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Mechanisms of Chemical Phosphorus Removal
0957-5820/01/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 79, Part B, November 2001
MECHANISMS OF CHEMICAL PHOSPHORUS REMOVAL 1—Iron (II) Salts J. THISTLETON1 , T. CLARK2 , P. PEARCE 2 and S. A. PARSONS 1 1
School of Water Sciences, Cran eld University, Cran eld, Bedfordshire, UK 2 Thames Water Research and Development, Reading, Berkshire, UK
A
series of jar tests and a full-scale assessment of iron (II) chloride dosing at Abingdon sewage treatment works were carried out to identify factors affecting phosphorus removal. Variables considered were pH, redox potential and dissolved oxygen concentration using crude sewage. As expected, there was a strong link between phosphorus removal and iron:phosphate ratio but this trend was not experienced with suspended solids removal. The main factor affecting the ef ciency of the phosphorus removal was pH, followed by dissolved oxygen (DO) and redox potential. This was assessed using a seven variable factorial design experiment. The conversion of iron (II) to iron (III) averaged close to 68.7% at optimal conditions which were: high DO (1.0–5.7 mg l 1), mid range redox potential (57–91 mV) and high pH (7.5–8.0). As before, the most important variables were pH and DO concentration. Full-scale observations appeared to support these ndings as iron conversion and phosphorus removal was good (85.6%) despite the very low redox potential ( 139 mV). This could be due to the high pH at point of dose (8.2) and the relatively high DO (4.6 mg l 1). In poor conditions, where less iron (II) is converted to iron (III), chemical consumption costs will increase, solids removal may decrease and the system is likely to become less reliable. Keywords: phosphorus removal; iron (II) chloride; redox; dissolved oxygen; pH.
INTRODUCTION
Iron (III) and aluminium (III) are most frequently used in wastewater treatment because they form easily settleable ocs within a short time. For iron (II), this settleability is dependent on its conversion to iron (III), as iron (III) complexes show considerably better occulation characteristics than those of iron (II). The latter often appear as nely dispersed colloids that can be washed out to the ef uent4. The oxidation of iron (II) ions to the iron (III) form is dependent on pH, oxygen concentration5, catalysis by micro-organisms and catalysis or inhibition by components such as sulphur and carbonate5. Llang et al.6 found that under low dissolved oxygen (DO) conditions iron (II) oxidation was relatively slow. It has been suggested that in treatment plants having adequate aeration capacity, iron (II) and iron (III) are equally effective for phosphorus removal7. However, Chung and Bhagat8 found that iron (II) addition without aeration gave better turbidity results than iron (II) with aeration. It has been suggested that because of the existence of oxidative conditions in conventional biological wastewater treatment, iron (II) added to wastewater is oxidized in situ resulting in the homogenous generation of iron (III). Recht and Ghassemi3 reported that homogeneously generated iron (III) is a more ef cient phosphate precipitant than iron (III) added from an external source. Gillberg et al.2 investigated the in uence of pH on the precipitation of orthophosphates when adding metal salts and also on the addition of fresh hydroxides produced from
The chemical precipitation of phosphorus is brought about by the addition of the salts of di- and tri-valent metal ions that form precipitates of sparingly soluble phosphates1. Three types of metal precipitants are commonly used for chemical phosphorus removal, namely iron (II), iron (III) and aluminium. The level of precipitation in a sewage treatment plant depends on the pH of the system, the type of metal salt used and on the degree of mixing of the metal salt into the sewage2. However mechanisms of chemical phosphorus removal are poorly understood and data reported in the literature have often been contradictory. Considerable disagreement exists between various investigators on the kinetics and stoichiometry of cation-phosphate reactions, and on the effect of parameters such as pH and ionic concentrations on the ef ciency of phosphate removal3. It is generally thought that when an iron=aluminium salt is added to sewage, the metal ions react with water molecules to form hydrolysis products. If the hydrolysis product is in proximity to an orthophosphate molecule, a reaction will occur. However, if the hydrolysis products do not react with an orthophosphate ion or other charged species they will continue to react with water molecules and form metal hydroxides2. The higher the pH the greater the probability that metal hydroxides are formed through reactions with the hydroxide ion2. 339
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the different salts. It should be noted, however, that the experiments were carried out using ‘pH-adjusted hydrogen carbonate water’ as opposed to sewage. At most pHs, more orthophosphate was precipitated by adding the metal salts to solutions of orthophosphate than by adding metal hydroxides. The iron (II) salt precipitated orthophosphate to the same extent as the iron (III) salt only in the pH range 7–8. At other pHs the iron (II) salt was considerably less effective. Stumm and Morgan9 reported that the rate of oxygenation of iron (II) in solutions of pH 5 was found to be rst order with respect to the concentration of both iron (II) and oxygen and second order with respect to the hydroxide ion. Thus a 100-fold increase in the rate of reaction occurs for a unit increase in pH. Finally, Strickland10 demonstrated that iron (II) salts were not only cheaper than iron (III) but that they are also less sensitive to dosing point and control methods in activated sludge plants. The aim of this paper is to develop an understanding of some important fundamental aspects of chemical phosphorus removal using iron (II) salts to allow the optimization of metal salt addition at phosphorus consented sites. MATERIALS AND METHODS Jar Tests Jar tests were performed using crude sewage taken from Pangbourne sewage treatment works and using standardized apparatus. Before commencing the jar tests, orthophosphate, iron (II) and iron (III) concentrations were measured using analytical test kits (LCK320C and LCK049C cuvette tests, Dr Lange, Basingstoke). The orthophosphate concentration was used to calculate the coagulant dose at different molar ratios of Fe:P. Redox, dissolved oxygen (DO), pH and temperature were also measured. Analysis of the crude sewage, measuring suspended solids, soluble and total phosphorus was carried out by Thames Water Laboratories. Iron (II) chloride (commercial grade) was diluted by 27 times prior to addition to the jar tests. Immediately after dosing, the jars were stirred at 325 rpm for 1 minute rapid mix. The speed was then reduced to 30 rpm for 30 minutes occulation time, after which the sewage was left to settle for 60 minutes. After settlement the supernatant was decanted and analysed for orthophosphate, iron (II) and (III). Analysis was also carried out for suspended solids, total phosphate and soluble phosphate by Thames Water Laboratories. Effects of Redox, DO and pH Conditions were altered to see how the above variables affected removals of suspended solids and orthophosphate. These tests were all conducted at molar ratios of approximately 2:1 Fe:P. The pH was lowered with hydrochloric acid (AnalaR) and raised with sodium hydroxide (AnalaR). A low pH was classed as 6.5–7.2 and a high pH was classed as 7.5–8.0. The dissolved oxygen was increased with a standard sh tank aerator and airstone. A reduction of redox was achieved by allowing the sewage to go septic (2 days). For an extremely negative redox, this time was increased to 4 days. A factorial matrix was used to consider all the different
