Enhanced phosphorus removal in the DAF process by flotation scum recycling for advanced treatment of municipal wastewater
Enhanced phosphorus removal in the DAF process by flotation scum recycling for advanced treatment of municipal wastewater
Dong-Heui Kwak and Ki-Cheol Lee
Phosphorus (P) discharged from wastewater treatment plants causes eutrophication and algal blooms in downstream watersheds. Recently, to enhance public confidence in water, many countries have increased their efforts to preserve water resources, prevent eutrophication of various bodies of water, and increase the water quality criteria. Therefore, additional tertiary treatment processes should be applied to remove P from secondary wastewater treatment plants. The total phosphorus (T-P) concentration in effluent water discharged from plants that utilize conventional biological processes is generally several mg/L (Haas et al. 2000).
Previous studies (Berkowitz et al. 2006; Vicente et al. 2008) have been conducted regarding the treatment of lake water or sediment with aluminum (Al) for lake restoration, but only a few studies have been performed on phosphate removal in the effluents from wastewater treatment plants. Chemical reactions are used to convert soluble P species into particulates, which must be removed from wastewater in order to eliminate the P. Dissolved air flotation (DAF) is a well-known, flexible process used for liquid‒solid separations, and the main advantages include its small space requirement and fast start-up (Kwak et al. 2010; Flippin et al. 2013; Zhang et al. 2014). In a trickling filter stream, the P content was reduced to less than 0.5 mg/L using DAF (Crossley & Valade 2006). Since alum sludge contains a large amount of dissolved aluminum hydroxide, it can act as a coagulant in primary clarifiers and improve the removal efficiencies of suspended solids (SS), chemical oxygen demand (COD), and phosphate, and reduce the burden for sludge treatment (Guan et al. 2005). When treating phosphate, alum sludge is less effective than alum, but the combination of alum and alum sludge is effective for treating SS and phosphate (Huang & Chiswell 2000; Georgantas & Grigoropoulou 2005). Similarly, it is possible to increase the removal efficiency of T-P by recycling flotation scum, because metal salts that do not react with P remain in the flotation scum, which is coagulated and floated upon injection of the metal salts.
Conversely, the physical adsorption on the surface of Al (OH)3 occurs differently depending on the concentration. On the adsorption sites of Al(OH)3, silicate competes with phosphate, which has been detected at low concentrations (less than 5 μM) in natural waters. However, at higher phosphate concentrations, bridging by metal ions such as aluminum ions is required to bind phosphate with the organic substance (Cheng et al. 2004). Berkowitz et al. (2006) established the isothermal adsorption of PO3 4 on the surface of Al(OH)3 particles. Also, according to the hypothesis of Butkus et al. (1998), organic polymers and Fe and Al oxides play important roles in the adsorption of P on ferric water treatment residuals. One model predicted that phosphate associated with quaternary polyamines constituted up to 40% of phosphate, which was bound by water with high P loadings. The adsorption was found to occur mainly through electrostatic interactions, although ligand exchange reactions between phosphate and hydroxyl or amine functionalities were also possible. Boisvert et al. (1997) reported that phosphate adsorption on hydrolyzed alum occurred via ligand exchange (Equation (1)), in addition to charge neutralization (Equation (2)) and proton transfer reactions (Equation (3)). The surface charges of the adsorbent with various OH/PO4 ratios and zeta potentials were used as evidence for these processes. The release of H+ (Equation (3)) occurred with decreasing OH/PO4 ratios, while an overall increase was controlled by the ligand exchange process.
Ligand exchange: Al(OH)+2 + HPO42- à Al(OH)HPO4 + OH- (1)
Charge neutralization: Al(OH)+2 + H2PO-4 à Al(OH)2H2PO4 (2)
Proton transfer: Al(OH)+2 + H2PO-4 à Al(OH)HPO-4 + H+ (3)
The T-P removal efficiency is limited in the treatment of water containing less than 1.0 mg/L P. Particles acting as coagulation nuclei are deficient in the effluents from secondary wastewater treatment plants and a large amount of additional coagulants is needed to form chemical precipitates. With few coagulation nuclei and chemical coagulant overdosing, flotation scum recycling can be a solution and an alternative to tertiary P treatment processes. In this study, for P removal in the effluents of municipal wastewater treatment plants (MWTPs), a series of experiments were carried out using a DAF pilot plant in the J MWTP, which applies the biological anaerobic-anoxic-oxic process. Based on the assumption that particles in recycled flotation scum may be useful coagulation nuclei, the P removal efficiency was investigated as a function of flotation scum recycling in the DAF process.
