The enhancement for sludge anaerobic digestion and dewaterability were investigated in sludge pretreated by microwave (MW) and its combined processes. The results showed that microwave and its combined processes can efficiently release soluble organic matter and thus enhance anaerobic digestion of sludge. The cumulative methane production in the test of the MW-H2O2-OH (0.2) process was increased by 13.34% compared with that of the control. The MW-H process was effective in improving sludge dewaterability, e.g., the capillary suction time (CST) at only 9.85S.
- anaerobic digestion
- sludge pretreatment
With the rapid development of municipal wastewater treatment, the amount of sewage sludge produced as a byproduct of biological wastewater treatment has increased sharply every year in China, e.g., the amount of dewatered sewage sludge at 80% of moisture content produced was up to 30 million tons in 2013. Owing to paying more attention to municipal wastewater treatment than sewage sludge treatment and disposal in the past, about 80% of the sewage sludge was not disposed of properly (Wang et al. 2010). According to the 12th Five-year Plan of municipal wastewater treatment and reuse in China, the sewage sludge treatment rate will increase from 25 to 80% in 36 key cities. Therefore, the treatment and disposal of sewage sludge has become a major focus of current environmental protection policies in China according to the principle of reduction, stabilization and resource, and thus more and more attention is currently being paid to enhanced sludge dewatering and sludge anaerobic digestion.
Anaerobic digestion and dewatering, which are related to sludge biodegradability and physicochemical characterization, are the main technologies to achieve both sludge reduction and energy recovery. Anaerobic digestion has become a major part of modern wastewater treatment plants which seem to be the most popular stabilization methods in Europe (Kelessidis & Stasinakis 2012). Sludge dewatering is an important step in sludge management because the moisture content of sewage sludge is crucial for transportation, and subsequent treatment and disposal of sludge, such as composting and incineration. In anaerobic digestion, biogas production is limited by hydrolysis of macromolecular organic matter which results in longer sludge retention time (SRT). On the other hand, sludge dewatering is also related to macromolecular organic matter especially extracellular polymeric substances (EPS) due to water being trapped in EPS and its negative charged characterization. Therefore, several pretreated methods (physical, biological and chemical treatment) have been investigated and applied to enhance the release of dissolved organic matter for improving hydrolysis, as well as sludge dewaterability.
Among other methods, microwave (MW) irradiation has attracted much more attention in recent years. Its combination with acid (Xiao et al. 2012a), alkali (Chang et al. 2011; Xiao et al. 2012a), or H2O2 (Wang et al. 2009; Xiao et al. 2012b) has synergetic effects on sludge disintegration, and then affects subsequent anaerobic digestion (Rani et al. 2013) and sludge dewaterability (Peng et al. 2013). Interestingly, it has been found that in several studies of the microwave-H2O2 (MW-H2O2) process for sludge pretreatment (Eskicioglu et al. 2008; Shahriari et al. 2012), the anaerobic digestion of sludge treatment was inhibited because of the excess of H2O2 dosage (>0.5 g H2O2/g total solids (TS)) or byproducts from chemical oxidation in the MW-H2O2 process, although the highest soluble organic matter release occurred. Eskicioglu et al. (2008) found that the methane yield of sludge treated by the MW-H2O2 process was 25% lower than that of the control, which indicates that the excess H2O2 dosage (1 g H2O2/g TS) under MW irradiation of 100 °C decreased the methane production rate. Shahriari et al. (2012) also reported that the anaerobic digestion was inhibited when municipal solid waste was treated by MW-H2O2 (0.66 g H2O2/g TS) at 85 °C. In anaerobic digestion, sludge pretreated by the MW-H2O2 process displayed a long lag phase and its cumulative methane production was usually lower than that of sludge pretreated only by the MW process. The inhibition of anaerobic digestion of the pretreated sludge may be due to residual H2O2 in sludge pretreated by the MW-H2O2 with high H2O2 dosage (>0.5 g H2O2/g TS). Therefore, further study is needed to investigate effects of MW and its combined processes on enhancement of methane production and sludge dewaterability, e.g., H2O2 dosage (<0.3 g H2O2/g TS) in the MW-H2O2 process.
