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Volatile fatty acid production dynamics during the acidification of pretreated olive mill wastewater
Accepted Manuscript Volatile fatty acid production dynamics during the acidification of pretreated olive mill wastewater Canan Can Yarımtepe, Nilgün Ayman Oz, Orhan Ince PII: DOI: Reference:
S0960-8524(17)30841-6 http://dx.doi.org/10.1016/j.biortech.2017.05.173 BITE 18199
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
27 March 2017 25 May 2017 26 May 2017
Please cite this article as: Yarımtepe, C.C., Oz, N.A., Ince, O., Volatile fatty acid production dynamics during the acidification of pretreated olive mill wastewater, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/ j.biortech.2017.05.173
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1
VOLATILE FATTY ACID PRODUCTION DYNAMICS DURING THE
2
ACIDIFICATION OF PRETREATED OLIVE MILL WASTEWATER
3
Canan Can Yarımtepe1,2, Nilgün Ayman Oz2,3* and Orhan Ince1
4 5
1
Istanbul Technical University, Department of Environmental Engineering, 34469, Maslak, Istanbul, Turkey
6 7
2
Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100, Çanakkale, Turkey
8 9
3
Center for Environmental Research and Application, Çanakkale, Turkey
10 11
ABSTRACT
12
This study focuses on the dynamics of VFA production from pretreated olive mill
13
wastewater (OMW). Acidogenic anaerobic sequencing batch reactors (AcASBR) were
14
operated with the pretreated OMW at hydraulic retention time (HRT) of 2 days and pH
15
of 5,5 for different organic loading rates (OLRs) ranging from 5 gr COD/L to 40 gr
16
COD/L. VFA production reached to the highest value of about 27000 mg/L with the
17
increase in the organic load applied (20 gr COD/L). The highest acidification rate was
18
calculated as 68%. Acetic acid was found to be as the dominant VFA compound for all
19
stage of the study. At steady state, VFA production during a day-cycle period (24th
20
hours) in AcASBR is also monitored, VFA production gradually increased after the 3rd
21
and 6th hours (about 20%) and a rapid increase (about %40) was observed reaching the
22
maximum at the end of the cycle (24th hours).
23
1
1
Keywords: VFA production, acidification, byproduct, electrocoagulation pretreatment,
2
olive mill wastewater, resource recovery.
3 4
*Corresponding author. Address: Çanakkale Onsekiz Mart University, Department of
5
Environmental Engineering, Çanakkale 17100, Turkey. Tel.: +90 286 218 0018 / 2177;
6
Fax: +90 286 218 0541. E-mail addresses: [email protected]
7 8
1. INTRODUCTION
9
Turkey is the 5th largest olive oil producer after Spain, Italy, Greece and Tunisia. Olive
10
mill wastewater (OMW) which is produced during the olive oil production process, is
11
one of the most important environmental problems for Mediterranean countries. It is
12
challenging to manage OMW, which occurs seasonally at high flow rate and with high
13
fluctuation of organic matter, suspended solids, oil and phenol concentrations. In
14
literature, several treatment processes and process combinations (El-Gohary et al., 2009;
15
Azbar et al., 2008) have been reported for OMW. However, both efficient and feasible
16
treatment process have not been addressed for the wastewater, so far. Especially for
17
OMW, diluted wastewaters are commonly used in several studies, because treatment
18
efficiency of the biological processes is directly affected by the high content of organic
19
matters, solid matters and toxic compounds of wastewaters. Due to the fact that dilution
20
can not be proposed as a management strategy for treatment of wastewaters,
21
pretreatment methods have been proposed prior to biological processes for OMW in
22
several studies such as ultrasound (Oz and Uzun, 2015), ozonation (Betinez et al.,
23
1997), fenton (El-Gohary et al., 2009), physicochemical pretreatment (Filidei et al.,
24
2003), aerobic treatment (Cereti et al, 2004) and electrochemical pretreatment (Khoufi
2
1
et al., 2006; Hanafi et al., 2011). Electrocoagulation (EC) is a electrolytic treatment
2
process which can remove the pollutants from wastewater such as suspended solid,
3
organic matter content, oil and phenolic compounds (Khoufi et al., 2007). The dominant
4
mechanism may vary throughout the process dynamics depending on treatment
5
conditions, operating parameters and pollutant types of wastewaters (Holt et al., 2005).
6
Several studies have been examined about EC for OMW as main, pre or post treatment
7
option (Khoufi et al., 2007; Adhoum and Monser, 2004; Hanafi et al., 2010). However,
8
integration of EC process with acidification reactors to maximize the VFAs from OMW
9
has not been reported in the literature, so far.
10
Instead of treating strength wastewaters such as OMW to provide effluent discharge
11
standards with a high cost, resource recovery which provide value-added products could
12
be an alternative or supplementary to wastewater treatment. Resource recovery from
13
waste/wastewater streams which should be considered as a source of valuable products
14
and energy, has been a major concept in recent days (Guest et al., 2009). Different
15
materials such as water, bioplastics, energy in terms of methane and hydrogen, phenolic
16
compounds and volatile fatty acids (VFAs) can be recovered from waste activated
17
sludge (Li et al., 2016; Wang et a., 2015), food wastes (Zhao et al., 2016) and especially
18
organic-rich wastewaters which are regarded as suitable renewable sources (Editors,
19
2009).
20
Short-chain fatty acids which consisting of six or fewer carbon, are identified as VFAs
21
(APHA, 1992). VFAs which formed during the acidification of organic-rich
22
wastewaters, can be used in many ways such as for bioplastics/bioenergy production
23
(Mengmeng et al., 2009; Uyar et al., 2009) and biological nitrogen and phosphorus
24
removal from wastewaters as a preferred carbon source (Tong and Chen, 2007; Chen et
3
1
al., 2016). Acidogenic reactor plays a crucial role in the conversion of complex organic
2
substrates to VFAs in OMW.
3
wastes/wastewaters, carbon based chemicals can alternatively produced and this would
4
represent a sustainable option for wastewater management. With the optimization of
5
VFA-producing bioreactors, valuable products with higher yields and purity can be
6
obtained from organic-rich wastes/wastewaters (Elmekawy et al., 2014). Various wastes
7
and wastewaters have been tested for VFA production in literature (Morgan-Sagastume
8
et al., 2015; Bayr et al., 2012; Ji et al., 2010; Cesaro et al., 2013; Kim et al., 2005).
9
However the studies focused on organic-rich wastes and wastewaters such as sludge,
10
food wastes and agricultural wastes/wastewaters (Lee et al., 2014) due to their high
11
COD content and possibility of producing VFAs at high rates (Dionisi et al., 2005). In
12
literatüre, there is limited study about VFA production from OMW (Dionisi et al., 2005;
13
Beccari et al., 2009; Scoma et al. 2013). In these studies, different pretreatment process
14
were investigated for production of biodegradable polymers from OMW and under
15
optimum conditions, acidification rates have reported as between 20% to 45%.
16
However, there is no reported study which evaluate VFA production and conversion
17
dynamics of OMW in acidogenic anaerobic sequencing batch reactor (AcASBR) in this
18
context. Since the composition of end products significantly differs based on the
19
reactor’s conditions during the acidification process, it is strategically important to
20
identify VFA production mechanism and conversion dynamics of different VFA types.
21
Therefore, the aim of this study is to optimise the acidification potential of pretreated
22
OMW via AcASBRs for higher VFAs productivity. tVFA production and VFAs
23
composition have also been monitored in detail in order to understand both process
By the recovery of VFAs from organic
4
1
efficiency in terms of acidification and the conversion dynamics during operation period
2
and for a-day-cycle period at different operating periods.
3 4
2. MATERIALS AND METHODS
5
2.1.Olive Mill Wastewater Characterization
6
Olive mill wastewater samples were taken from TARIS Olive and Olive Oil
7
Agricultural Sales Cooperatives Union in December, 2015 as a composite sample. For
8
the determination of the OMW’s composition; pH, conductivity, turbidity, color, total
9
phenol, total chemical oxygen demand (tCOD), soluble chemical oxygen demand
10
(sCOD) (after vacuum filtration through 0.45µm membrane), biochemical oxygen
11
demand (BOD), total organic carbon (TOC), total volatile fatty acid (tVFA), total solids
12
(TS), total volatile solids (TVS), total suspended solids (TSS) and volatile suspended
13
solids (VSS) were monitored. The characterization of olive mill wastewater sample are
14
given in Table 1.
15 16
Table 1. OMW Composition.
17 18
2.2.Experimental Set-up
19
Experimental set-up used during the study was shown in Figure 1. Since raw OMW
20
contains high concentration of pollutants (as seen from Table 1), OMW was pretreated
21
by EC in order to reduce pollutant load, especially solid content which is very
22
problematic for biological reactors.
23 24
Figure 1. Experimental set-up.
5
1 2
2.2.1. Electrocoagulation as a Pre-treatment Step
3
Electrocoagulation process was carried out in a lab-scale 1 L glass reactor with consists
4
a cover supporting two parallel aluminum electrodes. Selection of electrode type is the
5
main part of an electrochemical process and it is directly related to removal efficiencies.
6
The most common electrode materials for EC are aluminum and iron because they are
7
cheap, readily available and effective. In this study, aluminum electrodes are selected
8
based on the results of preliminary studies. Sample volume was 500 ml for experiment.
9
There were 2 cm distance between electrodes and 2 cm distance between the bottom of
10
the electrodes which allowed easy stirring of the effluent. A direct current as 2A with
11
different voltages including 4, 6, 8, 10 and 12 voltages were tested by using a power
12
supply (TT-T-ECHNI-C MCH-305D-II) for four hours. During the experiments, the
13
change in voltage and the current were monitored by multimeter (UNI-T UT61D Digital
14
Modern Multimeters UT-61D AC/DC Tester). After EC, samples were precipitated for
15
12 hours and supernatant was analyzed in terms of removal of major in pollutants
16
removal determination according to standard methods.
17 18
2.2.2. Acidification Process
19
Acidification process was performed with 1 L anaerobic sequencing batch reactors with
20
a total cycle period of 24 h consisting of 15 min for filling, 23 hours for reaction, 30
21
min for settling and 15 min for decant. Reactor with separate gas and sample ports was
22
submerged in a water bath for temperature control and mixing. Seed sludge which used
23
in acidification reactors was obtained from a yeast factory's acidification reactors. Seed
24
sludge's Total Solid and Total Volatile Solid concentration was determined as 163382
6
1
mg/l and 60612 mg/l, respectively. With aim of determination optimal conditions for
2
acidification; organic loading rate was increased gradually as 1 gr COD/ gr TVS to 2 gr
3
COD/ gr TVS, 4 gr COD/ gr TVS and 8 gr COD/ gr TVS for testing while
4
microorganism concentration was stable at 7500 TVS/L in reactors. Acidification
5
reactors were performed for 1.7±0.3 days hydraulic retention time at pH 5-5.5. The
6
daily alkalinity addition was carried out to keep the pH of the reactors at 5.5. Reactors
7
were fed with pretreated olive mill wastewater by EC. During the study; pH, sCOD,
8
TSS, tVFA concentration and composition were monitored daily. All analyzes were
9
carried out according to standard methods (APHA, 1992). tVFA concentration and
10
composition were monitored with GC. Acidification efficiencies were determined as
11
acidification rate and calculated by following equation (Oktem et al., 2006). The COD
12
equivalents for VFAs are considered as; 1.066 mg COD/l for mg/l acetic acid, 1.512
13
mg COD/l for mg/1 propionic acid, 1.816 mg COD/l for mg/1 butyric and isobutyric
14
acid, 2.036 mg COD/l for mg/ valeric and isovaleric acid, and 2.204 mg COD/l for mg/1
15
caproic acid (Oktem et al., 2006).
