KSCE Journal of Civil Engineering (2010) 14(5):693-697 DOI 10.1007/s12205-010-0934-6
Environmental Engineering
www.springer.com/12205
Reduction of Sewage Sludge by Ball Mill Pretreatment and Mn Catalytic Ozonation Myoung Joo Lee*, Tae Hyeong Kim**, Ga Young Yoo***, Boo Ki Min****, and Sun Jin Hwang***** Received July 21, 2009/Accepted December 31, 2009
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Abstract The effects of ball mill pretreatment and Mn catalytic ozonation on the reduction of sewage sludge were investigated. The ball mill pretreatment increased ∆SCOD from 2,000mg/L to 9,000 mg/L, as Total Solids (TS) increased from 1% to 4% at a specific energy of 75.8 KJ/g-TSS. The increase in disintegration ratio (DR), which is an index of the solubilization of sewage sludge, was consistent with that in TS, at the same specific energy input. With Mn catalytic ozonation treatment, the operational pH was varied from 3 to 6 using HCl at an ozone dose of 0.2 g-O3/g-TS; an optimum pH of 3 was determined. At pH 3, the removal efficiency of Total Suspended Solids (TSS) was 45%, the highest value compared with the other pH conditions. The combination of ball mill pretreatment and Mn catalyst ozonation could achieve 60% TSS removal efficiency. The removal efficiency by this combination was about twice as high as the case study of Mn catalyst ozonation treatment alone, without a ball mill pretreatment process. Keywords: sewage sludge reduction, solubilization, mn catalyst, ozonation, ball mill ···································································································································································································································
1. Introduction By the end of 2006, 7,450 total tons (wet base) of sewage sludge was produced every day by the 328 sewage disposal plants in Korea. The sewage sludge was disposed of by means of ocean dumping (71%), incineration (13%), recycling (15%) and landfill (1%) (Ministry of Environment, Republic of Korea, 2007). Ocean dumping, however, which constitutes the largest portion of sludge disposal, will be prohibited after 2012, in accordance with the London Dumping Convention. Anaerobic digestion has been widely used for the reduction and stabilization of sewage sludge, but it has some limitations, such as low biodegradability of organic matter and long retention time during the reduction process. Thus, alternative treatment technologies such as mechanical treatment, treatments using ultrasonic, ozone, alkaline, heat, enzyme, etc. have been intensively studied for their abilities to reduce and stabilize sludge (Zhang et al., 2008; Bougrier et al., 2006; Bougrier et al., 2007; Dursun et al., 2006; Kopp et al., 1998; Kwon et al., 2003). During the process of mechanical pretreatment including ball mill treatment, bacterial cells in the sludge are ruptured by a high
critical tension and destroyed. As this process is a physical process not involving any by-products nor additional reactions, the main and post treatments are not significantly affected. Also, the energy consumption during the ball mill treatment is lower than that of ultrasonic and heat treatment, considering that the same COD solubilization ratio is obtained (Mûller et al., 1998). Many researchers have investigated the Advanced Oxidation Process (AOP), in which a pollutant is decomposed and removed by a strong oxidizing agent, such as an OH radical. Various AOP agents include ozone/UV, UV/H2O2, Fenton, H2O2, photolysis, plasma, electron beam, ultrasonic, ozone/catalyst, etc. Among those, the ozone/catalyst method has recently been studied by many researchers (Pines et al., 2003). Mn catalytic ozonation, in which more OH radicals are produced than with the ozonation process alone, has a high oxidation capacity and also a relatively high efficiency, even in acidic conditions (Ma and Graham, 2000; Hwang et al., 2006). The main chemical reactions of the Mn catalytic ozonation process are as follows: Mn(II) + O3 + H+ Mn(III) + OH· + O2 − 2
OH· + M + O2 → M(oxide) + O · ( HO2·)
(1) (2)
*Member, Ph.D. Student, Dept. of Environmental Science and Engineering, Kyung Hee University, Yongin 446-701, Korea (E-mail:
