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Energy efficiency of pre-treating excess sewage sludge with microwave irradiation
Bioresource Technology 101 (2010) 5092–5097
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Energy efficiency of pre-treating excess sewage sludge with microwave irradiation Bing Tang *, Linfeng Yu, Shaosong Huang, Jianzhong Luo, Ying Zhuo Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 10 October 2009 Received in revised form 27 January 2010 Accepted 28 January 2010 Available online 18 February 2010 Keywords: Energy efficiency Excess sewage sludge Pre-treatment Microwave irradiation
a b s t r a c t The objective of this study is to investigate the energy consumption of pre-treating excess sewage sludge with microwave irradiation using several parameters, including temperature rise, degree of cell destruction, SCOD/TCOD ratio (solids solubilization), and biogas production to evaluate the energy efficiency. It was found that water content was the most important factor that influenced the energy efficiency of raising the temperature and promoting the solubilization of solid materials. Increasing specific energy (Es) accelerated the biogas production, but there was a limit to this process. For quantitative comparison to the energy efficiency of different pre-treatment steps, an empirical method was also proposed based on the experimental data. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Excess sewage sludge (ESS) containing corruptible organic substances and pathogenic microorganisms is an inevitable product of the biological conversion process in wastewater treatment plants (WWTPs). In the past decade, many WWTPs were constructed to satisfy the increasing demand for wastewater purification, in China, more than 600 WWTPs have been established countrywide till 2007 (Cai et al., 2007), and an enormous amount of ESS was produced, which led to serious environmental issues especially in some metropolitan cities such as Beijing, Shanghai, and Guangzhou. In fact, dealing with the environmental problems associated with ESS has become one of the biggest challenges in operating a WWTP and has already given rise to extensive concerns worldwide, Canada produces about 670,000 tons/year of dry sewage sludge (Eskicioglu et al., 2008), and in China, the amount of ESS is increasing by an average of 5% every year (Wang et al., 2008). Conventional treatment and disposal of ESS involves several steps, such as anaerobic digestion, chemical conditioning, and mechanical dewatering to 70–80% (wt.) water content, followed by disposal as landfill, application to cropland or incineration when faced with land shortage. The whole complex process, including pre-treatment, transportation and disposal, represents major capital and operational costs to every WWTP and is related directly to the volume of ESS, which emphasizes the importance of maximal volume reduction. ESS contains mainly water and various organic and inorganic solids. Because of the bioprocess at WWTPs, volatile components * Corresponding author. Tel.: +86 20 39322295; fax: +86 20 38457257. E-mail address: [email protected] (B. Tang). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.01.132
are often a large fraction of the solids in ESS, suggesting the possibility to accomplish solids biodegradation through anaerobic digestion. In a conventional volume reduction process, anaerobic digestion is often considered a requisite step to mineralize organic solids and improve dehydrate capability, which is so far considered to be the most preferable option for sludge treatment (Murray et al., 2008) and is widely used in most WWTPs around the world. However, the slow degradation of ESS by anaerobic digestion is a major problem, with a retention time of about 20 days in conventional digesters, which has significant space requirements in a WWTP, it is very necessary to enhance the efficiency of anaerobic digestion. So far, it has been determined that anaerobic digestion was a first order reaction (Eskicioglu et al., 2006), which included four major steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis, hydrolysis was the rate-limiting step and prolonged the overall process. It was also proved that cell membrane and extracellular polymeric substances (EPS) presented physical and chemical barriers to direct anaerobic degradation, respectively (Higgins and Novak, 1997; Novak et al., 2003; Ewa and Marcin, 2006; Sheng et al., 2008). The solid components in ESS are very complex, and include biomass produced by the biological conversion of organics, oil and grease, nutrients (nitrogen and phosphorus), heavy metals, synthetic organic compounds and pathogens. According to recent reports, different groups of microorganism, organic and inorganic matter are agglomerated into a polymeric network formed by EPS or divalent cations. Because of this special structure and the hydrophilic characteristic of ESS, it increases the difficulty to achieve effective volume reduction with anaerobic digestion and mechanical dehydration. A pre-treatment step to disrupt the network and increase the biodegradability of ESS is an important-priority for consideration.
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After World War II, microwave (MV) irradiation found many applications in industrial fields; in the field of environmental engineering, waste processing was a subject that attracted a lot of interests (Jones et al., 2002). The use of MV-assisted processes in ESS handling or treatment has been investigated during the last 15 years. In the electromagnetic spectrum, MV irradiation occurs at wavelengths of 1 mm–1 m at corresponding frequencies of 300–300 MHz. A shorter wavelength (2450 MHz, 12.24 cm) is often adopted when MV irradiation is used to treat materials such as ESS. One of the main effects of pre-treating ESS with MV irradiation comes from the thermal effect that is generated after the rotation of dipole molecules under an oscillating electromagnetic field, and provides rapid, energy-efficient heating of water to the boiling point where the cells of microorganisms are ruptured and the bound water is released. Another effect is the athermal effect, which was argued by several researchers (Hong et al., 2006; Dreyfuss and Chipley, 1980; Khalil and Villota, 1985; Kozempel et al., 1998). Banik et al. (2003) summarized the athermal effect of MV irradiation on microorganisms. Eskicioglu et al. (2007) determined that there was no discernible athermal effect on the COD solubilization of ESS, but it had a positive impact on the mesophilic anaerobic biodegradability of ESS. It was believed that the athermal effect of MV irradiation was caused by the polarized parts of macromolecules aligning with the poles of the electromagnetic field, resulting in possible breakage of hydrogen bonds (Loupy, 2002), which led to the death of microorganisms at lower temperatures (Dreyfuss and Chipley, 1980). For the purpose of enhancing the effect of volume reduction of ESS, the synergistic pre-treatment with MV irradiation and other strong oxidizers has been investigated (Eskicioglu et al., 2008; Yin et al., 2008, 2007). The experimental results indicated that the increased MV irradiation temperatures (>80 °C) further increased the decomposition of H2O2 into OH radicals and enhanced both the oxidation of COD and the solubilization of particulate COD (>0.45 lm) of ESS, but the soluble organics so generated were slower to biodegrade than those generated by MV irradiation alone. Dog˘an and Sanin (2009) tested the effectiveness of MV irradiation in combination with alkaline pretreatment and found synergistic effects in improving COD solubilization and biogas production. Many researchers are striving for a feasible approach to realize the volume reduction of ESS with MV irradiation. So far, studies of the treatment of ESS with MV irradiation have focused mainly on the following: (1) pre-treatment, including enhancement of anaerobic digestion (Hong et al., 2006; Climent et al., 2007) and dewaterability (Wojciechowska, 2005), cell destruction (Hong et al., 2004) and solubilization of organic solids (Eskicioglu et al., 2006); (2) pyrolysis of sewage sludge for the production of valuable gases and oils (Menéndez et al., 2004); (3) vitrification of solid residue for the purpose of decreasing the leaching of heavy metals (Menéndez et al., 2005). Energy efficiency is a crucial factor in determining the economical feasibility of a sewage sludge pre-treatment process. Wojciechowska (2005) indicated that the parameters characterizing the dewaterability of sludge were related to the exposure time to MV irradiation, and increasing the contact time beyond the optimal value not only consumed more energy but also worsened the pre-treating effects, which implied the pre-treatment effects had a close relationship with energy consumption. How to improve the energy efficiency is actually an essential concern in pre-treating ESS with MV irradiation, however, most reports only focused on the pre-treatment effects without considering the energy efficiency of the corresponding step. There was no information available on the efficiency of energy consumption in an ESS pretreatment process with MV irradiation, and still no practical criterion with which to evaluate the efficiency of energy consumption in different pre-treatment steps. We have investigated four funda-
mental steps in the pre-treatment of ESS with MV irradiation, including temperature rise, cell destruction, solubilization of organic solids and enhancing anaerobic digestion. Here, we focus mainly on the following two aspects: (1) to determine the main factor that influences the energy efficiency of MV irradiation during the treatment of ESS; (2) to propose a simple and convenient quantification criterion for evaluating the efficiency of energy consumption. We believe the results presented here provide useful information for understanding the energy efficiency that is needed for the pre-treatment of ESS as well as a method to compare the economical feasibility of different pre-treatment strategies.
