- Email: [email protected]
Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization
Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization Wei Fang a, Panyue Zhang a,⇑, Guangming Zhang b,c, Shuguang Jin a, Dongyi Li a, Meixia Zhang a, Xiangzhe Xu a a b c
Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China School of Environment and Resource, Renmin University of China, Beijing 100872, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, China
h i g h l i g h t s Combined pretreatment of alkaline and HPH significantly enhanced sludge digestion. Alkaline addition greatly improved organic matter degradation. Alkaline addition obviously promoted biogas production. Sludge disintegration degree was a key factor to improve anaerobic digestion.
a r t i c l e
i n f o
Article history: Received 10 January 2014 Received in revised form 9 March 2014 Accepted 12 March 2014 Available online xxxx Keywords: Alkaline pretreatment High pressure homogenization Sludge disintegration Organic degradation Biogas production
a b s t r a c t To improve anaerobic digestion efficiency, combination pretreatment of alkaline and high pressure homogenization was applied to pretreat sewage sludge. Effect of alkaline dosage on anaerobic sludge digestion was investigated in detail. SCOD of sludge supernatant significantly increased with the alkaline dosage increase after the combined pretreatment because of sludge disintegration. Organics were significantly degraded after the anaerobic digestion, and the maximal SCOD, TCOD and VS removal was 73.5%, 61.3% and 43.5%, respectively. Cumulative biogas production, methane content in biogas and biogas production rate obviously increased with the alkaline dosage increase. Considering both the biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. Relationships between biogas production and sludge disintegration showed that the accumulative biogas was mainly enhanced by the sludge disintegration. The methane yield linearly increased with the DDCOD increase as Methane yield (ml/gVS) = 4.66 DDCOD 9.69. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rapid urbanization and population growth in many areas of the world have resulted in an increasing production of sewage sludge in wastewater treatment plants (WWTPs). The sewage sludge usually contains a large amount of organics, heavy metals, harmful insect eggs and so on, which can cause serious environmental issues (Shehu et al., 2012). It’s estimated that more than ⇑ Corresponding author. Address: College of Environmental Science and Engineering, 22 Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing 100083, China. Tel.: +86 15001255497; fax: +86 10 62336900. E-mail addresses: [email protected] (W. Fang), [email protected] (P. Zhang), [email protected] (G. Zhang), [email protected] (S. Jin), lidy@ cnemc.cn (D. Li), [email protected] (M. Zhang), [email protected] (X. Xu).
30 million tons of dewatered sewage sludge with a water content of 80% were generated in China each year and kept increasing (Wang et al., 2010). In addition, the cost of sludge treatment is quite expensive and accounts for 20–60% of the total cost of WWTPs (Luo et al., 2013). Anaerobic digestion is an appropriate technique for sludge treatment and energy production before final sludge disposal, and thus is applied worldwide. The significant increase in sewage sludge production makes anaerobic digestion become more and more important as a stabilization and resourcelization process (Qiao et al., 2013). Anaerobic digestion is a multi-stage process includes hydrolysis, acidification and methane production (Metcalf, 2002). In the hydrolysis step, insoluble organic material and higher molecular compounds such as lipids, polysaccharides, proteins, fats and nucleic acids are transformed into soluble
http://dx.doi.org/10.1016/j.biortech.2014.03.050 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
2
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
organic materials. These smaller molecules are further broken down during the acidogenesis; the final products of this step are acetate, hydrogen and carbon dioxide. These molecules are the precursors of the methanogenesis; in this step, two groups of methanogenic organisms are involved into the methane production; one group split acetate into methane and carbon dioxide, and the second group use hydrogen as electron donor and carbon dioxide as electron acceptor to produce methane. The final obtained biogas consists of methane, carbon dioxide and some trace level gases. However, application of the anaerobic sludge digestion was often limited by a long retention time of 20–30 d and low degradation efficiency. It’s usually believed that the sludge hydrolysis limits the rate and extent of organic degradation in anaerobic sludge digestion. However, the sludge hydrolysis can be accelerated by pretreatment before anaerobic digestion. The pretreatment process can normally penetrate the microorganism wall to release the extracellular and intracellular materials into the supernatant. The most reasonable approach for sludge pretreatment is to break the sludge flocs and cells by chemical, thermal, mechanical, and biological attack. Various sludge pretreatment methods have hence been studied to promote the rate of sludge hydrolysis. Alkaline pretreatment with NaOH has been widely examined and is very effective in terms of damaging cellular substances and solubilizing extracellular polymeric substances (EPS). Both chemical oxygen demand (COD) and total solid (TS) elimination rates increase with the increase in alkaline dosage. However, extreme high pH condition may lead to a low biodegradability of sewage sludge (Penaud et al., 1999). Recently, alkaline pretreatment has often been combined with other pretreatment methods such as ultrasonic, microwave, and ozone treatment to accomplish a higher sludge disintegration degree. High-press homogenization (HPH) is a well-known mechanical method for cell disruption, and is mainly used for the stabilization of food and dairy emulsions (Jacquel et al., 2008; Paquin, 1999). Recently, HPH has been reported as an efficient pretreatment technology improving the rate and extent of sewage sludge degradation in anaerobic digestion (Onyeche et al., 2003; Rai and Rao, 2009; Zhang et al., 2012a). The HPH pretreatment mechanically disintegrates the sewage sludge by a combination of large pressure drop, highly focused turbulent eddies and strong shearing forces (Barjenbruch and Kopplow, 2003; Floury et al., 2004). As a mechanical method, HPH shows several advantages, such as no obvious chemical reaction, high efficiency, simple operation and so on (Camacho et al., 2002; Middelberg et al., 1991). However, HPH is energy-intensive, which may restrict its application (Zhang et al., 2012b). The high energy consumption can be reduced by combining with other pretreatment methods. Though the combination of chemical and mechanical treatments has been reported to improve the anaerobic sludge digestion, studies have rarely focused on the combination of alkaline and HPH treatment. In our previous studies, sludge disintegration model of combined alkaline and HPH treatment was established, showing a synergy effect between alkaline and HPH pretreatments (Zhang et al., 2012b). Besides, the studies showed that the alkaline dosage was the most important operating parameter for the sludge disintegration (Zhang et al., 2012b). The objective of this study was to further investigate the effect of alkaline dosage on the performances of anaerobic sludge digestion with combined pretreatment of alkaline and HPH in detail. The effect of alkaline dosage on anaerobic sludge digestion was evaluated with total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD) and volatile solids (VS) removal, biogas and methane production. The relationship between the sludge disintegration degree of the combined pretreatment and efficiency of anaerobic sludge digestion was analyzed.