[email protected]) **Member, Ph.D. Student, Dept. of Environmental Science and Engineering, Kyung Hee University, Yongin 446-701, Korea (E-mail: mskphs@khu. ac.kr) ***Assistant Professor, Dept. of Environmental Science and Engineering, Kyung Hee University, Yongin 446-701, Korea (E-mail:
[email protected]) ****Assistant Professor, Dept. of Environmental Science and Engineering, Kyung Hee University, Yongin 446-701, Korea (E-mail:
[email protected]) *****Member, Professor, Dept. of Environmental Science and Engineering, Kyung Hee University, Yongin 446-701, Korea (Corresponding Author, E-mail:
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Myoung Joo Lee, Tae Hyeong Kim, Ga Young Yoo, Boo Ki Min, and Sun Jin Hwang
O2−· + O3 → OH· + O2
(3)
Mn(III) + M Mn(II) + M(oxide)
(4)
As shown in the above reactions, Mn catalyst acts as the initiator of ozone decomposition when it coexists with O3. Ozone is converted to an OH radical and O2, which then react with organic compounds (M) and produce radicals such as O2−• and HO2•. The radicals then produce OH radicals through a chain reaction (Heising et al., 1997; Staehelin and Hoigne, 1985). Furthermore, the oxidized Mn can be used to oxidize organic compounds in electron transfer reactions (Legube and Leitner, 1999; Heisig et al., 1997; Nowell and Hoigne, 1987). While most research about Mn catalytic ozonation has been focused on removal of non-degradable matter such as oxalic acid, phenol, etc. (Liu et al., 2008; Dong et al., 2008), only few studies on organic wastes such as sewage sludge has been reported. Therefore, in this study, optimum operation conditions of ball mill pretreatment and Mn catalytic ozonation for the reduction of sludge were investigated. In addition, several combinations of Mn catalytic ozonation and ball mill pretreatment were tested to examine the optimal operation conditions for effective and economical sludge reduction.
2. Materials and Methods 2.1 Sludge Source and Characteristics Excess sludge was collected from a centrifugal concentrator at the B-city Municipal Wastewater Treatment Plant (MWTP), to be used in the reduction and solubilization tests. The TS, TSS, VSS, pH, TCOD and SCOD of the sludge were 4.0%, 38,000 mg/L (3.8%), 31,000 mg/L (3.1%), 6.4, 36,000 mg/L and 280 mg/L, respectively.
7) (Lehne et al., 2001). The energy input was determined by power (P), operation time (t) and the treated sludge working volume (V). P×t Espec [ KJ ⁄ ( g-TSS ) ] = -----------------TSS × V
(7)
The effect of TS on the ball mill pretreatment efficiency was examined with 1, 2, 3 and 4% TS diluted with distilled water. 2.3 Mn Catalytic Ozonation of Sludge A cylindrical reactor (10 cm diameter and 110 cm height) was designed for bringing excess sludge into contact with ozone, as shown in Fig. 1. Ozone was supplied to the reactor with up to 0.2 g-O3/g-TS by a high performance ozone generator (LAB2B, Ozonia Triogen Ltd., England) using 95% pure O2 gas. The inlet and outlet ozone concentrations were measured by the widely used KI method. The Mn catalyst (10 mg-catalyst/g-TS), whose titer was decided through a previous study, was reacted with ozone in a reactor filled with excess sludge (Hwang et al., 2006). Sludge removal was accomplished by the metal-catalytic ozone treatment, and the efficiency of the treatment was expressed by the measurement of TSS reduction of the sludge cake obtained after dewatering process. In addition, as it is known that Mn catalytic ozonation is sensitive to pH, the optimum operation pH was decided by comparing the TSS removal rate at pH 3, 4, 5 and 6. 2.4 Combination of Ball Mill and Mn Catalytic Ozonation In order to investigate the effect of the DR value on the sludge reduction by Mn catalytic ozonation, 3% sewage sludge having 10, 20 and 30% DR values obtained after ball mill pretreatment was further oxidized with Mn catalytic ozonation. With this experiment, inlet ozone quantities were adjusted to 0, 0.05, 0.1,
2.2 Ball Mill Pretreatment of Sludge Sewage sludge was pretreated by a ball mill (attrition ball mill, Deahwa Co., Korea) at 600 rpm to increase the solubility of the sludge. Zircon beads (1 mm diameter) filled the ball mill vessel at a bead-to-sludge ratio of 1:2 (v/v). In this study, the mechanical treatment was attempted as the pretreatment method prior to the sludge removal in the metal-catalytic ozone treatment. Ball mill pretreatment efficiency was expressed in the soluble COD (SCOD) and the disintegration ratio of SCOD (DRSCOD) (Kopp et al., 2006).
