Microwave (MW) technique is a promising process for sludge conditioning, numerous papers has verified the beneficial effects on the microbial degradation and anaerobic digestion of municipal sewage sludge, but cannot be found study specialized on the investigation of MW process with different intensities for sewage sludge generated in food industry wastewater purification technologies. Our research was focused on obtaining optimum parameters of MW pre-ireatment for meal processing sewage sludge (MPSS) using solubility index (SLI) and the 5 days biodegradability index (BDIs) as control parameters. For the modeling and optimization response surface methodology (RSM) and multiple linear regressions (MLR) were applied, respectively. The investigated factors were the irradiated MW energy (IMWE) and the specific MW power level (MWPL). The results shown, that since the large-scale biodegradability enhancement, the MW pre-treatment is suitable for MPSS conditioning. The MW pre-treatmcnts could be advantageous in numerous process based on biological transformation, such as activated sludge recycling processes, composting and anaerobic digestion

Beszédes, Sándor

László, Zsuzsanna

Seres, Zita

Beszédes Sándor

Seres Zita

László Zsuzsanna

University of Szeged

OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE Sándor Beszédes,' Zita Seres,2 Zsuzsanna László3 '• 'Department of Mechanical and Process Engineering, Faculty of Engineering, University of Szeged, H-6724, Szeged, Moszkvai krt. 5-7., Hungary 'Faculty of Technology, University of Novi Sad 21000 Novi Sad, Bulevar Cara Lazara 1., Serbia e-mail: beszedes@mk.u-szeged.hu ABSTRACT Microwave (MW) technique is a promising process for sludge conditioning, numerous papers has verified the beneficial effects on the microbial degradation and anaerobic digestion of munici-pal sewage sludge, but cannot be found study specialized on the investigation of MW process with different intensities for sewage sludge generated in food industry wastewater purification technolo-gies. Our research was focused on obtaining optimum parameters of MW pre-ireatment for meal processing sewage sludge (MPSS) using solubility index (SLI) and the 5 days biodegradability index (BDIs) as control parameters. For the modeling and optimization response surface methodol-ogy (RSM) and multiple linear regressions (MLR) were applied, respectively. The investigated factors were the irradiated MW energy (IMWE) and the specific MW power level (MWPL). The results shown, that since the large-scale biodegradability enhancement, the MW pre-treatment is suitable for MPSS conditioning. The MW pre-treatmcnts could be advantageous in numerous proc-ess based on biological transformation, such as activated sludge recycling processes, composting and anaerobic digestion. 1. INTRODUCTION Compared to the other industrial sectors, the food processing technologies output a great amount of wastewater because of the high water content of the raw materials processed, the commonly used dehydration operations, and in addition, the high water demand of flushing and cleaning procedures. The methods of chemical precipitation, biological treat-ment and their combination with mechanical processes have been widely used to purify municipal wastewater but because of the lack of biological stage in food industry wastewa-ter management systems a large amount of sludge with high organic matter content has been also generated. Several novel technologies have been investigated to develop flexible adaptable wastewater purification and sludge management technology for food industry effluents. The dosage of added chemicals can be mitigated by membrane separation, ac-cording to the minimal processing principle (Hodúr et al., 2004). Nanofiltration (NF) in combination with advanced oxidation processes (AOP) is suitable for achieving higher capacity of membrane purification due to the reduced membrane fouling (László et al., 2009). The concentrate remained in these hybrid processes has a lower environmental load, compared to the sludge produced in a commercial precipitation wastewater purification technology. Additionally, the NF process alone can be suitable for producing recyclable process water from high surfactant contented dairy wastewater, for instance (Kertész et al., 2008). Conventional treatment and disposal of sewage sludge involves several steps, such as anaerobic digestion, chemical conditioning, thermal