- Email: [email protected]
Effects and model of alkaline waste activated sludge treatment
Available online at www.sciencedirect.com
Bioresource Technology 99 (2008) 5140–5144
Short Communication
Effects and model of alkaline waste activated sludge treatment Huan Li, Yiying Jin *, RasoolBux Mahar, Zhiyu Wang, Yongfeng Nie Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Received 3 May 2007; received in revised form 5 September 2007; accepted 8 September 2007 Available online 22 October 2007
Abstract Due to lack of information about alkaline sludge treatment with high dose at ambient temperature, alkaline sludge treatment was investigated to know the effects of different doses (0–0.5 mol/L) of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] at 0–40 °C. Results showed that NaOH was more suitable than Ca(OH)2 for sludge disintegration. For NaOH treatment, most efficient dose was about 0.05 mol/L (0.16 g/g dry solid), and 60–71% solubilization of organic matters was achieved in first 30 min during its treatment of 24 h. The process is described by a transmutative power function model, and relation between reaction rate constant and temperature are according with Arrhenius equation. Low dose NaOH treatment (<0.2 mol/L) deteriorated sludge dewatering ability obviously, while the ability was recovered gradually with the increase of NaOH dose. For Ca(OH)2 treatment, the disintegrated floc fragments and soluble organic polymers can be re-flocculated with the help of calcium cations. Consequently sludge disintegration effect was counteracted and dewatering ability improved. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Alkaline treatment; Disintegration; Dewatering ability
1. Introduction Waste activated sludge (WAS) is the main by-product of wastewater treatment process, and it is about 0.5–1% of total influent water. In China 20–50% of capital and operation cost of wastewater treatment plants are spent on WAS treatment and disposal. So sludge reduction is very important for its economical treatment. Sludge disintegration, a pretreatment method of dewatering or anaerobic digestion, has been studied extensively and proved in improvement of sludge reduction processes. Sludge disintegration methods include mechanical, thermal, chemical and biological treatments. Comparing with other methods alkaline treatment has the several advantages, i.e. simple manufacturing of device, easy to operate and high efficiency (Weemaes and Verstraete, 1998). Alkaline sludge treatment can disrupt flocs and cells, release inner organic
*
Corresponding author. Tel.: +86 10 62782029; fax: +86 10 62789748. E-mail address: [email protected] (Y. Jin).
0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.09.019
matters and accelerate sludge hydrolysis, and consequently improve the performance of succedent anaerobic digestion (Novelli et al., 1995; Lin et al., 1997; Kim et al., 2003; Cassini et al., 2006). Besides this, alkaline sludge treatment can also release the water held inside floc and cell structure, which can not be removed by conventional dewatering processes. Therefore, alkaline treatment can improve sludge dewatering ability (Erdincler and Vesilind, 2000; Neyens et al., 2003). Alkaline sludge treatment depends on dissolution or destruction of floc structure and cell wall by hydroxy radicle. Extracellular polymer substances (EPS) hold on sludge particles together to form flocs. EPS include protein, humic substances, polysaccharid, lipid and nucleic acid (Dignac et al., 1998). An extreme of high pH causes protein to loose their natural shapes, saponification of lipid and hydrolysis of RNA (Zhang, 2002). Strong alkali solubilize gels not only because of chemical degradation but also ionization of the hydroxyl groups (–OH ! –O), which leads to extensive swelling and subsequent solubilization (Neyens et al., 2004). After destruction of EPS and gels, cells are
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
exposed to environment with extremes of pH thereby cannot keep the appropriate turgor pressure. Due to aforementioned reason and saponification of lipid, cells are disrupted and the inner matters are released. Therefore, alkaline treatment can solubilize sludge and release inner water. Most of the investigations exhibit increase of soluble chemical oxygen demand (SCOD) or decrease of volatile suspended solid (VSS) especially during low dose (<0.1 mol/L) alkaline treatment or its combining treatment with thermal treatment (Lin et al., 1997; Erdincler and Vesilind, 2000; Navia et al., 2002; Neyens et al., 2003; Vlyssides and Karlis, 2004; Cassini et al., 2006). But low dose alkali generally acts as auxiliaries in thermal hydrolysis (50– 200 °C), and less information is available on high dose alkaline treatment at ambient temperature range, especially its effect on sludge dewatering ability. The aim of this study is to fill up some gaps in the research. We examined both sludge disintegration effect and dewatering ability during alkaline treatment with low and high dose of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] at ambient temperature range (0–40 °C). Based on the analyses, this study can provide more insights into alkaline sludge treatment and establish an experimental model to describe the process. 