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Adapting dissolved air flotation for the clarification of seawater
Desalination 311 (2013) 90–94
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Adapting dissolved air flotation for the clarification of seawater Johannes Haarhoff a,⁎, James K. Edzwald b a b
Department of Civil Engineering Science, University of Johannesburg, Box 524, Auckland Park, 2006 Johannesburg, South Africa Department of Civil and Environmental Engineering, University of Massachusetts, Amherst, MA 01003, USA
H I G H L I G H T S ► ► ► ►
We compare the performance of dissolved air flotation in seawater and freshwater. The contact between particles, bubbles and water is slightly impaired in seawater. Air solubility is less in seawater, necessitating substantial design changes. Two charts are provided to make design corrections for seawater.
a r t i c l e
i n f o
Article history: Received 10 October 2012 Accepted 27 October 2012 Available online 20 December 2012 Keywords: Dissolved air flotation Saturation pressure Recycle rate Seawater Air solubility
a b s t r a c t The high ionic strength of seawater affects the performance of the dissolved air flotation (DAF) clarification process in a number of ways. At a reference temperature of 20 °C, the density of seawater is 3% higher than freshwater; the dynamic viscosity 8% higher; and the surface tension 1% higher. These differences cause very small changes in the rates of movement of particles and bubbles in both the contact and separation zones of the DAF reactor, that can be ignored for practical design purposes. Much more important are the differences in the solubility of air in seawater (controlled by Henry's constant) and the air transfer efficiency in pressure saturators or other air saturation devices (largely controlled by the molecular diffusivities of the air gases in water). At 20 °C, a typical air saturator only transfers 74% of the air to seawater relative to freshwater. This shortfall can be corrected by either increasing the recycle rate, or by operating the saturator at a higher pressure. The paper presents design charts for both these options. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dissolved air flotation (DAF) is an emerging process for the clarification of seawater before desalination by reverse osmosis. The success of DAF depends on a number of physical processes relating to the movement of bubbles and particles through water, and to the dissolution and precipitation of air. These processes have been studied in detail in relation to the clarification of freshwater and robust design procedures are available [2]. The much higher salinity of seawater, however, requires some adjustments to the conventional design guidelines to ensure the optimal efficiency of DAF. The objectives of this paper are to identify how seawater affects DAF performance and to propose methods whereby the design procedures are adjusted. The concentration of dissolved matter in seawater is described in terms of its salinity (S). It is expressed on a mass basis as the grammes of dissolved material per kilogramme of seawater. For seawater of average composition it is 35.16 g/kg, but varies in the open oceans from about 31 to 38 g/kg. The higher values are found in the ⁎ Corresponding author. Tel.: +27 11 559 2148; fax: +27 11 559 2395. E-mail addresses: [email protected] (J. Haarhoff), [email protected] (J.K. Edzwald). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.035
Mediterranean Sea, Red Sea (Arabian Gulf), and the Persian Gulf. Lower values are found near the mouths of rivers. For the purposes of this paper, a rounded salinity of S = 35 g/kg is used. Another measure of the ion content of seawater is the ionic strength (I). I for freshwaters range from about 5 × 10 −4 to 10 −2 M corresponding to total dissolved solids (TDS) of 20 to 400 mg/L. Brackish and estuarine waters have I starting at about 10−2 M up to less than that of seawater. The average I for seawater is 0.68 mol/kg. 2. Physical properties of seawater The density, dynamic viscosity and surface tension are all higher in seawater than in freshwater. The density difference ranges from 2.8% at 0 °C to 2.6% at 40 °C; for dynamic viscosity from 6.3% at 0 °C to 8.3% at 40 °C; and for surface tension from 0.7% at 0 °C to 1.4% at 40 °C. These ratios (seawater/freshwater) were calculated from reference values published for freshwater [9] and seawater at S = 35 g/kg [8] and shown in Fig. 1. The Henry's constants for the principal three gases in atmospheric air (nitrogen, oxygen and argon) are all significantly higher in seawater than in freshwater (and therefore less soluble). The values for oxygen were calculated from Garcia and Gordon [3] and the values for nitrogen
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
91
Fig. 1. Seawater/freshwater ratios for bulk water physical properties.