2. Methods 2.1. ESS samples ESS samples were obtained from the secondary sedimentation and the concentrating tank of the Lijiao municipal wastewater treatment plant located in the Haizhu District (Guangzhou, China). Sludge samples were collected at least 3 times a week from March 2007 to May 2008 and stored in a refrigerator at 4 °C for a maximum of 2 days during the experiments. Table 1 presents the basic characteristics of the raw ESS samples. The raw sludge contains highly volatile solids (VS), and the SCOD is increased 1.5–2.6-fold after being concentrated by gravity. 2.2. MV irradiation A domestic microwave oven (power, 0–1250 W; frequency, 2450 MHz; maximum temperature, 260 °C; G8023 CTL-K3, Galanze,) was used to provide MV irradiation for sludge pre-treatment. A thoroughly mixed sample of 200–800 mL volume in a polyfluortetraethylene container was placed in the microwave and exposed to MV irradiation for 30–300 s at power settings of 400–800 W. 2.3. Methods Measurement of temperature difference: the temperature of the sample was measured immediately before and after it was exposed to the MV irradiation. The temperature difference was calculated by subtracting the temperature of the sample before irradiation from that after irradiation. Cell damage observation: cell damage by MV irradiation was observed using a biological microscope with a digital camera (B203LEDR, Chongqing Optec Com. Ltd.). Total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD): COD was measured in the total ESS and in the supernatant using standard methods (APHA et al., 2005). The supernatant and solid substances were separated by centrifugation in a table-top centrifuge (TDL-50B, Shanghai Anke). The increment of SCOD/TCOD is the degree of ESS solubilization after exposure to MV irradiation.
Table 1 Characteristics of untreated ESS samples.
pH Water (wt.%) TS (g/L) VS (g/L) VS/TS TCOD (mg/L) SCOD (mg/L) SCOD/TCOD
Secondary sludge
Concentrated sludge
6.41–6.46 99.33–99.69 3.11–6.71 1.85–3.89 0.58–0.70 2021–2507 103–122 4.11 102–5.59 102
6.23–6.43 96.60–97.86 21.41–34.00 13.41–16.52 0.53–0.64 21,119–39,114 187–323 8.26 103–1.29 102
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Inoculation: inoculums (400 mL) from the anaerobic digester of Liede municipal wastewater treatment plant were placed into 5 digesters with 1000 mL volume, respectively, and then untreated sludge samples (400 mL) were added. Nitrogen sparging was applied to each digester after inoculums and sludge samples were mixed completely, then digesters were sealed with paraffin. Anaerobic digestion and biogas production: anaerobic digestion of untreated and MV irradiation pre-treated samples was done in batch mode under thermophilic conditions using five identical darkened digesters. At the beginning of each experiment, 60 mL of digested sludge was extracted each day at the same time, and replaced by 60 mL of untreated sludge. The gas production of each digester gradually became stable, and then anaerobic digestion began with the same inflow of treated ESS as substrate. Untreated ESS was added to one of the digesters to serve as a reference. Other digesters received ESS treated with MV irradiation for different lengths of time between 30 s and 300 s. Es was calculated according to the exposure time and the volume of the ESS samples. Biogas production was measured every 24 h by connecting a surge flask attached to a metering cylinder, the volume of daily biogas production was determined by measuring the volume of discharged liquors.
where SL is the solids solubilization index, (SCOD/TCOD)0 and (SCOD/TCOD)t is the SCOD/TCOD ratio of the untreated and treated ESS at time t, respectively.
2.4. Energy consumption evaluation
3.1. Energy consumption at elevated temperature
To describe the energy consumption in each step of pre-treatment of ESS, it is essential to identify a quantized indicator for comparison. In this study, the concept of specific energy Es (J/mL) was used to evaluate the energy consumption of each step on the basis of MV irradiation dose exposure and the volume of ESS, which is defined as:
In the treated samples, which include secondary and thickened sludge, the water content varies in the range 96.60–99.85% (wt.), which shows that water is the main component of ESS. Water has a dipolar molecular structure that can absorb energy from MV irradiation and gradually become warmer; thus, water content is related directly to the temperature rise of ESS during MV irradiation. Three samples of ESS with different original water content were irradiated to investigate this effect and the results are shown in Fig. 1. At ambient pressure, the highest temperature achieved is the boiling point of the ESS sample, which does not depend on the original water content. Fig. 1 shows that the temperature increases with increasing specific energy, but the incremental extent is quite different for different original water contents. At low water content, the temperature rise increases relatively faster than that at high water content, and the increment of temperature of the treated ESS is greater for samples with low water content. That is because water has a high thermal capacity and can absorb more energy with a relatively small increase in temperature. The results in Fig. 1 indicate that more energy is consumed in increasing the temperature of water in a sample with higher water content.
Es ¼
PMV t V ESS
ð1Þ
where PMV, is the MV irradiation power (W), t is the exposure time (s) of ESS to MV irradiation; VESS is the volume (mL) of the treated ESS sample. Energy consumption in each pre-treatment step is calculated with the given power and the exposure time. In some studies, Es is defined on the basis of the mass of solids contained in ESS. This can be converted conveniently according to Eq. (1) by considering the solid content in ESS. 2.5. Effect of temperature increase The energy input by MV irradiation is the origin of the increased temperature of the ESS. A dimensionless effective factor to describe the effect of the temperature increase is proposed as:
Te ¼
Tt T0 T 0 þ 273:15
2.7. Criterion for evaluation of the efficiency of energy consumption A criterion is proposed here to compare the efficiency of energy consumption in the different steps of pre-treatment with the following equation:
ge ¼
3. Results and discussion
ð2Þ
0.25 0.20 0.15 Te
2.6. Solids solubilization evaluation The SCOD/TCOD ratio is used as a general indication of the extent of solubilization of sludge in most reports, but it is influenced significantly by the characteristics of the raw sludge. To evaluate the degree of solids solubilization in ESS, a dimensionless parameter (SL, the solubilization index) for eliminating the effect of raw sludge from different origins, is used to evaluate the degree of solids solubilization of pre-treated ESS as follows:
ðSCOD=TCODÞt ðSCOD=TCODÞ0 ðSCOD=TCODÞ0
ð4Þ
where ge (mL J1) stands for the efficiency of energy consumption, Ef is the effective factor used to describe the response to MV irradidEf is the differential quotient of ation (Te and SL, respectively), and dE s the effective factor to specific energy, which can be used to evaluate the efficiency of energy consumption.
where Te is the temperature rise index, T0 and Tt are the temperature (°C) at the beginning of the treatment and at time t, respectively.
