3-1 Sludge source
The sewage sludge (composing primary and biological sludge) was collected from Di-Hua wastewater treatment plant in Taipei, Taiwan. The pH of the sludge was 6.70. Prior the experiments, the sludge was sieved through a mesh (no.16 with the pore size of 1.5 mm) to remove impurities and floating matters, then settled by gravity until the TS was about 3%.
Finally, the pre-adjusted sludge was refrigerated at 4oC. The characteristics of this concentrated sludge were shown in Table 3-1.
The ratio of volatile solids contents (VS) and TS was 70% while the total COD and soluble COD were 26,500 mg/L and 140 mg/L, respectively. Cheng et al. (2000) measured the DEHP concentration in sewage sludge collected from Di-Hua wastewater treatment plant, where the result of 153.15 mg/kg-dw sludge was higher than the EU limit value (Table 2-3). Also, the sewage sludge contained large amount of other PAEs, especially DBP.
Therefore, the sewage sludge collected from Di-Hua wastewater treatment plant was used as the sludge source to demonstrate the PAEs changes during alkalization and sonication pretreatment.
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Table 3-1 Characteristics of sewage sludge in Di-Hua wastewater treatment plant
Characteristics Data
TS 2.97%
VS 2.08%
COD 26,500 mg/L
SCOD 140 mg/L
pH 6.70
DBP 718 mg/kg-dw
DEHP 41 mg/kg-dw
BBP 8 mg/kg-dw
Sampling date: Aug. 20th, 2009
3-2 Chemicals and reagents
The chemicals used in the present study are listed in Table 3-2. Two organic solvents, n-hexane and dichloromethane, were used in GC-FID analysis for PAEs determination.
Three PAEs such as DBP, DEHP and BBP with the purist grade (purity > 98%) were used in this study as the target compounds. Other chemicals with the reagent grade were used in this study without further purification.
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Table 3-2 Experimental chemicals in this study
Chemical Purpose
Sodium hydroxide (NaOH): 98%, Panreac (EU) Alkalization
DBP (C16H22O4): 99%, Panreac (EU)
PAEs analysis DEHP (C24H38O4): 99%, Riedel-deHaen (Germany)
BBP (C19H20O4): 98%, Aldrich (USA) n-hexane (C6H14): 96%, Scharlau (EU)
Dichloromethane (CH2Cl2): 99.9%, Mallinckrodt (USA) Potassium dichromate (K2Cr2O7): 99.5%, Panreac (EU)
COD analysis Mercuric sulfate (HgSO4): 99%, Riedel-deHaen (Germany)
Sulfuric acid (H2SO4): 98%, Panreac (EU) Boiling stone: Hanawa (Japan)
Silver sulfate sulfuric acid (AgSO4): 10 g/L, Fluka (Germany)
1, 10 phenanthroline monohydrate (C12H8N2·H2O): 99.5%, Riedel-deHaen (Germany) Iron (II) sulfate 7-hydrate (FeSO4·7H2O): 99.5%, Ferak (Germany)
Ferrous ammonium sulfate 6-hydrate, fine crystal (Fe(NH4)2(SO4)2·6H2O) (USA)
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3-3 Experimental procedures and designs
Figure 3-1 shows the experimental design of this study. Experiments of alkalization and ultrasound pretreatment followed the steps as reported by Chiu et al. (1997). DEHP was the most quantitative PAEs in municipal sewage sludges for many cities around the world (Table 2-3). In order to understand the treatment efficiency of high strength DEHP sludge, DEHP was spiked to the collected sewage sludge with the level of 200 mg/kg-dw, which was higher than the limit value designed by the EU. Two experimental parameters, i.e.
NaOH concentration and sonication time were investigated to understand the effect on PAEs removal and SCOD increase (Table 3-3 and Table 3-4). The procedures of pretreatment experiment were as follows: (1) alkalization reaction for 24 hours by adding 1 M NaOH and (2) ultrasound reaction for designed sonication time. After a combined alkalization-sonication pretreatment experiments, sludge sample was collected to obtain the results of TS, VS, DBP, DEHP, BBP, COD, SCOD and pH changes.
The CCD was used to simplify the number of experiments and create response surface (Mpntgomery, 2006). CCD was operated by Minitab 14 because it not only calculated the natural variables in the range of different parameters but also randomized the experimental order of different variables. Besides, the software created the response surfaces and calculated the equation of response surface and R2 value. In this study, the changeable parameters were NaOH concentration (ranged between 0 and 80 mM) and sonication time (ranged between 0 and 15 min) listed in Table 3-3 and Table 3-4.
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Fundamental analysis: TS, VS, DBP, DEHP, BBP, COD, SCOD and pH
NaOH concentration Sonication time
Determine the best pretreatment condition Experimental design of pretreatments
Alkalization pretreatment for 24 hours
Ultrasound pretreatment for designed reaction time
Figure 3-1 Experimental processes in this study
Table 3-3 Ranges and levels of designed factors for CCD
Factors
Levels
-1.414 -1 0 1 1.414
A: NaOH concentration (mM) 0 12 40 68 80
U: Sonication time (min) 0.0 2.2 7.5 12.8 15
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Table 3-4 Sequence of runs for CCD
Run order NaOH concentration (mM) Sonication time(min)
1 68 12.8
2 40 0.0
3* 40 7.5
4 0 7.5
5 68 2.2
6 40 15.0
7* 40 7.5
8* 40 7.5
9 80 7.5
10 12 2.2
11 12 12.8
*Runs 3, 7 and 8 could be considered as the triplicate tests
3-4 Experimental apparatus
3-4-1 Alkalization experiment
The alkalization pretreatment was conducted in a glass reactor equipped with a mechanic mixer (Figure 3-2). The sewage sludge was added to glass reactor and agitated by the mechanic mixer. During agitation, 1 M NaOH was added into the reactor and mixed well for 24 hours at the mixing speed of 400 rpm. Adding 12, 40, 68 and 80 mM NaOH to sewage sludge was equal to adding 12.0, 41.5, 72.9 and 87.2 mL of 1 M NaOH to 1 L sewage sludge, individually. After alkalization reaction, the alkalized sludge was taken for
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further ultrasound pretreatment.
