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Re-use of water treatment works sludge to enhance particulate pollutant removal from sewage
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Water Research 39 (2005) 3433–3440 www.elsevier.com/locate/watres
Re-use of water treatment works sludge to enhance particulate pollutant removal from sewage Xiao-Hong Guan, Guang-Hao Chen, Chii Shang Department of Civil Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China Received 29 October 2003; received in revised form 25 June 2004; accepted 8 July 2004 Available online 10 August 2005
Abstract This paper attempted to study the feasibility of reusing water treatment works sludge (‘‘alum sludge’’) to improve particulate pollutant removal from sewage. The main issues focused upon were: (1) the appropriate dosage of the alum sludge, (2) the appropriate operating conditions, and (3) the possible mechanisms for enhancement by adding alum sludge. Actual alum sludge and sewage were applied to a series of jar tests conducted under various conditions. It has been found that both the SS and COD removal efficiencies could be improved by the addition of the alum sludge, which was mainly attributed to the removal of relatively fine particles with a size of 48–200 mm. The appropriate dosage of the alum sludge was determined to be 18–20 mg of Al/L. Increasing the mixing speed or reducing the floc size of the alum sludge enhanced the SS and COD removal and the dispersed alum sludge could remove particulate contaminants with smaller size than the raw sewage. ToF–SIMS evidence revealed that the aluminum species at the surface of the alum sludge were effectively utilized for improving the SS and COD removal. It was postulated that the sweep flocculation and/or the physical adsorption might play key roles in the enhancement of particulate pollutant removal from sewage. r 2005 Elsevier Ltd. All rights reserved. Keywords: Alum sludge re-use; Primary treatment; Jar test; Suspended solids; COD; Particle size distribution
1. Introduction In a typical primary treatment system, the removal efficiencies of suspended solids (SS) and chemical oxygen demand (COD) are around 50% and 30%, respectively. Such low removal efficiencies are mainly due to the insufficient removal of finely divided suspended particles that account for a large portion of the SS. Peavy pointed out that removal of small particles (o50 mm) is almost impossible in a conventional primary sedimentation tank (Peavy et al., 1985). Thus, to improve the removal of both SS and COD from Corresponding author. Tel.: +852 2358 8752; fax: +852 2358 1534. E-mail address: [email protected] (G.-H. Chen).
sewage in primary treatment, particle agglomeration through chemical coagulation/flocculation is necessary and has actually been applied to the primary sewage treatment in Hong Kong (Chen, 1996). It should be noted that such chemically enhanced primary sewage treatment imposes a high-operational cost incurring from the use of coagulants and the treatment and disposal of a large amount of chemical sludge. Therefore, it is necessary to find a low-cost alternative to induce the adsorption and enmeshment of fine particles in the primary sewage treatment. Re-use of aluminumladen sludge (referred to as ‘‘alum sludge’’ in this paper) from water treatment works may provide such a solution because the alum sludge contains a large portion of insoluble aluminum hydroxides (Chu, 1999, 2001) that can be utilized as a coagulant in the primary
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.07.033
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sewage treatment. Re-use of the alum sludge may not only improve the particulate pollutant removal efficiency of a primary sewage treatment, but also ease the burden of water treatment works relating to sludge treatment and disposal. In order to verify the possibility of the above alum sludge re-use idea discussed above, the following five issues need to be addressed: (1) feasibility of this approach, (2) impact of the alum sludge on the performance of the existing primary and secondary sewage treatment plants, (3) appropriate dosage of the alum sludge and the appropriate operating conditions for maximizing the effect of the alum sludge, (4) a possible mechanism responsible for the alum sludge effect, and (5) a cost-effective method to transfer alum sludge from a water treatment works to a sewage treatment works for re-use. Regarding the first and second issues, which are very important for future application, several studies have been carried out to investigate the effect of adding alum sludge in primary wastewater treatment on the performance of activated sludge process, anaerobic digestion of wastewater sludge, and the overall performance of wastewater treatment plant (Hsu and Pipes, 1972, 1973; Salotto et al., 1973; Wilson et al., 1975; Culp and Wilson, 1979). Culp and Wilson (1979) pointed out that no adverse effects on overall wastewater treatment plant performance were identified, which is consistent with the findings of Hsu and Pipes (1972, 1973) and Salotto et al. (1973). It was observed that there was an apparent reduction of scum and sludge bulking problems in the secondary clarifier (Hsu and Pipes, 1973; Culp and Wilson, 1979). The carry-over of the slower-settling solids from the alum sludge to the activated sludge process improved the settling of the waste activated sludge and increased the process efficiency (Salotto et al., 1973). It was also reported that alum sludge addition could increase the gas production in sludge digester and capacity of centrifuges for sludge dewatering (Culp and Wilson, 1979). As for the final issue, Montgomery (1985) and Qasim et al. (2000) suggested a controlled discharge of alum sludge into a sewer system, provided that the sewer system is monitored appropriately during the transfer of the sludge. The third and fourth issues have not been studied, though they are of particular importance for maximizing the benefits gained from the alum sludge re-use, such as the enhancement of SS and COD removal, the control of sludge bulking, and the improvement of sludge dewaterability. Therefore, the objectives of this study are to investigate the appropriate dosage of alum sludge, to evaluate the effect of mixing speed as a key operational factor, and to examine the possible mechanism involved in SS and COD removal enhancement by alum sludge. The effect of alum sludge on improving SS and COD removal may depend on both the dosage of sludge (as Al) and Al
content in alum sludge. Since the aluminum fractions in alum sludge has been found to be relatively constant in wastes produced at several plants (Warriner and O’Blenis, 1972), the effect of Al content in alum sludge on pollutants removal was not investigated.
2. Materials and methods 2.1. Characterization of the alum sludge and the sewage Real sewage and alum sludge were employed in this study to determine the appropriate dosage of alum sludge. The sewage was taken from an inlet of a primary sedimentation tank in the largest local sewage treatment plant, while the alum sludge in slurry form was obtained from the same works, where the alum sludge is transferred from a nearby water treatment works for co-dewatering with the sewage treatment sludge. Both the raw sewage and the alum sludge samples were preserved in a cold room at 4 1C. The pH, COD, SS and zeta potential were determined to characterize the raw sewage, and the pH, aluminum content, solid content and zeta potential of the sludge samples were measured to characterize the alum sludge. 2.2. Test procedure and condition Jar tests were performed with a standard jar testing device (Stuart Scientific) to simulate a conventional coagulation/flocculation process. The jar testing procedure, which was adopted from Bell-Ajy et al. (2000), was initiated with a rapid mixing at 120 rpm for 1 min, followed by a slow mixing at 40 rpm for 10 min and at 20 rpm for 10 min, consecutively; and finally there was a 30 min settling. Control tests without the alum sludge addition were also conducted in parallel. All the jar tests were carried out in a temperature controlled room at 2171 1C. The testing mainly focused on the effects of the alum sludge dosages, the rapid mixing speed, and the floc size of the alum sludge on the improvement of the SS and COD removal from the raw sewage. Thus, the tests were conducted with different amounts of raw alum sludge or alum sludge dispersed with an ultrasonic processor (GE130) under 60 W for 15 min, while the rapid mixing speeds ranged from 120 to 247 rpm. In all cases, the pH remained unadjusted. After each test, the supernatant was sampled for measuring SS, volatile suspended solids (VSS), and COD concentrations. The settled sludge volume, the initial solids content, the final solids content in the settled sludge before and after sludge dewatering, and the time and specific resistance to filtration of the settled sludge were also measured. In order to evaluate the COD release from the alum sludge during the tests, synthetic wastewater, prepared from a 400 mg/L NaHCO3 solution with a pH of 7.5 adjusted
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by 0.1 M NaOH, was employed in the COD release tests which were carried out in a temperature controlled room at 2171 1C. The COD release tests followed abovementioned jar test procedure, that is, initiated with a rapid mixing at 120 rpm for 1 min, followed by a slow mixing at 40 rpm for 10 min and at 20 rpm for 10 min, consecutively; and finally there was a 30 min settling.
