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Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater
Renewable Energy xxx (2015) 1e8
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Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater M. Mischopoulou a, b, P. Naidis b, S. Kalamaras c, T.A. Kotsopoulos c, *, P. Samaras a, ** a
Department of Food Technology, Alexander Technological Education Institute of Thessaloniki, GR-57400 Thessaloniki, Greece Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece c Department of Hydraulics, Soil Science and Agricultural Engineering, Faculty of Agriculture, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 20 November 2015 Accepted 23 November 2015 Available online xxx
Ozonation and sonication were applied to baker's yeast wastewater with high molasses content, under various operation conditions, in order to study the effect of them on COD (Chemical Oxygen Demand) removal and on methane enhancement. The ozonation treatment resulted in a significant reduction of the COD content; the COD removal was up to 38% after a reaction time of 5 h. Moreover, a remarkable decolorization was observed, at 20 min of ozonation. The effect of sonication on the physical characteristics of the wastewater was negligible and resulted in an increase of the COD value. The anaerobic experiment was carried out in 18 batch reactors at 37 C. The most efficient pretreatment method was sonication in a continuous mode, since it presented the highest methane production equal to 441.6 LCH4/ kgVS. It was found that this method was also effective on COD removal, when sonication is followed by anaerobic digestion. The ozonation as a pretreatment method affected negatively biomethanation, as it resulted in significant reduction of methane production compared to the samples without pretreatment. The findings of the present study proved that the sonication of molasses wastewater followed by anaerobic digestion is an efficient solution, capable of treating this type of wastewater. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic Ozonation Anaerobic digestion Molasses wastewater Methane enhancement
1. Introduction Molasses represents a by-product of sugar manufacturing and it is used as raw material in several industrial applications such as bioethanol production, baker's yeast fermentation etc. Molasses is made up from 10 to 15% minerals, 15e20% non sugar organic substances, 45e50% residual sugars, such as glucose, fructose and sucrose, and about 20% water [1,2]. During the baker's yeast fermentation process, a great part of the non-sugar substances is not assimilated by the yeasts, and it is released unaltered in the processing water. These compounds make up the effluent wastewater from the yeast production procedure [1]. The dark brown colored effluent from baker's yeast industry is generated in large quantities: 10e15 L of wastewater is produced per L of product. The main characteristics of the effluent are the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.A. Kotsopoulos), samaras@food. teithe.gr (P. Samaras).
high concentration of total Kjeldahl nitrogen, COD (chemical oxygen demand), BOD (biological oxygen demand), sulphate and trimethylglycine, the low levels of readily degradable acids and sugars, as well as the presence of highly colored melanoidin and phenolic substances [2e4]. Therefore, the wastewater produced by this process is highly polluted, and it is vital that it is treated before being discharged into the receiving water bodies, aiming to prevent significant environmental problems [2]. Several processes have been employed for the treatment of the molasses-based wastewater including anaerobic, aerobic as well as physico-chemical methods. Anaerobic treatment constitutes the most often used technique, as it is associated to energy recovery in the form of biogas and over 80% BOD reduction [5]. However the utilization of anaerobic processes for the treatment of molasses wastewater is often restricted due to the presence of a high content of non-biodegradable organic materials in the effluent, and only a partial decomposition of the organic fraction occurs under anaerobic digestion [6]. Therefore, combination of other processes with anaerobic digestion is required, aiming to the implementation of an integrated treatment scheme for the efficient degradation of
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Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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biorefractories [7]. Towards this target, pretreatment methods, prior to anaerobic digestion, could be beneficial in favoring the anaerobic digestion potential of molasses wastewater and enhancing the methane production rate. Several pretreatment methods can be used to improve the biodegradability of sludge and wastewater including alkali, acid and thermal processes [8e10]. In addition to more conventional pretreatment methods the potential of advanced oxidation technologies, such as sonication and ozonation, is investigated for enhancing the biodegradability and simultaneously reducing the organic and inorganic content of wastewaters with high biorefractories content [11]. High-power ultrasound at frequencies between 20 and 100 kHz is an efficient method for wastewater treatment due to cavitation phenomena and the formation of high intensity bubbles [12,13]. The energy waves, generated by the application of ultrasound irradiation to a liquid medium, are propagated by the alternation of compression (high pressure) and rarefraction (low pressure). At sufficiently high-power densities, the attractive forces are overcome by the rarefraction cycle and bubbles are formed [14]. These bubbles grow, expand and finally collapse, leading to the establishment of high local temperatures (up to 5000 K) and pressures (up to 180 MPa). Furthermore, high shearing phenomena are generated in the liquid phase, while radicals including OH., OH.2, and H. may be formed [15]. During wastewater treatment processes, a cavitation bubble may function as a micro reactor in which organic bonds may break down. In addition, H. and OH. radicals can react with specific pollutants in the bulk of the solution, resulting to additional reduction of organic loading [16]. Ultrasound technology has been used for the treatment of several types of wastewater, such as chlorophenolics, high fat dairy wastewater, herbicide wastewater and wastewater containing textile dyes [17e21]. Nevertheless, ultrasound irradiation has found application as a pretreatment method for the enhancement of biodegradability of wastewater with a high biorefractories content, such as olive mill or distilleries wastewater, ahead of biological treatment carried out by anaerobic digestion or by fungi [7,22,23]. Moreover, ultrasound technology was efficiently used in waste activated sludge to improve the biodegradability of the organic compounds [24] and for the enhancement of methane production when ultrasound technology was used prior to anaerobic digestion in the sludge [25]. Ozonation constitutes a promising method for the polishing of secondary effluents, due to its potential of degrading detrimental organic contaminants. Hence, it has been used for the removal of a large number of compounds such as pesticides, organochlorides, phenolic substances and dyes [26,27]. Moreover, ozonation has shown high COD removal on anaerobically treated molasses wastewater followed by aerobic treatment and therefore can be considered as an excellent way for COD removal [28]. However, limited efforts have been reported on the utilization of advanced oxidation processes for the enhancement of anaerobic digestion potential of wastewater with a high content of substances with low or even negligible biodegradability such as wastewater from the baker's yeast manufacturing with a high concentration of melanoidins. The objectives of this work were the examination of the effect of ultrasound irradiation and ozonation on the anaerobic biodegradability and methane production rate of molasses wastewater, the investigation of the reduction potential of COD content and decolorization capacity of raw molasses effluents due to these processes, and the determination of the optimum experimental conditions for the achievement of an integrated process with the highest efficiency for the treatment of molasses wastewater. The objectives of this work are illustrated in Fig. 1.
