Municipal wastewater treatment with anodizing solid waste (PDF ...
SM 154
Desalination 185 (2005) 341–350
Municipal wastewater treatment with anodizing solid waste A. Correia*, T. Chambino, L. Gonc¸alves, A. Franco, R. Gonc¸alves, A. Gonc¸alves, V. Limpo, F. Delmas, C. Nogueira, F. Bartolomeu Department of Materials and Production Technologies, National Institute of Engineering, Technology and Innovation, Estrada do Pac¸o do Lumiar, 1649–038 Lisboa, Portugal Tel. þ351 21 716 51 41x2327; Fax þ351 21 716 65 68; e-mail: [email protected] Received 21 March 2005; accepted 10 April 2005
Abstract In this study we have investigated the feasibility of the use of aluminium anodizing waste as coagulant for the treatment of municipal wastewaters instead of conventional inorganic materials. We have selected three different anodizing aluminium facilities for this study. The fresh mud was collected in the industrial facilities and prepared for tests. The anodizing muds were tested in two different forms: as powder and as mud suspension. For the powder form the fresh muds were submitted to a stabilisation process, homogenised and grinded. For the mud suspension form, water was added to the fresh mud in a way that the content of solids was known. A series of coagulation/ flocculation tests were made, being controlled important parameters as pH, coagulant dose, and impeller type, stirring conditions (time and speed) and settling time for the different sludges. The effective coagulant capacity of the anodizing sludges was verified based on the final turbidity and on the pollutant removal (COD) of the supernatant samples as a function of the coagulant dose and pH. Keywords: Coagulation; Anodizing waste; Municipal wastewater; Aluminium; Flocculation
1. Introduction In aluminium anodizing processes, the classical surface treatment of this metal, large quantities of waste (anodic mud) are generated. In the industrial facilities there are normally two kinds of effluents clearly differentiated: concentrated solutions from the chemical baths and washing *Corresponding author.
waters. In most installations there is only a settling tank where all wastes are blended, neutralized, flocculated and settled. The final product is the anodizing mud, which has a variable composition but that contains three main constituents: aluminium hydroxide, oxy-hydroxides and basic sulphates, namely of aluminium. This anodic sludge is a problem for many countries because it is relatively difficult to manage due
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved
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A. Correia et al. / Desalination 185 (2005) 341–350
to its complex nature. At the present time the disposal of this sludge on land is a common practice and the cost of this operation is getting increasingly higher. Today this situation represents a really great environmental problem whose resolution is necessary and urgent. The use of aluminium mineral salts in wastewater treatment is much spread. The chemical coagulation and flocculation followed by neutralization is a process well known and most common in water and wastewater treatment. For a long time we have been studying the feasibility of the aluminium anodizing waste as coagulant [1,2]. In previous works we have tested anodizing sludge from one facility in the treatment of municipal wastewaters [3] and in the treatment of paint industry wastewater [4] instead of the conventional inorganic coagulants. In this work three anodizing sludges were selected due to their chemical composition in aluminium and two forms of the coagulant were tested: powder and mud suspension. 2. Coagulation basic principles Coagulation is a well-known process which purpose, combined with a solid-liquid separation process, is the removal of turbidity, colour or micro-organisms that are present in the wastewaters as colloidal suspensions. These suspensions are a heterogeneous mixture of particles with different size, shape and chemical composition. A colloid has been defined as a dispersion of distinguishable particles in the size range of 0.01–10 mm in a medium that may be regarded as a structure less continuum [5]. Colloidal systems will usually scatter light, that is, they exhibit turbidity, which is related to the sizes of the particles involved. Colloidal suspensions in aqueous media appear cloudy, and the observed turbidity depends on both the particle size distribution and the mass concentration present. This type of particles tends to remain in
