Abstract
Phosphate removal from seawater is important for biofouling control on RO membranes because phosphorus is one of the nutrients for microbial growth. This paper is based on the hypothesis that multi-stage coagulation results in better phosphate removal. Therefore, comparison of phosphate removal with one-step and three-step dose coagulation from the aspects of both the theoretical calculation and experimental results is investigated in this paper. The result of theoretical calculation based on the Freundlich equation shows that the final phosphate concentration from a three-step dose, i.e. 0.43 μgP/L, is ten times lower than that from a one-step dose, i.e. 4.47 μgP/L. The experimental result shows that for the three-step dose, final phosphate concentration is 1.0 μgP/L, which is lower than for the one-step dose (i.e. 4.0 μgP/L), but not as low as the theoretical calculated value (0.43 μgP/L). This discrepancy between theoretical calculation and experimental result may be the impact of equilibrium phosphate concentration, different initial Fe:P molar ratio and NOM competition between one-step dose and three-step dose coagulation. Although this discrepancy exists, the experimental results still showed that multi-stage coagulation presented better phosphate removal in seawater to concentration levels that are lower than with conventional coagulation. In other words, the problem of the high coagulant dosage in the pretreatment process while removing phosphate from seawater may be solved by application of multi-stage coagulation instead of conventional coagulation.
HIGHLIGHTS
The hypothesis is tested that multi-stage coagulation results in better phosphate removal from seawater.
Phosphate removal with one-step and three-step dose coagulation is compared according to theoretical calculation and experimental results.
Theoretical calculation and experimental results both show better phosphate removal with three-step dose than with one-step dose coagulation.
Application of multi-stage coagulation instead of conventional coagulation may be a way to solve the problem of high coagulant dosage for phosphate removal from seawater.
INTRODUCTION
Biofouling is the formation of biofilms on a membrane surface by the deposition and growth of micro-organisms from raw water. Microbial growth needs nutrients such as (carbon (C), nitrogen (N), and phosphorus (P)). Therefore, limiting these nutritional compositions in the feed solution has been introduced in attempts to prevent or restrict microbial growth, instead of killing microorganisms using chemical biocide (van der Kooij et al. 1982; Mathieu et al. 1992; Niquette et al. 2000; Kim et al. 2014). Among these three nutrients, phosphate is a potentially significant limiting factor as the molar ratio of carbon, nitrogen and phosphorus is 100:20:1.7 in microbial biomass (Vrouwenvelder et al. 2010; Kim et al. 2014), an indication that a much lower concentration of phosphate than carbon is required for cell growth (van de Wende et al. 1989; Alphenaar et al. 1993; van der Aa et al. 2002). Therefore, even low concentrations of phosphate can more significantly affect microbial status than other major nutrients (carbon and nitrogen) (Kim et al. 2014).
Jacobson et al. (2009) suggested that phosphate limitation in RO systems might be an option in controlling biofouling which was only linked to decreased cleaning frequency (Table 1). It shows that chemical cleaning of the RO units was much less frequent in the non-summer months due to the lower phosphate concentration in these months than in summer both in Plant A and Plant B. And Vrouwenvelder et al. (2010) experimentally showed low phosphate concentration was able to control biofouling by measuring biomass concentration, as is summarized in Table 1 that when ‘limiting P’ (no P dose) in RO feed, the biomass concentration of 2.5 × 102 pgATP/cm2 is much lower than the 6 × 104 pgATP/cm2 with 20 μgP/L dose in the RO feed water. Later, Chang-Min Kim investigated the effects of phosphate limitation on the performance of FO. By measuring the biomass parameter of total cell number decrease, it was discovered that phosphate limitation in FO feed water greatly reduced microbial growth (Table 1). Jacobson and Vrouwenvelder proved that phosphate limitation can control biofouling. However, the problem is that a high chemical dosage in the pretreatment process for the removal of phosphate causes high operational costs. Moreover, in existing research, only surface water and synthetic water as raw water have been studied for phosphate limitation and no relevant research is currently available concerning seawater.
