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Sewage sludge disintegration by combined treatment of alkaline + high pressure homogenization
Bioresource Technology 123 (2012) 514–519
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Sewage sludge disintegration by combined treatment of alkaline + high pressure homogenization Yuxuan Zhang a, Panyue Zhang a,⇑, Guangming Zhang a,b, Weifang Ma a, Hao Wu a, Boqiang Ma a a b
Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China School of Natural Resource & Environment, Renmin University of China, 100872, China
h i g h l i g h t s " A synergy effect was achieved with combined sludge disintegration of alkaline + HPH. " Maximum sludge disintegration degree with combined treatment was 59.26%.
¼ 0:713C 0:334 P0:234 N 0:119 . " Energy efficiency with combined treatment significantly increased. " Sludge disintegration model for combined treatment was
a r t i c l e
i n f o
Article history: Received 21 April 2012 Received in revised form 13 July 2012 Accepted 22 July 2012 Available online 27 July 2012 Keywords: High pressure homogenization Alkaline treatment Sludge disintegration degree Sludge disintegration model Energy efficiency
1 1DDCOD
a b s t r a c t Alkaline pretreatment combined with high pressure homogenization (HPH) was applied to promote sewage sludge disintegration. For sewage sludge with a total solid content of 1.82%, sludge disintegration degree (DDCOD) with combined treatment was higher than the sum of DDCOD with single alkaline and single HPH treatment. NaOH dosage 60.04 mol/L, homogenization pressure 660 MPa and a single homogenization cycle were the suitable conditions for combined sludge treatment. The combined sludge treatment showed a maximum DDCOD of 59.26%. By regression analysis, the combined sludge disintegra1 ¼ 0:713C 0:334 P 0:234 N0:119 , showing that the effect of operating tion model was established as 1DD COD parameters on sludge disintegration followed the order: NaOH dosage > homogenization pressure > number of homogenization cycle. The energy efficiency with combined sludge treatment significantly increased compared with that with single HPH treatment, and the high energy efficiency was achieved at low homogenization pressure with a single homogenization cycle. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Despite myriad advances in biological wastewater treatment process, high sludge production has become one of the drawbacks impossible to ignore. Sewage sludge treatment accounts for more than 60% of total cost of municipal wastewater treatment plants (Wang et al., 2009). Therefore, new technologies have to be developed for efficient treatment and reuse of excess sludge in municipal wastewater treatment plants. For decades, an ideal way to solve sludge-associated problems is anaerobic digestion, which has been efficiently used for sludge degradation and energy generation in full scale municipal wastewater treatment plants (Fantozzi
⇑ Corresponding author. Address: College of Environmental Science and Engineering, Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing 100083, China. Tel.: +86 15001255497; fax: +86 10 62336900. E-mail addresses: [email protected] (Y. Zhang), [email protected], [email protected] (P. Zhang), [email protected] (G. Zhang), [email protected] (W. Ma), [email protected] (H. Wu), [email protected] (B. Ma). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.07.078
and Buratti, 2009; Gavala et al., 2003; Lin et al., 2009; Rubio-Loza and Noyola, 2010; Sialve et al., 2009). It is known that intracellular organic matters such as proteins, carbohydrates and nucleic acids in bacteria cytoplasm are relatively favorable for microbial digestion. However, resistance of bacterial cell walls generally limits the cell digestibility (Sialve et al., 2009). In order to improve anaerobic digestion performance, the sludge disintegration has been developed as the efficient pretreatment process. Various sludge disintegration technologies based on thermal, chemical, biological, and mechanical processes as well as their combinations can accomplish sludge disintegration and lysis (Pilli et al., 2011; Wang et al., 2010; Wei et al., 2003; Zhang et al., 2009). Among these methods, alkaline treatment is considered one of effective techniques for solubilizing extracellular polymers (EPS) and making bacterial cell walls more susceptible to be attacked (Dog˘an and Sanin, 2009; Li et al., 2008). Alkaline treatment becomes especially effective on sludge disintegration when combined with other technologies, and thermal-alkaline, alkalineultrasonic and microwave-alkaline pretreatments have been
Y. Zhang et al. / Bioresource Technology 123 (2012) 514–519
reported (Dog˘an and Sanin, 2009; Kim et al., 2010; Vlyssides and Karlis, 2004). Recently high pressure homogenization (HPH) for sludge disintegration has shown to be an efficient technology (Onyeche et al., 2003; Rai and Rao, 2009). The sludge disintegration using HPH treatment is generally based on the disruption of microbial cell walls by a combination of large pressure drop, highly focused turbulent eddies and strong shearing forces. Performance of HPH process is mainly influenced by homogenizing pressure, number of homogenizing cycles and characteristics of the treated materials (Donsì et al., 2009; Middelberg et al., 1991; Sauer et al., 1989). HPH treatment as a cell disruption method has several advantages, such as no chemical changes or denaturing during cell lysis, high disruption efficiency and simple operation (Barjenbruch and Kopplow, 2003; Li et al., 2008; Rai and Rao, 2009; Onyeche et al., 2003). Although the HPH treatment was more energy-efficient, compared with ultrasonic and microwave treatment (Zhang et al., 2012), the HPH is still energy-intensive as a mechanical treatment, forming the bottleneck for its application (Kim et al., 2010). The high energy requirement of HPH treatment can be reduced by combined with other treatment methods, especially with chemical methods (Anand et al., 2007). Meanwhile, the HPH performance can be significantly improved through these combinations. Though the combination of chemical and mechanical treatment has been reported to improve the sludge disintegration, few studies have focused on the combination of alkaline pretreatment and HPH treatment. In the present work, the combined treatment of alkaline + HPH was chosen based on solubilizing EPS and weakening bacterial cell walls with alkaline pretreatment, followed more efficiently disrupting sludge cells with HPH treatment. The objective of this study was to investigate the performance of the combined (alkaline + HPH) sewage sludge disintegration. A model for the combined sludge disintegration was established to evaluate the effects of alkaline dosage, homogenization pressure and homogenization cycle number. The energy efficiency with the combined treatment was calculated to optimize the operating parameters with low energy consumption.
2. Methods 2.1. Sewage sludge Sewage sludge was obtained from outlets of the aerobic tank in a municipal wastewater treatment plant in Beijing, China. The total solid content (TS) of sludge was thickened to 1.82% by gravity settling for 24 h as the experimental sludge sample. The sludge sample was stored at 4 °C before use. The characteristics of sludge sample were shown in Table 1.