2. Methods Sludge samples were collected from the outlet of aerobic and thickening tanks of a local full-scale municipal wastewater treatment plant and were stored at 4 °C before usage. The process of waste water treatment plant is anaerobic– anoxic–aerobic. The sludge samples had a water content of 98%, pH of 6.9, SCOD of 275 mg/L, suspended solid (SS) of 12,788 mg/L and volatile suspended solid (VSS) of 9630 mg/L. To analyze the effect of sludge concentration on alkaline treatment, sludge concentration was adjusted through moving or adding supernatant obtained by sludge settlement. Alkaline sludge treatment was carried out in a 2.0 L batch mixed reactor, which was put in a water bath (THZ95, SBL Ltd., China) to adjust the reaction temperature between 0 °C and 40 °C. NaOH doses were added between 0.05 mol/L to 1.0 mol/L, and Ca(OH)2 between 0.02 mol/L to 0.5 mol/L. Solubilization of sludge organic matters was measured with SCOD and VSS. Sludge disintegration degree was calculated as the ratio of the SCOD increase by alkaline treatment to the maximum possible SCOD increase: DDCOD = (SCOD SCOD0)/(TCOD SCOD0), in which SCOD0 is the SCOD of untreated sludge sample, TCOD is total chemical oxygen demand of sludge sample. Sludge dewatering ability was measured by three methods, i.e. capillary suction time (CST), filtration, centrifugation. Sludge samples were used directly for measurement of water content, SS, VSS and TCOD by following Standard Methods (APHA, 1995). The samples were centrifuged at
5141
5000g for 10 min, and then the supernatant was filtered through a membrane with a mesh size of 0.45 lm. The filtrate was used for the measurement of SCOD. pH was measured using pH meter (EUTECH Cyberscan510). Sludge particle size distributions were examined by a Malvern Mastersizer laser beam diffraction granulometer. Sludge CST was measured by CST instrument manufactured by Boshitong Ltd. China. Sludge filtration ability (V5 min) was measured by filtrate volume of 100 mL sludge sample through a medium-velocity quantitative filter paper after 5 min of vacuum filtration at 0.09 MPa. Sludge centrifugation ability was measured by moisture content of centrifugal (5000g, 10 min) sludge cake and turbidity of centrifugal supernatant. The turbidity was measured with a Nephelometer (HACH 2100P). 3. Results and discussion 3.1. Solubilization of organic matters The effects of NaOH dose on sludge disintegration are presented by SCOD change (Fig. 1). The SCOD increased with NaOH dose, and treatment was the most efficient when dose was about 0.05 mol/L (0.16 g/g DS). It indicated that NaOH was excess. In fact, sludge pH value was still over 12 after 0.05 mol/L NaOH treatment of 30 min. Consequently the quantity of solubilized organic matters was proportional to sludge concentration, and in this study the ratio of SCOD increment and sludge VSS was about 0.63 with 0.05 mol/L NaOH treatment. Similar results were also reported by Xiao and Liu (2006). The effect of NaOH treatment duration on sludge disintegration is also shown in Fig. 1. The increase of SCOD can be divided into two stages: an initial rapid stage of 0.5 h and a subsequent slow stage. The similar duration of the first stage can also be found in other papers (Navia et al., 2002; Cai et al., 2004; Xiao and Liu, 2006). In first 30 min, the solubilization quantity was 60–71% of total solubilized organic matters in 24 h. So most efficient treatment duration was 30 min.
Fig. 1. Variation of sludge SCOD with duration time during NaOH treatment.
5142
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
Ca(OH)2 can also disintegrate the sludge, but the effect was far lower than that of NaOH. After 30 min Ca(OH)2 treatment, sludge SCOD increased from 275 mg/L to 1375 mg/L, 1365 mg/L, 984 mg/L and 821 mg/L with the doses of 0.02 mol/L, 0.05 mol/L, 0.3 mol/L and 0.5 mol/ L, respectively. The results showed that soluble organic matters decreased gradually when Ca(OH)2 dose was over 0.02 mol/L. Since extracellular polymer substances (EPS) are important joining matter in sludge flocs, the dissolved organic polymers can also play the same role. The bivalent cations including calcium and magnesium are the key matter connecting cells with EPS (Neyens et al., 2003). So with the help of calcium cations, the dissolved organic polymers enhanced the re-flocculation of fragments produced by alkaline treatment. During the same process, the solubilized organic matters were transformed into sludge flocs again, and the floc size was increased. Based on the comparison analysis of NaOH and Ca(OH)2, it can be concluded that NaOH was more effective than Ca(OH)2 for sludge solubilization.