Phosphate limitation and biofouling control in RO or FO system
| Reference source | Raw water | Treatment | Phosphate concentration of RO or FO feed | Biofouling investigation |
|---|---|---|---|---|
| Jacobson et al. (2009) | Plant A: Canal water | RSF(8 mL/LPAC) +In-line coagulation (2–5 mg/LFeCl3 dose) +UF(100 kDa) +RO | Summer: 3.02 μgP/L | Summer (RO cleaning frequency): 2 |
| Spring: 1.56 μgP/L | Non-summer (RO cleaning frequency): 0.5 | |||
| Plant B: River water | In-line coagulation (6 mg/LFeCl3 dose) +Sand filter +UF(150–200 kDa) +RO | Summer: 3.95 μgP/L | Summer (RO cleaning frequency): 3 | |
| Spring: 2.44 μgP/L | Non-summer (RO cleaning frequency): 1 | |||
| Vrouwenvelder et al. (2010) | Surface water | MS+ Coagulation +RSF + BACF + UF+ RO | 20 μgP/L dose | Biomass concentration: 6 × 104 pg ATP/cm2 |
| ‘Limiting P’ (no P dose) | Biomass concentration: 2.5 × 102 pgATP/cm2 | |||
| Kim et al. (2014) | Synthetic secondary wastewater effluent | FO | 8.0 mgP/L dose | Biomass parameter: total cell number 9.6 × 106(±3.2 × 106)cells/cm2 |
| ‘Limiting P’ (no P dose) | Biomass parameter: total cell number 3.0 × 106(±5.5 × 105)cells/cm2 |
| Reference source | Raw water | Treatment | Phosphate concentration of RO or FO feed | Biofouling investigation |
|---|---|---|---|---|
| Jacobson et al. (2009) | Plant A: Canal water | RSF(8 mL/LPAC) +In-line coagulation (2–5 mg/LFeCl3 dose) +UF(100 kDa) +RO | Summer: 3.02 μgP/L | Summer (RO cleaning frequency): 2 |
| Spring: 1.56 μgP/L | Non-summer (RO cleaning frequency): 0.5 | |||
| Plant B: River water | In-line coagulation (6 mg/LFeCl3 dose) +Sand filter +UF(150–200 kDa) +RO | Summer: 3.95 μgP/L | Summer (RO cleaning frequency): 3 | |
| Spring: 2.44 μgP/L | Non-summer (RO cleaning frequency): 1 | |||
| Vrouwenvelder et al. (2010) | Surface water | MS+ Coagulation +RSF + BACF + UF+ RO | 20 μgP/L dose | Biomass concentration: 6 × 104 pg ATP/cm2 |
| ‘Limiting P’ (no P dose) | Biomass concentration: 2.5 × 102 pgATP/cm2 | |||
| Kim et al. (2014) | Synthetic secondary wastewater effluent | FO | 8.0 mgP/L dose | Biomass parameter: total cell number 9.6 × 106(±3.2 × 106)cells/cm2 |
| ‘Limiting P’ (no P dose) | Biomass parameter: total cell number 3.0 × 106(±5.5 × 105)cells/cm2 |
Phosphorus removal started in the 1950s in response to the growing problem of eutrophication and the level of phosphorus that needed to be reduced before discharge to surface water. Chemical precipitation was the initial method applied to remove phosphorus (Jiang & Graham 1998). Furthermore, biological phosphorus removal (Oehmen et al. 2007), coagulation (van der Wende et al. 1989) and adsorption (Zeng et al. 2004; Zhang et al. 2007; Liu et al. 2008) have been established gradually. Chemical precipitation and biological removal are generally suitable for removing phosphate with higher concentration and usually the concentration in the effluent is still high. Coagulation and adsorption are proposed as an effective removal process for a low concentration of phosphate (Tian et al. 2009). Therefore, for the low phosphate concentration in seawater, coagulation and adsorption are usually applied for phosphate removal.