2. Materials and methods 2.1. Ozonation and sonication experiments Wastewater samples were collected from the effluent of a baker's yeast industrial plant. About 10 L of samples were collected in plastic bottles, and they were transferred to the laboratory for further analysis or treatment; samples were stored at 4 C, prior to their usage. Ozonation experiments took place at room temperature, in a 2 L bubble flow plexi-glass reactor, under various reaction times, ranging from 5 up to 300 min. The gas flow rate was adjusted at 4 L/ min, the inlet ozone concentration was approximately 8.3 mg O3/L gas, while ozone was produced by an ozone generator. An ultrasonic homogenizer (SONOPLUS HD3400, BANDELIN, Germany) was used for the sonication experiments and the pretreatment of the samples. During sonication experiment, 500 mL of sample were added in a glass beaker and were treated at different conditions; sample temperature was monitored and was maintained at 25 C using an ice bath. The operation conditions examined included: ultrasound power 400 W, ultrasound frequency 20 kHz, amplitude of the ultrasound waves 90% and 50%, contact time 30, 60, 90, 120 min, continuous or intermitted operation mode (pulsation time on0.5 s, off-1 sec). In addition, SDS (Dodecyl hydrogen sulfate sodium salt, Merck) was added, at concentrations of 50 and 100 mg/L, in order to examine the effect of an anionic surfactant on reduction of organics concentration and on the enhancement of biodegradation potential. 2.2. Anaerobic biodegradation potential A batch assay was carried out to determine the anaerobic biodegradation potential of different samples of pretreated wastewater, corresponding to the biochemical methane potential (BMP). The experimental setup for the determination of the BMP test consisted in 1 L glass bottles representing bench scale anaerobic reactors. The working volume of each glass bottle was 500 mL; in each reactor, an appropriate volume of anaerobic sludge was added in order to obtain a sludge: substrate ratio of 2:1 v/v. Anaerobic sludge was collected from the full scale anaerobic digester of the wastewater treatment plant of the baker's yeast industry. Each sample was carried out in three replicates in order to receive representative results. Nevertheless, control reactors containing only sludge and water in the desired ratio were prepared aiming to the estimation of the background methane production due to organic matter content of the sludge itself. After inoculation of the sludge and the substrate in each reactor, the glass bottles were flushed with nitrogen gas to ensure anaerobic conditions, sealed with screw cap GL45, and finally were placed in an incubator operating at 37 ± 1 C. Once per day the reactors were vigorously mixed by hand. All the reactors were operated for 35 days until no further biogas production was observed. 2.3. Analytical methods Total and soluble COD content in each sample were determined by the Hach-Lange cuvette tests, along with a spectrophotometer (DR-2800). Soluble COD was determined by filtering the sample through a 0.45 mm filter. Total and volatile solids analysis carried out according to standard methods of analysis [29]. The pH of the samples was measured using a portable pH-meter (Nahita, model 902/4). The absorbance spectrum of each sample was measured by a UVeVis spectrophotometer (Helios Alpha, Thermo Electron Corporation) at wavelengths from 200 to 800 nm, using quartz cells
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
M. Mischopoulou et al. / Renewable Energy xxx (2015) 1e8
3
Fig. 1. Logic diagram of the study's objectives.
1 cm long. Biogas production was monitored each day, by the volumetric water displacement method: each bench scale anaerobic reactor was connected to 1 L glass cylinder, reversed into a plexi-glass water tank. The tank and the cylinders were filled with acidified water (pH < 2), to reduce the dissolution of the produced carbon dioxide in the water phase, allowing for the measurement of the biogas production rate. The methane content of biogas was determined using a Shimadzu Gas Chromatograph (GC-2010plusAT), with a Thermal Conductivity Detector (TCD) equipped with a GC-Column, (FS ValcoPLOT HayeSep D) 15 m long, with internal diameter 0.53 mm and a film thickness 20 mm. The GC had a VP-Molesieve Column, (FS, ValcoBond, 5A Fused Silica) 15 m long with internal diameter 0.53 mm, and a film thickness 20 mm. The temperature of the injector was adjusted at 100 C. Helium was used as a carrier gas and the temperature of the oven and TCD was at 32.5 C and 120 C accordingly. Standard gas mixtures were used for the quantification of methane and the sampling volume was 1 mL. 2.4. Statistical analysis of the data Data analysis was performed using SPSS version 21. Significant differences (p < 0.05) among treatments were based on one-way ANOVA (Analysis Of Variance). Duncan test was used to identify which treatments were significantly different. 3. Results and discussion The aim of this work was the investigation of ultrasonic irradiation and ozonation as potential pretreatment methods of molasses wastewater, for increasing the following biodegradation potential of the effluents. Therefore, preliminary experiments of the single processes were carried out, in order to identify the optimum conditions of sample treatment, prior to the measurement of BMP. 3.1. Effect of ozonation and sonication on the decolorization of the wastewater Molasses wastewater represents an effluent with a strong brownish color; thus, several efforts have been paid towards to its decolorization. The UV/VIS absorbance spectrums of raw effluent and of the sample subjected to 20 min ozonation are shown in Fig. 2(a), while the color of the samples is presented in Fig. 2(b). Ozonation represents a strong oxidative treatment method, able to remove color and aromatic compounds such as melanoidins; as a result, treatment by ozone of molasses wastewater resulted in color reduction of the sample, at short reaction times. As shown in Fig. 2(a), the ozone treated sample presented lower absorbance than the raw effluent at shorter wavelengths; the latter presented high absorbance at wavelengths up to 400 nm, and even small
absorbance at wavelengths as high as 800 nm. Ozonation at 20 min resulted in a yellowish effluent, as shown in Fig. 2(b), similar to the results of Pena et al. [26], who reported that molasses wastewater decolorization occurred mainly at the initial stage of the ozonation reaction. However, ultrasonic irradiation of the raw effluent was not efficient in color reduction of the sample; the absorption spectra of molasses wastewater before and after sonication for 120 min was almost the same. 3.2. Effect of ozonation and sonication on the COD removal of the wastewater The aim of the advanced oxidation processes is the reduction of the organic matter content of the effluents, through the disintegration of the organic compounds due to oxidation by free radicals or cavitation respectively. Therefore, the effect of ozonation and ultrasound irradiation on the COD of the molasses wastewater was examined in this section. The variation of the total COD as a function of reaction time during ozonation is presented in Fig. 3(a), while the corresponding removal of total COD is shown in Fig. 3(b). Various raw effluents were subjected to ozonation at reaction times as long as 300 min; effluents samples were collected from the full scale industrial plant and presented a great variation in COD content, depending upon the processing line, the raw materials used, etc. As shown in Fig. 3(a), ozonation resulted in a great reduction of COD content at short reaction times; during the first 5 min of the reaction COD removal of about 15% was observed. However, ozonation for longer times resulted only in slight organic matter removal, reaching to about 38% at 300 min of reaction. The effect of ozonation on sample soluble COD is shown in Fig. 4(a) as a function of reaction time, while the corresponding removal % is presented in Fig. 4(b). Similar to total COD removal, ozonation was efficient in the removal of dissolved compounds, even from the first 5 min of ozonation: soluble COD removal was about 13% after 5 min of ozonation reaching to 22 after 20 min of reaction. However, prolonged ozonation, from 30 to 120 min did not significantly reduced the content of organic compounds. These results indicate that a short ozonation time, up to 20 min, could be effective for molasses wastewater COD removal, as most of the soluble COD was removed during that period. Sonication of raw effluents carried out for reaction times up to 120 min, under various experimental conditions: two samples were treated at a continuous operation mode with 90% amplitude, one sample was treated at an intermitted operation mode with 90% amplitude, one sample was treated at a continuous operation mode with 50% amplitude and in two samples SDS was added at concentrations of 50 and 100 mg/L, following ultrasonic irradiation at continuous operation mode with 90% amplitude. The variation of the total COD as a function of reaction time during sonication is
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 2. Absorption spectra of molasses wastewater before and after ozonation for 20 min.