16 1.
17
Throughout the study, the amount of tVFA was indicated as acetic acid equivalent.
18 19
2.3.Analytical methods
20
Analyses were carried out according to the Standard Methods (APHA, 1992). Hach
21
Lange DR 5000 Spectrophotometer was used for tCOD, sCOD (after vacuum filtration
22
through 0.45µm membrane) measurements (2120 C). TS, TVS, TSS and VSS were
23
measured (2540 A-B-D) following the standard methods. pH and conductivity were
7
1
measured with ORION SA 520 pHmeter. Turbidity was measured with Hach 2100N
2
Turbidimeter. tVFA and its composition were determined by a gas chromatograph
3
(Agilent 7820A) equipped with a flame ionization detector (FID) and FFAP column
4
(Innowax 25 m x 0.25 mm x 0.50 mm). Samples taken from the acidification reactor
5
was filtered through 0.45µm membrane using vacuum filtration and 10% phosphoric
6
acid was added to the samples. VFA analysises were carried out in triplicates and
7
figures are given for average values. Total phenolic concentrations were quantified by
8
means of 4-Aminoantipyrine spectrophotometric method which based on the reaction
9
between phenolic materials and 4-aminoantipyrine at pH 10. In the presence of phenolic
10
substances, antipyrine dye which is a reddish-brown colored, is formed.
11 12
2.4. Statistical analysis
13
Statistical analysis were performed with IBM SPSS (version 22) software package. All
14
composition of wastewaters and seed sludge has been given with arithmetic mean
15
values and standard deviation. The Mann–Whitney U test which is a non-parametric
16
test, was used to determine the correlation between VFA concentrations and OLR with
17
a confidence level of 95%. The p-values less than 0.05 indicated a significant
18
correlation.
19 20 21
3. RESULTS AND DISCUSSION
22
OMW is classified as one of the most complex substrates and management of the
23
wastewater is problematic by biological reactors due to both its extremely high solid
24
content and also high organic and phenolic content. Composition of OMW can vary
8
1
depending on many factors such as origin, type and maturity of olives, climate
2
characteristic and olive oil producing technology (Oz and Uzun, 2014). OMW
3
compositions reported in the literature are given in the Table 2. As seen from Table 1,
4
the characteristics of OMW used in this study indicates extremely high strength
5
wastewater. Therefore, since pretreatment is inevitable for raw OMW, EC was
6
performed as a pretreatment step for the wastewater in this study in order to reduce
7
extremely high solid and organic content.
8 9
Table 2. OMW compositions reported in the literature
10 11
3.1.Electrocoagulation Pretreatment
12
In EC; 4, 6, 8, 10 and 12 V DC were used for raw OMW for four hours. The supernatant
13
was used in COD, sCOD, solid matters and turbidity determination according to
14
standard methods.
15
COD removal efficiencies were enhanced while applied DC voltage was increased from
16
4 to 10 V. In literature, it has been reported that EC efficiency is directly related to DC
17
voltage which increased the bubble production rate and size and enhanced the
18
flocculation effect (Kobya et al., 2003). However after 10 V; further increases in applied
19
DC voltages to 12 V, did not improved the effluent COD concentrations. Therefore, 10
20
V direct current for four hours is selected as optimum DC voltage. At this voltage, 91%
21
turbidity, 88% TSS, 56% COD and 22% sCOD removal efficiencies were obtained. EC
22
process involves the dissolution of metal cations (typically iron, stainless steel or
23
aluminium) from the anode and formation of hydroxyl ions and hydrogen gas at the
24
cathode, simultaneouslly. The overall reaction mechanism is a combination of
9
1
coagulation, flocculation, settlement and flotation (Kabdaşlı et al., 2012). Dominant
2
mechanism may vary depending on process conditions (Holt et al., 2005). In biological
3
treatment processes which were related to microbial activity; the treatment efficiency of
4
the process is directly affected by the content of organic matters, solid matters and toxic
5
compounds of wastewaters. Therefore, especially for OMW, pretratment step is of great
6
importance. EC prior to biological processes was proposed as a sufficient pretreatment
7
method with higher treatment efficiencies due to the removal of toxic compounds
8
((Khoufi et al., 2006; Hanafi et al., 2011). Therefore EC was selected as a pretreatment
9
method for OMW in order to remove both high organic matter and suspended solid
10
concentrations and also toxic effect which adversely affect the performance of the
11
bioreactors.
12
When pollutant removal efficiencies obtained in this study were considered, it is
13
obvious that most of the suspended solids and partial organic matter were removed by
14
EC process.
15
treatment for OMW. In these studies; at optimum conditions, COD removal efficiencies
16
were obtained between 45% to 76% (Adhoum and Monster, 2004; Kargı et al., 2011).
In literature there are two reported study about investigation of EC
17 18
After EC, pretreated OMW characteristics are given in Table 3.
19 20
Table 3. OMW composition after EC Pre-treatment.
21 22
The scheme which proposed in this study, allowed a notable suspended solid and
23
organic matter removal efficiency in EC step and it has been expected that an effective
24
conversion of the pretreated OMW into VFAs could be achieved in acidification step.
10
1 2
3.2. Acidification
3
After pretreatment, acidification reactors were operated with pretreated OMW.
4
Acidification is a second phase of anaerobic treatment where short chain VFAs such as
5
acetic, propionic, butyric, and valeric acids are produced. With the aim of determination
6
of optimum conditions for acidification; substrate concentrations in terms of COD, was
7
increased gradually as 5000 mg COD/L, 10000 mg COD/L, 20000 mg COD/L and
8
40000 mg COD/L while microorganism concentration was kept as constant at 5000 mg
9
TVS/L in reactors. In this case; study was started with 5 gr COD/L-day TVS as S/X
10
ratio and increased gradually to 2 gr COD/ gr TVS, 4 gr COD/ gr TVS and 8 gr COD/
11
gr TVS for testing.
12
Acidification reactors were performed for 2 days hydraulic retention time at pH 5-5.5. A
13
wide range of optimum pH from 5 to 11 have been reported in the literature for
14
acidification of complex organic matters by microbial activities (Dareioti and Kornaros,
15
2014). It has been stated that low pH values support the production of longer chain fatty
16
acids such as butyric, valeric, and caproic acids (Silva et al., 2013). In parallel, it has
17
been reported that concentrations of acetic acid and butyric acid are decreased while the
18
concentration of propionic acid is increased at higher pH (6 to 7) (Oktem et al., 2006;
19
Albuquerque et al., 2007). However specific pH values are highly related to type of
20
wastes/wastewaters. For instance, according to previous reported studies, although VFA
21
production from sludge is higher in alkaline condition, VFA production from food
22
wastes/wastewaters is better achieved in acidic conditions (Lee et al., 2014). Thus, in
23
this study, the pH at the beginning of the study, was set at 5.5 and measured varied
24
between 5.1 to 5.5 at the end of the all days of experiment. Since alkalinity is a key
11
1
parameter for VFA production and acidification rate (Gameiro et al., 2015), 1gr/L
2
Na2CO3 was added to reactors daily in order to maintain a constant pH in the reactor.
3 4
3.3.VFA Production
5
Acidification reactors were fed with pretreated olive mill wastewater at different OLRs.
6
In the literature, effect of OLR on VFA production has not been fully determined, so
7
far. Nevertheless it has been stated that, similar to pH, optimum OLR values changes
8
depending on waste/wastewater type (Lee et al., 2014).
9
In this study, as a results of gradually increase in organic loading rate (5 gr COD/L to
10
40 gr COD/L); acidification rate efficiencies were increased parallelly until 40 gr
11
COD/L. Total VFA concentrations in acidification reactor operated for 95 days at
12
optimal conditions are shown in Figure 2. VFA production with the increase in the
13
organic load, was reached the highest value of about 27000 mg/L at load applied (20 gr
14
sCOD/L). However, while OLR was increased to 40 gr COD/L, a dramatic decrease in
15
the VFA concentration to about 15000 mg/L was occured due to the acidogenic bacteria
16
are under stress (Oktem et al., 2006). Therefore, highest organic loading rate was
17
selected as 20 gr COD/L. Similar results have been also reported for OMW acidification
18
(Gameiro et al., 2015). It has been stated that the maximum VFA concentration was
19
accomplished when the highest substrate concentration was applied (14 g COD/L).
20
Consequentially, the optimum OLR which could be loaded to biological reactors should
21
be taken into consideration due to the possible limiting affect on the process.
22 23
Figure 2. Total VFA concentrations in acidification reactor
24
12
1
At the beginning of the study; total phenol concentration was about 1340 mg/l in
2
pretreated OMW. Nevertheless no inhibition was observed due to phenol concentration
3
during the acidification process. It has been expressed that phenolic compounds at
4
relatively low concentrations (<5000 mg/L), do not inhibit the conversion of organic
5
matter into VFAs (Dionisi et al., 2005; Cerrone et al., 2010).
6
Initial tVFA concentrations were about 3300 mg/l. As seen from Figure 2., tVFA
7
concentration was gradually increased from 3300 to 27195 mg/L after 95 operation
8
days. At the end of the start-up period; while influent tVFA concentrations were about
9
13000 mg/l, effluent VFA concentration was gradually increased to 15526 mg/L, 20953
10
mg/L, 24077 mg/L and 26000 mg/L up to the steady state. While during the steady state
11
condition, tVFA concentrations were determined as 27152±39.94 mg/L. Acidification
12
rate was defined as the ratio of COD equivalent of tVFA to tCOD of sample. The
13
maximum VFA concentration (about 27000 mg/L) was achieved when the substrate
14
concentration was 20 gr sCOD/L. Accordingly, as a result of acidification process;
15
while acidification rate was determined as 39% in the start-up phase, during the steady
16
state it has reached to 68% which was the highest value. Total VFAs concentration was
17
equal to 77% of the overall effluent sCOD in the steady state. Bertin et al. (2010) have
18
also studied the anaerobic acidogenic digestion of OMW and they have reported
19
acidification rate as 66% at 13 gr COD/L as OLR.