2. Methods 2.1. Sewage sludge Sewage sludge used in this study was taken from the secondary sedimentation tank of a municipal WWTP with an A2O process in Beijing, China. The influent of this WWTP includes only domestic sewage from inhabitant equivalents of 2,400,000. The sewage sludge was concentrated by gravity settling and stored at 4 °C for later use. The characteristics of sewage sludge used for subsequent experiments were as follows: SCOD of 525 mg/L; TCOD of 19,562 mg/L; total solids (TS) of 19,530 mg/L; VS of 11,920 mg/L; pH of 6.92; and alkalinity of 258 mg/L (as CaCO3). All values were the average value. 2.2. Combined sludge disintegration For the alkaline pretreatment, NaOH solution with a concentration of 4 mol/L was added to the sewage sludge in a 3 L plastic vessel, and the amount of added NaOH was 0.0, 0.8, 1.2, 1.6 and 2.0 g/ kg-sludge, respectively. The final pH was in the range of 7–12. Then the vessel was put in a stirring apparatus (HZS-HA, Nuoji Inc., China) at a speed of 180 r/min and 28 °C. According to the previous studies, the alkaline pretreatment time was selected as 0.5 h (Zhang et al., 2012b). After the alkaline pretreatment, the sludge was homogenized with a high pressure laboratory homogenizer (JJ-30, Shengtong Inc., China) at a homogenization pressure of 60 MPa based on our previous research (Zhang et al., 2012b). The efficiency of sludge disintegration was evaluated by disintegration degree (DDCOD), which was calculated as Eq. (1) (Bougrier et al., 2005):
DDCOD ð%Þ ¼
SCOD SCOD0 100% TCOD SCOD0
ð1Þ
where SCOD0 is the SCOD of sewage sludge before treatment. 2.3. Anaerobic sludge digestion Batch anaerobic digestion experiments were carried out in two identical plexiglass batch reactors with an effective volume of 5 L. In order to keep the temperature stable at 35 °C for mesophilic anaerobic digestion, the reactors were twined with a heater band, which was connected with a temperature sensor, and then were wrapped with a layer of asbestos. An agitator was used to mix the sludge at a stirring speed of 100 r/min. The digester was flushed with nitrogen gas for 5 min to replace the air, and the pretreated sludge was neutralized to a pH of 7 (±0.2) with 4 mol/L HCl before adding into the reactors. The inocula for anaerobic digestion were obtained from an anaerobic digester in a local WWTP. The average composition ratio of inocula and sewage sludge was 4:6. After the anaerobic digestion system became stable with an alkalinity of 2000–3000 mg/L, a pH of 6.6–7.3 and methane content in biogas above 50%, the pretreated sludge was put into the digesters for anaerobic digestion. The anaerobic digestion period was 15 d. The biogas production was daily recorded by displacement of a HCl solution with a pH value of 3. The methane content in the biogas was measured when the anaerobic digestion was stable. The SCOD, TCOD, VS, pH and alkaline were measured every other day. 2.4. Analysis The TS, VS, VFA and alkalinity were measured according to APHA Standard Methods (Eaton et al., 2005). The COD was measured with a COD meter (CTL-12, Huatong Inc., China). The sludge
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
3
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
sample was centrifuged at 5000 r/min for 30 min with a centrifuge (TGL-20B, Anting Inc., China), and then filtered through a 0.45 lm membrane. The filtrate obtained was used to determine the sludge SCOD. For determining the TCOD, the sludge sample was firstly treated with a NaOH dosage of 0.5 mol/L for 24 h, and then filtered through a 0.45 lm membrane, the COD of filtrate was used as the TCOD (Tiehm et al., 2001). The content of methane in biogas was analyzed with a gas chromatograph (GC-2014, Shimadzu, Japan) and a column carbon molecular sieve (TDX-01, HRBY Inc., China). The temperature for the injection port, column and detector was set at 100, 130 and 150 °C, respectively. Helium was used as the carrier gas at a flow rate of 25 ml/min. The bridge circuit was set at 150 mA. 3. Results and discussion 3.1. Effect of alkaline dosage on sludge disintegration DDCOD is a key parameter to evaluate the release of soluble organics from the sludge solids to the liquid phase. Sewage sludge was effectively disintegrated with the combined pretreatment of alkaline and HPH. Both SCOD and DDCOD rapidly increased with increasing the alkaline dosage. The SCOD and DDCOD almost linearly increased within an alkaline dosage between 0 and 0.04 mol/ L. With an alkaline dosage of 0.04 mol/L, the DDCOD and SCOD reached 51.5% and 10,326 mg/L, and increased by about 26% and 93%, respectively, compared with HPH pretreatment alone. The alkaline pretreatment showed a strong effectiveness for sewage sludge disintegration. However, the increment in SCOD and DDCOD became insignificant with a further increase of alkaline dosage from 0.04 to 0.05 mol/L, and the DDCOD and SCOD were 54.1% and 10,825 mg/L, respectively. After the weaker and more sensitive cells were selectively disrupted (Donsì et al., 2009), no significant sludge disintegration occurred even if the alkaline dosage further increased, which was in accordance with the combined sludge disintegration model from Zhang et al. (2012b). 3.2. Effect of alkaline dosage on anaerobic sludge digestion 3.2.1. Organic removal During the process of anaerobic digestion, there are two parameters affecting the sludge SCOD. On the one hand, organics in the solid phase can be dissolved into the liquid phase under the function of exocellular enzymes; on the other hand, the macromolecular organics in the liquid phase can be hydrolyzed and acidized into micromolecular substances, and further converted into methane and carbon dioxide by microorganisms. As shown in Fig. 1(a), the sludge SCOD did not show obvious change in the first 2 d, and then rapidly decreased, which was similar to the results from Wang et al. (2005). The initial sludge SCOD was higher when the sludge was disintegrated with a higher alkaline dosage, because the higher alkaline addition led to a higher sludge disintegration degree. The sludge SCOD decreased more quickly for the combined pretreatment with a higher alkaline dosage in the further sludge digestion period, because the soluble organic compounds released from the sludge solids were more easily and quickly decomposed. The SCOD reduction considerably slowed down after 6-day digestion for the HPH pretreatment alone, probably due to the low biodegradability of residual SCOD. However, the sludge SCOD showed an obvious reduction after 6-day sludge digestion with the combined pretreatment of alkaline and HPH, especially with an alkaline dosage of 0.04 and 0.05 mol/L. Probably more sludge organics became biodegradable after the combined pretreatment of alkaline and HPH. There was no significant difference in SCOD reduction between the alkaline dosage of 0.04 and 0.05 mol/L.
Fig. 1. Effect of alkaline dosage on sludge SCOD (a) and SCOD decrease rate (b) during anaerobic digestion (homogenization pressure of 60 MPa).
After 14-day anaerobic sludge digestion, the SCOD reduction reached 54%, 59%, 64%, 74% and 73% with the alkaline dosage of 0, 0.02, 0.03, 0.04 and 0.05 mol/L respectively (as shown in Fig. 1(b)). The SCOD reduction increased 19% for the combined pretreatment of alkaline (with 0.04 mol/L NaOH dosage) and HPH, compared with that for HPH pretreatment alone. The SCOD reduction in this study was lower than that with microwave-alkali pretreatment after 20-day digestion (but with a low initial SCOD of 2700 mg/L) (Tyagi and Lo, 2012), but was higher than that with ozone pretreatment (Braguglia et al., 2012). Degradation of organic matters during the anaerobic digestion leads to sludge reduction, which is commonly evaluated by VS removal and TCOD removal (Kim et al., 2003). The VS removal and TCOD removal after anaerobic digestion are shown in Table 1. The VS removal and TCOD removal was about 31% and 43% with the HPH pretreatment alone. When combining alkaline addition with HPH, both VS removal and TCOD removal increased. The higher the alkaline dosage was, the higher the VS removal and TCOD removal reached, which might be caused by the conversion of sludge to low-molecular weight compounds due to alkaline addition. Nah et al. (2000) reported that the low-molecular weight components
Table 1 Effect of alkaline dosage on TCOD removal and VS removal after anaerobic digestion (homogenization pressure of 60 MPa). Alkaline dosage
0
0.02
0.03
0.04
0.05
TCOD removal (%) VS removal (%)
43.28 30.95
47.53 35.83
56.38 39.47
61.25 42.67
60.84 43.52
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
4
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
were more rapidly and easily biodegraded. We concluded from Table 1 that the VS removal and TCOD removal, respectively, increased by 12% and 18%, when the sludge was pretreated with alkaline (with a NaOH dosage of 0.04 mol/L) combined with HPH, showing that the alkaline addition effectively improved the anaerobic biodegradation. Similar VS removal and TCOD removal were found by alkaline-microwave treatment (Dog˘an and Sanin, 2009). Comparing the alkaline dosage of 0.05 mol/L with 0.04 mol/L, the VS removal and TCOD removal showed no obvious improvement for the higher alkaline dosage, indicating that there was a limiting point for the combined pretreatment of alkaline and HPH to improve the anaerobic sludge biodegradation. 3.2.2. Effect of alkaline dosage on biogas production Application of anaerobic sludge digestion is usually limited by low biogas production and long retention time. The biogas production and methane content in biogas for the anaerobic sludge digestion with the combined pretreatment of alkaline and HPH are shown in Fig. 2. As shown in Fig 2(a), the biogas production increased insignificantly in the first three days, followed by a remarkable biogas production increase from day 4th to 10th, and no significant increase was observed with further digestion. After a short adaptation stage for methanogens, the biogas production became stable. The higher biogas yield was observed after 14day anaerobic digestion with the combined pretreatment of alkaline and HPH. The biogas yield increased as the alkaline dosage increased, however there was no significant difference between the biogas production for the alkaline dosage of 0.04 and 0.05 mol/L. As the alkaline dosage increasing from 0.04 to 0.05 mol/L, the
Fig. 2. Effect of alkaline dosage on cumulative biogas production (a) and biogas production rate (b) (homogenization pressure of 60 MPa).
