Removal of volatile organic compounds in distillation steam by DBD decomposition treatment for water recycling in fermentation industry
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Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 (8pp)
Regular Paper DOI: 10.34343/ijpest.2020.14.e02003
Removal of volatile organic compounds in distillation
steam by DBD decomposition treatment for water
recycling in fermentation industry
Takanori Tanino1, 2, Kazuaki Shibuki3, Keiichi Kubota1,Naokatsu Kannari1,
Masayoshi Matsui1,Takayuki Ohshima1,2, *
1
Graduate School of Science and Technology, Gunma University, Japan
2
Gunma University Center for Food Science and Wellness, Japan
3
Ajinomoto Co. Inc., Japan
* Corresponding author: tohshima@gunma-u.ac.jp (Takayuki Ohshima)
Received: 1 April 2020
Revised: 15 June 2020
Accepted: 15 June 2020
Published online: 19 June 2020
Abstract
We investigated the removal of various organic compounds in distillation steam by dielectric barrier discharge (DBD)
decomposition treatment. Fatty acids (C1–C5), alcohols (C1–C5), and ketones (C3 and C4) were used as volatile organic
compounds that could be by-products of fermentation and were volatilized together with water as a water recovery process
from fermentation wastewater. Total organic carbon (TOC) in water was successfully decreased by condensing DBD-
treated distillation steam (DBD-treated water) in all the experiments with each organic compound. The TOC in DBD-
treated water decreased with increasing retention time of distillation steam in the DBD generation area and was almost
completely removed (over 99%) at a retention time of 61 ms. In the analysis of intermediate products remained in the
DBD-treated water after the DBD decomposition experiment of valeric acid in distillation steam, it was confirmed that
the valeric acid was decomposed to shorter fatty acids. Also, the main components of the gas products in the experiment
with propionic acid were CO2, CO, CH4, and H2.
Keywords: Decomposition, fatty acid, steam, dielectric barrier discharge (DBD).
1. Introduction
Securing stable water resources is essential for the sustainable development of many industries including the
fermentation. In particular, the fermentation industry requires a large amount of water to prepare fermentation
media and purify fermentation products such as food additives and chemicals. Not only the quantity but also
the quality of water is important in fermentation because some compounds in water inhibit microbial growth
and fermentation. In addition, the production of fuel and building-block chemicals by the fermentation of
renewable biomass, especially lignocellulosic biomass, has recently been actively studied [1–4]. In the near
future, the fermentation industry is expected to grow, increasing the importance of securing stable water
resources. The recycling of water in the fermentation process is the most promising way of achieving this.
Today, the huge amount of water used in fermentation and purification processes is separated from the large
amount of soluble and involatile components such as microbial materials by distillation because the activated
sludge treatment of wastewater requires a long time and generates a large amount of sludge. However, volatile
organic compounds including short-chain fatty acids, alcohols, and ketones, which are by-products of
fermentation, also volatilize together with water and are mixed into the recovered water. When the recovered
water is reused in fermentation and purification processes, these organic compounds inhibit microbial growth
and fermentation, and contaminate the products being purified. Therefore, these compounds must be removed
from the recovered water to allow its reuse in fermentation.