2.2. Sludge disintegration Alkaline disintegration of 200 ml sludge sample was performed in a 500 ml conical flask, which was put in a horizontal shaking bath (HZS-HA, nuoji Inc., China) at 180 r/min and 28 °C. The NaOH dosage in sludge sample ranged from 0.01 to 0.06 mol/L, and the reaction time varied from 0.5 to 2 h. The sludge HPH treatment was carried out in a high pressure laboratory homogenizer (GJJ-0.03/100, Puzong Inc., China) with a working pressure range of 0–100 MPa. Sludge sample of 600 ml was homogenized at a homogenization pressure from 20 to 80 MPa with one to three homogenization cycles. To investigate the combined sludge disintegration, the NaOH dosage from 0.02 to 0.05 mol/L was added in the sludge sample to solubilize sludge for a certain time, then the sludge pretreated
515
Table 1 Characteristics of sludge sample. Parameter
Value
Total solids (TS) (%) Volatile solids (VS) (mg/L) Total suspended solids (TSS) (mg/L) pH Soluble chemical oxygen demand (SCOD) (mg/L) Total chemical oxygen demand (TCOD) (mg/L)
1.82 12216 17319 6.82 123 18051
by alkaline was disintegrated by the high pressure laboratory homogenizer. Sludge disintegration efficiency was represented by disintegration degree (DDCOD), which was calculated as Eq. (1) (Zhang et al., 2008):
DDCOD ¼
SCOD SCOD0 TCOD SCOD0
ð1Þ
where SCOD0 is the SCOD of the sludge sample before treatment (mg/L). 2.3. Energy efficiency Energy consumption of one HPH cycle (Et) was calculated in terms of the operating pressure P, the volumetric flow rate Q and the time of operation, according to Eq. (2) (Anand et al., 2007):
Et ¼ PQt
ð2Þ
In order to estimate total energy dissipated for multiple cycles, Et is multiplied by the number of homogenization cycles. By replacing the time of operation for one cycle by V/Q, the total energy consumption was obtained. Dividing by the total volume V, the energy consumption per unit volume E (MJ/m3) was given by Eq. (3):
E ¼ PN
ð3Þ
where P is the homogenization pressure (MPa) and N is the number of homogenization cycles. Therefore, the number of cycles and pressure contribute directly to the total energy consumption. Energy efficiency of sludge disintegration (EESCOD, g/MJ) was calculated by the ratio between the increase of sludge supernatant SCOD and the energy consumption per unit sludge volume, as Eq. (4):
EESCOD ¼
SCOD SCOD0 : E
ð4Þ
2.4. Analysis COD, TS, VS and TSS were determined according to APHA standard methods (Eaton et al., 2005). The sludge samples before and after treatment were centrifuged at 8000 r/min for 20 min with a centrifuge (TGL-20B, Anting Inc., China) and then filtered with a 0.45 lm filter membrane, and the filtrate obtained was used to determine SCOD. The raw sludge with the TS of 1.82% was treated by alkaline with a NaOH dosage of 0.5 mol/L for 24 h and then filtered through a quantitative paper, and the filtrate obtained was used to determine TCOD. 3. Results and discussion 3.1. Alkaline treatment Alkaline dosage and reaction time had significant impacts on the sludge disintegration. Sewage sludge disintegration efficiency by alkaline treatment is shown in Fig. 1. Alkaline treatment
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showed a strong positive effect on the sewage sludge disintegration. The DDCOD rapidly increased with the NaOH dosage increase until a NaOH dosage of 0.05 mol/L; there was no significant DDCOD increase when the NaOH dosage increased from 0.05 to 0.06 mol/L. Furthermore, it was observed that the sewage sludge was mainly disintegrated within the initial 0.5 h of alkaline treatment and the DDCOD had a significant increase; while the DDCOD increase became insignificant in the subsequent 1.5 h. The DDCOD reached about 24% after the alkaline treatment at a NaOH dosage of 0.06 mol/L for 2.0 h. Though the increase of NaOH dosage could enhance the sludge disintegration, high sludge pH would negatively influence the subsequent anaerobic digestion. It was reported that extremely high pH by alkaline treatment could lead to the formation of some refractory components in the liquid phase, which were regarded as an extra burden in the subsequent anaerobic digestion due to their low biodegradability (Penaud et al., 1999). In addition, a higher NaOH dosage led to a higher sludge pH, which could cause extra cost for neutralizing the sludge for subsequent sludge digestion (Jin et al., 2009). Table 2 shows the sludge pH before and after sludge alkaline disintegration for 0.5 h. It could be seen that the sludge pH significantly increased with the increase of NaOH dosage. After NaOH treatment for 0.5 h, only a slight reduction in sludge pH was observed, indicating that the concentration of hydroxide ions remained high in the sludge after alkaline disintegration. Considering the changes of sludge disintegration degree and pH, the NaOH dosage could be chosen between 0.02 and 0.05 mol/L and the alkaline treatment duration was chosen as 0.5 h.
