4-1 PAEs levels in collected sewage sludge
The sewage sludge from Di-Hua wastewater treatment plant was collected on Aug.
20th, 2009 and Jan. 4th, 2010. The order of original PAEs concentrations in the raw sewage sludge was DBP > DEHP > BBP (Table 4-1).
Even part of the PAEs concentration contained in raw sludge are low, the presence of PAEs should be put attention on, especially if the PAEs contained sewage sludge was used in land application, PAEs could transport from sludge to soil then be accumulated in plants/crops (Cecil et al., 1992). In recent years, PAEs had attracted much attention because even at low concentration levels they were suspected of interfering with reproductive and behavioral health in humans and wildlife, through disturbance of the endocrine system (Petrovic et al., 2001). After exposing PAEs by dermal contact, inhalation and ingestion of humans, PAEs could convert to monoesters of which toxicity was more than PAEs in human blood (Woodward, 1988). In order to moderate acute toxic, it is important to control PAEs of their high production volume and their ubiquitous occurrence (Heise and Litz, 2004). In order to decrease the toxicity of PAEs, sludge pretreatment was conducted with alkalization, sonication and a combination of alkalization-sonication in this study.
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Table 4-1 PAEs concentration in Di-Hua WWTP sewage sludge
PAEs
Concentration (mg/kg-dw)
Aug. 20th, 2009 Jan. 4th, 2010
DBP 718 1,971
DEHP 41 74
BBP 8 16
4-2 Influences of sludge qualities after pretreatments
Alkalization and ultrasonication not only effect the transformation between particulate and soluble organics and the degradation of recalcitrant and toxic organics, but also involve in solids mass, sludge pH and toxicity of reagent. The degrees of these effective factors are list in this:
1. Solids mass
During pretreatments, the solids mass could not be changed (Table 4-2). Even though the recalcitrant organics are removed by pretreatments, these recalcitrant organics are transformed to the low molecular weight compounds. Due to this reason, the soluble organics could increase by pretreatments. The values of TS and VS were not changed along with NaOH concentration and sonication time. The values shown in Table 4-2 reveal that pretreatments could maintain the solids mass balance of sewage sludge.
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Table 4-2 TS and VS of sludge during pretreatments
Run
NaOH concentration (mM)
Sonication time (min)
Initial After alkalization After alkalization-sonication
TS (%) VS (%) TS (%) VS (%) TS (%) VS (%)
1 68 12.8
2.97 2.08
2.98 1.99 3.08 2.07
2 40 0.0 2.96 1.94 2.96 1.94
3 40 7.5 2.96 1.94 2.90 1.90
4 0 7.5 2.97 2.08 2.89 1.99
5 68 2.2 2.98 1.99 2.94 1.99
6 40 15.0 2.96 1.94 3.03 1.98
7 40 7.5 2.96 1.94 3.04 2.02
8 40 7.5 2.96 1.94 3.05 2.04
9 80 7.5 3.10 1.99 3.05 1.95
10 12 2.2 2.99 2.02 2.88 1.97
11 12 12.8 2.99 2.02 2.99 2.04
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2. Sludge pH
In sludge pretreatments, NaOH alkalization not only increased soluble organics to facilitate biodegradation in digestion but also offered the alkalinity of sludge; hydroxides (OH-), carbonates (CO3
2-) and HCO3
were the sources of alkalinity (Metcalf and Eddy, 2004). The pH values of sludge before and after pretreatments were listed in Table 4-3.
After alkalization, the pH values increased from 6.7 (initial) to 7.8 - 11.5. The role of NaOH played in sludge treatment was not only the pretreatment before digestion but also chemical stabilization after pretreatment (Cecil et al., 1992). Ultrasound pretreatment would not affect the pH value of sludge significantly.
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Table 4-3 pH changes during pretreatments
Run
Initial After alkalization After alkalization-sonication
1 68 12.8
3. Toxicity of reagent
NaOH alkalization could increase the toxicity of sewage sludge which sodium is the toxic light metal. When adding more than 5,000 mg/L of sodium, high concentration of sodium could disturb biological treatment in anaerobic digestion (Cheremisinoff, 1994). In this study, adding 80 mM (or 3,200 mg/L NaOH) to sludge did not affect digestion significantly supposing that the initial sodium concentration of raw sewage sludge was very less. Although enough NaOH concentration could remove recalcitrant organics and harmful
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microorganisms, increase SCOD and stable sludge quality, the additional sodium concentration should be controlled during pretreatment or chemical stability.