MATERIALS AND METHODS
A DAF pilot plant was set up to determine the influence of flotation scum recycling on the treatment of the T-P content in the effluent water of the J MWTP (303,000 m3 /d, Jeonju, Republic of Korea) as shown in Figure 1. The experiments in this study were conducted based on the following parameters: a pilot plant capacity, Q, of 15 m3 /d, coagulant alum (Al2(SO4)·18H2O as Al2O3 8%), saturator pressure of 3.0–5.0 kgf/cm2 , saturated water recycle ratio of 0.05–0.2, and a flotation and mixing gradient, G, of 215 s-1 for rapid mixing and 37 s-1 for slow mixing. A portion of the flotation scum (bubble-floc agglomerates) in the scum hopper was recycled to reduce the coagulant dose and to improve the T-P removal. The coagulant doses were decided after the jar test, and rapid and slow mixing was carried out for 10 min to form flocs. After coagulation, the influent and milky water from the saturator were injected co-currently into the bottom of the contact zone in the flotation chamber. Flotation scum was collected along the water surface of the separation zone and was discharged into the scum hopper by the skimmer. The collected flotation scum in the hopper, which was composed of about 3% solid content, was returned to the coagulation tank using a diaphragm type pump. Meanwhile, there would be solid content changes in the condition of wastewater treatment during the experimental period. However, the amount of flotation scum recycling was constantly maintained, because it was predicted to insignificantly affect the P removal with SS change as injecting coagulant. Changes in the quality of the influent and effluent water were monitored by measuring the pH and turbidity. Samples from the DAF pilot plant was obtained using the grab sampling method, and all samples were analyzed based on Standard Methods (American Public Health Association (APHA) et al. 1998).
RESULTS AND DISCUSSION
Treatment characteristics and removal efficiency
For a previous study using ferric chloride (30–110 mg/L) from a pilot-scale anaerobic reactor treating domestic sewage (Penetra et al. 1999), the removal efficiencies were between 87 and 91% for the COD, between 95 and 96% for T-P, and 94% for the SS. Nardi et al. (2011) reported that an Fe/P molar ratio of 1.32 ± 0.19 was optimal for the removal of P; under these conditions >99% P and 65 ± 25% SS were removed in a laboratory-scale chemical DAF using ferric chloride from anaerobically pre-treated poultry slaughter-house wastewater.
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Table 1 and Figure 2 show the summarized results of the water treatment using the DAF pilot plant in the J MWTP over 1 year. Compared with previous studies, the removal without flotation scum recycling was low, since the raw water used in this study was the supernatant water from the secondary sedimentation basin in JMWTP, whose water quality was below the specified criteria for discharge water. Meanwhile, the removal efficiencies of T-P and PO4-P with flotation scum recycling were greatly improved; it may be considered that the efficiency of P removal was enhanced by (1) recycling the flotation scum including the residual aluminum in the solids and (2) inducing the physical adsorption reaction on the surface of Al(OH)3 particles. The flow rate of flotation scum recycling was 7.5 L/d and the recycled P concentration in the flotation scum was 25,000 mg/L. The average increases in the removal efficiencies due to flotation scum recycling were 22.61% for T-P and 18.34% for PO4-P.
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Table 1 Comparison of pollutant removal efficiency without and with flotation scum recycling
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Figure 2 P removal efficiency in effluent of J MWTP by DAF pilot plant for 1 year. Operating conditions: alum dose 38.1 ppm, scum recycle ratio 0.5, saturated water recycle ratio 0.152, air to solid ratio 0.78 on average for 1 year.
pH and solid loading
While the processes are expected to follow the fundamentals of solid separation, the pH dependency of the P precipitant and surface chemistry of metal hydroxide/P complexes are not established. Recent work by Szabó et al. (2008) showed that there was an optimum pH for P removal, but that the optimum range was relatively wide (ranging from pH 5 to pH 7) and deterioration occurred outside this pH range. For the removal of phosphate from water to induce precipitation, the molar ratio and pH were the key parameters in determining the residual phosphate concentration (Liu et al. 2009). The precipitation reactions are dependent on phosphate concentration and pH, depending on the composition of the wastewater (Denham 2007). In addition, other hydrolysis products of aluminum may form, depending mainly on the pH (Georgantas & Grigoropoulou 2006).