scenarios with redox, pH and DO. During some of the jar tests, samples were taken at intervals of 1, 2, 5, 10, 20, 30, 60 and 90 minutes to measure iron (II)=(III) levels. Full-scale Survey Abingdon sewage treatment works (Thames Water) is dosed with iron (II) chloride along the inlet channel after the screens. The dose is constant at 1.25 l min 1 (average incoming ow rate 14,000 m3 d 1). The site has two discharge points with different ef uent consents; see Figure 1. Treatment stream 1 relies on stone media trickling lters whereas treatment stream 2 utilizes high rate plastic media lters. A survey was conducted to examine phosphorus removal across both treatment streams. This was repeated three times on different days. On-site Analysis Spot analyses were carried out on each of the samples using analytical test kits and a portable photometer, Lasa 100 (Dr. Lange, Basingstoke). Orthophosphate, iron (II) and (III) measurements were recorded, using both ltered (Whatman lter papers 0.45 mm) and un ltered samples. The remaining sample was sent to Thames Laboratories for the analysis of ammoniacal-nitrogen, suspended solids, COD, BOD, total phosphate and soluble phosphate. The redox, DO and pH were also measured prior to the biological ltration stage. RESULTS Initial Tests Initial tests indicated that there was a strong relationship between orthophosphate removal and iron (II) chloride addition. These tests were performed at pH 7.55–7.81, DO 0.93–1.53 mg l 1 and redox 112–132 mV. The trendline shown in Figure 2 indicates that 85% removal could be expected at a molar ratio of 3.4:1; decreasing to 72% at 2:1, 60% at 1.5:1 and 52% at 1:1. As expected the relationship between iron (II) dose and total phosphorus and suspended solids removal was less apparent. The percentage conversion of iron (II) to iron (III) during these experiments did not vary greatly with increasing dosage. The average conversion was 54.4% 12.2%. The Effects of pH, DO and Redox A seven variable factorial design experiment11 was used to minimize repetitions and so that all possible combinations of pH, DO and redox could be examined. The results of these experiments are summarized in Table 1. It is apparent that in terms of both suspended solids and phosphorus removal, pH has the greatest effect (as demonstrated by the highest half effect (12E)). In both cases, a positive half effect has been produced, indicating that pH has a more bene cial effect on suspended solids and phosphorus removal when high than when low. Redox and DO had less effect on removal ef ciencies. However, interestingly, when all three variables were positive (i.e., high pH, DO added and positive redox), a positive effect on phosphorus removal was produced (12E 10.8). Trans IChemE, Vol 79, Part B, November 2001
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Figure 3. The effect of pH on phosphorus removal by iron (II) addition.
Figure 1. Schematic diagram of Abingdon sewage treatment works.
Figure 2. Percentage removal of orthophosphate at increasing molar ratios of Fe:P.
The in uence of pH on orthophosphate (7 mg l 1 in uent) and total phosphorus (12.3 mg l 1 in uent) removal is demonstrated in Figure 3. Factors Affecting Conversion of Iron (II) to Iron (III) Generally, it was found that low DO, low redox and low pH resulted in poor conversions of iron (II) to iron (III)
(Table 2). Wastewater with a very negative redox potential produced the lowest conversion, averaging just 32.4% over 30 min. The highest conversions (72.3%) were observed with high DO concentrations which (by nature of the relationship between DO and redox) coincided with redox potentials between 50 and 100 mV. Although the results obtained were consistent and repeatable, problems were experienced during iron analysis with interference with suspended solids. These problems did not appear to affect the analysis in terms of the relative proportions of iron (II):iron (III). However total iron concentrations were often measured to be higher than expected. As suction ltration for suspended solids removal appeared to increase oxidation of iron (II) to iron (III), samples were analysed un ltered as the total iron concentrations were of less importance than the relative proportions of iron (II):iron (III).
Full-scale Observations The phosphorus removal at Abingdon was similar for both sets of lters but the plastic media lters (treatment stream 2) produced slightly lower ef uent suspended solids and iron concentrations (Table 3). The redox potential at point of dose (sampling point 2) was very negative, possibly due to the recycle of sludge liquors from further down the treatment stream (Figure 4). Due to the high degree of
Table 1. Results of seven variable factorial design experiments. Run
DO
1 2 3 4 5 6 7 8
7
Redox
DO
redox
pH
DO
pH
Redox
pH
DO
7 7 7
Tot E 1 2E
277 229 48 12 6
233.5 272.4 38.9 9.7 4.9
261.3 244.6 16.7 4.2 2.1
288.4 217.5 70.9 17.7 8.9
239 266.9 27.9 7 3.5
253.2 252.7 0.5 0.13 0
267.6 238.3 29.3 7.3 3.7
Tot E 1 2E
179.9 164.9 15 3.8 1.9
182.5 162.3 20.2 5.1 2.5
190.6 154.2 36.4 9.1 4.6
253.8 91 162.8 40.7 20.4
174.9 169.9 5 1.25 0.6
164.9 179.9 15 3.8 1.9
129.4 215.4 86 21.5 10.8
Where: ‘ ’
high pH, positive redox or aerated sample and ‘ ’
Trans IChemE, Vol 79, Part B, November 2001
redox
pH
SS rem %
P rem%
48.2 70.4 41.5 57.4 80.1 73.7 59.0 75.6
32.4 4.3 10.6 43.7 54.1 71.5 67.8 60.4
63.2
low pH, negative redox or unaerated sample depending on variable.
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Table 2. Variables affecting the conversion of iron (II) to iron (III). Average conversion of iron (II) to iron (III), %
Standard deviation, %
36.5 72.3
19.9 16.7
Redox potential, mV 175 to 342 57 to 91 112–132
32.4 66.8 60.0
9.6 28.4 18.7
pH 6.5–7.5 7.5–8.0
46.0 67.0
15.9 18.4
Parameter DO concentration, mg l 0–1.0 1.0–5.7
1
Table 3. Averaged values from on-site tests at Abingdon sewage treatment works. Sampling point 1. 2. 3. 4. 5. 6. 7. 8.