Fig. 3. Effect of ozonation on molasses wastewater total COD: a. COD variation b. COD removal as a function of reaction time. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Fig. 4. Effect of ozonation time on molasses wastewater soluble COD: a. COD variation b. COD removal %. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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presented in Fig. 5(a), while the total COD removal is shown in Fig. 5(b). COD values of all the samples increased during sonication, especially for the two samples that were treated with 90% amplitude at a continuous operation mode. COD increased by sonication from 6 up to about 19% compared to the raw sample, resulting in effluents with COD values exceeding 20 g/L. It appears that ultrasound irradiation had a significant effect on the samples organic loading, that could be attributed to compounds that although presented in the influent, they were not measured by the corresponding COD method; sonication resulted in the dissociation of these substances, making them susceptible to COD measurement. Furthermore, the addition of SDS or the application of intermittent mode of operation, did not have a significant effect on COD change. The variation of soluble COD as a function of reaction time during sonication is presented in Fig. 6(a), while the % soluble COD change is shown in Fig. 6(b). It is observed that ultrasound irradiation resulted in the increase of the samples soluble COD values of all the samples, similar to total COD values. Soluble COD values of treated samples increased by sonication from 10 to about 21% depending upon the reaction timed, potentially due to the dissociation of organic substances that could then be measured by the analytical COD method. Similar results have been found by Appels et al. [25], who reported an increase of soluble COD in waste activated sludge when increasing the specific energy derived by the ultrasonic device. 3.3. Effect of pretreatment on the anaerobic methane production rate Preliminary treatment of raw molasses wastewater by ozonation and sonication carried out under different conditions in order to determine their effect on biogas production during the following anaerobic digestion tests, and to deduce the optimum experimental conditions for further analysis. During the preliminary runs it was found that the treatment of raw effluent by ozone at long reaction times, exceeding 60 min, revealed negligible biogas production, while ultrasound irradiation at reaction times shorter than 120 min resulted in low biogas production. Therefore, five samples were examined for their anaerobic digestion potential produced by ozone and ultrasonic irradiation pretreatment, under conditions shown in Table 1. Samples treated by ultrasonic irradiation were designated TSC and TSI for treatment in continuous and intermitted mode
5
respectively, while ozone treated samples were named according the time of ozonation as TO20, TO40 and TO60, where the index represents the reaction time in minutes of ozonation. The sample of the raw substrate without pretreatment was designated T0, while the characteristic properties of this sample are given in Table 2. After pretreatment, the produced samples were cooled to room temperature and stored at 4 C until the anaerobic digestion tests. For the examination of the anaerobic methane potential, each sample was added in the bench scale anaerobic reactors, together with inoculums of anaerobic sludge sample, collected from the full scale anaerobic digester of the baker's yeast wastewater treatment plant. The characteristics of the anaerobic sludge sample were: total solids content 86.9 g/L, volatile solids 13.7 g/L and pH 8.6. The characteristic properties of the samples subjected to various pretreatment modes are included in Table 3, including pH, COD, total and volatile solids. The pH of the raw effluent increased during the ultrasonic irradiation from 8.0 to 8.5 while ozonation resulted in the reduction of the pH; the longer the reaction time the higher the pH reduction. At the longer reaction time, 60 min, pH value of the treated sample was 7.8. In addition, ozonation resulted in the significant reduction of organic matter; the highest COD removal rate was observed at the longer reaction time and total COD concentration of the sample that was ozone treated at 60 min, was about 13 g/L, while soluble COD content was 11.5 g/L. However, ultrasonic irradiation under the specific conditions examined in these tests, had negligible effects on the wastewater samples: total and soluble COD values of treated samples were almost similar to the raw effluent. Nevertheless, total and volatile solids concentration of treated samples were only slightly reduced by the application of the particular processes. The accumulative biogas production rate during anaerobic digestion of pretreated samples is shown in Fig. 7(a), while the corresponding methane yield per kg of volatile solids in the corresponding samples before the pretreatment, as deduced by the analysis of biogas samples is presented in Fig. 7(b). All anaerobic reactors were operated for 35 days; however negligible biogas production was observed after the 30th day of operation. Nevertheless, biogas was not produced in the control reactor containing the inoculated anaerobic sludge and water at the specific ratio used for the anaerobic runs. However, significant biogas production was observed for the pretreated sample, even from the first day of the operation of the anaerobic reactors.
Fig. 5. Effect of sonication time on molasses wastewater total COD: a) COD variation b) % COD increase. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 6. Effect of sonication time on molasses wastewater soluble COD: a. COD variation b. COD increase %. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Table 1 Experimental pretreatment conditions used for the examination of anaerobic digestion potential of baker's yeast wastewater. Sample symbol
Pretreatment method
T0 TSC TSI TO20 TO40 TO60
Raw molasses wastewater without any pretreatment Sonication, amplitude 90%, continuous operation mode, 120 min Sonication, amplitude 90%, intermitted operation mode, 120 min Ozonation, 20 min Ozonation, 40 min Ozonation, 60 min
Table 2 Characteristic properties of raw effluent from the full scale baker's yeast manufacturing industrial plant, used for the examination of anaerobic digestion potential. Parameters
Values
TS (g/L) VS (g/L) PH COD total (g/L) COD soluble (g/L)
27.4 14.1 8.0 19.9 15.4
Intermittent sonication of the effluent was not as efficient as the continuous sonication, and methane yield of sample TSI was 420.5 L CH4/kg of VS, slightly higher than the corresponding value of the reference control sample T0, for which methane yield reached to 403.6 L CH4/kg VS. The difference in methane production rate between the sample TSC subjected to continuous sonication and the reference control T0, was statistically significant. These results indicate that pretreatment by ultrasonic irradiation enhanced the methane production of the molasses wastewater, and the process was more efficient especially under a continuous operation mode.
Table 3 Characteristic properties of baker yeast's wastewater samples after pretreatment by ozonation and ultrasonic irradiation. Wastewater sample
Total COD (g/L)
Soluble COD (g/L)
TS (g/L)
VS (g/L)
pH
Sonication pretreated (continuous) Sonication pretreated (intermitted) Ozonation pretreated, 20 min Ozonation pretreated, 40 min Ozonation pretreated, 60 min
18.9 18.9 13.6 13.4 13.1
15.8 15.6 12.3 11.8 11.5
21.8 22.2 26.1 24.7 23.1
11.9 12.4 13.1 13.2 11.4
8.5 8.5 8.3 8.0 7.8
Biogas production rate varied from 10 to 300 mL per day depending upon the particular sample. The cumulative biogas volumes for the samples T0, TSC, TSI, TO20, T040 and TO60 were 1400, 1510, 1440, 1270, 1120 and 790 mL, respectively. In general, methane production was significantly higher in samples pretreated by ultrasonic irradiation, than the samples pretreated by ozone. Especially sample TSC, produced by sonication of raw effluent under continuous operation mode, presented the highest methane yield reaching to about 441.6 L CH4/kg of VS.
It seems that ultrasonic cavitation pretreatment increased the biodegradability of some of the organic substances of the raw effluent, that are not readily biodegradable [7,30]. However, ozone pretreatment proved to result samples with low biogas production rates and methane yields. The examination of the corresponding methane yields of samples TO20, TO40 and TO60, revealed that the ozonation reaction adversely affected the methane production: as shown in Fig. 7(b) the methane production rate reduced significantly by the increase of ozonation time.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 7. Effect of pretreatment by ozonation and ultrasonic irradiation on the anaerobic biogas production rate of molasses wastewater: a) cumulative biogas production b) methane yield. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Ozonation is a strong oxidative process, and therefore, oxidation of biodegradable organic compounds of raw wastewater carried out during the ozone pretreatment; thus, ozone pretreated samples had low substrate concentration, that could be consumed by the anaerobic microorganisms during the following process, corresponding to a low methane production rate [31]. The COD total removal % of the effluents subjected to pretreatment followed by anaerobic digestion, is presented in Fig. 8. As shown, about 70% COD removal of raw wastewater sample was achieved by the examined processes. COD total removal % of reference control sample T0, subjected only to anaerobic digestion, was significantly low, reaching to about 58%. Samples pretreated by ultrasonic irradiation resulted in about 70e74% COD total removal, the most efficient being the treatment under continuous sonication mode. However, lower COD removal rates were observed for ozone pretreated samples, varying from 65 to 68%. Although in the ultrasound treatment the energy input probably will exceed the extra energy produced by the surplus of methane, through the anaerobic treatment, the combination of the COD reduction and the recovery of a part of the energy input renders the ultrasound pretreatment an interesting process for the treatment of the molasses based wastewater.