Many kinds of solid materials including iron-based compounds (Bastin et al. 1999), aluminum oxide hydroxide (Tanada et al. 2003), iron oxide tailings (Zeng et al. 2004), mesoporous (Liu et al. 2008), and Fe-Mn binary oxide adsorbent (Zhang et al. 2007) have been used to remove phosphate from water by adsorption. In adsorption modelling, the distribution of adsorbate between the two phases (the bulk solution and the adsorbent) is often described in terms of isotherms. The amount of solute adsorbed per unit of adsorbent (q) as a function of the equilibrium concentration of the solute in bulk solution (Ce), at a constant temperature, is called the adsorption isotherm. Five isotherm equations are used in practice, including the Freundlich, Langmuir, Temkin (two-parameter equations) and Redlich–Peterson and Langmuir–Freundlich (three-parameter equations), as described below:
Freundlich equation
Langmuir equation
Temkin equation
Redlich–Peterson equation
Langmuir–Freundlich equation
Although the three-parameter equations (Redlich–Peterson and Langmuir–Freundlich) often provide a better fit of the isotherm data, the two-parameter equations (Freundlich, Langmuir, Temkin) are more widely used in practice due to the convenience of evaluating two parameters rather than three parameters (Zeng et al. 2004).
In the case of phosphate removal by coagulation, two coagulants including ferric chloride (FeCl3.6H2O) and aluminum sulfate (Al2 (SO4)3.14H2O) are commonly used. Two major mechanisms occur when these are added in water (Jiang & Graham 1998):
Formation of Al/Fe-hydro-phosphate complexes with the general formula of Me(OH)x(PO4)y. (M = Al/Fe). These complexes either adsorb onto positively charged Al/Fe hydrolysis species or act as centres of precipitation for Al/Fe hydrolysis products.
Adsorption of phosphate ions onto the surface of Al/Fe hydrolysis species. In water treatment practice, amorphous Al(OH)3 or Fe(OH)3 are the predominant hydrolysis species.
Based on these mechanisms, coagulation and adsorption have similar mechanisms. Al/Fe hydrolysis species play a role of adsorbent. In order to obtain the phosphate removal capacity by coagulation, an adsorption isotherm is adopted to describe the amount of phosphate removed per unit of coagulant as a function of the equilibrium concentration of phosphate since the mechanism of phosphate removal by coagulation is the adsorption of phosphate and Fe-hydroxide-phosphate complexes onto iron hydrolysis species. Therefore, the aim of this study is: (1) to investigate phosphate removal capacity based on an adsorption isotherm; (2) to investigate multi-stage coagulation for phosphate removal.
MATERIALS AND METHODS
Raw water
Raw water was collected from the intake of a seawater-UF/RO pilot desalination plant in Zeeland Province, The Netherlands. Samples were collected in clean, dark-glass bottles and stored at 4 °C. Experiments were performed in the UNESCO-IHE laboratory within a week after sample collection. Prior to each experiment, bottles were shaken gently to ensure particulate matter was brought back to suspension.
Coagulant
There are two conventional chemicals used in coagulation for phosphate removal, i.e. ferric chloride and aluminum. In practice, a better phosphate removal by use of ferric chloride has been observed in desalination plants. Therefore, FeCl3.6H2O was used as the coagulant in this research.
Chemicals
The chemicals used for phosphate measurement together with the related preparations are presented in Table 2 (Jin et al. 2014).