sludge disintegration degree had no obvious variation, which meant that there were no more soluble organics transformed into the sludge liquid. On the other hand, the high alkaline dosage might inhibit the activity of microorganisms, and more Na+ concentration might produce sodium toxicity to methanogens (Chen et al., 2008; Feijoo et al., 1995; Wu et al., 2006). After the anaerobic digestion for 14 d, the cumulated biogas increased by about 60% for the combined pretreatment of alkaline (dosage of 0.04 mol/L) and HPH, compared with that for the HPH pretreatment alone. The alkaline pretreatment improved not only the organic solubilization, but also the surface area available for enzymatic action, which led to improvement of anaerobic digestion performances. More released soluble and low-molecular weight components were biodegraded and transformed into biogas with the combined pretreatment. Other authors also summarized that the biogas production increased with the increase in alkaline dosage when the sludge was pretreated with alkaline (Lin et al., 1997; Lopez Torres et al., 2008). The maximum cumulative biogas production of 5059 ml with an alkaline dosage of 0.05 mol/L and a homogenization pressure of 60 MPa was achieved on the 12th day, and there was no significant increase after 12-day sludge digestion, which suggested that the suitable time for methane production was 12 d. The suitable sludge digestion time was shorter than that for the alkaline treatment alone and microwave–alkaline pretreatment (Shao et al., 2012; Tyagi and Lo, 2012). Considering both biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. Methane content in the biogas was improved by the combined pretreatment of alkaline and HPH. For the anaerobic sludge digestion with the HPH pretreatment alone, the methane content in biogas was about 58%, and a methane content of about 68% was achieved for the sludge digestion with the combined pretreatment of alkaline (with a NaOH dosage of 0.04 mol/L) and HPH. Sludge solubilization and anaerobic biodegradability increased with combined pretreatment. What’s more, the low-molecular weight and soluble components were easily utilized by methanogens and further transformed into the methane (Ghose et al., 1979). Therefore, the methane production was higher than HPH pretreatment alone. The biogas production rate during anaerobic digestion is shown in Fig. 2(b), which indicated that the anaerobic sludge digestion was greatly accelerated by the combined pretreatment of alkaline and HPH. All biogas production rates reached the maximum on the day 5th, and then decreased. The biogas production rate increased with the alkaline dosage increase. The maximum biogas production rate for the HPH pretreatment alone was 735 ml/d, and increased to 1105 ml/d for the combined pretreatment of alkaline (with a NaOH dosage of 0.04 mol/L) and HPH, which was approximately improved by 50%. The significant increase in biogas production rate might shorten the digestion period. The biogas production rate insignificantly increased when the alkaline dosage was higher than 0.04 mol/L, agreeing with the organic substance degradation shown in Table 1. 3.2.3. pH value and alkalinity of sludge system Sludge pH is one of the most influence factors in anaerobic digestion. The fermentative microorganisms can function in a wider pH range between 5.0 and 8.5, while the pH range for methanogens is relatively narrow, and should be maintained in the range of 6.5–7.8. When the acid-producing bacteria and methanogens can coexist in a single-phase anaerobic digestion system, the most appropriate pH range is 7.0–7.6 (Hwang et al., 2004). The pH value in this study remained within the range from 6.8 to 8.0 during anaerobic digestion. The pH changes with different alkaline dosages were similar; dropping within the first 3 d and then slowly rising in the further digestion period. In the beginning digestion
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
period, pH might drop with VFA accumulation until the VFA began to be consumed for methane production. Compared with the alkaline treatment alone (Shao et al., 2012), we found that the combined pretreatment of alkaline and HPH showed a shorter acidification time, which was beneficial by shortening the whole digestion period. The pH value is not an effective parameter evaluating the stability of anaerobic process when the sludge system has a high buffering capacity (Björnsson et al., 2000). Alkalinity can be used to reflect the process performance directly. The alkalinity of a steady system is between 2000 and 5000 mg/L (as CaCO3). When alkalinity was lower than 1000 mg/L, the pH value in the bioreactors would decrease (Lin et al., 1997). In this study, all the alkalinity maintained stable within a rational range of 3000–5000 mg/L (as CaCO3). The sludge with the combined pretreatment of alkaline and HPH showed a higher alkalinity compared with that with HPH pretreatment alone due to the alkaline addition, which may keep the system more stable.
3.3. Relationship between sludge disintegration and anaerobic sludge digestion From Fig. 3(a) it can be seen that the cumulative biogas production showed a positive relationship with the DDCOD. The cumulative biogas production linearly increased with the increase in DDCOD, which indicated that the organic components released from sludge were biodegradable and converted to the biogas. The methane content in biogas with the combined pretreatment was higher than that with the HPH pretreatment alone. The methane content in biogas increased by 10% when DDCOD reached 51.5% (with addition of 0.04 mol/L alkaline). It was known that different organic components were released with different DDCOD due to the
5
selective sludge disruption (Donsì et al., 2009), so the methane content in biogas decreased when the DDCOD was higher than 51%. The linear relationship between the cumulative biogas production, methane content in biogas and DDCOD were obtained as Eqs. (2) and (3):
Cumulative biogas production ðmlÞ ¼ 71:10 DDCOD þ 1177
ð2Þ
Methane content in biogas ð%Þ ¼ 0:3263 DDCOD þ 50:71
ð3Þ
From Fig. 3(b) it can be seen that the biogas yield and methane yield per gram of VS biodegraded were improved with increasing the alkaline dosage. Within the experimental DDCOD range, the biogas and methane yield linearly increased with the increase of DDCOD, and the linear relationships were expressed with Eqs. (4) and (5). The organics released from the sludge solids were more easily degraded and converted to methane by microorganism with the DDCOD increase due to alkaline addition, because higher VS was removed with the higher alkaline addition (as shown in Table 1). The combined pretreatment of alkaline and HPH was an effective method for mesophilic anaerobic sludge digestion. When the alkaline dosage was 0.05 mol/L, the biogas and methane yield reached 406 ml/gVS and 247 ml/gVS, which was about 47% and 107% higher than that with HPH pretreatment alone, respectively.