Fig. 3. Schematic representation of the DAF process.
and argon from Hamme and Emerson [5]. Fig. 2 shows seawater/ freshwater ratios for the Henry's constants. The molecular diffusivity of the air gases in seawater is somewhat uncertain at present. Earlier experimental measurements showed that the molecular diffusion coefficient is lower in seawater, which was used to support a recommendation that seawater diffusivity should be taken as 6% lower than that of freshwater [6]. A more recent review pointed out that later experimental work failed to confirm this recommendation [7]. At this point, for practical engineering design, it seems prudent to assume the seawater/freshwater ratio of molecular diffusivity to be 0.94 for all the principal air gases.
algae, and oil and grease. It is particularly effective in removing algae so DAF is a good choice where harmful red and brown algal blooms occur. Fig. 4 shows SWRO pretreatment consisting of coagulation (ferric chloride is the coagulant of choice), flocculation, DAF, and dual-media granular filters. An alternative is to place DAF directly on top of the filters (DAFF) where the hydraulic loading rate on the DAFF system is 10 to 15 m/h. An alternative is horizontal separation of DAF from the filters (as depicted in Fig. 4) and use of high-rate DAF loading rates of 20 to 35 m/h. Returning to Fig. 3, the DAF reactor is conceptualised as having two consecutive steps. The first is the contact zone, where flocs and bubbles interact to form stable floc–bubble aggregates. The second is the separation zone, where the aggregates are allowed to rise to the top of the tank. In reality, there is some overlap between these two zones. For both these zones, comprehensive mathematical models have been developed which allowed much better understanding of the fundamental mechanisms involved in these complex three-phase systems. These models, coupled with a strong foundation of empirical experience, have evolved into robust design models for DAF in the drinking water business, recently summarised in a comprehensive handbook [2]. The mathematical models presented in this handbook form the basis for the analyses presented in the following sections.
3. Seawater effects on DAF 3.1. Process description DAF is a gravity-driven solids–liquid separation process which has gained wide acceptance in the drinking water treatment industry since the early 1990s. It relies on the buoyancy induced by adding air to a suspension of flocculated water, which drives the floc–bubble aggregates to the top of the DAF reactor towards the float layer, while the clarified water is withdrawn at the bottom of the DAF reactor. The air is injected with the pressurised recycle stream, which precipitates as small bubbles when the pressure is released across the recycle injection nozzles. The process is schematically shown in Fig. 3. Over the last 10 years or so DAF is being increasingly used as an important pretreatment process for seawater reverse osmosis (SWRO) plants in removing turbidity, total organic carbon (TOC),
3.2. Contact zone The contact zone model, initially developed by Edzwald et al. [1] and later reviewed by Haarhoff and Edzwald [4], indicates that the physical properties of the water affect its performance in two ways. The first is the rise velocity (vb) of the air bubbles through the contact zone, which is dependent on both water density and viscosity. The rise velocity is calculated from the classical Stokes expression: 2
vb ¼
gðρw −ρb Þdb 18μ w
ð1Þ
Eq. (1) is used to calculate the seawater/freshwater ratio of the rise velocity (vb ratio) in the contact zone (subscript S indicates seawater and F freshwater): vb ratio ¼
Fig. 2. Seawater/freshwater ratios for Henry's constants for the principal air gases.
μF ρF −ρb
ρS −ρb μS
ð2Þ
The second determinant of contact zone performance is the total single collector efficiency (ηT), which is the sum of the particle transport mechanisms due to Brownian diffusion (ηD), interception (ηI), and settling (ηS). Interception is the dominant mechanism, but it is
92
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
Fig. 4. Process schematic of dissolved air flotation pretreatment for SWRO plants.
not dependent on water's physical properties and thus is the same for freshwater and seawater. Brownian diffusion and settling, which are dependent on water density, play minor roles. In fact, the settling collector term plays such a negligible role in the contact zone that only Brownian diffusion needs to be considered. The collision efficiency due to Brownian diffusion is: 2 = " #2 =3 2 3 kb T 1 1 ηD ¼ 6:18 g ðρw −ρb Þ dp db
ð3Þ
The ratio of the Brownian diffusion term in seawater and freshwater (ηD ratio) follows:
ηD ratio ¼
2 ρF −ρb =3 ρS −ρb
ð4Þ
At a reference temperature of 20 °C and with particle or floc density assumed at 1050 kg/m 3: the vb ratio is 0.96 and the ηD ratio is 0.98. At a higher temperature of 35 °C (some seawater facilities are located in hot climates) these ratios approach unity. Therefore, the collision efficiency in seawater should be about the same as in freshwater. Assuming sufficient air is supplied (see Sections 3.4, 3.5, and 4), then the overall contact zone performance and contact zone design should be similar for seawater and freshwater. 3.3. Separation zone The efficiency of the DAF separation zone depends on the upward movement of the floc–bubble aggregates through the water—a process dependent on the density and viscosity of the water. Mathematical models for the rise rate of the floc–bubble aggregates (vfb) allow the calculation of the seawater/freshwater ratio for the rise rate (vfb ratio) [2]. For laminar flow (Reynolds number less than 1), vfb and vfb ratio are given by Eqs. (5) and (6):
vfb ðlaminarÞ ¼
4g ρw −ρfb d2fb
ð5Þ
3Kμ w
vfb ratio ðlaminarÞ ¼
ρS −ρfb ρF −ρfb
!