2.3. Chemical and physical analyses The SS and the VSS were measured according to the Standard Methods (APHA, 1995). COD was measured using a colorimetric test kit (HACH model) based on Method 5220D in the Standard Methods (APHA, 1995). The pH value of the samples was determined using a pH meter with a high-performance probe (Corning 350). The zeta potentials of the alum sludge and the raw sewage samples were measured with a zeta potential analyzer (Zetaplus). A particle size analyzer (Coulter LS230) was employed to determine the particle size distribution in the samples. The aluminum content in the alum sludge was analyzed using an atomic absorption spectrophotometer (Hitachi, Z-8200) equipped with a nitrous-oxide/acetylene flame after the sample pretreatment using a microwave digestion system (MDS2000). The settling rates of sludge flocs were determined according to the method proposed by Diamadopoulos and Benedek (1984) so that the settleability of suspended particles before and after the alum sludge addition could be evaluated. The solid content of the settled sludge was examined following the Standard Methods (APHA, 1995) except that a duration of 24 h was employed (Lai, 2001). The dewaterability of the settled sludge, which was characterized by the specific resistance to filtration (SRF) and the time to filtration (TTF), was determined by using the Bu¨chner funnel test (APHA, 1995). The final solid content was measured with the method proposed by Lee and Hsu (1994) and Matsuda et al. (1992). A time-of-flight secondary-ion mass spectrometer (ToF-SIMS) (Physical Electronics 7200) was adopted to investigate the surface characteristics of the alum sludge before and after the tests. To prepare the ToF-SIMS analysis, the sludge samples were dried in an oven at 103–105 1C for 72 h.
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3. Results and discussion 3.1. Characterization of the alum sludge and sewage Table 1 summarizes the characteristics of the raw sewage and the alum sludge used in this study. A high salinity was observed in the raw sewage which was due to the introduction of seawater for toilet flushing. A low solid content was typically found in the alum sludge, indicating that the alum sludge had not gone through any thickening and/or dewatering processes. Both the raw sewage and the alum sludge samples showed a negative zeta potential. 3.2. Effect of alum sludge dosage on SS and COD removal Fig. 1 shows the effect of the alum sludge dosage on the removal efficiencies of SS and COD. The SS removal efficiency was increased up to 72–90%, compared to 52–66% in the control tests. Similarly, the COD removal efficiency was improved by more than 15% compared to the control levels (28–38%). The enhancements were mainly attributed to the removal of fine particles with size ranging between 48 and 200 mm, as shown in Fig. 2, which illustrates the particle size distribution in the raw sewage and the supernatants with or without the alum sludge addition. The particles larger than 200 mm were removed but the smaller particles were partially removed when no alum sludge was applied. On the other hand, when alum sludge was introduced, the smaller particles (48–200 mm) disappeared in the supernatant, which indicated that the removal of all the particles with a size between 48 and 200 mm could be achieved through the alum sludge introduction. Obviously the SS removal efficiency increased when the alum sludge dosage increased, while the COD removal efficiency increased until a maximum level was reached at the alum sludge dosage of 70 mg/L. This may be associated with the nature of the alum sludge because some organic matters, called as COD, were released from the alum sludge, as shown in Fig. 3. The amount of organic matter released from the alum sludge was related to the dosage of the alum sludge. When the alum sludge was dosed at 18–20 mg Al/L, the COD concentration of the wastewater supernatant was
Table 1 Characteristics of the raw sewage and alum sludge Raw sewage Zeta potential (mV) SS (mg/L) VSS/SS ( ) COD (mg/L) pH
Alum sludge 0.24 to 236–356 0.85–0.9 390–490 7.2–7.8
12.39
Zeta potential (mV) Solid content (w/w%) VSS/SS ( ) Aluminum content (w/w%) pH
17.44 to 2.4–2.8 0.28–0.32 0.3–0.4 6.5–6.9
37.4
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40
SS
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1 COD
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30 60 90 120 Dosage of alum sludge (mg Al/L)
150
0 0
20
40
60
80
0.5 100
Dosage of alum sludge (mg Al/L)
Fig. 1. SS and COD removal efficiency at different dose of alum sludge.
Fig. 3. COD concentration and VSS/SS ratio in the supernatant of the tests with different dosage of alum sludge.
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Fig. 2. Particle size distribution in the raw sewage and the supernatant of the tests with and without the addition of alum sludge.