Non-thermal plasma (NTP) treatment is a promising technology for removing these organic compounds in
recovered water. NTP treatment can simultaneously decompose various organic compounds because the active
1
Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
species generated in NTP can interact with organic compounds at random. The organic compounds consisting
of C, H, and O atoms are finally decomposed to CO2 and H2O by thorough treatment; thus, there is no concern
about the presence of residual organic by-products in the water. Plasma discharge above water is the most
convenient way of applying NTP to water [5, 6], but its decomposition efficiency is limited by the high mass
transfer resistance of active species into the water. NTP generation in water using a submerged electrode and
bubbles as the trigger of plasma generation is an effective way of improving the mass transfer of active species
into water that has been studied for more than two decades [7–9]. Moreover, supplying water in a form that
can easily access the plasma generation field of the gas phase, such as a mist [10, 11] or a thin film [12, 13], is
also effective for facilitating the decomposition of organic compounds in water. Indeed, many studies on water
treatments using plasma to clean liquid water have been reported [14, 15]. However, it is difficult to treat a
huge amount of water using NTP because the stable and continuous generation of plasma in water is
challenging owing to the high breakdown voltage of liquid water. In addition, supplying water to a plasma
reactor in a controlled manner requires a large amount of energy as well as the operation of complex
apparatuses. Fortunately, the main process for recycling wastewater in the fermentation industry is distillation
at a low pressure, in which the water and organic compounds are gaseous. The low pressure makes it easy to
generate NTP in the gas phase. In this study, therefore, as a means of purifying recovered water in the
fermentation industry, the removal of volatile organic compounds in distillation steam by dielectric barrier
discharge (DBD) decomposition was investigated. Total organic carbon (TOC) was used as the index of the
organic compounds, and intermediates in the decomposition were analyzed by liquid and gas chromatography.
2. Materials and methods
2.1 Experimental setup and DBD decomposition treatment procedure
A schematic of the experimental setup used in this study is shown in Fig. 1a. A mantle heater was used to
boiling organic compound solution to generate steam containing volatile organic compounds. The flow path
of the steam and the constant-temperature chamber were kept at 110 ˚C to prevent the steam from condensing
to a liquid. The DBD reactor consisted of a quartz tube (I.D. 6 mm and O.D. 8 mm), a titanium rod (3 mm),
and a copper mesh (100 50 mm, 50 meshes/inch), which were used as a dielectric material, a high-voltage
electrode, and a ground electrode, respectively (Fig. 1b). The copper mesh was wrapped around the quartz tube
and connected to the ground. The titanium rod was at the center of the quartz tube and connected to an AC
high-voltage (AC H.V.) power supply (AGF-B10, KASUGA DENKI Inc., Kanagawa, Japan). A Graham
condenser was used to condense the steam after DBD treatment.
Fig. 1. Schematic of the experimental setup (a) and image of DBD reactor (b).
2
Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
The DBD decomposition treatment was carried out as follows. Two hundred and fifty milliliters of solution
containing 1000 ppm volatile organic compound was placed in a flask and heated by the mantle heater. After
confirming the generation of steam from the upper outlet of the three-way cock connected to the flask, DBD
was generated by applying an AC H.V. (6 kVp−p, 25 kHz). The consumed power of the DBD was 40 W. Then,
the steam was introduced into the flow path connected to the DBD reactor by turning the three-way cock. The
amount of steam introduced into the DBD reactor was controlled by draining part of the steam with a needle
valve. The steam was supplied to the DBD reactor for 30 min. The steam treated by DBD was condensed to a
liquid by cooling with the Graham condenser, and the resulting liquid (hereafter, DBD-treated water) was
collected in a flask connected to the Graham condenser. In this study, the amounts of volatile compound in
solution were sufficiently smaller than that of water, then we considered that there was no significant difference
in the amount of each steam generated from the water and solution containing various organic compounds.
The retention time of steam in the DBD generation area was calculated by using water and considering the
steam as an ideal gas. The mole number of water collected in the flask after supplying steam for 30 min was
determined, and the volume of the steam of that mole number at 110 ˚C was calculated using equation of state
of ideal gas. Then the flow rate was calculated from the value of this volume of the steam at 110 ˚C, and
retention time in the DBD generation area was determined.
2.2 Analysis of DBD-treated water and produced gas
TOC in the DBD-treated water was measured with a TOC analyzer (TOC−L, Shimadzu Co., Kyoto, Japan).