SS
Pre-dose Post-dose Pre-PST Settled sewage Pre-humus 1 Post-humus 1 Pre-humus 2 Post-humus 2
Where: PST
Iron P total Ortho-P (II)
382 11.8 462 12.4 430 13.0 119 4.6 53.0 3.4 27.4 1.6 52.2 2.4 12.4 1.8
6.2 5.3 7.9 2.8 1.6 1.2 1.8 1.1
Iron (III) Redox pH DO
2.4 0.2 34 18.0 10.2 139 16.1 0.8 51 4.3 2.3 11 2.9 2.3 – 1.2 0.4 – 2.7 1.0 – 0.9 0.1 –
8.2 7.6 7.5 7.5 – – – –
4.6 6.9 1.8 1.1 – – – –
primary settling tank.
Figure 4. Phosphorus removal using iron (II)=(III) at Abingdon sewage treatment works. Key: 1 pre-dose, 2 post-dose, 3 pre-PST, 4 settled sewage, 5 pre-humus 1, 6 post-humus 2, 7 pre-humus 1 and 8 posthumus 2.
turbulence at the point of dosing, the DO concentration was fairly high. Despite this, Figure 4 shows that 10 mg l 1 of the added iron (II) was immediately oxidized to iron (III). It should be noted that the pH at point of dose was also fairly high (8.2). This iron (III) was rapidly removed from the system (to a concentration of 2.3 mg l 1 in the settled sewage). The iron (II) concentration remained high until after the primary settlement tanks (PSTs), perhaps due to the inferior ability of the iron (II) precipitate to settle compared to iron (III). However, surprisingly the phosphorus concentration did not decrease at the same time as the iron (III) concentration. This may suggest that iron (III) hydroxide was formed instead of iron (III) phosphate.
DISCUSSION It has been suggested that conventional jar tests carried out using iron (II) salt as a phosphorus removal chemical cannot be expected to be representative of plant performance, as the oxygen supply in a jar test is usually de cient, and the iron will tend to remain mainly in the iron (II) state7. In a treatment plant with adequate aeration, the iron quickly oxidizes to the iron (III) state. Recht and Ghassemi3 suggested that homogeneously generated iron (III) is a more ef cient phosphate precipitant than iron (III) added from an external source. However these studies3,7 alongside many previous studies, were concerned with the use of iron (II) salts as simultaneous precipitants rather than as pre-precipitants as for this study. It is likely in this case that the oxygen supply to the system was greater during jar tests than that usually experienced pre-PSTs. Over the range of molar ratios iron conversion ranged from 40–80%. In two of the runs, iron conversion did not go above 60% suggesting that conversion was hindered perhaps by DO concentrations. However good orthophosphate removals may indicate that removal occurred as iron (II) phosphate. Iron (II) precipitates appear often as nely dispersed colloids which may partly be washed out to the ef uent4. This could explain the poor solids removal. However, Chung and Bhagat8 found that iron (II) without aeration gave better turbidity results than iron (II) with aeration. Explanations for this may be that the aeration was too vigorous, causing the break-up of ocs and the production of ne, unsettleable solids, or that the in-situ iron (III) itself produces poorly settleable solids. In a simultaneous precipitation environment, the poor settleability of in-situ iron (III) is unlikely to be noticed but would be more apparent during pre-precipitation. With decreasing pH, there is less dissociation of H2PO4 to release the phosphate ion. Conversely with increasing pH, there is stronger competition of the hydroxide ion with phosphate for iron (III). Both scenarios may impact the formation of iron phosphates12, depending on in uent pH. Recht and Ghassemi3 found the ef ciency of orthophosphate removal with iron (II) is strongly pH dependent with maximum removal obtained in the vicinity of pH 8. After 5 hours reaction time (at a ratio of 1:1 with a phosphate in uent of 12 mg l 1), orthophosphate removal was found to increase from 7 to 94% by increasing pH from 6 to 8. During this research, a similar experiment was performed (using an iron dose of molar ratio of 2:1 Fe:P) and average removals of orthophosphate increased from 5 to 71% by increasing pH from 6.5 to 8. These results agree with those of Recht and Ghassemi3: orthophosphate removal is strongly dependent on pH, and is ef cient at pH 8.