μF μS
ð6Þ
For DAF, it is likely that the rise rate of some of the larger floc–bubble aggregates will be in the transitional flow regime (Reynolds number larger than 1), which leads to different expressions for vfb and vfb ratio:
4 vfb ðtransitionalÞ ¼ 3K
2 3 0:8 g 0:8 ρ −ρ 0:8 d1:4 w fb fb 6 7 4 5 0:6 ρ0:2 w μw
ð7Þ
vfb ratio ðtransitionalÞ ¼
ρS −ρfb ρF −ρfb
!0:8
ρF ρS
0:2
μF μS
0:6
ð8Þ
Eqs. (6) and (8) require an estimate of the density of the floc–bubble aggregates. The difference between water density and floc–bubble density should be about the same in seawater and freshwater, and the first bracketed terms on the right of the equations are assumed to be one. For laminar flow, vfb ratio varies from 0.94 at 0 °C to 0.92 at 40 °C. For transitional flow, it varies from 0.96 at 0 °C to 0.95 at 40 °C. This analysis suggests a reduction in hydraulic loading rates for seawater between 5 and 8%. However, given the small differences and the conservative nature of design, we expect little or no change in the design of full-scale plants. 3.4. Air solubility The solubilities of gases in water are limited by their Henry's constants. Fig. 2 shows that the Henry's constants are considerably higher in seawater. Because the gas solubility (G) is inversely proportional to the Henry's constant, then the gas solubility seawater/freshwater ratio (G ratio) is: G ratio ¼
−1
HS H F −1
! ¼
HF HS
ð9Þ
Atmospheric air consists mainly of nitrogen (78.1% volume per volume), oxygen (21.0%) and argon (0.9%). Taking a weighted average of these fractions, the solubility of atmospheric air at 20 °C and sea level is 0.640 mol/m 3 in seawater compared to 0.837 mol/m 3 in freshwater — a G ratio of 0.765. 3.5. Air transfer in packed saturators Practical air saturation systems for DAF usually employ packed or unpacked pressure saturators. The majority of water clarification plants around the world use packed saturators with good effect, and their design is backed up with a rational design procedure, which is still lacking for unpacked saturators. This paper considers the effect of seawater on packed saturators only. Henry's constant provides the maximum amount of air that can be dissolved in water. In a real saturation system, less air is transferred, because saturators are not 100% efficient due to kinetic constraints. First, the air transfer efficiency in packed saturators depends on the wetted packing area in the saturator. The dry packing area is defined by the geometry of the packing pieces, but the wet packing area depends on how evenly the water spreads itself over the packing surfaces. This can be predicted with an Onda correlation [2]. The seawater/freshwater ratio of wetted packing area (aw ratio) is practically constant over all packing sizes and hydraulic loading rates and ranges between 0.97 and 0.98. Second, the rate of mass transfer at the air/water interface is influenced by the molecular diffusivity (D) of the
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
93
Table 1 Design parameters for typical packed saturators.
Hydraulic loading Saturator pressure Packing depth Packing size
kg/m2-s kPa gauge mm mm
Range
Typical
20–60 350–600 800–2000 25–90
40 500 1400 50
gas, which is discussed above. As suggested, the seawater/freshwater ratio of molecular diffusivity (D ratio) is taken conservatively as 0.94. The mass transfer constant (KL) reflects the combined effects of the wetted packing area and the molecular diffusivity, as well as the density and viscosity of the water. Using a second Onda correlation in Eq. (10), the seawater/freshwater ratio of the mass transfer constant (KL ratio) is predicted with Eq. (11): 1 1 2 2 = μ g =3 HLmass =3 ρ D =2 a dp 5 K L ¼ 0:0051 ρ μ aw μ K L ratio ¼
5 = 1 = 2 = 1 = 6 6 3 3 μF ρS aF DS μS ρF aS DF
ð10Þ
ð11Þ
Eq. (11) indicates that KL ratio ranges from 0.94 at 0 °C to 0.92 at 40 °C for all the air gases. All the ratios are now known to calculate the effect of seawater on the total mass of air transferred (M) in a typical packed air saturator, using well established methods reported elsewhere [2]. The design parameters of a typical packed saturator used in these calculations are shown in Table 1. The seawater/freshwater ratio for the total air mass transferred in a typical saturator (M ratio) is shown in Fig. 5. The relationship in Fig. 5 indicates that a typical saturator which produces 100% of air in freshwater will only produce 71% to 77% of air in seawater over the temperature range investigated. This is a large difference. This range stays about the same even if the saturator design is varied over the full range of the design parameters in Table 1. If one assumes that seawater DAF requires the same volumetric air concentration as freshwater DAF (there is no reason at this point to disagree), then the conventional saturation system, as commonly used for freshwater, has to be adapted for seawater.