Organic acids in the DBD-treated water were analyzed by high performance liquid chromatography (HPLC)
using an electrical conductivity detector (CDD−10A, Shimadzu) with a Shim-Pack SCR102H column
(Shimadzu) for separation. The HPLC apparatus was operated at 40 ˚C with flow rate of the mobile phase (5
mM p-toluenesulfonic acid monohydrate) and post-column buffer (5 mM p-toluenesulfonic acid monohydrate,
0.1 mM EDTA-4H, 20 mM Bis-Tris) of 0.8 mL min−1. The gas produced during DBD decomposition treatment
was analyzed using a gas chromatography–thermal conductivity detector (GC-TCD; GC−2014, Shimadzu) and
a gas chromatography–flame ionization detector equipped with a methane converter (GC-FID; GC−2014,
Shimadzu). The analysis conditions of the GC-TCD were as follows. A Shincarbon ST column (Shinwa
Chemical Industries Ltd., Kyoto, Japan) was used for separation. Ar at flow rate of 20 mL min−1 was used as
the carrier gas, and the temperatures of the column and detector were 60 and 200 ˚C, respectively. The analysis
using the GC-FID employed a Sunpak-A column (Shinwa Chemical Industries) for separation, N2 at a flow
rate of 20 mL min−1 as the carrier gas, and H2 at 400 ˚C as the methanizer. The temperature of the column was
first maintained at 50 ˚C for 5 min, then increased at a rate of 20 ˚C min−1 to 150 ˚C, and maintained at 150 ˚C
for 10 min. The temperature of the detector was 150 ˚C.
3. Results and discussion
3.1 Decomposition of organic compounds in distillation steam by DBD treatment
We selected several short-chain fatty acids (C1–C5), alcohols (C1–C5), and ketones (C3, C4) as volatile organic
compounds that might simultaneously evaporate during the distillation of wastewater in the fermentation
industry. The removal of these volatile organic compounds in distillation steam by DBD treatment was
investigated at a retention time of 16 ms in the DBD generation area (Fig. 2). TOC in the recovered water
without DBD treatment (hereafter, untreated water) depended on the volatility and carbon chain length of the
organic compound. A decrease in TOC in the DBD-treated water was confirmed in all the experiments with
each selected organic compound. The DBD-treated water was transparent and contained no insoluble solid,
and there was no difference in appearance between the untreated and DBD-treated water (data not shown).
This result shows that each gaseous organic compound in the distillation steam was decomposed by DBD
treatment and part of the organic compound was completely decomposed to CO2 and/or converted to insoluble
gaseous carbon compounds. The relative decrease in TOC in the DBD-treated water in the experiment with C1
organic compounds (formic acid and methanol) were higher than those with longer organic compounds. This
is considered to be because the decomposition of C1 organic compounds does not produce shorter-carbon-
chain intermediates by the cleavage of the carbon chain that can be easily oxidized and converted to CO and
CO2. DBD was generated in the distillation atmosphere which mainly consists of water, therefore, a large
3
Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
number of highly reactive hydroxyl radicals could be produced via inelastic electron collisions with water [16].
Norsic et al. reported that DBD treatment in humid air converted gaseous methanol to formaldehyde, formic
acid, CO, and CO2 [17]; formic acid in water can also be oxidized and decomposed to CO2 by hydroxyl radicals
[18, 19]. On the other hand, residual TOC in the DBD-treated water tended to increase with the length of the
carbon chain in the initial organic compound. This might have resulted from the production of intermediates
with a shorter-carbon-chain by the cleavage of the carbon chain of the initial organic compounds and the
incomplete decomposition of the produced intermediates.
1200
1200 1200
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(a) (b)
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1000 1000
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TOC (mgC/L)
TOC (mgC/L)
800
800 800
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600
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400
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Fig. 2. TOC in distillation steam with and without DBD treatment. Experiments were
carried out with fatty acids (a), ketones (b), and alcohols (c). White and gray bars show
the untreated and DBD-treated water, respectively.