Experiments of the DAF pilot plant performed without any pH adjustments led to less than 1.0 mg/L T-P in the effluent without recycled flotation scum and 0.1 mg/L T-P in the effluent with recycled flotation scum. As shown in Figure 3(a), for the influent pH range between 6.6 and 7.4, the removal efficiency of PO4-P increased with increasing pH values, while there was no relationship between pH and T-P removal efficiency under the operating conditions without or with flotation scum recycling. This result suggested that the removal of orthophosphate ions (PO4-P) was governed by pH, whereas other types of P compounds may not be removed by the formation of aluminum phosphate (AlPO4 ideally). Conversely, particles in the recycled flotation scum may be useful coagulation nuclei. Figure 3 (b) shows the removal efficiency depending on the solid loading rate. However, an obvious relationship between removal efficiency and solid loading rate was not observed without or with flotation scum recycling.
CONCLUSIONS
A series of experiments were conducted to investigate the feasibility of flotation scum recycling for effective P removal from a MWTP using a DAF pilot plant. The DAF experiments were performed using the effluent from secondary sedimentation in the J MWTP. Over 1 year, the T-P removal efficiency increased by 22.6% on average, and the T-P concentration was maintained below 0.1 mg/L by flotation scum recycling. A higher removal efficiency of T-P was induced by recycling the flotation scum because a significant amount of Al com[1]ponents remained in the flotation scum. The higher removal efficiency with recycling was due to the improved coagulation efficiency as Al components in the flotation scum acted as coagulants. Furthermore, the SS in the flotation scum enhanced the physical adsorption. The increase in T-P removal efficiency, due to the recycling of flotation scum, shifted from the boundary of the stoichiometric precipitate to the equilibrium control region, and the removal efficiency of T-P significantly increased as the molar ratio of Al:P was increased to 1.05 in the equilibrium control region. Flotation scum recycling may contribute to improving the quality of treated water and reducing treatment costs by minimizing the coagulant dosage required.
REFERENCES
American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF) 1998 Standard Methods for the Examination of Water and Wastewater 20th edn. American Public Health Association, Washington, DC.
Berkowitz, J., Anderson, M. A. & Amrhein, C. 2006 Influence of aging on phosphorus sorption to alum floc in lake water. Water Research 40 (5), 911–916.
Boisvert, J. P., To, T. C., Berrak, A. & Jolicoeur, C. 1997 Phosphate adsorption in flocculation processes of aluminium sulphate and poly-aluminium-silicate-sulphate. Water Research 31 (8), 1939–1946.
Butkus, M. A., Grasso, D., Schulthess, C. P. & Wijnja, H. 1998 Surface complexation modeling of phosphate adsorption by water treatment residual. Journal of Environmental Quality 27, 1055–1063.
Cheng, W. P., Chi, F. H. & Yu, R. F. 2004 Effect of phosphate on removal of humic substances by aluminum sulfate coagulant. Journal of Colloid and Interface Science 272 (1), 153–157.
Crossley, I. A. & Valade, M. T. 2006 A review of the technological developments of dissolved air flotation. Journal of Water Supply: Research and Technology-AQUA 55 (7–8), 479–491.
Denham, K. 2007 Chemical Phosphorus Removal and Control Strategies. MSc Thesis, Cranfield University, United Kingdom.
Flippin, H., Cuomo, L. & Petersen, L. 2013 Consolidating clarification Using dissolved-air flotation as secondary clarification improves activated sludge treatment and reduces space requirements. Water Environment & Technology 25 (6), 60–69.
Georgantas, D. A. & Grigoropoulou, H. P. 2005 Phosphorus removal from synthetic and municipal wastewater using spent alum sludge. Water Science and Technology 52 (10–11), 525–532.
Georgantas, D. A. & Grigoropoulou, H. P. 2006 Phosphorus removal from synthetic wastewater using alum and aluminum hydroxide. Global NEST Journal 8 (2), 121–130.