about 200 mg/L and the COD concentration of the supernatant in the release tests was about 15 mg/L. Therefore, the contaminants released by alum sludge contributed about 7.5% of the COD in the supernatant of wastewater if alum sludge was applied at 18–20 mg Al/L. There was a decrease in the VSS/SS ratio of the supernatant when the alum sludge dosage increased, as illustrated in Fig. 3, suggesting that the SS remained in the supernatant shifted from organic to inorganic substances due to the addition of the alum sludge. Since most of the particles in the alum sludge were inorganic as the VSS/SS ratio of the alum sludge was only 30%
while that of the raw sewage was 85–90%, the decrease in the VSS/SS ratio of the supernatant was caused by the release of inorganic particles from the alum sludge. It could be associated with the back diffusion of contaminants from the reused alum sludge as proposed by Chu (1999), who also observed similar phenomenon. Therefore, the enhancement of the SS and COD removal by alum sludge depends on the contaminants removed from the water phase as well as on those released from the alum sludge. When the dosage of the alum sludge was 18–20 mg Al/L, the contaminants removed by the alum sludge far exceeded the contaminants that were released, thereby resulting in a considerable enhancement in both the SS and COD removal efficiencies. Although the particulate contaminants released from the alum sludge exerted a negative impact on the SS and COD removal in the primary sewage treatment, these contaminants may be helpful in controlling sludge bulking in the secondary treatment (Hsu and Pipes, 1973; Salotto et al., 1973). It had been elucidated that the carry-over of suspended solids from alum sludge could increase the density and settleability of biological flocs in the following secondary clarifier (Salotto et al., 1973). The SS removal efficiency was as high as 90.3% by dosing the alum sludge at 120 mg Al/L and the COD removal efficiency to 66.0% by setting the alum sludge dosage at 75 mg Al/L, as shown in Fig. 1. However, this study found that the resulted sludge volume increased linearly with the increase in the dosage of alum sludge. Therefore, such high-alum dosages as 120 or 75 mg Al/L were not appropriate due to the huge sludge volume under these situations. To minimize the increase in sludge volume while maintaining the high-removal
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efficiencies of SS and COD, the alum sludge dosage of 18–20 mg Al/L was considered to be adequate in this study. At this dosage, the SS and COD removal efficiency was found to be around 85% and 52%, respectively. When the alum sludge was reused at this dosage, the volume of the settled sludge was increased by 50% only compared to the control tests without the alum sludge addition. Reduction in alum sludge dosage (smaller ALT ratio) could not only reduce the volume of the settled sludge but also increase the density of the sludge flocs (Watanabe and Toyoshima, 1988). However, the SS and COD removal efficiency would decrease at a smaller ALT ratio. Considering the variation of the characteristics of wastewater with time, 12 runs were performed to check the stability of the effluent quality with dosing alum sludge, as shown in Fig. 4. It was found that a stable effluent quality could be achieved and the SS concentration in the supernatant of the tests was in the range of 34–52 mg/L by dosing alum sludge at 18–20 mg Al/L. However, the appropriate dosage of alum sludge needed to be verified by a full-scale trial. To achieve similar removal efficiency of SS and COD (85% for SS and 52% for COD) in a settling tank, the minimal amount of pure alum might be 7–10 mg/L as Al (Hsu and Pipes, 1972, 1973), which was about 50% of the recommended dosage, 18–20 mg/L as Al, of the real
400
Raw sewage Supernatant of control tests Supernatant of tests with alum sludge dosed at 18 mg-Al/L
SS (mg/L)
300
100
0 2
3
4
alum sludge used this study. Such a difference reasonably resulted from the significant difference in the floc nature and properties of the pure alum and the alum sludge of a water treatment plant. Therefore, Wilson et al. (1975) suggested that the appropriate dosage of real alum sludge could be up to 40 mg/L as Al, depending on the sludge properties. The characteristics of the alum sludge, the settled primary sludge obtained from the control tests and the settled sludge produced from the alum sludge-added tests (combined sludge) are summarized in Table 2. It was found that with the introduction of the alum sludge the thickening and dewatering properties of the combined sludge were improved in terms of final solid content, time to filtration and specific resistance to filtration compared to those of the primary sludge. Watanabe et al. (1987) also found that reusing alum sludge in primary sedimentation tank could improve the thickening and dewatering ability of the combined sludge. It was observed in this study that the settling rates of the sludge flocs were increased by adding the alum sludge to the raw sewage. The experimental results showed that assuming an overflow rate of 1.5 m/h, the floc removal could be enhanced from 56% to 74%. In other words, even when the overflow rate was assumed to be 2 m/h, 71% of the flocs could be removed. This may be due to the inorganic aluminum hydroxide enmeshment that can increase the sludge density and the settling rate of flocs.
3.3. Effect of the mixing speed
200
1
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5 6 7 8 9 10 11 12 Number of runs
Fig. 4. Comparison of the SS concentrations in raw sewage, the supernatants of the control tests and the tests with alum sludge dosed at 18 mg Al/L.
Fig. 5 illustrates the particle size distribution in the samples of the raw alum sludge, the alum sludge after a rapid mixing, and the settled sludge after a jar test with the addition of the alum sludge. The particle size of the alum sludge after a rapid mixing became smaller than the particle size of the raw alum sludge. This was because the rapid mixing induced the temporal detachment of particles from the sludge nucleus. The difference between the particle size of the raw alum sludge and that of the settled sludge reflected that the alum sludge agglomerated more particles at the sludge surface during slow mixing or that the particles could be entrapped by alum sludge during the settling. Therefore, the re-use of
Table 2 Characteristics of different types of sludge used in this study
Solid content (%) Final solid content (%) Time to filtration (s) Specific resistance to filtration (m/kg)
Raw alum sludge
Combined sludge
Primary sludge
2.08 16.23 930 9.02 1012
2.06 28.15 846 1.17 1013
1.90 15.57 1181 1.85 1013
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6 Alum sludge after rapid mixing
5 Volumetric percent (%)
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150
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100 50
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Dispersed alum sludge Raw alum sludge
3
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1 0 0
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Fig. 5. Particle size distribution in the raw alum sludge, the alum sludge after the rapid mixing, and the settled sludge of the jar test.
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Particle size (µm) Fig. 7. Particle size distribution in the raw and dispersed alum sludge.
COD removal efficiency (%)
100
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40 120 rpm 190 rpm
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247 rpm 0 0
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100
Fig. 6. COD removal under different rapid mixing conditions.
alum sludge in the primary treatment is a process where particles in sewage are either attached onto the nucleation sites of the alum sludge or entrapped by it during the settling. Considering the decrease in the VSS/ SS ratio in the supernatant with an increase in the alum sludge dosage, a rearrangement of the particles at the surface of the alum sludge is also expected. A higher COD removal efficiency was achieved with a higher mixing speed, as depicted in Fig. 6. The range of mixing speed examined in this study was limited by the jar testing device. More particles were released from the alum sludge and the specific surface area of the alum sludge was increased accordingly under higher mixing speeds, thus creating more nucleation sites for aggregating and adsorbing particles and eventually increasing the
particulate pollutant removal. To verify this hypothesis, alum sludge samples were dispersed by employing an ultrasonic processor and were then subjected to the jar test. Fig. 7 shows that the particle size of the dispersed alum sludge decreased compared to the particle size of the raw alum sludge. The SS and COD concentrations of the supernatant after applying 18–20 mg/L dispersed alum sludge were found to be lower by about 15 and 5 mg/L, respectively, compared to those with applying same dose of raw alum sludge. Particle size analysis of the supernatants after applying alum sludge with/without dispersion was carried out and it was found that the dispersed alum sludge could remove smaller particles (size ranging from 9 to 48 mm) than the non-dispersed alum sludge, as shown in Fig. 8. The dispersed alum sludge had better performance with respect to SS and COD removal than the raw alum sludge, which may be attributable to the better distribution of dispersed alum sludge through the wastewater in the coagulation stage. It is necessary to disperse the coagulant properly and to promote the particle collisions for achieving good coagulation. The dispersed alum sludge with smaller size and larger specific surface area was easier to be distributed through the wastewater and the chances of collision between the alum sludge and the particles in the sewage were increased. Therefore, higher SS and COD removal could be achieved by applying dispersed alum sludge.