suspension for a long period of time and due to its great stability colloids do not form aggregates. The most important interactions affecting suspension stability are electrostatic repulsion and Van der Waals attraction. These two interactions are assumed to be additive and together establishing the total energy of interaction between particles as a function of separation distance. Attraction predominates at short distances and repulsion is more effective at greater distances. To eliminate these particles the electrostatic forces of the suspension must be destabilized. Then if there is enough kinetic energy available a separation distance can be reached where attraction becomes more effective and particle collision and aggregation can occur. Coagulation can be described as the agentinduced aggregation of particles suspended in liquid media into larger particles. The coagulation favours, with the help of slow stirring, the contacts between the destabilized particles. The particles aggregate to form flocs that are more easily removed. According to O’Melia [6] four mechanisms of coagulation are recognised: compression of the diffuse layer, adsorption to produce charge neutralization, enmeshment in a precipitate and adsorption to permit interparticle bridging. The destabilisation of colloids in water and wastewater is probably accomplished by adsorption of oppositely charged soluble and insoluble coagulant hydrolysis species on the colloid and subsequent destabilisation, enmeshment of colloid within hydroxide or carbonate precipitates, or both. The use of aluminium in the clarification of water is common practice in wastewater treatment. When aluminium salts are dissolved in water, dissociation into the constituent ions occurs. These ions are submitted to hydration reactions. The products of the
A. Correia et al. / Desalination 185 (2005) 341–350
hydrolyse reactions of the aluminium salts are the effective coagulant agents. Several hydrolysis species can be formed depending on the pH, temperature and the concentration of mineral salts. These hydroximetal complexes adsorb on the particle surfaces and the charges they carry may cause charge reversals of the surfaces they adsorb on, contributing to the destabilisation of the suspension. The hydrolysis reactions have a great tendency to release Hþ, lowering the pH. A different but important effect of the coagulation characteristics of aluminium is the formation in the alkaline range of a hydroxide precipitate that appears as a fine colloidal dispersion. These particles tend to aggregate forming hydroxide flocs and then enmesh the colloidal particles present in the wastewater. Which possibility will occur will depend on the concentration of aluminium, the final pH and the wastewater particle concentration as observed by Gregory [7]. It can be seen that in acid range it predominates the coagulation mechanism of adsorption and charge neutralization and in alkaline range it happens the mechanism of sweep floc with formation of a precipitate that involves and drags the suspended particles. 3. Characterization of the sludge and the municipal wastewater 3.1. Anodizing sludge characterization The anodizing muds were collected in three Portuguese different facilities having the operations of aluminium anodizing. The sludges came from the wastewaters treatment Table 1 Moisture and pH of the fresh aluminium anodizing sludges Parameter Moisture (%) pH
Results
343
of the anodizing plants and were collected at the press-filter discharge. A representative composite sample was collected, well mixed and homogenised. The sludges were characterised and the results obtained are presented in Tables 1 and 2. Like it was expected aluminium is one of the major constituents of the composition of the solid fraction along with the total sulphur expressed in sulphates. When using the sludges as mud suspensions, water was added to the fresh muds in a way that the solids percentage was known. The total solids, in the mud suspensions tested, were between 5 and 12.5% for the three different sludges. When the sludges were used in the powder form they were first stabilized. They were submitted to a process of drying at room temperature, for several days until constant weight loss. The time needed for the stabilization process was dependent on particle/cake size, room temperature, air humidity and sludge moisture. After stabilization the dried sludges were grinded in a hammer mill with a screen of 0.50 mm so that a homogeneous powder of constant characteristics was produced for the use in the wastewater treatment experiments. The stabilized muds have moisture values that are presented in Table 3. The particle size distribution of the three dried sludges tested as flocculants in this work was characterised and we have verified that 90% of the particles in S1 sample have a diameter smaller than 286.96 mm, in S2 sample have a diameter smaller than 50.18 mm and in S3 sample they have a diameter smaller than 35.10 mm (Fig. 1). 3.2. Municipal wastewater characterization
S1
S2
S3
81 7.4
69 6.9
70 6.7
The municipal wastewaters used in this study were collected in a large municipal wastewater treatment plant, near Lisbon. This
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A. Correia et al. / Desalination 185 (2005) 341–350
Table 2 Chemical analysis of the aluminium anodizing sludges after drying at 105 C Parameter
Results
Weight loss 600 C (%) Weight loss 1000 C (%) Sulphur, total (SO4) (%) Aluminium, total (Al) (%) Sodium (Na) (%) Calcium (Ca) (%) Iron (Fe) (%) Tin (Sn) (%) Manganese (Mn) (mg/kg) Chromium total (Cr) (mg/kg) Chromium VI (Cr) (mg/kg) Nickel (Ni) (mg/kg) Cooper (Cu) (mg/kg) Zinc (Zn) (mg/kg) Lead (Pb) (mg/kg) Strontium (Sr) (mg/kg) Cyanides (CN) (mg/kg)