3.2. Changes of sludge dewatering ability During alkaline sludge treatment, the changes of sludge flocs can be reflected by sludge dewatering ability indirectly, including CST, V5 min, moisture content of centrifugal cake, and turbidity of centrifugal supernatant (Figs. 2 and 3). As a whole, sludge dewatering ability deteriorated at first and then improved gradually when NaOH dose was increased from 0.05 mol/L to 0.5 mol/L. When NaOH dose was lower than 0.1 mol/L, sludge dewatering ability deteriorated obviously because of disruption of sludge flocs and cells. The change of CST value indicated that hydrophilic organic polymers and vicinal water content increased. The effects of sludge filtration and centrifugation showed
Fig. 2. Effects of alkaline kind and dose on sludge CST and filtration ability.
Fig. 3. Effects of alkaline kind and dose on sludge centrifugal ability.
that fine particles increased. The centrifugal supernatant seemed to be cleaner after 0.1 mol/L NaOH treatment. It might be because solubilized organic polymers formed gels containing a large amount of water and some fine particles, which can also be observed directly. As a result, the moisture content of centrifugal sludge cake was high and the turbidity of centrifugal supernatant was low. When NaOH dose increased over 0.2 mol/L, sludge dewatering ability was improved comparing with the effect of low dose NaOH treatment. The decrease of CST value showed that hydrophilic organic polymers decreased, and gels were solubilized by strong alkali (Neyens et al., 2004). The increase of V5 min and the decrease of supernatant turbidity indicated that fine particles were decreased partially. The particle size distributions can be used to verify the change. The value of d90 increased from 115.69 lm to 135.51 lm after 0.5 mol/L NaOH treatment. This showed that some disrupted floc fragments were re-flocculated with the assistance of soluble electropositive organic polymers produced by strong alkali treatment (Erdincler and Vesilind, 2000). The reduction of fine particles and the solubilization of gels lead to improvement of sludge dewatering ability. Although fine particles can be re-flocculated during high dose NaOH treatment, their quantity still increased comparing with untreated sludge samples, and the value of d10 decreased from 15.77 lm to 11.87 lm. So the filtration ability of treated sludge was still worse than that of untreated sludge. Although Ca(OH)2 can also lead to the disintegration of some flocs, it can improve sludge dewatering ability as a whole. The changes of CST and V5 min were not obvious. The water content of centrifugal sludge cake and turbidity of centrifugal supernatant both decreased obviously with Ca(OH)2 dose, which indicated that Ca(OH)2 can enhance the re-flocculation and increase sludge inorganic content. Although high dose Ca(OH)2 treatment was beneficial for sludge compactibility, the weight of sludge cake was also increased because of the addition of inorganic matters. When Ca(OH)2 dose was 0.05 mol/L, the weight of sludge
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
5143
cake increased by 24.3% (decreased 26% for NaOH treatment with the same dose). More doses could lead to more increase of sludge cake weight, which was disadvantageous for the following treatment. 3.3. Model of NaOH sludge disintegration Since calcium cation can reduce the dissolved organic polymers by re-flocculation, NaOH treatment was more efficient for sludge disintegration than Ca(OH)2. For describing the process of alkaline treatment with different NaOH dose (0–0.5 mol/L) at ambient temperature, this work was to provide an experimental model using DDCOD as the independent variable. According to above analysis, sludge concentration had no obvious effect on its disintegration degree. So the independent variables included NaOH dose, treatment duration and ambient temperature. The value of DDCOD changed between 0 and 1 during alkaline sludge treatment; moreover, the solubilization rate of organic matters decreased gradually with duration or NaOH dose. So a transmutative power function model is designed as Eq. (1) to describe the process of alkaline sludge treatment DDCOD ¼ 1
1 ; 1 þ k C aA tb
ð1Þ
where CA is concentration of NaOH, g/L; a and b are respectively the indexes of NaOH dose and duration time; k is reaction rate constant depending on ambient temperature, which may be expressed as Arrhenius equation. k ¼ A expðEa =RT Þ
ð2Þ
In Eq. (2), A is pre-exponential factor; Ea is activation energy, J/mol; R is gas constant, 8.31 J/(K mol); T is absolute temperature, K. When reaction temperature was increased further, the solubilization of organic matter was not mainly dependent on alkaline hydrolysis, but also on thermal hydrolysis, and the model maybe not suitable. For the convenience of calculation, Eq. (1) can be changed into Eq. (3) ln
DDCOD ¼ ln k þ a ln C A þ b ln t 1 DDCOD
Fig. 4. Comparison of experimental and simulated data for sludge DDCOD (the points are the experimental data, the lines are the calculation results, the numbers is the dose of NaOH, g/L).
to accelerate solubilization of sludge organic matters in the range of 0–40 °C. Based on these, the parameters of Eq. (2) were calculated by multiple line regression. The result is shown in Eq. (5). The correlation coefficient is 0.96. It shows that the relationship between the reaction rate constant and temperature is according with the Arrhenius equation 12030 ð5Þ k ¼ 72:12 exp 8:31 T Another 34 groups of data were used to verify Eqs. (4) and (5) at different ambient temperature, sludge concentration, alkaline dose and treatment duration. The simulated data and experimental data are compared (Fig. 5). They fit well and the correlation coefficient is 0.97. So the model could be used to calculate the disintegration degree during NaOH treatment based on Eqs. (4) and (5).