Chemicals used for phosphate measurement
| Chemicals | Preparation |
|---|---|
| H2SO4 2.5M | Add 70 mL concentrated H2SO4 to 420 mL H2O. Store in a 500 mL glass stoppered bottle. |
| Potassium antimonyl tartrate | Dissolve 1.3715 g K(SbO)C4H4O6.½H2O in 400 mL H2O and dilute to 500 mL in a volumetric flask. Store in a 500 mL glass stoppered bottle. |
| Ammonium heptamolybdate | Dissolve 20 g (NH4)6Mo7O24.4H2O in 500 mL H2O. Store in a 500 mL glass stoppered bottle. |
| Ascorbic acid 0.1M | Dissolve 1.76 g ascorbic acid in 100 mL H2O. This solution is stable for about 1 week at 4 °C. However, it is preferable to prepare daily. |
| Combined reagent | Mix the above reagents in the following order: 125 mL 2.5M H2SO4 + 125 mL H2SO4 + 12.5 mL K(SbO)C4H4O6.½H2O + 37.5 mL (NH4)6Mo7O24.4H2O + 75 mL ascorbic acid. Store in a 250 mL glass stoppered bottle. |
| Stock P Solution | Dissolve 439.0 mg KH2PO4 (dried at 105 °C for a few hours) in 1,000 mL. 1 mL = 100 μg PO43−-P. Store in a 1 L glass stoppered bottle. |
| Standard P Solution | Dilute 5.00 mL stock solution to 1,000 mL. 1 mL = 0.5 μg P. Store in a 1 L glass stoppered bottle. |
| N-hexanol | No preparation necessary. |
| Chemicals | Preparation |
|---|---|
| H2SO4 2.5M | Add 70 mL concentrated H2SO4 to 420 mL H2O. Store in a 500 mL glass stoppered bottle. |
| Potassium antimonyl tartrate | Dissolve 1.3715 g K(SbO)C4H4O6.½H2O in 400 mL H2O and dilute to 500 mL in a volumetric flask. Store in a 500 mL glass stoppered bottle. |
| Ammonium heptamolybdate | Dissolve 20 g (NH4)6Mo7O24.4H2O in 500 mL H2O. Store in a 500 mL glass stoppered bottle. |
| Ascorbic acid 0.1M | Dissolve 1.76 g ascorbic acid in 100 mL H2O. This solution is stable for about 1 week at 4 °C. However, it is preferable to prepare daily. |
| Combined reagent | Mix the above reagents in the following order: 125 mL 2.5M H2SO4 + 125 mL H2SO4 + 12.5 mL K(SbO)C4H4O6.½H2O + 37.5 mL (NH4)6Mo7O24.4H2O + 75 mL ascorbic acid. Store in a 250 mL glass stoppered bottle. |
| Stock P Solution | Dissolve 439.0 mg KH2PO4 (dried at 105 °C for a few hours) in 1,000 mL. 1 mL = 100 μg PO43−-P. Store in a 1 L glass stoppered bottle. |
| Standard P Solution | Dilute 5.00 mL stock solution to 1,000 mL. 1 mL = 0.5 μg P. Store in a 1 L glass stoppered bottle. |
| N-hexanol | No preparation necessary. |
Experiments for investigation of phosphate removal capacity by coagulation (for isotherm equation)
The experiments for investigation of phosphate removal capacity by coagulation were performed in seawater with six different initial phosphate concentrations, i.e. 34, 49, 77, 106, 329, 1,006 μgP/L, at the same coagulant dose of 1 mgFe3+/L. These six different initial phosphate concentrations were obtained by adding different amounts of stock P solution (see previous section) to seawater. Coagulation was conducted in a compact laboratory mixer (Model CLM4 EC Engineering, volume: 1,000 mL) with rapid mixing intensity RMG = 1,100 s−1, mixing time RMt = 20 s and slow mixing intensity SMG = 40 s−1, mixing time SMt = 30 min. Afterwards, the flocculated solution was taken and filtrated for the final phosphate concentration measurement. Figure 1 shows the main steps of the whole experimental process.
Schematic diagram of experiments for investigation of phosphate removal capacity by coagulation.
Schematic diagram of experiments for investigation of phosphate removal capacity by coagulation.
One-step dosing and three-step dosing coagulation
One-step dosing coagulation was done in seawater with initial phosphate concentration of 117 μgP/L at a coagulant dose of 1 mgFe3+/L. It was performed in a compact laboratory mixer (Model CLM4 EC Engineering) under conditions of rapid mixing intensity RMG = 1,100 s−1, mixing time RMt = 20 s and slow mixing intensity SMG = 40 s−1, mixing time SMt = 30 min.
Three-step dosing coagulation was performed in seawater with initial phosphate concentration of 117 μgP/L under conditions of rapid mixing intensity RMG = 1,100 s−1, mixing time RMt = 20 s, and slow mixing intensity SMG = 40s−1, mixing time SMt = 10 min, 0.35 mgFe3+/L dose for the first step. The same procedure was applied for the second and third steps and only the dose for the third step was changed to 0.30 mgFe3+/L. The total dose was thus 1 mgFe3+/L, which is the same dosing amount as applied in the one-step dosing.