SL ¼
dEf dEs
ð3Þ
Water content 96.60% 97.86% 99.59%
0.10 0.05 0.00 0
20
40
60
80 100 Es (J/mL)
120
Fig. 1. Effect of MV irradiation to temperature rise.
140
160
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3.2. Energy consumption in cell destruction
3.3. Energy consumption in solids solubilization SCOD refers to the soluble chemical oxygen demand in ESS, whereas TCOD is the total chemical oxygen demand. In most reports of the pre-treatment of ESS, the SCOD/TCOD ratio was used to evaluate the degree of solubilization of sludge solids. It was believed that the joint effects (thermal and athermal effect) under MV irradiation would disrupt the complex ESS flock structure and release extra- and intracellular biopolymers, such as proteins and sugars, into the soluble phase. To investigate the effectiveness of ESS solubilization under MV irradiation, ESS samples containing different water contents were tested with different Es inputs. The solubilization index (SL) expressed by Eq. (3) was used to evaluate
4.0 3.5 3.0 2.5 SL
Cell membranes consist of lipids, proteins and some carbohydrates. The lipids form a bilayer in which the proteins are embedded to various degrees. This structure envelops a cell, enclosing the cytoplasm and forming a selectively permeable barrier. Recent investigations (Novak et al., 2003; Ewa and Marcin, 2006; Sheng et al., 2008) have shown that the polymeric network formed by EPS or cations accompanying the cell membrane provides protection for microorganisms in ESS, is resistant to direct anaerobic degradation and is a major factor in energy consumption during MV irradiation. Damage to cells by MV irradiation may occur in two ways: one is the thermal effect caused by heating the intracellular liquor to boiling point; the other is the athermal effect caused by polarized chains of macromolecules wriggling rapidly under an oscillating microwave field, leading to possible breakage of the cell. To investigate the effect of MV irradiation on the cell membrane, samples before and after exposure to MV irradiation were observed using a biological microscope. For explicit illustration of the damage caused by MV irradiation, the presence of Arcella hemisphaerica, which is often used to evaluate the effluent quality of a WWTP, was selected as an indicator of the presence of microorganisms. The experimental results demonstrate the fact that the degree of cell membrane damage increases with increasing energy consumption. In an untreated sample, the original structure of a cell of A. hemisphaeric is covered by EPS, and other ESS structures remain compact, which demonstrates the morphology of ESS in its original state. Under low doses of MV irradiation (Es = 30 J/mL), the covering of a cell has disappeared accompanied by the loosened flock structure around the cell; the cell membrane is intact but the intracellular substances are destroyed. Given a relatively higher energy input, for example Es = 75 J/mL, the cell membrane is split and some intracellular substances have been released. When the energy input continues to be increased, the cell membrane suffers more serious damage (Es = 120 J/mL). After the cell membrane is broken, all of the intracellular contents are released, and the cell membrane will continue to be broken into increasingly smaller pieces under the athermal effect of MV irradiation. The results shown in our experiments actually reveal the degree of energy consumption in destroying the cell membrane. It is clear that cell membrane can be broken efficaciously only after the input specific energy is higher than a certain value (about 75 J/mL in our experiments). Similar result was also observed for ultrasonic treatment, in which a minimum specific energy of 1000 kJ/kg TSS was needed to destroy the cell membrane (Bourgrier et al., 2005). The degree of cell damage has a direct relationship to the energy input, but, because there are huge amounts of microorganisms within ESS, it is difficult to determine a suitable level of energy consumption solely from the degree of cell damage, Woo et al. (2000) also observed the phenomenon of differential damage in different cells by the same dosage of MV irradiation.
4.5
2.0 1.5
Water content 99.59% 97.86% 96.60%
1.0 0.5 0.0 0
15
30
45
60
75
90 105 120 135 150 165
Es (J/mL) Fig. 2. Energy consumption in solids solubilization.
the effect of MV irradiation on the solubilization of ESS, and the results are shown in Fig. 2. Fig. 2 shows that the increment of SL increases rapidly with increasing Es, which has been reported elsewhere. As expected, MV irradiation is the main factor determining the solubilization of solids, but it should be noted that the water content of ESS has a marked impact. Fig. 2 shows that a high water content is unfavourable for the solubilization of solids at a given energy input, whereas at a lower water content, less energy input would lead to a larger SL. Combined with the results in Figs. 1 and 2, it can be seen that there is a close relationship of water content to the energy efficiency in treating ESS with MV irradiation. More energy would be consumed in raising the temperature of the water at higher water contents, which decreases the efficiency of energy in the solubilization of solids.
3.4. Energy consumption in enhancing anaerobic digestion In the literature, cumulative biogas production (CBP) is a commonly used parameter to describe the anaerobic digestibility of ESS after pre-treatment. As a mass reduction method, anaerobic digestion has long been used as an economical and convenient way to treat both municipal and industrial sludge, but the process is time-consuming and often needs a large area for the retention of sludge. The difficulty in solubilizing ESS during anaerobic digestion lies mainly in the special structure of the sludge flock and the protection of the cell membrane, so the logical approach to accelerate the process of anaerobic digestion should be to disrupt the flock and destroy the protection of the cell membrane, as shown in Section 3.2. Anaerobic digestion after the pre-treatment is expected to progress more rapidly. In Fig. 3, it reveal the durable effect on biodegradation of ESS pre-treated by MV irradiation. After exposure to MV irradiation, different solid substances in ESS suffer different degrees of damage. The flock structure and the cell membrane are destroyed and solubilized. This process contributes to promotion of the biodegradability of ESS. Fig. 3 shows the accelerating effect of MV irradiation on anaerobic digestion. It is clear that higher Es improves the anaerobic digestion, while differences in the water content have relatively less impact on biogas production. Fig. 3 reveals that the accelerating effect of MV irradiation on anaerobic digestion is gradually reduced after Es attains a certain level. In terms of energy efficiency, there appears to be a limitation in promoting biogas production by MV irradiation. Such a phenomenon is relevant to the degree of destruction of the flock structure and cell
B. Tang et al. / Bioresource Technology 101 (2010) 5092–5097
3.5. Evaluation of energy efficiency in pre-treatment ESS with MV irradiation Several steps are generally involved simultaneously in the treatment of ESS with MV irradiation, which include temperature increase, damage of flock structure, and destruction of cells and solubilization of solids. These effects improve the biodegradability of ESS and accelerate biogas production. A quantification criterion is proposed for evaluating the efficiency of energy consumption in
0.007 0.006
Water content 99.59% 97.86% 96.60%
0.005 0.004 0.003 0.002 0.001 0.000 0
40
80
120
160
200
Es (J/mL)
3200 Es=0 (untreated sample) Es=20 (J/mL) Es=60 (J/mL) Es=120(J/mL)
2800 2400
b
0.06 0.05
2000
Water content 99.59% 97.86% 96.60%
0.04
ηe (mLJ )
1600
-1
Cumulative biogas production (mL)
a
a
-1
membranes in ESS. The factor responsible for limitation of the solubilization of solids in ESS is the breakage of cell membranes, damage to the flock structure of ESS and release of the extra- and intracellular substances. The energy efficiency decreases gradually when approaching the limitation point.
