Evaluation of Poly-Alumino-Iron Sulphate (PAFS) as a Coagulant for Water Treatment

J. Q. Jiang and N. J. D. Graham

Abstract

This paper is concerned with the performance of a new kind of pre-polymerised inorganic coagulant, poly-alumino-iron sulphate (PAFS), for use in drinking wa­ter treatment. Considerable laboratory work has been undertaken to evaluate the performance of PAFS in comparison with conventional coagulants such as ferric sulphate (FS) and aluminium sulphate (AS), for the coagulation of a model up­land coloured water and a typical lowland surface water in the UK. The results have shown that the PAFS coagulant was superior to the FS and AS in terms of the removal of colour, UV254-absorbing substances and dissolved organic carbon (DOC). PAFS achieved the lowest residual concentrations of Fe and Al in compar­ison with FS and AS, respectively. In addition, to achieve an equivalent removal percentage, a lower coagulant dose (in molar units) was required for PAFS in comparison with AS and FS.

1. Introduction

Coagulants used for water and wastewater treatment are predominantly inorganic salts of iron and aluminium. When dosed into water the iron or aluminium ions hydrolyse rapidly and in an uncontrolled manner, to form a range of metal hydrol­ysis species. A range of factors such as the characteristics of water, the coagulation pH and the dose of coagulant influence the coagulating species that are formed, and therefore the treatment performance. Considerable interest and attention have been paid in recent years to preparing pre-hydrolysed metal-ion coagulants, based on either aluminium (e.g. poly-aluminium chloride) or ferric iron (e.g. polyferric sulphate), and some of these have been found to perform better in some cases, in comparison with conventional coagulants such as alum (AS) or ferric sulphate (FS). The superior performance of pre-polymerised coagulants is attributed to their wider working pH range, a lower sensitivity to low water temperature, a relatively lower dose required to achieve the equivalent treatment efficiency, and lower residual metal-ion concentrations.

Development of the polymeric inorganic coagulant has been based on the consideration that the activated coagulants can be formed prior to their addition to

H. H. Hahn et al. (eds.), Chemical Water and Wastewater Treatment V© Springer-Verlag Berlin Heidelberg 1998

16 J.Q.Jiang and N.J.D.Graham

water, and the preparation conditions can thus be controlled and optimised. This paper mainly considers the potential advantages of the development and use of poly-alumina-iron-sulphate (PAPS) as a coagulant for water treatment. Particularly, this paper summarises the results of laboratory scale experiments to evaluate the coagulation performance of PAPS in comparison with AS and FS. The performance assessment was based on each coagulant'S ability to remove particulate materials and dissolved organic carbon at various pH values and coagulant dosages. A preliminary study of floc properties is also presented.

2. Experimental Procedures and Materials

PAPS was prepared in our laboratory by an optimised procedure arising from an extensive study of combinations of the relevant variables and their values (e.g., the molar ratio of Al to Fe, the ratio of hydroxide (OH) to the total metals (AI + Fe), the total metal concentration), which are believed to affect the properties and nature of the resulting PAFS. Characterisation of PAPS was carried out based on the determination of the molecular weight distribution (MWD) and the electrophoretic mobility (EM) of the partially hydrolysed species, using a membrane ultrafiltration technique and an electrophoresis method. The other coagulants used in this study were, (a) polyferric sulphate (PFS), which was prepared in our laboratory and had the following properties: stock concentration of 40 g Fen, molecular size of the predominant Fe(III) species (Fe (w /w > 83.5 %» = 500 Da, and an electrophoretic mobility of 6.0 (J.1mcmN s); (b) FS (commercial grade product, 10.5 % as Fe); and (c) AS (8.2 % Ah03, Alcan Chemical Ltd, Gerrards Cross, UK).

The coagulation tests used either a model upland coloured water or actual samples of a lowland surface water taken from Alton Reservoir (Anglian Water, Ipswich, England). Typical water quality values for the two raw waters can be found elsewhere [1].