ð3Þ
Based on SCOD data during alkaline treatment with different NaOH doses and duration at 9.8 °C (Fig. 4), k9.8 °C, a and b can be calculated by multiple line regression. The result is shown in Eq. (4) and correlation coefficient is 0.98, in which, k9.8 °C is 0.42. The simulated curve is also shown in Fig. 4: DDCOD ¼ 1
1 1 þ k C 0:22 t0:15 A
ð4Þ
The effect of ambient temperature on alkaline sludge treatment was also measured. When reaction temperatures were 0 °C, 13.3 °C, 20.4 °C, 30.6 °C and 40.6 °C, the values of k were 0.40, 0.41,0.55, 0.59 and 0.74, respectively. The results indicated that the rise of reaction temperature was helpful
Fig. 5. Correlation between simulated data and experimental data for DDCOD.
5144
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
4. Conclusions NaOH was more efficient for sludge disintegration. The process included two stages: a rapid initial phase of 0.5 h duration and subsequent slower phase. Sludge disintegration degree was decided by NaOH dose, and at the same dose the quantity of dissolved organic matters was proportional with sludge concentration. Low dose NaOH treatment deteriorated sludge dewatering ability obviously, while the ability can be restored at some degree by the treatment with high dose. Ca(OH)2 treatment was more appropriate for improving sludge dewatering ability. Calcium cations can enhance the re-flocculation of soluble organic polymers, which would counteract sludge disintegration effect. Ca(OH)2 treatment can not change sludge CST and filtration ability obviously, while can improve sludge centrifugal ability. The weight of sludge cake increased with Ca(OH)2 dose. In this study, the process of NaOH treatment can be described with a power function model and relation between reaction rate constant and ambient temperature has good agreement with Arrhenius equation. Acknowledgements This work was supported by China National Eleven Five-Year Scientific and Technical Support Plans (No. 2006BAC02A18). References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association Inc., Washington, DC, USA.
Cai, M.L., Wei, Y.S., Liu, J.X., 2004. Enhanced biohydrogen production from sewage sludge with alkaline pretreatment. Environ. Sci. Technol. 38, 3195–3202. Cassini, S.T., Andrade, M.C.E., Abreu, T.A., et al., 2006. Alkaline and acid hydrolytic processes in aerobic and anaerobic sludges: effect on total EPS and fractions. Water Sci. Technol. 53, 51–58. Dignac, M.F., Urbain, V., Rybacki, D., et al., 1998. Chemical description of extracellular polymers: implication on activated sludge floc structure. Water Sci. Technol. 38, 45–53. Erdincler, A., Vesilind, P.A., 2000. Effect of sludge cell disruption on compactibility of biological sludge. Water Sci. Technol. 42, 119–126. Kim, J.S., Park, C.H., Kim, T.H., et al., 2003. Effects of various pretreatment 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. Bioresource Technol. 62, 85–90. Navia, R., Soto, M., Vidal, G., et al., 2002. Alkaline pretreatment of kraft mill sludge to improve its anaerobic digestion. Bull. Environ. Contam. Toxicol. 69, 869–876. Neyens, E., Baeyens, J., Creemers, C., 2003. Alkaline thermal sludge hydrolysis. J. Hazard. Mater. 97, 295–314. Neyens, E., Baeyens, J., Dewil, R., et al., 2004. Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering. J. Hazard. Mater. 106, 83–92. Novelli, A., Ottonello, F., Converti, A., et al., 1995. Alkaline hydrolysis for the treatment of the organic fraction of municipal solid wastes and sludges. Chem. Biochem. Eng. Quart. 9, 195. Vlyssides, A.G., Karlis, P.K., 2004. Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion. Bioresource Technol. 91, 201–206. Weemaes, M.P., Verstraete, J., 1998. Evaluation of current wet sludge disintegration techniques. J. Chem. Technol. Biotechnol. 73, 83–92. Xiao, B., Liu, J., 2006. Study on treatment of excess sludge under alkaline condition (Chinese). Environ. Sci. 27, 319–323. Zhang, H., 2002. Biochemistry Tutorial, third ed. Sichuan University Publishing Company, Chengdu, China, pp. 22–23, 33–34, 103 and 124.