4. Conclusions The present work showed that the ozonation pretreatment resulted in a remarkable decolorization of the high strength wastewater after 20 min of reaction time. Furthermore, a significant removal of the COD content was observed, reaching up to 38%; COD removal increased by the reaction time. However, ozonation as a pretreatment method prior to anaerobic digestion did not enhance methane production rate. The effect of ultrasonic irradiation on the properties of the raw wastewater was negligible, and treated samples presented similar color compared to the raw samples. Total and soluble COD values of treated samples increased by sonication up to about 21% of the raw sample, potentially due to the formation of compounds that were susceptible to the specific COD analysis method. Sonication followed by anaerobic digestion was effective for the removal of organic compounds, resulting to more than 70% COD total removal, and high methane yields, reaching up to 441.6 L CH4/kg VS. This study revealed that ozonation constitutes an effective method for the decolorization and COD removal of molasses wastewater when it's used itself, whereas it shows limited effectiveness when it is followed by anaerobic digestion. Sonication is a more favorable choice when it's coupled with anaerobic digestion than when it's applied on its own, although the energy required for its implementation exceeds in many cases the extra energy produced by the surplus of biogas. However, sonication could be regarded in general terms as an effective process for the treatment of molasses wastewater if the environmental parameters are also taken into consideration. Nevertheless economic and energy analyses are needed in order to clarify which of the above mentioned methods is preferable, in accordance with the desirable goal. Acknowledgements The financial support through the co financed by the European Union and the Greek State Program EPAN-II (OPC-II)/ESPA (NSRF): 'SYNERGASIA II0 , Project MOL-TREAT-Integrated treatment of high molasses wastewater for recovery of high added value products and reduction of pollutant loading (11SYN-8-699) is gratefully appreciated.
Fig. 8. COD total removal % of molasses wastewater samples subjected to ultrasonic irradiation and ozonation followed by anaerobic digestion. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
References [1] M. Kobya, S. Delipinar, Treatment of the baker's yeast wastewater by electrocoagulation, J. Hazard Mater 154 (2008) 1133e1140.
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[2] M.E. Ersahin, R.K. Dereli, H. Ozgun, B.G. Donmez, I. Koyuncu, M. Altinbas, et al., Source Based Characterization and Pollution Profile of a Baker's Yeast Industry, CLEANeSoil Air Water 39 (2011) 543e548. [3] S. Zub, T. Kurissoo, A. Menert, V. Blonskaja, Combined biological treatment of high-sulphate wastewater from yeast production, Water Environ. J. 22 (2008) 274e286. [4] S. Kalyuzhnyi, M. Gladchenko, E. Starostina, S. Shcherbakov, B. Versprille, Integrated biological (anaerobic-aerobic) and physico-chemical treatment of baker's yeast wastewater, in: Presented at the 4th World Water Congress: Innovation in Wastewater Treatment Processes, 52, 2004, pp. 273e280. [5] Y. Satyawali, M. Balakrishnan, Wastewater treatment in molasses-based alcohol distilleries for COD and color removal: a review, J. Environ. Manage 86 (2008) 481e497. ve, R. Dewil, Principles and potential of the [6] L. Appels, J. Baeyens, J. Degre anaerobic digestion of waste-activated sludge, Prog. Energy Combust. Sci. 34 (2008) 755e781. [7] P.C. Sangave, A.B. Pandit, Ultrasound pre-treatment for enhanced biodegradability of the distillery wastewater, Ultrason. Sonochem 11 (2004) 197e203. [8] E. Neyens, J. Baeyens, A review of thermal sludge pre-treatment processes to improve dewaterability, J. Hazard Mater 98 (2003) 51e67. [9] E. Neyens, J. Baeyens, C. Creemers, Alkaline thermal sludge hydrolysis, J. Hazard Mater 97 (2003) 295e314. [10] E. Neyens, J. Baeyens, M. Weemaes, Hot acid hydrolysis as a potential treatment of thickened sewage sludge, J. Hazard Mater 98 (2003) 275e293. n, J. Encinar, J.F. Gonza lez, Industrial wastewater advanced oxidation. [11] F.J. Beltra Part 2. Ozone combined with hydrogen peroxide or UV radiation, Water Res. 31 (1997) 2415e2428. mez-Lo pez, M.I. Gil, A. Allende, J. Blancke, L. Schouteten, M.V. Selma, [12] V.M. Go Disinfection capacity of high-power ultrasound against E. coli O157: H7 in process water of the fresh-cut industry, Food Bioprocess Technol. 7 (2014) 3390e3397. [13] S. Papoutsakis, S. Miralles-Cuevas, N. Gondrexon, S. Baup, S. Malato, C. Pulgarin, Coupling between high-frequency ultrasound and solar photoFenton at pilot scale for the treatment of organic contaminants: An initial approach, Ultrason. Sonochem 22 (2015) 527e534. [14] F. Hogan, S. Mormede, P. Clark, M. Crane, Ultrasonic sludge treatment for enhanced anaerobic digestion, Water Sci. Technol. 50 (2004) 25e32. [15] J. Laurent, M. Casellas, M.-N. Pons, C. Dagot, Flocs surface functionality assessment of sonicated activated sludge in relation with physico-chemical properties, Ultrason. Sonochem 16 (2009) 488e494. [16] C. Petrier, A. Francony, Incidence of wave-frequency on the reaction rates during ultrasonic wastewater treatment, Water Sci. Technol. 35 (1997) 175e180.
[17] D.-M. Zhao, X.-H. Xu, L.-C. Lei, D.-H. Wang, Degradation of 4-chiorophenol solution by synergetic effect of dual-frequency ultrasound with fenton reagent, Chin. J. Chem. Eng. 13 (2005) 204e210. [18] T.V. Adulkar, V.K. Rathod, Ultrasound assisted enzymatic pre-treatment of high fat content dairy wastewater, Ultrason. Sonochem 21 (2014) 1083e1089. [19] C. Wang, C. Liu, Decontamination of alachlor herbicide wastewater by a continuous dosing mode ultrasound/Fe(2þ)/H2O2 process, J. Environ. Sci. 26 (2014) 1332e1339. [20] M. Siddique, R. Farooq, G.J. Price, Synergistic effects of combining ultrasound with the Fenton process in the degradation of Reactive Blue 19, Ultrason. Sonochem 21 (2014) 1206e1212. [21] J. Monteagudo, A. Duran, I. San Martín, S. Garcia, Ultrasound-assisted homogeneous photocatalytic degradation of reactive blue 4 in aqueous solution, Appl. Catal. B Environ. 152 (2014) 59e67. [22] N.A. Oz, A.C. Uzun, Ultrasound pretreatment for enhanced biogas production from olive mill wastewater, Ultrason. Sonochem 22 (2015) 565e572. [23] A.K. Verma, C. Raghukumar, C.G. Naik, A novel hybrid technology for remediation of molasses-based raw effluents, Bioresour. Technol. 102 (2011) 2411e2418. [24] R. Dewil, J. Baeyens, R. Goutvrind, The use of ultrasonics in the treatment of waste activated sludge, Chin. J. Chem. Eng. 14 (2006) 105e113. ve, Ultrasonically enhanced anaerobic [25] L. Appels, R. Dewil, J. Baeyens, J. Degre digestion of waste activated sludge, Int. J. Sustain Eng. 1 (2008) 94e104. [26] M. Pena, M. Coca, G. Gonzalez, R. Rioja, M. Garcıa, Chemical oxidation of wastewater from molasses fermentation with ozone, Chemosphere 51 (2003) 893e900. [27] A. Battimelli, D. Loisel, D. Garcia-Bernet, H. Carrere, J. Delgenes, Combined ozone pretreatment and biological processes for removal of colored and biorefractory compounds in wastewater from molasses fermentation industries, J. Chem. Technol. Biotechnol. 85 (2010) 968e975. [28] C. Tsioptsias, D.C. Banti, P. Samaras, Experimental study of degradation of molasses wastewater by biological treatment combined with ozonation, J. Chem. Technol. Biotechnol. (2015), http://dx.doi.org/10.1002/jctb.4648. [29] E.W. Rice, American Public Health Association, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, 2012. [30] J. Bohdziewicz, A. Kwarciak, E. Neczaj, Influence of ultrasound field on landfill leachate treatment by means of anaerobic process, Environ. Prot. Eng. 31 (2005) 61e71. ~ o, A.P. Rollon, The Effect of ozonation pre-treatment in enhancing [31] E.E. Ordon the biodegradability of effluent from distillery of ethanol fermented from molasses, Int. J. Eng. 1 (2012) 47e52.