Fig. 6. Required seawater recycle rate to match the air produced by a similar air saturator for freshwater.
other saturator variables remaining the same. To end up with the same air concentration in the combined flow entering the contact zone (A), the first obvious solution is to increase the recycle rate (r). The design equation linking M, A and r is provided by Eq. (12): M¼
1þr A r
ð12Þ
Assuming that the required air concentration A should be the same for seawater and freshwater, an expression for the required recycle rate for seawater is obtained: −1 r r S ¼ ðM ratioÞ −1 1þr F
ð13Þ
The M ratio in Fig. 5 cannot be meaningfully adjusted with the design parameters in Table 1. This means that the air concentration leaving the saturator (M) is less for seawater then for freshwater, all
Fig. 6 provides a chart to obtain the necessary recycle rate for seawater to match the air produced by a similar saturator for freshwater. To match 10% at 20 °C for freshwater, for example, the seawater recycle rate has to be 14%. The above analysis holds if the saturation pressure stays the same. An alternative option would be to keep the recycle rate constant, and increase the pressure within the saturator. Based on the saturator design equations presented by Edzwald and Haarhoff [2], Fig. 7 provides a chart to determine the required pressure for a seawater saturator to match the air transfer of a freshwater saturator. To match 500 kPa at 20 °C for freshwater, for example, the seawater saturation pressure has to be 675 kPa.
Fig. 5. Seawater/freshwater ratio for the mass of air transferred in a typical packed saturator.
Fig. 7. Required pressure for a seawater saturator to match the air produced by a freshwater saturator.
4. Suggested design corrections for seawater DAF
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Table 2 Summary of effects of seawater relative to freshwater, at 20 °C with salinity S= 35 g/kg. Property
Change
Comment
Physical properties Density Dynamic viscosity Surface tension
Plus 3% Plus 8% Plus 1%
– – –
Contact zone Bubble rise rate Collision efficiency Brownian Collision efficiency interception Collision efficiency total
Minus 4% Minus 2% No effect Very small
Not significant Minor importance Major importance Dominated by interception
Separation zone Floc/bubble rise rate (laminar) Floc/bubble rise rate (transitional)
Minus 7% Minus 4%
Not likely to affect design Not likely to affect design
Air solubility Henry's constant nitrogen Henry's constant oxygen Henry's constant argon Atmospheric air solubility
Plus 32% Plus 30% Plus 29% Minus 24%
Large Large Large Large
Packed saturators Wetted packing area Mass transfer constant Air mass transferred
Minus 2% Minus 7% Minus 26%
Minor importance Important Large difference
difference difference difference difference
Nomenclature a packing area A available air concentration entering the contact zone d diameter D molecular diffusivity g gravitational acceleration G gas solubility H Henry's constant HLmass mass hydraulic loading kb Boltzmann's constant K coefficient for drag constant KL mass transfer constant M available air concentration leaving the saturator r recycle rate S salinity T absolute temperature v rise velocity ρ density η collision efficiency μ dynamic viscosity subscript b bubble subscript D Brownian diffusion subscript fb floc–bubble subscript F freshwater subscript I inertia subscript S seawater subscript T total subscript w water or wetted
5. Summary and conclusions The high salinity of seawater affects its physical properties in relation to freshwater. The changes that are relevant to the design of DAF systems for water clarification are summarised in Table 2. The changes have a small, but negligible, effect on the design of the contact zone. The effects on the design of the separation zone are greater, but are not likely to change the design of full-scale systems. The design of the air saturation systems, however, definitely needs to be adapted due to the lower solubility of air in seawater. Two possible ways of adapting the design procedure for seawater systems, or combinations thereof, are suggested. In both cases, a saturator should first be designed for freshwater, using standard published procedures (see [2]). To account for seawater, the first option is to keep the saturation pressure constant, but to increase the recycle rate by using the design chart in Fig. 6. The second option is to keep the recycle rate constant, but to increase the saturation pressure by using the design chart in Fig. 7.