20
For statistical analysis, Mann–Whitney U test was applied to determine the correlation
21
between VFA production and OLR and it showed that VFA production was not
22
significant at OLR of 5 and 10 gr COD/L. However, when OLR was increased to 20 gr
23
COD/L, tVFA production showed a significant increase (p < 0.05).
13
1
During acidification process, changes in sCOD concentration were monitored daily. In
2
these experiments, only about 10% (between 8% to 13%) of the sCOD was utilized.
3
Therefore loss of organic matter or a major changes in sCOD were not detected.
4
Therefore, since removal of organic matter has not occurred, high amounts of gas
5
production has not taken place. Gas production was monitored from time to time during
6
the study (during the steady-state conditions at highest OLR. The volume of daily
7
hydrogen gas produced was about 120±25 mL. Furthermore, it is obvious that
8
significant COD removal efficiencies are not expected by the acidification process
9
(Ntaikou et al., 2009). In contradistinction to anaerobic treatment where the organic
10
matter is converted to methane at the end of the process, in anaerobic acidification
11
process, organic matters are converted into the soluble organics thus no COD removal is
12
occured (Gottschalk, 1986). Similarly, Beccari et al. (2009) reported only an 8% sCOD
13
loss during acidification of OMW and indicated that this is a ordinary result for the
14
acidification of OMW (Beccari et al., 2009).
15
Although there was a limited change in organic matter concentration, a significant
16
increase in tVFA concentration has been detected in our study. Therefore it can be
17
deduced that organic matter changed form and acidification process was achieved
18
successfully.
19
In the reactor, due to high conversion efficiency of organic matters to VFA, there was
20
no significant COD removal in acidification reactor (<8%). Some fraction of COD
21
removal was used for biomass production. Similar results were also reported in the
22
literature (Monti et al., 2005; Silva et al., 2013).
23
3.4.Changes in VFA Composition
14
1
In acidification reactor, VFA composition was monitored daily using GC with FID
2
dedector. Figure 3. shows the changes in tVFA composition during process. As shown
3
in Figure 3., after acidification process, changes in VFA composition have been
4
observed. At the beginning of the study; VFA compounds’ concentrations were 1231
5
mg/l, 659 mg/l, 435 mg/l and 263 mg/l for acetic acid, propionic acid, isobutyric acid
6
and butyric acid, respectively. At the end of the study, these concentrations increased to
7
12121 mg/l, 4540 mg/l, 3100 mg/l and 1604 mg/l for acetic acid, propionic acid,
8
isobutyric acid and butyric acid, respectively. While each VFA concentration were
9
increased, also other VFA compounds, not detected in influent, were determined in
10
effluent such as isovaleric and isocaproic. This change in composition and amount of
11
VFA can be attributed to the increase in OLR. Statistical analyses (Mann–Whitney U
12
test) indicate that low OLRs (5 and 10 gr COD/L) did not make a significant change in
13
the composition of VFAs while higher OLR (20 gr COD/L) was found to be
14
significantly effective (p < 0.05) for the type of VFAs.
15
composition of VFA is directly related to both substrate and seed sludge composition,
16
and also operational conditions including hydraulic retention time (HRT), organic
17
loading rate (OLR), temperature and pH (Lee et al., 2014).
During the acidification,
18 19
Figure 3. Changes in tVFA composition during process
20 21
In the literature, it has been stated that unlike the other VFAs, isocaproic and isovaleric
22
acids which have higher molecular weight, are mostly produced during the acidification
23
of proteinaceous organic matters (Zoetemeyer et al., 1982). In this study; acetic,
24
propionic, butyric and valeric acids were all detected in both influent and effluent
15
1
samples. It has been stated that acetic, propionic, butyric and caproic acids were the
2
dominant VFA compounds in the acidification of OMW during the co-digestion
3
(Fezzani et al., 2010). In all stages of this study (start-up, progress and steady state)
4
VFAs were mainly composed of acetic, propionic, butyric and isobutyric acids. No
5
significant change was observed in other VFA compounds’ portions except acetic acid.
6
Nevertheless, initially 47% of the tVFA was acetic acid, this portion increased to 56%
7
in steady state. In literature, it has been reported that acetic and propionic acid
8
concentrations are affected by the changes in OLR (Bertin et al., 2010). Since type of
9
VFA is important in terms of possible recovery applications, it is important to achieve
10
acidification process and determine the type of VFAs at each operation condition. If
11
PHA production is the first aim, composition of produced VFA is an important factor.
12
Different PHAs can be obtained from different VFA species such as 3-hydroxybutyrate
13
can be obtained from acetic and butyric acids while 3- hydroxyvaleate can be obtained
14
from propionic and valeric acids (Cavinato et al., 2017). Therefore it is essential to find
15
a correlation between operation conditions and VFA compositions for producing the
16
specific VFA compositions.
17 18
3.5.Conversion of VFAs during a day cycle
19
During the acidification process, VFA concentration was also monitored during the 24-
20
hour AcSBR cycle in order to investigate the dynamics of VFA production. tVFA
21
production for the period of a day cycle operation at different selected operation days
22
are shown in Figure 4. At the beginning of the study (20.-21. days), a rapid increase of
23
21% was observed after 3 operation hours in VFA production during 24 hours-cycle
24
operation period. After the 3rd hour, VFA production gradually increased and reached
16
1
the maximum at the end of the cycle period (24th hour). Unlike at operating days on 40-
2
41, 80-81 and 90-91, VFA production gradually increased after the 3rd and 6th hours
3
(about 20%) and a rapid increase (about %40) was observed reaching the maximum at
4
the end of the cycle period (24th hour). According to our knowledge, there is no
5
reported study about VFA production dynamics for an AcASBR treating OMW during
6
a day cycle period, so far.
7
Figure 4. Changes in tVFA concentrations during a day cycle
8 9
Figure 5 shows the changes in acidification rate during one-day-cycle period. In start up
10
period; acidification rate was determined as 34% at the end of 3rd hour and was slightly
11
increased to 39% at the end of a day cycle (24th hour). In steady state, acidification rate
12
was determined as 40% and %48 in 3rd and 6th hours, respectively. In addition, at the
13
end of the day cycle, it reached 68% with a increase of 40%. There is no reported study
14
for acidification rate at different time periods during the operation of AcSBR.
15 16
Figure 5. Changes in acidification rate during a day cycle
17 18
Figure 6. shows the changes in VFA composition during a day cycle at start-up (20.-21.
19
days) and at the end of the operation period (90.-91. days). As can be seen from figure;
20
acetic acid was the dominant VFA compound for the whole study. In start up phase;
21
while other VFA compounds’ concentrations significantly increased after 3rd hours but
22
did not show an increase, acetic acid concentration was significantly increased after 6th
23
hour and continued to increase at the end of the day cycle. Through to the end of the
24
operation period, it can be seen that there is a different tendency on VFA composition in
17
1
the reactor. In this phase; while acetic acid gradually increased and reached the
2
maximum at the end of the cycle, propionic and isobutyric acids showed a significant
3
increase after 6th hour and no major changes were observed at the end of the cycle. In
4
all the monitored cycles, the levels of acetic acid gradually increased toward the end of
5
the cycles. The increase was more evident on day 90. In addition, isovaleric and
6
isocaproic acids which were undetected VFA compounds in influent, were detected at
7
the end of the 6th hours in start-up period, were detected earlier (In 3rd hour) at the end
8
of the operation period.
9 10
Figure 6. Changes in VFA composition during a day cycle.
11 12
3.6. Effluent composition of acidification reactor
13
After acidification process, composition of acidified OMW is given in Table 4.
14 15
Table 4. OMW composition after Acidification
16 17
As seen from the table, organic matter in OMW is successfully converted to VFAs.
18
tVFA amount is around 26000 mg/L. VFAs can be used in the production of
19
polyhydroxyalkanoates (biodegradable plastics), formation of electricity, biogas and
20
hydrogen. The composition of the produced VFA is one of the important factors which
21
affect the efficiency of the successive processes. For PHA production, acetic and
22
butyric acids especially favor the process (Albuquerque et al., 2007). It has been
23
reported that two times higher electricity can be produced with acetate-fed reactors
24
compared to the other higher molecular weight of VFAs and also stated that valeric acid
18
1
decreases the efficiency of the electricity production from the reactors (Freguia et al.,
2
2010).
3
concentrations of propionic acid higher than 5000 mg/L has been reported as to be
4
inhibitory to methanogens and block the biogas production (Doğan and Demirer, 2009).
5
Methane content has been found to be correlated with acetic acid concentration and
6
increase the energy efficiency (Doğan and Demirer, 2009). For the production of
7
hydrogen from VFAs; four times higher hydrogen production rate can obtained with
8
acetate and propionate compared to butyrate (Uyar et al., 2009). In this study; at the end
9
of the acidification process; acetic, propionic, butyric, isobutyric, isovaleric and
10
isocaproic acids represented the approximately 56%; 21%; 14%; 7%; 0,44% and 0,42%
11
of the whole detected VFAs, respectively. Bertin et al. (2010)
12
similar results for VFA production from OMW using anaerobic packed bed biofilm
13
reactors packed with ceramic filters. In this study, the main part of the produced VFA
14
consists of acetic acid. Therefore, it has been concluded that the high acetic acid content
15
(56%) of produced VFA from OMW using AcSBR can be alternatively used in different
16
processes with a high efficiency. After recovery of tVFAs from acidified OMW by all
17
these proposed processes, remaining sCOD content will be sharply decreased, as
18
expected. However, at the end of the processes, composition of OMW may not still
19
maintain discharge standards. Therefore, a final treatment process is inevitable.
20
Conventional anaerobic treatment systems combined with aerobic processes can be
21
effectively used for the remaining organic matter. In this perspective, application of
22
acidification and recovery of VFAs make it possible for OMW producers to use
23
conventional treatment processes for remaining wastewater.
If VFAs will be used in biogas production via methanogenic reactors,
have also reported
19
1
All things considered, besides many positive environmental impacts promoting green
2
technology, the results of the study will also generate real benefits to the olive oil
3
manufacturers in terms of both lowering cost of treatment including chemical, energy
4
etc. and obtaining valuable byproducts from OMW. Since legislation on discharge and
5
control of industrial wastewaters is becoming more strict, economic evaluation of the
6
wastewaters is inevitable in terms of olive oil producers.
7 8
4. CONCLUSION
9 10
The results indicated that AcSBRs can be used for VFA production from pretreated
11
OMW with high efficiency when pH/alkalinity and organic loading rate, which are main
12
parameters affecting acidification performance, are adjusted properly. Produced VFA
13
can represent as a valuable source for further processes. Therefore, VFAs production is
14
an important step of an integrated management system for OMW in order to enhance
15
successive treatment systems and other alternative recovery/energy processes. Microbial
16
community structure playing a critical role in VFA production and conversion from
17
OMW should be determined in order to improve our understanding of the mechanisms
18
for further studies.