The Conversion of Iron (II) to Iron (III) The oxidation of iron (II) ions to the iron (III) is dependent on pH, oxygen concentration, catalysis by micro-organisms or inhibition by components sulphate and carbonate5. This oxidation process was examined to discover whether the reaction was most strongly in uenced by pH, dissolved oxygen or redox. It was evident that the reaction rates were strongly pH dependent. Oxidation of iron (II) is very slow below pH 6. The dependence of the Trans IChemE, Vol 79, Part B, November 2001
MECHANISMS OF CHEMICAL PHOSPHORUS REMOVAL oxidation rate on pH can be accounted for by assuming that hydrolysed iron (II) reacts faster with oxygen than nonhydrolysed iron (II)9. Since the oxidation inversely depends on the hydrogen ion concentration to a second power, a drop in pH reduces the oxidation6. In this study, a trend could not be seen with the oxidation of iron (II) and increasing pH. In the studies carried out by Llang et al.6, DO concentrations were low. A greater number of tests conducted at a DO concentration of 1– 2 mg l 1 with differing pHs may produce a better trend. Above 3.5 mg l 1 DO, an average conversion of 77% of iron (II) to iron (III) occurred after 30 minutes. Below 1 mg l 1 DO, an average conversion of 22% of iron (II) to iron (III) occurred after 30 minutes. Llang et al.6 also found that under low DO conditions iron (II) oxidation was relatively slow. In septic conditions, very poor oxidation of iron (II) was found. This is most probably a function of DO because poor oxidation was also found at a positive redox where no aeration was applied. High conversions were found at a positive redox where aeration had been applied. However it should be noted that jar tests attempted at 0 mg l 1 DO may in fact have contained a small amount of oxygen by entrained air during mixing. In summary, a high pH produces good orthophosphate removal and a high DO concentration is necessary for the conversion of iron (II) to iron (III). It was found that when aeration was applied to the jar tests, the pH increased due to the loss of carbon dioxide. Therefore, if aeration is applied to the point of dosing this will aid mixing, increase the oxidation of iron (II), and increase the pH to improve solids capture and orthophosphate removal. The best case scenario requires a high pH (8.0), high DO (above 3.5 mg l 1) and a positive redox. A worst case scenario occurs with a low pH (6.5), low DO (below 1 mg l 1) and a negative redox. Full-scale Survey For biological lters, the dosing of iron (II) sulphate to primary sedimentation tanks has been demonstrated to be
capable of achieving the 2 mg l 1 total phosphorus standard10. This is illustrated in the Abingdon results where nal ef uent results are below 2 mg l 1. However, the performance of any plant is strongly in uenced by the suspended solids content of the ef uent10. Total phosphorus removals can give misleading results as the majority of the phosphorus may be particulate which can settle out. This could explain why, in one case, 83% removal has occurred with iron (II) chloride at a 0.6:1 ratio which is less than the stoichiometric ratio. If a large proportion of phosphate is being removed by the lters then it is possible that on older stone lter beds, the bound phosphate could accumulate. This may lead to a sudden release of particulate phosphorus during high ow conditions, perhaps leading to a breach of phosphorus and=or iron discharge consent conditions . In runs 2 and 3 the orthophosphate concentrations in settled sewage were approximately 3.5 mg l 1. At an iron (III) dosed site, the orthophosphate concentration before the lters may be expected to be much lower. This suggests that perhaps vivianite (iron (II) phosphate precipitate) is formed and then slowly oxidizes downstream to iron (III) phosphate, as shown by the decreasing iron (II) concentrations. However, although the grab samples were taken at the same time of day to minimize diurnal variations, it is appreciated that there are not suf cient data to con rm this theory. It may be that initially iron (II) phosphate is precipitated, followed by slow oxidation of remaining iron (II) to (III) which may remove extra phosphorus (particularly the orthophosphate produced by hydrolysis downstream in the lters). Alternatively iron (II) or (III) hydroxide may be formed which may or may not remove phosphorus by adsorption. Further work is required to con rm any of these theories. The number of pathways available and the complexity of the reactions is demonstrated in simpli ed form in Figure 5. This survey and the jar tests have indicated that the condition of the sewage strongly in uences the removal of orthophosphate. It is apparent from both sets of data that the pH at point of dose is a major parameter affecting the iron (II)–(III) conversion. In poor conditions, iron (II) phosphate
Figure 5. Simpli ed diagram of possible iron (II) chloride reactions pathways in wastewater.
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may not convert to iron (III) phosphate resulting in a higher chemical requirement. If changing conditions in the inlet do have such a strong in uence as suggested than there is less control over the phosphate removal with iron (II) than with iron (III). If inlet pH is low then high Fe (II) concentrations may leave the PST. Following this, if the retention time in the lters is not suf cient to oxidize to iron (III), this may result in high iron concentrations in the ef uent. However, note that although the analytical test kits appear to be able to determine the relative fractions of iron (II) and iron (III) during on-site tests, there was a high degree of interference with suspended solids in the samples. Therefore these tests should not be relied upon for absolute concentrations of iron, only as a guide to the relative proportions of iron (II):iron (III).
CONCLUSIONS (1) The optimum ratio for orthophosphate and total phosphorus removal was 3:1 Fe:P (molar ratio). (2) The removal of orthophosphate and suspended solids by iron (II) chloride is most strongly dependent on pH, reaching high removals at pH 8.0 and very low removals at pH 6.5. (3) The conversion of iron (II) to iron (III) was greatest at higher pH and DO concentrations and mid range redox potentials. (4) Abingdon site survey showed that iron (II) chloride added as primary precipitation can reduce total phosphate levels to below 2 mg l 1. (5) The pH and the DO at point of dosing may in uence the ef ciency of the iron (II) to (III) conversion and the overall phosphorus removal ef ciency more than redox potential.
REFERENCES 1. Metcalf and Eddy Inc., 1991, Wastewater Engineering—Treatment, Disposal, Reuse, 3rd edition (McGraw-Hill International Editions, New York). 2. Gillberg, L., Nilsson, D. and Akesson, M., 1996, The in uence of pH when precipitating orthophosphate with aluminium and iron salts, in Chemical Water and Wastewater Treatment IV (Springer-Verlag, Berlin Heidelberg). 3. Recht, H. L. and Ghassemi, M., 1971, Phosphate precipitation with iron (II) iron, Water Pollution Control Series, 17010 EKI 09=71. 4. Maurer, M. and Boller, M., 1999, Phosphorus precipitation in wastewater treatment plants, Wat Sci Tech, 39(1): 147–163. 5. Yeoman, S., Stephenson, T., Lester, J. N. and Perry, R., 1988, The removal of phosphorus during wastewater treatment: a review, Environ Pollut, 49: 183–233. 6. Llang, L., McNabb, J. A., Paulk, J. M., Gu, B. and McCarthy, J. F., 1993, Kinetics of Iron (II) oxygenation at low partial pressure of oxygen in the presence of natural organic matter, Environ Sci Tech, 27: 1864–1870. 7. Scott, D. S., 1973, Use and Production of Iron Salts for Phosphorus Removal, Research Report No. 5 (Environment Canada). 8. Chung, S. K. and Bhagat, S. K., 1973, Wastewater treatment properties of Iron (II) and Iron (III) compounds, Wat Sewage Wks, February. 9. Stumm, J. J. and Morgan, W., 1996, Aquatic Chemistry, 3rd edition (J. Wiley & Sons, Inc., New York). 10. Strickland, J., 1998, The development and application of phosphorus removal from wastewater using biological and metal precipitation techniques, JIWEM, 12: 30–37. 11. Clark, T. and Stephenson, T., 1999, Development of a jar testing protocol for chemical phosphorus removal in activated sludge using statistical experimental design, Wat Res, 33(7): 1730–1734. 12. Hsu, P. H., 1976, Comparison of iron (III) and aluminium in the precipitation of phosphate from solution, Wat Res, 10: 903–907.
ADDRESS Correspondence concerning this paper should be addressed to Dr. T. Clark, Thames Water Research and Technology, Spencer House, Manor Farm Road, Reading, Berkshire RG2 0JN. E-mail: [email protected] The manuscript was received 22 January 2001 and accepted for publication after revision 22 October 2001.