∆SCOD[mg/L] = SCODT − SCOD0
(5)
SCOD T – SCODO - × 100 DRSCOD[ % ] = --------------------------------------------SCODmax – SCOD0
(6)
where SCODo is the soluble COD of untreated excess sludge, and SCODT is the soluble COD of treated excess sludge. SCODmax is the maximum releasable SCOD during the 48 hours after NaOH treatment at 80 meq/L; this was measured to be about 80% of the TCOD. Also, the specific energy (Espec) of the ball mill showed the energy input in relation to sludge TSS (Eq.
Fig. 1. Schematic Diagram of the Mn Catalytic Ozonation Reactor
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KSCE Journal of Civil Engineering
Reduction of Sewage Sludge by Ball Mill Pretreatment and Mn Catalytic Ozonation
0.15 and 0.2 g-O3/g-TS to examine the effect of ozone quantity on sludge reduction.
respectively. This result suggests that pretreatment with relatively high TS sludge will show better sludge reduction or solubilization at low specific energy inputs.
3. Results and Discussion 3.1 Ball Mill Pretreatment of Excess Sludge Ball mill pretreatment of 1, 2, 3 and 4% TS excess sludge was conducted in order to see the effects of the TS on the Disintegration Ratio (DR) (Fig. 2). At the initial stage of the reaction, ∆SCOD increased rapidly as a function of the specific energy input due to the disintegration of the solids in the sludge, but the rate of increase slowed gradually as more specific energy was applied. Especially, at the laboratory scale test with TS 1%, no further increase in SCOD was observed at more energy input than the value of 75.8 KJ/g-TSS. However, the significant effect of the mechanical pretreatment on the TS changes could be clearly observed at the specific energy of 75.8 KJ/g-TSS. As the TS of the sludge increased from 1 to 4%, ∆SCOD increased from approximately 2,000 mg/L to 9,000 mg/L at a specific energy of 75.8 KJ/g-TSS. The reason for the higher ∆SCOD in high TS sludge could be explained by the more frequent contact between cells and beads which occurs at relatively higher TS concentrations. DR also increased as TS was increased from 1 to 4%, but the increase in DR for the TS change from 3 to 4% was relatively small in comparison with other TS increases. The maximum DR was 32% for 3 and 4% TS sludge at 100 and 75.8 KJ/g-TSS, respectively. However, with low TS (1 and 2%), the DR value was less than 30%, even at high specific energies of 150 and 300,
Fig. 2. Disintegration Ratios (DR) and SCODs of Excess Sludge According to Specific Energies, in Ball Mill Treatment Vol. 14, No. 5 / September 2010
3.2 Mn Catalytic Ozonation of Excess Sludge at Different pH Levels Mn catalytic ozonation of the sludge was tested at different ozone doses (0 to 0.2 mg-O3/g-TS) and pH levels (3, 4, 5, and 6) (Fig. 3). As the ozone dose increased, more TSS removal was achieved at each pH condition. At the same ozone dose, more TSS removal was observed at lower pH conditions. This result might be caused by increased OH radical formation at lower pH, due to the reactions of high concentration H+ ions, Mn and ozone (equation 1) (Heising et al., 1997; Colin and Robbie, 1999). The maximum TSS removal was 45%, occurring at pH 3 with an ozone dose of 0.2 g-O3/g-TS. Based on the investigation of the graphs obtained with different kinetics, TSS removal by catalytic ozonation revealed a pseudo second-order reaction. These results also agree well with those of Hoigne Bader (1985) and Legube and Leitner (1999). The rate constant between ozone and TSS increased as pH decreased. The rate constant at pH 3 was 5.0×104 L/mg·min (r2=0.97), which was 1.7 times higher than at pH 6 (3.0×104 L/ mg·min; r2=0.93). Therefore, in this study, pH 3 was determined to be the optimum pH for the Mn catalytic ozonation of excess sewage sludge. 