Biogas yield ðml=gVSÞ ¼ 4:51 DDCOD þ 153:76
ð4Þ
Methane yield ðml=gVSÞ ¼ 4:66 DDCOD 9:69
ð5Þ
4. Conclusion SCOD in sludge supernatant with combined pretreatment of alkaline and high pressure homogenization (HPH) was significantly higher than that with HPH pretreatment alone. After anaerobic digestion, SCOD, TCOD and VS removal increased, biogas production process accelerated, and methane content in biogas increased with the combined pretreatment, because the organic components released from cells might be more biodegradable. Higher alkaline dosage within 0–0.05 mol/L showed greater enhancement on the sludge digestion. Considering both biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. The sludge disintegration degree was a key factor to improve the anaerobic sludge digestion efficiency. Acknowledgements This research was funded by the China–Israel Joint Research Program of MOST of China, National Natural Science Foundation of China (51178047 and 51278489), and Key Laboratory for Solid Waste Management and Environment Safety (SWMES). References
Fig. 3. Cumulative biogas production and methane content in biogas (a), biogas yield and methane yield (b) with DDCOD increase.
Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pretreatment of surplus sludge. Adv. Environ. Res. 7, 715–720. Björnsson, L., Murto, M., Mattiasson, B., 2000. Evaluation of parameters for monitoring an anaerobic co-digestion process. Appl. Microbiol. Biotechnol. 54, 844–849. Bougrier, C., Carrere, H., Delgenes, J., 2005. Solubilisation of waste-activated sludge by ultrasonic treatment. Chem. Eng. J. 106, 163–169. Braguglia, C., Gianico, A., Mininni, G., 2012. Comparison between ozone and ultrasound disintegration on sludge anaerobic digestion. J. Environ. Manag. 95, S139–S143. Camacho, P., Déléris, S., Geaugey, V., Ginestet, P., Paul, E., 2002. A comparative study between mechanical, thermal and oxidative disintegration techniques of waste activated sludge. Water Sci. Technol. 46, 79–87. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Dog˘an, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43, 2139–2148.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
6
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
Donsì, F., Ferrari, G., Lenza, E., Maresca, P., 2009. Main factors regulating microbial inactivation by high-pressure homogenization: operating parameters and scale of operation. Chem. Eng. Sci. 64, 520–532. Eaton, A.D., Clesceri, L.S., Rice, E.W., Greenberg, A.E., 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American Water Works Association & Water Environment Federation, Washington, DC, USA. Feijoo, G., Soto, M., Méndez, R., Lema, J.M., 1995. Sodium inhibition in the anaerobic digestion process: antagonism and adaptation phenomena. Enzyme Microb. Technol. 17, 180–188. Floury, J., Bellettre, J., Legrand, J., Desrumaux, A., 2004. Analysis of a new type of high pressure homogeniser. A study of the flow pattern. Chem. Eng. Sci. 59, 843–853. Ghose, T., Singh, A., Mukhopadhyay, S., 1979. Increased methane production in biogas. Biotechnol. Lett. 1, 275–280. Hwang, M.H., Jang, N.J., Hyun, S.H., Kim, I.S., 2004. Anaerobic bio-hydrogen production from ethanol fermentation: the role of pH. J. Biotechnol. 111, 297–309. Jacquel, N., Lo, C.W., Wei, Y.H., Wu, H.S., Wang, S.S., 2008. Isolation and purification of bacterial poly (3-hydroxyalkanoates). Biochem. Eng. J. 39, 15–27. Kim, J., Park, C., Kim, T.H., Lee, M., Kim, S., Kim, S.W., Lee, J., 2003. Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95, 271–275. Lin, J.G., Chang, C.N., Chang, S.C., 1997. Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol. 62, 85–90. Lopez Torres, M., Espinosa Lloréns, M.d.C., 2008. Effect of alkaline pretreatment on anaerobic digestion of solid wastes. Waste Manag. 28, 2229–2234. Luo, K., Ye, Q., Yi, X., Yang, Q., Li, X.M., Chen, H.B., Liu, X., Zeng, G.M., 2013. Hydrolysis and acidification of waste-activated sludge in the presence of biosurfactant rhamnolipid: effect of pH. Appl. Microb. Biotechnol. 97, 5597– 5604. Metcalf, E., 2002. Wastewater Engineering: Treatment and Reuse. McGraw-Hill, New York. Middelberg, A.P., O’Neill, B.K., Bogle, L., David, I., Snoswell, M.A., 1991. A novel technique for the measurement of disruption in high-pressure homogenization: studies on E. coli containing recombinant inclusion bodies. Biotechnol. Bioeng. 38, 363–370. Nah, I.W., Kang, Y.W., Hwang, K.Y., Song, W.K., 2000. Mechanical pretreatment of waste activated sludge for anaerobic digestion process. Water Res. 34, 2362– 2368.
Onyeche, T., Schläfer, O., Sievers, M., 2003. Advanced anaerobic digestion of sludge through high pressure homogenisation. J. Solid Waste Technol. Manag. 29, 56– 61. Paquin, P., 1999. Technological properties of high pressure homogenizers: the effect of fat globules, milk proteins, and polysaccharides. Int. Dairy J. 9, 329–335. Penaud, V., Delgenes, J., Moletta, R., 1999. Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability. Enzyme Microb. Technol. 25, 258–263. Qiao, W., Takayanagi, K., Shofie, M., Niu, Q., Yu, H.Q., Li, Y.Y., 2013. Thermophilic anaerobic digestion of coffee grounds with and without waste activated sludge as co-substrate using a submerged AnMBR: system amendments and membrane performance. Bioresour. Technol. 150, 249–258. Rai, C.L., Rao, P.G., 2009. Influence of sludge disintegration by high pressure homogenizer on microbial growth in sewage sludge: an approach for excess sludge reduction. Clean Technol. Environ. Policy 11, 437–446. Shao, L., Wang, X., Xu, H., He, P., 2012. Enhanced anaerobic digestion and sludge dewaterability by alkaline pretreatment and its mechanism. J. Environ. Sci. 24, 1731–1738. Shehu, M.S., Abdul Manan, Z., Wan Alwi, S.R., 2012. Optimization of thermo-alkaline disintegration of sewage sludge for enhanced biogas yield. Bioresour. Technol. 114, 69–74. Tiehm, A., Nickel, K., Zellhorn, M., Neis, U., 2001. Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization. Water Res. 35, 2003–2009. Tyagi, V.K., Lo, S.L., 2012. Enhancement in mesophilic aerobic digestion of waste activated sludge by chemically assisted thermal pretreatment method. Bioresour. Technol. 119, 105–113. Wang, J., Zhang, H., Stabnikova, O., Tay, J., 2005. Comparison of lab-scale and pilotscale hybrid anaerobic solid–liquid systems operated in batch and semicontinuous modes. Process Biochem. 40, 3580–3586. Wang, W., Luo, Y., Qiao, W., 2010. Possible solutions for sludge dewatering in China. Front. Environ. Sci. Eng. China 4, 102–107. Wu, J., Xu, L., Xie, J., 2006. The effect of alkali-pretreatment on anaerobic digestion of solid organic waste. Acta Sci. Circumst. 26 (2), 252–255 (in Chinese). Zhang, S., Zhang, P., Zhang, G., Fan, J., Zhang, Y., 2012a. Enhancement of anaerobic sludge digestion by high-pressure homogenization. Bioresour. Technol. 118, 496–501. Zhang, Y., Zhang, P., Zhang, G., Ma, W., Wu, H., Ma, B., 2012b. Sewage sludge disintegration by combined treatment of alkaline + high pressure homogenization. Bioresour. Technol. 123, 514–519.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization Wei Fang a, Panyue Zhang a,⇑, Guangming Zhang b,c, Shuguang Jin a, Dongyi Li a, Meixia Zhang a, Xiangzhe Xu a a b c
Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China School of Environment and Resource, Renmin University of China, Beijing 100872, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, China
h i g h l i g h t s Combined pretreatment of alkaline and HPH significantly enhanced sludge digestion. Alkaline addition greatly improved organic matter degradation. Alkaline addition obviously promoted biogas production. Sludge disintegration degree was a key factor to improve anaerobic digestion.