Guan, X. H., Chen, G. H. & Shang, C. 2005 Re-use of water treatment works sludge to enhance particulate pollutant from sewage. Water Research 39 (15), 3433–3440
Dong-Heui Kwak and Ki-Cheol Lee
Phosphorus (P) discharged from wastewater treatment plants causes eutrophication and algal blooms in downstream watersheds. Recently, to enhance public confidence in water, many countries have increased their efforts to preserve water resources, prevent eutrophication of various bodies of water, and increase the water quality criteria. Therefore, additional tertiary treatment processes should be applied to remove P from secondary wastewater treatment plants. The total phosphorus (T-P) concentration in effluent water discharged from plants that utilize conventional biological processes is generally several mg/L (Haas et al. 2000).
Previous studies (Berkowitz et al. 2006; Vicente et al. 2008) have been conducted regarding the treatment of lake water or sediment with aluminum (Al) for lake restoration, but only a few studies have been performed on phosphate removal in the effluents from wastewater treatment plants. Chemical reactions are used to convert soluble P species into particulates, which must be removed from wastewater in order to eliminate the P. Dissolved air flotation (DAF) is a well-known, flexible process used for liquid‒solid separations, and the main advantages include its small space requirement and fast start-up (Kwak et al. 2010; Flippin et al. 2013; Zhang et al. 2014). In a trickling filter stream, the P content was reduced to less than 0.5 mg/L using DAF (Crossley & Valade 2006). Since alum sludge contains a large amount of dissolved aluminum hydroxide, it can act as a coagulant in primary clarifiers and improve the removal efficiencies of suspended solids (SS), chemical oxygen demand (COD), and phosphate, and reduce the burden for sludge treatment (Guan et al. 2005). When treating phosphate, alum sludge is less effective than alum, but the combination of alum and alum sludge is effective for treating SS and phosphate (Huang & Chiswell 2000; Georgantas & Grigoropoulou 2005). Similarly, it is possible to increase the removal efficiency of T-P by recycling flotation scum, because metal salts that do not react with P remain in the flotation scum, which is coagulated and floated upon injection of the metal salts.
Conversely, the physical adsorption on the surface of Al (OH)3 occurs differently depending on the concentration. On the adsorption sites of Al(OH)3, silicate competes with phosphate, which has been detected at low concentrations (less than 5 μM) in natural waters. However, at higher phosphate concentrations, bridging by metal ions such as aluminum ions is required to bind phosphate with the organic substance (Cheng et al. 2004). Berkowitz et al. (2006) established the isothermal adsorption of PO3 4 on the surface of Al(OH)3 particles. Also, according to the hypothesis of Butkus et al. (1998), organic polymers and Fe and Al oxides play important roles in the adsorption of P on ferric water treatment residuals. One model predicted that phosphate associated with quaternary polyamines constituted up to 40% of phosphate, which was bound by water with high P loadings. The adsorption was found to occur mainly through electrostatic interactions, although ligand exchange reactions between phosphate and hydroxyl or amine functionalities were also possible. Boisvert et al. (1997) reported that phosphate adsorption on hydrolyzed alum occurred via ligand exchange (Equation (1)), in addition to charge neutralization (Equation (2)) and proton transfer reactions (Equation (3)). The surface charges of the adsorbent with various OH/PO4 ratios and zeta potentials were used as evidence for these processes. The release of H+ (Equation (3)) occurred with decreasing OH/PO4 ratios, while an overall increase was controlled by the ligand exchange process.
Ligand exchange: Al(OH)+2 + HPO42- à Al(OH)HPO4 + OH- (1)
Charge neutralization: Al(OH)+2 + H2PO-4 à Al(OH)2H2PO4 (2)
Proton transfer: Al(OH)+2 + H2PO-4 à Al(OH)HPO-4 + H+ (3)
The T-P removal efficiency is limited in the treatment of water containing less than 1.0 mg/L P. Particles acting as coagulation nuclei are deficient in the effluents from secondary wastewater treatment plants and a large amount of additional coagulants is needed to form chemical precipitates. With few coagulation nuclei and chemical coagulant overdosing, flotation scum recycling can be a solution and an alternative to tertiary P treatment processes. In this study, for P removal in the effluents of municipal wastewater treatment plants (MWTPs), a series of experiments were carried out using a DAF pilot plant in the J MWTP, which applies the biological anaerobic-anoxic-oxic process. Based on the assumption that particles in recycled flotation scum may be useful coagulation nuclei, the P removal efficiency was investigated as a function of flotation scum recycling in the DAF process.