19 20
ACKNOWLEDGEMENT
21
Financial support by The Scientific and Technological Research Council of Turkey
22
(TUBITAK) was gratefully acknowledged (Project No:114Y179) and partly (Project
23
No:111Y112) for laboratory equipments. The authors wish to thank to TARIS Olive and
20
1
Olive Oil Agricultural Sales Cooperatives Union and Pakmaya baker’s yeast plant for
2
their support in supplying Olive Mill Wastewater and seed sludge samples, respectively.
3
21
REFERENCES 1. Adhoum, N., Monser, L., 2004. Decolourization and removal of phenolic compounds from olive mill wastewater by electrocoagulation. Chem. Eng. Process. Process Intensif. 43, 1281–1287. 2. Albuquerque, M.G.E., Eiroa, M., Torres, C., Nunes, B.R., Reis, M.A.M., 2007. Strategies for the development of a side stream process for polyhydroxyalkanoate (PHA) production from sugar cane molasses. J. Biotechnol. 130, 411–421. 3. APHA, 1992. American Public Health Association (APHA) method 9221: Standard methods for the examination of water and wastewater, Standard Methods. 4. Azbar, N., Keskin, T., Yuruyen, A., 2008. Enhancement of biogas production from olive mill effluent (OME) by co-digestion. Biomass and Bioenergy 32, 1195–1201. 5. Bayr, S., Rantanen, M., Kaparaju, P., Rintala, J., 2012. Mesophilic and thermophilic anaerobic co-digestion of rendering plant and slaughterhouse wastes. Bioresour. Technol. 104, 28–36. 6. Beccari, M., Bertin, L., Dionisi, D., Fava, F., Lampis, S., Majone, M., Valentino, F., Vallini, G., Villano, M., 2009. Exploiting olive oil mill effluents as a renewable resource for production of biodegradable polymers through a combined anaerobicaerobic process. J. Chem. Technol. Biotechnol. 84, 901–908. 7. Benitez, F.J., Beltran-Heredia, J., Torregrosa, J., Acero, J.L., 1997. Improvement of the anaerobic biodegradation of olive mill wastewaters by prior ozonation pretreatment. Bioprocess Eng. 17, 169–175. 8. Bertin, L., Lampis, S., Todaro, D., Scoma, A., Vallini, G., Marchetti, L., Majone, M., Fava, F., 2010. Anaerobic acidogenic digestion of olive mill wastewaters in biofilm reactors packed with ceramic filters or granular activated carbon. Water Res. 44, 4537–4549. 1
9. Cavinato, C., Da Ros, C., Pavan, P., Bolzonella, D., 2017. Influence of temperature and hydraulic retention on the production of volatile fatty acids during anaerobic fermentation of cow manure and maize silage. Bioresour. Technol. 223, 59–64. 10. Cereti, C.F., Rossini, F., Federici, F., Quaratino, D., Vassilev, N., Fenice, M., 2004. Reuse of microbially treated olive mill wastewater as fertiliser for wheat (Triticum durum Desf.). Bioresour. Technol. 91, 135–140. 11. Cerrone, F., del Mar Sánchez-Peinado, M., Juárez-Jimenez, B., González-López, J., Pozo, C., 2010. Biological treatment of two-phase olive mill wastewater (TPOMW, alpeorujo): Polyhydroxyalkanoates (PHAs) production by azotobacter strains. J. Microbiol. Biotechnol. 20, 594–601. 12. Cesaro, A., Belgiorno, V., 2013. Sonolysis and ozonation as pretreatment for anaerobic digestion of solid organic waste. Ultrason. Sonochem. 20, 931–936. 13. Chen, H., Liu, Y., Ni, B.J., Wang, Q., Wang, D., Zhang, C., Li, X., Zeng, G., 2016. Full-scale evaluation of aerobic/extended-idle regime inducing biological phosphorus removal and its integration with intermittent sand filter to treat domestic sewage discharged from highway rest area. Biochem. Eng. J. 113, 114–122. 14. Dareioti, M.A., Kornaros, M., 2014. Effect of hydraulic retention time (HRT) on the anaerobic co-digestion of agro-industrial wastes in a two-stage CSTR system. Bioresour. Technol. 167, 407–415. 15. Dionisi, D., Carucci, G., Petrangeli Papini, M., Riccardi, C., Majone, M., Carrasco, F., 2005. Olive oil mill effluents as a feedstock for production of biodegradable polymers. Water Res. 39, 2076–2084. 16. Dogan, E., Demirer, G.N., 2009. Volatile Fatty Acid Production from Organic Fraction of Municipal Solid Waste Through Anaerobic Acidogenic Digestion. Environ. Eng. Sci. 26, 1443–1450.
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17. Editors, G., Nocera, D., Guldi, D., 2009. Renewable Energy issue. Chem. Soc. Rev. 38, 165–84. 18. El-Gohary, F.A., Badawy, M.I., El-Khateeb, M.A., El-Kalliny, A.S., 2009. Integrated treatment of olive mill wastewater (OMW) by the combination of Fenton’s reaction and anaerobic treatment. J. Hazard. Mater. 162, 1536–1541. 19. Elmekawy, A., Diels, L., Bertin, L., De Wever, H., Pant, D., 2014. Potential biovalorization techniques for olive mill biorefinery wastewater. Biofuels, Bioprod. Biorefining. 20. F. Kargi, E.C. Catalkaya, Hydrogen gas production from olive mill wastewater by electrohydrolysis with simultaneous COD removal, Int. J. Hydrogen Energy. 36 (2011) 3457–3464. 21. Fezzani, B., Cheikh, R. Ben, 2010. Two-phase anaerobic co-digestion of olive mill wastes in semi-continuous digesters at mesophilic temperature. Bioresour. Technol. 101, 1628–1634. 22. Filidei, S., Masciandaro, G., Ceccanti, B., 2003. Anaerobic digestion of olive oil mill effluents: Evaluation of wastewater organic load and phytotoxicity reduction. Water. Air. Soil Pollut. 145, 79–94. 23. Freguia, S., Teh, E.H., Boon, N., Leung, K.M., Keller, J., Rabaey, K., 2010. Microbial fuel cells operating on mixed fatty acids. Bioresour. Technol. 101, 1233–1238. 24. Gameiro, T., Sousa, F., Silva, F.C., Couras, C., Lopes, M., Louros, V., Nadais, H., Capela, I., 2015. Olive oil mill wastewater to volatile fatty acids: Statistical study of the acidogenic process. Water. Air. Soil Pollut. 226. 25. Gottschalk, G., 1986. Bacterial Fermentation. Springer Ser. Microbiol. 208–282. 26. Guest, J.S., Skerlos, S.J., Barnard, J.L., Beck, M.B., Daigger, G.T., Hilger, H., Jackson, S.J., Karvazy, K., Kelly, L., Macpherson, L., Mihelcic, J.R., Pramanik, A.,
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Raskin, L., Van Loosdrecht, M.C.M., Yeh, D., Love, N.G., 2009. A new planning and design paradigm to achieve sustainable resource recovery from wastewater. Environ. Sci. Technol. 27. Hanafi, F., Assobhei, O., Mountadar, M., 2010. Detoxification and discoloration of Moroccan olive mill wastewater by electrocoagulation. J. Hazard. Mater. 174, 807– 812. 28. Hanafi, F., Belaoufi, A., Mountadar, M., Assobhei, O., 2011. Augmentation of biodegradability of olive mill wastewater by electrochemical pre-treatment: Effect on phytotoxicity and operating cost. J. Hazard. Mater. 190, 94–99. 29. Holt, P.K., Barton, G.W., Mitchell, C.A., 2005. The future for electrocoagulation as a localised water treatment technology. Chemosphere. 30. Kabdaşlı, I. Arslan-Alaton, T. Ölmez-Hancı, O. Tünay, Electrocoagulation applications for industrial wastewaters: a critical review, Environ. Technol. Rev. 1 (2012) 2–45. 31. Ji, Z., Chen, G., Chen, Y., 2010. Effects of waste activated sludge and surfactant addition on primary sludge hydrolysis and short-chain fatty acids accumulation. Bioresour. Technol. 101, 3457–3462. 32. Khoufi, S., Aloui, F., Sayadi, S., 2006. Treatment of olive oil mill wastewater by combined process electro-Fenton reaction and anaerobic digestion. Water Res. 40, 2007–2016. 33. Khoufi, S., Feki, F., Sayadi, S., 2007. Detoxification of olive mill wastewater by electrocoagulation and sedimentation processes. J. Hazard. Mater. 142, 58–67. 34. Kim, H.J., Choi, Y.G., Kim, G.D., Kim, S.H., Chung, T.H., 2005. Effect of enzymatic pretreatment on solubilization and volatile fatty acid production in fermentation of food waste. Water Sci. Technol. 52, 51–59.
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35. Lee, W.S., Chua, A.S.M., Yeoh, H.K., Ngoh, G.C., 2014. A review of the production and applications of waste-derived volatile fatty acids. Chem. Eng. J. 36. Li, X., Zhao, J., Wang, D., Yang, Q., Xu, Q., Deng, Y., Yang, W., Zeng, G., 2016. An efficient and green pretreatment to stimulate short-chain fatty acids production from waste activated sludge anaerobic fermentation using free nitrous acid. Chemosphere 144, 160–167. 37. M. Kobya, O.T. Can, M. Bayramoglu, Treatment of textile wastewaters by electrocoagulation using iron and aluminum electrodes, J. Hazard. Mater. 100 (2003) 163–178. 38. Mengmeng, C., Hong, C., Qingliang, Z., Shirley, S.N., Jie, R., 2009. Optimal production of polyhydroxyalkanoates (PHA) in activated sludge fed by volatile fatty acids (VFAs) generated from alkaline excess sludge fermentation. Bioresour. Technol. 100, 1399–1405. 39. Morgan-Sagastume, F., Hjort, M., Cirne, D., Gérardin, F., Lacroix, S., Gaval, G., Karabegovic, L., Alexandersson, T., Johansson, P., Karlsson, A., Bengtsson, S., Arcos-Hernández, M. V., Magnusson, P., Werker, A., 2015. Integrated production of polyhydroxyalkanoates (PHAs) with municipal wastewater and sludge treatment at pilot scale. Bioresour. Technol. 181, 78–89. 40. Ntaikou, I., Kourmentza, C., Koutrouli, E.C., Stamatelatou, K., Zampraka, A., Kornaros, M., Lyberatos, G., 2009. Exploitation of olive oil mill wastewater for combined biohydrogen and biopolymers production. Bioresour. Technol. 100, 3724– 3730. 41. Oktem, Y.A., Ince, O., Donnelly, T., Sallis, P., Ince, B.K., 2006. Determination of optimum operating conditions of an acidification reactor treating a chemical synthesis-based pharmaceutical wastewater. Process Biochem. 41, 2258–2263.