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Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater M. Mischopoulou a, b, P. Naidis b, S. Kalamaras c, T.A. Kotsopoulos c, *, P. Samaras a, ** a
Department of Food Technology, Alexander Technological Education Institute of Thessaloniki, GR-57400 Thessaloniki, Greece Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece c Department of Hydraulics, Soil Science and Agricultural Engineering, Faculty of Agriculture, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 20 November 2015 Accepted 23 November 2015 Available online xxx
Ozonation and sonication were applied to baker's yeast wastewater with high molasses content, under various operation conditions, in order to study the effect of them on COD (Chemical Oxygen Demand) removal and on methane enhancement. The ozonation treatment resulted in a significant reduction of the COD content; the COD removal was up to 38% after a reaction time of 5 h. Moreover, a remarkable decolorization was observed, at 20 min of ozonation. The effect of sonication on the physical characteristics of the wastewater was negligible and resulted in an increase of the COD value. The anaerobic experiment was carried out in 18 batch reactors at 37 C. The most efficient pretreatment method was sonication in a continuous mode, since it presented the highest methane production equal to 441.6 LCH4/ kgVS. It was found that this method was also effective on COD removal, when sonication is followed by anaerobic digestion. The ozonation as a pretreatment method affected negatively biomethanation, as it resulted in significant reduction of methane production compared to the samples without pretreatment. The findings of the present study proved that the sonication of molasses wastewater followed by anaerobic digestion is an efficient solution, capable of treating this type of wastewater. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic Ozonation Anaerobic digestion Molasses wastewater Methane enhancement
1. Introduction Molasses represents a by-product of sugar manufacturing and it is used as raw material in several industrial applications such as bioethanol production, baker's yeast fermentation etc. Molasses is made up from 10 to 15% minerals, 15e20% non sugar organic substances, 45e50% residual sugars, such as glucose, fructose and sucrose, and about 20% water [1,2]. During the baker's yeast fermentation process, a great part of the non-sugar substances is not assimilated by the yeasts, and it is released unaltered in the processing water. These compounds make up the effluent wastewater from the yeast production procedure [1]. The dark brown colored effluent from baker's yeast industry is generated in large quantities: 10e15 L of wastewater is produced per L of product. The main characteristics of the effluent are the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.A. Kotsopoulos), samaras@food. teithe.gr (P. Samaras).
high concentration of total Kjeldahl nitrogen, COD (chemical oxygen demand), BOD (biological oxygen demand), sulphate and trimethylglycine, the low levels of readily degradable acids and sugars, as well as the presence of highly colored melanoidin and phenolic substances [2e4]. Therefore, the wastewater produced by this process is highly polluted, and it is vital that it is treated before being discharged into the receiving water bodies, aiming to prevent significant environmental problems [2]. Several processes have been employed for the treatment of the molasses-based wastewater including anaerobic, aerobic as well as physico-chemical methods. Anaerobic treatment constitutes the most often used technique, as it is associated to energy recovery in the form of biogas and over 80% BOD reduction [5]. However the utilization of anaerobic processes for the treatment of molasses wastewater is often restricted due to the presence of a high content of non-biodegradable organic materials in the effluent, and only a partial decomposition of the organic fraction occurs under anaerobic digestion [6]. Therefore, combination of other processes with anaerobic digestion is required, aiming to the implementation of an integrated treatment scheme for the efficient degradation of
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Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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biorefractories [7]. Towards this target, pretreatment methods, prior to anaerobic digestion, could be beneficial in favoring the anaerobic digestion potential of molasses wastewater and enhancing the methane production rate. Several pretreatment methods can be used to improve the biodegradability of sludge and wastewater including alkali, acid and thermal processes [8e10]. In addition to more conventional pretreatment methods the potential of advanced oxidation technologies, such as sonication and ozonation, is investigated for enhancing the biodegradability and simultaneously reducing the organic and inorganic content of wastewaters with high biorefractories content [11]. High-power ultrasound at frequencies between 20 and 100 kHz is an efficient method for wastewater treatment due to cavitation phenomena and the formation of high intensity bubbles [12,13]. The energy waves, generated by the application of ultrasound irradiation to a liquid medium, are propagated by the alternation of compression (high pressure) and rarefraction (low pressure). At sufficiently high-power densities, the attractive forces are overcome by the rarefraction cycle and bubbles are formed [14]. These bubbles grow, expand and finally collapse, leading to the establishment of high local temperatures (up to 5000 K) and pressures (up to 180 MPa). Furthermore, high shearing phenomena are generated in the liquid phase, while radicals including OH., OH.2, and H. may be formed [15]. During wastewater treatment processes, a cavitation bubble may function as a micro reactor in which organic bonds may break down. In addition, H. and OH. radicals can react with specific pollutants in the bulk of the solution, resulting to additional reduction of organic loading [16]. Ultrasound technology has been used for the treatment of several types of wastewater, such as chlorophenolics, high fat dairy wastewater, herbicide wastewater and wastewater containing textile dyes [17e21]. Nevertheless, ultrasound irradiation has found application as a pretreatment method for the enhancement of biodegradability of wastewater with a high biorefractories content, such as olive mill or distilleries wastewater, ahead of biological treatment carried out by anaerobic digestion or by fungi [7,22,23]. Moreover, ultrasound technology was efficiently used in waste activated sludge to improve the biodegradability of the organic compounds [24] and for the enhancement of methane production when ultrasound technology was used prior to anaerobic digestion in the sludge [25]. Ozonation constitutes a promising method for the polishing of secondary effluents, due to its potential of degrading detrimental organic contaminants. Hence, it has been used for the removal of a large number of compounds such as pesticides, organochlorides, phenolic substances and dyes [26,27]. Moreover, ozonation has shown high COD removal on anaerobically treated molasses wastewater followed by aerobic treatment and therefore can be considered as an excellent way for COD removal [28]. However, limited efforts have been reported on the utilization of advanced oxidation processes for the enhancement of anaerobic digestion potential of wastewater with a high content of substances with low or even negligible biodegradability such as wastewater from the baker's yeast manufacturing with a high concentration of melanoidins. The objectives of this work were the examination of the effect of ultrasound irradiation and ozonation on the anaerobic biodegradability and methane production rate of molasses wastewater, the investigation of the reduction potential of COD content and decolorization capacity of raw molasses effluents due to these processes, and the determination of the optimum experimental conditions for the achievement of an integrated process with the highest efficiency for the treatment of molasses wastewater. The objectives of this work are illustrated in Fig. 1.
2. Materials and methods 2.1. Ozonation and sonication experiments Wastewater samples were collected from the effluent of a baker's yeast industrial plant. About 10 L of samples were collected in plastic bottles, and they were transferred to the laboratory for further analysis or treatment; samples were stored at 4 C, prior to their usage. Ozonation experiments took place at room temperature, in a 2 L bubble flow plexi-glass reactor, under various reaction times, ranging from 5 up to 300 min. The gas flow rate was adjusted at 4 L/ min, the inlet ozone concentration was approximately 8.3 mg O3/L gas, while ozone was produced by an ozone generator. An ultrasonic homogenizer (SONOPLUS HD3400, BANDELIN, Germany) was used for the sonication experiments and the pretreatment of the samples. During sonication experiment, 500 mL of sample were added in a glass beaker and were treated at different conditions; sample temperature was monitored and was maintained at 25 C using an ice bath. The operation conditions examined included: ultrasound power 400 W, ultrasound frequency 20 kHz, amplitude of the ultrasound waves 90% and 50%, contact time 30, 60, 90, 120 min, continuous or intermitted operation mode (pulsation time on0.5 s, off-1 sec). In addition, SDS (Dodecyl hydrogen sulfate sodium salt, Merck) was added, at concentrations of 50 and 100 mg/L, in order to examine the effect of an anionic surfactant on reduction of organics concentration and on the enhancement of biodegradation potential. 2.2. Anaerobic biodegradation potential A batch assay was carried out to determine the anaerobic biodegradation potential of different samples of pretreated wastewater, corresponding to the biochemical methane potential (BMP). The experimental setup for the determination of the BMP test consisted in 1 L glass bottles representing bench scale anaerobic reactors. The working volume of each glass bottle was 500 mL; in each reactor, an appropriate volume of anaerobic sludge was added in order to obtain a sludge: substrate ratio of 2:1 v/v. Anaerobic sludge was collected from the full scale anaerobic digester of the wastewater treatment plant of the baker's yeast industry. Each sample was carried out in three replicates in order to receive representative results. Nevertheless, control reactors containing only sludge and water in the desired ratio were prepared aiming to the estimation of the background methane production due to organic matter content of the sludge itself. After inoculation of the sludge and the substrate in each reactor, the glass bottles were flushed with nitrogen gas to ensure anaerobic conditions, sealed with screw cap GL45, and finally were placed in an incubator operating at 37 ± 1 C. Once per day the reactors were vigorously mixed by hand. All the reactors were operated for 35 days until no further biogas production was observed. 2.3. Analytical methods Total and soluble COD content in each sample were determined by the Hach-Lange cuvette tests, along with a spectrophotometer (DR-2800). Soluble COD was determined by filtering the sample through a 0.45 mm filter. Total and volatile solids analysis carried out according to standard methods of analysis [29]. The pH of the samples was measured using a portable pH-meter (Nahita, model 902/4). The absorbance spectrum of each sample was measured by a UVeVis spectrophotometer (Helios Alpha, Thermo Electron Corporation) at wavelengths from 200 to 800 nm, using quartz cells
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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3
Fig. 1. Logic diagram of the study's objectives.