3.4. ToF–SIMS analysis results Fig. 9 shows the ToF–SIMS analysis results of the raw alum sludge and the settled sludge after the addition
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of aluminum species were detected at the surface of the raw alum sludge while no aluminum species were detected after re-use. This suggested that the coagulation capacity of the alum sludge was not fully utilized in the water treatment plant and that it was more extensively used after having been reused in the primary sewage treatment. Apparently the aluminum species played an important role in the re-use of the alum sludge in primary sewage treatment. In the alum sludge re-use process, charge neutralization will not contribute to particulate pollutant removal enhancement since both alum sludge and particles in the sewage carry negative surface charges (see Table 1). Guan et al. (2002) reported that pH had little impact on the improvement of SS and COD removal by alum sludge and that few soluble aluminum ions were found in the alum sludge re-use tests. Thus, it was reasonable to postulate that the removal of SS and COD, enhanced by alum sludge, was mainly due to a combination of floc sweeping and physical adsorption. An explanation for the improvement of SS and COD removal from sewage through the re-use of alum sludge may be as follows: (1) the surface of raw alum sludge is not entirely enclosed by particles, which leaves some of the aluminum species exposed; (2) during a rapid mixing suspended solids and water molecules that are attached
of the alum sludge, which revealed that aluminum hydroxides in the alum sludge were responsible for agglomerating small particles, as a substantial amount 5
Supernatant of the test with dispersed alum sludge
Supernatant of the test with raw alum sludge
3
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Particle size (µm) Fig. 8. Particle size distribution in the supernatant of the tests with the raw and dispersed alum sludge.
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x 10 4 350
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300 250 200 150 100 50 0
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Fig. 9. (a) Aluminum species on the surface of raw alum sludge and (b) aluminum species on the surface of combined sludge.
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to the alum sludge through electrostatic bonds or physically entrapped by aluminum precipitates may detach themselves from the alum sludge surface temporarily; thereby enabling many of the surface sites to be activated; (3) the alum sludge adsorbs and enmeshes particles (including some of the particles released from the alum sludge and those originally existing in the sewage) to form large flocs with high settling rates; and (4) after re-use the surface sites of the alum sludge are utilized more effectively. 4. Conclusions Real sewage and the actual alum sludge were applied to the jar tests for evaluating the effect of the alum sludge on the removal of SS and COD from the sewage under various conditions. It was found that both SS and COD removal efficiencies were improved by 20% and 15%, respectively, due to the introduction of alum sludge, which was mainly contributed by the removal of particles with size ranging from 48 to 200 mm. The appropriate dosage of the alum sludge was determined to be 18–20 mg Al/L. By increasing the rapid mixing speed or reducing the floc size of the alum sludge the COD removal could be further enhanced. The thickening and dewatering properties of the combined sludge were improved and the settling rates of the flocs were increased after alum sludge introduction. ToF–SIMS analysis revealed that the aluminum species at the surface of the alum sludge were effectively utilized after reusing in the primary sewage treatment. Sweep flocculation and/or physical adsorption were expected to play key roles in the enhancement of particulate pollutant removal from the sewage. Acknowledgements The authors would like to extend their gratitude to the Department of Drainage Services of the HKSAR Government for provision of both sewage and alum sludge samples. References APHA Standard Methods for the Examination of Water and Wastewater, 19th Ed. American Public Health Association, American Water Works Association, Water Environmental Federation, Washington D.C. 1995. Bell-Ajy, K., Abbaszadegan, M., Ibrahim, E., Verges, D., LeChevallier, M., 2000. Conventional and optimized coagulation for NOM removal. J Am Water Works Assoc. 92, 44–57. Chen, G.H., 1996. Introduction of HK University of Science & Technology. Environ. Sani. Eng. Res. (Kyoto University) 10, 43–46.
Chu, W., 1999. Lead metal removal by recycled alum sludge. Water Res. 33 (13), 3019–3025. Chu, W., 2001. Dye removal from textile dye wastewater using recycled alum sludge. Water Res. 35 (13), 3147–3152. Culp, R.L., Wilson, W.I., 1979. Is alum sludge advantageous in wastewater treatment? Water Wastes Eng. 16, 16–19. Diamadopoulos, E., Benedek, A., 1984. Aluminum hydrolysis effects on phosphorus removal from wastewaters. J. Water Pollut. Control Fed. 56, 1165–1172. Guan, X.H., Shang, C., Yu, S.M., Chen, G.H., 2002. Exploratory study on reusing water treatment works sludge to enhance primary sewage treatment. Proceedings of the International Specialized Conference on Creative Water and Wastewater Treatment Technologies for Densely Populated Urban Areas, 189–195. Hsu, D.Y., Pipes, W.O., 1972. The effects of aluminum hydroxide on primary wastewater treatment process. Presented at 27th Ind. Waste Conference, Purdue University, West Lafayette, Ind. Hsu, D.Y., Pipes, W.O., 1973. Aluminum hydroxide effects on wastewater treatment processes. J. Water Pollut. Control Fed. 45, 681–697. Lai, C.K., 2001. Salinity Effect on Biological Sludge Dewatering. M.Phil Thesis, Department of Chemical Engineering, Hong Kong University of Science and Technology. Lee, D.J., Hsu, Y.H., 1994. Fast freeze/thaw treatment on excess activated sludges: floc structure and sludge dewaterability. Environ. Sci. Technol. 28, 1444–1449. Matsuda, A., Kawasaki, K., Mizukawa, Y., 1992. Measurement of bound water in excess activated sludges and effect of freezing and thawing process on it. J. Chem. Eng. Jpn. 25, 100–103. Montgomery, J.M., 1985. Water Treatment Principles and Design. Wiley, New York. Peavy, H.S., Rowe, D.R., Tchobanoglous, G., 1985. Environmental Engineering. Mc-Graw Hill Book Company, Singapore 207–228. Qasim, S.R., Motley, E.M., Zhu, G., 2000. Water Works Engineering: Planning, Design, and Operation, Chiang. Patel & Yerby Inc, Prentice-Hall PTR. Salotto, B.V., Farrell, J.B., Dean, R.B., 1973. The effect of water-utility sludge on the activated-sludge process. J. Am. Water Works Assoc. 65, 428–431. Warriner, T.R., O’Blenis, J.D., 1972. The physical and chemical characteristics of water treatment plant washwater from sever plants in Estern Ontario. Civil Engineering Research Report No. CE 72-4, Royal Military College of Canada. Watanabe, Y., Toyoshima, A., Fukuda, Y., Nakaishi, K., 1987. Improvement of physical properties of biological sludge and chemical adsorption of orthophosphate by chemical sludge. Proc. Environ. Sani. Eng. Res. 23, 149–156 [in Japanese]. Watanabe, Y., Toyoshima, A., 1988. Reuse of chemical sludge for conditioning of biological sludges. In: Hahn, H.H., Klute, R. (Eds.), Pretreatment in Chemical Water and Wastewater Treatment, p. 291. Wilson, T.E., Bizzarri, R.E., Burke, T., Langdon, P.E.J., Courson, C.M., 1975. Upgrading primary treatment with chemicals and water treatment plant sludge. J. Water Pollut. Control Fed. 47, 2820–2833.