ηe (mLJ )
5096
1200 800 400
0.03 0.02 0.01
0 0
2
4
6
8
10
12
14
16
18
20
Cumulative biogas production (mL)
b
40
80 120 160 200 240 280 320 360 Es (J/mL)
Es=0 (untreated sample) Es=20 (J/mL) Es=60 (J/mL) Es=120 (J/mL)
3200 2800 2400
Fig. 4. Curves of energy efficiency: (a) curves based on temperature rise index (Te); (b) curves based on solids solubilization index (SL).
2000 1600 1200 800 400 0 0
2
4
6
8
10
12
14
16
18
20
Digestion time (d) Fig. 3. Energy consumption in enhancing anaerobic digestion: (a) water content = 99.22%; (b) water content = 96.76%.
Table 2 Evaluation of the efficiency of energy consumption under different water content. Effective factor (Ef)
Relational expression between Ef and Es
R2
Original water content (wt.%)
Te
T e ¼ 0:264ð1 expð0:033Es ÞÞ1:90
0.9871
99.59
T e ¼ 0:258ð1 expð0:040Es ÞÞ1:70
0.9953
97.86
T e ¼ 0:256ð1 expð0:051Es ÞÞ1:69
0.9950
96.60
SL ¼ 3:43ð1 expð0:024Es ÞÞ4:18
0.9898
99.59
SL ¼ 4:08ð1 expð0:020Es ÞÞ2:19
0.9863
97.86
SL ¼ 4:76ð1 expð0:021Es ÞÞ1:82
0.9942
96.60
SL
0.00 0
Digestion time (d)
the pre-treatment of ESS. The relation of the effective factors Te and SL between the input Es values can be obtained by fitting the data from Figs. 1 and 2, respectively, which are shown in Table 2. According to the fitting expressions shown in Table 2, the relevant expressions of derived functions can be obtained, and the curves of derived functions are shown in Fig. 4, which illustrates the energy efficiency based on the temperature rise index and the solids solubilization index. It is clear from Fig. 4a and b that there is maximum energy efficiency that can be attained while treating ESS with MV irradiation. The water content of ESS is an important factor that influences the energy efficiency, which is not dependent on the temperature rise index or the solids solubilization index. In ESS with high water content, the energy efficiency is relatively low, and the maximum energy efficiency will be achieved only with more energy consumption. Such results are shown in Table 3, which demonstrates the direct relationship between maximum energy efficiency and water content in ESS. Temperature increase and solubilization of solids are the direct effects induced by MV irradiation, which will accelerate the subsequent biogas production process. From the results given in Fig. 4 and Table 3, it is clear that the rate of change of effective factors (Te or SL) by MV irradiation is quite different for different water contents. No matter what the effective factor, there is a maximum energy efficiency that can be attained, which implies this is the limit value. Such data are in accord with the experimental results in Section 3.4. Park et al. (2010) obtained a maximum degree of solubilization (17.9%) under optimal conditions, including temper-
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B. Tang et al. / Bioresource Technology 101 (2010) 5092–5097 Table 3 Calculation of maximum energy efficiency under different water content. Expression of ge
Te
dT e dEs dT e dEs dT e dEs
¼ 0:0166ð1 expð0:033Es ÞÞ0:90 expð0:033Es Þ
99.59
0.00446
¼ 0:0175ð1 expð0:040Es ÞÞ0:70 expð0:040Es Þ
97.86
0.00553
¼ 0:0221ð1 expð0:051Es ÞÞ0:69 expð0:051Es Þ
96.60
0.00705
dSL dEs dSL dEs dSL dEs
¼ 0:344ð1 expð0:024Es ÞÞ3:18 expð0:024Es Þ
99.59
0.0345
¼ 0:179ð1 expð0:020Es ÞÞ1:19 expð0:020Es Þ
97.86
0.0395
¼ 0:182ð1 expð0:021Es ÞÞ0:82 expð0:021Es Þ
96.60
0.0520
SL
a
ge;Max a (mL J1)
Effective factor (Ef)
Original water content (wt.%)
ge;Max is the maximum energy efficiency calculating from Fig. 4 or the expression of ge in this table.
ature (102 °C), output power (400 W) and concentration of solids (2.3%), which also supported the results presented here. 4. Conclusions The most important factor that influenced the efficiency of energy consumption for pre-treating ESS with MV irradiation was the water content. So, reducing the water content was necessary for improving the efficiency of energy use. In terms of the energy efficiency, there was a limit in treating ESS with MV irradiation, which was also related to the water content of ESS. While treating ESS with MV irradiation, it was suggested that every operational parameter should be based on the optimal energy efficiency, which could be calculated by the criterion proposed here. References APHA, AWWA, WPCF, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association, Washington, DC. Banik, S., Bandyopadhyay, S., Ganguly, S., 2003. Bioeffects of microwave – a brief review. Bioresour. Technol. 87 (2), 155–159. Bourgrier, C., Carrère, H., Delgenès, J.P., 2005. Solubilisation of waste-activated sludge by ultrasonic treatment. Chem. Eng. J. 106 (2), 163–169. Cai, Q.Y., Mo, C.H., Wu, Q.T., Zeng, Q.Y., Katsoyiannis, A., 2007. Occurrence of organic contaminants in sewage sludges from eleven wastewater treatment plants, China. Chemosphere 68 (9), 1751–1762. Climent, M., Ferrer, I., Baeza, M.M., Artola, A., Vázquez, F., Font, X., 2007. Effects of thermal and mechanical pretreatments of secondary sludge on biogas production under thermophilic conditions. Chem. Eng. J. 133 (1-3), 335–342. Dreyfuss, M.S., Chipley, J.R., 1980. Comparison of effects of sublethal microwave radiation and conventional heating on the metabolic activity of Staphylococcus aureus. Appl. Environ. Microb. 39, 13–16. Dog˘an, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43 (8), 2139–2148. Eskicioglu, C., Kennedy, K.J., Droste, R.L., 2006. Characterization of soluble organic matter of waste activated sludge before and after thermal pretreatment. Water Res. 40 (20), 3725–3736. Eskicioglu, C., Prorot, A., Marin, J., Droste, R.L., Kennedy, K.J., 2008. Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion. Water Res. 42 (18), 4674–4682. Eskicioglu, C., Terzian, N., Kennedy, K.J., Droste, R.L., Hamoda, M., 2007. Athermal microwave effects for enhancing digestibility of waste activated sludge. Water Res. 41 (11), 2457–2466.