1 cm long. Biogas production was monitored each day, by the volumetric water displacement method: each bench scale anaerobic reactor was connected to 1 L glass cylinder, reversed into a plexi-glass water tank. The tank and the cylinders were filled with acidified water (pH < 2), to reduce the dissolution of the produced carbon dioxide in the water phase, allowing for the measurement of the biogas production rate. The methane content of biogas was determined using a Shimadzu Gas Chromatograph (GC-2010plusAT), with a Thermal Conductivity Detector (TCD) equipped with a GC-Column, (FS ValcoPLOT HayeSep D) 15 m long, with internal diameter 0.53 mm and a film thickness 20 mm. The GC had a VP-Molesieve Column, (FS, ValcoBond, 5A Fused Silica) 15 m long with internal diameter 0.53 mm, and a film thickness 20 mm. The temperature of the injector was adjusted at 100 C. Helium was used as a carrier gas and the temperature of the oven and TCD was at 32.5 C and 120 C accordingly. Standard gas mixtures were used for the quantification of methane and the sampling volume was 1 mL. 2.4. Statistical analysis of the data Data analysis was performed using SPSS version 21. Significant differences (p < 0.05) among treatments were based on one-way ANOVA (Analysis Of Variance). Duncan test was used to identify which treatments were significantly different. 3. Results and discussion The aim of this work was the investigation of ultrasonic irradiation and ozonation as potential pretreatment methods of molasses wastewater, for increasing the following biodegradation potential of the effluents. Therefore, preliminary experiments of the single processes were carried out, in order to identify the optimum conditions of sample treatment, prior to the measurement of BMP. 3.1. Effect of ozonation and sonication on the decolorization of the wastewater Molasses wastewater represents an effluent with a strong brownish color; thus, several efforts have been paid towards to its decolorization. The UV/VIS absorbance spectrums of raw effluent and of the sample subjected to 20 min ozonation are shown in Fig. 2(a), while the color of the samples is presented in Fig. 2(b). Ozonation represents a strong oxidative treatment method, able to remove color and aromatic compounds such as melanoidins; as a result, treatment by ozone of molasses wastewater resulted in color reduction of the sample, at short reaction times. As shown in Fig. 2(a), the ozone treated sample presented lower absorbance than the raw effluent at shorter wavelengths; the latter presented high absorbance at wavelengths up to 400 nm, and even small
absorbance at wavelengths as high as 800 nm. Ozonation at 20 min resulted in a yellowish effluent, as shown in Fig. 2(b), similar to the results of Pena et al. [26], who reported that molasses wastewater decolorization occurred mainly at the initial stage of the ozonation reaction. However, ultrasonic irradiation of the raw effluent was not efficient in color reduction of the sample; the absorption spectra of molasses wastewater before and after sonication for 120 min was almost the same. 3.2. Effect of ozonation and sonication on the COD removal of the wastewater The aim of the advanced oxidation processes is the reduction of the organic matter content of the effluents, through the disintegration of the organic compounds due to oxidation by free radicals or cavitation respectively. Therefore, the effect of ozonation and ultrasound irradiation on the COD of the molasses wastewater was examined in this section. The variation of the total COD as a function of reaction time during ozonation is presented in Fig. 3(a), while the corresponding removal of total COD is shown in Fig. 3(b). Various raw effluents were subjected to ozonation at reaction times as long as 300 min; effluents samples were collected from the full scale industrial plant and presented a great variation in COD content, depending upon the processing line, the raw materials used, etc. As shown in Fig. 3(a), ozonation resulted in a great reduction of COD content at short reaction times; during the first 5 min of the reaction COD removal of about 15% was observed. However, ozonation for longer times resulted only in slight organic matter removal, reaching to about 38% at 300 min of reaction. The effect of ozonation on sample soluble COD is shown in Fig. 4(a) as a function of reaction time, while the corresponding removal % is presented in Fig. 4(b). Similar to total COD removal, ozonation was efficient in the removal of dissolved compounds, even from the first 5 min of ozonation: soluble COD removal was about 13% after 5 min of ozonation reaching to 22 after 20 min of reaction. However, prolonged ozonation, from 30 to 120 min did not significantly reduced the content of organic compounds. These results indicate that a short ozonation time, up to 20 min, could be effective for molasses wastewater COD removal, as most of the soluble COD was removed during that period. Sonication of raw effluents carried out for reaction times up to 120 min, under various experimental conditions: two samples were treated at a continuous operation mode with 90% amplitude, one sample was treated at an intermitted operation mode with 90% amplitude, one sample was treated at a continuous operation mode with 50% amplitude and in two samples SDS was added at concentrations of 50 and 100 mg/L, following ultrasonic irradiation at continuous operation mode with 90% amplitude. The variation of the total COD as a function of reaction time during sonication is
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 2. Absorption spectra of molasses wastewater before and after ozonation for 20 min.