MATERIALS AND METHODS
A DAF pilot plant was set up to determine the influence of flotation scum recycling on the treatment of the T-P content in the effluent water of the J MWTP (303,000 m3 /d, Jeonju, Republic of Korea) as shown in Figure 1. The experiments in this study were conducted based on the following parameters: a pilot plant capacity, Q, of 15 m3 /d, coagulant alum (Al2(SO4)·18H2O as Al2O3 8%), saturator pressure of 3.0–5.0 kgf/cm2 , saturated water recycle ratio of 0.05–0.2, and a flotation and mixing gradient, G, of 215 s-1 for rapid mixing and 37 s-1 for slow mixing. A portion of the flotation scum (bubble-floc agglomerates) in the scum hopper was recycled to reduce the coagulant dose and to improve the T-P removal. The coagulant doses were decided after the jar test, and rapid and slow mixing was carried out for 10 min to form flocs. After coagulation, the influent and milky water from the saturator were injected co-currently into the bottom of the contact zone in the flotation chamber. Flotation scum was collected along the water surface of the separation zone and was discharged into the scum hopper by the skimmer. The collected flotation scum in the hopper, which was composed of about 3% solid content, was returned to the coagulation tank using a diaphragm type pump. Meanwhile, there would be solid content changes in the condition of wastewater treatment during the experimental period. However, the amount of flotation scum recycling was constantly maintained, because it was predicted to insignificantly affect the P removal with SS change as injecting coagulant. Changes in the quality of the influent and effluent water were monitored by measuring the pH and turbidity. Samples from the DAF pilot plant was obtained using the grab sampling method, and all samples were analyzed based on Standard Methods (American Public Health Association (APHA) et al. 1998).
RESULTS AND DISCUSSION
Treatment characteristics and removal efficiency
For a previous study using ferric chloride (30–110 mg/L) from a pilot-scale anaerobic reactor treating domestic sewage (Penetra et al. 1999), the removal efficiencies were between 87 and 91% for the COD, between 95 and 96% for T-P, and 94% for the SS. Nardi et al. (2011) reported that an Fe/P molar ratio of 1.32 ± 0.19 was optimal for the removal of P; under these conditions >99% P and 65 ± 25% SS were removed in a laboratory-scale chemical DAF using ferric chloride from anaerobically pre-treated poultry slaughter-house wastewater.
Table 1 and Figure 2 show the summarized results of the water treatment using the DAF pilot plant in the J MWTP over 1 year. Compared with previous studies, the removal without flotation scum recycling was low, since the raw water used in this study was the supernatant water from the secondary sedimentation basin in JMWTP, whose water quality was below the specified criteria for discharge water. Meanwhile, the removal efficiencies of T-P and PO4-P with flotation scum recycling were greatly improved; it may be considered that the efficiency of P removal was enhanced by (1) recycling the flotation scum including the residual aluminum in the solids and (2) inducing the physical adsorption reaction on the surface of Al(OH)3 particles. The flow rate of flotation scum recycling was 7.5 L/d and the recycled P concentration in the flotation scum was 25,000 mg/L. The average increases in the removal efficiencies due to flotation scum recycling were 22.61% for T-P and 18.34% for PO4-P.
Table 1 Comparison of pollutant removal efficiency without and with flotation scum recycling
Figure 2 P removal efficiency in effluent of J MWTP by DAF pilot plant for 1 year. Operating conditions: alum dose 38.1 ppm, scum recycle ratio 0.5, saturated water recycle ratio 0.152, air to solid ratio 0.78 on average for 1 year.
pH and solid loading
While the processes are expected to follow the fundamentals of solid separation, the pH dependency of the P precipitant and surface chemistry of metal hydroxide/P complexes are not established. Recent work by Szabó et al. (2008) showed that there was an optimum pH for P removal, but that the optimum range was relatively wide (ranging from pH 5 to pH 7) and deterioration occurred outside this pH range. For the removal of phosphate from water to induce precipitation, the molar ratio and pH were the key parameters in determining the residual phosphate concentration (Liu et al. 2009). The precipitation reactions are dependent on phosphate concentration and pH, depending on the composition of the wastewater (Denham 2007). In addition, other hydrolysis products of aluminum may form, depending mainly on the pH (Georgantas & Grigoropoulou 2006).
