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Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge
Journal of Environmental Management 213 (2018) 271e278
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge Liyu Peng a, b, Lise Appels b, *, 1, Haijia Su a, **, 1 a b
Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing, China Process and Environmental Technology Lab, Department of Chemical Engineering, KU Leuven, Sint-Katelijne-Waver, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2017 Received in revised form 6 February 2018 Accepted 14 February 2018
This study investigated the synergistic effect of sodium citrate (SC; Na3C3H5O(COO)3) and microwave (MW) treatment on the efficiency of the anaerobic digestion of excess sewage sludge. In terms of the methane yield, an increase of the digestion's efficiency was observed. Taking into account the cost for the MW energy supplied to the system, the optimum treatment conditions were a MW energy input of 20 MJ/kg TS and a SC concentration of 0.11 g/g TS, obtaining a methane yield of 218.88 ml/g VS, i.e., an increase of 147.7% compared to the control. MW treatment was found to break the sludge structure, thereby improving the release of extracellular polymeric substances (EPS) and volatile fatty acids (VFAs). The treatment of sodium citrate further strengthened the breakage of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS). The increased VFA content stressed the improved digestion by this pretreatment. Furthermore, the preliminary economic analysis showed that at this point in the research, only operational but no financial gains were achieved. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Sewage sludge Microwave Sodium citrate Anaerobic digestion EPS
1. Introduction The disposal of excess sewage sludge from wastewater treatment plants (WWTP) is of increasing importance, since (i) the amount produced on yearly basis is steadily growing, and (ii) its final disposal possibilities are limited. The stringent legislation regarding the presence of heavy metals and toxic nonbiodegradable organics leads to the fact that only a small amount of this sludge can be used in agriculture as fertilizer (Hanc, A. et al., 2009). For final disposal, large amounts of sludge must be combusted in dedicated sludge incinerators, co-digested with other types of waste, or dried and utilized as a secondary fuel in cement kilns or coal fired power stations. Due to the high water content and poor mechanical dewatering properties of the excess sludge, with typically only 20e25% DS-content achievable, these final disposal
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Appels), [email protected] (H. Su). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jenvman.2018.02.053 0301-4797/© 2018 Elsevier Ltd. All rights reserved.
routes are only feasible after an energy consuming pre-drying step (Appels et al., 2008). It is reported that sludge disposal represents over 60% of total treatment plant operating expense (Appels et al., 2008; Neyens and Baeyens, 2003), which is a major drawback for waste water treatment. As a treatment process of waste sludge, anaerobic digestion (AD) is generally recognized as a stable, and energy-yielding process with the production of biogas containing 55e70% CH4 (Appels et al., 2008). AD can moreover (i) decrease the total sludge volume for disposal, (ii) improve the sludge dewaterability and (iii) stabilize the sludge (Appels et al., 2011). There are four major steps in anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The hydrolysis phase is often regarded as the ratelimiting step in the whole process (Appels et al., 2010; Abelleira et al., 2012). However, pretreatment of waste activated sludge can accelerate this step. In this regard, a great number of studies investigated pre-treatment methods to disintegrate the sludge and to release organic matter into the aqueous sludge phase, aiming to accelerate the hydrolytic stage and produce more biogas. Various treatment methods have been conducted for sludge digestion on pilot-scale and lab-scale. They comprise ultrasound (Pham et al., 2009; Bougrier et al., 2005), mechanical disintegration, (Zhang
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et al., 2012), chemical action (Dewil et al., 2007), ozone oxidation (Ahn et al., 2002), Fenton peroxidation (Kaynak and Filibeli, 2008), bio-hydrolysis (Gopi Kumar et al., 2012), thermal treatment (Val del Río et al., 2011), alkaline pre-treatment (Lin et al., 2007) and thermo-alkaline pre-treatment (Uma Rani et al., 2012), and combinations thereof Most results not only showed an improvement of biodegradability and sludge dewatering with accelerated hydrolysis, but also revealed a reduction of pathogens and foaming. Applying microwaves as sludge pretreatment process is quite novel and promising. Microwaves are electromagnetic waves (oscillating frequency about 0.3e300 GHz) that can promote structure alteration of micro-organisms and organic matter in the sludge, and further generate heat (Climent et al., 2007). In addition to their thermal effect, microwaves also cause an additional athermal effect by altering the dipole orientation in the polarized side chains of the cell membrane molecules. The a-thermal effect results in the cleavage of the hydrogen bonds, so that the floc matrix of the micro-organisms disintegrates and the structure of the present proteins break (Park et al., 2004, 2010; Tang et al., 2010; Eskicioglu et al., 2007a). The effect of microwave radiation is influenced by four parameters, i.e. radiation time, strength (frequency), penetration depth and concentration (Park et al., 2010). Microwave technology has been widely used in several aspects like synthesis, extraction, digestion and stabilization (Huang et al., 2015), for many advantages of microwave such as: 1) reduction of energy consumption, 2) reduction of reaction time, 3) environmentally friendly heating (Huang et al., 2015; Wang et al., 2015). MW pre-treatment can contribute to an augmented biogas yield, as shown by several studies. The summary of these studies have been listed in Table 1. Other previous research has investigated how microwave treatment influences other characteristics of sludge, like pathogen destruction (Kuglarz et al., 2013), energy efficiency (Tang et al., 2010), and solubilization (Chang et al., 2011). It is proven that MW causes the breakage of the sludge structure and the release of organics while improving its dewaterability. However, the cell structure of microorganisms could be excessively broken when too much radiation power or too long radiation time is used. This excessive breakage will lead to the over-leakage of intracellular organics. Hence, a combination treatment like microwaves with sodium citrate could avoid these drawbacks, since less energy needs to be applied to the sludge in order to reach the same effect. Sodium citrate is generally used as anticoagulant of blood in pharmaceuticals and as acidity regulator in food. In this study it can be used as a cation-binding agent for sludge treatment. It mainly affects the mineral components of EPS (Ca2þ, Mg2þ, Fe3þ) by complexation, according to the following reaction: nþ
EPS M
reduced the irradiation energy of microwave by 1.4 times. This provided a way for sodium citrate to pretreat the excess sludge. The goal of this study is to improve the efficiency of MW pretreatment for subsequent AD. The addition of sodium citrate prior to MW treatment disrupts the sludge biomass structure and releases COD and EPS into the aqueous phase. The outcome of the study is assessed through biomethane potential tests, COD and VFA measurements and characterization of the EPS components.
2. Materials and methods 2.1. Sludge sampling and characterization The excess sludge was obtained from the buffer tank of a municipal WWTP located in Mechelen, Belgium. After sampling, the sludge was immediately stored at 4 C prior to further treatment and analysis. The characteristics of the raw sludge are shown in Table 2.
2.2. Sodium citrate pre-treatment Dosage optimization for sodium citrate was conducted in 60 conical flasks with a volume of 250 mL, filled with 100 g sludge. The range of SC addition was 0.05e0.14 g/g TS, according to (Ebenezer et al., 2015b). The sludge and sodium citrate were continuously mixed for 2 h at 70 rpm. After the reaction was terminated, the microwave pre-treatment was applied, as described further. Soluble COD (SCOD) concentrations were determined before and after the pre-treatment. The experiments were conducted in triplicate.
2.3. Microwave pre-treatment A microwave synthesis reactor (Anton Paar, Monowave 300, max power of 850 W) was used to treat the sludge; the treatment was conducted in a glass vial filled with 30 mL of sludge and different energy levels were applied. The power was kept constant at 850 W. The irradiation duration was varied between 3 and 30 s, thus leading to a specific energy (SE) input ranging from 10 to 40 MJ/kg TS, calculated according to the formula below.
Table 2 Characterization of the raw sludge.