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42. Oz, N.A., Uzun, A.C., 2015. Ultrasound pretreatment for enhanced biogas production from olive mill wastewater. Ultrason. Sonochem. 22, 565–572. 43. Scoma, A., Bertin, L., Fava, F., 2013. Effect of hydraulic retention time on biohydrogen and volatile fatty acids production during acidogenic digestion of dephenolized olive mill wastewaters. Biomass and Bioenergy 48, 51–58. 44. Silva, F.C., Serafim, L.S., Nadais, H., Arroja, L., Capela, I., 2013. Acidogenic fermentation towards valorisation of organic waste streams into volatile fatty acids. Chem. Biochem. Eng. Q. 27, 467–476. 45. Tong, J., Chen, Y., 2007. Enhanced biological phosphorus removal driven by shortchain fatty acids produced from waste activated sludge alkaline fermentation. Environ. Sci. Technol. 41, 7126–7130. 46. Uyar, B., Eroglu, I., Yücel, M., Gündüz, U., 2009. Photofermentative hydrogen production from volatile fatty acids present in dark fermentation effluents. Int. J. Hydrogen Energy 34, 4517–4523. 47. Uyar, B., Eroglu, I., Yücel, M., Gündüz, U., 2009. Photofermentative hydrogen production from volatile fatty acids present in dark fermentation effluents. Int. J. Hydrogen Energy 34, 4517–4523. 48. Wang, D., Zhao, J., Zeng, G., Chen, Y., Bond, P.L., Li, X., 2015. How Does Poly(hydroxyalkanoate) Affect Methane Production from the Anaerobic Digestion of Waste-Activated Sludge? Environ. Sci. Technol. 49, 12253–12262. 49. Zhao, J., Zhang, C., Wang, D., Li, X., An, H., Xie, T., Chen, F., Xu, Q., Sun, Y., Zeng, G., Yang, Q., 2016. Revealing the underlying mechanisms of how sodium chloride affects short-chain fatty acid production from the cofermentation of waste activated sludge and food waste. ACS Sustain. Chem. Eng. 4, 4675–4684. 50. Zoetemeyer, R.J., van den Heuvel, J.C., Cohen, A., 1982. pH influence on acidogenic
6
dissimilation of glucose in an anaerobic digestor. Water Res. 16, 303–311.
7
FIGURE CAPTIONS
Figure 1. Experimental set up Figure 2. Total VFA concentrations in acidification reactor Figure 3. Changes in tVFA composition during process Figure 4. Changes in tVFA concentrations during a day cycle Figure 5. Changes in acidification rate during a day cycle Figure 6. Changes in VFA composition during a day cycle
Figure 1. Experimental set up.
30000 Start-up
Steady state
tVFA, mg Acetic acid/L
25000
20000
15000
10000
5000
0 0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time, day
Figure 2. Total VFA concentrations in acidification reactor
15000
VFA concentration, mg/l
12000 Acetic A. 9000
Propionic A. isobutyric A.
6000
Butyric A. Isovaleric A. Isocaproic A.
3000
0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time, day Figure 3. Changes in tVFA composition during process
Influent
3rd hour
6th hour
24th hour
tVFA concentations, mg Acetic acid/L
30000 25000 20000 15000 10000 5000 0 20-21
40-41
80-81 Days
Figure 4. Changes in tVFA concentrations during a day cycle
90-91
24th hour
6th hour
3rd hour
90-91
Days
80-81 40-41 20-21 0
20
40 60 Acidification rate, %
80
100
Figure 5. Changes in acidification rate during a day cycle
14000
Acetic A.
Propionic A.
Isobutyric A.
Butyric A.
Isovaleric A.
Isocaproic A.
VFAs concentations, mg /L
12000 10000 8000 6000 4000 2000 0
0,00
3,00
6,00
24,00
0,00
20th-21st days Figure 6. Changes in VFA composition during a day cycle
3,00
6,00
90th-91st
24,00
TABLE CAPTIONS Table 1. OMW Composition. Table 2. OMW compositions reported in the literature Table 3. OMW composition after Electrocoagulation Pre-treatment. Table 4. OMW composition after Acidification.
Table 1. OMW Composition. Parameters
Unit
pH
Wastewater Sample* 5.05±0.02
Conductivity
µs/cm2
14460±30
Turbidity
NTU
19200±55
Color
ptCO
60300±120
TS
mg/L
56325±310
TVS
mg/L
39846±203
TSS
mg/L
21350±107
VSS
mg/L
19600±50
COD
mg/L
110393±271
TOC
mg/L
23883±20
SCOD
mg/L
57460±190
BOD
mg/L
28210±105
8
TVFA
mg/L
15045±60
Total Phenol
mg/L
6560±11
n=10
Table 2. OMW compositions reported in the literature Adhoum and
Dionisi et
Azbar et
Bertin et
Scoma et
Oz A.,
Monser, 2004
al., 2005
al., 2008 al., 2010
al., 2013
Uzun, 2013
pH
4,96
5,2
4,8
5,2
4,43
5,14
COD, g/L
57,8
113,8
100
35
51,66
40,51
TSS, g/L
-
-
16
32
-
12,59
2,42
2,2
4,1
2
1,21
5,06
Parameter
Phenol, g/L
Table 3. OMW composition after Electrocoagulation Pre-treatment Parameters Unit Pretreated OMW pH
7.2±0.1
Conductivity
µs/cm2
16280±35
Turbidity
NTU
2680±45
Color
ptCO
26400±130
TS
mg/L
37420±100
TVS
mg/L
23420±85
TSS
mg/L
3700±55
TVS
mg/L
3310±32
COD
mg/L
50499±148
SCOD
mg/L
40232±112
BOD
mg/L
19505±97
TVFA
mg/L
13035±55
9
Total Phenol
mg/L
1340±17
n=10.
Table 4. OMW composition after Acidification Parameters Unit Acidified OMW pH
5.45±0.03
Conductivity
µs/cm2
14910±20
Turbidity
NTU
2005±15
Color
ptCO
7200±40
TSS
mg/L
1900±30
sCOD
mg/L
35968±217
tVFA
mg/L
26007±193
n=10
10
Highlights
Acidification of olive mill wastewater was investigated at different loading rates.
Dynamics of VFA production has been investigated.
tVFA production and VFAs conversion were monitored for a cycle period (a day).
A high VFA production was obtained at 20 gr sCOD/ L with %68 acidification rate.
Possible alternative VFA recovery methods were discussed.
11
S0960-8524(17)30841-6 http://dx.doi.org/10.1016/j.biortech.2017.05.173 BITE 18199
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
27 March 2017 25 May 2017 26 May 2017
Please cite this article as: Yarımtepe, C.C., Oz, N.A., Ince, O., Volatile fatty acid production dynamics during the acidification of pretreated olive mill wastewater, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/ j.biortech.2017.05.173
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1
VOLATILE FATTY ACID PRODUCTION DYNAMICS DURING THE
2
ACIDIFICATION OF PRETREATED OLIVE MILL WASTEWATER
3
Canan Can Yarımtepe1,2, Nilgün Ayman Oz2,3* and Orhan Ince1
4 5
1
Istanbul Technical University, Department of Environmental Engineering, 34469, Maslak, Istanbul, Turkey
6 7
2
Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100, Çanakkale, Turkey
8 9
3
Center for Environmental Research and Application, Çanakkale, Turkey
10 11
ABSTRACT
12
This study focuses on the dynamics of VFA production from pretreated olive mill
13
wastewater (OMW). Acidogenic anaerobic sequencing batch reactors (AcASBR) were
14
operated with the pretreated OMW at hydraulic retention time (HRT) of 2 days and pH
15
of 5,5 for different organic loading rates (OLRs) ranging from 5 gr COD/L to 40 gr
16
COD/L. VFA production reached to the highest value of about 27000 mg/L with the
17
increase in the organic load applied (20 gr COD/L). The highest acidification rate was
18
calculated as 68%. Acetic acid was found to be as the dominant VFA compound for all
19
stage of the study. At steady state, VFA production during a day-cycle period (24th
20
hours) in AcASBR is also monitored, VFA production gradually increased after the 3rd
21
and 6th hours (about 20%) and a rapid increase (about %40) was observed reaching the
22
maximum at the end of the cycle (24th hours).
23
1
1
Keywords: VFA production, acidification, byproduct, electrocoagulation pretreatment,
2
olive mill wastewater, resource recovery.
3 4
*Corresponding author. Address: Çanakkale Onsekiz Mart University, Department of
5
Environmental Engineering, Çanakkale 17100, Turkey. Tel.: +90 286 218 0018 / 2177;
6
Fax: +90 286 218 0541. E-mail addresses: [email protected]
7 8
1. INTRODUCTION
9
Turkey is the 5th largest olive oil producer after Spain, Italy, Greece and Tunisia. Olive
10
mill wastewater (OMW) which is produced during the olive oil production process, is
11
one of the most important environmental problems for Mediterranean countries. It is
12
challenging to manage OMW, which occurs seasonally at high flow rate and with high
13
fluctuation of organic matter, suspended solids, oil and phenol concentrations. In
14
literature, several treatment processes and process combinations (El-Gohary et al., 2009;
15
Azbar et al., 2008) have been reported for OMW. However, both efficient and feasible
16
treatment process have not been addressed for the wastewater, so far. Especially for
17
OMW, diluted wastewaters are commonly used in several studies, because treatment
18
efficiency of the biological processes is directly affected by the high content of organic
19
matters, solid matters and toxic compounds of wastewaters. Due to the fact that dilution
20
can not be proposed as a management strategy for treatment of wastewaters,
21
pretreatment methods have been proposed prior to biological processes for OMW in
22
several studies such as ultrasound (Oz and Uzun, 2015), ozonation (Betinez et al.,
23
1997), fenton (El-Gohary et al., 2009), physicochemical pretreatment (Filidei et al.,
24
2003), aerobic treatment (Cereti et al, 2004) and electrochemical pretreatment (Khoufi
2
1
et al., 2006; Hanafi et al., 2011). Electrocoagulation (EC) is a electrolytic treatment
2
process which can remove the pollutants from wastewater such as suspended solid,
3
organic matter content, oil and phenolic compounds (Khoufi et al., 2007). The dominant
4
mechanism may vary throughout the process dynamics depending on treatment
5
conditions, operating parameters and pollutant types of wastewaters (Holt et al., 2005).
6
Several studies have been examined about EC for OMW as main, pre or post treatment
7
option (Khoufi et al., 2007; Adhoum and Monser, 2004; Hanafi et al., 2010). However,
8
integration of EC process with acidification reactors to maximize the VFAs from OMW
9
has not been reported in the literature, so far.