Bioresource Technology 99 (2008) 5140–5144
Short Communication
Effects and model of alkaline waste activated sludge treatment Huan Li, Yiying Jin *, RasoolBux Mahar, Zhiyu Wang, Yongfeng Nie Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Received 3 May 2007; received in revised form 5 September 2007; accepted 8 September 2007 Available online 22 October 2007
Abstract Due to lack of information about alkaline sludge treatment with high dose at ambient temperature, alkaline sludge treatment was investigated to know the effects of different doses (0–0.5 mol/L) of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] at 0–40 °C. Results showed that NaOH was more suitable than Ca(OH)2 for sludge disintegration. For NaOH treatment, most efficient dose was about 0.05 mol/L (0.16 g/g dry solid), and 60–71% solubilization of organic matters was achieved in first 30 min during its treatment of 24 h. The process is described by a transmutative power function model, and relation between reaction rate constant and temperature are according with Arrhenius equation. Low dose NaOH treatment (<0.2 mol/L) deteriorated sludge dewatering ability obviously, while the ability was recovered gradually with the increase of NaOH dose. For Ca(OH)2 treatment, the disintegrated floc fragments and soluble organic polymers can be re-flocculated with the help of calcium cations. Consequently sludge disintegration effect was counteracted and dewatering ability improved. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Alkaline treatment; Disintegration; Dewatering ability
1. Introduction Waste activated sludge (WAS) is the main by-product of wastewater treatment process, and it is about 0.5–1% of total influent water. In China 20–50% of capital and operation cost of wastewater treatment plants are spent on WAS treatment and disposal. So sludge reduction is very important for its economical treatment. Sludge disintegration, a pretreatment method of dewatering or anaerobic digestion, has been studied extensively and proved in improvement of sludge reduction processes. Sludge disintegration methods include mechanical, thermal, chemical and biological treatments. Comparing with other methods alkaline treatment has the several advantages, i.e. simple manufacturing of device, easy to operate and high efficiency (Weemaes and Verstraete, 1998). Alkaline sludge treatment can disrupt flocs and cells, release inner organic
*
Corresponding author. Tel.: +86 10 62782029; fax: +86 10 62789748. E-mail address: [email protected] (Y. Jin).
0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.09.019
matters and accelerate sludge hydrolysis, and consequently improve the performance of succedent anaerobic digestion (Novelli et al., 1995; Lin et al., 1997; Kim et al., 2003; Cassini et al., 2006). Besides this, alkaline sludge treatment can also release the water held inside floc and cell structure, which can not be removed by conventional dewatering processes. Therefore, alkaline treatment can improve sludge dewatering ability (Erdincler and Vesilind, 2000; Neyens et al., 2003). Alkaline sludge treatment depends on dissolution or destruction of floc structure and cell wall by hydroxy radicle. Extracellular polymer substances (EPS) hold on sludge particles together to form flocs. EPS include protein, humic substances, polysaccharid, lipid and nucleic acid (Dignac et al., 1998). An extreme of high pH causes protein to loose their natural shapes, saponification of lipid and hydrolysis of RNA (Zhang, 2002). Strong alkali solubilize gels not only because of chemical degradation but also ionization of the hydroxyl groups (–OH ! –O), which leads to extensive swelling and subsequent solubilization (Neyens et al., 2004). After destruction of EPS and gels, cells are
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
exposed to environment with extremes of pH thereby cannot keep the appropriate turgor pressure. Due to aforementioned reason and saponification of lipid, cells are disrupted and the inner matters are released. Therefore, alkaline treatment can solubilize sludge and release inner water. Most of the investigations exhibit increase of soluble chemical oxygen demand (SCOD) or decrease of volatile suspended solid (VSS) especially during low dose (<0.1 mol/L) alkaline treatment or its combining treatment with thermal treatment (Lin et al., 1997; Erdincler and Vesilind, 2000; Navia et al., 2002; Neyens et al., 2003; Vlyssides and Karlis, 2004; Cassini et al., 2006). But low dose alkali generally acts as auxiliaries in thermal hydrolysis (50– 200 °C), and less information is available on high dose alkaline treatment at ambient temperature range, especially its effect on sludge dewatering ability. The aim of this study is to fill up some gaps in the research. We examined both sludge disintegration effect and dewatering ability during alkaline treatment with low and high dose of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] at ambient temperature range (0–40 °C). Based on the analyses, this study can provide more insights into alkaline sludge treatment and establish an experimental model to describe the process. 