a r t i c l e
i n f o
Article history: Received 10 January 2014 Received in revised form 9 March 2014 Accepted 12 March 2014 Available online xxxx Keywords: Alkaline pretreatment High pressure homogenization Sludge disintegration Organic degradation Biogas production
a b s t r a c t To improve anaerobic digestion efficiency, combination pretreatment of alkaline and high pressure homogenization was applied to pretreat sewage sludge. Effect of alkaline dosage on anaerobic sludge digestion was investigated in detail. SCOD of sludge supernatant significantly increased with the alkaline dosage increase after the combined pretreatment because of sludge disintegration. Organics were significantly degraded after the anaerobic digestion, and the maximal SCOD, TCOD and VS removal was 73.5%, 61.3% and 43.5%, respectively. Cumulative biogas production, methane content in biogas and biogas production rate obviously increased with the alkaline dosage increase. Considering both the biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. Relationships between biogas production and sludge disintegration showed that the accumulative biogas was mainly enhanced by the sludge disintegration. The methane yield linearly increased with the DDCOD increase as Methane yield (ml/gVS) = 4.66 DDCOD 9.69. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rapid urbanization and population growth in many areas of the world have resulted in an increasing production of sewage sludge in wastewater treatment plants (WWTPs). The sewage sludge usually contains a large amount of organics, heavy metals, harmful insect eggs and so on, which can cause serious environmental issues (Shehu et al., 2012). It’s estimated that more than ⇑ Corresponding author. Address: College of Environmental Science and Engineering, 22 Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing 100083, China. Tel.: +86 15001255497; fax: +86 10 62336900. E-mail addresses: [email protected] (W. Fang), [email protected] (P. Zhang), [email protected] (G. Zhang), [email protected] (S. Jin), lidy@ cnemc.cn (D. Li), [email protected] (M. Zhang), [email protected] (X. Xu).
30 million tons of dewatered sewage sludge with a water content of 80% were generated in China each year and kept increasing (Wang et al., 2010). In addition, the cost of sludge treatment is quite expensive and accounts for 20–60% of the total cost of WWTPs (Luo et al., 2013). Anaerobic digestion is an appropriate technique for sludge treatment and energy production before final sludge disposal, and thus is applied worldwide. The significant increase in sewage sludge production makes anaerobic digestion become more and more important as a stabilization and resourcelization process (Qiao et al., 2013). Anaerobic digestion is a multi-stage process includes hydrolysis, acidification and methane production (Metcalf, 2002). In the hydrolysis step, insoluble organic material and higher molecular compounds such as lipids, polysaccharides, proteins, fats and nucleic acids are transformed into soluble
http://dx.doi.org/10.1016/j.biortech.2014.03.050 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
2
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
organic materials. These smaller molecules are further broken down during the acidogenesis; the final products of this step are acetate, hydrogen and carbon dioxide. These molecules are the precursors of the methanogenesis; in this step, two groups of methanogenic organisms are involved into the methane production; one group split acetate into methane and carbon dioxide, and the second group use hydrogen as electron donor and carbon dioxide as electron acceptor to produce methane. The final obtained biogas consists of methane, carbon dioxide and some trace level gases. However, application of the anaerobic sludge digestion was often limited by a long retention time of 20–30 d and low degradation efficiency. It’s usually believed that the sludge hydrolysis limits the rate and extent of organic degradation in anaerobic sludge digestion. However, the sludge hydrolysis can be accelerated by pretreatment before anaerobic digestion. The pretreatment process can normally penetrate the microorganism wall to release the extracellular and intracellular materials into the supernatant. The most reasonable approach for sludge pretreatment is to break the sludge flocs and cells by chemical, thermal, mechanical, and biological attack. Various sludge pretreatment methods have hence been studied to promote the rate of sludge hydrolysis. Alkaline pretreatment with NaOH has been widely examined and is very effective in terms of damaging cellular substances and solubilizing extracellular polymeric substances (EPS). Both chemical oxygen demand (COD) and total solid (TS) elimination rates increase with the increase in alkaline dosage. However, extreme high pH condition may lead to a low biodegradability of sewage sludge (Penaud et al., 1999). Recently, alkaline pretreatment has often been combined with other pretreatment methods such as ultrasonic, microwave, and ozone treatment to accomplish a higher sludge disintegration degree. High-press homogenization (HPH) is a well-known mechanical method for cell disruption, and is mainly used for the stabilization of food and dairy emulsions (Jacquel et al., 2008; Paquin, 1999). Recently, HPH has been reported as an efficient pretreatment technology improving the rate and extent of sewage sludge degradation in anaerobic digestion (Onyeche et al., 2003; Rai and Rao, 2009; Zhang et al., 2012a). The HPH pretreatment mechanically disintegrates the sewage sludge by a combination of large pressure drop, highly focused turbulent eddies and strong shearing forces (Barjenbruch and Kopplow, 2003; Floury et al., 2004). As a mechanical method, HPH shows several advantages, such as no obvious chemical reaction, high efficiency, simple operation and so on (Camacho et al., 2002; Middelberg et al., 1991). However, HPH is energy-intensive, which may restrict its application (Zhang et al., 2012b). The high energy consumption can be reduced by combining with other pretreatment methods. Though the combination of chemical and mechanical treatments has been reported to improve the anaerobic sludge digestion, studies have rarely focused on the combination of alkaline and HPH treatment. In our previous studies, sludge disintegration model of combined alkaline and HPH treatment was established, showing a synergy effect between alkaline and HPH pretreatments (Zhang et al., 2012b). Besides, the studies showed that the alkaline dosage was the most important operating parameter for the sludge disintegration (Zhang et al., 2012b). The objective of this study was to further investigate the effect of alkaline dosage on the performances of anaerobic sludge digestion with combined pretreatment of alkaline and HPH in detail. The effect of alkaline dosage on anaerobic sludge digestion was evaluated with total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD) and volatile solids (VS) removal, biogas and methane production. The relationship between the sludge disintegration degree of the combined pretreatment and efficiency of anaerobic sludge digestion was analyzed.