4-3 PAEs changes
4-3-1 PAEs changes after sonication
According to the plots of peak distribution of sewage sludge in GC-FID analysis (Figure 4-1), the peak areas of each compound between without and with sonication didn’t change significantly. In fact, after sonication, only DBP was decreased from 718 to 687.8 mg/kg-dw while DEHP and BBP were almost same before and after sonication. In order to remove some recalcitrant organics of sewage sludge by sonication, other physical or chemical methods must be attached before sonication to facilitate these compounds removal.
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Figure 4-1 Peak distribution of sewage sludge in GC-FID: (a) without sonication and (b) sonication for 7.5 min
4-3-2 PAEs changes after alkalization
PAEs concentrations after the alkalization pretreatment are shown in Table 4-4. In Table 4-4, only 9% of DBP was decreased by alkalization with 12 mM NaOH addition. The concentration of DEHP and BBP were almost constant before and after alkalization.
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However, there was an exponential relationship between NaOH concentration and DBP removal (Figure 4-2). If 75 mM of NaOH was added to sludge, removal of DBP achieved nearly to 100%. This result could be explained by the reaction between hydroxyl ions and DBP where the DBP was converted to hydrophilic organics, such as monobutyl phthalate (MBP) so that the DBP was effectively removed by alkalization (Yim et al., 2002). The more hydroxyl ions addition leads to more DBP degradation. Since the BBP and DEHP were not decreased with NaOH addition, it was understood that lower molecular weight PAE such as DBP could be easily degraded by hydroxyl ions.
Table 4-4 PAEs concentration after alkalization NaOH concentration
(mM)
Initial PAE (mg/kg-dw) PAE after alkalization (mg/kg-dw)
DBP DEHP BBP DBP DEHP BBP
12 1,971a 215b 8b 1,794 214 8
40
718b 215b 8b
298 212 8
68 9 196 6
80 0 209 8
aSewage sludge collected on Jan. 4th, 2010
bSewage sludge collected on Aug. 20th, 2009
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0 10 20 30 40 50 60 70 80 90 100
0 20 40 60 80 100
DBP removal (%)
NaOH concentration (mM) DBP removal (%)
Exponential of DBP removal
Exp. equation: y = 115.76 - 153.67 e- 0.03 x R2 = 0.992
Figure 4-2 DBP removal during alkalization pretreatment
4-3-3 PAEs changes after alkalization and sonication
PAEs removal efficiencies after both alkalization and ultrasound pretreatment are given in Table 4-5, where a response surface contour is plotted by Minitab 14 (Figure 4-3).
The response surface equations of three contour plots calculated from Minitab 14 are listed in Table 4-6.
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Table 4-5 PAEs removal after pretreatments Run NaOH concentration
(mM)
Sonication time (min)
Removal (%)
DBP DEHP BBP
1 68 12.8 100.0 8.3 22.7
2 40 0.0 58.6 1.7 0.0
3 40 7.5 64.1 1.3 0.0
4 0 7.5 4.2 0.0 0.0
5 68 2.2 100.0 6.0 16.0
6 40 15.0 80.5 0.0 0.0
7 40 7.5 62.9 0.0 0.0
8 40 7.5 64.0 0.0 0.0
9 80 7.5 100.0 4.6 0.0
10 12 2.2 0.8 5.0 16.0
11 12 12.8 0.0 0.0 0.0
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Figure 4-3-1 Contour plot of DBP removal after pretreatments (%)
NaOH concentration (mM)
Figure 4-3-2 Contour plot of DEHP removal after pretreatments (%)
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Figure 4-3-3 Contour plot of BBP removal after pretreatments (%)
Table 4-6 Response surface equations of PAEs removal
PAEs Response surface equation R2
DBP z=-16.739+2.313x+0.506y–0.011x2+0.010y2+0.001xy 0.944 DEHP z=7.725–0.196x–1.151y+0.002x2+0.036y2+0.012xy 0.741 BBP z=22.656–0.525x–3.556y+0.004x2+0.122y2+0.038xy 0.405 x = NaOH concentration (mM)
y = Sonication time (min) z = PAEs removal (%)
According to the response surface equations, the equation of DBP removal was applicable for finding the optimal NaOH dosage and sonication time. However, the other equations of DEHP and BBP removal were not used because their R2 values were lower
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than 0.8. In Figure 4-4, the extracted liquid after alkalization-sonication contained not only DBP, DEHP and BBP but also the other unknown compounds. The compounds with retention times of GC-FID analysis lower than 8 minutes were degraded significantly by pretreatments. The lower molecular weight compounds such as DBP was easily degraded by pretreatments than the higher ones such as DEHP and BBP (Neis, 2002). Combining alkalization and ultrasound pretreatment could facilitate DBP removal of sewage sludge.