The effect of the retention time of the distillation steam in the DBD generation area on the removal of
organic compounds from DBD-treated water was investigated using fatty acids (Fig. 3). TOC in the DBD-
treated water decreased with increasing retention time in all experiments with each fatty acid. The initial
decrease in TOC was rapid and a large part of the TOC in the DBD-treated water was removed within a
retention time of 25 ms. The TOC in DBD-treated water was completely removed at a retention time of 38 ms
in the experiment with formic acid, but there was a small amount of TOC remaining in the DBD-treated water
in the experiment with other fatty acids. The relative decrease in TOC in the experiment with isovaleric acid
was lower than that with valeric acid for the retention time of 38 ms, even though both compounds contain the
same number of carbons (C5). This was due to the branched-chain structure of carbon in the isovaleric acid,
in which the surrounding hydrogen atoms might make it more difficult for radicals to interact with the carbon
skeleton. Almost complete removal of TOC in DBD-treated water (over 99%) was achieved in the experiments
with fatty acids except for formic acid at the retention time of 61 ms. The amounts of TOC in the untreated
water varied from 329 mgC L−1 (acetic acid) to 926 mgC L−1 (valeric acid); however, the relative decreases in
TOC as a function of retention time showed almost the same tendency for all compounds. In addition, the
4Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
relative decrease in TOC became lower when the amount of remaining TOC was small. As one of the
promising hypotheses we consider, it is considered that radicals including hydroxyl radicals generated from
the surrounding gaseous water might be much larger than that of organic compound molecules in the
distillation steam used for DBD treatment, and a factor controlling the decomposition of organic compounds
might be the contact frequency between radical and organic compound molecules. This hypothesis could
explain the observed behavior of the decrease in TOC in DBD treatment, therefore further studies including
measurement of number of radicals to verify this hypothesis would be necessary.
Fig. 3. Effect of retention time of distillation steam in DBD generation area on TOC removal
from DBD-treated water. Graphs show the TOC (a) and its relative decrease (b) in DBD-
treated water.
Fig. 4. Effect of amount of TOC in distillation steam on removal of TOC. Graphs show the TOC
(a) and its relative decrease ratio (b) in DBD-treated water.
The effect of the amounts of organic compounds in distillation steam on the removal of organic compounds
from DBD-treated water was investigated with 1000, 5000, and 10000 ppm propionic acid solutions used to
produce the distillation steam (Fig. 4). TOCs in the untreated water in the experiments with 1000, 5000, and
10000 ppm propionic acid solutions were 600, 3017, and 6271 mgC L−1, respectively. The decrease in TOC in
the DBD-treated water in the experiment with 5000 ppm solution at a retention time of 16 ms was 727 mgC
L−1, which is greater than the TOC in the untreated water in the experiment with 1000 ppm solution (mgC L−1).
Moreover, the decrease in the TOC in DBD-treated water in the experiment with 10000 ppm solution at a
retention time of 25 ms was 3430 mgC L−1, which was greater than the TOC in the untreated water in the
experiment with 5000 ppm solution. These results show that radicals including hydroxyl radicals able to
contribute to the decomposition of organic compounds are produced in large numbers, and that contact between
radicals and organic compounds is the rate-limiting factor in the removal of organic compounds in DBD-
treated water. TOC in DBD-treated water was almost completely eliminated in the experiment with 5000 ppm
solution at a retention time of 61 ms. The maximum decrease in TOC in the DBD-treated water was 5422 mgC
L−1 in the experiment with 10000 ppm solution at a retention time of 61 ms. The generation of DBD in the
steam decomposed a large amount of organic compounds in the distillation steam in a relatively short retention
5Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
time (ms) and was very effective for removing TOC from water. The distillation process is already utilized in
the fermentation industry, and the generation of DBD in steam to decompose organic compounds is suitable
for this process. On the other hand, the distillation process and the maintenance of steam in the gaseous state
require energy, so it is not economical or environmentally friendly to apply this technique to a simple water
treatment to decompose a small amount of organic compounds in water. Other promising applications of
plasma to a high-temperature gaseous phase in which exhaust heat can be used to generate steam, such as the
treatment of engine exhaust gas by plasma, have already been reported [20, 21]. Moreover, steam is used in
other processes in food industry, such as superheated steam sterilization. Therefore, the generation of DBD
using the steam employed in these processes is expected to improve the treatment efficiency.