S1
S2
S3
28 37 14 22 0.17 0.062 0.34 5 124 110
Desalination 185 (2005) 341–350
Municipal wastewater treatment with anodizing solid waste A. Correia*, T. Chambino, L. Gonc¸alves, A. Franco, R. Gonc¸alves, A. Gonc¸alves, V. Limpo, F. Delmas, C. Nogueira, F. Bartolomeu Department of Materials and Production Technologies, National Institute of Engineering, Technology and Innovation, Estrada do Pac¸o do Lumiar, 1649–038 Lisboa, Portugal Tel. þ351 21 716 51 41x2327; Fax þ351 21 716 65 68; e-mail: [email protected] Received 21 March 2005; accepted 10 April 2005
Abstract In this study we have investigated the feasibility of the use of aluminium anodizing waste as coagulant for the treatment of municipal wastewaters instead of conventional inorganic materials. We have selected three different anodizing aluminium facilities for this study. The fresh mud was collected in the industrial facilities and prepared for tests. The anodizing muds were tested in two different forms: as powder and as mud suspension. For the powder form the fresh muds were submitted to a stabilisation process, homogenised and grinded. For the mud suspension form, water was added to the fresh mud in a way that the content of solids was known. A series of coagulation/ flocculation tests were made, being controlled important parameters as pH, coagulant dose, and impeller type, stirring conditions (time and speed) and settling time for the different sludges. The effective coagulant capacity of the anodizing sludges was verified based on the final turbidity and on the pollutant removal (COD) of the supernatant samples as a function of the coagulant dose and pH. Keywords: Coagulation; Anodizing waste; Municipal wastewater; Aluminium; Flocculation
1. Introduction In aluminium anodizing processes, the classical surface treatment of this metal, large quantities of waste (anodic mud) are generated. In the industrial facilities there are normally two kinds of effluents clearly differentiated: concentrated solutions from the chemical baths and washing *Corresponding author.
waters. In most installations there is only a settling tank where all wastes are blended, neutralized, flocculated and settled. The final product is the anodizing mud, which has a variable composition but that contains three main constituents: aluminium hydroxide, oxy-hydroxides and basic sulphates, namely of aluminium. This anodic sludge is a problem for many countries because it is relatively difficult to manage due
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved
342
A. Correia et al. / Desalination 185 (2005) 341–350
to its complex nature. At the present time the disposal of this sludge on land is a common practice and the cost of this operation is getting increasingly higher. Today this situation represents a really great environmental problem whose resolution is necessary and urgent. The use of aluminium mineral salts in wastewater treatment is much spread. The chemical coagulation and flocculation followed by neutralization is a process well known and most common in water and wastewater treatment. For a long time we have been studying the feasibility of the aluminium anodizing waste as coagulant [1,2]. In previous works we have tested anodizing sludge from one facility in the treatment of municipal wastewaters [3] and in the treatment of paint industry wastewater [4] instead of the conventional inorganic coagulants. In this work three anodizing sludges were selected due to their chemical composition in aluminium and two forms of the coagulant were tested: powder and mud suspension. 2. Coagulation basic principles Coagulation is a well-known process which purpose, combined with a solid-liquid separation process, is the removal of turbidity, colour or micro-organisms that are present in the wastewaters as colloidal suspensions. These suspensions are a heterogeneous mixture of particles with different size, shape and chemical composition. A colloid has been defined as a dispersion of distinguishable particles in the size range of 0.01–10 mm in a medium that may be regarded as a structure less continuum [5]. Colloidal systems will usually scatter light, that is, they exhibit turbidity, which is related to the sizes of the particles involved. Colloidal suspensions in aqueous media appear cloudy, and the observed turbidity depends on both the particle size distribution and the mass concentration present. This type of particles tends to remain in