þ
þ RCOONa/EPS Na þðRCOOÞn M
This loosens the EPS structure and makes it prone to a more direct disrupture when MW is applied. (Ebenezer et al., 2015b) used sodium citrate as deflocculant to remove the EPS of sludge and
Parameter
Value
pH Total Solids (TS) (g TS/kg sludge) Volatile Solids (VS) (g VS/kg sludge) Soluble COD (SCOD) (mg O2/L) Total COD (TCOD) (mg O2/L) Soluble proteins (mg/L) Soluble carbohydrates (mg Glu-eq/L)
7.56 ± 0.02 48.5 ± 0.2 33.3 ± 0.1 2519 ± 122 84,263 ± 1043 17,379 ± 1060 3066 ± 159
Table 1 Summary and comparison of the previous studies on MW treatment. Reference
Treatment method
Results
des et al. (2011) Besze Zhou et al. (2010) Yu et al. (2009) Yu et al. (2010) Ebenezer et al. (2015a) Zhang et al. (2016)
MW MW MW MW MW MW
Methane content increased up to 60%, COD solubility enhanced to a maximum of 57%. SCOD concentration increased 1.8e4.0-fold. EPS concentration (1500e2000 mg/L) and disintegration (1.5e2%) accompanied. Settle ablity increased from 39.58 mm/h to 45.08 mm/h. COD solubilization increased 28%, and reduction in SS increased 38%. The methane production was 338.44 mL CH4/gVS.
pretreatment (5 W/g and 30 min) treatment (700 W and 9min) treatment (900 W and 60s) treatment (900 W and 60s) pretreatment (14,000 kJ/kg TS) pretreatment (600 We100 C)
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SE ¼
Pt V TS
with P ¼ power [W], t ¼ duration of the treatment [s], V ¼ volume of the treated sludge [kg], TS ¼ TS content of the sludge [g TS/kg sludge].
supernatant obtained was regarded as the loosely bound EPS (LBEPS). For the tightly bound EPS (TB-EPS), the sludge pellet was diluted to 30 ml with 0.05% NaCl solution and re-suspended, and placed in a water bath at 60 C for 30 min. After centrifugation at 8000g for 15 min at 4 C, the supernatant collected was the TB-EPS fraction.
2.4. Biochemical methane potential assay
3. Results and discussion
The biochemical methane potential (BMP) assay was carried out in 24 batch reactors (volume of 1 L) at a constant temperature of 37 C, and a retention time of 30 days. Table 3 depicts the set-up of the BMP assay experiments, with each experiment conducted in triplicate. Prior to the start of the digestion experiment, the reactors were filled with digested sludge that serves as inoculum from the full-scale digester of the WWTP of Antwerpen-Zuid (Belgium). An inoculum:substrate ratio of 2:1 on VS-basis was used for the experiments. The VFA concentration, biogas production and methane content were measured daily.
3.1. Effect of the combined pre-treatment on sewage sludge characteristics
2.5. Analytical methods TS and VS were measured gravimetrically according to the Standard Methods (APHA, 2005). COD was measured both on the total sludge (i.e., total fractions) and on the sludge supernatant (i.e., soluble fractions). The supernatant was obtained by centrifugation of the sludge at 8000 g for 15 min, while maintaining the temperature inside the centrifuge at 4 C to minimize decomposition during sample handling. The degree of disintegration COD (DDCOD) was calculated as follows:
DDSCOD ¼
273
3.1.1. SCOD concentration and degree of disintegration The influence of MWþSC on sludge characteristics was evaluated using the release of SCOD and variation in VFA concentration (see further) of the untreated (control) and pretreated sludge. Fig. 1 depicts the change in SCOD due to the treatment, which represents the sludge solubilization (Ahn et al., 2009; Kuglarz et al., 2013). The first data point of each curve represents the treatment with solely MW radiation. The following data points are the treatments with increasing SC dosage. It is clear that the combination of MW and SC results in a synergistic effect on the COD release and hence solubilization of the sludge organic matter, though the effect is more pronounced in the lower MW-regions, as can be seen in Fig. 2. Table 4 shows the degree of disintegration COD (DDCOD) measured in the sludge supernatant. It is clear that with increasing MW-power, the DDCOD increases quasi linearly, except for the lowest SC-dosage. Furthermore, it can be deduced from the DDCOD values that SC has an extra positive effect on the COD
SCODtreated SCODcontrol 100% TCOD
The theoretical COD of SC was calculated first and then substracted from the total SCOD, since SC is a soluble organic compound. The COD was determined colorimetrically using a digital photometer Nanocolor® 500D (Macherey-Nagel) and Nanocolor® COD 1500 test tubes (Macherey-Nagel). Carbohydrates were analyzed with the Anthrone method, according to Gerhardt et al. (1994), using glucose as the standard. The amount of proteins was determined by measuring total Kjeldahl-nitrogen (TKN), and adopting a correction factor of 6.25. The analysis of volatile fatty acids (VFA) was performed with GC-FID, the methane content was determined with GC-TCD, both based on the methods described by Westerholm et al. (2016). The EPS extraction method was modified according to Luo et al. (2005). 30 mL of sludge was centrifuged at 8000 g for 15 min at 4 C. The supernatant was collected as the slime EPS. The sludge pellet was diluted to 30 ml with a 0.05% NaCl solution at 50 C and immediately re-suspended by vortexing for 1 min. The sludge was subsequently centrifuged again at 8000 g for 10 min at 4 C. The
Fig. 1. Change in SCOD concentration for the different treatment conditions.
Table 3 Set-up of the BMP assay experiments. Experiment
Sodium citrate dosage (g SC/g TS sludge)
Microwave energy input (MJ/kg TS sludge)
Control SC MW10 MW10þSC MW20 MW20þSC MW40 MW40þSC
0 0.11 0 0.11 0 0.11 0 0.11
0 0 10 10 20 20 40 40
BMP ¼ Biochemical Methane Potential; MW ¼ microwave; SC ¼ sodium citrate; Temperature ¼ 37 C; Retention time ¼ 30 days.
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Fig. 2. The EPS release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Table 4 Degree of disintegration (DDCOD) (expressed in %). SC dosage (g/g TS sludge)
Microwave energy input (MJ/kg TS sludge) 0
10
20
40
0 0.05 0.08 0.11 0.14
0 4.5 2.8 3.8 4.3
2.0 4.2 4.9 7.5 8.9
5.1 4.6 8.1 11.4 11.7
9.2 6.1 13.3 14.6 15.0
disintegration, up to a dosage of 0.11 g/g TS. A DDCOD of 11.4% at an MW energy input of 20 MJ/kg TS and SC dosage of 0.11 g/g TS was considered acceptable, although somewhat lower than suggested in literature, and was selected for further investigation. At an elevated energy input however, evaporation of organics occurs (Eskicioglu et al., 2007c), and this may cause the lower increase of SCOD observed in this study. All further tests were hence performed at an SE of 20 MJ/kg TS and an SCdosage of 0.11 g/g TS. The deflocculating action of SC is seen as the reason of the improved breakage of the sludge floc matrix, and the elevated MW energy input further improves the solubilization of this deflocculated organic matter. The treatment thereby increases the cell's permeability and improves the release of EPS. An obvious difference of SCOD release was clearly noted between untreated and SCtreated sludge. On the contrary, studies performed by Kuglarz et al. (2013) and Chang et al. (2011) did not obtain an increased sludge solubilization at elevated MW energy inputs. The present study showed that solubilization was more affected at elevated power inputs, instead of the effect by SC. 3.1.2. EPS, carbohydrates and proteins Next, the shares of proteins and carbohydrates in the EPS were analyzed. The results are depicted in Fig. 3 (proteins) and 4 (carbohydrates). The protein concentration in the TB-EPS decreases from 1934 (11%) to 608 mg/L (3%) as a function of treatment intensity, and again the effect is most pronounced at the highest intensities. Likewise, the proteins in the slime-EPS increases from 11,498 (66%) to 16,380 mg/L (82%). This is not surprising since proteins make out an important fraction of the EPS (Nielsen et al., 1996), and solubilization of the EPS is induced by the treatments.