References [1] J.K. Edzwald, J.P. Malley Jr., C. Yu, A conceptual model for dissolved air flotation in water treatment, Water Supply 8 (1990) 141–150. [2] J.K. Edzwald, J. Haarhoff, Dissolved Air Flotation for Water Clarification, McGraw-Hill, New York and AWWA, Denver, 2012. [3] H.E. García, L.I. Gordon, Oxygen solubility in seawater: better fitting equations, Limnol. Oceanogr. 37 (6) (1992) 1307–1312. [4] J. Haarhoff, J.K. Edzwald, Dissolved air flotation modelling: insights and shortcomings, J. Water Supply Res. Technol. AQUA 53 (3) (2004) 127–150. [5] R.C. Hamme, S.R. Emerson, The solubility of neon, nitrogen and argon in distilled water and seawater, Deep Sea Res. 51 (11) (2004) 1517–1528. [6] B. Jähne, G. Heinz, W. Dietrich, Measurement of the diffusion coefficients of sparingly soluble gases in water, J. Geophys. Res. 92 (1987) 10767–10776. [7] D.B. King, W.J. De Bryun, M. Zheng, E.S. Salzman, Uncertainties in the molecular diffusion coefficient of gases in water for use in the estimation of air-sea exchange, in: B. Jähne, E. Monahan (Eds.), Air–Water Gas Transfer, AEON Verlag, 1995. [8] M.H. Sharqawy, J.H. Lienhard, S.M. Zubair, Thermophysical properties of seawater: a review of the existing correlations and data, Desalin. Water Treat. 16 (2010) 354–380. [9] The Chemical Rubber Company, in: R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 16th ed., CRC Press Inc., Boca Raton, Florida, 1985.
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Adapting dissolved air flotation for the clarification of seawater Johannes Haarhoff a,⁎, James K. Edzwald b a b
Department of Civil Engineering Science, University of Johannesburg, Box 524, Auckland Park, 2006 Johannesburg, South Africa Department of Civil and Environmental Engineering, University of Massachusetts, Amherst, MA 01003, USA
H I G H L I G H T S ► ► ► ►
We compare the performance of dissolved air flotation in seawater and freshwater. The contact between particles, bubbles and water is slightly impaired in seawater. Air solubility is less in seawater, necessitating substantial design changes. Two charts are provided to make design corrections for seawater.
a r t i c l e
i n f o
Article history: Received 10 October 2012 Accepted 27 October 2012 Available online 20 December 2012 Keywords: Dissolved air flotation Saturation pressure Recycle rate Seawater Air solubility
a b s t r a c t The high ionic strength of seawater affects the performance of the dissolved air flotation (DAF) clarification process in a number of ways. At a reference temperature of 20 °C, the density of seawater is 3% higher than freshwater; the dynamic viscosity 8% higher; and the surface tension 1% higher. These differences cause very small changes in the rates of movement of particles and bubbles in both the contact and separation zones of the DAF reactor, that can be ignored for practical design purposes. Much more important are the differences in the solubility of air in seawater (controlled by Henry's constant) and the air transfer efficiency in pressure saturators or other air saturation devices (largely controlled by the molecular diffusivities of the air gases in water). At 20 °C, a typical air saturator only transfers 74% of the air to seawater relative to freshwater. This shortfall can be corrected by either increasing the recycle rate, or by operating the saturator at a higher pressure. The paper presents design charts for both these options. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dissolved air flotation (DAF) is an emerging process for the clarification of seawater before desalination by reverse osmosis. The success of DAF depends on a number of physical processes relating to the movement of bubbles and particles through water, and to the dissolution and precipitation of air. These processes have been studied in detail in relation to the clarification of freshwater and robust design procedures are available [2]. The much higher salinity of seawater, however, requires some adjustments to the conventional design guidelines to ensure the optimal efficiency of DAF. The objectives of this paper are to identify how seawater affects DAF performance and to propose methods whereby the design procedures are adjusted. The concentration of dissolved matter in seawater is described in terms of its salinity (S). It is expressed on a mass basis as the grammes of dissolved material per kilogramme of seawater. For seawater of average composition it is 35.16 g/kg, but varies in the open oceans from about 31 to 38 g/kg. The higher values are found in the ⁎ Corresponding author. Tel.: +27 11 559 2148; fax: +27 11 559 2395. E-mail addresses: [email protected] (J. Haarhoff), [email protected] (J.K. Edzwald). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.035
Mediterranean Sea, Red Sea (Arabian Gulf), and the Persian Gulf. Lower values are found near the mouths of rivers. For the purposes of this paper, a rounded salinity of S = 35 g/kg is used. Another measure of the ion content of seawater is the ionic strength (I). I for freshwaters range from about 5 × 10 −4 to 10 −2 M corresponding to total dissolved solids (TDS) of 20 to 400 mg/L. Brackish and estuarine waters have I starting at about 10−2 M up to less than that of seawater. The average I for seawater is 0.68 mol/kg. 2. Physical properties of seawater The density, dynamic viscosity and surface tension are all higher in seawater than in freshwater. The density difference ranges from 2.8% at 0 °C to 2.6% at 40 °C; for dynamic viscosity from 6.3% at 0 °C to 8.3% at 40 °C; and for surface tension from 0.7% at 0 °C to 1.4% at 40 °C. These ratios (seawater/freshwater) were calculated from reference values published for freshwater [9] and seawater at S = 35 g/kg [8] and shown in Fig. 1. The Henry's constants for the principal three gases in atmospheric air (nitrogen, oxygen and argon) are all significantly higher in seawater than in freshwater (and therefore less soluble). The values for oxygen were calculated from Garcia and Gordon [3] and the values for nitrogen
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
91
Fig. 1. Seawater/freshwater ratios for bulk water physical properties.
