling and
draw solution leakage must be low. To improve the FO membrane performance for the
OMBR process, the thickness and structure of the porous substructure of the FOmembrane
must be optimized by membrane development.
Membrane fouling and high cost are the major obstacles to the widespread use of
membrane bioreactors in waste water treatment. The membrane is contaminated rapidly
because of the high concentration of waste water passing through it. This decreases the
membrane lifespan and increases aeration energy demands and operating costs. Different
strategies are being developed to overcome these limitations. Novel nonwoven textiles may
be a more economical alternative to the usual ultra- or microfiltration membranes, due to
lower production costs and potentially higher permeability, which apart from smaller
filtration surface areas would also lead to savings in aeration requirements. The application
of nonwovens has recently been investigated in lab-scale membrane bioreactors [46], but
research is at an early stage and no membrane bioreactor textiles are commercially
available as yet.
Various strategies for fouling control have been developed, including:
Feed waste water pre-treatment by physical (aquatic substance size increase), chemical
(coagulants, oxidants and adsorbents) or biological (biodegradation) mechanisms.
Biomass characteristic modification according to biomass properties.
Optimization of operational conditions (operation below critical flux, aeration).
Antifouling membranes, obtained by modifying hydrophobic membranes into relatively
hydrophilic membranes.
8.4.3 Waste Valorization to Produce High-added-value Compounds
In this section, two case studies will be described which have gained a lot of interest in the
green chemistry field: lactic acid production and phenolic compound valorization from
waste materials.
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