2. Methods 2.1. Sewage sludge Sewage sludge used in this study was taken from the secondary sedimentation tank of a municipal WWTP with an A2O process in Beijing, China. The influent of this WWTP includes only domestic sewage from inhabitant equivalents of 2,400,000. The sewage sludge was concentrated by gravity settling and stored at 4 °C for later use. The characteristics of sewage sludge used for subsequent experiments were as follows: SCOD of 525 mg/L; TCOD of 19,562 mg/L; total solids (TS) of 19,530 mg/L; VS of 11,920 mg/L; pH of 6.92; and alkalinity of 258 mg/L (as CaCO3). All values were the average value. 2.2. Combined sludge disintegration For the alkaline pretreatment, NaOH solution with a concentration of 4 mol/L was added to the sewage sludge in a 3 L plastic vessel, and the amount of added NaOH was 0.0, 0.8, 1.2, 1.6 and 2.0 g/ kg-sludge, respectively. The final pH was in the range of 7–12. Then the vessel was put in a stirring apparatus (HZS-HA, Nuoji Inc., China) at a speed of 180 r/min and 28 °C. According to the previous studies, the alkaline pretreatment time was selected as 0.5 h (Zhang et al., 2012b). After the alkaline pretreatment, the sludge was homogenized with a high pressure laboratory homogenizer (JJ-30, Shengtong Inc., China) at a homogenization pressure of 60 MPa based on our previous research (Zhang et al., 2012b). The efficiency of sludge disintegration was evaluated by disintegration degree (DDCOD), which was calculated as Eq. (1) (Bougrier et al., 2005):
DDCOD ð%Þ ¼
SCOD SCOD0 100% TCOD SCOD0
ð1Þ
where SCOD0 is the SCOD of sewage sludge before treatment. 2.3. Anaerobic sludge digestion Batch anaerobic digestion experiments were carried out in two identical plexiglass batch reactors with an effective volume of 5 L. In order to keep the temperature stable at 35 °C for mesophilic anaerobic digestion, the reactors were twined with a heater band, which was connected with a temperature sensor, and then were wrapped with a layer of asbestos. An agitator was used to mix the sludge at a stirring speed of 100 r/min. The digester was flushed with nitrogen gas for 5 min to replace the air, and the pretreated sludge was neutralized to a pH of 7 (±0.2) with 4 mol/L HCl before adding into the reactors. The inocula for anaerobic digestion were obtained from an anaerobic digester in a local WWTP. The average composition ratio of inocula and sewage sludge was 4:6. After the anaerobic digestion system became stable with an alkalinity of 2000–3000 mg/L, a pH of 6.6–7.3 and methane content in biogas above 50%, the pretreated sludge was put into the digesters for anaerobic digestion. The anaerobic digestion period was 15 d. The biogas production was daily recorded by displacement of a HCl solution with a pH value of 3. The methane content in the biogas was measured when the anaerobic digestion was stable. The SCOD, TCOD, VS, pH and alkaline were measured every other day. 2.4. Analysis The TS, VS, VFA and alkalinity were measured according to APHA Standard Methods (Eaton et al., 2005). The COD was measured with a COD meter (CTL-12, Huatong Inc., China). The sludge
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
3
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
sample was centrifuged at 5000 r/min for 30 min with a centrifuge (TGL-20B, Anting Inc., China), and then filtered through a 0.45 lm membrane. The filtrate obtained was used to determine the sludge SCOD. For determining the TCOD, the sludge sample was firstly treated with a NaOH dosage of 0.5 mol/L for 24 h, and then filtered through a 0.45 lm membrane, the COD of filtrate was used as the TCOD (Tiehm et al., 2001). The content of methane in biogas was analyzed with a gas chromatograph (GC-2014, Shimadzu, Japan) and a column carbon molecular sieve (TDX-01, HRBY Inc., China). The temperature for the injection port, column and detector was set at 100, 130 and 150 °C, respectively. Helium was used as the carrier gas at a flow rate of 25 ml/min. The bridge circuit was set at 150 mA. 3. Results and discussion 3.1. Effect of alkaline dosage on sludge disintegration DDCOD is a key parameter to evaluate the release of soluble organics from the sludge solids to the liquid phase. Sewage sludge was effectively disintegrated with the combined pretreatment of alkaline and HPH. Both SCOD and DDCOD rapidly increased with increasing the alkaline dosage. The SCOD and DDCOD almost linearly increased within an alkaline dosage between 0 and 0.04 mol/ L. With an alkaline dosage of 0.04 mol/L, the DDCOD and SCOD reached 51.5% and 10,326 mg/L, and increased by about 26% and 93%, respectively, compared with HPH pretreatment alone. The alkaline pretreatment showed a strong effectiveness for sewage sludge disintegration. However, the increment in SCOD and DDCOD became insignificant with a further increase of alkaline dosage from 0.04 to 0.05 mol/L, and the DDCOD and SCOD were 54.1% and 10,825 mg/L, respectively. After the weaker and more sensitive cells were selectively disrupted (Donsì et al., 2009), no significant sludge disintegration occurred even if the alkaline dosage further increased, which was in accordance with the combined sludge disintegration model from Zhang et al. (2012b). 3.2. Effect of alkaline dosage on anaerobic sludge digestion 3.2.1. Organic removal During the process of anaerobic digestion, there are two parameters affecting the sludge SCOD. On the one hand, organics in the solid phase can be dissolved into the liquid phase under the function of exocellular enzymes; on the other hand, the macromolecular organics in the liquid phase can be hydrolyzed and acidized into micromolecular substances, and further converted into methane and carbon dioxide by microorganisms. As shown in Fig. 1(a), the sludge SCOD did not show obvious change in the first 2 d, and then rapidly decreased, which was similar to the results from Wang et al. (2005). The initial sludge SCOD was higher when the sludge was disintegrated with a higher alkaline dosage, because the higher alkaline addition led to a higher sludge disintegration degree. The sludge SCOD decreased more quickly for the combined pretreatment with a higher alkaline dosage in the further sludge digestion period, because the soluble organic compounds released from the sludge solids were more easily and quickly decomposed. The SCOD reduction considerably slowed down after 6-day digestion for the HPH pretreatment alone, probably due to the low biodegradability of residual SCOD. However, the sludge SCOD showed an obvious reduction after 6-day sludge digestion with the combined pretreatment of alkaline and HPH, especially with an alkaline dosage of 0.04 and 0.05 mol/L. Probably more sludge organics became biodegradable after the combined pretreatment of alkaline and HPH. There was no significant difference in SCOD reduction between the alkaline dosage of 0.04 and 0.05 mol/L.
Fig. 1. Effect of alkaline dosage on sludge SCOD (a) and SCOD decrease rate (b) during anaerobic digestion (homogenization pressure of 60 MPa).
After 14-day anaerobic sludge digestion, the SCOD reduction reached 54%, 59%, 64%, 74% and 73% with the alkaline dosage of 0, 0.02, 0.03, 0.04 and 0.05 mol/L respectively (as shown in Fig. 1(b)). The SCOD reduction increased 19% for the combined pretreatment of alkaline (with 0.04 mol/L NaOH dosage) and HPH, compared with that for HPH pretreatment alone. The SCOD reduction in this study was lower than that with microwave-alkali pretreatment after 20-day digestion (but with a low initial SCOD of 2700 mg/L) (Tyagi and Lo, 2012), but was higher than that with ozone pretreatment (Braguglia et al., 2012). Degradation of organic matters during the anaerobic digestion leads to sludge reduction, which is commonly evaluated by VS removal and TCOD removal (Kim et al., 2003). The VS removal and TCOD removal after anaerobic digestion are shown in Table 1. The VS removal and TCOD removal was about 31% and 43% with the HPH pretreatment alone. When combining alkaline addition with HPH, both VS removal and TCOD removal increased. The higher the alkaline dosage was, the higher the VS removal and TCOD removal reached, which might be caused by the conversion of sludge to low-molecular weight compounds due to alkaline addition. Nah et al. (2000) reported that the low-molecular weight components
Table 1 Effect of alkaline dosage on TCOD removal and VS removal after anaerobic digestion (homogenization pressure of 60 MPa). Alkaline dosage
0
0.02
0.03
0.04
0.05
TCOD removal (%) VS removal (%)
43.28 30.95
47.53 35.83
56.38 39.47
61.25 42.67
60.84 43.52
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
4
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
were more rapidly and easily biodegraded. We concluded from Table 1 that the VS removal and TCOD removal, respectively, increased by 12% and 18%, when the sludge was pretreated with alkaline (with a NaOH dosage of 0.04 mol/L) combined with HPH, showing that the alkaline addition effectively improved the anaerobic biodegradation. Similar VS removal and TCOD removal were found by alkaline-microwave treatment (Dog˘an and Sanin, 2009). Comparing the alkaline dosage of 0.05 mol/L with 0.04 mol/L, the VS removal and TCOD removal showed no obvious improvement for the higher alkaline dosage, indicating that there was a limiting point for the combined pretreatment of alkaline and HPH to improve the anaerobic sludge biodegradation. 3.2.2. Effect of alkaline dosage on biogas production Application of anaerobic sludge digestion is usually limited by low biogas production and long retention time. The biogas production and methane content in biogas for the anaerobic sludge digestion with the combined pretreatment of alkaline and HPH are shown in Fig. 2. As shown in Fig 2(a), the biogas production increased insignificantly in the first three days, followed by a remarkable biogas production increase from day 4th to 10th, and no significant increase was observed with further digestion. After a short adaptation stage for methanogens, the biogas production became stable. The higher biogas yield was observed after 14day anaerobic digestion with the combined pretreatment of alkaline and HPH. The biogas yield increased as the alkaline dosage increased, however there was no significant difference between the biogas production for the alkaline dosage of 0.04 and 0.05 mol/L. As the alkaline dosage increasing from 0.04 to 0.05 mol/L, the
Fig. 2. Effect of alkaline dosage on cumulative biogas production (a) and biogas production rate (b) (homogenization pressure of 60 MPa).