Fig. 3. Effect of ozonation on molasses wastewater total COD: a. COD variation b. COD removal as a function of reaction time. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Fig. 4. Effect of ozonation time on molasses wastewater soluble COD: a. COD variation b. COD removal %. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
M. Mischopoulou et al. / Renewable Energy xxx (2015) 1e8
presented in Fig. 5(a), while the total COD removal is shown in Fig. 5(b). COD values of all the samples increased during sonication, especially for the two samples that were treated with 90% amplitude at a continuous operation mode. COD increased by sonication from 6 up to about 19% compared to the raw sample, resulting in effluents with COD values exceeding 20 g/L. It appears that ultrasound irradiation had a significant effect on the samples organic loading, that could be attributed to compounds that although presented in the influent, they were not measured by the corresponding COD method; sonication resulted in the dissociation of these substances, making them susceptible to COD measurement. Furthermore, the addition of SDS or the application of intermittent mode of operation, did not have a significant effect on COD change. The variation of soluble COD as a function of reaction time during sonication is presented in Fig. 6(a), while the % soluble COD change is shown in Fig. 6(b). It is observed that ultrasound irradiation resulted in the increase of the samples soluble COD values of all the samples, similar to total COD values. Soluble COD values of treated samples increased by sonication from 10 to about 21% depending upon the reaction timed, potentially due to the dissociation of organic substances that could then be measured by the analytical COD method. Similar results have been found by Appels et al. [25], who reported an increase of soluble COD in waste activated sludge when increasing the specific energy derived by the ultrasonic device. 3.3. Effect of pretreatment on the anaerobic methane production rate Preliminary treatment of raw molasses wastewater by ozonation and sonication carried out under different conditions in order to determine their effect on biogas production during the following anaerobic digestion tests, and to deduce the optimum experimental conditions for further analysis. During the preliminary runs it was found that the treatment of raw effluent by ozone at long reaction times, exceeding 60 min, revealed negligible biogas production, while ultrasound irradiation at reaction times shorter than 120 min resulted in low biogas production. Therefore, five samples were examined for their anaerobic digestion potential produced by ozone and ultrasonic irradiation pretreatment, under conditions shown in Table 1. Samples treated by ultrasonic irradiation were designated TSC and TSI for treatment in continuous and intermitted mode
5
respectively, while ozone treated samples were named according the time of ozonation as TO20, TO40 and TO60, where the index represents the reaction time in minutes of ozonation. The sample of the raw substrate without pretreatment was designated T0, while the characteristic properties of this sample are given in Table 2. After pretreatment, the produced samples were cooled to room temperature and stored at 4 C until the anaerobic digestion tests. For the examination of the anaerobic methane potential, each sample was added in the bench scale anaerobic reactors, together with inoculums of anaerobic sludge sample, collected from the full scale anaerobic digester of the baker's yeast wastewater treatment plant. The characteristics of the anaerobic sludge sample were: total solids content 86.9 g/L, volatile solids 13.7 g/L and pH 8.6. The characteristic properties of the samples subjected to various pretreatment modes are included in Table 3, including pH, COD, total and volatile solids. The pH of the raw effluent increased during the ultrasonic irradiation from 8.0 to 8.5 while ozonation resulted in the reduction of the pH; the longer the reaction time the higher the pH reduction. At the longer reaction time, 60 min, pH value of the treated sample was 7.8. In addition, ozonation resulted in the significant reduction of organic matter; the highest COD removal rate was observed at the longer reaction time and total COD concentration of the sample that was ozone treated at 60 min, was about 13 g/L, while soluble COD content was 11.5 g/L. However, ultrasonic irradiation under the specific conditions examined in these tests, had negligible effects on the wastewater samples: total and soluble COD values of treated samples were almost similar to the raw effluent. Nevertheless, total and volatile solids concentration of treated samples were only slightly reduced by the application of the particular processes. The accumulative biogas production rate during anaerobic digestion of pretreated samples is shown in Fig. 7(a), while the corresponding methane yield per kg of volatile solids in the corresponding samples before the pretreatment, as deduced by the analysis of biogas samples is presented in Fig. 7(b). All anaerobic reactors were operated for 35 days; however negligible biogas production was observed after the 30th day of operation. Nevertheless, biogas was not produced in the control reactor containing the inoculated anaerobic sludge and water at the specific ratio used for the anaerobic runs. However, significant biogas production was observed for the pretreated sample, even from the first day of the operation of the anaerobic reactors.
Fig. 5. Effect of sonication time on molasses wastewater total COD: a) COD variation b) % COD increase. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 6. Effect of sonication time on molasses wastewater soluble COD: a. COD variation b. COD increase %. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Table 1 Experimental pretreatment conditions used for the examination of anaerobic digestion potential of baker's yeast wastewater. Sample symbol
Pretreatment method
T0 TSC TSI TO20 TO40 TO60
Raw molasses wastewater without any pretreatment Sonication, amplitude 90%, continuous operation mode, 120 min Sonication, amplitude 90%, intermitted operation mode, 120 min Ozonation, 20 min Ozonation, 40 min Ozonation, 60 min
Table 2 Characteristic properties of raw effluent from the full scale baker's yeast manufacturing industrial plant, used for the examination of anaerobic digestion potential. Parameters
Values
TS (g/L) VS (g/L) PH COD total (g/L) COD soluble (g/L)
27.4 14.1 8.0 19.9 15.4
Intermittent sonication of the effluent was not as efficient as the continuous sonication, and methane yield of sample TSI was 420.5 L CH4/kg of VS, slightly higher than the corresponding value of the reference control sample T0, for which methane yield reached to 403.6 L CH4/kg VS. The difference in methane production rate between the sample TSC subjected to continuous sonication and the reference control T0, was statistically significant. These results indicate that pretreatment by ultrasonic irradiation enhanced the methane production of the molasses wastewater, and the process was more efficient especially under a continuous operation mode.
Table 3 Characteristic properties of baker yeast's wastewater samples after pretreatment by ozonation and ultrasonic irradiation. Wastewater sample
Total COD (g/L)
Soluble COD (g/L)
TS (g/L)
VS (g/L)
pH
Sonication pretreated (continuous) Sonication pretreated (intermitted) Ozonation pretreated, 20 min Ozonation pretreated, 40 min Ozonation pretreated, 60 min
18.9 18.9 13.6 13.4 13.1
15.8 15.6 12.3 11.8 11.5
21.8 22.2 26.1 24.7 23.1
11.9 12.4 13.1 13.2 11.4
8.5 8.5 8.3 8.0 7.8
Biogas production rate varied from 10 to 300 mL per day depending upon the particular sample. The cumulative biogas volumes for the samples T0, TSC, TSI, TO20, T040 and TO60 were 1400, 1510, 1440, 1270, 1120 and 790 mL, respectively. In general, methane production was significantly higher in samples pretreated by ultrasonic irradiation, than the samples pretreated by ozone. Especially sample TSC, produced by sonication of raw effluent under continuous operation mode, presented the highest methane yield reaching to about 441.6 L CH4/kg of VS.
It seems that ultrasonic cavitation pretreatment increased the biodegradability of some of the organic substances of the raw effluent, that are not readily biodegradable [7,30]. However, ozone pretreatment proved to result samples with low biogas production rates and methane yields. The examination of the corresponding methane yields of samples TO20, TO40 and TO60, revealed that the ozonation reaction adversely affected the methane production: as shown in Fig. 7(b) the methane production rate reduced significantly by the increase of ozonation time.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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Fig. 7. Effect of pretreatment by ozonation and ultrasonic irradiation on the anaerobic biogas production rate of molasses wastewater: a) cumulative biogas production b) methane yield. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
Ozonation is a strong oxidative process, and therefore, oxidation of biodegradable organic compounds of raw wastewater carried out during the ozone pretreatment; thus, ozone pretreated samples had low substrate concentration, that could be consumed by the anaerobic microorganisms during the following process, corresponding to a low methane production rate [31]. The COD total removal % of the effluents subjected to pretreatment followed by anaerobic digestion, is presented in Fig. 8. As shown, about 70% COD removal of raw wastewater sample was achieved by the examined processes. COD total removal % of reference control sample T0, subjected only to anaerobic digestion, was significantly low, reaching to about 58%. Samples pretreated by ultrasonic irradiation resulted in about 70e74% COD total removal, the most efficient being the treatment under continuous sonication mode. However, lower COD removal rates were observed for ozone pretreated samples, varying from 65 to 68%. Although in the ultrasound treatment the energy input probably will exceed the extra energy produced by the surplus of methane, through the anaerobic treatment, the combination of the COD reduction and the recovery of a part of the energy input renders the ultrasound pretreatment an interesting process for the treatment of the molasses based wastewater.
4. Conclusions The present work showed that the ozonation pretreatment resulted in a remarkable decolorization of the high strength wastewater after 20 min of reaction time. Furthermore, a significant removal of the COD content was observed, reaching up to 38%; COD removal increased by the reaction time. However, ozonation as a pretreatment method prior to anaerobic digestion did not enhance methane production rate. The effect of ultrasonic irradiation on the properties of the raw wastewater was negligible, and treated samples presented similar color compared to the raw samples. Total and soluble COD values of treated samples increased by sonication up to about 21% of the raw sample, potentially due to the formation of compounds that were susceptible to the specific COD analysis method. Sonication followed by anaerobic digestion was effective for the removal of organic compounds, resulting to more than 70% COD total removal, and high methane yields, reaching up to 441.6 L CH4/kg VS. This study revealed that ozonation constitutes an effective method for the decolorization and COD removal of molasses wastewater when it's used itself, whereas it shows limited effectiveness when it is followed by anaerobic digestion. Sonication is a more favorable choice when it's coupled with anaerobic digestion than when it's applied on its own, although the energy required for its implementation exceeds in many cases the extra energy produced by the surplus of biogas. However, sonication could be regarded in general terms as an effective process for the treatment of molasses wastewater if the environmental parameters are also taken into consideration. Nevertheless economic and energy analyses are needed in order to clarify which of the above mentioned methods is preferable, in accordance with the desirable goal. Acknowledgements The financial support through the co financed by the European Union and the Greek State Program EPAN-II (OPC-II)/ESPA (NSRF): 'SYNERGASIA II0 , Project MOL-TREAT-Integrated treatment of high molasses wastewater for recovery of high added value products and reduction of pollutant loading (11SYN-8-699) is gratefully appreciated.