conditioning, and mechanical dewater-ing followed by disposal as landfill, application to cropland or incineration (Tang et al., 2010). The commonly used treatment with polyelectrolyte is mentioned as an expensive method, and furthermore the added chemicals contribute to form extracellular polymeric 9 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE substance (EPS) (Higgins and Novak, 1997). Different species of microorganisms, biomass produced by the degradation of grease, nitrogen, and phosphorus; heavy metals and syn-thetic organic compounds agglomerated together with EPS into the polymeric netw ork of sludge. It causes hydrophilic characteristic of sludge, and increases the difficulty to achieve effective byconversion during anaerobic condition (AD) or aerobic processes (compost-ing, for instance). Various alternative methods such as sonication, AOP's, freezing, elec-trolysis, and thermal pre-treatments have been investigated to improve dehydrate capability and the disintegration of sludge (Na et al.. 2007: Yuan et al.. 2010). Many researches were focused on examining the efficiency of microwave (MW) treat-ment on sludge characteristic. Effects of high frequency electromagnetic field can be mani-fested in the change of the dipole orientation of the molecules, and it can lead to polarized side-chains of macromolecules, and the breakage of hydrogen bounds, which has an effect on biodegradability. anaerobic digestion efficiency, and disinfection ability (Hong et al.. 2004; Sz6p et al., 2007; Toreci et al., 2009). However, several results show that the MW irradiation affected the enzyme activity as well (Nemenyi et al, 2008). The MW irradiation in combination with alkaline pre-treatment has a synergetic effects on the biogas genera-tion from municipal sewage sludge (Dogan and Sanin, 2009), and the MW irradiation of sludge contented oxidizer (H2O2) could accelerate the decomposition of H2O2 into hy-droxy! radicals what is manifested in enhanced organic matter solubilization and increased disintegration rate of sludge flock (Eskicioglu et al., 2008). Investigating the dewatering characteristic of MW irradiated sludge, Wojciechowska (2005) concluded that increase the exposure time beyond the optimal value the pre-treatment effects has been worsened and the specific energy consumption was notably increased. In last decade, numerous papers have dealt with the examination of dewaterability and AD characteristic of MW irradiated sludge, but study cannot be found that specialize on the effects of MW irradiation with various power levels on the biodegradability of food industry sludge. In our work we focused on the examination, and optimization of MW pre-treatment for meat processing sewage sludge (MPSS). 2. MATERIALS AND METHODS Dewatered (27.24 ± 1.8% TS) MPSS came from tertiary wastewater purification stage of a meal processing company located in Szeged (Hungary). Before the measurements the samples were frozen at -20 °C. The initial BOD5 and TCOD of sludge was 112.48 ± 9.2, and 478.36 ± 6.6 [kg m-3], respectively. The MW pre-treatments were performed in a microwave cavity resonator equipped with a 700 W magnetron operating at a frequency of 2.45 GHz. The magnetron power (Рпмрмюп) is changeable continuously from 50 to 700 W through varying the heating voltage with a toroidal-core transformer. For modelling and to optimize the process parameters response surface methodology (RSM) with central composite facc centered (CCF) experimental design was performed using MODDE 8.0 statistical experimental design software (Umetrics. Sweden). The stud-ied factors were the microwave power level (MWPL), and the irradiated MW energy (IMWE). MWPL (Wg ') was defined as the ratio of magnetron power to the quantity of treated sludge. IMWE was calculated as the product of magnetron power (Pnugmiron) and the exposure time (Хот) = / > . „ * ' „ [ * / ] Eq . ( l ) 10 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE The selected responses were the solubilisation index (SLI) and the aerobic biodegrad-ability for 5 days (BDI5). The solubilization index for organic matters was determined by indirect COD measurement method (SCOD/TCOD), - (SCOP/TCOD) 0 (SCOD/TCOD) 0 E q ( 2 ) where (SCOD/TCOD),, and (SCOD/TCOD), is the solubility ratio of the untreated and MW pre-treated MPSS, respectively. The total COD (tCOD) was measured by the standard dichromate method (APHA 5250D, 1995) sampled from the total sludge. The soluble COD (sCOD) was determined after centrifugation (6000 rpm, 20 min). For the separation of the water soluble phase a 0.45 pm pore-sized disc filter (Millipore) was used. The biodegradability index (BDI5) was calculated by the following expression (BOD, / tCODD) - (BOD0 / tCOD,) BDI, = Eq. (3) ( B O D 0 / t C O D 0 ) M The biochemical oxygen demand (BOD5) measurements were carried out in a respi-rometric BOD system at 20 °C for 5 days. To ensure the consistency of the experiments acclimatized standard microbes (BOD SEED, Cole-Parmer, U.S.) were used as inoculums for the measurements. 