Hence, solubilized proteins are transferred to the slime-EPS. The results of the carbohydrates are in line with those of the proteins: TB-EPS decreases from 22% to 11% and slime-EPS increases from 46% to 62% by cell lysis, releasing cell wall polysaccharides and proteins (Ahn et al., 2009; Eskicioglu et al., 2007b). This is confirmed by the values of the total concentrations of carbohydrates and proteins, which were elevated for the treatment compared to the control (data shown in Table 5). In contrast to the overall EPS, the TB-EPS fractions of proteins and carbohydrates are also affected by the treatment, albeit less than the other fractions, as can be seen in Fig. 4. Ahn et al. (2009) studied the effect of MW (2.45 GHZ, 700 W) in a SE range of 10e49 MJ/kg TS. They also observed an increase in soluble carbohydrates and soluble proteins, with a factor 11 and 8 respectively, for the highest MW energy input. Based on our experiments, microwave pretreatment at an energy input of 20 MJ/kg TS and 40 MJ/kg TS could be considered as treatment. 3.1.3. Volatile fatty acids (VFA) Volatile fatty acids (VFA) are important intermediary compounds during anaerobic digestion, formed during acidogenesis and further transformed to biogas during the acetogenesis and methanogenesis phase. Even though they are crucial to the process, elevated concentrations may hamper and eventually inhibit the process completely. Generally, concentrations of 7000e10,500 are considered inhibitory (Appels et al., 2008). It is therefore important to assess whether or not the treatments in this study can cause elevated VFA concentrations and to which extent. Fig. 5a,b depicts the variation of total VFA concentration and acetic acid concentration under the various MW energy inputs and SC dosages. It must be noted that the untreated sludge sample already has a VFA concentration of 990 mg/L, which is quite high. A first observation is that acetic acid comprises the larger part of the total VFA in the treated sludge samples, and concentrations as high as 1400 mg/L for the MW20 þ SC0.11 treatment are obtained. Furthermore, a treatment with solely SC (black curve in Fig. 5a) is able to achieve an increase in VFA, whereas treating the sludge with solely MW does not result in any change. Focusing on the combined treatments, it is clear that only the lower MW-regions are inducing an effect on VFA concentrations, especially on the acetic acid concentration. MW/SC at 20 MJ/kg and 0.11 g/g might be optimal for the treatment for leading to the highest VFA concentration.
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Fig. 3. The protein release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Fig. 4. The carbohydrates release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Table 5 Total concentrations of EPS, carbohydrates and proteins.
Control SC 0.11 MW 10 MJ MW10MJþSC0.11 MW 20 MJ MW20MJþSC0.11 MW 40 MJ MW40MJþSC0.11
Total EPS (mg/L)
Total protein-EPS (mg/L)
Total carbohydrate-EPS (mg/L)
20,446 21,147 18,138 20,857 20,675 23,260 25,158 25,451
17,380 17,846 15,010 17,468 17,569 19,354 20,443 19,912
3067 3300 3128 3389 3105 3906 4715 5538
3.2. Effect of the combined pre-treatment on the anaerobic digestibility of sewage sludge 3.2.1. Methane production (BMP) In order to investigate the influence of organic matter solubilization and elevated VFA concentration on the anaerobic digestibility, batch assays were performed for an SC dosage of 0.11 g/g TS and different MW energy inputs. The results are shown in Fig. 6 and Table 6. It is clear that the increase in biogas yield can be attributed to the difference in the structure of organic material, rather than to the amount of organic matter introduced into the digesters. On the other hand, the solubilization of organic matter
and increase in VFA concentration caused by the pre-treatments did not cause overload or acidification of the process. According to Fig. 6, the treatments can be divided in three groups: (1) Low-intensity treatments: SC0.11 (without MW) and MW10 (without SC). They achieve an increase in methane yield of a factor 1.5: 52.6 and 56.5% respectively. This shows that both the SC and MW have a limited effect on the anaerobic digestibility of sewage sludge. (2) Medium-intensity treatments: MW10 þ SC0.11, MW20 (without SC) and MW40 (without SC). They achieve a 2-fold
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increase in methane yield: 82.2, 93.7 and 107% respectively. Two conclusions can be drawn from these results: (i) a lower MW energy input combined with SC can achieve the same effect on the digestion process as a higher MW energy input without SC (cfr. 82.2% vs. 93.7%); (ii) the degrees of improvement for MW20 and MW40 are nearly the same (cfr. 93.7% vs. 107.0%), indicating that a MW energy input higher than 20 MJ/kg TS is not beneficial for sludge pre-treatment. (3) High-intensity treatments: MW20 þ SC0.11 and MW40 þ SC0.11. Here, a 2.5-fold increase in methane yield was obtained (147.7 and 155.9% respectively), supporting the hypothesis that a MW energy input higher than 20 MJ/kg TS is not beneficial for sludge pre-treatment. Comparing the percentage increase in methane production depicted in Table 6, it is observed that for a given MW energy input, the yield is consistently higher in the digester treated with the combined pre-treatment compared to a sole MW pre-treatment. This proves the synergistic effect of the combination of microwaves and sodium citrate on the anaerobic digestibility of sewage sludge. The methane production of MW20 þ SC0.11 is even higher than that of MW40: 218.88 vs. 182.92 ml/g VS, supporting the beneficial effect of sodium citrate addition. 3.2.2. TS, VS removal Table 7 presents the data on TS and VS removal for both untreated and pre-treated sludge under different levels of energy inputs, with and without SC dosage. The effect on TS removal was mostly noticeable for the MW treatments with SC dosage: an increase from 50.73% up to 59.33% could be observed. A larger difference can be found for the VS removal. Uma et al. (2013) employed similar conditions by treating dairy sludge with a TS content of 11.6 g/L for 12 min (2450 MHz frequency, 900 W). The experiments proved that the reduction of suspended solids in the treated sludge was 14% higher than that in the untreated sludge. Fig. 5. Variation of the concentration of VFA and acetic acid with sodium citrate dosage for different microwave energy input. (a). Variation of the VFA concentration with sodium citrate dosage for different MW energy inputs. (b). Variation of the concentration of acetic acid with sodium citrate dosage for different microwave energy input.
Fig. 6. Cumulative methane production (ml/g VS) for the different treatment conditions.
3.2.3. Economic evaluation A simplified economic analysis is made to compare the cost and benefits from the combined treatments with a business-as-usual scenario. Two treatments, i.e. MW20 þ SC0.11 and MW40 þ SC0.11 were chosen for the analysis. The TS and VS content used for the calculations can be found in Table 2. Table 8 shows the energy content of the extra biogas produced from the treated sludge and the energy needed for these pre-treatments. It is clear that the MW treatments demands a lot more energy than the extra methane produced can provide: MW20 demands the 5-fold in energy, MW40 requires almost 10 times as much energy. The cost for the SC dosage is counterbalanced by the decrease in sludge disposal costs. It is clear that the current energy balance is negative for the studied treatments, even though a large gain in methane yield was achieved. However, for the calculation of the operational costs and benefits of the treatment, the decrease in sludge to be disposed of must be taken into account, as shown in Table 8. It must be noted that a possibly improved sludge dewaterability was not included in this brief analysis, which also an important cost for the WWTP and could have an influence on the final balance. For both combined pre-treatments, there is no positive net gain
Table 6 Comparison of methane production (ml/g VS) and the methane yield increase compared to the control (%). Methane Production
MW 0 MJ/kg TS
MW 10 MJ/kg TS
MW 20 MJ/kg TS
MW 40 MJ/kg TS
SC 0 g/g TS SC 0.11 g/g TS
88.36 () 134.88 (52.6%)
138.28 (56.5%) 161.00 (82.2%)
171.19 (93.7%) 218.88 (147.7%)
182.92 (107.0%) 226.15 (155.9%)
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Table 7 Comparison of TS and VS removal after digestion, for the different treatment conditions (expressed in %).