2. Methods Sludge samples were collected from the outlet of aerobic and thickening tanks of a local full-scale municipal wastewater treatment plant and were stored at 4 °C before usage. The process of waste water treatment plant is anaerobic– anoxic–aerobic. The sludge samples had a water content of 98%, pH of 6.9, SCOD of 275 mg/L, suspended solid (SS) of 12,788 mg/L and volatile suspended solid (VSS) of 9630 mg/L. To analyze the effect of sludge concentration on alkaline treatment, sludge concentration was adjusted through moving or adding supernatant obtained by sludge settlement. Alkaline sludge treatment was carried out in a 2.0 L batch mixed reactor, which was put in a water bath (THZ95, SBL Ltd., China) to adjust the reaction temperature between 0 °C and 40 °C. NaOH doses were added between 0.05 mol/L to 1.0 mol/L, and Ca(OH)2 between 0.02 mol/L to 0.5 mol/L. Solubilization of sludge organic matters was measured with SCOD and VSS. Sludge disintegration degree was calculated as the ratio of the SCOD increase by alkaline treatment to the maximum possible SCOD increase: DDCOD = (SCOD SCOD0)/(TCOD SCOD0), in which SCOD0 is the SCOD of untreated sludge sample, TCOD is total chemical oxygen demand of sludge sample. Sludge dewatering ability was measured by three methods, i.e. capillary suction time (CST), filtration, centrifugation. Sludge samples were used directly for measurement of water content, SS, VSS and TCOD by following Standard Methods (APHA, 1995). The samples were centrifuged at
5141
5000g for 10 min, and then the supernatant was filtered through a membrane with a mesh size of 0.45 lm. The filtrate was used for the measurement of SCOD. pH was measured using pH meter (EUTECH Cyberscan510). Sludge particle size distributions were examined by a Malvern Mastersizer laser beam diffraction granulometer. Sludge CST was measured by CST instrument manufactured by Boshitong Ltd. China. Sludge filtration ability (V5 min) was measured by filtrate volume of 100 mL sludge sample through a medium-velocity quantitative filter paper after 5 min of vacuum filtration at 0.09 MPa. Sludge centrifugation ability was measured by moisture content of centrifugal (5000g, 10 min) sludge cake and turbidity of centrifugal supernatant. The turbidity was measured with a Nephelometer (HACH 2100P). 3. Results and discussion 3.1. Solubilization of organic matters The effects of NaOH dose on sludge disintegration are presented by SCOD change (Fig. 1). The SCOD increased with NaOH dose, and treatment was the most efficient when dose was about 0.05 mol/L (0.16 g/g DS). It indicated that NaOH was excess. In fact, sludge pH value was still over 12 after 0.05 mol/L NaOH treatment of 30 min. Consequently the quantity of solubilized organic matters was proportional to sludge concentration, and in this study the ratio of SCOD increment and sludge VSS was about 0.63 with 0.05 mol/L NaOH treatment. Similar results were also reported by Xiao and Liu (2006). The effect of NaOH treatment duration on sludge disintegration is also shown in Fig. 1. The increase of SCOD can be divided into two stages: an initial rapid stage of 0.5 h and a subsequent slow stage. The similar duration of the first stage can also be found in other papers (Navia et al., 2002; Cai et al., 2004; Xiao and Liu, 2006). In first 30 min, the solubilization quantity was 60–71% of total solubilized organic matters in 24 h. So most efficient treatment duration was 30 min.
Fig. 1. Variation of sludge SCOD with duration time during NaOH treatment.
5142
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
Ca(OH)2 can also disintegrate the sludge, but the effect was far lower than that of NaOH. After 30 min Ca(OH)2 treatment, sludge SCOD increased from 275 mg/L to 1375 mg/L, 1365 mg/L, 984 mg/L and 821 mg/L with the doses of 0.02 mol/L, 0.05 mol/L, 0.3 mol/L and 0.5 mol/ L, respectively. The results showed that soluble organic matters decreased gradually when Ca(OH)2 dose was over 0.02 mol/L. Since extracellular polymer substances (EPS) are important joining matter in sludge flocs, the dissolved organic polymers can also play the same role. The bivalent cations including calcium and magnesium are the key matter connecting cells with EPS (Neyens et al., 2003). So with the help of calcium cations, the dissolved organic polymers enhanced the re-flocculation of fragments produced by alkaline treatment. During the same process, the solubilized organic matters were transformed into sludge flocs again, and the floc size was increased. Based on the comparison analysis of NaOH and Ca(OH)2, it can be concluded that NaOH was more effective than Ca(OH)2 for sludge solubilization.