Water Research 39 (2005) 3433–3440 www.elsevier.com/locate/watres
Re-use of water treatment works sludge to enhance particulate pollutant removal from sewage Xiao-Hong Guan, Guang-Hao Chen, Chii Shang Department of Civil Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China Received 29 October 2003; received in revised form 25 June 2004; accepted 8 July 2004 Available online 10 August 2005
Abstract This paper attempted to study the feasibility of reusing water treatment works sludge (‘‘alum sludge’’) to improve particulate pollutant removal from sewage. The main issues focused upon were: (1) the appropriate dosage of the alum sludge, (2) the appropriate operating conditions, and (3) the possible mechanisms for enhancement by adding alum sludge. Actual alum sludge and sewage were applied to a series of jar tests conducted under various conditions. It has been found that both the SS and COD removal efficiencies could be improved by the addition of the alum sludge, which was mainly attributed to the removal of relatively fine particles with a size of 48–200 mm. The appropriate dosage of the alum sludge was determined to be 18–20 mg of Al/L. Increasing the mixing speed or reducing the floc size of the alum sludge enhanced the SS and COD removal and the dispersed alum sludge could remove particulate contaminants with smaller size than the raw sewage. ToF–SIMS evidence revealed that the aluminum species at the surface of the alum sludge were effectively utilized for improving the SS and COD removal. It was postulated that the sweep flocculation and/or the physical adsorption might play key roles in the enhancement of particulate pollutant removal from sewage. r 2005 Elsevier Ltd. All rights reserved. Keywords: Alum sludge re-use; Primary treatment; Jar test; Suspended solids; COD; Particle size distribution
1. Introduction In a typical primary treatment system, the removal efficiencies of suspended solids (SS) and chemical oxygen demand (COD) are around 50% and 30%, respectively. Such low removal efficiencies are mainly due to the insufficient removal of finely divided suspended particles that account for a large portion of the SS. Peavy pointed out that removal of small particles (o50 mm) is almost impossible in a conventional primary sedimentation tank (Peavy et al., 1985). Thus, to improve the removal of both SS and COD from Corresponding author. Tel.: +852 2358 8752; fax: +852 2358 1534. E-mail address: [email protected] (G.-H. Chen).
sewage in primary treatment, particle agglomeration through chemical coagulation/flocculation is necessary and has actually been applied to the primary sewage treatment in Hong Kong (Chen, 1996). It should be noted that such chemically enhanced primary sewage treatment imposes a high-operational cost incurring from the use of coagulants and the treatment and disposal of a large amount of chemical sludge. Therefore, it is necessary to find a low-cost alternative to induce the adsorption and enmeshment of fine particles in the primary sewage treatment. Re-use of aluminumladen sludge (referred to as ‘‘alum sludge’’ in this paper) from water treatment works may provide such a solution because the alum sludge contains a large portion of insoluble aluminum hydroxides (Chu, 1999, 2001) that can be utilized as a coagulant in the primary
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.07.033
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sewage treatment. Re-use of the alum sludge may not only improve the particulate pollutant removal efficiency of a primary sewage treatment, but also ease the burden of water treatment works relating to sludge treatment and disposal. In order to verify the possibility of the above alum sludge re-use idea discussed above, the following five issues need to be addressed: (1) feasibility of this approach, (2) impact of the alum sludge on the performance of the existing primary and secondary sewage treatment plants, (3) appropriate dosage of the alum sludge and the appropriate operating conditions for maximizing the effect of the alum sludge, (4) a possible mechanism responsible for the alum sludge effect, and (5) a cost-effective method to transfer alum sludge from a water treatment works to a sewage treatment works for re-use. Regarding the first and second issues, which are very important for future application, several studies have been carried out to investigate the effect of adding alum sludge in primary wastewater treatment on the performance of activated sludge process, anaerobic digestion of wastewater sludge, and the overall performance of wastewater treatment plant (Hsu and Pipes, 1972, 1973; Salotto et al., 1973; Wilson et al., 1975; Culp and Wilson, 1979). Culp and Wilson (1979) pointed out that no adverse effects on overall wastewater treatment plant performance were identified, which is consistent with the findings of Hsu and Pipes (1972, 1973) and Salotto et al. (1973). It was observed that there was an apparent reduction of scum and sludge bulking problems in the secondary clarifier (Hsu and Pipes, 1973; Culp and Wilson, 1979). The carry-over of the slower-settling solids from the alum sludge to the activated sludge process improved the settling of the waste activated sludge and increased the process efficiency (Salotto et al., 1973). It was also reported that alum sludge addition could increase the gas production in sludge digester and capacity of centrifuges for sludge dewatering (Culp and Wilson, 1979). As for the final issue, Montgomery (1985) and Qasim et al. (2000) suggested a controlled discharge of alum sludge into a sewer system, provided that the sewer system is monitored appropriately during the transfer of the sludge. The third and fourth issues have not been studied, though they are of particular importance for maximizing the benefits gained from the alum sludge re-use, such as the enhancement of SS and COD removal, the control of sludge bulking, and the improvement of sludge dewaterability. Therefore, the objectives of this study are to investigate the appropriate dosage of alum sludge, to evaluate the effect of mixing speed as a key operational factor, and to examine the possible mechanism involved in SS and COD removal enhancement by alum sludge. The effect of alum sludge on improving SS and COD removal may depend on both the dosage of sludge (as Al) and Al
content in alum sludge. Since the aluminum fractions in alum sludge has been found to be relatively constant in wastes produced at several plants (Warriner and O’Blenis, 1972), the effect of Al content in alum sludge on pollutants removal was not investigated.