sludge disintegration degree had no obvious variation, which meant that there were no more soluble organics transformed into the sludge liquid. On the other hand, the high alkaline dosage might inhibit the activity of microorganisms, and more Na+ concentration might produce sodium toxicity to methanogens (Chen et al., 2008; Feijoo et al., 1995; Wu et al., 2006). After the anaerobic digestion for 14 d, the cumulated biogas increased by about 60% for the combined pretreatment of alkaline (dosage of 0.04 mol/L) and HPH, compared with that for the HPH pretreatment alone. The alkaline pretreatment improved not only the organic solubilization, but also the surface area available for enzymatic action, which led to improvement of anaerobic digestion performances. More released soluble and low-molecular weight components were biodegraded and transformed into biogas with the combined pretreatment. Other authors also summarized that the biogas production increased with the increase in alkaline dosage when the sludge was pretreated with alkaline (Lin et al., 1997; Lopez Torres et al., 2008). The maximum cumulative biogas production of 5059 ml with an alkaline dosage of 0.05 mol/L and a homogenization pressure of 60 MPa was achieved on the 12th day, and there was no significant increase after 12-day sludge digestion, which suggested that the suitable time for methane production was 12 d. The suitable sludge digestion time was shorter than that for the alkaline treatment alone and microwave–alkaline pretreatment (Shao et al., 2012; Tyagi and Lo, 2012). Considering both biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. Methane content in the biogas was improved by the combined pretreatment of alkaline and HPH. For the anaerobic sludge digestion with the HPH pretreatment alone, the methane content in biogas was about 58%, and a methane content of about 68% was achieved for the sludge digestion with the combined pretreatment of alkaline (with a NaOH dosage of 0.04 mol/L) and HPH. Sludge solubilization and anaerobic biodegradability increased with combined pretreatment. What’s more, the low-molecular weight and soluble components were easily utilized by methanogens and further transformed into the methane (Ghose et al., 1979). Therefore, the methane production was higher than HPH pretreatment alone. The biogas production rate during anaerobic digestion is shown in Fig. 2(b), which indicated that the anaerobic sludge digestion was greatly accelerated by the combined pretreatment of alkaline and HPH. All biogas production rates reached the maximum on the day 5th, and then decreased. The biogas production rate increased with the alkaline dosage increase. The maximum biogas production rate for the HPH pretreatment alone was 735 ml/d, and increased to 1105 ml/d for the combined pretreatment of alkaline (with a NaOH dosage of 0.04 mol/L) and HPH, which was approximately improved by 50%. The significant increase in biogas production rate might shorten the digestion period. The biogas production rate insignificantly increased when the alkaline dosage was higher than 0.04 mol/L, agreeing with the organic substance degradation shown in Table 1. 3.2.3. pH value and alkalinity of sludge system Sludge pH is one of the most influence factors in anaerobic digestion. The fermentative microorganisms can function in a wider pH range between 5.0 and 8.5, while the pH range for methanogens is relatively narrow, and should be maintained in the range of 6.5–7.8. When the acid-producing bacteria and methanogens can coexist in a single-phase anaerobic digestion system, the most appropriate pH range is 7.0–7.6 (Hwang et al., 2004). The pH value in this study remained within the range from 6.8 to 8.0 during anaerobic digestion. The pH changes with different alkaline dosages were similar; dropping within the first 3 d and then slowly rising in the further digestion period. In the beginning digestion
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
period, pH might drop with VFA accumulation until the VFA began to be consumed for methane production. Compared with the alkaline treatment alone (Shao et al., 2012), we found that the combined pretreatment of alkaline and HPH showed a shorter acidification time, which was beneficial by shortening the whole digestion period. The pH value is not an effective parameter evaluating the stability of anaerobic process when the sludge system has a high buffering capacity (Björnsson et al., 2000). Alkalinity can be used to reflect the process performance directly. The alkalinity of a steady system is between 2000 and 5000 mg/L (as CaCO3). When alkalinity was lower than 1000 mg/L, the pH value in the bioreactors would decrease (Lin et al., 1997). In this study, all the alkalinity maintained stable within a rational range of 3000–5000 mg/L (as CaCO3). The sludge with the combined pretreatment of alkaline and HPH showed a higher alkalinity compared with that with HPH pretreatment alone due to the alkaline addition, which may keep the system more stable.
3.3. Relationship between sludge disintegration and anaerobic sludge digestion From Fig. 3(a) it can be seen that the cumulative biogas production showed a positive relationship with the DDCOD. The cumulative biogas production linearly increased with the increase in DDCOD, which indicated that the organic components released from sludge were biodegradable and converted to the biogas. The methane content in biogas with the combined pretreatment was higher than that with the HPH pretreatment alone. The methane content in biogas increased by 10% when DDCOD reached 51.5% (with addition of 0.04 mol/L alkaline). It was known that different organic components were released with different DDCOD due to the
5
selective sludge disruption (Donsì et al., 2009), so the methane content in biogas decreased when the DDCOD was higher than 51%. The linear relationship between the cumulative biogas production, methane content in biogas and DDCOD were obtained as Eqs. (2) and (3):
Cumulative biogas production ðmlÞ ¼ 71:10 DDCOD þ 1177
ð2Þ
Methane content in biogas ð%Þ ¼ 0:3263 DDCOD þ 50:71
ð3Þ
From Fig. 3(b) it can be seen that the biogas yield and methane yield per gram of VS biodegraded were improved with increasing the alkaline dosage. Within the experimental DDCOD range, the biogas and methane yield linearly increased with the increase of DDCOD, and the linear relationships were expressed with Eqs. (4) and (5). The organics released from the sludge solids were more easily degraded and converted to methane by microorganism with the DDCOD increase due to alkaline addition, because higher VS was removed with the higher alkaline addition (as shown in Table 1). The combined pretreatment of alkaline and HPH was an effective method for mesophilic anaerobic sludge digestion. When the alkaline dosage was 0.05 mol/L, the biogas and methane yield reached 406 ml/gVS and 247 ml/gVS, which was about 47% and 107% higher than that with HPH pretreatment alone, respectively.