In our previous study, a new H2O2 dosing strategy in the MW-H2O2 process with MW irradiation operated at ambient pressure and lower heating temperature was developed by Wang et al. (2009) to pretreat sludge with a low dosage of H2O2, such as the ratio of H2O2 and total suspended solids (TSS) was decreased from 1.0 to 0.2 with optimization (Xiao et al. 2012b). Therefore the purpose of this study was to investigate the MW-H2O2 process with the new H2O2 dosing strategy as to whether it is effective in enhancing anaerobic digestion and sludge dewatering through comparing it with other methods (MW, MW-H) in terms of sludge solubilization, anaerobic digestion and sludge dewaterability.
MATERIAL AND METHODS
Sludge pretreated by microwave and its combined methods
A laboratory industrial microwave reactor operating at 2,450 MHz, ambient pressure, with a maximum power and temperature of 1,000 W, 100 °C, respectively, was used in this study. The raw thickened waste activated sludge was collected before anaerobic digestion at one municipal wastewater treatment plant in Beijing (Table 1), and passed through an 18-mesh screen. In the MW-acid process (MW-H), raw sludge of 500 mL was adjusted to pH 2.5 by addition of 5 mol/L HCl, and then heated to 100 °C by the MW reactor operated at 600 W. According to the H2O2 dosing strategy (Wang et al. 2009) for reducing the H2O2 dosage (Xiao et al. 2012b), NaOH solution at 5 mol/L was first added to adjust pH of the raw sludge to 10.0, and the sludge was heated to 80 °C by microwave irradiation and the H2O2 (30%) was then dosed at a H2O2/TS ratio of 0.06 and 0.2, respectively, and finally the sludge was continuously heated to 100 °C with microwave irradiation operated at 600 W. After the MW-H and MW-H2O2-OH pretreatments, pH of the sludge was adjusted close to 7.
Analysis of anaerobic biodegradability
The anaerobic degradability of pretreated sludge was determined by batch-mesophilic biochemical methane potential (BMP) assays. The BMP tests were performed by a AMPTS II instrument made by Bioprocess Control Company, Sweden. The 650 mL serums with 400 mL working volume were fed in 400 mL raw sludge as the control, and 280 mL untreated sludge plus 120 mL pretreated sludge as the tests. All of these serums were kept in a water bath at 38 °C.
TS and volatile solids (VS) were measured according to Standard Methods procedure (APHA 2005). Alkalinity was determined by potentiometric titration, with titrating the pH to 3.7. The total chemical oxygen demand (TCOD) concentration of the waste activated sludge and soluble chemical oxygen demand (SCOD) concentration of the filtrate through a 0.45 μm membrane were determined by a DR2800 spectrophotometer (HACH Co., Loveland, CO, USA). The soluble protein (<0.45 μm) concentration was measured by the modified Lowry method (Frolund et al. 1995) using bovine serum albumin as protein standard. Concentration of soluble carbohydrates (<0.45 μm) was measured according to the method of Dubois et al. (1956) with glucose used as the standard. Capillary suction time (CST) was measured by a CST Instrument (Model 304M, Triton Electronics Ltd, Dunmow, UK). Particle size distribution and zeta potential were analyzed by a Malvern Mastersizer 2000 (Malvern Instruments, Worcestershire, UK). Surface charge density was measured according to the colloidal titration method (Jin et al. 2003).
RESULTS AND DISCUSSION
Solubilization of organics in sludge after pretreatment
The disintegration efficiency of microwave and its combined processes was indicated by the solubilization rate of particulate COD in sludge. As shown in Table 1, the sludge pretreated by MW, MW-H, MW-H2O2-OH (0.06) and MW-H2O2-OH (0.2) released 454.12, 328.24, 869.41 and 1,142.35% more of soluble COD, respectively, than the control. The maximum COD solubilization was obtained in sludge pretreated by the MW-H2O2-OH (0.2) process. The SCOD/TCOD ratio after the MW-H2O2-OH (0.2) pretreatment was about 13.9% which was much higher than that in the MW and MW-H pretreatment processes, but slightly lower than the MW-H2O2-OH (0.06) process. The results of protein and carbohydrate solubilization rates under different pretreatment processes were consistent with COD solubilization. The highest concentration of soluble organic matter was also obtained under the MW-H2O2-OH (0.2) pretreatment process. According to the equivalent relationships between COD and substrates as follows: 1.5 g COD/g protein, 1.06 g COD/g carbohydrate (Lu et al. 2012), the soluble protein accounted for 47.43, 35.73, 15.77, 62.84 and 59.13% of the released SCOD for the control, and sludge pretreated by MW, MW-H, MW-H2O2-OH (0.06) and MW-H2O2-OH (0.2) processes, respectively. Accordingly, the soluble carbohydrate accounted for 4.08, 7.73, 8.85, 13.42 and 11.01% of the released SCOD for the control and sludge pretreated by these four processes. This result was in accordance with Wilen's study (Wilen et al. 2003), who reported that protein was the dominant organic compound, constituting more than 43% of the total amount of polymeric materials in the sludge, and carbohydrate constituted only 10–18% of the polymeric materials. The combination of MW, H2O2 and alkaline increased the proportion of protein and carbohydrate in SCOD. Otherwise, the EPS which consisted of 45–55% protein and approximately 10% carbohydrates, and constituted 50–60% of the organic fraction, the cell biomass only make up 2–20% of the organic fraction (Frølund et al. 1996). Therefore, the increased proportion of protein and carbohydrate may mostly derive from the EPS disintegration. These results indicated that the combination of MW, H2O2 and alkaline pretreatment process was more efficient than both the individual MW pretreatment and MW-H process. With the new H2O2 dosing strategy proposed by Wang et al. (2009), the highest COD solubilization rate was obtained but lower H2O2 consumed compared with other studies (Xiao et al. 2012b). This higher COD solubilization with lower H2O2 dosing rate would be in favor of the degradation of organic matters in subsequent mesophilic anaerobic digestion.