Fig. 3. Schematic representation of the DAF process.
and argon from Hamme and Emerson [5]. Fig. 2 shows seawater/ freshwater ratios for the Henry's constants. The molecular diffusivity of the air gases in seawater is somewhat uncertain at present. Earlier experimental measurements showed that the molecular diffusion coefficient is lower in seawater, which was used to support a recommendation that seawater diffusivity should be taken as 6% lower than that of freshwater [6]. A more recent review pointed out that later experimental work failed to confirm this recommendation [7]. At this point, for practical engineering design, it seems prudent to assume the seawater/freshwater ratio of molecular diffusivity to be 0.94 for all the principal air gases.
algae, and oil and grease. It is particularly effective in removing algae so DAF is a good choice where harmful red and brown algal blooms occur. Fig. 4 shows SWRO pretreatment consisting of coagulation (ferric chloride is the coagulant of choice), flocculation, DAF, and dual-media granular filters. An alternative is to place DAF directly on top of the filters (DAFF) where the hydraulic loading rate on the DAFF system is 10 to 15 m/h. An alternative is horizontal separation of DAF from the filters (as depicted in Fig. 4) and use of high-rate DAF loading rates of 20 to 35 m/h. Returning to Fig. 3, the DAF reactor is conceptualised as having two consecutive steps. The first is the contact zone, where flocs and bubbles interact to form stable floc–bubble aggregates. The second is the separation zone, where the aggregates are allowed to rise to the top of the tank. In reality, there is some overlap between these two zones. For both these zones, comprehensive mathematical models have been developed which allowed much better understanding of the fundamental mechanisms involved in these complex three-phase systems. These models, coupled with a strong foundation of empirical experience, have evolved into robust design models for DAF in the drinking water business, recently summarised in a comprehensive handbook [2]. The mathematical models presented in this handbook form the basis for the analyses presented in the following sections.
3. Seawater effects on DAF 3.1. Process description DAF is a gravity-driven solids–liquid separation process which has gained wide acceptance in the drinking water treatment industry since the early 1990s. It relies on the buoyancy induced by adding air to a suspension of flocculated water, which drives the floc–bubble aggregates to the top of the DAF reactor towards the float layer, while the clarified water is withdrawn at the bottom of the DAF reactor. The air is injected with the pressurised recycle stream, which precipitates as small bubbles when the pressure is released across the recycle injection nozzles. The process is schematically shown in Fig. 3. Over the last 10 years or so DAF is being increasingly used as an important pretreatment process for seawater reverse osmosis (SWRO) plants in removing turbidity, total organic carbon (TOC),
3.2. Contact zone The contact zone model, initially developed by Edzwald et al. [1] and later reviewed by Haarhoff and Edzwald [4], indicates that the physical properties of the water affect its performance in two ways. The first is the rise velocity (vb) of the air bubbles through the contact zone, which is dependent on both water density and viscosity. The rise velocity is calculated from the classical Stokes expression: 2
vb ¼
gðρw −ρb Þdb 18μ w
ð1Þ
Eq. (1) is used to calculate the seawater/freshwater ratio of the rise velocity (vb ratio) in the contact zone (subscript S indicates seawater and F freshwater): vb ratio ¼
Fig. 2. Seawater/freshwater ratios for Henry's constants for the principal air gases.
μF ρF −ρb
ρS −ρb μS
ð2Þ
The second determinant of contact zone performance is the total single collector efficiency (ηT), which is the sum of the particle transport mechanisms due to Brownian diffusion (ηD), interception (ηI), and settling (ηS). Interception is the dominant mechanism, but it is
92
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
Fig. 4. Process schematic of dissolved air flotation pretreatment for SWRO plants.
not dependent on water's physical properties and thus is the same for freshwater and seawater. Brownian diffusion and settling, which are dependent on water density, play minor roles. In fact, the settling collector term plays such a negligible role in the contact zone that only Brownian diffusion needs to be considered. The collision efficiency due to Brownian diffusion is: 2 = " #2 =3 2 3 kb T 1 1 ηD ¼ 6:18 g ðρw −ρb Þ dp db
ð3Þ
The ratio of the Brownian diffusion term in seawater and freshwater (ηD ratio) follows:
ηD ratio ¼
2 ρF −ρb =3 ρS −ρb
ð4Þ
At a reference temperature of 20 °C and with particle or floc density assumed at 1050 kg/m 3: the vb ratio is 0.96 and the ηD ratio is 0.98. At a higher temperature of 35 °C (some seawater facilities are located in hot climates) these ratios approach unity. Therefore, the collision efficiency in seawater should be about the same as in freshwater. Assuming sufficient air is supplied (see Sections 3.4, 3.5, and 4), then the overall contact zone performance and contact zone design should be similar for seawater and freshwater. 3.3. Separation zone The efficiency of the DAF separation zone depends on the upward movement of the floc–bubble aggregates through the water—a process dependent on the density and viscosity of the water. Mathematical models for the rise rate of the floc–bubble aggregates (vfb) allow the calculation of the seawater/freshwater ratio for the rise rate (vfb ratio) [2]. For laminar flow (Reynolds number less than 1), vfb and vfb ratio are given by Eqs. (5) and (6):
vfb ðlaminarÞ ¼
4g ρw −ρfb d2fb
ð5Þ
3Kμ w
vfb ratio ðlaminarÞ ¼
ρS −ρfb ρF −ρfb
!