TS removal VS removal
SC SC SC SC
0 g/g TS 0.11 g/g TS 0 g/g TS 0.11 g/g TS
MW 0 MJ/kg TS
MW 10 MJ/kg TS
MW 20 MJ/kg TS
MW 40 MJ/kg TS
48.90 50.73 56.06 62.94
36.71 50.80 55.94 75.04
41.13 56.85 70.06 73.77
55.22 59.33 76.90 79.73
Table 8 Energy and economic balance.
Energy balance (per ton of sludge) Increase in methane production Energy content of the extra methane* Energy applied Net energy production Cost calculation (per ton of sludge) Energy cost (at 0.08 V/kWh) Cost of SC dosage (at 0.2V/kg) Decrease in TS to be disposed Reduced cost for sludge disposal (at 1.55 V/kg TS**) Net cost
MW20 þ SC0.11
MW40 þ SC0.11
4.4 43.6 269.4 225.8
4.6 45.6 538.9 493.3
m3 kWh kWh kWh
18.1 1.1 3.9 5.9 13.3
39.5 1.1 5.1 7.7 32.9
V V kg V V
* Heating value CH4 ¼ 35.7 MJ/Nm3, ** Sludge disposal cost ¼ 75 V/ton sludge (Houtmeyers et al., 2014).
at this point. They are not attractive from an economic point of view. 4. Conclusions The biodegradability of the sewage sludge was significantly improved through the synergistic treatment with sodium citrate and microwave radiation. It was proven that the pre-treatment leads to a larger degree of solubilization of the sludge organic matter (COD) and furthermore, improves the deflocculation of the different fractions of the sludge EPS. The degree of solubilization of carbohydrates was larger than that of proteins, although the protein concentration is significantly higher than that of carbohydrates. Pre-treating the sludge with the combination of MW and SC, the concentration of VFA is considerably increased, but the BMP batch assays did not show an inhibitory effect of these elevated concentrations. The amenability of sewage sludge for anaerobic digestion was improved by the combined pre-treatment. Although the methane production could be enhanced with up to 155%, the energy needed for the MW treatment transcended the energy gained from the extra methane. Therefore, the combined pretreatment as investigated in this study is, at this point, not yet attractive from an economic point of view. Acknowledgements The authors express their thanks for the support from the National Natural Science Foundation of China (21525625), the National Basic Research Program (973 Program) of China (2014CB745100), the (863) High Technology Project (2013AA020302). References rez-Elvira, S.I., Portela, J.R., Sa nchez-Oneto, J., Nebot, E., 2012. Abelleira, J., Pe Advanced thermal hydrolysis: optimization of a noval thermochemical process to aid sewage sludge treatment. Environ. Sci. Technol. 46, 6158e6166. Ahn, K.H., Park, K.Y., Maeng, S.K., Hwang, J.G., Lee, J.W., Song, K.G., Chai, S., 2002. Ozonation of wastewater sludge for reduction and recycling. Water Sci. Technol. 46, 71e77. Ahn, J.H., Shin, S.G., Hwang, S., 2009. Effect of microwave irradiation on the disintegration and acidogenesis of municipal secondary sludge. Chem. Eng. J. 153,
145e150. APHA, 2006. Standard Methods for the Examination of Water and Wastewater, eighteenth ed. American Public Health Association, Washington, DC. ve, J., Dewil, R., 2008. Principles and potential of the Appels, L., Baeyens, J., Degre anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34, 755e781. ve, J., Van der Bruggen, B., Van Impe, J., Dewil, R., 2010. Influence of Appels, L., Degre low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion. Bioresour. Technol. 101, 5743e5748. ve, J., Helsen, L., Lievens, B., Willems, K., Van Impe, J., Appels, L., Lauwers, J., Degre Dewil, R., 2011. Anaerobic digestion in global bio-energy production: potential and research challenges. Renew. Sustain. Energy Rev. 15, 4295e4301. des, S., La szlo , Z., Horv , G., Hodúr, C., 2011. Comparison of the Besze ath, Z.H., Szabo effects of microwave irradiation with different intensities on the biodegradability of sludge from the dairy- and meat-industry. Bioresour. Technol. 102, 814e821. re, H., Degene s, J.P., 2005. Solubilisation of waste-activated sludge Bougrier, C., Carre by ultrasonic treatment. Chem. Eng. J. 106, 163e169. Chang, C.J., Tyagi, Vinay Kumar, Lo, S.L., 2011. Effects of MW-alkali induced pretreatment on sludge solubilization and subsequent aerobic digestion. Bioresour. Technol. 102, 7633e7640. Climent, M., Ferrer, I., Baeza, M.M., Artola, A., Vazquez, F., Font, X., 2007. Effects of thermal and mechanical pretreatments of secondary sludge on biogas production under thermophilic conditions. Chem. Eng. J. 133, 335e342. ve, J., 2007. Peroxidation enhances the biogas Dewil, R., Appels, L., Baeyens, J., Degre production in the anaerobic digestion of biosolids. J. Hazard Mater. 146, 577e581. Ebenezer, A.V., Arulazhagan, P., Kumar, S.A., Yeom, I.T., Banu, J.R., 2015a. Effect of deflocculation on the efficiency of low-energy microwave pretreatment and anaerobic biodegradation of waste activated sludge. Appl. Energy 145 (1), 104e110. Ebenezer, A.V., Kaliappan, S., Kumar, S.A., Yeom, I., Banu, J.R., 2015b. Influence of deflocculation on microwave disintegration and anaerobic biodegradability of waste activated sludge. Bioresour. Technol. 185, 194e201. Eskicioglu, C., Terzian, N., Kennedy, K.J., Droste, R.L., Hamoda, M., 2007a. Athermal microwave effects for enhancing digestibility of waste activated sludge. Water Res. 41, 2457e2466. Eskicioglu, C., Droste, R.L., Kennedy, K.J., 2007b. Performance of anaerobic waste activated sludge digesters after microwave pre-treatment. Water Environ. Res. 79, 2265e2273. Eskicioglu, C., Kennedy, K.J., Droste, R.L., 2007c. Enhancement of batch waste activated sludge digestion by microwave pretreatment. Water Environ. Res. 79 (11), 2304e2317. Gerhardt, P., Murray, R.G.E., Wood, W.A., Krieg, N.R., 1994. Methods for General and Molecular Bacteriology. ASM, Washington, DC. Gopi kumar, S., Merrylin, J., Kaliappan, S., Adish Kumar, S., Yeom, I.T., Rajesh Banu, J., 2012. Effect of cation binding agents on sludge solubilization potential of bacteria. Biotechnol. Bioproc. Eng. 17, 346e352. Han c, A., Komorowicz, I., Sek, K., Baralkiewicz, D., 2009. Test of the relationships between the content of heavy metals in sewage sludge and source of their pollution by chemometric methods. J. Environ. Sci. Health Part A Toxic/Hazard. Substanc. Environ. Eng. 44 (13), 1441e1448. ve, J., Willems, K., Dewil, R., Appels, L., 2014. Comparing the Houtmeyers, S., Degre