A jar test experimental procedure was used to evaluate the coagulation perfor­mance of PAFS in comparison with other coagulants. The fast mixing time was set to 1 min at a paddle speed of 300 rpm, the flocculation period was 20min at a speed of 35 rpm, and the sedimentation period was 1 hour. Supernatant samples after settling were withdrawn from a position of 5 cm below the surface of the 300 ml test water samples for the analysis of water quality parameters. The range of coagulant dosages applied was 0 to 0.4 mM. The pH values of the coagulation experiment were 5 and 6 for the model coloured water, and 7.5 for the Alton water, which simulated the operational pH at the Alton Water Treatment Works. For the jar test experiments carried out at a low water temperature of 4°C, the test beakers were placed in a cooling bath, with recirculating water chilled by a Thermo Circulator (Betta-Tech Controls, Buckinghamshire, UK).

All water quality parameters such as the concentrations of dissolved organic carbon (DOC), UV254-absorbance, colour, and turbidity were measured in accor­dance with standard methods [2]. Floc size development and settling rate (defined as the reduction in the total particle numbers per given time) were determined by

Evaluation of Poly-Alumino-Iron Sulphate (PAFS) 17

Coulter Counter Particle Analysis (Model TA II, Coulter electronics Ltd, Luton, UK). Two different PAFS coagulants, PAPS 1 and 2, with an equivalent metal concentration of 1.4 M and with significantly different cationic charge, were used to evaluate the significance of cationic charge on the resulting floc density. Mea­surements of floc density were conducted externally at the Department of Civil Engineering, Strathclyde University, using a specially developed sedimentation velocity measurement technique which is described elsewhere [3].

3. Results

3.1 Evaluation of Coagulation Performance

The coagulation performance of two particular PAFS stock solutions (PAFSa and PAFSb), believed to have approximately the optimum properties, was evaluated in comparison with the other coagulants (i.e. FS, AS and PFS) with the model coloured water and this is summarised in Figures 1 to 6. At pH 5, the PAFS achieved greater removals of UV -absorbance and DOC than the other coagulants, but the differences between the PAPS and PFS were slight; both the PAFS and PFS performed much better than the FS and AS (Fig. 1). At pH 6, the superior perfor­mance of PAFS was clearly evident with about 10-20 % more removal of colour, UV-absorbance and DOC (Fig. 2). The expected advantage with PAFS of lower residual concentrations of Al and Fe was confirmed from the tests. For example (Fig. 3), at pH 6 and a dose of 0.15 mM as total metal, the residual concentration of Fe for PAFS (0.08 mgll) was similar to that for PFS but much lower than that for FS (0.21 mg/l), and the residual concentration of Al for PAFS (0.1 mg/l) was significantly lower than that for AS (0.35 mg/l). At pH 5, a similar effect was observed with the PAFS giving the lowest residual metal ion concentrations.

An example of the comparative performance of four coagulants at two water temperatures (18 DC and 4 DC) for treating model coloured water can be seen in Table 1. Generally, the performances of PAFS and PFS were unaffected by the water temperature, but FS and AS were significantly affected with evidence of a reduction in the removal percentages of colour, UV-abs and DOC, and an increase in residual concentrations of the metal in the treated waters at 4 DC.

Table 1. Comparison of coagulation perfonnancet using model coloured water at 18 DC and 4 DC (at pH 5)

Coagulant Colour UV-Abs DOC Residual Fe Residual Al (420 nm) R %* (254 nm) R % R% (mgll) (mg/l) 18 DC 4°C 18°C 4°C 18 °C 4°C 18 DC 4°C 18°C 4°C

PAFS 92 91 86 83 75 72 0.07 0.08 0.12 0.14

PFS 89 87 82 81 72 70 0.14 0.15 0.01 0.01

FS 73 67 70 63 61 56 0.23 0.28 0.01 0.01

AS 75 64 72 60 63 55 0.01 0.01 0.39 0.49

t Coagulant dose = 0.15 mM as AI + Fe, or as AI, or Fe; * R % = Percentage reduction