2. Materials and methods 2.1. Characterization of the alum sludge and the sewage Real sewage and alum sludge were employed in this study to determine the appropriate dosage of alum sludge. The sewage was taken from an inlet of a primary sedimentation tank in the largest local sewage treatment plant, while the alum sludge in slurry form was obtained from the same works, where the alum sludge is transferred from a nearby water treatment works for co-dewatering with the sewage treatment sludge. Both the raw sewage and the alum sludge samples were preserved in a cold room at 4 1C. The pH, COD, SS and zeta potential were determined to characterize the raw sewage, and the pH, aluminum content, solid content and zeta potential of the sludge samples were measured to characterize the alum sludge. 2.2. Test procedure and condition Jar tests were performed with a standard jar testing device (Stuart Scientific) to simulate a conventional coagulation/flocculation process. The jar testing procedure, which was adopted from Bell-Ajy et al. (2000), was initiated with a rapid mixing at 120 rpm for 1 min, followed by a slow mixing at 40 rpm for 10 min and at 20 rpm for 10 min, consecutively; and finally there was a 30 min settling. Control tests without the alum sludge addition were also conducted in parallel. All the jar tests were carried out in a temperature controlled room at 2171 1C. The testing mainly focused on the effects of the alum sludge dosages, the rapid mixing speed, and the floc size of the alum sludge on the improvement of the SS and COD removal from the raw sewage. Thus, the tests were conducted with different amounts of raw alum sludge or alum sludge dispersed with an ultrasonic processor (GE130) under 60 W for 15 min, while the rapid mixing speeds ranged from 120 to 247 rpm. In all cases, the pH remained unadjusted. After each test, the supernatant was sampled for measuring SS, volatile suspended solids (VSS), and COD concentrations. The settled sludge volume, the initial solids content, the final solids content in the settled sludge before and after sludge dewatering, and the time and specific resistance to filtration of the settled sludge were also measured. In order to evaluate the COD release from the alum sludge during the tests, synthetic wastewater, prepared from a 400 mg/L NaHCO3 solution with a pH of 7.5 adjusted
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by 0.1 M NaOH, was employed in the COD release tests which were carried out in a temperature controlled room at 2171 1C. The COD release tests followed abovementioned jar test procedure, that is, initiated with a rapid mixing at 120 rpm for 1 min, followed by a slow mixing at 40 rpm for 10 min and at 20 rpm for 10 min, consecutively; and finally there was a 30 min settling.
2.3. Chemical and physical analyses The SS and the VSS were measured according to the Standard Methods (APHA, 1995). COD was measured using a colorimetric test kit (HACH model) based on Method 5220D in the Standard Methods (APHA, 1995). The pH value of the samples was determined using a pH meter with a high-performance probe (Corning 350). The zeta potentials of the alum sludge and the raw sewage samples were measured with a zeta potential analyzer (Zetaplus). A particle size analyzer (Coulter LS230) was employed to determine the particle size distribution in the samples. The aluminum content in the alum sludge was analyzed using an atomic absorption spectrophotometer (Hitachi, Z-8200) equipped with a nitrous-oxide/acetylene flame after the sample pretreatment using a microwave digestion system (MDS2000). The settling rates of sludge flocs were determined according to the method proposed by Diamadopoulos and Benedek (1984) so that the settleability of suspended particles before and after the alum sludge addition could be evaluated. The solid content of the settled sludge was examined following the Standard Methods (APHA, 1995) except that a duration of 24 h was employed (Lai, 2001). The dewaterability of the settled sludge, which was characterized by the specific resistance to filtration (SRF) and the time to filtration (TTF), was determined by using the Bu¨chner funnel test (APHA, 1995). The final solid content was measured with the method proposed by Lee and Hsu (1994) and Matsuda et al. (1992). A time-of-flight secondary-ion mass spectrometer (ToF-SIMS) (Physical Electronics 7200) was adopted to investigate the surface characteristics of the alum sludge before and after the tests. To prepare the ToF-SIMS analysis, the sludge samples were dried in an oven at 103–105 1C for 72 h.
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3. Results and discussion 3.1. Characterization of the alum sludge and sewage Table 1 summarizes the characteristics of the raw sewage and the alum sludge used in this study. A high salinity was observed in the raw sewage which was due to the introduction of seawater for toilet flushing. A low solid content was typically found in the alum sludge, indicating that the alum sludge had not gone through any thickening and/or dewatering processes. Both the raw sewage and the alum sludge samples showed a negative zeta potential. 3.2. Effect of alum sludge dosage on SS and COD removal Fig. 1 shows the effect of the alum sludge dosage on the removal efficiencies of SS and COD. The SS removal efficiency was increased up to 72–90%, compared to 52–66% in the control tests. Similarly, the COD removal efficiency was improved by more than 15% compared to the control levels (28–38%). The enhancements were mainly attributed to the removal of fine particles with size ranging between 48 and 200 mm, as shown in Fig. 2, which illustrates the particle size distribution in the raw sewage and the supernatants with or without the alum sludge addition. The particles larger than 200 mm were removed but the smaller particles were partially removed when no alum sludge was applied. On the other hand, when alum sludge was introduced, the smaller particles (48–200 mm) disappeared in the supernatant, which indicated that the removal of all the particles with a size between 48 and 200 mm could be achieved through the alum sludge introduction. Obviously the SS removal efficiency increased when the alum sludge dosage increased, while the COD removal efficiency increased until a maximum level was reached at the alum sludge dosage of 70 mg/L. This may be associated with the nature of the alum sludge because some organic matters, called as COD, were released from the alum sludge, as shown in Fig. 3. The amount of organic matter released from the alum sludge was related to the dosage of the alum sludge. When the alum sludge was dosed at 18–20 mg Al/L, the COD concentration of the wastewater supernatant was
Table 1 Characteristics of the raw sewage and alum sludge Raw sewage Zeta potential (mV) SS (mg/L) VSS/SS ( ) COD (mg/L) pH
Alum sludge 0.24 to 236–356 0.85–0.9 390–490 7.2–7.8
12.39
Zeta potential (mV) Solid content (w/w%) VSS/SS ( ) Aluminum content (w/w%) pH
17.44 to 2.4–2.8 0.28–0.32 0.3–0.4 6.5–6.9
37.4
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40
SS
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30 60 90 120 Dosage of alum sludge (mg Al/L)
150
0 0
20
40
60
80
0.5 100
Dosage of alum sludge (mg Al/L)
Fig. 1. SS and COD removal efficiency at different dose of alum sludge.
Fig. 3. COD concentration and VSS/SS ratio in the supernatant of the tests with different dosage of alum sludge.
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Fig. 2. Particle size distribution in the raw sewage and the supernatant of the tests with and without the addition of alum sludge.