Ewa, L.B., Marcin, B., 2006. Effect of selected anionic surfactants on activated sludge flocs. Enzyme Microb. Tech. 39 (4), 660–668. Higgins, M.J., Novak, J.T., 1997. Characterization of extracellular protein and its role in bioflocculation. ASCE J. Environ. Eng., 479–485. Hong, S.M., Park, J.K., Lee, Y.O., 2004. Mechanisms of microwave irradiation involved in the destruction of fecal coliforms from biosolids. Water Res. 38 (6), 1615– 1625. Hong, S.M., Park, J.K., Teeradej, N., Lee, Y.O., Cho, Y.K., Park, C.H., 2006. Pretreatment of sludge with microwaves for pathogen destruction and improved anaerobic digestion performance. Water Environ. Res. 78 (1), 76–83. Jones, D.A., Lelyveld, T.P., Mavrofidis, S.D., Kingman, S.W., Miles, N.J., 2002. Microwave heating applications in environmental engineering – a review. Resour. Conserv. Recy. 34 (2), 75–90. Khalil, H., Villota, R., 1985. A comparative study on the thermal inactivation of Bacillus stearothermophilus spores in microwave and conventional heating. In: Fourth International Congress on Engineered Food. Applied Science Publishers, Essex, England. Kozempel, M.F., Annous, B.A., Cook, R.D., Scullen, O.J., Whitting, R.C., 1998. Inactivation of microorganisms with microwaves at reduced temperatures. J. Food Protect. 61, 582–585. Loupy, A., 2002. Microwaves in Organic Synthesis. Wiley-VCH, France. Menéndez, J.A., Domínguez, A., Inguanzo, M., Pis, J.J., 2004. Microwave pyrolysis of sewage sludge: analysis of the gas fraction. J. Anal. Appl. Pyrol. 71 (2), 657–667. Menéndez, J.A., Domínguez, A., Inguanzo, M., Pis, J.J., 2005. Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: vitrification of the solid residue. J. Anal. Appl. Pyrol. 74 (1-2), 406–412. Murray, A., Horvath, A., Nelson, K.L., 2008. Hybrid life-cycle environmental and cost inventory of sewage sludge treatment and end-use scenarios: a case study from China. Environ. Sci. Technol. 42 (9), 3163–3169. Novak, J.T., Sadler, M.E., Murty, S.N., 2003. Mechanisms of loc destruction during anaerobic digestion and the effect on conditioning and dewatering of biosolids. Water Res. 37 (13), 3136–3144. Park, W.J., Ahn, J.H., Hwang, S., Lee, C.K., 2010. Effect of output power, target temperature, and solid concentration on the solubilization of waste activated sludge using microwave irradiation. Bioresour. Technol. 101 (1), S13–S16. Sheng, G.P., Yu, H.Q., Li, X.Y., 2008. Stability of sludge flocs under shear conditions. Biochem. Eng. J. 38 (3), 302–308. Wang, X.R., Jin, Y.Y., Wang, Z.Y., Mahar, R.B., Nie, Y.F., 2008. A research on sintering characteristics and mechanisms of dried sewage sludge. J. Hazard. Mater. 160 (2-3), 489–494. Wojciechowska, E., 2005. Application of microwaves for sewage sludge conditioning. Water Res. 39 (19), 4749–4754. Woo, I.S., Rhee, I.K., Park, H.D., 2000. Differential damage in bacterial cells by microwave radiation on the basis of cell wall structure. Appl. Environ. Microbiol. 60 (5), 2243–2247. Yin, G.Q., Liao, P.H., Lo, K.V., 2008. Sewage sludge treatment using microwaveenhanced advanced oxidation process. J. Environ. Sci. Heal. A 43 (2), 191–201. Yin, G.Q., Liao, P.H., Lo, K.V., 2007. An ozone/hydrogen peroxide/microwaveenhanced advanced oxidation process for sewage sludge treatment. J. Environ. Sci. Heal. A 42 (8), 1177–1181.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Energy efficiency of pre-treating excess sewage sludge with microwave irradiation Bing Tang *, Linfeng Yu, Shaosong Huang, Jianzhong Luo, Ying Zhuo Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 10 October 2009 Received in revised form 27 January 2010 Accepted 28 January 2010 Available online 18 February 2010 Keywords: Energy efficiency Excess sewage sludge Pre-treatment Microwave irradiation
a b s t r a c t The objective of this study is to investigate the energy consumption of pre-treating excess sewage sludge with microwave irradiation using several parameters, including temperature rise, degree of cell destruction, SCOD/TCOD ratio (solids solubilization), and biogas production to evaluate the energy efficiency. It was found that water content was the most important factor that influenced the energy efficiency of raising the temperature and promoting the solubilization of solid materials. Increasing specific energy (Es) accelerated the biogas production, but there was a limit to this process. For quantitative comparison to the energy efficiency of different pre-treatment steps, an empirical method was also proposed based on the experimental data. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Excess sewage sludge (ESS) containing corruptible organic substances and pathogenic microorganisms is an inevitable product of the biological conversion process in wastewater treatment plants (WWTPs). In the past decade, many WWTPs were constructed to satisfy the increasing demand for wastewater purification, in China, more than 600 WWTPs have been established countrywide till 2007 (Cai et al., 2007), and an enormous amount of ESS was produced, which led to serious environmental issues especially in some metropolitan cities such as Beijing, Shanghai, and Guangzhou. In fact, dealing with the environmental problems associated with ESS has become one of the biggest challenges in operating a WWTP and has already given rise to extensive concerns worldwide, Canada produces about 670,000 tons/year of dry sewage sludge (Eskicioglu et al., 2008), and in China, the amount of ESS is increasing by an average of 5% every year (Wang et al., 2008). Conventional treatment and disposal of ESS involves several steps, such as anaerobic digestion, chemical conditioning, and mechanical dewatering to 70–80% (wt.) water content, followed by disposal as landfill, application to cropland or incineration when faced with land shortage. The whole complex process, including pre-treatment, transportation and disposal, represents major capital and operational costs to every WWTP and is related directly to the volume of ESS, which emphasizes the importance of maximal volume reduction. ESS contains mainly water and various organic and inorganic solids. Because of the bioprocess at WWTPs, volatile components * Corresponding author. Tel.: +86 20 39322295; fax: +86 20 38457257. E-mail address: [email protected] (B. Tang). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.01.132
are often a large fraction of the solids in ESS, suggesting the possibility to accomplish solids biodegradation through anaerobic digestion. In a conventional volume reduction process, anaerobic digestion is often considered a requisite step to mineralize organic solids and improve dehydrate capability, which is so far considered to be the most preferable option for sludge treatment (Murray et al., 2008) and is widely used in most WWTPs around the world. However, the slow degradation of ESS by anaerobic digestion is a major problem, with a retention time of about 20 days in conventional digesters, which has significant space requirements in a WWTP, it is very necessary to enhance the efficiency of anaerobic digestion. So far, it has been determined that anaerobic digestion was a first order reaction (Eskicioglu et al., 2006), which included four major steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis, hydrolysis was the rate-limiting step and prolonged the overall process. It was also proved that cell membrane and extracellular polymeric substances (EPS) presented physical and chemical barriers to direct anaerobic degradation, respectively (Higgins and Novak, 1997; Novak et al., 2003; Ewa and Marcin, 2006; Sheng et al., 2008). The solid components in ESS are very complex, and include biomass produced by the biological conversion of organics, oil and grease, nutrients (nitrogen and phosphorus), heavy metals, synthetic organic compounds and pathogens. According to recent reports, different groups of microorganism, organic and inorganic matter are agglomerated into a polymeric network formed by EPS or divalent cations. Because of this special structure and the hydrophilic characteristic of ESS, it increases the difficulty to achieve effective volume reduction with anaerobic digestion and mechanical dehydration. A pre-treatment step to disrupt the network and increase the biodegradability of ESS is an important-priority for consideration.