18 J.Q.Jiang and N.J.D.Graham

100

80

j 60 0 .., ... ., 0

J" 40 PAFSa 5~ -£J--

~ ~ --+-- PAFSb

--It- PFS 20

--0-- FS __�t_ AS

0.05 0.10 0.15 0.20 0.25 0.30

100

80

., ----t:J-- PAFSa ~

~ 60 ~ PAFSb

--It- PFS ~'S

40 --0-- FS

f~ --It- AS

t ~ ~ ..

20

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30

100

pHS

80

u 0 60 0 ...

~o

i~ 40 -£J-- PAFSa

~ ~ ~ PAFSb ---- PFS

20 ---- AS

--9-- FS

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Dose (mMas AI+Fe, Fe or AI)

Fig. I. Comparative coagulation performance of model coloured water at pH 5

Evaluation of Poly-Alumino-Iron Sulphat~ (PAFS) 19

100

80

g '8

60

Ii 40 --0- PAFSa

~ --+-- PAFSb p., ... --- PFS

20 _��__

AS ~ FS

0.05 0.10 0.15 0.20 0.25 0.30

100

pH6

80 .. ~

~ 60

t'o'S ~i 40

PAFSa

~ ~ PAFSb

p., ... PFS AS

20 FS

0.05 0.10 0.15 0.20 0.25 0.30

100

pH6

80

u 0 60 ~ ....

" 0 ~ .. i(~ 40

--0- PAFSa Q ~ ~ --+-- PAFSb --- PFS

20 -II-- AS ~ FS

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Dose (mM as Al+Fe, Fe or Al)

Fig.2. Comparative coagulation performance of model coloured water at pH 6

20 lQ.Jiang and N.J.D.Oraham

0.4

pH6 - FS

0.3 - PFS -- PAFSb

.j -- PAFSa

0.2

§i ~~ .... ~

0.1 ~'S

0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30

1.0 pH6 - AS

0.8 PAFSa --

.~ -- PAFSb

i 0.6

~~

1~ 0.4

]< ~'S 0.2

Fig. 3. Residual concen-tration of coagulants for

0.0 the treatment of model 0.00 0.05 0.10 O.IS 0.20 0.25 0.30

coloured water at pH 6 Dose (mMas AI+Fe or AI)

The coagulation performance of the two PAFS solutions, PAFSa and PAFSb, was compared at a specific dose and pH with a mixed solution of AS and FS, denoted as ref. The molar ratio of Al to Fe of the ref. solution was the same as that of the PAFSa and PAFSb. The results can be seen in Figures 4 to 6. It is clear that PAFS performed better than the ref. solution of FS and AS, with an almost 20 % greater removal percentage of colour, UV -absorbance and DOC for

100

80 : "

I 60

1 40

! ___ Colour

20 Model coloured water ~ UV2S4-abs coagulant dose = O.IS mM --a- DOC coagulation pH = S

Fig. 4. Comparative per-0

PAFSa PAFSb ref formance of PAFS at

coagulants pHS

Evaluation of Poly-Alumino-Iron Sulphate (PAFS) 21

100

~ 80

u

f 60 ." 'ii

~ 40

~ --+-- Colour

20 Model coloured water -0- UV2S4-abs

coagulant dose = 0.15 mM -D- DOC coagulation pH = 6 Fig. 5. Comparative per-0

PAFSa PAFSb ref formance of PAFS at

Coagulants pH 6

100 --+-- Co our -0- UV2S4-abs

80 -D-noc u on

i 60 i

] 40 g ~ 20 Alton surface water

coagulant dose = 0.15 mM Fig. 6. Comparative per-coagulation pH = 7.s

formance ofPAFS at pH 0 PAFSa PAFSb ref 7.5, using Alton surface

Coagulants water as the test water

the coagulation of the model coloured water. This was also the case fOT the Alton surface water treatment (Fig. 6) and more detailed results of the treatment of this type of water by PAFS can be seen elsewhere [4].