about 200 mg/L and the COD concentration of the supernatant in the release tests was about 15 mg/L. Therefore, the contaminants released by alum sludge contributed about 7.5% of the COD in the supernatant of wastewater if alum sludge was applied at 18–20 mg Al/L. There was a decrease in the VSS/SS ratio of the supernatant when the alum sludge dosage increased, as illustrated in Fig. 3, suggesting that the SS remained in the supernatant shifted from organic to inorganic substances due to the addition of the alum sludge. Since most of the particles in the alum sludge were inorganic as the VSS/SS ratio of the alum sludge was only 30%
while that of the raw sewage was 85–90%, the decrease in the VSS/SS ratio of the supernatant was caused by the release of inorganic particles from the alum sludge. It could be associated with the back diffusion of contaminants from the reused alum sludge as proposed by Chu (1999), who also observed similar phenomenon. Therefore, the enhancement of the SS and COD removal by alum sludge depends on the contaminants removed from the water phase as well as on those released from the alum sludge. When the dosage of the alum sludge was 18–20 mg Al/L, the contaminants removed by the alum sludge far exceeded the contaminants that were released, thereby resulting in a considerable enhancement in both the SS and COD removal efficiencies. Although the particulate contaminants released from the alum sludge exerted a negative impact on the SS and COD removal in the primary sewage treatment, these contaminants may be helpful in controlling sludge bulking in the secondary treatment (Hsu and Pipes, 1973; Salotto et al., 1973). It had been elucidated that the carry-over of suspended solids from alum sludge could increase the density and settleability of biological flocs in the following secondary clarifier (Salotto et al., 1973). The SS removal efficiency was as high as 90.3% by dosing the alum sludge at 120 mg Al/L and the COD removal efficiency to 66.0% by setting the alum sludge dosage at 75 mg Al/L, as shown in Fig. 1. However, this study found that the resulted sludge volume increased linearly with the increase in the dosage of alum sludge. Therefore, such high-alum dosages as 120 or 75 mg Al/L were not appropriate due to the huge sludge volume under these situations. To minimize the increase in sludge volume while maintaining the high-removal
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efficiencies of SS and COD, the alum sludge dosage of 18–20 mg Al/L was considered to be adequate in this study. At this dosage, the SS and COD removal efficiency was found to be around 85% and 52%, respectively. When the alum sludge was reused at this dosage, the volume of the settled sludge was increased by 50% only compared to the control tests without the alum sludge addition. Reduction in alum sludge dosage (smaller ALT ratio) could not only reduce the volume of the settled sludge but also increase the density of the sludge flocs (Watanabe and Toyoshima, 1988). However, the SS and COD removal efficiency would decrease at a smaller ALT ratio. Considering the variation of the characteristics of wastewater with time, 12 runs were performed to check the stability of the effluent quality with dosing alum sludge, as shown in Fig. 4. It was found that a stable effluent quality could be achieved and the SS concentration in the supernatant of the tests was in the range of 34–52 mg/L by dosing alum sludge at 18–20 mg Al/L. However, the appropriate dosage of alum sludge needed to be verified by a full-scale trial. To achieve similar removal efficiency of SS and COD (85% for SS and 52% for COD) in a settling tank, the minimal amount of pure alum might be 7–10 mg/L as Al (Hsu and Pipes, 1972, 1973), which was about 50% of the recommended dosage, 18–20 mg/L as Al, of the real
400
Raw sewage Supernatant of control tests Supernatant of tests with alum sludge dosed at 18 mg-Al/L
SS (mg/L)
300
100
0 2
3
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alum sludge used this study. Such a difference reasonably resulted from the significant difference in the floc nature and properties of the pure alum and the alum sludge of a water treatment plant. Therefore, Wilson et al. (1975) suggested that the appropriate dosage of real alum sludge could be up to 40 mg/L as Al, depending on the sludge properties. The characteristics of the alum sludge, the settled primary sludge obtained from the control tests and the settled sludge produced from the alum sludge-added tests (combined sludge) are summarized in Table 2. It was found that with the introduction of the alum sludge the thickening and dewatering properties of the combined sludge were improved in terms of final solid content, time to filtration and specific resistance to filtration compared to those of the primary sludge. Watanabe et al. (1987) also found that reusing alum sludge in primary sedimentation tank could improve the thickening and dewatering ability of the combined sludge. It was observed in this study that the settling rates of the sludge flocs were increased by adding the alum sludge to the raw sewage. The experimental results showed that assuming an overflow rate of 1.5 m/h, the floc removal could be enhanced from 56% to 74%. In other words, even when the overflow rate was assumed to be 2 m/h, 71% of the flocs could be removed. This may be due to the inorganic aluminum hydroxide enmeshment that can increase the sludge density and the settling rate of flocs.
3.3. Effect of the mixing speed
200
1
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5 6 7 8 9 10 11 12 Number of runs
Fig. 4. Comparison of the SS concentrations in raw sewage, the supernatants of the control tests and the tests with alum sludge dosed at 18 mg Al/L.
Fig. 5 illustrates the particle size distribution in the samples of the raw alum sludge, the alum sludge after a rapid mixing, and the settled sludge after a jar test with the addition of the alum sludge. The particle size of the alum sludge after a rapid mixing became smaller than the particle size of the raw alum sludge. This was because the rapid mixing induced the temporal detachment of particles from the sludge nucleus. The difference between the particle size of the raw alum sludge and that of the settled sludge reflected that the alum sludge agglomerated more particles at the sludge surface during slow mixing or that the particles could be entrapped by alum sludge during the settling. Therefore, the re-use of
Table 2 Characteristics of different types of sludge used in this study
Solid content (%) Final solid content (%) Time to filtration (s) Specific resistance to filtration (m/kg)
Raw alum sludge
Combined sludge
Primary sludge
2.08 16.23 930 9.02 1012
2.06 28.15 846 1.17 1013
1.90 15.57 1181 1.85 1013
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5 Volumetric percent (%)
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150
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100 50
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3
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Fig. 5. Particle size distribution in the raw alum sludge, the alum sludge after the rapid mixing, and the settled sludge of the jar test.
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Particle size (µm) Fig. 7. Particle size distribution in the raw and dispersed alum sludge.
COD removal efficiency (%)
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40 120 rpm 190 rpm
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247 rpm 0 0
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Fig. 6. COD removal under different rapid mixing conditions.
alum sludge in the primary treatment is a process where particles in sewage are either attached onto the nucleation sites of the alum sludge or entrapped by it during the settling. Considering the decrease in the VSS/ SS ratio in the supernatant with an increase in the alum sludge dosage, a rearrangement of the particles at the surface of the alum sludge is also expected. A higher COD removal efficiency was achieved with a higher mixing speed, as depicted in Fig. 6. The range of mixing speed examined in this study was limited by the jar testing device. More particles were released from the alum sludge and the specific surface area of the alum sludge was increased accordingly under higher mixing speeds, thus creating more nucleation sites for aggregating and adsorbing particles and eventually increasing the
particulate pollutant removal. To verify this hypothesis, alum sludge samples were dispersed by employing an ultrasonic processor and were then subjected to the jar test. Fig. 7 shows that the particle size of the dispersed alum sludge decreased compared to the particle size of the raw alum sludge. The SS and COD concentrations of the supernatant after applying 18–20 mg/L dispersed alum sludge were found to be lower by about 15 and 5 mg/L, respectively, compared to those with applying same dose of raw alum sludge. Particle size analysis of the supernatants after applying alum sludge with/without dispersion was carried out and it was found that the dispersed alum sludge could remove smaller particles (size ranging from 9 to 48 mm) than the non-dispersed alum sludge, as shown in Fig. 8. The dispersed alum sludge had better performance with respect to SS and COD removal than the raw alum sludge, which may be attributable to the better distribution of dispersed alum sludge through the wastewater in the coagulation stage. It is necessary to disperse the coagulant properly and to promote the particle collisions for achieving good coagulation. The dispersed alum sludge with smaller size and larger specific surface area was easier to be distributed through the wastewater and the chances of collision between the alum sludge and the particles in the sewage were increased. Therefore, higher SS and COD removal could be achieved by applying dispersed alum sludge.
