RECOVERY OF POTASSIUM FROM DUNDER USING DIFFERENT MEMBRANE
OPTIONS
Alice Antony1 Kylie Lim1, Lyn Darius1, Audrey Luiz2, John Kavanagh2, Greg Leslie1
1
2
UNESCO Centre for Membrane Science and Technology, The University of New South
Wales, Sydney 2052, Australia.
School of Chemical and Biomolecular Engineering, Chemical Engineering Building J01,
The University of Sydney, NSW, 2006, Australia.
ABSTRACT
The benefits of potassium recovery from molasses
dunder include improved methane production from
biomethanation during anaerobic treatment and the
generation of additional supplies of potassium
based fertiliser. A feasibility study of three
membrane based techniques, electrodialysis (ED),
nanofltration (NF) and dialysis (D) was conducted
to assess the maxium potassium recoverable from
a molasses dunder stream.
ED, an energy intensitive process conventionally
used for deionisation of process streams, resulted
in a maximum recovery of 76% from diluted dunder
(20% molasses dunder). Nanofiltration, a
pressurised membrane operation with tubular
membrane configuration resulted in recovering as
high as 91% of potassium recovery from diluted
dunder (10% molasses dunder). Dialysis,
a
diffusion process driven by the concentration
gradient resulted in recovery of 46% of potassium
from undiluted dunder.
While NF is identified to be the best process for
maximum potassium recovery, the necessity to
dilute the feed and frequent membrane cleaning
limits it’s utility in this application. In contrast,
dialysis warrents further development to increase
potassium recovery based on promising separation
of undiluted high strength, complex distillery effluent
into an inorganic rich stream (recovered dialysate)
and organic rich (treated feed).
INTRODUCTION
The disposal of production waste is a major
problem faced by industries. Wastewater from
fermentation industries like breweries, distilleries,
yeast production and wineries are generally dark
brown coloured and viscous in nature. In addition,
they possess high BODs and CODs due to the
presence of various organic compounds like
polyphenols, sugars and organic acids (Arimi et al.,
2014; Lameloise & Lewandowski, 2012). Various
adverse effects on soil, water, air, and health might
occur if the untreated wastes from distillery industry
are disposed into natural environment (Lele et al.,
1989; Wilkie et al., 2000). The discharge of distillery
effluent to the seas, rivers, or lakes can lead to
eutrophication due to its high organic content
(Chaudhari). In addition, the dark colour of the
waste water can block the sunlight penetration to
the water which decreases photosynthetic activity
and dissolved oxygen concentration, harming
aquatic biota (Kaushik 2009). Disposal on land can
impact the soil fertility (Chandra et al., 2008).
Notwithstanding the environmental imperatives of
treating industrial waste prior to discharge, there
also exists an opportunity to incorporate both
energy and nutrient recovery into the treatment
process to derive additional value from the waste.
For example, nutrients in the distillery wastes could
potentially be re-used in agriculture, aquaculture,
and other activities (Hussain, 2001) while the
calorific content of the organics could be converted
to methane via anaerobic treatment.
One of the methods to recover energy from
distillery waste is biomethanation, a process that
involves a number of bacteria species to generate
methane from distillery waste under anaerobic
condition (Krishania et al., 2013). However, the
presence of inhibitors that include potassium,
ammonia, calcium, and sodium can possibly impact
the anaerobic process. Potassium has the biggest
impact over the other inhibitors due to its amount in
the spent wash (Nataraj et al., 2006). On the other
hand, potassium can be utilized as fertilizer when it
is recovered from distillery waste. Presence of high
potassium in this stream (up to 35 g/L) therefore
provides a great opportunity to convert the waste
stream into a valuable resource.
Different technological processes have been tested
for the recovery of potassium from sugar cane juice
and molasses dunder, like membrane filtration, ion
exchange and, electrodialysis. ED
selectively
separates potassium and/or other salt ions, whilst
leaving the organic stream behind to be further
processed. This is advantageous when streams
from ED can further utilise downstream, for
example by biomethanation. Similarly, nanofiltration
(NF) has the advantage of selectively passing
monovalent ions while rejecting organics and
divalents ions. Dialysis is generally applied in the
hemodialysis process, acid recovery from metal
industry and alkali recovery from aluminium ore
industry. However, there are limited reports on
theapplication of dialysis for the recovery of
resources from fermentation industry effluents In
this paper the efficiency of three techniques, ED,
NF (flat sheet and tubular configurations) and
dislysis are compared in recovering potassium from
a molasses Dunder.