Trans IChemE, Vol 79, Part B, November 2001
MECHANISMS OF CHEMICAL PHOSPHORUS REMOVAL 1—Iron (II) Salts J. THISTLETON1 , T. CLARK2 , P. PEARCE 2 and S. A. PARSONS 1 1
School of Water Sciences, Cran eld University, Cran eld, Bedfordshire, UK 2 Thames Water Research and Development, Reading, Berkshire, UK
A
series of jar tests and a full-scale assessment of iron (II) chloride dosing at Abingdon sewage treatment works were carried out to identify factors affecting phosphorus removal. Variables considered were pH, redox potential and dissolved oxygen concentration using crude sewage. As expected, there was a strong link between phosphorus removal and iron:phosphate ratio but this trend was not experienced with suspended solids removal. The main factor affecting the ef ciency of the phosphorus removal was pH, followed by dissolved oxygen (DO) and redox potential. This was assessed using a seven variable factorial design experiment. The conversion of iron (II) to iron (III) averaged close to 68.7% at optimal conditions which were: high DO (1.0–5.7 mg l 1), mid range redox potential (57–91 mV) and high pH (7.5–8.0). As before, the most important variables were pH and DO concentration. Full-scale observations appeared to support these ndings as iron conversion and phosphorus removal was good (85.6%) despite the very low redox potential ( 139 mV). This could be due to the high pH at point of dose (8.2) and the relatively high DO (4.6 mg l 1). In poor conditions, where less iron (II) is converted to iron (III), chemical consumption costs will increase, solids removal may decrease and the system is likely to become less reliable. Keywords: phosphorus removal; iron (II) chloride; redox; dissolved oxygen; pH.
INTRODUCTION
Iron (III) and aluminium (III) are most frequently used in wastewater treatment because they form easily settleable ocs within a short time. For iron (II), this settleability is dependent on its conversion to iron (III), as iron (III) complexes show considerably better occulation characteristics than those of iron (II). The latter often appear as nely dispersed colloids that can be washed out to the ef uent4. The oxidation of iron (II) ions to the iron (III) form is dependent on pH, oxygen concentration5, catalysis by micro-organisms and catalysis or inhibition by components such as sulphur and carbonate5. Llang et al.6 found that under low dissolved oxygen (DO) conditions iron (II) oxidation was relatively slow. It has been suggested that in treatment plants having adequate aeration capacity, iron (II) and iron (III) are equally effective for phosphorus removal7. However, Chung and Bhagat8 found that iron (II) addition without aeration gave better turbidity results than iron (II) with aeration. It has been suggested that because of the existence of oxidative conditions in conventional biological wastewater treatment, iron (II) added to wastewater is oxidized in situ resulting in the homogenous generation of iron (III). Recht and Ghassemi3 reported that homogeneously generated iron (III) is a more ef cient phosphate precipitant than iron (III) added from an external source. Gillberg et al.2 investigated the in uence of pH on the precipitation of orthophosphates when adding metal salts and also on the addition of fresh hydroxides produced from
The chemical precipitation of phosphorus is brought about by the addition of the salts of di- and tri-valent metal ions that form precipitates of sparingly soluble phosphates1. Three types of metal precipitants are commonly used for chemical phosphorus removal, namely iron (II), iron (III) and aluminium. The level of precipitation in a sewage treatment plant depends on the pH of the system, the type of metal salt used and on the degree of mixing of the metal salt into the sewage2. However mechanisms of chemical phosphorus removal are poorly understood and data reported in the literature have often been contradictory. Considerable disagreement exists between various investigators on the kinetics and stoichiometry of cation-phosphate reactions, and on the effect of parameters such as pH and ionic concentrations on the ef ciency of phosphate removal3. It is generally thought that when an iron=aluminium salt is added to sewage, the metal ions react with water molecules to form hydrolysis products. If the hydrolysis product is in proximity to an orthophosphate molecule, a reaction will occur. However, if the hydrolysis products do not react with an orthophosphate ion or other charged species they will continue to react with water molecules and form metal hydroxides2. The higher the pH the greater the probability that metal hydroxides are formed through reactions with the hydroxide ion2. 339
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the different salts. It should be noted, however, that the experiments were carried out using ‘pH-adjusted hydrogen carbonate water’ as opposed to sewage. At most pHs, more orthophosphate was precipitated by adding the metal salts to solutions of orthophosphate than by adding metal hydroxides. The iron (II) salt precipitated orthophosphate to the same extent as the iron (III) salt only in the pH range 7–8. At other pHs the iron (II) salt was considerably less effective. Stumm and Morgan9 reported that the rate of oxygenation of iron (II) in solutions of pH 5 was found to be rst order with respect to the concentration of both iron (II) and oxygen and second order with respect to the hydroxide ion. Thus a 100-fold increase in the rate of reaction occurs for a unit increase in pH. Finally, Strickland10 demonstrated that iron (II) salts were not only cheaper than iron (III) but that they are also less sensitive to dosing point and control methods in activated sludge plants. The aim of this paper is to develop an understanding of some important fundamental aspects of chemical phosphorus removal using iron (II) salts to allow the optimization of metal salt addition at phosphorus consented sites. MATERIALS AND METHODS Jar Tests Jar tests were performed using crude sewage taken from Pangbourne sewage treatment works and using standardized apparatus. Before commencing the jar tests, orthophosphate, iron (II) and iron (III) concentrations were measured using analytical test kits (LCK320C and LCK049C cuvette tests, Dr Lange, Basingstoke). The orthophosphate concentration was used to calculate the coagulant dose at different molar ratios of Fe:P. Redox, dissolved oxygen (DO), pH and temperature were also measured. Analysis of the crude sewage, measuring suspended solids, soluble and total phosphorus was carried out by Thames Water Laboratories. Iron (II) chloride (commercial grade) was diluted by 27 times prior to addition to the jar tests. Immediately after dosing, the jars were stirred at 325 rpm for 1 minute rapid mix. The speed was then reduced to 30 rpm for 30 minutes occulation time, after which the sewage was left to settle for 60 minutes. After settlement the supernatant was decanted and analysed for orthophosphate, iron (II) and (III). Analysis was also carried out for suspended solids, total phosphate and soluble phosphate by Thames Water Laboratories. Effects of Redox, DO and pH Conditions were altered to see how the above variables affected removals of suspended solids and orthophosphate. These tests were all conducted at molar ratios of approximately 2:1 Fe:P. The pH was lowered with hydrochloric acid (AnalaR) and raised with sodium hydroxide (AnalaR). A low pH was classed as 6.5–7.2 and a high pH was classed as 7.5–8.0. The dissolved oxygen was increased with a standard sh tank aerator and airstone. A reduction of redox was achieved by allowing the sewage to go septic (2 days). For an extremely negative redox, this time was increased to 4 days. A factorial matrix was used to consider all the different