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influence of low power ultrasonic and microwave pre-treatments on the solubilisation and semi-continuous anaerobic digestion of waste activated sludge. Bioresour. Technol. 171, 44e49. Huang, Y.F., Chunhao, S., Peite, C., Shanglien, L., 2015. Microwave co-pyrolysis of sewage sludge and rice straw. Energy 87, 638e644. Kaynak, G.E., Filibeli, A., 2008. Assessment of Fenton process as a minimization technique for biological sludge: effects on anaerobic sludge bioprocessing. J. Res. Sci. Technol. 5, 151e160. Kuglarz, M., Karakashev, D., Angelidaki, I., 2013. Microwave and thermal pretreatment as methods for increasing the biogas potential of secondary sludge from municipal wastewater treatment plants. Bioresour. Technol. 134 (2), 290e297. Lin, J.G., Chang, C.N., Chang, S.C., 2007. Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol. 62, 85e90. Luo, X., Lei, Z.F., Zhang, Z.Y., Sugiura, N., 2005. Study on the extraction of extracellular polymeric substances (eps) from aerobic/anaerobic sludges. Acta Sci. Circumstantiae 25 (12), 1624e1629. Neyens, E., Baeyens, J., 2003. A review of thermal sludge pre-treatment processes to improve dewaterability. J. Hazard Mater. 98, 51e67. Nielsen, P.H., Frølund, B., Keiding, K., 1996. Changes in the composition of extracellular polymeric substances in activated sludge during anaerobic storage. Appl. Microbiol. Biotechnol. 44 (6), 823. Park, B., Ahn, J.H., Kim, J., Hwang, S., 2004. Use of microwave pre-treatment for enhanced anaerobiosis of secondary sludge. Water Sci. Technol. 50, 17e23. 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, S13eS16. Pham, T.T.H., Brar, S.K., Tyagi, R.D., Surampalli, R.Y., 2009. Ultrasonication of wastewater sludge consequences on biodegradability and flowability. J. Hazard Mater. 163, 891e898. Tang, B., Yu, L.F., Huang, S.S., Luo, J.Z., Zhuo, Y., 2010. Energy efficiency of pretreating excess sewage sludge with microwave irradiation. Bioresour. Technol. 101 (14),
5092e5097. Uma Rani, R., Adish kumar, S., Kaliappan, S., Yeom, Ick-Tae, Rajesh Banu, J., 2012. Low temperature thermo e chemical pretreatment of dairy waste activated sludge for anaerobic digestion process. Bioresour. Technol. 103, 415e424. Uma, R.R., Adish Kumar, S., Kaliappan, S., Yeom, I.T., Rajesh Banu, J., 2013. Impacts of microwave pre-treatments on the semi-continuous anaerobic digestion of dairy waste activated sludge. Waste Manag. 33, 1119e1127. Val del Río, A., Morales, N., Isanta, E., Mosquera-Corral, A., Campos, J.L., Steyer, J.P., re, H., 2011. Thermal pre-treatment of aerobic granular sludge: impact on Carre anaerobic biodegradability. Water Res. 45, 6011e6020. Wang, Y.W., Gui, C.M., Ni, X.T., Chen, M.X., Wei, Y.X., 2015. Multivariate analysis of sludge disintegration by microwave-hydrogen peroxide pretreatment process. J. Hazard Mater. 283 (283C), 856e864. Westerholm, M., Crauwels, S., Geel, M.V., Dewil, R., Lievens, B., Appels, L., 2016. Microwave and ultrasound pre-treatments influence microbial community structure and digester performance in anaerobic digestion of waste activated sludge. Appl. Microbiol. Biotechnol. 100 (12), 5339e5352. Yu, Q., Lei, H., Yu, G., Feng, X., Li, Z., Wu, Z., 2009. Influence of microwave irradiation on sludge dewaterability. Chem. Eng. J. 155, 88e93. Yu, Q., Lei, H.Y., Li, Z., Li, H.L., Chen, K., Zhang, X.H., Liang, R.L., 2010. Physical and chemical properties of waste-activated sludge after microwave treatment. Water Res. 44 (9), 2841e2849. Zhang, J., Lv, C., Tong, J., Liu, J., Liu, J., Yu, D., et al., 2016. Optimization and microbial community analysis of anaerobic co-digestion of food waste and sewage sludge based on microwave pretreatment. Bioresour. Technol. 200 (2), 253e261. Zhang, S., Zhang, P., Zhang, G., Fan, J., Zhang, Y., 2012. Enhancement of anaerobic sludge digestion by high-pressure homogenization. Bioresour. Technol. 118, 496e501. Zhou, B.W., Shin, S.G., Hwang, K.H., Ahn, J.H., Hwang, S., 2010. Effect of MW irradiation on cellular disintegration of Gram positive and negative cells. Appl. Microbiol. Biotechnol. 87, 765e770.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge Liyu Peng a, b, Lise Appels b, *, 1, Haijia Su a, **, 1 a b
Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing, China Process and Environmental Technology Lab, Department of Chemical Engineering, KU Leuven, Sint-Katelijne-Waver, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2017 Received in revised form 6 February 2018 Accepted 14 February 2018
This study investigated the synergistic effect of sodium citrate (SC; Na3C3H5O(COO)3) and microwave (MW) treatment on the efficiency of the anaerobic digestion of excess sewage sludge. In terms of the methane yield, an increase of the digestion's efficiency was observed. Taking into account the cost for the MW energy supplied to the system, the optimum treatment conditions were a MW energy input of 20 MJ/kg TS and a SC concentration of 0.11 g/g TS, obtaining a methane yield of 218.88 ml/g VS, i.e., an increase of 147.7% compared to the control. MW treatment was found to break the sludge structure, thereby improving the release of extracellular polymeric substances (EPS) and volatile fatty acids (VFAs). The treatment of sodium citrate further strengthened the breakage of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS). The increased VFA content stressed the improved digestion by this pretreatment. Furthermore, the preliminary economic analysis showed that at this point in the research, only operational but no financial gains were achieved. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Sewage sludge Microwave Sodium citrate Anaerobic digestion EPS
1. Introduction The disposal of excess sewage sludge from wastewater treatment plants (WWTP) is of increasing importance, since (i) the amount produced on yearly basis is steadily growing, and (ii) its final disposal possibilities are limited. The stringent legislation regarding the presence of heavy metals and toxic nonbiodegradable organics leads to the fact that only a small amount of this sludge can be used in agriculture as fertilizer (Hanc, A. et al., 2009). For final disposal, large amounts of sludge must be combusted in dedicated sludge incinerators, co-digested with other types of waste, or dried and utilized as a secondary fuel in cement kilns or coal fired power stations. Due to the high water content and poor mechanical dewatering properties of the excess sludge, with typically only 20e25% DS-content achievable, these final disposal
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Appels), [email protected] (H. Su). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jenvman.2018.02.053 0301-4797/© 2018 Elsevier Ltd. All rights reserved.