10
Instead of treating strength wastewaters such as OMW to provide effluent discharge
11
standards with a high cost, resource recovery which provide value-added products could
12
be an alternative or supplementary to wastewater treatment. Resource recovery from
13
waste/wastewater streams which should be considered as a source of valuable products
14
and energy, has been a major concept in recent days (Guest et al., 2009). Different
15
materials such as water, bioplastics, energy in terms of methane and hydrogen, phenolic
16
compounds and volatile fatty acids (VFAs) can be recovered from waste activated
17
sludge (Li et al., 2016; Wang et a., 2015), food wastes (Zhao et al., 2016) and especially
18
organic-rich wastewaters which are regarded as suitable renewable sources (Editors,
19
2009).
20
Short-chain fatty acids which consisting of six or fewer carbon, are identified as VFAs
21
(APHA, 1992). VFAs which formed during the acidification of organic-rich
22
wastewaters, can be used in many ways such as for bioplastics/bioenergy production
23
(Mengmeng et al., 2009; Uyar et al., 2009) and biological nitrogen and phosphorus
24
removal from wastewaters as a preferred carbon source (Tong and Chen, 2007; Chen et
3
1
al., 2016). Acidogenic reactor plays a crucial role in the conversion of complex organic
2
substrates to VFAs in OMW.
3
wastes/wastewaters, carbon based chemicals can alternatively produced and this would
4
represent a sustainable option for wastewater management. With the optimization of
5
VFA-producing bioreactors, valuable products with higher yields and purity can be
6
obtained from organic-rich wastes/wastewaters (Elmekawy et al., 2014). Various wastes
7
and wastewaters have been tested for VFA production in literature (Morgan-Sagastume
8
et al., 2015; Bayr et al., 2012; Ji et al., 2010; Cesaro et al., 2013; Kim et al., 2005).
9
However the studies focused on organic-rich wastes and wastewaters such as sludge,
10
food wastes and agricultural wastes/wastewaters (Lee et al., 2014) due to their high
11
COD content and possibility of producing VFAs at high rates (Dionisi et al., 2005). In
12
literatüre, there is limited study about VFA production from OMW (Dionisi et al., 2005;
13
Beccari et al., 2009; Scoma et al. 2013). In these studies, different pretreatment process
14
were investigated for production of biodegradable polymers from OMW and under
15
optimum conditions, acidification rates have reported as between 20% to 45%.
16
However, there is no reported study which evaluate VFA production and conversion
17
dynamics of OMW in acidogenic anaerobic sequencing batch reactor (AcASBR) in this
18
context. Since the composition of end products significantly differs based on the
19
reactor’s conditions during the acidification process, it is strategically important to
20
identify VFA production mechanism and conversion dynamics of different VFA types.
21
Therefore, the aim of this study is to optimise the acidification potential of pretreated
22
OMW via AcASBRs for higher VFAs productivity. tVFA production and VFAs
23
composition have also been monitored in detail in order to understand both process
By the recovery of VFAs from organic
4
1
efficiency in terms of acidification and the conversion dynamics during operation period
2
and for a-day-cycle period at different operating periods.
3 4
2. MATERIALS AND METHODS
5
2.1.Olive Mill Wastewater Characterization
6
Olive mill wastewater samples were taken from TARIS Olive and Olive Oil
7
Agricultural Sales Cooperatives Union in December, 2015 as a composite sample. For
8
the determination of the OMW’s composition; pH, conductivity, turbidity, color, total
9
phenol, total chemical oxygen demand (tCOD), soluble chemical oxygen demand
10
(sCOD) (after vacuum filtration through 0.45µm membrane), biochemical oxygen
11
demand (BOD), total organic carbon (TOC), total volatile fatty acid (tVFA), total solids
12
(TS), total volatile solids (TVS), total suspended solids (TSS) and volatile suspended
13
solids (VSS) were monitored. The characterization of olive mill wastewater sample are
14
given in Table 1.
15 16
Table 1. OMW Composition.
17 18
2.2.Experimental Set-up
19
Experimental set-up used during the study was shown in Figure 1. Since raw OMW
20
contains high concentration of pollutants (as seen from Table 1), OMW was pretreated
21
by EC in order to reduce pollutant load, especially solid content which is very
22
problematic for biological reactors.
23 24
Figure 1. Experimental set-up.
5
1 2
2.2.1. Electrocoagulation as a Pre-treatment Step
3
Electrocoagulation process was carried out in a lab-scale 1 L glass reactor with consists
4
a cover supporting two parallel aluminum electrodes. Selection of electrode type is the
5
main part of an electrochemical process and it is directly related to removal efficiencies.
6
The most common electrode materials for EC are aluminum and iron because they are
7
cheap, readily available and effective. In this study, aluminum electrodes are selected
8
based on the results of preliminary studies. Sample volume was 500 ml for experiment.
9
There were 2 cm distance between electrodes and 2 cm distance between the bottom of
10
the electrodes which allowed easy stirring of the effluent. A direct current as 2A with
11
different voltages including 4, 6, 8, 10 and 12 voltages were tested by using a power
12
supply (TT-T-ECHNI-C MCH-305D-II) for four hours. During the experiments, the
13
change in voltage and the current were monitored by multimeter (UNI-T UT61D Digital
14
Modern Multimeters UT-61D AC/DC Tester). After EC, samples were precipitated for
15
12 hours and supernatant was analyzed in terms of removal of major in pollutants
16
removal determination according to standard methods.
17 18
2.2.2. Acidification Process
19
Acidification process was performed with 1 L anaerobic sequencing batch reactors with
20
a total cycle period of 24 h consisting of 15 min for filling, 23 hours for reaction, 30
21
min for settling and 15 min for decant. Reactor with separate gas and sample ports was
22
submerged in a water bath for temperature control and mixing. Seed sludge which used
23
in acidification reactors was obtained from a yeast factory's acidification reactors. Seed
24
sludge's Total Solid and Total Volatile Solid concentration was determined as 163382
6
1
mg/l and 60612 mg/l, respectively. With aim of determination optimal conditions for
2
acidification; organic loading rate was increased gradually as 1 gr COD/ gr TVS to 2 gr
3
COD/ gr TVS, 4 gr COD/ gr TVS and 8 gr COD/ gr TVS for testing while
4
microorganism concentration was stable at 7500 TVS/L in reactors. Acidification
5
reactors were performed for 1.7±0.3 days hydraulic retention time at pH 5-5.5. The
6
daily alkalinity addition was carried out to keep the pH of the reactors at 5.5. Reactors
7
were fed with pretreated olive mill wastewater by EC. During the study; pH, sCOD,
8
TSS, tVFA concentration and composition were monitored daily. All analyzes were
9
carried out according to standard methods (APHA, 1992). tVFA concentration and
10
composition were monitored with GC. Acidification efficiencies were determined as
11
acidification rate and calculated by following equation (Oktem et al., 2006). The COD
12
equivalents for VFAs are considered as; 1.066 mg COD/l for mg/l acetic acid, 1.512
13
mg COD/l for mg/1 propionic acid, 1.816 mg COD/l for mg/1 butyric and isobutyric
14
acid, 2.036 mg COD/l for mg/ valeric and isovaleric acid, and 2.204 mg COD/l for mg/1
15
caproic acid (Oktem et al., 2006).
16 1.
17
Throughout the study, the amount of tVFA was indicated as acetic acid equivalent.
18 19
2.3.Analytical methods
20
Analyses were carried out according to the Standard Methods (APHA, 1992). Hach
21
Lange DR 5000 Spectrophotometer was used for tCOD, sCOD (after vacuum filtration
22
through 0.45µm membrane) measurements (2120 C). TS, TVS, TSS and VSS were
23
measured (2540 A-B-D) following the standard methods. pH and conductivity were
7
1
measured with ORION SA 520 pHmeter. Turbidity was measured with Hach 2100N
2
Turbidimeter. tVFA and its composition were determined by a gas chromatograph
3
(Agilent 7820A) equipped with a flame ionization detector (FID) and FFAP column
4
(Innowax 25 m x 0.25 mm x 0.50 mm). Samples taken from the acidification reactor
5
was filtered through 0.45µm membrane using vacuum filtration and 10% phosphoric
6
acid was added to the samples. VFA analysises were carried out in triplicates and
7
figures are given for average values. Total phenolic concentrations were quantified by
8
means of 4-Aminoantipyrine spectrophotometric method which based on the reaction
9
between phenolic materials and 4-aminoantipyrine at pH 10. In the presence of phenolic
10
substances, antipyrine dye which is a reddish-brown colored, is formed.
11 12
2.4. Statistical analysis
13
Statistical analysis were performed with IBM SPSS (version 22) software package. All
14
composition of wastewaters and seed sludge has been given with arithmetic mean
15
values and standard deviation. The Mann–Whitney U test which is a non-parametric
16
test, was used to determine the correlation between VFA concentrations and OLR with
17
a confidence level of 95%. The p-values less than 0.05 indicated a significant
18
correlation.
19 20 21
3. RESULTS AND DISCUSSION
22
OMW is classified as one of the most complex substrates and management of the
23
wastewater is problematic by biological reactors due to both its extremely high solid
24
content and also high organic and phenolic content. Composition of OMW can vary
8
1
depending on many factors such as origin, type and maturity of olives, climate
2
characteristic and olive oil producing technology (Oz and Uzun, 2014). OMW
3
compositions reported in the literature are given in the Table 2. As seen from Table 1,
4
the characteristics of OMW used in this study indicates extremely high strength
5
wastewater. Therefore, since pretreatment is inevitable for raw OMW, EC was
6
performed as a pretreatment step for the wastewater in this study in order to reduce
7
extremely high solid and organic content.
8 9
Table 2. OMW compositions reported in the literature
10 11
3.1.Electrocoagulation Pretreatment
12
In EC; 4, 6, 8, 10 and 12 V DC were used for raw OMW for four hours. The supernatant
13
was used in COD, sCOD, solid matters and turbidity determination according to
14
standard methods.
15
COD removal efficiencies were enhanced while applied DC voltage was increased from
16
4 to 10 V. In literature, it has been reported that EC efficiency is directly related to DC
17
voltage which increased the bubble production rate and size and enhanced the
18
flocculation effect (Kobya et al., 2003). However after 10 V; further increases in applied
19
DC voltages to 12 V, did not improved the effluent COD concentrations. Therefore, 10
20
V direct current for four hours is selected as optimum DC voltage. At this voltage, 91%
21
turbidity, 88% TSS, 56% COD and 22% sCOD removal efficiencies were obtained. EC
22
process involves the dissolution of metal cations (typically iron, stainless steel or
23
aluminium) from the anode and formation of hydroxyl ions and hydrogen gas at the
24
cathode, simultaneouslly. The overall reaction mechanism is a combination of
9
1
coagulation, flocculation, settlement and flotation (Kabdaşlı et al., 2012). Dominant
2
mechanism may vary depending on process conditions (Holt et al., 2005). In biological
3
treatment processes which were related to microbial activity; the treatment efficiency of
4
the process is directly affected by the content of organic matters, solid matters and toxic
5
compounds of wastewaters. Therefore, especially for OMW, pretratment step is of great
6
importance. EC prior to biological processes was proposed as a sufficient pretreatment
7
method with higher treatment efficiencies due to the removal of toxic compounds
8
((Khoufi et al., 2006; Hanafi et al., 2011). Therefore EC was selected as a pretreatment
9
method for OMW in order to remove both high organic matter and suspended solid
10
concentrations and also toxic effect which adversely affect the performance of the
11
bioreactors.