Figure 3-2 Experiments setup for alkalization pretreatment
3-4-2 Ultrasound experiment
In this study, K-Sonic sonicator was used to conduct the ultrasound pretreatment of sewage sludge. Frequency, power output and surface diameter of horn of this sonicator were 20 kHz, 1 kW and 48 mm, respectively. The schematic diagram of the sonicator is shown in Figure 3-3 and the operation parameters in sonication are given in Table 3-5. The converter was used to convert the electrical energy into ultrasound energy. The booster was a mechanical amplifier that helped to increase the amplitude (vibration) to the horn. The horn was used to deliver the ultrasonic energy to the sludge. During sonication, the distance
NaOH addition
Sewage sludge
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between the surface of sludge and the rim of beaker was kept as higher as possible to avoid the splashing of sludge around the horn.
Figure 3-3 Diagram of K-sonic sonicator
Table 3-5 Fixing parameters during sonication
Parameters Data
Frequency 20 kHz
Power density 1 W/mL
Power intensity 55 W/cm2
TS of sludge 3%
20 kHz
Sonicator
2 L beaker containing 1 L sludge
Horn Booster Converter
Power Frequency test
Nodal plan
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3-5 Analytical methods
3-5-1 Analysis of PAEs
1. Extraction steps
Analysis of PAEs followed the steps reported by Heise and Litz (2004). Sludge was dried at 105oC for 16 hours prior to extraction, then the dried sludge was ground by a grinder. Because of the high boiling point of PAEs, the characteristics of them are very stable during sludge drying. After grinding, 2 g dried sludge was added to Teflon centrifugal tube, then added with 10 mL of solvent n-hexane and dichloromethane at a volume ratio of 1:1. The sample was shaken by a shaker for 24 hours at ambient temperature. After shaking, the extracted sample was centrifuged by Harmonic Series centrifuge machine for 10 minutes at 3,500 rpm. After centrifugation, the supernatant of extracted solvent was collected to analyze PAEs concentration by GC-FID. The recovery of spiked DEHP was 85.4%.
2. GC-FID analysis
A GC-FID (Agilent Technology 7890A) equipped with a HP-5 capillary column (Agilent 19091J-413, 30 m long, 0.32 mm inner diameter, 0.25 m film thickness) was used in this study for PAEs determination. The operating conditions of GC-FID are listed in Table 3-6. The temperature of oven was programmed as followed: initial temperature of 120oC and hold for 1 min, then raise to 300oC with a rate of 20oC/min and hold for 5 min.
During GC-FID analysis, the retention times of DBP, BBP and DEHP were 6.66, 8.42 and
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9.17 min, respectively (Figure 3-4).
Table 3-6 Fixing parameters of GC-FID conditions
Parameters Data
Front inlet temperature 280oC
Front detector temperature 280oC
Makeup gas flow rate (He) 3 mL/min
N2 flow rate 22 mL/min
H2 flow rate 40 mL/min
Air flow rate 450 mL/min
Injection volume 1 L
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Figure 3-4 Peak locations of standards: (a) DBP and BBP and (b) DEHP
3. Calibration curves
Five different PAEs standards (5, 10, 25, 50 and 100 mg/L) were prepared to develop the calibration curves (Figure 3-5). In Figure 3-5, R2 values were all higher than 0.9991. In addition, 1 mg/L PAEs standards was prepared to get the method detective limits (MDL), where the MDLs of DBP, DEHP and BBP were 0.43, 0.27 and 0.98 mg/L, respectively.
30 Linear Fit of Peak area Linear equation: y = 11.966 x
R2 = 0.9997 MDL = 0.43 mg/L (a)
Linear equation: y = 6.056 x R2 = 0.9991 Linear Fit of Peak area
Linear equation: y = 13.413 x R2 = 0.9999 Linear Fit of Peak area
Figure 3-5 Calibration curves of GC-FID analysis: (a) DBP, (b) DEHP and (c) BBP
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3-5-2 Analysis of TS, VS, COD and pH
The experimental apparatus used for TS, VS, COD and pH analysis were shown in Table 3-7. TS, VS and COD analyses were according to 2540 B, 2540 E and 5220 B of standard methods, respectively (AWWA, APHA and WEF, 2005). Before analyzing TS and VS, evaporating dishes are prepared. In order to analyze SCOD, sludge sample was centrifuged by Harmonic Series centrifuge machine for 10 min at 3,500 rpm at ambient temperature to separate liquid and solid phase. The liquid phase was filtrated using 0.45 m Advantec membrane filter. The standard method as reported in AWWA, APHA and WEF, (2005), i.e. open reflux method, was adopted for COD analysis.
Table 3-7 Experimental equipments of fundamental analysis in this study
Equipment Purpose
Channel 105oC drying oven (DV-602) TS
Nabertherm 550oC muffle furnace (L9/R) VS
Sartorius electrical balance (BP221S) (capable of weighing to 0.1 mg)
TS and VS
Den Yng reflux apparatus COD
Suntex pH meter pH
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