routes are only feasible after an energy consuming pre-drying step (Appels et al., 2008). It is reported that sludge disposal represents over 60% of total treatment plant operating expense (Appels et al., 2008; Neyens and Baeyens, 2003), which is a major drawback for waste water treatment. As a treatment process of waste sludge, anaerobic digestion (AD) is generally recognized as a stable, and energy-yielding process with the production of biogas containing 55e70% CH4 (Appels et al., 2008). AD can moreover (i) decrease the total sludge volume for disposal, (ii) improve the sludge dewaterability and (iii) stabilize the sludge (Appels et al., 2011). There are four major steps in anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The hydrolysis phase is often regarded as the ratelimiting step in the whole process (Appels et al., 2010; Abelleira et al., 2012). However, pretreatment of waste activated sludge can accelerate this step. In this regard, a great number of studies investigated pre-treatment methods to disintegrate the sludge and to release organic matter into the aqueous sludge phase, aiming to accelerate the hydrolytic stage and produce more biogas. Various treatment methods have been conducted for sludge digestion on pilot-scale and lab-scale. They comprise ultrasound (Pham et al., 2009; Bougrier et al., 2005), mechanical disintegration, (Zhang
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et al., 2012), chemical action (Dewil et al., 2007), ozone oxidation (Ahn et al., 2002), Fenton peroxidation (Kaynak and Filibeli, 2008), bio-hydrolysis (Gopi Kumar et al., 2012), thermal treatment (Val del Río et al., 2011), alkaline pre-treatment (Lin et al., 2007) and thermo-alkaline pre-treatment (Uma Rani et al., 2012), and combinations thereof Most results not only showed an improvement of biodegradability and sludge dewatering with accelerated hydrolysis, but also revealed a reduction of pathogens and foaming. Applying microwaves as sludge pretreatment process is quite novel and promising. Microwaves are electromagnetic waves (oscillating frequency about 0.3e300 GHz) that can promote structure alteration of micro-organisms and organic matter in the sludge, and further generate heat (Climent et al., 2007). In addition to their thermal effect, microwaves also cause an additional athermal effect by altering the dipole orientation in the polarized side chains of the cell membrane molecules. The a-thermal effect results in the cleavage of the hydrogen bonds, so that the floc matrix of the micro-organisms disintegrates and the structure of the present proteins break (Park et al., 2004, 2010; Tang et al., 2010; Eskicioglu et al., 2007a). The effect of microwave radiation is influenced by four parameters, i.e. radiation time, strength (frequency), penetration depth and concentration (Park et al., 2010). Microwave technology has been widely used in several aspects like synthesis, extraction, digestion and stabilization (Huang et al., 2015), for many advantages of microwave such as: 1) reduction of energy consumption, 2) reduction of reaction time, 3) environmentally friendly heating (Huang et al., 2015; Wang et al., 2015). MW pre-treatment can contribute to an augmented biogas yield, as shown by several studies. The summary of these studies have been listed in Table 1. Other previous research has investigated how microwave treatment influences other characteristics of sludge, like pathogen destruction (Kuglarz et al., 2013), energy efficiency (Tang et al., 2010), and solubilization (Chang et al., 2011). It is proven that MW causes the breakage of the sludge structure and the release of organics while improving its dewaterability. However, the cell structure of microorganisms could be excessively broken when too much radiation power or too long radiation time is used. This excessive breakage will lead to the over-leakage of intracellular organics. Hence, a combination treatment like microwaves with sodium citrate could avoid these drawbacks, since less energy needs to be applied to the sludge in order to reach the same effect. Sodium citrate is generally used as anticoagulant of blood in pharmaceuticals and as acidity regulator in food. In this study it can be used as a cation-binding agent for sludge treatment. It mainly affects the mineral components of EPS (Ca2þ, Mg2þ, Fe3þ) by complexation, according to the following reaction: nþ
EPS M
reduced the irradiation energy of microwave by 1.4 times. This provided a way for sodium citrate to pretreat the excess sludge. The goal of this study is to improve the efficiency of MW pretreatment for subsequent AD. The addition of sodium citrate prior to MW treatment disrupts the sludge biomass structure and releases COD and EPS into the aqueous phase. The outcome of the study is assessed through biomethane potential tests, COD and VFA measurements and characterization of the EPS components.
2. Materials and methods 2.1. Sludge sampling and characterization The excess sludge was obtained from the buffer tank of a municipal WWTP located in Mechelen, Belgium. After sampling, the sludge was immediately stored at 4 C prior to further treatment and analysis. The characteristics of the raw sludge are shown in Table 2.
2.2. Sodium citrate pre-treatment Dosage optimization for sodium citrate was conducted in 60 conical flasks with a volume of 250 mL, filled with 100 g sludge. The range of SC addition was 0.05e0.14 g/g TS, according to (Ebenezer et al., 2015b). The sludge and sodium citrate were continuously mixed for 2 h at 70 rpm. After the reaction was terminated, the microwave pre-treatment was applied, as described further. Soluble COD (SCOD) concentrations were determined before and after the pre-treatment. The experiments were conducted in triplicate.
2.3. Microwave pre-treatment A microwave synthesis reactor (Anton Paar, Monowave 300, max power of 850 W) was used to treat the sludge; the treatment was conducted in a glass vial filled with 30 mL of sludge and different energy levels were applied. The power was kept constant at 850 W. The irradiation duration was varied between 3 and 30 s, thus leading to a specific energy (SE) input ranging from 10 to 40 MJ/kg TS, calculated according to the formula below.
Table 2 Characterization of the raw sludge.
þ
þ RCOONa/EPS Na þðRCOOÞn M
This loosens the EPS structure and makes it prone to a more direct disrupture when MW is applied. (Ebenezer et al., 2015b) used sodium citrate as deflocculant to remove the EPS of sludge and
Parameter
Value
pH Total Solids (TS) (g TS/kg sludge) Volatile Solids (VS) (g VS/kg sludge) Soluble COD (SCOD) (mg O2/L) Total COD (TCOD) (mg O2/L) Soluble proteins (mg/L) Soluble carbohydrates (mg Glu-eq/L)
7.56 ± 0.02 48.5 ± 0.2 33.3 ± 0.1 2519 ± 122 84,263 ± 1043 17,379 ± 1060 3066 ± 159
Table 1 Summary and comparison of the previous studies on MW treatment. Reference
Treatment method
Results
des et al. (2011) Besze Zhou et al. (2010) Yu et al. (2009) Yu et al. (2010) Ebenezer et al. (2015a) Zhang et al. (2016)
MW MW MW MW MW MW
Methane content increased up to 60%, COD solubility enhanced to a maximum of 57%. SCOD concentration increased 1.8e4.0-fold. EPS concentration (1500e2000 mg/L) and disintegration (1.5e2%) accompanied. Settle ablity increased from 39.58 mm/h to 45.08 mm/h. COD solubilization increased 28%, and reduction in SS increased 38%. The methane production was 338.44 mL CH4/gVS.
pretreatment (5 W/g and 30 min) treatment (700 W and 9min) treatment (900 W and 60s) treatment (900 W and 60s) pretreatment (14,000 kJ/kg TS) pretreatment (600 We100 C)
L. Peng et al. / Journal of Environmental Management 213 (2018) 271e278
SE ¼
Pt V TS
with P ¼ power [W], t ¼ duration of the treatment [s], V ¼ volume of the treated sludge [kg], TS ¼ TS content of the sludge [g TS/kg sludge].
supernatant obtained was regarded as the loosely bound EPS (LBEPS). For the tightly bound EPS (TB-EPS), the sludge pellet was diluted to 30 ml with 0.05% NaCl solution and re-suspended, and placed in a water bath at 60 C for 30 min. After centrifugation at 8000g for 15 min at 4 C, the supernatant collected was the TB-EPS fraction.
2.4. Biochemical methane potential assay
3. Results and discussion
The biochemical methane potential (BMP) assay was carried out in 24 batch reactors (volume of 1 L) at a constant temperature of 37 C, and a retention time of 30 days. Table 3 depicts the set-up of the BMP assay experiments, with each experiment conducted in triplicate. Prior to the start of the digestion experiment, the reactors were filled with digested sludge that serves as inoculum from the full-scale digester of the WWTP of Antwerpen-Zuid (Belgium). An inoculum:substrate ratio of 2:1 on VS-basis was used for the experiments. The VFA concentration, biogas production and methane content were measured daily.