Analytical method
Filtration of flocculated solution
Flocculated solution was filtrated through a 0.45 μm cellulose acetate filter before the phosphate measurement. The filtration procedure for the flocculated solution is described below:
The filter holder and syringe were soaked in (1 + 4) HCl or 10% H2SO4 for at least one hour and then rinsed with ultra-pure water at least three times to take out any traces of phosphate.
The 0.45 um filter was set into the filter holder.
A certain amount of flocculated solution was drawn into the syringe, then the syringe was connected to the filter holder and the flocculated solution was injected from the syringe into the filter holder for filtration.
The first 5 mL of permeate was discarded and the rest of the permeate collected into a clean PE bottle for phosphate measurement.
Phosphate measurement
An existing analytical method of phosphate measurement is the ascorbic acid method in which ‘orthophosphate reacts with ammonium molybdate to form molybdophosphoric acid. Through reductant (ascorbic acid), the molybdophosphoric acid is transformed into molybdenum blue whose absorbance can be measured on a spectrophotometer at 880 nm’. However, for phosphate concentrations below 10 μg/L it is necessary to extract the molybdenum blue via hexanol and measure the extracted solution on a spectrophotometer at 680 nm, which is known as the modified ascorbic acid method (Jacobson et al. 2009).
RESULTS AND DISCUSSION
Phosphate removal capacity based on adsorption isotherm
The experimental results of phosphate removal by coagulation are presented in Table 3. It can be observed that the final phosphate concentration after coagulation is 1.1, 1.8, 2.7, 3.7, 67 and 605 μgP/L when the initial Fe:P is 16:1, 11:1, 7.2:1, 5:1, 1.7:1 and 0.5:1, respectively. Phosphate removal efficiency is increased with increase in initial Fe:P molar ratio when the ratio is below 5:1. Phosphate removal efficiency becomes almost constant at molar ratio (Fe:P) in excess of 5:1, which indicates that the initial Fe:P molar ratio of 5:1 is the critical point for phosphate removal. Thistleton et al. (2002) also showed a similar result for phosphate removal as a function of initial Fe:P molar ratio, for wastewater at pH 7. Therefore, there will be two isotherm equations for phosphate removal capacity based on this critical point of the Fe:P molar ratio of 5:1.
The experimental data of phosphate removal capacity by coagulation
| Sample no. | Wt of adsorbent Fe3+ (g) | Initial P conc. Ci (μg/L) | Initial Fe:P | Final P conc. Ce (μg/L) | P removal effciency | Total amount of phosphate adsorbed, x (μg) | Amount of removal of phosphate per unit wt of Fe3+, q (μg/g) |
|---|---|---|---|---|---|---|---|
| 1 | 0.001 | 34 | 16.0 | 1.1 | 0.967 | 32.8 | 32,818.5 |
| 2 | 0.001 | 49 | 11.0 | 1.8 | 0.964 | 47.0 | 46,975.5 |
| 3 | 0.001 | 77 | 7.2 | 2.7 | 0.965 | 74.3 | 74,324.3 |
| 4 | 0.001 | 106 | 5.0 | 3.7 | 0.965 | 102.0 | 101,994.9 |
| 5 | 0.001 | 185 | 3.0 | 18.6 | 0.899 | 166.4 | 166,400 |
| 6 | 0.001 | 329 | 1.7 | 67.0 | 0.796 | 262.0 | 262,000.0 |
| 7 | 0.001 | 609 | 1.0 | 243 | 0.601 | 366 | 366,000 |
| 8 | 0.001 | 1,006 | 0.5 | 605.0 | 0.399 | 401.0 | 401,000.0 |
| Sample no. | Wt of adsorbent Fe3+ (g) | Initial P conc. Ci (μg/L) | Initial Fe:P | Final P conc. Ce (μg/L) | P removal effciency | Total amount of phosphate adsorbed, x (μg) | Amount of removal of phosphate per unit wt of Fe3+, q (μg/g) |
|---|---|---|---|---|---|---|---|