3.2. HPH treatment HPH process is regarded as a mechanical method for sludge disintegration and lysis. The performance of HPH treatment is shown in Fig. 2. The DDCOD increased with the increase of homogenization pressure and homogenization cycle number. It was noticed that the highest DDCOD in this study reached 21% at a homogenization pressure of 80 MPa with three homogenization cycles, indicating that single HPH treatment could not effectively disintegrate the sludge sample. Though the sludge disintegration was improved with the increase of homogenization pressure and homogenization cycle number, it is estimated from Eq. (3) that the energy consumption would significantly increase with the increase of homogenization pressure and homogenization cycle number. Therefore,
Fig. 1. DDCOD of sludge alkaline treatment (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
Table 2 Sludge pH variation after alkaline disintegration (Sludge TS, 1.82%). C (mol/L)
0
0.01
0.02
0.03
0.04
0.05
0.06
Initial pH pH after 0.5 h alkaline treatment
6.82 –
9.65 8.47
10.91 9.86
11.56 10.54
11.89 11.22
12.01 11.68
12.23 11.98
Fig. 2. DDCOD of sludge HPH treatment (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
non-mechanical method was suggested as the pretreatment of HPH treatment to improve sludge disintegration and to reduce energy consumption. The alkaline pretreatment combined with HPH treatment could be chosen as a promising sludge disintegration process. 3.3. Combined sludge disintegration of alkaline + HPH 3.3.1. Improvement of combined treatment on sludge disintegration To assess the effect of the combined treatment of alkaline + HPH, the DDCOD of the combined treatment was compared with that of the single alkaline treatment and the single HPH treatment. Six groups of experiments were arranged, including different alkaline dosages, homogenization pressures and homogenization cycles as shown in Table 3. To ensure the sample representativeness, the six groups covered the boundary conditions, e.g. the minimum and maximum levels for the sludge disintegration by combined treatment in this study. The combined sludge disintegration results are shown in Table 3. The combined treatment of alkaline + HPH was more effective to disintegrate sewage sludge than the single alkaline treatment and the single HPH treatment, and a higher DDCOD by combined treatment was achieved. The higher the alkaline dosage, homogenization pressure and homogenization cycle number was, the larger the DDCOD increase was by the combined treatment. The maximum DDCOD of 59.26% was achieved with the combined treatment at a homogenization pressure of 80 MPa with three homogenization cycles combined with the alkaline pretreatment at a NaOH dosage of 0.05 mol/L for 0.5 h. Furthermore, the DDCOD with the combined treatment was higher than the sum of DDCOD with the single alkaline treatment and the single HPH treatment under all experimental conditions. It was seen from Table 3 that the sum of DDCOD with the single alkaline treatment at a 0.05 mol/L NaOH dosage and the single HPH treatment at a 80 MPa homogenization pressure with three homogenization cycles was 40.07%; correspondingly, the DDCOD
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Table 3 Effect of combined sludge treatment on DDCOD. Single treatment Alkaline
HPH
C (mol/ L)
P (MPa)
N
0.02 0.03 0.04 0.04 0.04 0.05
20 40 40 60 60 80
1 1 1 1 2 3
Sum of DDCOD (%)
11.75 22.08 25.93 29.37 32.15 40.07
Combined treatment DDCOD (%)
DDCOD increase (%)
14.11 33.94 40.66 46.15 50.60 59.26
3.36 11.86 14.73 16.78 18.45 19.19
with the combined treatment increased by 19.19%. The results indicated that the combined sludge disintegration effect was not a simple sum of alkaline treatment and HPH treatment, and a synergy effect occurred in the combined treatment, which could adequately utilize the effect of chemical treatment on mechanical treatment (Jin et al., 2009). The alkaline pretreatment solubilized EPS and weakened the structure of bacterial cell walls, enhancing the efficiency of the following HPH treatment. 3.3.2. Optimization of combined treatment The sewage sludge was pretreated with different NaOH dosages for 0.5 h, then was disintegrated by HPH process with a single homogenization cycle. The results in Fig. 3 show that the DDCOD rapidly increased with the increase of NaOH dosage from 0.02 to 0.04 mol/L. When the NaOH dosage increased from 0.04 to 0.05 mol/L, the DDCOD increase became insignificant with only 1– 3% at different homogenization pressures. Table 2 shows that the sludge pH was higher than 11 after alkaline treatment with a NaOH dosage of 0.04 mol/L for 0.5 h, indicating that the NaOH dosage of 0.04 mol/L was enough for the subsequent sludge HPH disintegration. Considering both of sludge disintegration improvement and NaOH dosage saving, the suitable NaOH dosage should not be higher than 0.04 mol/L. With the pretreatment with a given NaOH dosage of 0.04 mol/L for 0.5 h, influences of homogenization pressure and number of homogenization cycles on the combined sludge disintegration are shown in Fig. 4. The higher the homogenization pressure was, the higher the DDCOD was after one to three homogenization cycles. However, the sludge disintegration was only slightly improved,
Fig. 4. Effect of homogenization pressure on DDCOD with different homogenization cycles after 0.04 mol/L NaOH pretreatment (sludge TS, 1.82%; sludge pH, 11.22; sludge temperature, 28 °C).
when the homogenization pressure increased from 60 to 80 MPa. Therefore, the suitable homogenization pressure should not be higher than 60 MPa based on sludge disintegration performance and energy conservation. The DDCOD of 54.16% was achieved using HPH treatment at 60 MPa with three homogenization cycles. Furthermore, Fig. 4 shows that the increase of homogenization cycle number insignificantly enhanced the sludge disintegration, and the DDCOD increased by about 5 and 8% with two and three homogenization cycles respectively, compared with that with a single homogenization cycle. The combined sludge disintegration was more significantly enhanced through the adjustment of NaOH dosage and homogenization pressure rather than that of homogenization cycle number. In addition, the energy consumption would increase in multiple with the increase of homogenization cycle number, as shown in Eq. (3). Therefore, the suitable homogenization cycle number was a single homogenization cycle. 3.3.3. Model analysis of combined sludge disintegration Previous researches reported relevant models for single alkaline treatment and single HPH treatment. For the alkaline sludge treatment, a model was designed by Li et al. (2008) to calculate the DDCOD depending on NaOH dosage (C) and reaction time (t), which is shown as Eq. (5):
1 a ¼ 1 þ kC t b 1 DDCOD
ð5Þ
where C is the NaOH concentration (g/L); a and b are, respectively the indexes of NaOH dose and duration time; k is the reaction rate constant depending on ambient temperature. For HPH treatment, a model was established for Escherichia coli cell disruption as Eq. (6) (Sauer et al., 1989):
ln
Fig. 3. Effect of NaOH dosage on DDCOD at different homogenization pressures with a single homogenization cycle (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
1 ¼ KPd Ng 1R
ð6Þ
where N represents the number of homogenization cycle; P is the homogenization pressure (MPa); K is the rate constant of sludge disintegrate; d and g are, respectively the exponents of homogenization cycle number and homogenization pressure; R is the fraction of cells disrupted, which is calculated by the ratio between the protein content in supernatant after disruption and that after complete disruption. As shown in Fig. 3 and Fig. 4, the NaOH dosage, homogenization pressure and homogenization cycle number had significant impacts on the sludge disintegration for the combined treatment of
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alkaline + HPH. The alkaline treatment duration was not considered, which was chosen as 0.5 h under all combined treatment conditions. In this study, factorial experiment method was employed to investigate the simultaneous effect of the alkaline dosage, homogenization pressure and homogenization cycle number. In order to establish a model of combined treatment of alkaline + HPH, the NaOH dosage, homogenization pressure and homogenization cycle number were chosen as independent variables, and the DDCOD was chosen as dependent variable. Based on the factorial experiment date, a transmutative power function model was designed as Eq. (7) to describe the combined treatment of alkaline + HPH:
1 ¼ KC a Pb Nc 1 DDCOD
ð8Þ
then the linear equation was fitted to calculate the relevant parameters by the multivariable linear regression method. The DDCOD of the combined treatment for the sludge sample was fitted according to Eq. (8) based on test data, which was obtained by factorial design for four groups of NaOH dosage, five groups of homogenization pressure and three groups of homogenization cycle number. Comparing the multiple correlation coefficients, the standard deviation and average value of relative errors, the best model of alkaline pretreatment combined with HPH treatment was obtained as Eq. (9):
1 ¼ 0:713C 0:334 P0:234 N0:119 1 DDCOD
Treatment
C (mol/L)
P (MPa)
N
SCOD (mg/L)
EESCOD (g/MJ)
HPH NaOH + HPH
0 0.02
80 30
2 1
3087 3147
19.29 104.90
ð7Þ
in which, C is the NaOH dosage (mol/L); P represents the homogenization pressure (MPa); N is the homogenization cycle number; a, b and c are, respectively the influence exponent for NaOH dosage, homogenization pressure and homogenization cycle number; K is the rate constant of sludge disintegration. In Eq. (7), DDCOD, C, P and N were known quantities, which were obtained by the factorial experiment. The coefficients a, b, c and K were calculated by the multivariable linear regression method. Firstly, a linearized Eq. (8) was obtained by transforming the Eq. (7) logarithmically:
1 ln ¼ ln K þ a ln C þ b ln P þ c ln N 1 DDCOD
Table 4 Comparison of energy efficiency under different operating mode.