3.1. Effect of the combined pre-treatment on sewage sludge characteristics
2.5. Analytical methods TS and VS were measured gravimetrically according to the Standard Methods (APHA, 2005). COD was measured both on the total sludge (i.e., total fractions) and on the sludge supernatant (i.e., soluble fractions). The supernatant was obtained by centrifugation of the sludge at 8000 g for 15 min, while maintaining the temperature inside the centrifuge at 4 C to minimize decomposition during sample handling. The degree of disintegration COD (DDCOD) was calculated as follows:
DDSCOD ¼
273
3.1.1. SCOD concentration and degree of disintegration The influence of MWþSC on sludge characteristics was evaluated using the release of SCOD and variation in VFA concentration (see further) of the untreated (control) and pretreated sludge. Fig. 1 depicts the change in SCOD due to the treatment, which represents the sludge solubilization (Ahn et al., 2009; Kuglarz et al., 2013). The first data point of each curve represents the treatment with solely MW radiation. The following data points are the treatments with increasing SC dosage. It is clear that the combination of MW and SC results in a synergistic effect on the COD release and hence solubilization of the sludge organic matter, though the effect is more pronounced in the lower MW-regions, as can be seen in Fig. 2. Table 4 shows the degree of disintegration COD (DDCOD) measured in the sludge supernatant. It is clear that with increasing MW-power, the DDCOD increases quasi linearly, except for the lowest SC-dosage. Furthermore, it can be deduced from the DDCOD values that SC has an extra positive effect on the COD
SCODtreated SCODcontrol 100% TCOD
The theoretical COD of SC was calculated first and then substracted from the total SCOD, since SC is a soluble organic compound. The COD was determined colorimetrically using a digital photometer Nanocolor® 500D (Macherey-Nagel) and Nanocolor® COD 1500 test tubes (Macherey-Nagel). Carbohydrates were analyzed with the Anthrone method, according to Gerhardt et al. (1994), using glucose as the standard. The amount of proteins was determined by measuring total Kjeldahl-nitrogen (TKN), and adopting a correction factor of 6.25. The analysis of volatile fatty acids (VFA) was performed with GC-FID, the methane content was determined with GC-TCD, both based on the methods described by Westerholm et al. (2016). The EPS extraction method was modified according to Luo et al. (2005). 30 mL of sludge was centrifuged at 8000 g for 15 min at 4 C. The supernatant was collected as the slime EPS. The sludge pellet was diluted to 30 ml with a 0.05% NaCl solution at 50 C and immediately re-suspended by vortexing for 1 min. The sludge was subsequently centrifuged again at 8000 g for 10 min at 4 C. The
Fig. 1. Change in SCOD concentration for the different treatment conditions.
Table 3 Set-up of the BMP assay experiments. Experiment
Sodium citrate dosage (g SC/g TS sludge)
Microwave energy input (MJ/kg TS sludge)
Control SC MW10 MW10þSC MW20 MW20þSC MW40 MW40þSC
0 0.11 0 0.11 0 0.11 0 0.11
0 0 10 10 20 20 40 40
BMP ¼ Biochemical Methane Potential; MW ¼ microwave; SC ¼ sodium citrate; Temperature ¼ 37 C; Retention time ¼ 30 days.
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Fig. 2. The EPS release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Table 4 Degree of disintegration (DDCOD) (expressed in %). SC dosage (g/g TS sludge)
Microwave energy input (MJ/kg TS sludge) 0
10
20
40
0 0.05 0.08 0.11 0.14
0 4.5 2.8 3.8 4.3
2.0 4.2 4.9 7.5 8.9
5.1 4.6 8.1 11.4 11.7
9.2 6.1 13.3 14.6 15.0
disintegration, up to a dosage of 0.11 g/g TS. A DDCOD of 11.4% at an MW energy input of 20 MJ/kg TS and SC dosage of 0.11 g/g TS was considered acceptable, although somewhat lower than suggested in literature, and was selected for further investigation. At an elevated energy input however, evaporation of organics occurs (Eskicioglu et al., 2007c), and this may cause the lower increase of SCOD observed in this study. All further tests were hence performed at an SE of 20 MJ/kg TS and an SCdosage of 0.11 g/g TS. The deflocculating action of SC is seen as the reason of the improved breakage of the sludge floc matrix, and the elevated MW energy input further improves the solubilization of this deflocculated organic matter. The treatment thereby increases the cell's permeability and improves the release of EPS. An obvious difference of SCOD release was clearly noted between untreated and SCtreated sludge. On the contrary, studies performed by Kuglarz et al. (2013) and Chang et al. (2011) did not obtain an increased sludge solubilization at elevated MW energy inputs. The present study showed that solubilization was more affected at elevated power inputs, instead of the effect by SC. 3.1.2. EPS, carbohydrates and proteins Next, the shares of proteins and carbohydrates in the EPS were analyzed. The results are depicted in Fig. 3 (proteins) and 4 (carbohydrates). The protein concentration in the TB-EPS decreases from 1934 (11%) to 608 mg/L (3%) as a function of treatment intensity, and again the effect is most pronounced at the highest intensities. Likewise, the proteins in the slime-EPS increases from 11,498 (66%) to 16,380 mg/L (82%). This is not surprising since proteins make out an important fraction of the EPS (Nielsen et al., 1996), and solubilization of the EPS is induced by the treatments.
Hence, solubilized proteins are transferred to the slime-EPS. The results of the carbohydrates are in line with those of the proteins: TB-EPS decreases from 22% to 11% and slime-EPS increases from 46% to 62% by cell lysis, releasing cell wall polysaccharides and proteins (Ahn et al., 2009; Eskicioglu et al., 2007b). This is confirmed by the values of the total concentrations of carbohydrates and proteins, which were elevated for the treatment compared to the control (data shown in Table 5). In contrast to the overall EPS, the TB-EPS fractions of proteins and carbohydrates are also affected by the treatment, albeit less than the other fractions, as can be seen in Fig. 4. Ahn et al. (2009) studied the effect of MW (2.45 GHZ, 700 W) in a SE range of 10e49 MJ/kg TS. They also observed an increase in soluble carbohydrates and soluble proteins, with a factor 11 and 8 respectively, for the highest MW energy input. Based on our experiments, microwave pretreatment at an energy input of 20 MJ/kg TS and 40 MJ/kg TS could be considered as treatment. 3.1.3. Volatile fatty acids (VFA) Volatile fatty acids (VFA) are important intermediary compounds during anaerobic digestion, formed during acidogenesis and further transformed to biogas during the acetogenesis and methanogenesis phase. Even though they are crucial to the process, elevated concentrations may hamper and eventually inhibit the process completely. Generally, concentrations of 7000e10,500 are considered inhibitory (Appels et al., 2008). It is therefore important to assess whether or not the treatments in this study can cause elevated VFA concentrations and to which extent. Fig. 5a,b depicts the variation of total VFA concentration and acetic acid concentration under the various MW energy inputs and SC dosages. It must be noted that the untreated sludge sample already has a VFA concentration of 990 mg/L, which is quite high. A first observation is that acetic acid comprises the larger part of the total VFA in the treated sludge samples, and concentrations as high as 1400 mg/L for the MW20 þ SC0.11 treatment are obtained. Furthermore, a treatment with solely SC (black curve in Fig. 5a) is able to achieve an increase in VFA, whereas treating the sludge with solely MW does not result in any change. Focusing on the combined treatments, it is clear that only the lower MW-regions are inducing an effect on VFA concentrations, especially on the acetic acid concentration. MW/SC at 20 MJ/kg and 0.11 g/g might be optimal for the treatment for leading to the highest VFA concentration.
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Fig. 3. The protein release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Fig. 4. The carbohydrates release of the different EPS fractions (slime-EPS, LB-EPS and TB-EPS) due to the different treatments. The data table shows the absolute values of the different fractions (mg/L).
Table 5 Total concentrations of EPS, carbohydrates and proteins.