| 1 | 0.001 | 34 | 16.0 | 1.1 | 0.967 | 32.8 | 32,818.5 |
| 2 | 0.001 | 49 | 11.0 | 1.8 | 0.964 | 47.0 | 46,975.5 |
| 3 | 0.001 | 77 | 7.2 | 2.7 | 0.965 | 74.3 | 74,324.3 |
| 4 | 0.001 | 106 | 5.0 | 3.7 | 0.965 | 102.0 | 101,994.9 |
| 5 | 0.001 | 185 | 3.0 | 18.6 | 0.899 | 166.4 | 166,400 |
| 6 | 0.001 | 329 | 1.7 | 67.0 | 0.796 | 262.0 | 262,000.0 |
| 7 | 0.001 | 609 | 1.0 | 243 | 0.601 | 366 | 366,000 |
| 8 | 0.001 | 1,006 | 0.5 | 605.0 | 0.399 | 401.0 | 401,000.0 |
Three models are mainly used in practice for adsorption isotherms as described in the equations below (López-Ramírez et al. 2003):
Freundlich equation
Langmuir equation
Temkin equation
where q is the equilibrium adsorption capacity (μgP/g); Ce is the equilibrium concentration of phosphate in the aqueous phase (μgP/L); and the rest of the parameters are different isotherm constants which are determined by regression of experimental data. The parameters and correlation coefficients for each model at initial Fe:P molar ratio of 5:1 to 16:1 are shown in Table 4. It can be seen that the correlation coefficients of the Freundlich and Langmuir isotherms are higher than for the Temkin isotherm, which indicates data-fitting was better with the Freundlich and Langmuir equations. Therefore, these two models are equally valid to fit the experimental data, and the Freundlich equation is used for theoretical calculation in this paper.
Estimated isotherm parameters for phosphate removal by coagulation at initial Fe:P molar ratio of 5:1 to 16:1
Freundlich  | Langmuir  | Temkin  | ||||||
|---|---|---|---|---|---|---|---|---|
| K | 1/n | R2 | qm | b | R2 | A | B | R2 |
| 28,425 | 0.960 | 0.995 | 106 | 0.033 | 0.993 | 20,718 | 57,670 | 0.956 |
| Freundlich | Langmuir | Temkin | ||||||
|---|---|---|---|---|---|---|---|---|
| K | 1/n | R2 | qm | b | R2 | A | B | R2 |
| 28,425 | 0.960 | 0.995 | 106 | 0.033 | 0.993 | 20,718 | 57,670 | 0.956 |
Therefore, the Freundlich equation is q = 28,425Ce0.963 for the phosphate removal capacity for initial Fe:P molar ratio of 5:1. For initial Fe:P molar ratio of 0.5:1 to 5:1, the Freundlich equation is q = 74,023x0.2794 (see Figure 2) which was obtained by experimental data from Table 3.
Coagulation isotherm for varying range of initial Fe:P molar ratio at pH = 6.
Theoretical calculations of final phosphate concentration of one-step and three-step dose coagulation based on Freundlich equation above
A three-step dose instead of a one-step dose is superior for phosphate removal. This hypothesis was based on the mechanisms involved in the coagulation of phosphate: the predominant mechanism for phosphate removal at pH 6 is (a) the formation of Fe-hydroxo-phosphate complexes (Fe(OH)x(PO4)y) and (b) these complexes adsorbing onto iron hydrolysis species. Three-step dosing is aimed to create three times the formation of iron hydrolysis species, which provides more chance for formation of Fe-hydroxo-phosphate complexes (Fe(OH)x(PO4)y) and more external mass transfer for complexes to adsorb onto iron hydrolysis species than one-step dosing. Based on this consideration, this section presents the calculation of phosphate removal by one-step dosing and three-step dosing with the isotherm equation above.
Calculation of final phosphate concentration for coagulation with one-step dose
can be used for calculation:
Therefore, the final theoretical phosphate concentration is 4.47 μgP/L when the initial phosphate concentration is 117 μgP/L for one-step dosing.