METHODS AND MATERIALS
Dunder sample
Molasses dunder was sourced from an Australian
plant, a schematic of the treatment processes and
the point of sample collection is shown in
Figure 1.
The sample was centrifuged at 3,000 rpm for 20
minutes and stored at 4°C to prevent fermentation.
The sample possessed 1,424 mg/L of potassium.
anion-exchange membranes and PC-SK-ED 64004
cation-exchange membranes are used in the ED
unit. One litre diluate/feed solution of molasses
dunder was circulated through the ED stack at a
flow rate of 10 L h-1. One litre of analyte solution of
KCl at 1/10th of the K+ concentration of the feed was
circulated around the cathode and anode at 30 l/h.
ED experiments were performed by monitoring
changes in electrical potential (V) across the ED
stack at a constant current of 0.2 A. Diffusion
experiments were performed with the full strength
molasses dunder, and a synthetic solution of
30,000 mg/L of K+ in 6,000 mg/L of acetate. The
diffusion experiments were performed in the same
ED stack mounted with 10 membrane pairs under
no applied current. The current and power
efficiency may be calculated (not discussed in this
paper) with either the diluate (feed) stream which
contains K+ and organic ions, while the concentrate
stream contains the recovered K+. The Concentrate
will be the focus for this study as it deals with the
recovered K+. Total K Recovery (%) was calculated
as
%
,
–
,
,
NF
NF experiments were performed with diluted
dunder. Flat sheet and tubular configurations of
same materials namely, MPF-36 and MPT-36 are
used for filtration (Figure 2).
Dunder
Figure 1 Schematic diagram of treatment process
and sampling point of dunder sample.
ED
ED experiments were performed with 20%, 40%
and 80% of full strength molasses dunder in a
bench scale ED unit (PC Cell GmbH, Germany)
configured to house up to 20 pairs of anion and
cation exchange membranes, The diluate (feed)
and concentrate (Potassium enriched stream) inlet
and outlet are concurrent in the same cell face, to
reduce transmembrane pressure. PC-SA-ED 64004
Figure 2 Flat sheet (MPF-36) and tubular (MPT-36)
configurations of NF membrane used in this study.
The filtration experiments were performed in a
cross flow mode, at a constant pressure of 8 bar.
The fouling propensity and performance efficiency
was assessed from membrane resistance and
potassium permeability.
Membraneresistance,
∆
Where, J is the permeate flux, ∆P is the
transmembrane pressure, µ is the dynamic
viscosity of permeate, Rm denotes the membrane
intrinsic resistance (calculated during filtration with
pure water) and Rf represents the resistance due to
the fouling layer.
%
100
Dialysis
Dialysis experiment was performed with dialysis
tubing membranes of active material Glycerol and
a-cellulose. The molecular weight cut off of the
membrane is 14 kDa. Undiluted dunder was used
as the feed and milliQ water as the dialysate
without any pressure. The feed and dialysate were
circulated in a co-current mode in a cross flow cell
at a constant flow rate of 50 ml min-1 and at room
temperature. The change in the dialysate volume
and conductivity was monitored for recorded
periodically.
At the end of the experiment,
concentration of potassium recovered in the
dialysate was measure as
%
100
Where, Md is the mass of normalized potassium
recovered in dialysate and Mf is the mass of
potassium in feed. Md is calculated by multiplying
the final concentration of potassium in dialysate
(Cd) and final volume of dialysate (Vd). The
potassium transport coefficient (Volume/area/time)
can be defined in the equation as,
. .
,
where U is the potassium transport coefficient
(m/h), W is the mass of potassium recovered (mg),
A is the membrane active area (m2), t is time (h)
and M is expression to normalise a change of the
concentration driving force overtime and is
calculated with the following equation. M
C
C
ln
C
C
C
C
where Cf0 is the concentration of potassium in feed
at time 0, Cpt is the concentration of potassium in
dialysate at time t, Cft is the concentration of
potassium in feed at time t. It should be noted that
Cf0 – Cpt – Cft ≠0 because of the change of volume
occur in the feed and dialysate because of the
water movement during the experiment (Stachera
et al., 1998; Tuwiner et al., 1962).
RESULTS AND DISCUSSION
ED performance
The potassium recovery efficiency during the ED of
molasses dunder, (20%, 40%, 80% and full
strength), and the two diffusion tests (control) are
presented in Figure 4. Both the full strength
molasses dunder batches had a recovery of 42%
after 4 hours, while the diluted dunder resulted in
76, 59 and 41% for 20, 40 and 80% respectively.