12
When pollutant removal efficiencies obtained in this study were considered, it is
13
obvious that most of the suspended solids and partial organic matter were removed by
14
EC process.
15
treatment for OMW. In these studies; at optimum conditions, COD removal efficiencies
16
were obtained between 45% to 76% (Adhoum and Monster, 2004; Kargı et al., 2011).
In literature there are two reported study about investigation of EC
17 18
After EC, pretreated OMW characteristics are given in Table 3.
19 20
Table 3. OMW composition after EC Pre-treatment.
21 22
The scheme which proposed in this study, allowed a notable suspended solid and
23
organic matter removal efficiency in EC step and it has been expected that an effective
24
conversion of the pretreated OMW into VFAs could be achieved in acidification step.
10
1 2
3.2. Acidification
3
After pretreatment, acidification reactors were operated with pretreated OMW.
4
Acidification is a second phase of anaerobic treatment where short chain VFAs such as
5
acetic, propionic, butyric, and valeric acids are produced. With the aim of determination
6
of optimum conditions for acidification; substrate concentrations in terms of COD, was
7
increased gradually as 5000 mg COD/L, 10000 mg COD/L, 20000 mg COD/L and
8
40000 mg COD/L while microorganism concentration was kept as constant at 5000 mg
9
TVS/L in reactors. In this case; study was started with 5 gr COD/L-day TVS as S/X
10
ratio and increased gradually to 2 gr COD/ gr TVS, 4 gr COD/ gr TVS and 8 gr COD/
11
gr TVS for testing.
12
Acidification reactors were performed for 2 days hydraulic retention time at pH 5-5.5. A
13
wide range of optimum pH from 5 to 11 have been reported in the literature for
14
acidification of complex organic matters by microbial activities (Dareioti and Kornaros,
15
2014). It has been stated that low pH values support the production of longer chain fatty
16
acids such as butyric, valeric, and caproic acids (Silva et al., 2013). In parallel, it has
17
been reported that concentrations of acetic acid and butyric acid are decreased while the
18
concentration of propionic acid is increased at higher pH (6 to 7) (Oktem et al., 2006;
19
Albuquerque et al., 2007). However specific pH values are highly related to type of
20
wastes/wastewaters. For instance, according to previous reported studies, although VFA
21
production from sludge is higher in alkaline condition, VFA production from food
22
wastes/wastewaters is better achieved in acidic conditions (Lee et al., 2014). Thus, in
23
this study, the pH at the beginning of the study, was set at 5.5 and measured varied
24
between 5.1 to 5.5 at the end of the all days of experiment. Since alkalinity is a key
11
1
parameter for VFA production and acidification rate (Gameiro et al., 2015), 1gr/L
2
Na2CO3 was added to reactors daily in order to maintain a constant pH in the reactor.
3 4
3.3.VFA Production
5
Acidification reactors were fed with pretreated olive mill wastewater at different OLRs.
6
In the literature, effect of OLR on VFA production has not been fully determined, so
7
far. Nevertheless it has been stated that, similar to pH, optimum OLR values changes
8
depending on waste/wastewater type (Lee et al., 2014).
9
In this study, as a results of gradually increase in organic loading rate (5 gr COD/L to
10
40 gr COD/L); acidification rate efficiencies were increased parallelly until 40 gr
11
COD/L. Total VFA concentrations in acidification reactor operated for 95 days at
12
optimal conditions are shown in Figure 2. VFA production with the increase in the
13
organic load, was reached the highest value of about 27000 mg/L at load applied (20 gr
14
sCOD/L). However, while OLR was increased to 40 gr COD/L, a dramatic decrease in
15
the VFA concentration to about 15000 mg/L was occured due to the acidogenic bacteria
16
are under stress (Oktem et al., 2006). Therefore, highest organic loading rate was
17
selected as 20 gr COD/L. Similar results have been also reported for OMW acidification
18
(Gameiro et al., 2015). It has been stated that the maximum VFA concentration was
19
accomplished when the highest substrate concentration was applied (14 g COD/L).
20
Consequentially, the optimum OLR which could be loaded to biological reactors should
21
be taken into consideration due to the possible limiting affect on the process.
22 23
Figure 2. Total VFA concentrations in acidification reactor
24
12
1
At the beginning of the study; total phenol concentration was about 1340 mg/l in
2
pretreated OMW. Nevertheless no inhibition was observed due to phenol concentration
3
during the acidification process. It has been expressed that phenolic compounds at
4
relatively low concentrations (<5000 mg/L), do not inhibit the conversion of organic
5
matter into VFAs (Dionisi et al., 2005; Cerrone et al., 2010).
6
Initial tVFA concentrations were about 3300 mg/l. As seen from Figure 2., tVFA
7
concentration was gradually increased from 3300 to 27195 mg/L after 95 operation
8
days. At the end of the start-up period; while influent tVFA concentrations were about
9
13000 mg/l, effluent VFA concentration was gradually increased to 15526 mg/L, 20953
10
mg/L, 24077 mg/L and 26000 mg/L up to the steady state. While during the steady state
11
condition, tVFA concentrations were determined as 27152±39.94 mg/L. Acidification
12
rate was defined as the ratio of COD equivalent of tVFA to tCOD of sample. The
13
maximum VFA concentration (about 27000 mg/L) was achieved when the substrate
14
concentration was 20 gr sCOD/L. Accordingly, as a result of acidification process;
15
while acidification rate was determined as 39% in the start-up phase, during the steady
16
state it has reached to 68% which was the highest value. Total VFAs concentration was
17
equal to 77% of the overall effluent sCOD in the steady state. Bertin et al. (2010) have
18
also studied the anaerobic acidogenic digestion of OMW and they have reported
19
acidification rate as 66% at 13 gr COD/L as OLR.
20
For statistical analysis, Mann–Whitney U test was applied to determine the correlation
21
between VFA production and OLR and it showed that VFA production was not
22
significant at OLR of 5 and 10 gr COD/L. However, when OLR was increased to 20 gr
23
COD/L, tVFA production showed a significant increase (p < 0.05).
13
1
During acidification process, changes in sCOD concentration were monitored daily. In
2
these experiments, only about 10% (between 8% to 13%) of the sCOD was utilized.
3
Therefore loss of organic matter or a major changes in sCOD were not detected.
4
Therefore, since removal of organic matter has not occurred, high amounts of gas
5
production has not taken place. Gas production was monitored from time to time during
6
the study (during the steady-state conditions at highest OLR. The volume of daily
7
hydrogen gas produced was about 120±25 mL. Furthermore, it is obvious that
8
significant COD removal efficiencies are not expected by the acidification process
9
(Ntaikou et al., 2009). In contradistinction to anaerobic treatment where the organic
10
matter is converted to methane at the end of the process, in anaerobic acidification
11
process, organic matters are converted into the soluble organics thus no COD removal is
12
occured (Gottschalk, 1986). Similarly, Beccari et al. (2009) reported only an 8% sCOD
13
loss during acidification of OMW and indicated that this is a ordinary result for the
14
acidification of OMW (Beccari et al., 2009).
15
Although there was a limited change in organic matter concentration, a significant
16
increase in tVFA concentration has been detected in our study. Therefore it can be
17
deduced that organic matter changed form and acidification process was achieved
18
successfully.
19
In the reactor, due to high conversion efficiency of organic matters to VFA, there was
20
no significant COD removal in acidification reactor (<8%). Some fraction of COD
21
removal was used for biomass production. Similar results were also reported in the
22
literature (Monti et al., 2005; Silva et al., 2013).
23
3.4.Changes in VFA Composition
14
1
In acidification reactor, VFA composition was monitored daily using GC with FID
2
dedector. Figure 3. shows the changes in tVFA composition during process. As shown
3
in Figure 3., after acidification process, changes in VFA composition have been
4
observed. At the beginning of the study; VFA compounds’ concentrations were 1231
5
mg/l, 659 mg/l, 435 mg/l and 263 mg/l for acetic acid, propionic acid, isobutyric acid
6
and butyric acid, respectively. At the end of the study, these concentrations increased to
7
12121 mg/l, 4540 mg/l, 3100 mg/l and 1604 mg/l for acetic acid, propionic acid,
8
isobutyric acid and butyric acid, respectively. While each VFA concentration were
9
increased, also other VFA compounds, not detected in influent, were determined in
10
effluent such as isovaleric and isocaproic. This change in composition and amount of
11
VFA can be attributed to the increase in OLR. Statistical analyses (Mann–Whitney U
12
test) indicate that low OLRs (5 and 10 gr COD/L) did not make a significant change in
13
the composition of VFAs while higher OLR (20 gr COD/L) was found to be
14
significantly effective (p < 0.05) for the type of VFAs.
15
composition of VFA is directly related to both substrate and seed sludge composition,
16
and also operational conditions including hydraulic retention time (HRT), organic
17
loading rate (OLR), temperature and pH (Lee et al., 2014).
During the acidification,
18 19
Figure 3. Changes in tVFA composition during process
20 21
In the literature, it has been stated that unlike the other VFAs, isocaproic and isovaleric
22
acids which have higher molecular weight, are mostly produced during the acidification
23
of proteinaceous organic matters (Zoetemeyer et al., 1982). In this study; acetic,
24
propionic, butyric and valeric acids were all detected in both influent and effluent
15
1
samples. It has been stated that acetic, propionic, butyric and caproic acids were the
2
dominant VFA compounds in the acidification of OMW during the co-digestion
3
(Fezzani et al., 2010). In all stages of this study (start-up, progress and steady state)
4
VFAs were mainly composed of acetic, propionic, butyric and isobutyric acids. No
5
significant change was observed in other VFA compounds’ portions except acetic acid.
6
Nevertheless, initially 47% of the tVFA was acetic acid, this portion increased to 56%
7
in steady state. In literature, it has been reported that acetic and propionic acid
8
concentrations are affected by the changes in OLR (Bertin et al., 2010). Since type of
9
VFA is important in terms of possible recovery applications, it is important to achieve
10
acidification process and determine the type of VFAs at each operation condition. If
11
PHA production is the first aim, composition of produced VFA is an important factor.
12
Different PHAs can be obtained from different VFA species such as 3-hydroxybutyrate
13
can be obtained from acetic and butyric acids while 3- hydroxyvaleate can be obtained
14
from propionic and valeric acids (Cavinato et al., 2017). Therefore it is essential to find
15
a correlation between operation conditions and VFA compositions for producing the
16
specific VFA compositions.