μF μS
ð6Þ
For DAF, it is likely that the rise rate of some of the larger floc–bubble aggregates will be in the transitional flow regime (Reynolds number larger than 1), which leads to different expressions for vfb and vfb ratio:
4 vfb ðtransitionalÞ ¼ 3K
2 3 0:8 g 0:8 ρ −ρ 0:8 d1:4 w fb fb 6 7 4 5 0:6 ρ0:2 w μw
ð7Þ
vfb ratio ðtransitionalÞ ¼
ρS −ρfb ρF −ρfb
!0:8
ρF ρS
0:2
μF μS
0:6
ð8Þ
Eqs. (6) and (8) require an estimate of the density of the floc–bubble aggregates. The difference between water density and floc–bubble density should be about the same in seawater and freshwater, and the first bracketed terms on the right of the equations are assumed to be one. For laminar flow, vfb ratio varies from 0.94 at 0 °C to 0.92 at 40 °C. For transitional flow, it varies from 0.96 at 0 °C to 0.95 at 40 °C. This analysis suggests a reduction in hydraulic loading rates for seawater between 5 and 8%. However, given the small differences and the conservative nature of design, we expect little or no change in the design of full-scale plants. 3.4. Air solubility The solubilities of gases in water are limited by their Henry's constants. Fig. 2 shows that the Henry's constants are considerably higher in seawater. Because the gas solubility (G) is inversely proportional to the Henry's constant, then the gas solubility seawater/freshwater ratio (G ratio) is: G ratio ¼
−1
HS H F −1
! ¼
HF HS
ð9Þ
Atmospheric air consists mainly of nitrogen (78.1% volume per volume), oxygen (21.0%) and argon (0.9%). Taking a weighted average of these fractions, the solubility of atmospheric air at 20 °C and sea level is 0.640 mol/m 3 in seawater compared to 0.837 mol/m 3 in freshwater — a G ratio of 0.765. 3.5. Air transfer in packed saturators Practical air saturation systems for DAF usually employ packed or unpacked pressure saturators. The majority of water clarification plants around the world use packed saturators with good effect, and their design is backed up with a rational design procedure, which is still lacking for unpacked saturators. This paper considers the effect of seawater on packed saturators only. Henry's constant provides the maximum amount of air that can be dissolved in water. In a real saturation system, less air is transferred, because saturators are not 100% efficient due to kinetic constraints. First, the air transfer efficiency in packed saturators depends on the wetted packing area in the saturator. The dry packing area is defined by the geometry of the packing pieces, but the wet packing area depends on how evenly the water spreads itself over the packing surfaces. This can be predicted with an Onda correlation [2]. The seawater/freshwater ratio of wetted packing area (aw ratio) is practically constant over all packing sizes and hydraulic loading rates and ranges between 0.97 and 0.98. Second, the rate of mass transfer at the air/water interface is influenced by the molecular diffusivity (D) of the
J. Haarhoff, J.K. Edzwald / Desalination 311 (2013) 90–94
93
Table 1 Design parameters for typical packed saturators.
Hydraulic loading Saturator pressure Packing depth Packing size
kg/m2-s kPa gauge mm mm
Range
Typical
20–60 350–600 800–2000 25–90
40 500 1400 50
gas, which is discussed above. As suggested, the seawater/freshwater ratio of molecular diffusivity (D ratio) is taken conservatively as 0.94. The mass transfer constant (KL) reflects the combined effects of the wetted packing area and the molecular diffusivity, as well as the density and viscosity of the water. Using a second Onda correlation in Eq. (10), the seawater/freshwater ratio of the mass transfer constant (KL ratio) is predicted with Eq. (11): 1 1 2 2 = μ g =3 HLmass =3 ρ D =2 a dp 5 K L ¼ 0:0051 ρ μ aw μ K L ratio ¼
5 = 1 = 2 = 1 = 6 6 3 3 μF ρS aF DS μS ρF aS DF
ð10Þ
ð11Þ
Eq. (11) indicates that KL ratio ranges from 0.94 at 0 °C to 0.92 at 40 °C for all the air gases. All the ratios are now known to calculate the effect of seawater on the total mass of air transferred (M) in a typical packed air saturator, using well established methods reported elsewhere [2]. The design parameters of a typical packed saturator used in these calculations are shown in Table 1. The seawater/freshwater ratio for the total air mass transferred in a typical saturator (M ratio) is shown in Fig. 5. The relationship in Fig. 5 indicates that a typical saturator which produces 100% of air in freshwater will only produce 71% to 77% of air in seawater over the temperature range investigated. This is a large difference. This range stays about the same even if the saturator design is varied over the full range of the design parameters in Table 1. If one assumes that seawater DAF requires the same volumetric air concentration as freshwater DAF (there is no reason at this point to disagree), then the conventional saturation system, as commonly used for freshwater, has to be adapted for seawater.