ð9Þ
Eq. (9) shows that the order of the exponent of each independent variable was: C > P > N, indicating that the effect of operating parameters on sludge disintegration was as follows: NaOH dosage > homogenization pressure > homogenization cycle number. Finally, the regression coefficient test was employed to verify the Eq. (9). The multiple correlation coefficients (R2) was 0.958, indicating that the fitted degree was excellent between regression model and data samples in this study. All partial correlation coefficients (v) between every independent variable and dependent variable were more than 90%, indicating that NaOH dosage, homogenization pressure and homogenization cycle number correlated well with the DDCOD. Moreover, all t values between three independent variables and dependent variable passed the t-test with a degree of confidence of 95%, showing that the NaOH dosage, homogenization pressure and homogenization cycle number were all the significant impact indexes on sludge disintegration (Li et al., 2010; Wang et al., 2005). Therefore, this model could excellently predict the DDCOD as a function of the NaOH dosage, homogenization pressure and homogenization cycle number within the experimental range. 3.3.4. Energy efficiency analysis It is well known that HPH treatment was a technology with high energy requirement. The energy consumption of HPH treatment was far higher than that of alkaline treatment for mixing. For the convenience of calculation, the energy consumption of alkaline
Fig. 5. EESCOD change with homogenization pressure and homogenization cycle number after 0.04 mol/L NaOH pretreatment (Sludge TS, 1.82%; sludge pH, 11.22; sludge temperature, 28 °C).
treatment was ignored in this study, thus the energy consumption for the combined sludge disintegration of alkaline + HPH could be calculated by Eq. (3). The NaOH pretreatment in combination with HPH showed a significant increase in energy efficiency compared to the single HPH treatment. Table 4 shows that the energy efficiency approximately increased by 5.44-fold, when the similar release of SCOD was achieved by HPH at 30 MPa with a single homogenization cycle combined with alkaline pretreatment with a NaOH dosage of 0.02 mol/L for 0.5 h reaction instead of that by single HPH treatment at a homogenization pressure of 80 MPa with two homogenization cycles. Clearly, the combined treatment of alkaline + HPH was advantageous for reducing energy consumption. Higher homogenization pressure and multiple homogenization cycle improved the sludge disintegration, while the energy efficiency decreased with the increase of homogenization pressure and homogenization cycle number. Fig. 5 reports the energy efficiency for the combined treatment with a given NaOH dosage of 0.04 mol/L. The highest energy efficiency was obtained at a homogenization pressure of 20 MPa with a single homogenization cycle. Moreover, it could be seen that the energy efficiency reduction became more and more insignificant with the increase of homogenization pressure form 20 to 80 MPa. The energy efficiencies only presented a slight decrease of 10–30 g/MJ when the homogenization pressures increased from 60 to 80 MPa, indicating that further increase of homogenization pressure insignificantly impacted the energy efficiency. Fig. 5 also shows that the energy efficiency of the operation with a single homogenization cycle was approximately 2-fold and 3-fold higher than that with two and three homogenization cycles, respectively, confirming that the energy efficiency of a single homogenization cycle was prior to that of multiple cycle operation. 4. Conclusions The combined sludge disintegration showed a synergy effect rather than the simple sum of the single alkaline treatment and the single HPH treatment. The NaOH dosage not be higher than
Y. Zhang et al. / Bioresource Technology 123 (2012) 514–519
0.04 mol/L, the homogenization pressure not be higher than 60 MPa, and a single homogenization cycle were considered as the appropriate operation mode. The combined sludge disintegra1 tion model of 1DD ¼ 0:713C 0:334 P 0:234 N 0:119 was established. The COD energy efficiency of combined sludge treatment was calculated by energy consumption per SCOD increase, which significantly increased compared with that for the single HPH treatment, and decreased with increasing the homogenization pressure or homogenization cycle number. Acknowledgements This research was funded by the National Natural Science Foundation of China (51178047), China-Israel Joint Research Program of MOST of China, and the Foundation of Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China (No. SWMES 2010-2). References Anand, H., Balasundaram, B., Pandit, A.B., Harrison, S.T.L., 2007. The effect of chemical pretreatment combined with mechanical disruption on the extent of disruption and release of intracellular protein from E. coli. Biochem. Eng. J. 35, 166–173. Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pretreatment of surplus sludge. Adv. Environ. Res. 7, 715–720. Dog˘an, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43, 2139–2148. Donsì, F., Ferrari, G., Lenza, E., Maresca, P., 2009. Main factors regulating microbial inactivation by high-pressure homogenization: operating parameters and scale of operation. Chem. Eng. Sci. 64, 520–532. Eaton, A.D., Clesceri, L.S., Rice, E.W., Greenberg A.E., 2005. Standard methods for the examination of water and wastewater, 21st ed. American Public Health Association, American Water Works Association & Water Environment Federation, Washington, DC, USA. Fantozzi, F., Buratti, C., 2009. Biogas production from different substrates in an experimental continuously stirred tank reactor anaerobic digester. Bioresour. Technol. 100, 5783–5789. Gavala, H.N., Yenal, U., Skiadas, I.V., Westermann, P., Ahring, B.K., 2003. Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature. Water Res. 37, 4561–4572. Jin, Y.Y., Li, H., Mahar, R.B., Wang, Z.Y., Nie, Y.F., 2009. Combined alkaline and ultrasonic pretreatment of sludge before aerobic digestion. J. Environ. Sci. 21, 279–284. Kim, D.H., Jeong, E., Oh, S.E., Shin, H.S., 2010. Combined (alkaline + ultrasonic) pretreatment effect on sewage sludge disintegration. Water Res. 44, 3093– 3100.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Sewage sludge disintegration by combined treatment of alkaline + high pressure homogenization Yuxuan Zhang a, Panyue Zhang a,⇑, Guangming Zhang a,b, Weifang Ma a, Hao Wu a, Boqiang Ma a a b
Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China School of Natural Resource & Environment, Renmin University of China, 100872, China
h i g h l i g h t s " A synergy effect was achieved with combined sludge disintegration of alkaline + HPH. " Maximum sludge disintegration degree with combined treatment was 59.26%.