17 18
3.5.Conversion of VFAs during a day cycle
19
During the acidification process, VFA concentration was also monitored during the 24-
20
hour AcSBR cycle in order to investigate the dynamics of VFA production. tVFA
21
production for the period of a day cycle operation at different selected operation days
22
are shown in Figure 4. At the beginning of the study (20.-21. days), a rapid increase of
23
21% was observed after 3 operation hours in VFA production during 24 hours-cycle
24
operation period. After the 3rd hour, VFA production gradually increased and reached
16
1
the maximum at the end of the cycle period (24th hour). Unlike at operating days on 40-
2
41, 80-81 and 90-91, VFA production gradually increased after the 3rd and 6th hours
3
(about 20%) and a rapid increase (about %40) was observed reaching the maximum at
4
the end of the cycle period (24th hour). According to our knowledge, there is no
5
reported study about VFA production dynamics for an AcASBR treating OMW during
6
a day cycle period, so far.
7
Figure 4. Changes in tVFA concentrations during a day cycle
8 9
Figure 5 shows the changes in acidification rate during one-day-cycle period. In start up
10
period; acidification rate was determined as 34% at the end of 3rd hour and was slightly
11
increased to 39% at the end of a day cycle (24th hour). In steady state, acidification rate
12
was determined as 40% and %48 in 3rd and 6th hours, respectively. In addition, at the
13
end of the day cycle, it reached 68% with a increase of 40%. There is no reported study
14
for acidification rate at different time periods during the operation of AcSBR.
15 16
Figure 5. Changes in acidification rate during a day cycle
17 18
Figure 6. shows the changes in VFA composition during a day cycle at start-up (20.-21.
19
days) and at the end of the operation period (90.-91. days). As can be seen from figure;
20
acetic acid was the dominant VFA compound for the whole study. In start up phase;
21
while other VFA compounds’ concentrations significantly increased after 3rd hours but
22
did not show an increase, acetic acid concentration was significantly increased after 6th
23
hour and continued to increase at the end of the day cycle. Through to the end of the
24
operation period, it can be seen that there is a different tendency on VFA composition in
17
1
the reactor. In this phase; while acetic acid gradually increased and reached the
2
maximum at the end of the cycle, propionic and isobutyric acids showed a significant
3
increase after 6th hour and no major changes were observed at the end of the cycle. In
4
all the monitored cycles, the levels of acetic acid gradually increased toward the end of
5
the cycles. The increase was more evident on day 90. In addition, isovaleric and
6
isocaproic acids which were undetected VFA compounds in influent, were detected at
7
the end of the 6th hours in start-up period, were detected earlier (In 3rd hour) at the end
8
of the operation period.
9 10
Figure 6. Changes in VFA composition during a day cycle.
11 12
3.6. Effluent composition of acidification reactor
13
After acidification process, composition of acidified OMW is given in Table 4.
14 15
Table 4. OMW composition after Acidification
16 17
As seen from the table, organic matter in OMW is successfully converted to VFAs.
18
tVFA amount is around 26000 mg/L. VFAs can be used in the production of
19
polyhydroxyalkanoates (biodegradable plastics), formation of electricity, biogas and
20
hydrogen. The composition of the produced VFA is one of the important factors which
21
affect the efficiency of the successive processes. For PHA production, acetic and
22
butyric acids especially favor the process (Albuquerque et al., 2007). It has been
23
reported that two times higher electricity can be produced with acetate-fed reactors
24
compared to the other higher molecular weight of VFAs and also stated that valeric acid
18
1
decreases the efficiency of the electricity production from the reactors (Freguia et al.,
2
2010).
3
concentrations of propionic acid higher than 5000 mg/L has been reported as to be
4
inhibitory to methanogens and block the biogas production (Doğan and Demirer, 2009).
5
Methane content has been found to be correlated with acetic acid concentration and
6
increase the energy efficiency (Doğan and Demirer, 2009). For the production of
7
hydrogen from VFAs; four times higher hydrogen production rate can obtained with
8
acetate and propionate compared to butyrate (Uyar et al., 2009). In this study; at the end
9
of the acidification process; acetic, propionic, butyric, isobutyric, isovaleric and
10
isocaproic acids represented the approximately 56%; 21%; 14%; 7%; 0,44% and 0,42%
11
of the whole detected VFAs, respectively. Bertin et al. (2010)
12
similar results for VFA production from OMW using anaerobic packed bed biofilm
13
reactors packed with ceramic filters. In this study, the main part of the produced VFA
14
consists of acetic acid. Therefore, it has been concluded that the high acetic acid content
15
(56%) of produced VFA from OMW using AcSBR can be alternatively used in different
16
processes with a high efficiency. After recovery of tVFAs from acidified OMW by all
17
these proposed processes, remaining sCOD content will be sharply decreased, as
18
expected. However, at the end of the processes, composition of OMW may not still
19
maintain discharge standards. Therefore, a final treatment process is inevitable.
20
Conventional anaerobic treatment systems combined with aerobic processes can be
21
effectively used for the remaining organic matter. In this perspective, application of
22
acidification and recovery of VFAs make it possible for OMW producers to use
23
conventional treatment processes for remaining wastewater.
If VFAs will be used in biogas production via methanogenic reactors,
have also reported
19
1
All things considered, besides many positive environmental impacts promoting green
2
technology, the results of the study will also generate real benefits to the olive oil
3
manufacturers in terms of both lowering cost of treatment including chemical, energy
4
etc. and obtaining valuable byproducts from OMW. Since legislation on discharge and
5
control of industrial wastewaters is becoming more strict, economic evaluation of the
6
wastewaters is inevitable in terms of olive oil producers.
7 8
4. CONCLUSION
9 10
The results indicated that AcSBRs can be used for VFA production from pretreated
11
OMW with high efficiency when pH/alkalinity and organic loading rate, which are main
12
parameters affecting acidification performance, are adjusted properly. Produced VFA
13
can represent as a valuable source for further processes. Therefore, VFAs production is
14
an important step of an integrated management system for OMW in order to enhance
15
successive treatment systems and other alternative recovery/energy processes. Microbial
16
community structure playing a critical role in VFA production and conversion from
17
OMW should be determined in order to improve our understanding of the mechanisms
18
for further studies.
19 20
ACKNOWLEDGEMENT
21
Financial support by The Scientific and Technological Research Council of Turkey
22
(TUBITAK) was gratefully acknowledged (Project No:114Y179) and partly (Project
23
No:111Y112) for laboratory equipments. The authors wish to thank to TARIS Olive and
20
1
Olive Oil Agricultural Sales Cooperatives Union and Pakmaya baker’s yeast plant for
2
their support in supplying Olive Mill Wastewater and seed sludge samples, respectively.
3
21
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7
FIGURE CAPTIONS
Figure 1. Experimental set up Figure 2. Total VFA concentrations in acidification reactor Figure 3. Changes in tVFA composition during process Figure 4. Changes in tVFA concentrations during a day cycle Figure 5. Changes in acidification rate during a day cycle Figure 6. Changes in VFA composition during a day cycle
Figure 1. Experimental set up.
30000 Start-up
Steady state
tVFA, mg Acetic acid/L
25000
20000
15000
10000
5000
0 0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time, day
Figure 2. Total VFA concentrations in acidification reactor
15000
VFA concentration, mg/l
12000 Acetic A. 9000
Propionic A. isobutyric A.
6000
Butyric A. Isovaleric A. Isocaproic A.
3000
0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time, day Figure 3. Changes in tVFA composition during process
Influent
3rd hour
6th hour
24th hour
tVFA concentations, mg Acetic acid/L
30000 25000 20000 15000 10000 5000 0 20-21
40-41
80-81 Days
Figure 4. Changes in tVFA concentrations during a day cycle
90-91
24th hour
6th hour
3rd hour
90-91
Days
80-81 40-41 20-21 0
20
40 60 Acidification rate, %
80
100
Figure 5. Changes in acidification rate during a day cycle
14000
Acetic A.
Propionic A.
Isobutyric A.
Butyric A.
Isovaleric A.
Isocaproic A.
VFAs concentations, mg /L
12000 10000 8000 6000 4000 2000 0
0,00
3,00
6,00
24,00
0,00
20th-21st days Figure 6. Changes in VFA composition during a day cycle
3,00
6,00
90th-91st
24,00
TABLE CAPTIONS Table 1. OMW Composition. Table 2. OMW compositions reported in the literature Table 3. OMW composition after Electrocoagulation Pre-treatment. Table 4. OMW composition after Acidification.
Table 1. OMW Composition. Parameters
Unit
pH
Wastewater Sample* 5.05±0.02
Conductivity
µs/cm2
14460±30
Turbidity
NTU
19200±55
Color
ptCO
60300±120
TS
mg/L
56325±310
TVS
mg/L
39846±203
TSS
mg/L
21350±107
VSS
mg/L
19600±50
COD
mg/L
110393±271
TOC
mg/L
23883±20
SCOD
mg/L
57460±190
BOD
mg/L
28210±105
8
TVFA
mg/L
15045±60
Total Phenol
mg/L
6560±11
n=10
Table 2. OMW compositions reported in the literature Adhoum and
Dionisi et
Azbar et
Bertin et
Scoma et
Oz A.,
Monser, 2004
al., 2005
al., 2008 al., 2010
al., 2013
Uzun, 2013
pH
4,96
5,2
4,8
5,2
4,43
5,14
COD, g/L
57,8
113,8
100
35
51,66
40,51
TSS, g/L
-
-
16
32
-
12,59
2,42
2,2
4,1
2
1,21
5,06
Parameter
Phenol, g/L
Table 3. OMW composition after Electrocoagulation Pre-treatment Parameters Unit Pretreated OMW pH
7.2±0.1
Conductivity
µs/cm2
16280±35
Turbidity
NTU
2680±45
Color
ptCO
26400±130
TS
mg/L
37420±100
TVS
mg/L
23420±85
TSS
mg/L
3700±55
TVS
mg/L
3310±32
COD
mg/L
50499±148
SCOD
mg/L
40232±112
BOD
mg/L
19505±97
TVFA
mg/L
13035±55
9
Total Phenol
mg/L
1340±17
n=10.
Table 4. OMW composition after Acidification Parameters Unit Acidified OMW pH
5.45±0.03
Conductivity
µs/cm2
14910±20
Turbidity
NTU
2005±15
Color
ptCO
7200±40
TSS
mg/L
1900±30
sCOD
mg/L
35968±217
tVFA
mg/L
26007±193
n=10
10
Highlights
Acidification of olive mill wastewater was investigated at different loading rates.
Dynamics of VFA production has been investigated.
tVFA production and VFAs conversion were monitored for a cycle period (a day).
A high VFA production was obtained at 20 gr sCOD/ L with %68 acidification rate.
Possible alternative VFA recovery methods were discussed.
11