Experiments of the DAF pilot plant performed without any pH adjustments led to less than 1.0 mg/L T-P in the effluent without recycled flotation scum and 0.1 mg/L T-P in the effluent with recycled flotation scum. As shown in Figure 3(a), for the influent pH range between 6.6 and 7.4, the removal efficiency of PO4-P increased with increasing pH values, while there was no relationship between pH and T-P removal efficiency under the operating conditions without or with flotation scum recycling. This result suggested that the removal of orthophosphate ions (PO4-P) was governed by pH, whereas other types of P compounds may not be removed by the formation of aluminum phosphate (AlPO4 ideally). Conversely, particles in the recycled flotation scum may be useful coagulation nuclei. Figure 3 (b) shows the removal efficiency depending on the solid loading rate. However, an obvious relationship between removal efficiency and solid loading rate was not observed without or with flotation scum recycling.
CONCLUSIONS
A series of experiments were conducted to investigate the feasibility of flotation scum recycling for effective P removal from a MWTP using a DAF pilot plant. The DAF experiments were performed using the effluent from secondary sedimentation in the J MWTP. Over 1 year, the T-P removal efficiency increased by 22.6% on average, and the T-P concentration was maintained below 0.1 mg/L by flotation scum recycling. A higher removal efficiency of T-P was induced by recycling the flotation scum because a significant amount of Al com[1]ponents remained in the flotation scum. The higher removal efficiency with recycling was due to the improved coagulation efficiency as Al components in the flotation scum acted as coagulants. Furthermore, the SS in the flotation scum enhanced the physical adsorption. The increase in T-P removal efficiency, due to the recycling of flotation scum, shifted from the boundary of the stoichiometric precipitate to the equilibrium control region, and the removal efficiency of T-P significantly increased as the molar ratio of Al:P was increased to 1.05 in the equilibrium control region. Flotation scum recycling may contribute to improving the quality of treated water and reducing treatment costs by minimizing the coagulant dosage required.
REFERENCES
American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF) 1998 Standard Methods for the Examination of Water and Wastewater 20th edn. American Public Health Association, Washington, DC.
Berkowitz, J., Anderson, M. A. & Amrhein, C. 2006 Influence of aging on phosphorus sorption to alum floc in lake water. Water Research 40 (5), 911–916.
Boisvert, J. P., To, T. C., Berrak, A. & Jolicoeur, C. 1997 Phosphate adsorption in flocculation processes of aluminium sulphate and poly-aluminium-silicate-sulphate. Water Research 31 (8), 1939–1946.
Butkus, M. A., Grasso, D., Schulthess, C. P. & Wijnja, H. 1998 Surface complexation modeling of phosphate adsorption by water treatment residual. Journal of Environmental Quality 27, 1055–1063.
Cheng, W. P., Chi, F. H. & Yu, R. F. 2004 Effect of phosphate on removal of humic substances by aluminum sulfate coagulant. Journal of Colloid and Interface Science 272 (1), 153–157.
Crossley, I. A. & Valade, M. T. 2006 A review of the technological developments of dissolved air flotation. Journal of Water Supply: Research and Technology-AQUA 55 (7–8), 479–491.
Denham, K. 2007 Chemical Phosphorus Removal and Control Strategies. MSc Thesis, Cranfield University, United Kingdom.
Flippin, H., Cuomo, L. & Petersen, L. 2013 Consolidating clarification Using dissolved-air flotation as secondary clarification improves activated sludge treatment and reduces space requirements. Water Environment & Technology 25 (6), 60–69.
Georgantas, D. A. & Grigoropoulou, H. P. 2005 Phosphorus removal from synthetic and municipal wastewater using spent alum sludge. Water Science and Technology 52 (10–11), 525–532.
Georgantas, D. A. & Grigoropoulou, H. P. 2006 Phosphorus removal from synthetic wastewater using alum and aluminum hydroxide. Global NEST Journal 8 (2), 121–130.
Guan, X. H., Chen, G. H. & Shang, C. 2005 Re-use of water treatment works sludge to enhance particulate pollutant from sewage. Water Research 39 (15), 3433–3440
09 Jul 2024