scenarios with redox, pH and DO. During some of the jar tests, samples were taken at intervals of 1, 2, 5, 10, 20, 30, 60 and 90 minutes to measure iron (II)=(III) levels. Full-scale Survey Abingdon sewage treatment works (Thames Water) is dosed with iron (II) chloride along the inlet channel after the screens. The dose is constant at 1.25 l min 1 (average incoming ow rate 14,000 m3 d 1). The site has two discharge points with different ef uent consents; see Figure 1. Treatment stream 1 relies on stone media trickling lters whereas treatment stream 2 utilizes high rate plastic media lters. A survey was conducted to examine phosphorus removal across both treatment streams. This was repeated three times on different days. On-site Analysis Spot analyses were carried out on each of the samples using analytical test kits and a portable photometer, Lasa 100 (Dr. Lange, Basingstoke). Orthophosphate, iron (II) and (III) measurements were recorded, using both ltered (Whatman lter papers 0.45 mm) and un ltered samples. The remaining sample was sent to Thames Laboratories for the analysis of ammoniacal-nitrogen, suspended solids, COD, BOD, total phosphate and soluble phosphate. The redox, DO and pH were also measured prior to the biological ltration stage. RESULTS Initial Tests Initial tests indicated that there was a strong relationship between orthophosphate removal and iron (II) chloride addition. These tests were performed at pH 7.55–7.81, DO 0.93–1.53 mg l 1 and redox 112–132 mV. The trendline shown in Figure 2 indicates that 85% removal could be expected at a molar ratio of 3.4:1; decreasing to 72% at 2:1, 60% at 1.5:1 and 52% at 1:1. As expected the relationship between iron (II) dose and total phosphorus and suspended solids removal was less apparent. The percentage conversion of iron (II) to iron (III) during these experiments did not vary greatly with increasing dosage. The average conversion was 54.4% 12.2%. The Effects of pH, DO and Redox A seven variable factorial design experiment11 was used to minimize repetitions and so that all possible combinations of pH, DO and redox could be examined. The results of these experiments are summarized in Table 1. It is apparent that in terms of both suspended solids and phosphorus removal, pH has the greatest effect (as demonstrated by the highest half effect (12E)). In both cases, a positive half effect has been produced, indicating that pH has a more bene cial effect on suspended solids and phosphorus removal when high than when low. Redox and DO had less effect on removal ef ciencies. However, interestingly, when all three variables were positive (i.e., high pH, DO added and positive redox), a positive effect on phosphorus removal was produced (12E 10.8). Trans IChemE, Vol 79, Part B, November 2001
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Figure 3. The effect of pH on phosphorus removal by iron (II) addition.
Figure 1. Schematic diagram of Abingdon sewage treatment works.
Figure 2. Percentage removal of orthophosphate at increasing molar ratios of Fe:P.
The in uence of pH on orthophosphate (7 mg l 1 in uent) and total phosphorus (12.3 mg l 1 in uent) removal is demonstrated in Figure 3. Factors Affecting Conversion of Iron (II) to Iron (III) Generally, it was found that low DO, low redox and low pH resulted in poor conversions of iron (II) to iron (III)
(Table 2). Wastewater with a very negative redox potential produced the lowest conversion, averaging just 32.4% over 30 min. The highest conversions (72.3%) were observed with high DO concentrations which (by nature of the relationship between DO and redox) coincided with redox potentials between 50 and 100 mV. Although the results obtained were consistent and repeatable, problems were experienced during iron analysis with interference with suspended solids. These problems did not appear to affect the analysis in terms of the relative proportions of iron (II):iron (III). However total iron concentrations were often measured to be higher than expected. As suction ltration for suspended solids removal appeared to increase oxidation of iron (II) to iron (III), samples were analysed un ltered as the total iron concentrations were of less importance than the relative proportions of iron (II):iron (III).
Full-scale Observations The phosphorus removal at Abingdon was similar for both sets of lters but the plastic media lters (treatment stream 2) produced slightly lower ef uent suspended solids and iron concentrations (Table 3). The redox potential at point of dose (sampling point 2) was very negative, possibly due to the recycle of sludge liquors from further down the treatment stream (Figure 4). Due to the high degree of
Table 1. Results of seven variable factorial design experiments. Run
DO
1 2 3 4 5 6 7 8
7
Redox
DO
redox
pH
DO
pH
Redox
pH
DO
7 7 7
Tot E 1 2E
277 229 48 12 6
233.5 272.4 38.9 9.7 4.9
261.3 244.6 16.7 4.2 2.1
288.4 217.5 70.9 17.7 8.9
239 266.9 27.9 7 3.5
253.2 252.7 0.5 0.13 0
267.6 238.3 29.3 7.3 3.7
Tot E 1 2E
179.9 164.9 15 3.8 1.9
182.5 162.3 20.2 5.1 2.5
190.6 154.2 36.4 9.1 4.6
253.8 91 162.8 40.7 20.4
174.9 169.9 5 1.25 0.6
164.9 179.9 15 3.8 1.9
129.4 215.4 86 21.5 10.8
Where: ‘ ’
high pH, positive redox or aerated sample and ‘ ’
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redox
pH
SS rem %
P rem%
48.2 70.4 41.5 57.4 80.1 73.7 59.0 75.6
32.4 4.3 10.6 43.7 54.1 71.5 67.8 60.4
63.2
low pH, negative redox or unaerated sample depending on variable.
43.1
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Table 2. Variables affecting the conversion of iron (II) to iron (III). Average conversion of iron (II) to iron (III), %
Standard deviation, %
36.5 72.3
19.9 16.7
Redox potential, mV 175 to 342 57 to 91 112–132
32.4 66.8 60.0
9.6 28.4 18.7
pH 6.5–7.5 7.5–8.0
46.0 67.0
15.9 18.4
Parameter DO concentration, mg l 0–1.0 1.0–5.7
1
Table 3. Averaged values from on-site tests at Abingdon sewage treatment works. Sampling point 1. 2. 3. 4. 5. 6. 7. 8.