Anaerobic biodegradability after pretreatment
The biodegradability of sludge pretreated under different conditions was tested by BMP assays. In activated sludge batch testing, the inoculum to substrate ratio (ISR) is one of the most important parameters, which represents the initial substrate to microorganism ratio. A lower ISR value will result in imbalance of anaerobic digestion because of the VFA accumulation. In this work, the ISR value, which was recommended as 2 by Raposo et al. (2012), was 2.41, 2.19, 2.69, 2.50 and 2.33 for the control and sludge pretreated by MW, MW-H, MW-H2O2-OH (0.06) and MW-H2O2-OH (0.2) processes, respectively. The result of the BMP test represented by cumulative methane production per VSadded indicated that MW and its combined processes enhanced the sludge anaerobic biodegradability to some degree (Figure 1). The test of MW-H2O2-OH (0.2) not only achieved significant enhancement of accumulated methane production which was 13.34% higher than the control, but also produced a higher methane production rate (Figure 2) than the other methods. The enhancement of sludge anaerobic digestion was coincident with the higher soluble organics (SCOD, proteins, carbohydrate) of sludge pretreated by the MW-H2O2-OH (0.2) process compared to the control. Interestingly, this result was extremely different from other studies (Eskicioglu et al. 2008; Shahriari et al. 2012), indicating that a lower H2O2 dosage (<0.3 g H2O2/g TS) in the MW-H2O2 process was effective in enhancing anaerobic sludge digestion. Slight inhibition of anaerobic digestion was observed only in the first 3 days, and then the released soluble COD was metabolized more rapidly than the control and other methods. Therefore, the complete inhibition observed in previous studies was mostly due to the residual H2O2 in sludge pretreated by the MW-H2O2 process. The H2O2 dosing strategy used in this work was proposed by Wang et al. (2009) and thus the H2O2 dosage was optimized by Xiao et al. (2012b). With optimization, the H2O2 dosage was reduced by 80% and the utilization rate was greatly improved by 3.87 times when the H2O2:MLSS (mixed liquor suspended solids) dosage ratio was decreased from 1.0 to 0.2, in which the SCOD release rate was nearly the same with an H2O2 dosage of 0.5.
It was interesting to observe that although sludge pretreated by the MW-H process contained significantly lower soluble organics compared to sludges pretreated by the MW, MW-H2O2-OH (0.06) and MW-H2O2-OH (0.2) processes, the particle fractions were still easily biodegraded with higher accumulated methane production rate than the MW treated sludge. The enhancement of sludge anaerobic digestion by acid pretreatment was also proved in a previous study (Devlin et al. 2011) which achieved 14.3% increase in methane yield compared with the untreated sludge in a semi-continuous digestion experiment. This finding demonstrates that solubilization of particle COD is not the essential precondition to judge the pretreatment effectiveness in anaerobic sludge digestion. The lower soluble organics but with higher biodegradability may be due to the sedimentation of released soluble organics especially protein related to its isoelectric point.