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After World War II, microwave (MV) irradiation found many applications in industrial fields; in the field of environmental engineering, waste processing was a subject that attracted a lot of interests (Jones et al., 2002). The use of MV-assisted processes in ESS handling or treatment has been investigated during the last 15 years. In the electromagnetic spectrum, MV irradiation occurs at wavelengths of 1 mm–1 m at corresponding frequencies of 300–300 MHz. A shorter wavelength (2450 MHz, 12.24 cm) is often adopted when MV irradiation is used to treat materials such as ESS. One of the main effects of pre-treating ESS with MV irradiation comes from the thermal effect that is generated after the rotation of dipole molecules under an oscillating electromagnetic field, and provides rapid, energy-efficient heating of water to the boiling point where the cells of microorganisms are ruptured and the bound water is released. Another effect is the athermal effect, which was argued by several researchers (Hong et al., 2006; Dreyfuss and Chipley, 1980; Khalil and Villota, 1985; Kozempel et al., 1998). Banik et al. (2003) summarized the athermal effect of MV irradiation on microorganisms. Eskicioglu et al. (2007) determined that there was no discernible athermal effect on the COD solubilization of ESS, but it had a positive impact on the mesophilic anaerobic biodegradability of ESS. It was believed that the athermal effect of MV irradiation was caused by the polarized parts of macromolecules aligning with the poles of the electromagnetic field, resulting in possible breakage of hydrogen bonds (Loupy, 2002), which led to the death of microorganisms at lower temperatures (Dreyfuss and Chipley, 1980). For the purpose of enhancing the effect of volume reduction of ESS, the synergistic pre-treatment with MV irradiation and other strong oxidizers has been investigated (Eskicioglu et al., 2008; Yin et al., 2008, 2007). The experimental results indicated that the increased MV irradiation temperatures (>80 °C) further increased the decomposition of H2O2 into OH radicals and enhanced both the oxidation of COD and the solubilization of particulate COD (>0.45 lm) of ESS, but the soluble organics so generated were slower to biodegrade than those generated by MV irradiation alone. Dog˘an and Sanin (2009) tested the effectiveness of MV irradiation in combination with alkaline pretreatment and found synergistic effects in improving COD solubilization and biogas production. Many researchers are striving for a feasible approach to realize the volume reduction of ESS with MV irradiation. So far, studies of the treatment of ESS with MV irradiation have focused mainly on the following: (1) pre-treatment, including enhancement of anaerobic digestion (Hong et al., 2006; Climent et al., 2007) and dewaterability (Wojciechowska, 2005), cell destruction (Hong et al., 2004) and solubilization of organic solids (Eskicioglu et al., 2006); (2) pyrolysis of sewage sludge for the production of valuable gases and oils (Menéndez et al., 2004); (3) vitrification of solid residue for the purpose of decreasing the leaching of heavy metals (Menéndez et al., 2005). Energy efficiency is a crucial factor in determining the economical feasibility of a sewage sludge pre-treatment process. Wojciechowska (2005) indicated that the parameters characterizing the dewaterability of sludge were related to the exposure time to MV irradiation, and increasing the contact time beyond the optimal value not only consumed more energy but also worsened the pre-treating effects, which implied the pre-treatment effects had a close relationship with energy consumption. How to improve the energy efficiency is actually an essential concern in pre-treating ESS with MV irradiation, however, most reports only focused on the pre-treatment effects without considering the energy efficiency of the corresponding step. There was no information available on the efficiency of energy consumption in an ESS pretreatment process with MV irradiation, and still no practical criterion with which to evaluate the efficiency of energy consumption in different pre-treatment steps. We have investigated four funda-
mental steps in the pre-treatment of ESS with MV irradiation, including temperature rise, cell destruction, solubilization of organic solids and enhancing anaerobic digestion. Here, we focus mainly on the following two aspects: (1) to determine the main factor that influences the energy efficiency of MV irradiation during the treatment of ESS; (2) to propose a simple and convenient quantification criterion for evaluating the efficiency of energy consumption. We believe the results presented here provide useful information for understanding the energy efficiency that is needed for the pre-treatment of ESS as well as a method to compare the economical feasibility of different pre-treatment strategies.
2. Methods 2.1. ESS samples ESS samples were obtained from the secondary sedimentation and the concentrating tank of the Lijiao municipal wastewater treatment plant located in the Haizhu District (Guangzhou, China). Sludge samples were collected at least 3 times a week from March 2007 to May 2008 and stored in a refrigerator at 4 °C for a maximum of 2 days during the experiments. Table 1 presents the basic characteristics of the raw ESS samples. The raw sludge contains highly volatile solids (VS), and the SCOD is increased 1.5–2.6-fold after being concentrated by gravity. 2.2. MV irradiation A domestic microwave oven (power, 0–1250 W; frequency, 2450 MHz; maximum temperature, 260 °C; G8023 CTL-K3, Galanze,) was used to provide MV irradiation for sludge pre-treatment. A thoroughly mixed sample of 200–800 mL volume in a polyfluortetraethylene container was placed in the microwave and exposed to MV irradiation for 30–300 s at power settings of 400–800 W. 2.3. Methods Measurement of temperature difference: the temperature of the sample was measured immediately before and after it was exposed to the MV irradiation. The temperature difference was calculated by subtracting the temperature of the sample before irradiation from that after irradiation. Cell damage observation: cell damage by MV irradiation was observed using a biological microscope with a digital camera (B203LEDR, Chongqing Optec Com. Ltd.). Total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD): COD was measured in the total ESS and in the supernatant using standard methods (APHA et al., 2005). The supernatant and solid substances were separated by centrifugation in a table-top centrifuge (TDL-50B, Shanghai Anke). The increment of SCOD/TCOD is the degree of ESS solubilization after exposure to MV irradiation.
Table 1 Characteristics of untreated ESS samples.
pH Water (wt.%) TS (g/L) VS (g/L) VS/TS TCOD (mg/L) SCOD (mg/L) SCOD/TCOD
Secondary sludge
Concentrated sludge
6.41–6.46 99.33–99.69 3.11–6.71 1.85–3.89 0.58–0.70 2021–2507 103–122 4.11 102–5.59 102
6.23–6.43 96.60–97.86 21.41–34.00 13.41–16.52 0.53–0.64 21,119–39,114 187–323 8.26 103–1.29 102
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Inoculation: inoculums (400 mL) from the anaerobic digester of Liede municipal wastewater treatment plant were placed into 5 digesters with 1000 mL volume, respectively, and then untreated sludge samples (400 mL) were added. Nitrogen sparging was applied to each digester after inoculums and sludge samples were mixed completely, then digesters were sealed with paraffin. Anaerobic digestion and biogas production: anaerobic digestion of untreated and MV irradiation pre-treated samples was done in batch mode under thermophilic conditions using five identical darkened digesters. At the beginning of each experiment, 60 mL of digested sludge was extracted each day at the same time, and replaced by 60 mL of untreated sludge. The gas production of each digester gradually became stable, and then anaerobic digestion began with the same inflow of treated ESS as substrate. Untreated ESS was added to one of the digesters to serve as a reference. Other digesters received ESS treated with MV irradiation for different lengths of time between 30 s and 300 s. Es was calculated according to the exposure time and the volume of the ESS samples. Biogas production was measured every 24 h by connecting a surge flask attached to a metering cylinder, the volume of daily biogas production was determined by measuring the volume of discharged liquors.
where SL is the solids solubilization index, (SCOD/TCOD)0 and (SCOD/TCOD)t is the SCOD/TCOD ratio of the untreated and treated ESS at time t, respectively.
