Performance evaluation of hybrid OMBR-MD using organic
and inorganic draw solutions
Sher Jamal Khan, Muhammad Saboor Siddique and
Hafiz Muhammad Aamir Shahzad
ABSTRACT
The performance of two inorganic divalent salts (CaCl
2,
and MgCl
2
) and two organic salts (CH
3
COONa
and Mg(CH
3
COO)
2
) was compared with commonly used NaCl in an osmotic membrane bioreactor
(OMBR) integrated with a membrane distillation (MD) system. The system was investigated in terms of
salinity buildup, flux stability, draw solution (DS) recovery and contaminants removal efficiency. Results
indicated that organic DSs not only lessen the salt accumulation within the bioreactor but also
increase the pollutant removal efficiency by improving biological treatment. Of all the draw solutions,
NaCl and CaCl
2
produced rapid declines in water flux because of the high salt accumulation in the bio-
tank as compared to other salts. The DCMD system successfully recovered all organic and inorganic
draw solute concentrations as per OMBR requirements. Membrane flushing frequency for the MD
system followed the order Mg(CH
3
COO)
2
> CH
3
COONa > CaCl
2
> MgCl
2
> NaCl. More than 90%
removal of chemical oxygen demand (COD), NH
4
þ
-N, and PO
4
3
-P was achieved in the permeate for each
salt because of the dual barriers of high-retention membranes i.e., forward osmosis and MD.
Sher Jamal Khan (corresponding author)
Muhammad Saboor Siddique
Hafiz Muhammad Aamir Shahzad
Institute of Environmental Sciences and
Engineering (IESE),
National University of Science and Technology
(NUST),
Islamabad,
Pakistan
E-mail: s.jamal@iese.nust.edu.pk;
sherjamal77@gmail.com
Key words | draw solutes, flux stability, reverse solute transport, salinity buildup
INTRODUCTION
In the last decade, forward osmosis (FO) membranes have
been investigated in membrane bioreactor (MBR) processes
for the development of an innovative idea i.e., the osmotic
membrane bioreactor (OMBR) to increase dissolved solid
rejection, obtain superior effluent quality, minimize power
consumption and lower membrane fouling propensity
(Achilli et al. ). Additionally, osmotic pressure as an
alternative for hydraulic force is used in the FO system
resulting in reduced and compact biofilm as compared to
the biofilm generated by hydraulic pressure (Lay et al.
a; Kazner et al. ). In spite of these advantages,
OMBR also presents some technical challenges such as
lack of appropriate draw solution (DS) recovery method,
membrane fouling during prolonged filtration runs and sal-
inity buildup in the bioreactor due to reverse salt flux. Salt
accumulation within the bioreactor severely affects the bio-
activities of microbial flora, as the majority of the functional
bacteria are extremely sensitive to high saline stress environ-
ment (Luo et al. , ). Researchers have applied
different approaches to confronting these challenges of
salinity buildup within the bioreactor, such as: sludge
wastage by optimizing the solids retention time (SRT),
removal of supernatant from settled biomass, integration
of ultrafiltration (UF) or microfiltration (MF) units with
the OMBR to bleed out the extra salt content via the MF/
UF membranes, employment of organic DSs instead of inor-
ganic salts, etc. (Wang et al. ; Aftab et al. ; Luo et al.
). Recently, the use of organic DS in FO systems has
emerged as a promising approach to reducing the saline
stress within the bioreactor because organic ions diffusing
into the bioreactor across the semi-permeable membrane
can easily be biodegraded by the activated biomass
(Bowden et al. ; Holloway et al. ; Ansari et al. ).
Moreover, the maintenance of an osmotic gradient
between the feed and DS sides is also considered to be a
hurdle for continuous OMBR operation. To date, separation
processes such as electrodialysis (ED), reverse osmosis
(RO), nanofiltration (NF), UF and membrane distillation
(MD) have been evaluated for the recovery of DSs. Regard-
less of being well-established and having a low energy
requirement, UF membranes may not effectively reject the
draw solutes because of their large pore size (>10 nm)
776 © IWA Publishing 2018 Water Science & Technology | 78.4 | 2018
doi: 10.2166/wst.2018.345
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