The changes of organic fraction in sludge before and after anaerobic digestion are shown in Table 2. The TS, VS and TCOD were all reduced after 30 days of anaerobic digestion. However, the total organic matter reduction did not show great difference between the control and pretreated sludges, e.g., the VS reduction rates for the control, MW, MW-H, MW-H2O2-OH (0.06) and MW-H2O2-OH (0.2) sludges were 35.67%, 35.46%, 39.03%, 36.68% and 37.26%, respectively. It is possible that bacteria are capable of biodegrading all of the biodegradable organics in such a long SRT as reported in Coelho's study (Coelho et al. 2011). What the function of MW and its combined processes is to accelerate hydrolysis rate in anaerobic digestion. As shown in Table 2, the changes of soluble COD, protein and carbohydrate were significantly different. The biggest reduction of soluble COD, protein and carbohydrate was observed in sludge pretreated by the MW-H2O2-OH (0.2) process, followed by sludge pretreated by the MW-H2O2-OH (0.06) and MW-H processes, although the concentrations of soluble COD and protein were increased after 30 days of anaerobic digestion in the test groups of the control and MW pretreatment. This result showed that the combination of MW, H2O2 and alkaline would result in a more rapid and thorough hydrolysis with remarkable SCOD reduction rather than the SCOD increase because of the continuous hydrolysis during the whole 30 days of anaerobic digestion.
Besides, another study (Tong et al. in preparation) showed that the diversity and abundance of specific functional microflora including acetogenic bacteria and methanogen in anaerobic digestion of sludge were increased in the MW-H2O2-OH (0.06) test compared with the control. This finding indicated that the enhancement of sludge anaerobic digestion by the combination of MW, H2O2 and alkaline was not only caused by the disintegration of sludge floc and release of soluble COD, but also resulted from the changes of diversity and abundance of specific functional microflora.
Dewaterability and physical characteristics of pretreated sludge
Sludge dewaterability was indicated by the CST measurement. As shown in Figure 3, CST values showed great changes under different pretreatment conditions. Compared with the control, the MW-H process greatly improved sludge dewaterability, e.g., its CST at 9.85S, only 19.2% of raw sludge. While in the MW and MW-H2O2-OH methods, the dewaterability of sludge deteriorated tremendously with the CST value at 104.10S, 723.45S and 228.4S, respectively. The significant enhancement of sludge dewaterability by the MW-H process has never been reported before. This result was mostly coincident with the effect of hot-acid hydrolysis process reported in a previous study (Neyens et al. 2003). However, in the hot-acid hydrolysis process, the DS-solid content of the dewatered cake will increase from 22.5% DS (initial untreated) to approximately 70% DS at a high temperature of above 120 °C, pH = 3 and a 60 min reaction time, which took a higher reaction temperature and longer time compared with the MW-H process in this work.
To explain the changes of sludge dewaterability, physical characteristics of raw and pretreated sludge, including zeta potential, surface charge density and particle size distribution were investigated. As shown in Figure 4, the surface electrical property characterized by zeta potential and surface charge density was minimized under the MW-H treatment process, while the MW and MW-H2O2-OH treatment processes greatly increased the net negative surface charge. As Neyens et al. (2003) reported, the presence of negative surface charge creates electrostatic repulsion that prevents close contact of sludge particles especially the colloidal fraction (1–100 μm). At the pH range of 2–4, near the isoelectric point of organic matter such as protein, the repulsive interactions are minimized due to the lower net negative surface charge. Therefore, the released organic matter of the sludge pretreated by the MW-H process would flocculate again to form larger particles, as shown in Figure 5. The sludge dewaterability was therefore greatly enhanced under the MW-H pretreatment process.
It is also worth noting that the sludge dewaterability was nearly the same for different treated samples after 30 days’ anaerobic digestion except for the MW-H treated sludge (Figure 3). Its CST value was 28.57S, much lower than that of the control and the other methods. These results indicate that the MW-H process can not only improve the sludge biodegradability, but also is an efficient method for improving sludge dewaterability.
(1) A lower dosage (0.2 g H2O2/g TS) of H2O2 in the MW-H2O2 process significantly enhanced methane production compared with the control and other methods. This result was related to a more rapid and thorough hydrolysis with remarkable SCOD, soluble protein and carbohydrate reduction after 30 days’ anaerobic digestion.
(2) The MW-H process enhanced sludge dewaterability, compared with the deteriorated sludge dewaterability by the MW and MW-H2O2-OH processes, and it still worked even after anaerobic digestion. The released organic matter of the sludge pretreated by the MW-H would flocculate again to form larger particles due to the minimization of the repulsive interaction at the pH close to the isoelectric point.
- First received 1 December 2014.
- Accepted in revised form 17 December 2014.
- © IWA Publishing 2015