2.4. Energy consumption evaluation
3.1. Energy consumption at elevated temperature
To describe the energy consumption in each step of pre-treatment of ESS, it is essential to identify a quantized indicator for comparison. In this study, the concept of specific energy Es (J/mL) was used to evaluate the energy consumption of each step on the basis of MV irradiation dose exposure and the volume of ESS, which is defined as:
In the treated samples, which include secondary and thickened sludge, the water content varies in the range 96.60–99.85% (wt.), which shows that water is the main component of ESS. Water has a dipolar molecular structure that can absorb energy from MV irradiation and gradually become warmer; thus, water content is related directly to the temperature rise of ESS during MV irradiation. Three samples of ESS with different original water content were irradiated to investigate this effect and the results are shown in Fig. 1. At ambient pressure, the highest temperature achieved is the boiling point of the ESS sample, which does not depend on the original water content. Fig. 1 shows that the temperature increases with increasing specific energy, but the incremental extent is quite different for different original water contents. At low water content, the temperature rise increases relatively faster than that at high water content, and the increment of temperature of the treated ESS is greater for samples with low water content. That is because water has a high thermal capacity and can absorb more energy with a relatively small increase in temperature. The results in Fig. 1 indicate that more energy is consumed in increasing the temperature of water in a sample with higher water content.
Es ¼
PMV t V ESS
ð1Þ
where PMV, is the MV irradiation power (W), t is the exposure time (s) of ESS to MV irradiation; VESS is the volume (mL) of the treated ESS sample. Energy consumption in each pre-treatment step is calculated with the given power and the exposure time. In some studies, Es is defined on the basis of the mass of solids contained in ESS. This can be converted conveniently according to Eq. (1) by considering the solid content in ESS. 2.5. Effect of temperature increase The energy input by MV irradiation is the origin of the increased temperature of the ESS. A dimensionless effective factor to describe the effect of the temperature increase is proposed as:
Te ¼
Tt T0 T 0 þ 273:15
2.7. Criterion for evaluation of the efficiency of energy consumption A criterion is proposed here to compare the efficiency of energy consumption in the different steps of pre-treatment with the following equation:
ge ¼
3. Results and discussion
ð2Þ
0.25 0.20 0.15 Te
2.6. Solids solubilization evaluation The SCOD/TCOD ratio is used as a general indication of the extent of solubilization of sludge in most reports, but it is influenced significantly by the characteristics of the raw sludge. To evaluate the degree of solids solubilization in ESS, a dimensionless parameter (SL, the solubilization index) for eliminating the effect of raw sludge from different origins, is used to evaluate the degree of solids solubilization of pre-treated ESS as follows:
ðSCOD=TCODÞt ðSCOD=TCODÞ0 ðSCOD=TCODÞ0
ð4Þ
where ge (mL J1) stands for the efficiency of energy consumption, Ef is the effective factor used to describe the response to MV irradidEf is the differential quotient of ation (Te and SL, respectively), and dE s the effective factor to specific energy, which can be used to evaluate the efficiency of energy consumption.
where Te is the temperature rise index, T0 and Tt are the temperature (°C) at the beginning of the treatment and at time t, respectively.
SL ¼
dEf dEs
ð3Þ
Water content 96.60% 97.86% 99.59%
0.10 0.05 0.00 0
20
40
60
80 100 Es (J/mL)
120
Fig. 1. Effect of MV irradiation to temperature rise.
140
160
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3.2. Energy consumption in cell destruction
3.3. Energy consumption in solids solubilization SCOD refers to the soluble chemical oxygen demand in ESS, whereas TCOD is the total chemical oxygen demand. In most reports of the pre-treatment of ESS, the SCOD/TCOD ratio was used to evaluate the degree of solubilization of sludge solids. It was believed that the joint effects (thermal and athermal effect) under MV irradiation would disrupt the complex ESS flock structure and release extra- and intracellular biopolymers, such as proteins and sugars, into the soluble phase. To investigate the effectiveness of ESS solubilization under MV irradiation, ESS samples containing different water contents were tested with different Es inputs. The solubilization index (SL) expressed by Eq. (3) was used to evaluate
4.0 3.5 3.0 2.5 SL
Cell membranes consist of lipids, proteins and some carbohydrates. The lipids form a bilayer in which the proteins are embedded to various degrees. This structure envelops a cell, enclosing the cytoplasm and forming a selectively permeable barrier. Recent investigations (Novak et al., 2003; Ewa and Marcin, 2006; Sheng et al., 2008) have shown that the polymeric network formed by EPS or cations accompanying the cell membrane provides protection for microorganisms in ESS, is resistant to direct anaerobic degradation and is a major factor in energy consumption during MV irradiation. Damage to cells by MV irradiation may occur in two ways: one is the thermal effect caused by heating the intracellular liquor to boiling point; the other is the athermal effect caused by polarized chains of macromolecules wriggling rapidly under an oscillating microwave field, leading to possible breakage of the cell. To investigate the effect of MV irradiation on the cell membrane, samples before and after exposure to MV irradiation were observed using a biological microscope. For explicit illustration of the damage caused by MV irradiation, the presence of Arcella hemisphaerica, which is often used to evaluate the effluent quality of a WWTP, was selected as an indicator of the presence of microorganisms. The experimental results demonstrate the fact that the degree of cell membrane damage increases with increasing energy consumption. In an untreated sample, the original structure of a cell of A. hemisphaeric is covered by EPS, and other ESS structures remain compact, which demonstrates the morphology of ESS in its original state. Under low doses of MV irradiation (Es = 30 J/mL), the covering of a cell has disappeared accompanied by the loosened flock structure around the cell; the cell membrane is intact but the intracellular substances are destroyed. Given a relatively higher energy input, for example Es = 75 J/mL, the cell membrane is split and some intracellular substances have been released. When the energy input continues to be increased, the cell membrane suffers more serious damage (Es = 120 J/mL). After the cell membrane is broken, all of the intracellular contents are released, and the cell membrane will continue to be broken into increasingly smaller pieces under the athermal effect of MV irradiation. The results shown in our experiments actually reveal the degree of energy consumption in destroying the cell membrane. It is clear that cell membrane can be broken efficaciously only after the input specific energy is higher than a certain value (about 75 J/mL in our experiments). Similar result was also observed for ultrasonic treatment, in which a minimum specific energy of 1000 kJ/kg TSS was needed to destroy the cell membrane (Bourgrier et al., 2005). The degree of cell damage has a direct relationship to the energy input, but, because there are huge amounts of microorganisms within ESS, it is difficult to determine a suitable level of energy consumption solely from the degree of cell damage, Woo et al. (2000) also observed the phenomenon of differential damage in different cells by the same dosage of MV irradiation.