suspension for a long period of time and due to its great stability colloids do not form aggregates. The most important interactions affecting suspension stability are electrostatic repulsion and Van der Waals attraction. These two interactions are assumed to be additive and together establishing the total energy of interaction between particles as a function of separation distance. Attraction predominates at short distances and repulsion is more effective at greater distances. To eliminate these particles the electrostatic forces of the suspension must be destabilized. Then if there is enough kinetic energy available a separation distance can be reached where attraction becomes more effective and particle collision and aggregation can occur. Coagulation can be described as the agentinduced aggregation of particles suspended in liquid media into larger particles. The coagulation favours, with the help of slow stirring, the contacts between the destabilized particles. The particles aggregate to form flocs that are more easily removed. According to O’Melia [6] four mechanisms of coagulation are recognised: compression of the diffuse layer, adsorption to produce charge neutralization, enmeshment in a precipitate and adsorption to permit interparticle bridging. The destabilisation of colloids in water and wastewater is probably accomplished by adsorption of oppositely charged soluble and insoluble coagulant hydrolysis species on the colloid and subsequent destabilisation, enmeshment of colloid within hydroxide or carbonate precipitates, or both. The use of aluminium in the clarification of water is common practice in wastewater treatment. When aluminium salts are dissolved in water, dissociation into the constituent ions occurs. These ions are submitted to hydration reactions. The products of the
A. Correia et al. / Desalination 185 (2005) 341–350
hydrolyse reactions of the aluminium salts are the effective coagulant agents. Several hydrolysis species can be formed depending on the pH, temperature and the concentration of mineral salts. These hydroximetal complexes adsorb on the particle surfaces and the charges they carry may cause charge reversals of the surfaces they adsorb on, contributing to the destabilisation of the suspension. The hydrolysis reactions have a great tendency to release Hþ, lowering the pH. A different but important effect of the coagulation characteristics of aluminium is the formation in the alkaline range of a hydroxide precipitate that appears as a fine colloidal dispersion. These particles tend to aggregate forming hydroxide flocs and then enmesh the colloidal particles present in the wastewater. Which possibility will occur will depend on the concentration of aluminium, the final pH and the wastewater particle concentration as observed by Gregory [7]. It can be seen that in acid range it predominates the coagulation mechanism of adsorption and charge neutralization and in alkaline range it happens the mechanism of sweep floc with formation of a precipitate that involves and drags the suspended particles. 3. Characterization of the sludge and the municipal wastewater 3.1. Anodizing sludge characterization The anodizing muds were collected in three Portuguese different facilities having the operations of aluminium anodizing. The sludges came from the wastewaters treatment Table 1 Moisture and pH of the fresh aluminium anodizing sludges Parameter Moisture (%) pH
Results
343
of the anodizing plants and were collected at the press-filter discharge. A representative composite sample was collected, well mixed and homogenised. The sludges were characterised and the results obtained are presented in Tables 1 and 2. Like it was expected aluminium is one of the major constituents of the composition of the solid fraction along with the total sulphur expressed in sulphates. When using the sludges as mud suspensions, water was added to the fresh muds in a way that the solids percentage was known. The total solids, in the mud suspensions tested, were between 5 and 12.5% for the three different sludges. When the sludges were used in the powder form they were first stabilized. They were submitted to a process of drying at room temperature, for several days until constant weight loss. The time needed for the stabilization process was dependent on particle/cake size, room temperature, air humidity and sludge moisture. After stabilization the dried sludges were grinded in a hammer mill with a screen of 0.50 mm so that a homogeneous powder of constant characteristics was produced for the use in the wastewater treatment experiments. The stabilized muds have moisture values that are presented in Table 3. The particle size distribution of the three dried sludges tested as flocculants in this work was characterised and we have verified that 90% of the particles in S1 sample have a diameter smaller than 286.96 mm, in S2 sample have a diameter smaller than 50.18 mm and in S3 sample they have a diameter smaller than 35.10 mm (Fig. 1). 3.2. Municipal wastewater characterization
S1
S2
S3
81 7.4
69 6.9
70 6.7
The municipal wastewaters used in this study were collected in a large municipal wastewater treatment plant, near Lisbon. This
344
A. Correia et al. / Desalination 185 (2005) 341–350
Table 2 Chemical analysis of the aluminium anodizing sludges after drying at 105 C Parameter
Results
Weight loss 600 C (%) Weight loss 1000 C (%) Sulphur, total (SO4) (%) Aluminium, total (Al) (%) Sodium (Na) (%) Calcium (Ca) (%) Iron (Fe) (%) Tin (Sn) (%) Manganese (Mn) (mg/kg) Chromium total (Cr) (mg/kg) Chromium VI (Cr) (mg/kg) Nickel (Ni) (mg/kg) Cooper (Cu) (mg/kg) Zinc (Zn) (mg/kg) Lead (Pb) (mg/kg) Strontium (Sr) (mg/kg) Cyanides (CN) (mg/kg)
S1
S2
S3
28 37 14 22 0.17 0.062 0.34 5 124 110