3.2. Changes of sludge dewatering ability During alkaline sludge treatment, the changes of sludge flocs can be reflected by sludge dewatering ability indirectly, including CST, V5 min, moisture content of centrifugal cake, and turbidity of centrifugal supernatant (Figs. 2 and 3). As a whole, sludge dewatering ability deteriorated at first and then improved gradually when NaOH dose was increased from 0.05 mol/L to 0.5 mol/L. When NaOH dose was lower than 0.1 mol/L, sludge dewatering ability deteriorated obviously because of disruption of sludge flocs and cells. The change of CST value indicated that hydrophilic organic polymers and vicinal water content increased. The effects of sludge filtration and centrifugation showed
Fig. 2. Effects of alkaline kind and dose on sludge CST and filtration ability.
Fig. 3. Effects of alkaline kind and dose on sludge centrifugal ability.
that fine particles increased. The centrifugal supernatant seemed to be cleaner after 0.1 mol/L NaOH treatment. It might be because solubilized organic polymers formed gels containing a large amount of water and some fine particles, which can also be observed directly. As a result, the moisture content of centrifugal sludge cake was high and the turbidity of centrifugal supernatant was low. When NaOH dose increased over 0.2 mol/L, sludge dewatering ability was improved comparing with the effect of low dose NaOH treatment. The decrease of CST value showed that hydrophilic organic polymers decreased, and gels were solubilized by strong alkali (Neyens et al., 2004). The increase of V5 min and the decrease of supernatant turbidity indicated that fine particles were decreased partially. The particle size distributions can be used to verify the change. The value of d90 increased from 115.69 lm to 135.51 lm after 0.5 mol/L NaOH treatment. This showed that some disrupted floc fragments were re-flocculated with the assistance of soluble electropositive organic polymers produced by strong alkali treatment (Erdincler and Vesilind, 2000). The reduction of fine particles and the solubilization of gels lead to improvement of sludge dewatering ability. Although fine particles can be re-flocculated during high dose NaOH treatment, their quantity still increased comparing with untreated sludge samples, and the value of d10 decreased from 15.77 lm to 11.87 lm. So the filtration ability of treated sludge was still worse than that of untreated sludge. Although Ca(OH)2 can also lead to the disintegration of some flocs, it can improve sludge dewatering ability as a whole. The changes of CST and V5 min were not obvious. The water content of centrifugal sludge cake and turbidity of centrifugal supernatant both decreased obviously with Ca(OH)2 dose, which indicated that Ca(OH)2 can enhance the re-flocculation and increase sludge inorganic content. Although high dose Ca(OH)2 treatment was beneficial for sludge compactibility, the weight of sludge cake was also increased because of the addition of inorganic matters. When Ca(OH)2 dose was 0.05 mol/L, the weight of sludge
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
5143
cake increased by 24.3% (decreased 26% for NaOH treatment with the same dose). More doses could lead to more increase of sludge cake weight, which was disadvantageous for the following treatment. 3.3. Model of NaOH sludge disintegration Since calcium cation can reduce the dissolved organic polymers by re-flocculation, NaOH treatment was more efficient for sludge disintegration than Ca(OH)2. For describing the process of alkaline treatment with different NaOH dose (0–0.5 mol/L) at ambient temperature, this work was to provide an experimental model using DDCOD as the independent variable. According to above analysis, sludge concentration had no obvious effect on its disintegration degree. So the independent variables included NaOH dose, treatment duration and ambient temperature. The value of DDCOD changed between 0 and 1 during alkaline sludge treatment; moreover, the solubilization rate of organic matters decreased gradually with duration or NaOH dose. So a transmutative power function model is designed as Eq. (1) to describe the process of alkaline sludge treatment DDCOD ¼ 1
1 ; 1 þ k C aA tb
ð1Þ
where CA is concentration of NaOH, g/L; a and b are respectively the indexes of NaOH dose and duration time; k is reaction rate constant depending on ambient temperature, which may be expressed as Arrhenius equation. k ¼ A expðEa =RT Þ
ð2Þ
In Eq. (2), A is pre-exponential factor; Ea is activation energy, J/mol; R is gas constant, 8.31 J/(K mol); T is absolute temperature, K. When reaction temperature was increased further, the solubilization of organic matter was not mainly dependent on alkaline hydrolysis, but also on thermal hydrolysis, and the model maybe not suitable. For the convenience of calculation, Eq. (1) can be changed into Eq. (3) ln
DDCOD ¼ ln k þ a ln C A þ b ln t 1 DDCOD
Fig. 4. Comparison of experimental and simulated data for sludge DDCOD (the points are the experimental data, the lines are the calculation results, the numbers is the dose of NaOH, g/L).
to accelerate solubilization of sludge organic matters in the range of 0–40 °C. Based on these, the parameters of Eq. (2) were calculated by multiple line regression. The result is shown in Eq. (5). The correlation coefficient is 0.96. It shows that the relationship between the reaction rate constant and temperature is according with the Arrhenius equation 12030 ð5Þ k ¼ 72:12 exp 8:31 T Another 34 groups of data were used to verify Eqs. (4) and (5) at different ambient temperature, sludge concentration, alkaline dose and treatment duration. The simulated data and experimental data are compared (Fig. 5). They fit well and the correlation coefficient is 0.97. So the model could be used to calculate the disintegration degree during NaOH treatment based on Eqs. (4) and (5).