Fig. 6. Required seawater recycle rate to match the air produced by a similar air saturator for freshwater.
other saturator variables remaining the same. To end up with the same air concentration in the combined flow entering the contact zone (A), the first obvious solution is to increase the recycle rate (r). The design equation linking M, A and r is provided by Eq. (12): M¼
1þr A r
ð12Þ
Assuming that the required air concentration A should be the same for seawater and freshwater, an expression for the required recycle rate for seawater is obtained: −1 r r S ¼ ðM ratioÞ −1 1þr F
ð13Þ
The M ratio in Fig. 5 cannot be meaningfully adjusted with the design parameters in Table 1. This means that the air concentration leaving the saturator (M) is less for seawater then for freshwater, all
Fig. 6 provides a chart to obtain the necessary recycle rate for seawater to match the air produced by a similar saturator for freshwater. To match 10% at 20 °C for freshwater, for example, the seawater recycle rate has to be 14%. The above analysis holds if the saturation pressure stays the same. An alternative option would be to keep the recycle rate constant, and increase the pressure within the saturator. Based on the saturator design equations presented by Edzwald and Haarhoff [2], Fig. 7 provides a chart to determine the required pressure for a seawater saturator to match the air transfer of a freshwater saturator. To match 500 kPa at 20 °C for freshwater, for example, the seawater saturation pressure has to be 675 kPa.
Fig. 5. Seawater/freshwater ratio for the mass of air transferred in a typical packed saturator.
Fig. 7. Required pressure for a seawater saturator to match the air produced by a freshwater saturator.
4. Suggested design corrections for seawater DAF
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Table 2 Summary of effects of seawater relative to freshwater, at 20 °C with salinity S= 35 g/kg. Property
Change
Comment
Physical properties Density Dynamic viscosity Surface tension
Plus 3% Plus 8% Plus 1%
– – –
Contact zone Bubble rise rate Collision efficiency Brownian Collision efficiency interception Collision efficiency total
Minus 4% Minus 2% No effect Very small
Not significant Minor importance Major importance Dominated by interception
Separation zone Floc/bubble rise rate (laminar) Floc/bubble rise rate (transitional)
Minus 7% Minus 4%
Not likely to affect design Not likely to affect design
Air solubility Henry's constant nitrogen Henry's constant oxygen Henry's constant argon Atmospheric air solubility
Plus 32% Plus 30% Plus 29% Minus 24%
Large Large Large Large
Packed saturators Wetted packing area Mass transfer constant Air mass transferred
Minus 2% Minus 7% Minus 26%
Minor importance Important Large difference
difference difference difference difference
Nomenclature a packing area A available air concentration entering the contact zone d diameter D molecular diffusivity g gravitational acceleration G gas solubility H Henry's constant HLmass mass hydraulic loading kb Boltzmann's constant K coefficient for drag constant KL mass transfer constant M available air concentration leaving the saturator r recycle rate S salinity T absolute temperature v rise velocity ρ density η collision efficiency μ dynamic viscosity subscript b bubble subscript D Brownian diffusion subscript fb floc–bubble subscript F freshwater subscript I inertia subscript S seawater subscript T total subscript w water or wetted
5. Summary and conclusions The high salinity of seawater affects its physical properties in relation to freshwater. The changes that are relevant to the design of DAF systems for water clarification are summarised in Table 2. The changes have a small, but negligible, effect on the design of the contact zone. The effects on the design of the separation zone are greater, but are not likely to change the design of full-scale systems. The design of the air saturation systems, however, definitely needs to be adapted due to the lower solubility of air in seawater. Two possible ways of adapting the design procedure for seawater systems, or combinations thereof, are suggested. In both cases, a saturator should first be designed for freshwater, using standard published procedures (see [2]). To account for seawater, the first option is to keep the saturation pressure constant, but to increase the recycle rate by using the design chart in Fig. 6. The second option is to keep the recycle rate constant, but to increase the saturation pressure by using the design chart in Fig. 7.
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