SS
Pre-dose Post-dose Pre-PST Settled sewage Pre-humus 1 Post-humus 1 Pre-humus 2 Post-humus 2
Where: PST
Iron P total Ortho-P (II)
382 11.8 462 12.4 430 13.0 119 4.6 53.0 3.4 27.4 1.6 52.2 2.4 12.4 1.8
6.2 5.3 7.9 2.8 1.6 1.2 1.8 1.1
Iron (III) Redox pH DO
2.4 0.2 34 18.0 10.2 139 16.1 0.8 51 4.3 2.3 11 2.9 2.3 – 1.2 0.4 – 2.7 1.0 – 0.9 0.1 –
8.2 7.6 7.5 7.5 – – – –
4.6 6.9 1.8 1.1 – – – –
primary settling tank.
Figure 4. Phosphorus removal using iron (II)=(III) at Abingdon sewage treatment works. Key: 1 pre-dose, 2 post-dose, 3 pre-PST, 4 settled sewage, 5 pre-humus 1, 6 post-humus 2, 7 pre-humus 1 and 8 posthumus 2.
turbulence at the point of dosing, the DO concentration was fairly high. Despite this, Figure 4 shows that 10 mg l 1 of the added iron (II) was immediately oxidized to iron (III). It should be noted that the pH at point of dose was also fairly high (8.2). This iron (III) was rapidly removed from the system (to a concentration of 2.3 mg l 1 in the settled sewage). The iron (II) concentration remained high until after the primary settlement tanks (PSTs), perhaps due to the inferior ability of the iron (II) precipitate to settle compared to iron (III). However, surprisingly the phosphorus concentration did not decrease at the same time as the iron (III) concentration. This may suggest that iron (III) hydroxide was formed instead of iron (III) phosphate.
DISCUSSION It has been suggested that conventional jar tests carried out using iron (II) salt as a phosphorus removal chemical cannot be expected to be representative of plant performance, as the oxygen supply in a jar test is usually de cient, and the iron will tend to remain mainly in the iron (II) state7. In a treatment plant with adequate aeration, the iron quickly oxidizes to the iron (III) state. Recht and Ghassemi3 suggested that homogeneously generated iron (III) is a more ef cient phosphate precipitant than iron (III) added from an external source. However these studies3,7 alongside many previous studies, were concerned with the use of iron (II) salts as simultaneous precipitants rather than as pre-precipitants as for this study. It is likely in this case that the oxygen supply to the system was greater during jar tests than that usually experienced pre-PSTs. Over the range of molar ratios iron conversion ranged from 40–80%. In two of the runs, iron conversion did not go above 60% suggesting that conversion was hindered perhaps by DO concentrations. However good orthophosphate removals may indicate that removal occurred as iron (II) phosphate. Iron (II) precipitates appear often as nely dispersed colloids which may partly be washed out to the ef uent4. This could explain the poor solids removal. However, Chung and Bhagat8 found that iron (II) without aeration gave better turbidity results than iron (II) with aeration. Explanations for this may be that the aeration was too vigorous, causing the break-up of ocs and the production of ne, unsettleable solids, or that the in-situ iron (III) itself produces poorly settleable solids. In a simultaneous precipitation environment, the poor settleability of in-situ iron (III) is unlikely to be noticed but would be more apparent during pre-precipitation. With decreasing pH, there is less dissociation of H2PO4 to release the phosphate ion. Conversely with increasing pH, there is stronger competition of the hydroxide ion with phosphate for iron (III). Both scenarios may impact the formation of iron phosphates12, depending on in uent pH. Recht and Ghassemi3 found the ef ciency of orthophosphate removal with iron (II) is strongly pH dependent with maximum removal obtained in the vicinity of pH 8. After 5 hours reaction time (at a ratio of 1:1 with a phosphate in uent of 12 mg l 1), orthophosphate removal was found to increase from 7 to 94% by increasing pH from 6 to 8. During this research, a similar experiment was performed (using an iron dose of molar ratio of 2:1 Fe:P) and average removals of orthophosphate increased from 5 to 71% by increasing pH from 6.5 to 8. These results agree with those of Recht and Ghassemi3: orthophosphate removal is strongly dependent on pH, and is ef cient at pH 8.
The Conversion of Iron (II) to Iron (III) The oxidation of iron (II) ions to the iron (III) is dependent on pH, oxygen concentration, catalysis by micro-organisms or inhibition by components sulphate and carbonate5. This oxidation process was examined to discover whether the reaction was most strongly in uenced by pH, dissolved oxygen or redox. It was evident that the reaction rates were strongly pH dependent. Oxidation of iron (II) is very slow below pH 6. The dependence of the Trans IChemE, Vol 79, Part B, November 2001
MECHANISMS OF CHEMICAL PHOSPHORUS REMOVAL oxidation rate on pH can be accounted for by assuming that hydrolysed iron (II) reacts faster with oxygen than nonhydrolysed iron (II)9. Since the oxidation inversely depends on the hydrogen ion concentration to a second power, a drop in pH reduces the oxidation6. In this study, a trend could not be seen with the oxidation of iron (II) and increasing pH. In the studies carried out by Llang et al.6, DO concentrations were low. A greater number of tests conducted at a DO concentration of 1– 2 mg l 1 with differing pHs may produce a better trend. Above 3.5 mg l 1 DO, an average conversion of 77% of iron (II) to iron (III) occurred after 30 minutes. Below 1 mg l 1 DO, an average conversion of 22% of iron (II) to iron (III) occurred after 30 minutes. Llang et al.6 also found that under low DO conditions iron (II) oxidation was relatively slow. In septic conditions, very poor oxidation of iron (II) was found. This is most probably a function of DO because poor oxidation was also found at a positive redox where no aeration was applied. High conversions were found at a positive redox where aeration had been applied. However it should be noted that jar tests attempted at 0 mg l 1 DO may in fact have contained a small amount of oxygen by entrained air during mixing. In summary, a high pH produces good orthophosphate removal and a high DO concentration is necessary for the conversion of iron (II) to iron (III). It was found that when aeration was applied to the jar tests, the pH increased due to the loss of carbon dioxide. Therefore, if aeration is applied to the point of dosing this will aid mixing, increase the oxidation of iron (II), and increase the pH to improve solids capture and orthophosphate removal. The best case scenario requires a high pH (8.0), high DO (above 3.5 mg l 1) and a positive redox. A worst case scenario occurs with a low pH (6.5), low DO (below 1 mg l 1) and a negative redox. Full-scale Survey For biological lters, the dosing of iron (II) sulphate to primary sedimentation tanks has been demonstrated to be