3.2 Analysis of intermediates in decomposition of fatty acids by DBD generation in distillation
steam
Carboxylic acids produced by the decomposition of valeric acid in distillation steam by DBD treatment were
investigated by HPLC analysis. A chromatogram obtained by HPLC analysis with untreated water showed a
single peak corresponding to valeric acid (data not shown). Figure 5 shows the chromatograms obtained in
HPLC analysis with appropriately diluted DBD-treated water in the experiment at retention times of 16, 25,
and 61 ms. It was confirmed that shorter-chain fatty acids and succinic acid (C4), a dicarboxylic acid, were
produced by the decomposition of valeric acid at the retention time of 16 ms (Fig. 5a). The production of
succinic acid indicates cleavage at the end carbon of valeric acid and the oxidation of the newly exposed carbon
chain terminal. The peak of valeric acid disappeared and the peak of acetic acid increased in magnitude at the
retention time of 25 ms (Fig. 5b). A small peak of acetic acid also remained even at the retention time of 61
ms (Fig. 5c). Under the conditions employed in this study, acetic acid was easily produced by the
decomposition of fatty acids, which was relatively difficult to be decompose by DBD treatment in distillation
steam. For all retention times, TOC calculated from the concentrations of fatty acids analyzed by HPLC was
lower than that measured using the TOC analyzer. It has been reported that small amounts of various chemical
species, in addition to the carboxylic acids measured in this study, are produced through the decomposition of
valeric acid by a photocatalyst [22] or an electron beam [23]. Therefore, the difference in TOC might result
from the existence of these chemical species in the DBD-treated water.
Fig. 5. Chromatograms of HPLC analysis of DBD-treated water at retention times of 16
(a), 25 (b), and 61 (c) ms.
6Int. J. Plasma Environ. Sci. Technol. 14 (2020) e02003 T. Tanino et al.
Table 1 Composition of gas produced in decomposition of propionic acid in distillation
steam by DBD treatment.
Compound Ratio (%)
CO2 20
CO 18
H2 47
CH4 13
C2H4 4.7 10−2
C2H6 2.6 10−1
C3H6 9.7 10−3
C3H8 1.5 10−1
others 1.1
The gas produced in the decomposition of propionic acid was investigated using a GC-TCD and a GC-FID
(Table 1). The main components of the produced gas were CO2, CO, H2 and CH4. CO2 and CO were produced
by the oxidation of the cleaved carbon chain of propionic acid by oxygen atoms provided by H2O. H2 originated
from the hydrogen atoms provided by H2O and propionic acid. In addition to CH4, small amounts of C2H4,
C2H6, C3H6, and C3H8 were also detected. This indicates that part of the propionic acid was removed from the
DBD-treated water by conversion into insoluble gaseous organic compounds, and these compounds can be
oxidized and converted to H2 and CO2 by increasing the retention time of distillation steam in the DBD
generation area. In the fermentation industry, the H2 produced in the DBD treatment of distillation steam might
contribute to the recovery of energy used in the distillation process.
4. Conclusion
We successfully decomposed various volatile organic compounds in distillation steam by DBD treatment. TOC
in DBD-treated water in the experiment with C1 organic compounds was more easily removed than that with
C2–C5 organic compounds. In experiments with C1–C5 fatty acids, extending the retention time of distillation
steam in the DBD generation area improved the efficiency of TOC removal from DBD-treated water, and an
almost complete removal of TOC (over 99%) was achieved at a retention time of 61 ms. Increasing the TOC
in distillation steam increased the TOC removed at the same retention time. From these results, it was argued
that the radicals including hydroxyl radicals produced in DBD were highly abundant and that the rate-limiting
factor in the decomposition of organic compounds was the contact between radicals and organic compounds.
The analysis of intermediates in the decomposition of valeric acid indicated the production of shorter fatty
acids, especially acetic acid, which remained in the DBD-treated water. The analysis of gas products in the
experiment on propionic acid decomposition showed that CO2, CO, H2, and CH4 were the main components
in the DBD decomposition treatment of distillation steam. Although it remains necessary to actually test
whether DBD-treated water can be reused in the fermentation process, the application of DBD treatment to the
distillation process could be effective for the removal of volatile organic compounds in water recycling in the
fermentation industry.
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