3.3 Combination of Ball Mill Pretreatment and Mn Catalytic Ozonation The removal of sludge at 3% TS was examined by the combination of ball mill pretreatment and Mn catalytic ozonation (Fig. 4). While the greater DR value guaranteed higher efficiency of biological or chemical post-treatment, in this study, Mn catalytic ozonation was carried out at DR values less than 30%, taking into consideration the economic aspects for scale-up and field applications of the ball mill process. At 0 g-O3/g-TS, the TSS removal efficiencies of the sludge initially having 10, 20 and 30% DR by ball mill pretreatment
Fig. 3. Effects of pH on Mn Catalytic Ozonation (Catalyst dose: 10 mg-Mn/g-TS)
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Myoung Joo Lee, Tae Hyeong Kim, Ga Young Yoo, Boo Ki Min, and Sun Jin Hwang
as the treatment processes at the field.
Acknowledgements This research was supported by the Kyung Hee University Research Fund in 2008 (KHU-20080628).
References
Fig. 4. The Effects of Ball Mill Preteatment on TSS Removal by Mn Catalytic Ozonation Treatment (TS 3% of excess sludge, Mn dosage: 10 mg-Mn/g-TS, pH: 3)
were about 25, 34 and 40%, respectively. When the Mn catalytic ozonation process followed, higher TSS removal efficiencies were observed at all conditions. The maximum removal efficiency of 60% of the sludge (30% DR) was obtained at the ozone dose of 0.2 g-O3/g-TS, which was about 30% higher in value than that of Mn catalytic ozonation treatment alone, i.e. without ball mill pretreatment. These results indicate that the combination of ball mill pretreatment and Mn catalytic ozonation can enhance sludge reduction dramatically.
4. Conclusions In this study, mechanical ball mill pretreatment was applied to excess sewage sludge for sludge solubilization and for sludge reduction by Mn catalytic ozonation treatment. The conclusions are as follows: 1. In the ball mill pretreatment test with 1~4% TS of excess sludge, as TS increased, the ∆SCOD value and DR increased up to 9,000 mg/L and 32%, respectively. The ball mill pretreatment with higher TS values of 3% and 4% were more effective for sludge solubilization, and at the same specific energy, a higher DR was achieved compared with the pretreatment of low TS sludge. This result indicates that at the field, mechanical pretreatment with relatively high TS (about 3~4%) of excess sludge will probably have better sludge reduction and thus be more economical. 2. At the same ozone dose, more sludge reduction was observed at lower pH conditions, with pH 3 determined to be the optimum value from an economical standpoint. Maximum TSS removal efficiency at pH 3 was about 45% at an ozone dose of 0.2 g-O3/g-TS. 3. The Mn catalytic ozonation treatment after ball mill pretreatment process showed better performance in excess sludge reduction (about 60% in this study) compared to the case without the pretreatment process (about 30% in this study). Thus, this combination process may guarantee the system becomes compact for sewage sludge reduction when applied
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