¼ 0:713C 0:334 P0:234 N 0:119 . " Energy efficiency with combined treatment significantly increased. " Sludge disintegration model for combined treatment was
a r t i c l e
i n f o
Article history: Received 21 April 2012 Received in revised form 13 July 2012 Accepted 22 July 2012 Available online 27 July 2012 Keywords: High pressure homogenization Alkaline treatment Sludge disintegration degree Sludge disintegration model Energy efficiency
1 1DDCOD
a b s t r a c t Alkaline pretreatment combined with high pressure homogenization (HPH) was applied to promote sewage sludge disintegration. For sewage sludge with a total solid content of 1.82%, sludge disintegration degree (DDCOD) with combined treatment was higher than the sum of DDCOD with single alkaline and single HPH treatment. NaOH dosage 60.04 mol/L, homogenization pressure 660 MPa and a single homogenization cycle were the suitable conditions for combined sludge treatment. The combined sludge treatment showed a maximum DDCOD of 59.26%. By regression analysis, the combined sludge disintegra1 ¼ 0:713C 0:334 P 0:234 N0:119 , showing that the effect of operating tion model was established as 1DD COD parameters on sludge disintegration followed the order: NaOH dosage > homogenization pressure > number of homogenization cycle. The energy efficiency with combined sludge treatment significantly increased compared with that with single HPH treatment, and the high energy efficiency was achieved at low homogenization pressure with a single homogenization cycle. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Despite myriad advances in biological wastewater treatment process, high sludge production has become one of the drawbacks impossible to ignore. Sewage sludge treatment accounts for more than 60% of total cost of municipal wastewater treatment plants (Wang et al., 2009). Therefore, new technologies have to be developed for efficient treatment and reuse of excess sludge in municipal wastewater treatment plants. For decades, an ideal way to solve sludge-associated problems is anaerobic digestion, which has been efficiently used for sludge degradation and energy generation in full scale municipal wastewater treatment plants (Fantozzi
⇑ Corresponding author. Address: College of Environmental Science and Engineering, Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing 100083, China. Tel.: +86 15001255497; fax: +86 10 62336900. E-mail addresses: [email protected] (Y. Zhang), [email protected], [email protected] (P. Zhang), [email protected] (G. Zhang), [email protected] (W. Ma), [email protected] (H. Wu), [email protected] (B. Ma). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.07.078
and Buratti, 2009; Gavala et al., 2003; Lin et al., 2009; Rubio-Loza and Noyola, 2010; Sialve et al., 2009). It is known that intracellular organic matters such as proteins, carbohydrates and nucleic acids in bacteria cytoplasm are relatively favorable for microbial digestion. However, resistance of bacterial cell walls generally limits the cell digestibility (Sialve et al., 2009). In order to improve anaerobic digestion performance, the sludge disintegration has been developed as the efficient pretreatment process. Various sludge disintegration technologies based on thermal, chemical, biological, and mechanical processes as well as their combinations can accomplish sludge disintegration and lysis (Pilli et al., 2011; Wang et al., 2010; Wei et al., 2003; Zhang et al., 2009). Among these methods, alkaline treatment is considered one of effective techniques for solubilizing extracellular polymers (EPS) and making bacterial cell walls more susceptible to be attacked (Dog˘an and Sanin, 2009; Li et al., 2008). Alkaline treatment becomes especially effective on sludge disintegration when combined with other technologies, and thermal-alkaline, alkalineultrasonic and microwave-alkaline pretreatments have been
Y. Zhang et al. / Bioresource Technology 123 (2012) 514–519
reported (Dog˘an and Sanin, 2009; Kim et al., 2010; Vlyssides and Karlis, 2004). Recently high pressure homogenization (HPH) for sludge disintegration has shown to be an efficient technology (Onyeche et al., 2003; Rai and Rao, 2009). The sludge disintegration using HPH treatment is generally based on the disruption of microbial cell walls by a combination of large pressure drop, highly focused turbulent eddies and strong shearing forces. Performance of HPH process is mainly influenced by homogenizing pressure, number of homogenizing cycles and characteristics of the treated materials (Donsì et al., 2009; Middelberg et al., 1991; Sauer et al., 1989). HPH treatment as a cell disruption method has several advantages, such as no chemical changes or denaturing during cell lysis, high disruption efficiency and simple operation (Barjenbruch and Kopplow, 2003; Li et al., 2008; Rai and Rao, 2009; Onyeche et al., 2003). Although the HPH treatment was more energy-efficient, compared with ultrasonic and microwave treatment (Zhang et al., 2012), the HPH is still energy-intensive as a mechanical treatment, forming the bottleneck for its application (Kim et al., 2010). The high energy requirement of HPH treatment can be reduced by combined with other treatment methods, especially with chemical methods (Anand et al., 2007). Meanwhile, the HPH performance can be significantly improved through these combinations. Though the combination of chemical and mechanical treatment has been reported to improve the sludge disintegration, few studies have focused on the combination of alkaline pretreatment and HPH treatment. In the present work, the combined treatment of alkaline + HPH was chosen based on solubilizing EPS and weakening bacterial cell walls with alkaline pretreatment, followed more efficiently disrupting sludge cells with HPH treatment. The objective of this study was to investigate the performance of the combined (alkaline + HPH) sewage sludge disintegration. A model for the combined sludge disintegration was established to evaluate the effects of alkaline dosage, homogenization pressure and homogenization cycle number. The energy efficiency with the combined treatment was calculated to optimize the operating parameters with low energy consumption.
2. Methods 2.1. Sewage sludge Sewage sludge was obtained from outlets of the aerobic tank in a municipal wastewater treatment plant in Beijing, China. The total solid content (TS) of sludge was thickened to 1.82% by gravity settling for 24 h as the experimental sludge sample. The sludge sample was stored at 4 °C before use. The characteristics of sludge sample were shown in Table 1.