ð3Þ
Based on SCOD data during alkaline treatment with different NaOH doses and duration at 9.8 °C (Fig. 4), k9.8 °C, a and b can be calculated by multiple line regression. The result is shown in Eq. (4) and correlation coefficient is 0.98, in which, k9.8 °C is 0.42. The simulated curve is also shown in Fig. 4: DDCOD ¼ 1
1 1 þ k C 0:22 t0:15 A
ð4Þ
The effect of ambient temperature on alkaline sludge treatment was also measured. When reaction temperatures were 0 °C, 13.3 °C, 20.4 °C, 30.6 °C and 40.6 °C, the values of k were 0.40, 0.41,0.55, 0.59 and 0.74, respectively. The results indicated that the rise of reaction temperature was helpful
Fig. 5. Correlation between simulated data and experimental data for DDCOD.
5144
H. Li et al. / Bioresource Technology 99 (2008) 5140–5144
4. Conclusions NaOH was more efficient for sludge disintegration. The process included two stages: a rapid initial phase of 0.5 h duration and subsequent slower phase. Sludge disintegration degree was decided by NaOH dose, and at the same dose the quantity of dissolved organic matters was proportional with sludge concentration. Low dose NaOH treatment deteriorated sludge dewatering ability obviously, while the ability can be restored at some degree by the treatment with high dose. Ca(OH)2 treatment was more appropriate for improving sludge dewatering ability. Calcium cations can enhance the re-flocculation of soluble organic polymers, which would counteract sludge disintegration effect. Ca(OH)2 treatment can not change sludge CST and filtration ability obviously, while can improve sludge centrifugal ability. The weight of sludge cake increased with Ca(OH)2 dose. In this study, the process of NaOH treatment can be described with a power function model and relation between reaction rate constant and ambient temperature has good agreement with Arrhenius equation. Acknowledgements This work was supported by China National Eleven Five-Year Scientific and Technical Support Plans (No. 2006BAC02A18). References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association Inc., Washington, DC, USA.
Cai, M.L., Wei, Y.S., Liu, J.X., 2004. Enhanced biohydrogen production from sewage sludge with alkaline pretreatment. Environ. Sci. Technol. 38, 3195–3202. Cassini, S.T., Andrade, M.C.E., Abreu, T.A., et al., 2006. Alkaline and acid hydrolytic processes in aerobic and anaerobic sludges: effect on total EPS and fractions. Water Sci. Technol. 53, 51–58. Dignac, M.F., Urbain, V., Rybacki, D., et al., 1998. Chemical description of extracellular polymers: implication on activated sludge floc structure. Water Sci. Technol. 38, 45–53. Erdincler, A., Vesilind, P.A., 2000. Effect of sludge cell disruption on compactibility of biological sludge. Water Sci. Technol. 42, 119–126. Kim, J.S., Park, C.H., Kim, T.H., et al., 2003. Effects of various pretreatment 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. Bioresource Technol. 62, 85–90. Navia, R., Soto, M., Vidal, G., et al., 2002. Alkaline pretreatment of kraft mill sludge to improve its anaerobic digestion. Bull. Environ. Contam. Toxicol. 69, 869–876. Neyens, E., Baeyens, J., Creemers, C., 2003. Alkaline thermal sludge hydrolysis. J. Hazard. Mater. 97, 295–314. Neyens, E., Baeyens, J., Dewil, R., et al., 2004. Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering. J. Hazard. Mater. 106, 83–92. Novelli, A., Ottonello, F., Converti, A., et al., 1995. Alkaline hydrolysis for the treatment of the organic fraction of municipal solid wastes and sludges. Chem. Biochem. Eng. Quart. 9, 195. Vlyssides, A.G., Karlis, P.K., 2004. Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion. Bioresource Technol. 91, 201–206. Weemaes, M.P., Verstraete, J., 1998. Evaluation of current wet sludge disintegration techniques. J. Chem. Technol. Biotechnol. 73, 83–92. Xiao, B., Liu, J., 2006. Study on treatment of excess sludge under alkaline condition (Chinese). Environ. Sci. 27, 319–323. Zhang, H., 2002. Biochemistry Tutorial, third ed. Sichuan University Publishing Company, Chengdu, China, pp. 22–23, 33–34, 103 and 124.