The potassium recovery reported for the diluted
dunder was achieved after 2, 3 and 3.5 hours. As
the potassium concentration increased in the
diluate (feed), the processing time increased and
the potassium recovery decreased since the run
time and residence time of solution is dependent on
the amount of initial potassium concentration.
During the ED experiments with dunder stream,
current and power efficiency values were over
100% indicating transport of potassium by diffusion
in addition movement induced by the electric field.
Additional diffusion experiments were performed
with synthetic salt mixture to observe any effects of
the complex organic matrix on the diffusion
process. After 4 hours, the potassium recovery
(diffusion) was 19.6% (9,518 mg/L) for the full
strength dunder and 23% (10,905 mg/L) for the
synthetic solution. This indicated strong diffusion of
potassium ions due to concentration polarisation,
and a subtle difference in the absence of dissolved
organics.
NF performance
Change in membrane resistance as a function of
permeate recovery during the filtration is presented
in Figure 2. This represents the degree of fouling
and its consequence on the permeate recovery.
With MPF-36, a sudden increase in the membrane
ressitance was observed after 41% pf permeate
recovery. With MPT-36, the resistance was
comparatively low from the start also sudden
increase in the resistance was not bserved. With
the tubular configuration, permeate recovery as
high as 56% could be achieved while this is not
possible with the flat sheet configuration, MPF-36.
Potassium recovered with the two membrane
configurations are similar, 90.8% for the flat sheet
and 91.2% for the tubular membrane. In the initial
stages of the filtration the potassium recovery
(permeability to permeate) was close to 100% but
this decreased as the filtration progressed. The
rejection behaviour of the NF membranes should
be collectively viewed as sieving and charge effects
for feed containing organics and ionic species. The
rejection of organics, generally large sized complex
aromatic structures are expected to be mostly
through the size exclusion mechanism and the
rejection of ionic species are by diffusion
mechanism. Progressive accumulation of organic
fouling layer acts as a secondary filtration layer and
therefore further separation is dominantly controlled
by this secondary layer during the later stages of
the process. Therefore, the potassium recovery is
expected to be decreased as a function of
permeate recovery.
.
Dialysis performance
Dialysis is a concentration driven process involving
the counter current diffusion of ions and water
molecules across the membrane. Movement of
potassium ions from the feed to the dialysate (low
potassium concentration solution) over 96h is
presented in (Figure 6, a). The conductivity of
dialysate increased rapidly for the first 8 hours (a);
a maximum was reached at around 26 h (b). Over
96 h 46.66% of potassium was recovered along
with 5.83% of organics and 36.91% of divalent ions.
Potassium transport coefficient for the first 6h is
presented in Figure 6, b. The decline in the
potassium transport coefficient over time is due to
the decreasing of potassium concentration in feed.
CONCLUSION
The recovery of potassium increased with increase
in the dilution of Dunder during ED, highest
achieved for 20% of molasses resulted with
recovering 76% of potassium. The diffusion tests
proved that there was transfer of potassium through
the ED stack without the need for applying current.
The diffusion tests also refer to the organic matrix
of molasses dunder having subtle effects on
efficiency and potassium recovery.
Among the three techniques, NF resulted in the
maximum recovery of potassium. The significant
feature of NF membranes is the selective
separation of monovalents from multivalents and
organics. This complemented an effective recovery
of Potassium >90%. The tubular NF membrane had
less fouling and therefore could be operated to
higher recoveries.
Although the recovery of potassium was not high
with dialysis, the recovery achieved, 21% is
beneficial since there is no energy invested.
ACKNOWLEDGMENT
This project is funded by Commonwealth of
Australia, Department of Industry, Innovation,
Science, Research and Tertiary Education, through
Australia-India Strategic Research Fund (Grant No.
ST060095). The authors thank Wilmar BioEthanol
(Australia) Pty Ltd for providing dunder sample.
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Figure 3. Schematic of Electrodialysis Stack including Diluate [A], Concentrate [B] and Analyte [C]
recirculation streams.
Figure 4 Potassium recovery as a function of time during the ED.
5
Figure 5 Membrane resistance as a function of permeate recovery during the NF of Dunder in tubular
and flat sheet configurations.
Figure 6 (a) Change in dialysate conductivity for 96 h (b) Potassium transport coefficient calculated for
the first 6 h.
6