2.2. Sludge disintegration Alkaline disintegration of 200 ml sludge sample was performed in a 500 ml conical flask, which was put in a horizontal shaking bath (HZS-HA, nuoji Inc., China) at 180 r/min and 28 °C. The NaOH dosage in sludge sample ranged from 0.01 to 0.06 mol/L, and the reaction time varied from 0.5 to 2 h. The sludge HPH treatment was carried out in a high pressure laboratory homogenizer (GJJ-0.03/100, Puzong Inc., China) with a working pressure range of 0–100 MPa. Sludge sample of 600 ml was homogenized at a homogenization pressure from 20 to 80 MPa with one to three homogenization cycles. To investigate the combined sludge disintegration, the NaOH dosage from 0.02 to 0.05 mol/L was added in the sludge sample to solubilize sludge for a certain time, then the sludge pretreated
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Table 1 Characteristics of sludge sample. Parameter
Value
Total solids (TS) (%) Volatile solids (VS) (mg/L) Total suspended solids (TSS) (mg/L) pH Soluble chemical oxygen demand (SCOD) (mg/L) Total chemical oxygen demand (TCOD) (mg/L)
1.82 12216 17319 6.82 123 18051
by alkaline was disintegrated by the high pressure laboratory homogenizer. Sludge disintegration efficiency was represented by disintegration degree (DDCOD), which was calculated as Eq. (1) (Zhang et al., 2008):
DDCOD ¼
SCOD SCOD0 TCOD SCOD0
ð1Þ
where SCOD0 is the SCOD of the sludge sample before treatment (mg/L). 2.3. Energy efficiency Energy consumption of one HPH cycle (Et) was calculated in terms of the operating pressure P, the volumetric flow rate Q and the time of operation, according to Eq. (2) (Anand et al., 2007):
Et ¼ PQt
ð2Þ
In order to estimate total energy dissipated for multiple cycles, Et is multiplied by the number of homogenization cycles. By replacing the time of operation for one cycle by V/Q, the total energy consumption was obtained. Dividing by the total volume V, the energy consumption per unit volume E (MJ/m3) was given by Eq. (3):
E ¼ PN
ð3Þ
where P is the homogenization pressure (MPa) and N is the number of homogenization cycles. Therefore, the number of cycles and pressure contribute directly to the total energy consumption. Energy efficiency of sludge disintegration (EESCOD, g/MJ) was calculated by the ratio between the increase of sludge supernatant SCOD and the energy consumption per unit sludge volume, as Eq. (4):
EESCOD ¼
SCOD SCOD0 : E
ð4Þ
2.4. Analysis COD, TS, VS and TSS were determined according to APHA standard methods (Eaton et al., 2005). The sludge samples before and after treatment were centrifuged at 8000 r/min for 20 min with a centrifuge (TGL-20B, Anting Inc., China) and then filtered with a 0.45 lm filter membrane, and the filtrate obtained was used to determine SCOD. The raw sludge with the TS of 1.82% was treated by alkaline with a NaOH dosage of 0.5 mol/L for 24 h and then filtered through a quantitative paper, and the filtrate obtained was used to determine TCOD. 3. Results and discussion 3.1. Alkaline treatment Alkaline dosage and reaction time had significant impacts on the sludge disintegration. Sewage sludge disintegration efficiency by alkaline treatment is shown in Fig. 1. Alkaline treatment
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showed a strong positive effect on the sewage sludge disintegration. The DDCOD rapidly increased with the NaOH dosage increase until a NaOH dosage of 0.05 mol/L; there was no significant DDCOD increase when the NaOH dosage increased from 0.05 to 0.06 mol/L. Furthermore, it was observed that the sewage sludge was mainly disintegrated within the initial 0.5 h of alkaline treatment and the DDCOD had a significant increase; while the DDCOD increase became insignificant in the subsequent 1.5 h. The DDCOD reached about 24% after the alkaline treatment at a NaOH dosage of 0.06 mol/L for 2.0 h. Though the increase of NaOH dosage could enhance the sludge disintegration, high sludge pH would negatively influence the subsequent anaerobic digestion. It was reported that extremely high pH by alkaline treatment could lead to the formation of some refractory components in the liquid phase, which were regarded as an extra burden in the subsequent anaerobic digestion due to their low biodegradability (Penaud et al., 1999). In addition, a higher NaOH dosage led to a higher sludge pH, which could cause extra cost for neutralizing the sludge for subsequent sludge digestion (Jin et al., 2009). Table 2 shows the sludge pH before and after sludge alkaline disintegration for 0.5 h. It could be seen that the sludge pH significantly increased with the increase of NaOH dosage. After NaOH treatment for 0.5 h, only a slight reduction in sludge pH was observed, indicating that the concentration of hydroxide ions remained high in the sludge after alkaline disintegration. Considering the changes of sludge disintegration degree and pH, the NaOH dosage could be chosen between 0.02 and 0.05 mol/L and the alkaline treatment duration was chosen as 0.5 h.