3.2 Study of Floc Properties

Two PAFS (1 and 2) were selected in order to compare their resulting floc proper­ties in the coagulation of the model coloured water since the two PAFS had signif­icantly different EM values, 4.7 (~cmN s) for PAFS 1 and 8.1 (~cmN s) for PAFS 2, respectively. It was considered likely that this could affect floc density, for example.

Use of the two PAFS lead to the formation of flocs whose size-density distri­bution complied approximately with the empirical form {!e = Ad-m over the size range of 60 ~ to 1000 11m, with {!e as the effective density, d as a representa­tive size, and A and m as fitted coefficients. Figure 7 summarises the difference between the size-density trends for the differing coagulants (PAFS 1 and 2) over the range of conditions analysed. It can be seen that PAFS 2, which had a higher cationic charge, had a greater floc density than PAFS 1, especially in the floc

22 J. Q. Jiang and N. J. D. Graham

0.' f" E ... ~

~ :z 0 .0 1 ~

'" ~ <..> Il! It

O.OOt

/

!'At'Sl

PAFS2 Pc - l.Ox l O"'d -1.10 R'--O.8846

Po - 2 .Ox 1O-'d- I ..51 R2 _ 0 .738

0000 ' L-______________________ ~--------~

0 ,00' 0\

FLOC DIAMETER (e m,

40

l\ --&- PAYS I --- PAYS 2 :l. 30

g !l ][ ·s

20 ~ ~ .q "'~

--0-- PFS ----9-- FS - All

JlJwwatcr

10

20 40 60 80

Particl. size (~",)

200000

I2IPAfS I 160000

DPAfS2

j- [ljPFS

~ il 120000 ElFS

~ '3 DAS II 80000

s ~ 40000

0 10 25 60

Settling time (min)

100

Fig_ 7. Effective density vs. floc size for the two PAFS coagulants

Fig. 8. (a) Comparison of particle size distributions after 5 min flocculation, (b) Comparison of the settling rate. (Model coloured water, coagu-lant dose=O.15 mM, co-agulation pH = 5)

size range of 60 to 500 J-lm. It is believed that coagulating species with a high cationic charge are more effective in neutralising the negative charge potential of the colloidal impurities, resulting in a more rapid floc size development and

Evaluation of Poly-Alumino-Iron Sulphate (PAFS) 23

leading to the formation of compact flocs with high density. In contrast, coagulating species of a lower charge may only partly neutralise the colloidal particles and the remaining repulsive forces may hinder particle aggregation. Thus, the floc size development for PAFS 1, FS and AS was relatively slow (see Fig.8(a», and the resulting settling rate (defined as the reduction in the total particle numbers per a given time) of PAFS 2 was greater than that of the other coagulants, especially after a settling period of 25 min (see Fig. 8(b».

4. Discussion and Conclusions

A major drawback in using conventional AlIFe(m) salts in water and wastewater treatment is the inability to control the nature of the coagulant species formed, and the performance of AlIFe(m) coagulants may worsen with changes in the nature of the raw water and water temperature. One successful and important method of improving the effectiveness of the conventional Al/Fe(ill) coagulants is to partially hydrolyse them prior to their addition to the raw water and thus pre-form the optimal coagulating species. In this way, the coagulant chemistry can be controlled and the proper solution conditions for the formation of the desired coagulant species can be maintained. The resulting pre-formed polymeric AlIFe(m) coagulants have high cationic charge and medium to high molecular size [5]. After dosing into the water to be treated, the preformed polymeric species are believed to persist long enough to enhance substantially the rate of colloidal charge neutralisation and chemical complexation [6] and therefore, to improve the overall water treatment performance.