3. RESULTS AND DISCUSSION The range and the levels of the experimental variables investigated, and the responses are shown in Table 1. To evaluate the reproducibility of the Fitted model the experiments were conducted duplicated with six center points. In order to reduce the systematic error the run of the experiments were randomized. Solubilization index indicates the change of water soluble fraction of sludge organic matters. Preliminary researches had reported that MW pre-treatments may disintegrate the flock structure of municipal sludge and, therefore, the solubilisation of the organic matters was enhanced (Bougrier et al., 2008; Eskicioglu et al., 2006). Because of the destruction of sludge flock, the specific surface of the sludge particles and the efficiency of biological degradation increased. In our research the effect of IMWE (from 100 k j to 1050 kJ) and MWPE (from 0.5 to 5 Wg"1) on COD solubilization efficiency were investigated. The ex-perimental data are shown as the contour plot of fitted models. Our results show, that increased IMWE from 100 to 500 kJ at a MWPL range of 1.5 to 4 Wg"1 could increase the solubility index to 0.9 (Fig. L). The increment of COD solubility can be explained by the hydrolysis of the large molecular weight organic compounds, the lysis of the cell walls, and the disintegration of the sludge flock, which was intensified by the applied MW irradiation. It was found, that beside the energy carried by MW irradia-tion, the applied MWPL had effect on the change of soluble fraction of the organic com-pounds of sludge as well. The lower than 2.5 Wg"1 MWPL was not enough to achieve the maximum organic matter solubilization. 11 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE Table I. RSM for two variables and its experimental responses Exp No. Run ord. IMWE |kJ | MWPL [ W g ' l SLI BDI, N1 5 90 0.50 0.303 0.886 N2 18 1050 0.50 0.734 1.186 N3 14 90 5.00 0.175 0.918 N4 17 1050 5.00 0.907 1.136 N5 8 90 2.75 0.253 1.005 N6 7 1050 2.75 0.903 1.246 N7 16 570 0.50 0.751 1.004 N8 20 570 5.00 0.793 1.619 N9 9 570 2.75 0.800 1.754 N10 11 570 2.75 0.798 1.749 N11 4 570 275 0.803 1.751 N12 IS 90 0.50 0.297 0.891 N13 10 1050 0.50 0.745 1.213 N14 21 90 5.00 0.186 0.921 N15 6 1050 5.00 0.899 1.159 N16 22 90 2.75 0.267 0 991 N17 19 1050 2.75 0.902 1.259 N18 3 570 0.50 0.776 1.613 N19 13 570 5.00 0.809 1.594 N20 12 570 2.75 0.803 1.741 N21 2 570 2.75 0.801 1.76 N22 1 570 2.75 0.807 1.753 100 200 300 400 S 0 0 6 0 0 7 0 0 8 0 0 9 0 0 KME Figure J. Contour plot of solubilization index (SLIJ showing the effects of variables MWPL and 1MWE Since the biodegradability is linked to the solubility, the advantage of stronger MW ir-radiation predicts similar non-linear trends in the change of BDI5, as well. But in the case of biodegradability limited increasing was found: applied MWPL over 4 Wg ' and/or irra-diated MW energy was more than 750 kJ the BDl? was worsened (Fig.2.). 12 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE The observed unfavourable effect of strong irradiation on biodegradability supposed to be due to the mineralization effects of MW heating. Furthermore, Eskicioglu et al. (2007) reported decreasing in the sugar and protein content of the soluble phase of sludge at an elevated temperature, explained by the Maillard reactions occurring between amino acids and reducing sugars. In our case the MPSS samples contained proteins with carbohydrate compounds; therefore the longer MW irradiation could manifest in Maillard reactions with a lower biodegradability. The change of BDIs caused by the most efficient MW pre-treatment process parameters is corresponded to about 118% increment, relate to the biodegradability of untreated MPSS. 100 200 300 400 500 600 700 800 900 IMWE Figure 2. Contour plot of biodegradability index (BDIs) The applied empirical second order multiple linear