3.2. HPH treatment HPH process is regarded as a mechanical method for sludge disintegration and lysis. The performance of HPH treatment is shown in Fig. 2. The DDCOD increased with the increase of homogenization pressure and homogenization cycle number. It was noticed that the highest DDCOD in this study reached 21% at a homogenization pressure of 80 MPa with three homogenization cycles, indicating that single HPH treatment could not effectively disintegrate the sludge sample. Though the sludge disintegration was improved with the increase of homogenization pressure and homogenization cycle number, it is estimated from Eq. (3) that the energy consumption would significantly increase with the increase of homogenization pressure and homogenization cycle number. Therefore,
Fig. 1. DDCOD of sludge alkaline treatment (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
Table 2 Sludge pH variation after alkaline disintegration (Sludge TS, 1.82%). C (mol/L)
0
0.01
0.02
0.03
0.04
0.05
0.06
Initial pH pH after 0.5 h alkaline treatment
6.82 –
9.65 8.47
10.91 9.86
11.56 10.54
11.89 11.22
12.01 11.68
12.23 11.98
Fig. 2. DDCOD of sludge HPH treatment (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
non-mechanical method was suggested as the pretreatment of HPH treatment to improve sludge disintegration and to reduce energy consumption. The alkaline pretreatment combined with HPH treatment could be chosen as a promising sludge disintegration process. 3.3. Combined sludge disintegration of alkaline + HPH 3.3.1. Improvement of combined treatment on sludge disintegration To assess the effect of the combined treatment of alkaline + HPH, the DDCOD of the combined treatment was compared with that of the single alkaline treatment and the single HPH treatment. Six groups of experiments were arranged, including different alkaline dosages, homogenization pressures and homogenization cycles as shown in Table 3. To ensure the sample representativeness, the six groups covered the boundary conditions, e.g. the minimum and maximum levels for the sludge disintegration by combined treatment in this study. The combined sludge disintegration results are shown in Table 3. The combined treatment of alkaline + HPH was more effective to disintegrate sewage sludge than the single alkaline treatment and the single HPH treatment, and a higher DDCOD by combined treatment was achieved. The higher the alkaline dosage, homogenization pressure and homogenization cycle number was, the larger the DDCOD increase was by the combined treatment. The maximum DDCOD of 59.26% was achieved with the combined treatment at a homogenization pressure of 80 MPa with three homogenization cycles combined with the alkaline pretreatment at a NaOH dosage of 0.05 mol/L for 0.5 h. Furthermore, the DDCOD with the combined treatment was higher than the sum of DDCOD with the single alkaline treatment and the single HPH treatment under all experimental conditions. It was seen from Table 3 that the sum of DDCOD with the single alkaline treatment at a 0.05 mol/L NaOH dosage and the single HPH treatment at a 80 MPa homogenization pressure with three homogenization cycles was 40.07%; correspondingly, the DDCOD
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Table 3 Effect of combined sludge treatment on DDCOD. Single treatment Alkaline
HPH
C (mol/ L)
P (MPa)
N
0.02 0.03 0.04 0.04 0.04 0.05
20 40 40 60 60 80
1 1 1 1 2 3
Sum of DDCOD (%)
11.75 22.08 25.93 29.37 32.15 40.07
Combined treatment DDCOD (%)
DDCOD increase (%)
14.11 33.94 40.66 46.15 50.60 59.26
3.36 11.86 14.73 16.78 18.45 19.19
with the combined treatment increased by 19.19%. The results indicated that the combined sludge disintegration effect was not a simple sum of alkaline treatment and HPH treatment, and a synergy effect occurred in the combined treatment, which could adequately utilize the effect of chemical treatment on mechanical treatment (Jin et al., 2009). The alkaline pretreatment solubilized EPS and weakened the structure of bacterial cell walls, enhancing the efficiency of the following HPH treatment. 3.3.2. Optimization of combined treatment The sewage sludge was pretreated with different NaOH dosages for 0.5 h, then was disintegrated by HPH process with a single homogenization cycle. The results in Fig. 3 show that the DDCOD rapidly increased with the increase of NaOH dosage from 0.02 to 0.04 mol/L. When the NaOH dosage increased from 0.04 to 0.05 mol/L, the DDCOD increase became insignificant with only 1– 3% at different homogenization pressures. Table 2 shows that the sludge pH was higher than 11 after alkaline treatment with a NaOH dosage of 0.04 mol/L for 0.5 h, indicating that the NaOH dosage of 0.04 mol/L was enough for the subsequent sludge HPH disintegration. Considering both of sludge disintegration improvement and NaOH dosage saving, the suitable NaOH dosage should not be higher than 0.04 mol/L. With the pretreatment with a given NaOH dosage of 0.04 mol/L for 0.5 h, influences of homogenization pressure and number of homogenization cycles on the combined sludge disintegration are shown in Fig. 4. The higher the homogenization pressure was, the higher the DDCOD was after one to three homogenization cycles. However, the sludge disintegration was only slightly improved,
Fig. 4. Effect of homogenization pressure on DDCOD with different homogenization cycles after 0.04 mol/L NaOH pretreatment (sludge TS, 1.82%; sludge pH, 11.22; sludge temperature, 28 °C).
when the homogenization pressure increased from 60 to 80 MPa. Therefore, the suitable homogenization pressure should not be higher than 60 MPa based on sludge disintegration performance and energy conservation. The DDCOD of 54.16% was achieved using HPH treatment at 60 MPa with three homogenization cycles. Furthermore, Fig. 4 shows that the increase of homogenization cycle number insignificantly enhanced the sludge disintegration, and the DDCOD increased by about 5 and 8% with two and three homogenization cycles respectively, compared with that with a single homogenization cycle. The combined sludge disintegration was more significantly enhanced through the adjustment of NaOH dosage and homogenization pressure rather than that of homogenization cycle number. In addition, the energy consumption would increase in multiple with the increase of homogenization cycle number, as shown in Eq. (3). Therefore, the suitable homogenization cycle number was a single homogenization cycle. 3.3.3. Model analysis of combined sludge disintegration Previous researches reported relevant models for single alkaline treatment and single HPH treatment. For the alkaline sludge treatment, a model was designed by Li et al. (2008) to calculate the DDCOD depending on NaOH dosage (C) and reaction time (t), which is shown as Eq. (5):
1 a ¼ 1 þ kC t b 1 DDCOD
ð5Þ
where C is the NaOH concentration (g/L); a and b are, respectively the indexes of NaOH dose and duration time; k is the reaction rate constant depending on ambient temperature. For HPH treatment, a model was established for Escherichia coli cell disruption as Eq. (6) (Sauer et al., 1989):
ln
Fig. 3. Effect of NaOH dosage on DDCOD at different homogenization pressures with a single homogenization cycle (sludge TS, 1.82%; sludge pH, 6.28; sludge temperature, 28 °C).
1 ¼ KPd Ng 1R
ð6Þ
where N represents the number of homogenization cycle; P is the homogenization pressure (MPa); K is the rate constant of sludge disintegrate; d and g are, respectively the exponents of homogenization cycle number and homogenization pressure; R is the fraction of cells disrupted, which is calculated by the ratio between the protein content in supernatant after disruption and that after complete disruption. As shown in Fig. 3 and Fig. 4, the NaOH dosage, homogenization pressure and homogenization cycle number had significant impacts on the sludge disintegration for the combined treatment of
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alkaline + HPH. The alkaline treatment duration was not considered, which was chosen as 0.5 h under all combined treatment conditions. In this study, factorial experiment method was employed to investigate the simultaneous effect of the alkaline dosage, homogenization pressure and homogenization cycle number. In order to establish a model of combined treatment of alkaline + HPH, the NaOH dosage, homogenization pressure and homogenization cycle number were chosen as independent variables, and the DDCOD was chosen as dependent variable. Based on the factorial experiment date, a transmutative power function model was designed as Eq. (7) to describe the combined treatment of alkaline + HPH:
1 ¼ KC a Pb Nc 1 DDCOD
ð8Þ
then the linear equation was fitted to calculate the relevant parameters by the multivariable linear regression method. The DDCOD of the combined treatment for the sludge sample was fitted according to Eq. (8) based on test data, which was obtained by factorial design for four groups of NaOH dosage, five groups of homogenization pressure and three groups of homogenization cycle number. Comparing the multiple correlation coefficients, the standard deviation and average value of relative errors, the best model of alkaline pretreatment combined with HPH treatment was obtained as Eq. (9):
1 ¼ 0:713C 0:334 P0:234 N0:119 1 DDCOD
Treatment
C (mol/L)
P (MPa)
N
SCOD (mg/L)
EESCOD (g/MJ)
HPH NaOH + HPH
0 0.02
80 30
2 1
3087 3147
19.29 104.90
ð7Þ
in which, C is the NaOH dosage (mol/L); P represents the homogenization pressure (MPa); N is the homogenization cycle number; a, b and c are, respectively the influence exponent for NaOH dosage, homogenization pressure and homogenization cycle number; K is the rate constant of sludge disintegration. In Eq. (7), DDCOD, C, P and N were known quantities, which were obtained by the factorial experiment. The coefficients a, b, c and K were calculated by the multivariable linear regression method. Firstly, a linearized Eq. (8) was obtained by transforming the Eq. (7) logarithmically:
1 ln ¼ ln K þ a ln C þ b ln P þ c ln N 1 DDCOD
Table 4 Comparison of energy efficiency under different operating mode.