The results presented in this study have shown the greater coagulation perfor­mance of PAFS for treating two kinds of raw waters. In particular, substantially lower metal-ion residual concentrations were achieved by PAFS. Since aluminium and iron are treated as separate parameters in the European Standards [7] and the World Health Organisation (WHO) water quality guidelines [8], PAFS will con­tain less of either individual metal-ion for the same overall dose of metal-ion as a conventional coagulant. In addition, PAFS may well produce flocs of a greater density as shown in this study, partially because of the expected greater charge mobility of PAFS species. This, together with a likely reduced coagulant dose requirement, should lead to a substantial reduction in sludge production.

In comparison with another pre-polymerised coagulant, polyferric sulphate (PFS), PAFS sometimes performed better under particular operational conditions (e.g., at pH 6 for treating model coloured water), and produced less residual metal­ion concentrations in the treated waters. Pre-polymerised metal coagulants are believed to interact initially by a charge neutralisation mechanism, with cationic species adsorbing onto colloidal particles and precipitating dissolved organic mate­rials. Thus, coagulating species with higher cationic charge are believed to perform better than those with lower cationic charge. Comparing the electrophoretic mo­bility (EM) value of the optimal PAFS (EM = 8 J.1DlcmN s) with that of PFS (EM = 6.2 J.1Dl cmN s) helps to explain this. In addition to this, the pre-polymerisation

24 J. Q. Jiang and N. J. D. Graham

of a mixed solution of aluminium and iron (ill) under optimal conditions of prepa­ration may produce polymeric species which are significantly different from those of either the polymeric Al species or polyferric species; this is to be the subject of further work.

In summary, the PAFS used in this study was superior to conventional co­agulants (i.e., AS and FS) in terms of the overall water treatment performance under the conditions used. Preliminary tests have indicated that the resulting floc properties are related to the nature of the coagulant species, and especially to the cationic charge.

Acknowledgements. The authors are very grateful to the Engineering and Physical Sciences Research Council (EPSRC), UK, for funding this study.

References

[1] Jiang, J.Q., Graham, N.J.D., Harward, C.: Preliminary Evaluation of Polyferric Sulphate as a Coagulant for Surface Water Treatment. In: Chemical Water and Wastewater Treat­ment III, R. Klute and H.H. Hahn (Eds.). Springer, Berlin Heidelberg 1994, pp. 71-93

[2] Standard Methods for the Examination of Water and Wastewater (18th ed.). AWWA, 1992

[3] Bache, D.H., Hossain, M.D: Optimum Coagulation Conditions for Coloured Water in Terms of Floc Properties. J. Water SRT-Aqua 40 (3) (1991), 170-178

[4] Jiang, J.Q., Graham, N.J.D.: Preliminary Evaluation of the Performance of New Pre­Polymerised Inorganic Coagulants for Lowland Surface Water Treatment. Water Sci. Technol. 37 (2) (1998) 121-128

[5] Jiang, J.Q., Graham, N.J.D.: Enhanced Coagulation Using AlIFe(III) Coagulants: Effect of Coagulant Chemistry on the Removal of Natural Organic Matter. Environm. Technol. 17 (1996) 937-950

[6] Jiang, J.Q., Graham, N.J.D.: Observations of the Comparative HydrolysisiPrecipitation Behaviour of Polyferric Sulphate and Ferric Sulphate. Water Res. 32 (3) (1998) 930-935

[7] Council of the European Communities: Directive of i5 July 1980 Relating to the Quality of Water Intended for Human Consumption. 8017781EEC, 1980

[8] WHO: Guidelines for Drinking Water Quality (2nd Ed.). Vol. 1, Recommendations. World Health Organisation, Geneva, 1993

Dr. Jia-Qian Jiang and Professor Nigel J. D. Graham Environmental and Water Resource Engineering Department of Civil Engineering Imperial College of Science, Technology and Medicine London SW7 2BU, UK