Control SC 0.11 MW 10 MJ MW10MJþSC0.11 MW 20 MJ MW20MJþSC0.11 MW 40 MJ MW40MJþSC0.11
Total EPS (mg/L)
Total protein-EPS (mg/L)
Total carbohydrate-EPS (mg/L)
20,446 21,147 18,138 20,857 20,675 23,260 25,158 25,451
17,380 17,846 15,010 17,468 17,569 19,354 20,443 19,912
3067 3300 3128 3389 3105 3906 4715 5538
3.2. Effect of the combined pre-treatment on the anaerobic digestibility of sewage sludge 3.2.1. Methane production (BMP) In order to investigate the influence of organic matter solubilization and elevated VFA concentration on the anaerobic digestibility, batch assays were performed for an SC dosage of 0.11 g/g TS and different MW energy inputs. The results are shown in Fig. 6 and Table 6. It is clear that the increase in biogas yield can be attributed to the difference in the structure of organic material, rather than to the amount of organic matter introduced into the digesters. On the other hand, the solubilization of organic matter
and increase in VFA concentration caused by the pre-treatments did not cause overload or acidification of the process. According to Fig. 6, the treatments can be divided in three groups: (1) Low-intensity treatments: SC0.11 (without MW) and MW10 (without SC). They achieve an increase in methane yield of a factor 1.5: 52.6 and 56.5% respectively. This shows that both the SC and MW have a limited effect on the anaerobic digestibility of sewage sludge. (2) Medium-intensity treatments: MW10 þ SC0.11, MW20 (without SC) and MW40 (without SC). They achieve a 2-fold
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increase in methane yield: 82.2, 93.7 and 107% respectively. Two conclusions can be drawn from these results: (i) a lower MW energy input combined with SC can achieve the same effect on the digestion process as a higher MW energy input without SC (cfr. 82.2% vs. 93.7%); (ii) the degrees of improvement for MW20 and MW40 are nearly the same (cfr. 93.7% vs. 107.0%), indicating that a MW energy input higher than 20 MJ/kg TS is not beneficial for sludge pre-treatment. (3) High-intensity treatments: MW20 þ SC0.11 and MW40 þ SC0.11. Here, a 2.5-fold increase in methane yield was obtained (147.7 and 155.9% respectively), supporting the hypothesis that a MW energy input higher than 20 MJ/kg TS is not beneficial for sludge pre-treatment. Comparing the percentage increase in methane production depicted in Table 6, it is observed that for a given MW energy input, the yield is consistently higher in the digester treated with the combined pre-treatment compared to a sole MW pre-treatment. This proves the synergistic effect of the combination of microwaves and sodium citrate on the anaerobic digestibility of sewage sludge. The methane production of MW20 þ SC0.11 is even higher than that of MW40: 218.88 vs. 182.92 ml/g VS, supporting the beneficial effect of sodium citrate addition. 3.2.2. TS, VS removal Table 7 presents the data on TS and VS removal for both untreated and pre-treated sludge under different levels of energy inputs, with and without SC dosage. The effect on TS removal was mostly noticeable for the MW treatments with SC dosage: an increase from 50.73% up to 59.33% could be observed. A larger difference can be found for the VS removal. Uma et al. (2013) employed similar conditions by treating dairy sludge with a TS content of 11.6 g/L for 12 min (2450 MHz frequency, 900 W). The experiments proved that the reduction of suspended solids in the treated sludge was 14% higher than that in the untreated sludge. Fig. 5. Variation of the concentration of VFA and acetic acid with sodium citrate dosage for different microwave energy input. (a). Variation of the VFA concentration with sodium citrate dosage for different MW energy inputs. (b). Variation of the concentration of acetic acid with sodium citrate dosage for different microwave energy input.
Fig. 6. Cumulative methane production (ml/g VS) for the different treatment conditions.
3.2.3. Economic evaluation A simplified economic analysis is made to compare the cost and benefits from the combined treatments with a business-as-usual scenario. Two treatments, i.e. MW20 þ SC0.11 and MW40 þ SC0.11 were chosen for the analysis. The TS and VS content used for the calculations can be found in Table 2. Table 8 shows the energy content of the extra biogas produced from the treated sludge and the energy needed for these pre-treatments. It is clear that the MW treatments demands a lot more energy than the extra methane produced can provide: MW20 demands the 5-fold in energy, MW40 requires almost 10 times as much energy. The cost for the SC dosage is counterbalanced by the decrease in sludge disposal costs. It is clear that the current energy balance is negative for the studied treatments, even though a large gain in methane yield was achieved. However, for the calculation of the operational costs and benefits of the treatment, the decrease in sludge to be disposed of must be taken into account, as shown in Table 8. It must be noted that a possibly improved sludge dewaterability was not included in this brief analysis, which also an important cost for the WWTP and could have an influence on the final balance. For both combined pre-treatments, there is no positive net gain
Table 6 Comparison of methane production (ml/g VS) and the methane yield increase compared to the control (%). Methane Production
MW 0 MJ/kg TS
MW 10 MJ/kg TS
MW 20 MJ/kg TS
MW 40 MJ/kg TS
SC 0 g/g TS SC 0.11 g/g TS
88.36 () 134.88 (52.6%)
138.28 (56.5%) 161.00 (82.2%)
171.19 (93.7%) 218.88 (147.7%)
182.92 (107.0%) 226.15 (155.9%)
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Table 7 Comparison of TS and VS removal after digestion, for the different treatment conditions (expressed in %).
TS removal VS removal
SC SC SC SC
0 g/g TS 0.11 g/g TS 0 g/g TS 0.11 g/g TS
MW 0 MJ/kg TS
MW 10 MJ/kg TS
MW 20 MJ/kg TS
MW 40 MJ/kg TS
48.90 50.73 56.06 62.94
36.71 50.80 55.94 75.04
41.13 56.85 70.06 73.77
55.22 59.33 76.90 79.73
Table 8 Energy and economic balance.
Energy balance (per ton of sludge) Increase in methane production Energy content of the extra methane* Energy applied Net energy production Cost calculation (per ton of sludge) Energy cost (at 0.08 V/kWh) Cost of SC dosage (at 0.2V/kg) Decrease in TS to be disposed Reduced cost for sludge disposal (at 1.55 V/kg TS**) Net cost
MW20 þ SC0.11
MW40 þ SC0.11
4.4 43.6 269.4 225.8
4.6 45.6 538.9 493.3
m3 kWh kWh kWh
18.1 1.1 3.9 5.9 13.3
39.5 1.1 5.1 7.7 32.9
V V kg V V
* Heating value CH4 ¼ 35.7 MJ/Nm3, ** Sludge disposal cost ¼ 75 V/ton sludge (Houtmeyers et al., 2014).
at this point. They are not attractive from an economic point of view. 4. Conclusions The biodegradability of the sewage sludge was significantly improved through the synergistic treatment with sodium citrate and microwave radiation. It was proven that the pre-treatment leads to a larger degree of solubilization of the sludge organic matter (COD) and furthermore, improves the deflocculation of the different fractions of the sludge EPS. The degree of solubilization of carbohydrates was larger than that of proteins, although the protein concentration is significantly higher than that of carbohydrates. Pre-treating the sludge with the combination of MW and SC, the concentration of VFA is considerably increased, but the BMP batch assays did not show an inhibitory effect of these elevated concentrations. The amenability of sewage sludge for anaerobic digestion was improved by the combined pre-treatment. Although the methane production could be enhanced with up to 155%, the energy needed for the MW treatment transcended the energy gained from the extra methane. Therefore, the combined pretreatment as investigated in this study is, at this point, not yet attractive from an economic point of view. Acknowledgements The authors express their thanks for the support from the National Natural Science Foundation of China (21525625), the National Basic Research Program (973 Program) of China (2014CB745100), the (863) High Technology Project (2013AA020302). References rez-Elvira, S.I., Portela, J.R., Sa nchez-Oneto, J., Nebot, E., 2012. Abelleira, J., Pe Advanced thermal hydrolysis: optimization of a noval thermochemical process to aid sewage sludge treatment. Environ. Sci. Technol. 46, 6158e6166. Ahn, K.H., Park, K.Y., Maeng, S.K., Hwang, J.G., Lee, J.W., Song, K.G., Chai, S., 2002. Ozonation of wastewater sludge for reduction and recycling. Water Sci. Technol. 46, 71e77. Ahn, J.H., Shin, S.G., Hwang, S., 2009. Effect of microwave irradiation on the disintegration and acidogenesis of municipal secondary sludge. Chem. Eng. J. 153,
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