ð9Þ
Eq. (9) shows that the order of the exponent of each independent variable was: C > P > N, indicating that the effect of operating parameters on sludge disintegration was as follows: NaOH dosage > homogenization pressure > homogenization cycle number. Finally, the regression coefficient test was employed to verify the Eq. (9). The multiple correlation coefficients (R2) was 0.958, indicating that the fitted degree was excellent between regression model and data samples in this study. All partial correlation coefficients (v) between every independent variable and dependent variable were more than 90%, indicating that NaOH dosage, homogenization pressure and homogenization cycle number correlated well with the DDCOD. Moreover, all t values between three independent variables and dependent variable passed the t-test with a degree of confidence of 95%, showing that the NaOH dosage, homogenization pressure and homogenization cycle number were all the significant impact indexes on sludge disintegration (Li et al., 2010; Wang et al., 2005). Therefore, this model could excellently predict the DDCOD as a function of the NaOH dosage, homogenization pressure and homogenization cycle number within the experimental range. 3.3.4. Energy efficiency analysis It is well known that HPH treatment was a technology with high energy requirement. The energy consumption of HPH treatment was far higher than that of alkaline treatment for mixing. For the convenience of calculation, the energy consumption of alkaline
Fig. 5. EESCOD change with homogenization pressure and homogenization cycle number after 0.04 mol/L NaOH pretreatment (Sludge TS, 1.82%; sludge pH, 11.22; sludge temperature, 28 °C).
treatment was ignored in this study, thus the energy consumption for the combined sludge disintegration of alkaline + HPH could be calculated by Eq. (3). The NaOH pretreatment in combination with HPH showed a significant increase in energy efficiency compared to the single HPH treatment. Table 4 shows that the energy efficiency approximately increased by 5.44-fold, when the similar release of SCOD was achieved by HPH at 30 MPa with a single homogenization cycle combined with alkaline pretreatment with a NaOH dosage of 0.02 mol/L for 0.5 h reaction instead of that by single HPH treatment at a homogenization pressure of 80 MPa with two homogenization cycles. Clearly, the combined treatment of alkaline + HPH was advantageous for reducing energy consumption. Higher homogenization pressure and multiple homogenization cycle improved the sludge disintegration, while the energy efficiency decreased with the increase of homogenization pressure and homogenization cycle number. Fig. 5 reports the energy efficiency for the combined treatment with a given NaOH dosage of 0.04 mol/L. The highest energy efficiency was obtained at a homogenization pressure of 20 MPa with a single homogenization cycle. Moreover, it could be seen that the energy efficiency reduction became more and more insignificant with the increase of homogenization pressure form 20 to 80 MPa. The energy efficiencies only presented a slight decrease of 10–30 g/MJ when the homogenization pressures increased from 60 to 80 MPa, indicating that further increase of homogenization pressure insignificantly impacted the energy efficiency. Fig. 5 also shows that the energy efficiency of the operation with a single homogenization cycle was approximately 2-fold and 3-fold higher than that with two and three homogenization cycles, respectively, confirming that the energy efficiency of a single homogenization cycle was prior to that of multiple cycle operation. 4. Conclusions The combined sludge disintegration showed a synergy effect rather than the simple sum of the single alkaline treatment and the single HPH treatment. The NaOH dosage not be higher than
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0.04 mol/L, the homogenization pressure not be higher than 60 MPa, and a single homogenization cycle were considered as the appropriate operation mode. The combined sludge disintegra1 tion model of 1DD ¼ 0:713C 0:334 P 0:234 N 0:119 was established. The COD energy efficiency of combined sludge treatment was calculated by energy consumption per SCOD increase, which significantly increased compared with that for the single HPH treatment, and decreased with increasing the homogenization pressure or homogenization cycle number. Acknowledgements This research was funded by the National Natural Science Foundation of China (51178047), China-Israel Joint Research Program of MOST of China, and the Foundation of Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China (No. SWMES 2010-2). References Anand, H., Balasundaram, B., Pandit, A.B., Harrison, S.T.L., 2007. The effect of chemical pretreatment combined with mechanical disruption on the extent of disruption and release of intracellular protein from E. coli. Biochem. Eng. J. 35, 166–173. Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pretreatment of surplus sludge. Adv. Environ. Res. 7, 715–720. Dog˘an, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43, 2139–2148. Donsì, F., Ferrari, G., Lenza, E., Maresca, P., 2009. Main factors regulating microbial inactivation by high-pressure homogenization: operating parameters and scale of operation. Chem. Eng. Sci. 64, 520–532. Eaton, A.D., Clesceri, L.S., Rice, E.W., Greenberg A.E., 2005. Standard methods for the examination of water and wastewater, 21st ed. American Public Health Association, American Water Works Association & Water Environment Federation, Washington, DC, USA. Fantozzi, F., Buratti, C., 2009. Biogas production from different substrates in an experimental continuously stirred tank reactor anaerobic digester. Bioresour. Technol. 100, 5783–5789. Gavala, H.N., Yenal, U., Skiadas, I.V., Westermann, P., Ahring, B.K., 2003. Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature. Water Res. 37, 4561–4572. Jin, Y.Y., Li, H., Mahar, R.B., Wang, Z.Y., Nie, Y.F., 2009. Combined alkaline and ultrasonic pretreatment of sludge before aerobic digestion. J. Environ. Sci. 21, 279–284. Kim, D.H., Jeong, E., Oh, S.E., Shin, H.S., 2010. Combined (alkaline + ultrasonic) pretreatment effect on sewage sludge disintegration. Water Res. 44, 3093– 3100.
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