Even the combination of alkalization and ultrasound pretreatment showed the better result for PAEs removal, the contribution by alkalization and ultrasound individually were different (Table 4-7). In combination pretreatments, alkalization contributed more than 90%
for DBP removal in most of runs.
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Figure 4-4 Peak distributions: (a) without pretreatment; (b) alkalization for 68 mM NaOH and (c) alkalization for 68 mM NaOH and sonication for 12.8 min
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Table 4-7 Contribution percentages for DBP removal between two pretreatments
Run
4-4-1 Variation of COD after alkalization
Figure 4-5-1 shows the relationship between NaOH concentration and SCOD after alkalization. The results indicate that the more hydroxyl increasing the NaOH concentration leads to the better SCOD increase; 8.37 mg/L of SCOD increase was observed as per 1 mM of NaOH addition. In addition, the result of total COD was slightly changed in alkalization, which indicated that the particulate COD could be transferred to SCOD (Figure 4-5-2).
47 Linear equation: y = 423.14 + 8.37 x
R2 = 0.962
Figure 4-5-1 SCOD after alkalization pretreatment
0 10 20 30 40 50 60 70 80 90 100
Figure 4-5-2 COD after alkalization pretreatment
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4-4-2 Variation of COD after alkalization and sonication
Table 4-8 shows the result of SCOD concentration and SCOD/COD ratio after alkalization-sonication pretreatments. It is found that the higher NaOH concentration addition and longer sonication time get the higher SCOD concentration and SCOD/COD ratio. The contour plot of Figure 4-6 is drawn by Minitab 14 according to the results of Table 4-8 of SCOD concentration and the response surface equation of the contour plot is
z=–288.165+125.622x+599.105y–0.847x2–24.978y2–1.567xy (6)
Table 4-8 SCOD and SCOD/COD after alkalization and sonication
Run
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Figure 4-6 Contour plot of SCOD after pretreatments (mg/L)
The optimal point of NaOH concentration and sonication time could be found based on the equation, where the best NaOH concentration and sonication time were 68 mM and 10 min. Even though the NaOH concentration and sonication time increased to 80 mM and 15 min, SCOD increased about 200 mg/L more than that of the optimal pretreatment parameters. In other words, SCOD/COD could increase less than 1% while NaOH concentration and sonication time were 80 mM and 15 min. Total COD could not be changed significantly along with NaOH concentration and sonication time (Table 4-9).
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Table 4-9 COD during pretreatments
Run
Initial After alkalization After alkalization-sonication
1 68 12.8
If pretreatment combines both alkalization and ultrasound, the contribution percentages were different between the two pretreatments (Table 4-10). If NaOH concentration was 12 mM, ultrasound reaction could contribute more than 80% for SCOD increase. If NaOH concentration was more than 40 mM, ultrasound reaction contributed less than 50% for SCOD increase. Kim et al. (2002) demonstrated the comparison between alkalization and sonication in which soluble organic carbon could increase in alkalization pretreatment (2 M NaOH for 10 min reaction) more than the ultrasound pretreatment (4 W/mL density, 20 kHz for 30 min reaction). In this study, SCOD increase with 40 mM
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alkalization was more than with 7.5 min ultrasound reaction (Table 4-11). Combination of two pretreatments could get better result for SCOD increase. The optimal point of response surface plot could be applied for the reaction application.
Table 4-10 Contribution percentages for SCOD increase between two pretreatments
Run
Table 4-11 SCOD increase in single pretreatment
Pretreatment method SCOD increase (mg/L) Alkalization in 40 mM NaOH concentration for 24 hours 3,610
Ultrasound reaction for 7.5 minutes for 1 W/mL density 3,210
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