4.5
2.0 1.5
Water content 99.59% 97.86% 96.60%
1.0 0.5 0.0 0
15
30
45
60
75
90 105 120 135 150 165
Es (J/mL) Fig. 2. Energy consumption in solids solubilization.
the effect of MV irradiation on the solubilization of ESS, and the results are shown in Fig. 2. Fig. 2 shows that the increment of SL increases rapidly with increasing Es, which has been reported elsewhere. As expected, MV irradiation is the main factor determining the solubilization of solids, but it should be noted that the water content of ESS has a marked impact. Fig. 2 shows that a high water content is unfavourable for the solubilization of solids at a given energy input, whereas at a lower water content, less energy input would lead to a larger SL. Combined with the results in Figs. 1 and 2, it can be seen that there is a close relationship of water content to the energy efficiency in treating ESS with MV irradiation. More energy would be consumed in raising the temperature of the water at higher water contents, which decreases the efficiency of energy in the solubilization of solids.
3.4. Energy consumption in enhancing anaerobic digestion In the literature, cumulative biogas production (CBP) is a commonly used parameter to describe the anaerobic digestibility of ESS after pre-treatment. As a mass reduction method, anaerobic digestion has long been used as an economical and convenient way to treat both municipal and industrial sludge, but the process is time-consuming and often needs a large area for the retention of sludge. The difficulty in solubilizing ESS during anaerobic digestion lies mainly in the special structure of the sludge flock and the protection of the cell membrane, so the logical approach to accelerate the process of anaerobic digestion should be to disrupt the flock and destroy the protection of the cell membrane, as shown in Section 3.2. Anaerobic digestion after the pre-treatment is expected to progress more rapidly. In Fig. 3, it reveal the durable effect on biodegradation of ESS pre-treated by MV irradiation. After exposure to MV irradiation, different solid substances in ESS suffer different degrees of damage. The flock structure and the cell membrane are destroyed and solubilized. This process contributes to promotion of the biodegradability of ESS. Fig. 3 shows the accelerating effect of MV irradiation on anaerobic digestion. It is clear that higher Es improves the anaerobic digestion, while differences in the water content have relatively less impact on biogas production. Fig. 3 reveals that the accelerating effect of MV irradiation on anaerobic digestion is gradually reduced after Es attains a certain level. In terms of energy efficiency, there appears to be a limitation in promoting biogas production by MV irradiation. Such a phenomenon is relevant to the degree of destruction of the flock structure and cell
B. Tang et al. / Bioresource Technology 101 (2010) 5092–5097
3.5. Evaluation of energy efficiency in pre-treatment ESS with MV irradiation Several steps are generally involved simultaneously in the treatment of ESS with MV irradiation, which include temperature increase, damage of flock structure, and destruction of cells and solubilization of solids. These effects improve the biodegradability of ESS and accelerate biogas production. A quantification criterion is proposed for evaluating the efficiency of energy consumption in
0.007 0.006
Water content 99.59% 97.86% 96.60%
0.005 0.004 0.003 0.002 0.001 0.000 0
40
80
120
160
200
Es (J/mL)
3200 Es=0 (untreated sample) Es=20 (J/mL) Es=60 (J/mL) Es=120(J/mL)
2800 2400
b
0.06 0.05
2000
Water content 99.59% 97.86% 96.60%
0.04
ηe (mLJ )
1600
-1
Cumulative biogas production (mL)
a
a
-1
membranes in ESS. The factor responsible for limitation of the solubilization of solids in ESS is the breakage of cell membranes, damage to the flock structure of ESS and release of the extra- and intracellular substances. The energy efficiency decreases gradually when approaching the limitation point.
ηe (mLJ )
5096
1200 800 400
0.03 0.02 0.01
0 0
2
4
6
8
10
12
14
16
18
20
Cumulative biogas production (mL)
b
40
80 120 160 200 240 280 320 360 Es (J/mL)
Es=0 (untreated sample) Es=20 (J/mL) Es=60 (J/mL) Es=120 (J/mL)
3200 2800 2400
Fig. 4. Curves of energy efficiency: (a) curves based on temperature rise index (Te); (b) curves based on solids solubilization index (SL).
2000 1600 1200 800 400 0 0
2
4
6
8
10
12
14
16
18
20
Digestion time (d) Fig. 3. Energy consumption in enhancing anaerobic digestion: (a) water content = 99.22%; (b) water content = 96.76%.
Table 2 Evaluation of the efficiency of energy consumption under different water content. Effective factor (Ef)
Relational expression between Ef and Es
R2
Original water content (wt.%)
Te
T e ¼ 0:264ð1 expð0:033Es ÞÞ1:90
0.9871
99.59
T e ¼ 0:258ð1 expð0:040Es ÞÞ1:70
0.9953
97.86
T e ¼ 0:256ð1 expð0:051Es ÞÞ1:69
0.9950
96.60
SL ¼ 3:43ð1 expð0:024Es ÞÞ4:18
0.9898
99.59
SL ¼ 4:08ð1 expð0:020Es ÞÞ2:19
0.9863
97.86
SL ¼ 4:76ð1 expð0:021Es ÞÞ1:82
0.9942
96.60
SL
0.00 0
Digestion time (d)
the pre-treatment of ESS. The relation of the effective factors Te and SL between the input Es values can be obtained by fitting the data from Figs. 1 and 2, respectively, which are shown in Table 2. According to the fitting expressions shown in Table 2, the relevant expressions of derived functions can be obtained, and the curves of derived functions are shown in Fig. 4, which illustrates the energy efficiency based on the temperature rise index and the solids solubilization index. It is clear from Fig. 4a and b that there is maximum energy efficiency that can be attained while treating ESS with MV irradiation. The water content of ESS is an important factor that influences the energy efficiency, which is not dependent on the temperature rise index or the solids solubilization index. In ESS with high water content, the energy efficiency is relatively low, and the maximum energy efficiency will be achieved only with more energy consumption. Such results are shown in Table 3, which demonstrates the direct relationship between maximum energy efficiency and water content in ESS. Temperature increase and solubilization of solids are the direct effects induced by MV irradiation, which will accelerate the subsequent biogas production process. From the results given in Fig. 4 and Table 3, it is clear that the rate of change of effective factors (Te or SL) by MV irradiation is quite different for different water contents. No matter what the effective factor, there is a maximum energy efficiency that can be attained, which implies this is the limit value. Such data are in accord with the experimental results in Section 3.4. Park et al. (2010) obtained a maximum degree of solubilization (17.9%) under optimal conditions, including temper-
5097
B. Tang et al. / Bioresource Technology 101 (2010) 5092–5097 Table 3 Calculation of maximum energy efficiency under different water content. Expression of ge
Te
dT e dEs dT e dEs dT e dEs
¼ 0:0166ð1 expð0:033Es ÞÞ0:90 expð0:033Es Þ
99.59
0.00446
¼ 0:0175ð1 expð0:040Es ÞÞ0:70 expð0:040Es Þ
97.86
0.00553
¼ 0:0221ð1 expð0:051Es ÞÞ0:69 expð0:051Es Þ
96.60
0.00705
dSL dEs dSL dEs dSL dEs
¼ 0:344ð1 expð0:024Es ÞÞ3:18 expð0:024Es Þ
99.59
0.0345
¼ 0:179ð1 expð0:020Es ÞÞ1:19 expð0:020Es Þ
97.86
0.0395
¼ 0:182ð1 expð0:021Es ÞÞ0:82 expð0:021Es Þ
96.60
0.0520
SL
a
ge;Max a (mL J1)
Effective factor (Ef)
Original water content (wt.%)
ge;Max is the maximum energy efficiency calculating from Fig. 4 or the expression of ge in this table.
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