regression model (MLR) for two factors was in general form according to Eq. 4. (Myers, Montgomery, 2002). Y = p o + X P , X , + l P B X ? + H P ( i X i X j ^ ( 4 ) where Y is the predicted response, Xj and Xj are the independent variables (IMWE and MWPL), and p„, Pi, Pk and py are the regression coefficients of the fitted model. Since our main object was to examine and optimize the MW pre-treatments to enhance the biode-gradability of MPSS, the modelling was performed using BDI5 as response parameter. Based on our experimental data the fitted model for biodegradability index (BDI5) was the following equation BDIS = 1.7393 + 0.1323X, -0 .0383X 2 -0 .5959Xf - 0 . 1 1 3 7 X j -0 .0208X,X 2 E q ( 5 ) The response function was significant at confidence level of 0.95; the R2 for BDI5 was 0.9976, and in addition the goodness of fit (Q2) was 0.995, which indicate good predictive 13 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE power of the models (Fig.3.). The reproducibility was over 99.9% and the standard deviations of the fitted models were higher than the standard deviation of the residuals ( R ^ > 0.997). Biodegradability with Experiment Number labels a > o l/> .o O 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Predicted »-22 R2-0.998 R2 Adj.-0.997 DF-16 Q2-0.995 RSD-0.01892 Figure J. Observed versus predicted values for biodegradability index Using the fitted model and based on the date obtained from the response surface analy-sis the optimal condition of MW process of MPSS for highest biodegradability with mini-mum irradiated energy and lowest MWPL were determined at IMWE of 621.9 kJ at MWPL of 2.684 Wg The MW pre-treatments with determined optimum process parame-ters caused a 114% increment of BDI5. The final temperature of sludge irradiated with op-timum parameters was about 95°C, therefore, for comparison purpose a commercial heat treatment was also carried out in laboratory heating equipment, at 95 °C for 120 min. The BDI5 of commercial heated MPSS was obtained at 1.023 ± 0.23; also the MW pre-treatment has advantage over the conventional heating method. 4. CONCLUSION In our work we focused on the examination of the effects of microwave (MW) pre-treatment on the solubility and biodegradability of meat processing sludge. For the experimen-tal design and optimization MODDE 8.0 software was used, investigating the effects of the specific microwave power level (MWPL) and irradiated MW energy (IMWE) on the res-ponses of solubility index of organic matters (SLI) and biodegradability index (BDI5). Our results show that the MW irradiation could enhance the soluble and biodegradable fraction of MPSS and beside the IMWE the MWPL also afreets the MW process. It was found, that in spite of the solubility increment, the MW pre-treaunent with MWPL over 4 Wg'1 or IMWE over 750 kJ had an unfavorable effect on biodegradability. Using Response Surface Methodol-ogy (RSM) with Central Composite Face (CCF) centered design the optimal process parame-ters of MW pre-treatments were determined at IMWE of 621.9 kJ with MWPL of 2.684 Wg1. After MW pre-treatments with optimum conditions the solubility index (SLI) and biodegrada-bility index (BDI5) was enhanced by 194% and 117%, respectively. Ackno wledgements The authors acknowledge the financial support provided by the project GVOP-3.2.1-2004-04/0252/3.0. 14 Sándor Beszédes. Zita Seres. Zsuzsanna László: OPTIMIZATION OF MICROWAVE PROCESS TO IMPROVE THE BIODEGRADABILITY OF MEAT PROCESSING SLUDGE REFERENCES 1. Bougrier, C., Delgenes J. P., Carrere H. (2008): Effects of thermal treatments on five different waste activated sludge samples solubilisation, physical properties and an-aerobic digestion. Chem. Eng. J. 2008:139 (2), 236-244. p. 2. Dogan I., Sanin F., D. (2009): Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 2009:43, 2139-2148. p. 3. Eskicioglu, C., Kennedy K., J., Droste R. L. (2006): Characterization of soluble or-ganic matter of waste activated sludge before and after thermal pretreatment. Water Res. 2006:40, 3725-3736. p. 4. Eskicioglu, C., Terzian N„ Kennedy K„ J., Droste, R., L„ Hamoda M. 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Optimization of microwave process to improve the biodegradability of meat processing sludge

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Optimization of microwave process to improve the biodegradability of meat processing sludge

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