Biogas yield ðml=gVSÞ ¼ 4:51 DDCOD þ 153:76
ð4Þ
Methane yield ðml=gVSÞ ¼ 4:66 DDCOD 9:69
ð5Þ
4. Conclusion SCOD in sludge supernatant with combined pretreatment of alkaline and high pressure homogenization (HPH) was significantly higher than that with HPH pretreatment alone. After anaerobic digestion, SCOD, TCOD and VS removal increased, biogas production process accelerated, and methane content in biogas increased with the combined pretreatment, because the organic components released from cells might be more biodegradable. Higher alkaline dosage within 0–0.05 mol/L showed greater enhancement on the sludge digestion. Considering both biogas production and alkaline dosage, the optimal alkaline dosage was selected as 0.04 mol/L. The sludge disintegration degree was a key factor to improve the anaerobic sludge digestion efficiency. Acknowledgements This research was funded by the China–Israel Joint Research Program of MOST of China, National Natural Science Foundation of China (51178047 and 51278489), and Key Laboratory for Solid Waste Management and Environment Safety (SWMES). References
Fig. 3. Cumulative biogas production and methane content in biogas (a), biogas yield and methane yield (b) with DDCOD increase.
Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pretreatment of surplus sludge. Adv. Environ. Res. 7, 715–720. Björnsson, L., Murto, M., Mattiasson, B., 2000. Evaluation of parameters for monitoring an anaerobic co-digestion process. Appl. Microbiol. Biotechnol. 54, 844–849. Bougrier, C., Carrere, H., Delgenes, J., 2005. Solubilisation of waste-activated sludge by ultrasonic treatment. Chem. Eng. J. 106, 163–169. Braguglia, C., Gianico, A., Mininni, G., 2012. Comparison between ozone and ultrasound disintegration on sludge anaerobic digestion. J. Environ. Manag. 95, S139–S143. Camacho, P., Déléris, S., Geaugey, V., Ginestet, P., Paul, E., 2002. A comparative study between mechanical, thermal and oxidative disintegration techniques of waste activated sludge. Water Sci. Technol. 46, 79–87. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Dog˘an, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43, 2139–2148.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050
6
W. Fang et al. / Bioresource Technology xxx (2014) xxx–xxx
Donsì, F., Ferrari, G., Lenza, E., Maresca, P., 2009. Main factors regulating microbial inactivation by high-pressure homogenization: operating parameters and scale of operation. Chem. Eng. Sci. 64, 520–532. Eaton, A.D., Clesceri, L.S., Rice, E.W., Greenberg, A.E., 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American Water Works Association & Water Environment Federation, Washington, DC, USA. Feijoo, G., Soto, M., Méndez, R., Lema, J.M., 1995. Sodium inhibition in the anaerobic digestion process: antagonism and adaptation phenomena. Enzyme Microb. Technol. 17, 180–188. Floury, J., Bellettre, J., Legrand, J., Desrumaux, A., 2004. Analysis of a new type of high pressure homogeniser. A study of the flow pattern. Chem. Eng. Sci. 59, 843–853. Ghose, T., Singh, A., Mukhopadhyay, S., 1979. Increased methane production in biogas. Biotechnol. Lett. 1, 275–280. Hwang, M.H., Jang, N.J., Hyun, S.H., Kim, I.S., 2004. Anaerobic bio-hydrogen production from ethanol fermentation: the role of pH. J. Biotechnol. 111, 297–309. Jacquel, N., Lo, C.W., Wei, Y.H., Wu, H.S., Wang, S.S., 2008. Isolation and purification of bacterial poly (3-hydroxyalkanoates). Biochem. Eng. J. 39, 15–27. Kim, J., Park, C., Kim, T.H., Lee, M., Kim, S., Kim, S.W., Lee, J., 2003. Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95, 271–275. Lin, J.G., Chang, C.N., Chang, S.C., 1997. Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol. 62, 85–90. Lopez Torres, M., Espinosa Lloréns, M.d.C., 2008. Effect of alkaline pretreatment on anaerobic digestion of solid wastes. Waste Manag. 28, 2229–2234. Luo, K., Ye, Q., Yi, X., Yang, Q., Li, X.M., Chen, H.B., Liu, X., Zeng, G.M., 2013. Hydrolysis and acidification of waste-activated sludge in the presence of biosurfactant rhamnolipid: effect of pH. Appl. Microb. Biotechnol. 97, 5597– 5604. Metcalf, E., 2002. Wastewater Engineering: Treatment and Reuse. McGraw-Hill, New York. Middelberg, A.P., O’Neill, B.K., Bogle, L., David, I., Snoswell, M.A., 1991. A novel technique for the measurement of disruption in high-pressure homogenization: studies on E. coli containing recombinant inclusion bodies. Biotechnol. Bioeng. 38, 363–370. Nah, I.W., Kang, Y.W., Hwang, K.Y., Song, W.K., 2000. Mechanical pretreatment of waste activated sludge for anaerobic digestion process. Water Res. 34, 2362– 2368.
Onyeche, T., Schläfer, O., Sievers, M., 2003. Advanced anaerobic digestion of sludge through high pressure homogenisation. J. Solid Waste Technol. Manag. 29, 56– 61. Paquin, P., 1999. Technological properties of high pressure homogenizers: the effect of fat globules, milk proteins, and polysaccharides. Int. Dairy J. 9, 329–335. Penaud, V., Delgenes, J., Moletta, R., 1999. Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability. Enzyme Microb. Technol. 25, 258–263. Qiao, W., Takayanagi, K., Shofie, M., Niu, Q., Yu, H.Q., Li, Y.Y., 2013. Thermophilic anaerobic digestion of coffee grounds with and without waste activated sludge as co-substrate using a submerged AnMBR: system amendments and membrane performance. Bioresour. Technol. 150, 249–258. Rai, C.L., Rao, P.G., 2009. Influence of sludge disintegration by high pressure homogenizer on microbial growth in sewage sludge: an approach for excess sludge reduction. Clean Technol. Environ. Policy 11, 437–446. Shao, L., Wang, X., Xu, H., He, P., 2012. Enhanced anaerobic digestion and sludge dewaterability by alkaline pretreatment and its mechanism. J. Environ. Sci. 24, 1731–1738. Shehu, M.S., Abdul Manan, Z., Wan Alwi, S.R., 2012. Optimization of thermo-alkaline disintegration of sewage sludge for enhanced biogas yield. Bioresour. Technol. 114, 69–74. Tiehm, A., Nickel, K., Zellhorn, M., Neis, U., 2001. Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization. Water Res. 35, 2003–2009. Tyagi, V.K., Lo, S.L., 2012. Enhancement in mesophilic aerobic digestion of waste activated sludge by chemically assisted thermal pretreatment method. Bioresour. Technol. 119, 105–113. Wang, J., Zhang, H., Stabnikova, O., Tay, J., 2005. Comparison of lab-scale and pilotscale hybrid anaerobic solid–liquid systems operated in batch and semicontinuous modes. Process Biochem. 40, 3580–3586. Wang, W., Luo, Y., Qiao, W., 2010. Possible solutions for sludge dewatering in China. Front. Environ. Sci. Eng. China 4, 102–107. Wu, J., Xu, L., Xie, J., 2006. The effect of alkali-pretreatment on anaerobic digestion of solid organic waste. Acta Sci. Circumst. 26 (2), 252–255 (in Chinese). Zhang, S., Zhang, P., Zhang, G., Fan, J., Zhang, Y., 2012a. Enhancement of anaerobic sludge digestion by high-pressure homogenization. Bioresour. Technol. 118, 496–501. Zhang, Y., Zhang, P., Zhang, G., Ma, W., Wu, H., Ma, B., 2012b. Sewage sludge disintegration by combined treatment of alkaline + high pressure homogenization. Bioresour. Technol. 123, 514–519.
Please cite this article in press as: Fang, W., et al. Effect of alkaline addition on anaerobic sludge digestion with combined pretreatment of alkaline and high pressure homogenization. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.03.050