Calculation of final phosphate concentration for coagulation with three-step dose
Calculation of the final phosphate concentration from coagulation with the three-step dose is in the same way as that for the one-step dose. The results of the calculation are shown in Table 5.
Final phosphate concentration for coagulation with three-step dose
| First step | Second step | Third step | |
|---|---|---|---|
| Fe:P molar ratio | 1.7:1 | 4.5:1 | 39.4:1 |
| Applied isotherm |  |  |  |
| Final P concentration | Ce1 = 42.93 | Ce2 = 4.21 | Ce3 = 0.43 |
| First step | Second step | Third step | |
|---|---|---|---|
| Fe:P molar ratio | 1.7:1 | 4.5:1 | 39.4:1 |
| Applied isotherm | | | |
| Final P concentration | Ce1 = 42.93 | Ce2 = 4.21 | Ce3 = 0.43 |
It can be observed that the final theoretical phosphate concentration is 0.43 μgP/L when the initial phosphate concentration is 117 μgP/L with the three-step dose. Comparing the results, the final phosphate concentration with the three-step dose, i.e. 0.43 μgP/L, is ten times lower than that with the one-step dose, i.e. 4.47 μgP/L. Therefore, theoretically phosphate removal with three-step dose is much better than that with the one-step dose, at the same total iron concentration.
Experimental results of final phosphate concentration of one-step and three-step dose coagulation
The experimental results of final phosphate concentration from one-step and three-step doses are shown in Figure 3. It can be seen that the experimental results of final phosphate concentration from one-step dose and three-step dose were different from calculation.
- (1)
For one-step dose, it can be observed that final phosphate concentration is 4.0 μgP/L, which is almost the same as the results from calculation of 4.47 μgP/L.
- (2)
For three-step dose, final phosphate concentration is 1.0 μgP/L which is better than with one-step dose of 4.0 μgP/L. This experimental result is similar to calculation, but the final phosphate concentration (1.0 μgP/L) is not as low as the calculation value (0.43 μgP/L). The reason may be that the equilibrium phosphate concentration formed in first and second steps impacted the final phosphate concentration because aged flocs with high equilibrium phosphate concentration may release phosphate back into the solution, which causes the real phosphate concentration to be higher than the theoretical value. Moreover, natural organic matter in seawater may compete with phosphate to adsorb on iron hydrolysis species. And a low initial molar ratio of Fe:P in the first step results in limiting phosphate removal, which may also be the reason for the discrepancy between the theoretical and experimental results.
Experimental results of final phosphate concentration of one-step and three-step dose coagulation at pH 6.
Experimental results of final phosphate concentration of one-step and three-step dose coagulation at pH 6.
CONCLUSIONS
Both theoretical calculation and experimental results show that three-step dosing coagulation is better than one-step dosing. Theoretically, three-step dosing coagulation proved to be ten times as efficient as one-step dosing coagulation, i.e. the 4.47 μgP/L by one-step dosing coagulation reduced to 0.43 μgP/L by three-step dosing coagulation, for a coagulant concentration of 1 mgFe3+/L. However, experimental results show that the final phosphate concentration is 1 μgP/L. The reasons for this discrepancy between theoretical calculation and experimental results may be the effect of equilibrium phosphate concentration, initial Fe:P molar ratio and NOM competition. Investigation of these reasons is recommended in follow-up study.
Although the experimental value of final phosphate concentration by three-step coagulation is higher than the theoretical value, compared with one-step coagulation under the same dosage of 1 mgFe3+/L, phosphate removal by three-step coagulation is still better. In other words, if the same phosphate removal efficiency is expected to be achieved, dosage of coagulant by three-step dose is lower than that by one-step dose, which will play an important role in saving the actual operating cost of pretreatment of a seawater RO system.
ACKNOWLEDGEMENT
This research was supported by UNESCO-IHE and seawater-UF/RO pilot desalination plant in Zeeland Province, The Netherlands. The authors wish to acknowledge the assistance of colleagues in the laboratory of UNESCO-IHE and seawater-UF/RO pilot desalination plant in Zeeland Province, The Netherlands.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
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