3.4. ToF–SIMS analysis results Fig. 9 shows the ToF–SIMS analysis results of the raw alum sludge and the settled sludge after the addition
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of aluminum species were detected at the surface of the raw alum sludge while no aluminum species were detected after re-use. This suggested that the coagulation capacity of the alum sludge was not fully utilized in the water treatment plant and that it was more extensively used after having been reused in the primary sewage treatment. Apparently the aluminum species played an important role in the re-use of the alum sludge in primary sewage treatment. In the alum sludge re-use process, charge neutralization will not contribute to particulate pollutant removal enhancement since both alum sludge and particles in the sewage carry negative surface charges (see Table 1). Guan et al. (2002) reported that pH had little impact on the improvement of SS and COD removal by alum sludge and that few soluble aluminum ions were found in the alum sludge re-use tests. Thus, it was reasonable to postulate that the removal of SS and COD, enhanced by alum sludge, was mainly due to a combination of floc sweeping and physical adsorption. An explanation for the improvement of SS and COD removal from sewage through the re-use of alum sludge may be as follows: (1) the surface of raw alum sludge is not entirely enclosed by particles, which leaves some of the aluminum species exposed; (2) during a rapid mixing suspended solids and water molecules that are attached
of the alum sludge, which revealed that aluminum hydroxides in the alum sludge were responsible for agglomerating small particles, as a substantial amount 5
Supernatant of the test with dispersed alum sludge
Supernatant of the test with raw alum sludge
3
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300 250 200 150 100 50 0
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Fig. 9. (a) Aluminum species on the surface of raw alum sludge and (b) aluminum species on the surface of combined sludge.
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to the alum sludge through electrostatic bonds or physically entrapped by aluminum precipitates may detach themselves from the alum sludge surface temporarily; thereby enabling many of the surface sites to be activated; (3) the alum sludge adsorbs and enmeshes particles (including some of the particles released from the alum sludge and those originally existing in the sewage) to form large flocs with high settling rates; and (4) after re-use the surface sites of the alum sludge are utilized more effectively. 4. Conclusions Real sewage and the actual alum sludge were applied to the jar tests for evaluating the effect of the alum sludge on the removal of SS and COD from the sewage under various conditions. It was found that both SS and COD removal efficiencies were improved by 20% and 15%, respectively, due to the introduction of alum sludge, which was mainly contributed by the removal of particles with size ranging from 48 to 200 mm. The appropriate dosage of the alum sludge was determined to be 18–20 mg Al/L. By increasing the rapid mixing speed or reducing the floc size of the alum sludge the COD removal could be further enhanced. The thickening and dewatering properties of the combined sludge were improved and the settling rates of the flocs were increased after alum sludge introduction. ToF–SIMS analysis revealed that the aluminum species at the surface of the alum sludge were effectively utilized after reusing in the primary sewage treatment. Sweep flocculation and/or physical adsorption were expected to play key roles in the enhancement of particulate pollutant removal from the sewage. Acknowledgements The authors would like to extend their gratitude to the Department of Drainage Services of the HKSAR Government for provision of both sewage and alum sludge samples. References APHA Standard Methods for the Examination of Water and Wastewater, 19th Ed. American Public Health Association, American Water Works Association, Water Environmental Federation, Washington D.C. 1995. Bell-Ajy, K., Abbaszadegan, M., Ibrahim, E., Verges, D., LeChevallier, M., 2000. Conventional and optimized coagulation for NOM removal. J Am Water Works Assoc. 92, 44–57. Chen, G.H., 1996. Introduction of HK University of Science & Technology. Environ. Sani. Eng. Res. (Kyoto University) 10, 43–46.
Chu, W., 1999. Lead metal removal by recycled alum sludge. Water Res. 33 (13), 3019–3025. Chu, W., 2001. Dye removal from textile dye wastewater using recycled alum sludge. Water Res. 35 (13), 3147–3152. Culp, R.L., Wilson, W.I., 1979. Is alum sludge advantageous in wastewater treatment? Water Wastes Eng. 16, 16–19. Diamadopoulos, E., Benedek, A., 1984. Aluminum hydrolysis effects on phosphorus removal from wastewaters. J. Water Pollut. Control Fed. 56, 1165–1172. Guan, X.H., Shang, C., Yu, S.M., Chen, G.H., 2002. Exploratory study on reusing water treatment works sludge to enhance primary sewage treatment. Proceedings of the International Specialized Conference on Creative Water and Wastewater Treatment Technologies for Densely Populated Urban Areas, 189–195. Hsu, D.Y., Pipes, W.O., 1972. The effects of aluminum hydroxide on primary wastewater treatment process. Presented at 27th Ind. Waste Conference, Purdue University, West Lafayette, Ind. Hsu, D.Y., Pipes, W.O., 1973. Aluminum hydroxide effects on wastewater treatment processes. J. Water Pollut. Control Fed. 45, 681–697. Lai, C.K., 2001. Salinity Effect on Biological Sludge Dewatering. M.Phil Thesis, Department of Chemical Engineering, Hong Kong University of Science and Technology. Lee, D.J., Hsu, Y.H., 1994. Fast freeze/thaw treatment on excess activated sludges: floc structure and sludge dewaterability. Environ. Sci. Technol. 28, 1444–1449. Matsuda, A., Kawasaki, K., Mizukawa, Y., 1992. Measurement of bound water in excess activated sludges and effect of freezing and thawing process on it. J. Chem. Eng. Jpn. 25, 100–103. Montgomery, J.M., 1985. Water Treatment Principles and Design. Wiley, New York. Peavy, H.S., Rowe, D.R., Tchobanoglous, G., 1985. Environmental Engineering. Mc-Graw Hill Book Company, Singapore 207–228. Qasim, S.R., Motley, E.M., Zhu, G., 2000. Water Works Engineering: Planning, Design, and Operation, Chiang. Patel & Yerby Inc, Prentice-Hall PTR. Salotto, B.V., Farrell, J.B., Dean, R.B., 1973. The effect of water-utility sludge on the activated-sludge process. J. Am. Water Works Assoc. 65, 428–431. Warriner, T.R., O’Blenis, J.D., 1972. The physical and chemical characteristics of water treatment plant washwater from sever plants in Estern Ontario. Civil Engineering Research Report No. CE 72-4, Royal Military College of Canada. Watanabe, Y., Toyoshima, A., Fukuda, Y., Nakaishi, K., 1987. Improvement of physical properties of biological sludge and chemical adsorption of orthophosphate by chemical sludge. Proc. Environ. Sani. Eng. Res. 23, 149–156 [in Japanese]. Watanabe, Y., Toyoshima, A., 1988. Reuse of chemical sludge for conditioning of biological sludges. In: Hahn, H.H., Klute, R. (Eds.), Pretreatment in Chemical Water and Wastewater Treatment, p. 291. Wilson, T.E., Bizzarri, R.E., Burke, T., Langdon, P.E.J., Courson, C.M., 1975. Upgrading primary treatment with chemicals and water treatment plant sludge. J. Water Pollut. Control Fed. 47, 2820–2833.