Fig. 8. COD total removal % of molasses wastewater samples subjected to ultrasonic irradiation and ozonation followed by anaerobic digestion. The bars designate standard deviations. Different letters above the bars signify distinct statistical groups (p < 0.05) between the different treatments.
References [1] M. Kobya, S. Delipinar, Treatment of the baker's yeast wastewater by electrocoagulation, J. Hazard Mater 154 (2008) 1133e1140.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060
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[2] M.E. Ersahin, R.K. Dereli, H. Ozgun, B.G. Donmez, I. Koyuncu, M. Altinbas, et al., Source Based Characterization and Pollution Profile of a Baker's Yeast Industry, CLEANeSoil Air Water 39 (2011) 543e548. [3] S. Zub, T. Kurissoo, A. Menert, V. Blonskaja, Combined biological treatment of high-sulphate wastewater from yeast production, Water Environ. J. 22 (2008) 274e286. [4] S. Kalyuzhnyi, M. Gladchenko, E. Starostina, S. Shcherbakov, B. Versprille, Integrated biological (anaerobic-aerobic) and physico-chemical treatment of baker's yeast wastewater, in: Presented at the 4th World Water Congress: Innovation in Wastewater Treatment Processes, 52, 2004, pp. 273e280. [5] Y. Satyawali, M. Balakrishnan, Wastewater treatment in molasses-based alcohol distilleries for COD and color removal: a review, J. Environ. Manage 86 (2008) 481e497. ve, R. Dewil, Principles and potential of the [6] L. Appels, J. Baeyens, J. Degre anaerobic digestion of waste-activated sludge, Prog. Energy Combust. Sci. 34 (2008) 755e781. [7] P.C. Sangave, A.B. Pandit, Ultrasound pre-treatment for enhanced biodegradability of the distillery wastewater, Ultrason. Sonochem 11 (2004) 197e203. [8] E. Neyens, J. Baeyens, A review of thermal sludge pre-treatment processes to improve dewaterability, J. Hazard Mater 98 (2003) 51e67. [9] E. Neyens, J. Baeyens, C. Creemers, Alkaline thermal sludge hydrolysis, J. Hazard Mater 97 (2003) 295e314. [10] E. Neyens, J. Baeyens, M. Weemaes, Hot acid hydrolysis as a potential treatment of thickened sewage sludge, J. Hazard Mater 98 (2003) 275e293. n, J. Encinar, J.F. Gonza lez, Industrial wastewater advanced oxidation. [11] F.J. Beltra Part 2. Ozone combined with hydrogen peroxide or UV radiation, Water Res. 31 (1997) 2415e2428. mez-Lo pez, M.I. Gil, A. Allende, J. Blancke, L. Schouteten, M.V. Selma, [12] V.M. Go Disinfection capacity of high-power ultrasound against E. coli O157: H7 in process water of the fresh-cut industry, Food Bioprocess Technol. 7 (2014) 3390e3397. [13] S. Papoutsakis, S. Miralles-Cuevas, N. Gondrexon, S. Baup, S. Malato, C. Pulgarin, Coupling between high-frequency ultrasound and solar photoFenton at pilot scale for the treatment of organic contaminants: An initial approach, Ultrason. Sonochem 22 (2015) 527e534. [14] F. Hogan, S. Mormede, P. Clark, M. Crane, Ultrasonic sludge treatment for enhanced anaerobic digestion, Water Sci. Technol. 50 (2004) 25e32. [15] J. Laurent, M. Casellas, M.-N. Pons, C. Dagot, Flocs surface functionality assessment of sonicated activated sludge in relation with physico-chemical properties, Ultrason. Sonochem 16 (2009) 488e494. [16] C. Petrier, A. Francony, Incidence of wave-frequency on the reaction rates during ultrasonic wastewater treatment, Water Sci. Technol. 35 (1997) 175e180.
[17] D.-M. Zhao, X.-H. Xu, L.-C. Lei, D.-H. Wang, Degradation of 4-chiorophenol solution by synergetic effect of dual-frequency ultrasound with fenton reagent, Chin. J. Chem. Eng. 13 (2005) 204e210. [18] T.V. Adulkar, V.K. Rathod, Ultrasound assisted enzymatic pre-treatment of high fat content dairy wastewater, Ultrason. Sonochem 21 (2014) 1083e1089. [19] C. Wang, C. Liu, Decontamination of alachlor herbicide wastewater by a continuous dosing mode ultrasound/Fe(2þ)/H2O2 process, J. Environ. Sci. 26 (2014) 1332e1339. [20] M. Siddique, R. Farooq, G.J. Price, Synergistic effects of combining ultrasound with the Fenton process in the degradation of Reactive Blue 19, Ultrason. Sonochem 21 (2014) 1206e1212. [21] J. Monteagudo, A. Duran, I. San Martín, S. Garcia, Ultrasound-assisted homogeneous photocatalytic degradation of reactive blue 4 in aqueous solution, Appl. Catal. B Environ. 152 (2014) 59e67. [22] N.A. Oz, A.C. Uzun, Ultrasound pretreatment for enhanced biogas production from olive mill wastewater, Ultrason. Sonochem 22 (2015) 565e572. [23] A.K. Verma, C. Raghukumar, C.G. Naik, A novel hybrid technology for remediation of molasses-based raw effluents, Bioresour. Technol. 102 (2011) 2411e2418. [24] R. Dewil, J. Baeyens, R. Goutvrind, The use of ultrasonics in the treatment of waste activated sludge, Chin. J. Chem. Eng. 14 (2006) 105e113. ve, Ultrasonically enhanced anaerobic [25] L. Appels, R. Dewil, J. Baeyens, J. Degre digestion of waste activated sludge, Int. J. Sustain Eng. 1 (2008) 94e104. [26] M. Pena, M. Coca, G. Gonzalez, R. Rioja, M. Garcıa, Chemical oxidation of wastewater from molasses fermentation with ozone, Chemosphere 51 (2003) 893e900. [27] A. Battimelli, D. Loisel, D. Garcia-Bernet, H. Carrere, J. Delgenes, Combined ozone pretreatment and biological processes for removal of colored and biorefractory compounds in wastewater from molasses fermentation industries, J. Chem. Technol. Biotechnol. 85 (2010) 968e975. [28] C. Tsioptsias, D.C. Banti, P. Samaras, Experimental study of degradation of molasses wastewater by biological treatment combined with ozonation, J. Chem. Technol. Biotechnol. (2015), http://dx.doi.org/10.1002/jctb.4648. [29] E.W. Rice, American Public Health Association, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, 2012. [30] J. Bohdziewicz, A. Kwarciak, E. Neczaj, Influence of ultrasound field on landfill leachate treatment by means of anaerobic process, Environ. Prot. Eng. 31 (2005) 61e71. ~ o, A.P. Rollon, The Effect of ozonation pre-treatment in enhancing [31] E.E. Ordon the biodegradability of effluent from distillery of ethanol fermented from molasses, Int. J. Eng. 1 (2012) 47e52.
Please cite this article in press as: M. Mischopoulou, et al., Effect of ultrasonic and ozonation pretreatment on methane production potential of raw molasses wastewater, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.11.060