capable of achieving the 2 mg l 1 total phosphorus standard10. This is illustrated in the Abingdon results where nal ef uent results are below 2 mg l 1. However, the performance of any plant is strongly in uenced by the suspended solids content of the ef uent10. Total phosphorus removals can give misleading results as the majority of the phosphorus may be particulate which can settle out. This could explain why, in one case, 83% removal has occurred with iron (II) chloride at a 0.6:1 ratio which is less than the stoichiometric ratio. If a large proportion of phosphate is being removed by the lters then it is possible that on older stone lter beds, the bound phosphate could accumulate. This may lead to a sudden release of particulate phosphorus during high ow conditions, perhaps leading to a breach of phosphorus and=or iron discharge consent conditions . In runs 2 and 3 the orthophosphate concentrations in settled sewage were approximately 3.5 mg l 1. At an iron (III) dosed site, the orthophosphate concentration before the lters may be expected to be much lower. This suggests that perhaps vivianite (iron (II) phosphate precipitate) is formed and then slowly oxidizes downstream to iron (III) phosphate, as shown by the decreasing iron (II) concentrations. However, although the grab samples were taken at the same time of day to minimize diurnal variations, it is appreciated that there are not suf cient data to con rm this theory. It may be that initially iron (II) phosphate is precipitated, followed by slow oxidation of remaining iron (II) to (III) which may remove extra phosphorus (particularly the orthophosphate produced by hydrolysis downstream in the lters). Alternatively iron (II) or (III) hydroxide may be formed which may or may not remove phosphorus by adsorption. Further work is required to con rm any of these theories. The number of pathways available and the complexity of the reactions is demonstrated in simpli ed form in Figure 5. This survey and the jar tests have indicated that the condition of the sewage strongly in uences the removal of orthophosphate. It is apparent from both sets of data that the pH at point of dose is a major parameter affecting the iron (II)–(III) conversion. In poor conditions, iron (II) phosphate
Figure 5. Simpli ed diagram of possible iron (II) chloride reactions pathways in wastewater.
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may not convert to iron (III) phosphate resulting in a higher chemical requirement. If changing conditions in the inlet do have such a strong in uence as suggested than there is less control over the phosphate removal with iron (II) than with iron (III). If inlet pH is low then high Fe (II) concentrations may leave the PST. Following this, if the retention time in the lters is not suf cient to oxidize to iron (III), this may result in high iron concentrations in the ef uent. However, note that although the analytical test kits appear to be able to determine the relative fractions of iron (II) and iron (III) during on-site tests, there was a high degree of interference with suspended solids in the samples. Therefore these tests should not be relied upon for absolute concentrations of iron, only as a guide to the relative proportions of iron (II):iron (III).
CONCLUSIONS (1) The optimum ratio for orthophosphate and total phosphorus removal was 3:1 Fe:P (molar ratio). (2) The removal of orthophosphate and suspended solids by iron (II) chloride is most strongly dependent on pH, reaching high removals at pH 8.0 and very low removals at pH 6.5. (3) The conversion of iron (II) to iron (III) was greatest at higher pH and DO concentrations and mid range redox potentials. (4) Abingdon site survey showed that iron (II) chloride added as primary precipitation can reduce total phosphate levels to below 2 mg l 1. (5) The pH and the DO at point of dosing may in uence the ef ciency of the iron (II) to (III) conversion and the overall phosphorus removal ef ciency more than redox potential.
REFERENCES 1. Metcalf and Eddy Inc., 1991, Wastewater Engineering—Treatment, Disposal, Reuse, 3rd edition (McGraw-Hill International Editions, New York). 2. Gillberg, L., Nilsson, D. and Akesson, M., 1996, The in uence of pH when precipitating orthophosphate with aluminium and iron salts, in Chemical Water and Wastewater Treatment IV (Springer-Verlag, Berlin Heidelberg). 3. Recht, H. L. and Ghassemi, M., 1971, Phosphate precipitation with iron (II) iron, Water Pollution Control Series, 17010 EKI 09=71. 4. Maurer, M. and Boller, M., 1999, Phosphorus precipitation in wastewater treatment plants, Wat Sci Tech, 39(1): 147–163. 5. Yeoman, S., Stephenson, T., Lester, J. N. and Perry, R., 1988, The removal of phosphorus during wastewater treatment: a review, Environ Pollut, 49: 183–233. 6. Llang, L., McNabb, J. A., Paulk, J. M., Gu, B. and McCarthy, J. F., 1993, Kinetics of Iron (II) oxygenation at low partial pressure of oxygen in the presence of natural organic matter, Environ Sci Tech, 27: 1864–1870. 7. Scott, D. S., 1973, Use and Production of Iron Salts for Phosphorus Removal, Research Report No. 5 (Environment Canada). 8. Chung, S. K. and Bhagat, S. K., 1973, Wastewater treatment properties of Iron (II) and Iron (III) compounds, Wat Sewage Wks, February. 9. Stumm, J. J. and Morgan, W., 1996, Aquatic Chemistry, 3rd edition (J. Wiley & Sons, Inc., New York). 10. Strickland, J., 1998, The development and application of phosphorus removal from wastewater using biological and metal precipitation techniques, JIWEM, 12: 30–37. 11. Clark, T. and Stephenson, T., 1999, Development of a jar testing protocol for chemical phosphorus removal in activated sludge using statistical experimental design, Wat Res, 33(7): 1730–1734. 12. Hsu, P. H., 1976, Comparison of iron (III) and aluminium in the precipitation of phosphate from solution, Wat Res, 10: 903–907.
ADDRESS Correspondence concerning this paper should be addressed to Dr. T. Clark, Thames Water Research and Technology, Spencer House, Manor Farm Road, Reading, Berkshire RG2 0JN. E-mail: [email protected] The manuscript was received 22 January 2001 and accepted for publication after revision 22 October 2001.
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