J Water SRTÐ Aqua Vol. 48, No. 4, pp. 167±175, 1999

OPERATIONAL/PRACTICAL PAPER

Factors in¯uencing residual aluminium levels at the Bu�alo Pound

Water Treatment Plant, Saskatchewan, Canada

P. T. Srinivasan*, T. Viraraghavan* and J. Bergman{, *Faculty of Engineering, University of Regina, Regina, Canada S4S 0A2;

{Bu�alo Pound Water Treatment Plant, Saskatchewan, Canada

ABSTRACT: One of the current concerns for water treatment plants using alum as a coagulant is elevated

concentration of residual aluminium in ®nished water. The objective of the study is to examine the seasonal

variations and factors in¯uencing residual aluminium at the Bu�alo Pound Water Treatment Plant

(BPWTP). Analysis of BPWTP data for two years related to seasonal variations of total/dissolved

aluminium levels showed that much of the raw water total aluminium were in particulate form. Raw water

particulate aluminium is correlated well (r2=0.79 for 1996 and r2=0.82 for 1997) with raw water total

suspended solids indicating much of the particulate aluminium is derived from the suspended solids present

in raw water. An analysis of eight-year data on total/dissolved aluminium, turbidity, dissolved organic

carbon and applied alum dosage showed that dissolved organic carbon present in the raw water played a

major role in controlling e�cacy of alum coagulation at BPWTP. The data showed that when alum/DOC

ratio is less than 7, insu�cient alum addition led to incomplete coagulation resulting in colloidal material

mostly consisting of organic aluminium in particulate form. Hence particulate aluminium increased in

treated water. But this increase in particulate aluminium did not increase the turbidity of treated water. This

indicated that an adequate alum dose in response to dissolved organic carbon change is important in

minimising residual aluminium in treated water at BPWTP.

INTRODUCTION

The Bu�alo PoundWater Treatment Plant (BPWTP) is located

approximately 85 km west of Regina. The plant provides all of

Moose Jaw's water supply and 50±70% of the water supply to

Regina. Water for the plant is drawn from Bu�alo Pound Lake,

a shallow reservoir in the Qu'Appelle Valley. The lake is 29 km

long, 1 kmwide and has a mean depth of 3 m. The lake is rich in

nutrients (phosphate, organic nitrogen and organic carbon)

which encourage the occurrence of algae blooms (diatoms in

winter/spring and blue-green algae in summer). Weed growth is

also extensive. The algae and weeds cause many treatment

problems, notably bad taste and odour, high chemical

demands and treatment interferences. Raw water from the

Bu�alo Pound Lake passes through a series of unit operations

and processes including prechlorination, cascade degassi®ca-

tion, coagulation and ¯occulation, clari®cation, ®ltration and

carbon adsorption to remove impurities such as clay particles,

bacteria, algae and dissolved organic material. Figure 1 shows a

schematic of the BPWTP. The sources of aluminium in this

water treatment plant include: (i) natural aluminium present in

lake water, and (ii) aluminium that is derived due to the use of

aluminium sulphate (better known as alum) as a coagulant

during treatment.

Recently there has been a concern over the use of aluminium

salts as coagulants in the treatment of potable water. The use of

alum as a coagulant in the treatment of drinking water

increased the aluminium concentration in ®nished water [1].

There is a 40±50% chance for increase in aluminium concentra-

tions in drinking water over the concentrations in the raw water

in plants using aluminium-based coagulants [2]. In a USEPA

survey of 186 water utilities, it was found that after coagulation

with aluminium salts, the aluminium concentration in the

treated water varied from 0.01 to 2.37 mg/L [2]. Approximately

11% of the aluminium input (through raw water and alum)

remained in the ®nished water as residual aluminium (at a

water treatment plant) and was transported through the dis-

tribution system without any signi®cant loss [3]. High concen-

trations of aluminium in drinking water were related to both

raw water concentrations and high treated-water turbidity [3].

Surveys of residual aluminium in the United States [2,3] and in

Europe [4] have also shown similar results. The major ®ndings

of the studies were that alum increased treated-water concen-

trations of aluminium, with the mean concentration values of

# 1999 IWSA 167

aluminium from facilities using alum as a coagulant being

approximately 0.1 mg/L; the aluminium concentrations in

treated waters were however, highly variable (0.05±0.25 mg/

L). It can be concluded that alum treated waters generally

contain more aluminium than raw surface waters. Numerous

water quality and supply problems are caused by increased

aluminium levels in ®nished water. Guideline values for alumi-

nium in drinking water are currently under review by Health

Canada.

The objective of the paper is to analyse the factors in-

¯uencing residual aluminium at BPWTP based on plant data

and suggest suitable strategies for minimising residual alumi-

nium.

SEASONAL VARIATIONS OF TOTAL/

DISSOLVED ALUMINIUM AT BPWTP

Seasonal variations of total and dissolved aluminium in the

(chlorinated) raw water of the BPWTP for 1996 and 1997 are

shown in Fig. 2. It can be seen from Fig. 2 that from May to

October (for 1996) dissolved aluminium concentrations were

slightly higher (30 mg/L) than the rest of the year. Amore or less

similar trend was observed during May through to July 1997.

The Bu�alo Pound Lake is eutrophic and hence during summer

periods high algal growth occurs. This high algae content

coupled with phytoplanktons and soil vegetation give rise to

an increase in dissolved organic carbon. This can be seen in Fig.

Fig. 1 Schematic of the Bu�alo Pound

Water Treatment Plant (BPWTP).

Fig. 2 Seasonal variations of raw water

total and dissolved aluminium at the

BPWTP for 1996 and 1997.

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# 1999 IWSA, J Water SRTÐAqua 48, 167±175

3 which shows seasonal variations of raw and treated water

dissolved organic carbon (DOC) for 1996 and 1997, respec-

tively. DOC, which has the ability to complex soil bound

aluminium, gives rise to an increase in dissolved aluminium.

However, raw water (chlorinated) dissolved aluminium con-

centrations were less than 50 mg/L throughout 1996 and 1997,

respectively. It is also seen from Fig. 2 that total aluminium

concentrations were also very high during May to November

(4200 mg/L) with a peak in September (750 mg/L) for 1996 aswell as during April to November with a peak in October

(1180 mg/L) for 1997. Particulate aluminium levels were also

high during May to November for 1996 (with a peak in

particulate aluminium in September; Fig. 4) and closely fol-

lowed the trend of total aluminium shown in Fig. 2. Particulate

aluminium levels were also high during April to November for

1997 (with a peak in particulate aluminium in October; Fig. 4)

and closely followed the trend of total aluminium shown in Fig.

Fig. 3 Seasonal variations of raw and

treated water dissolved organic carbon (in

mg/L) at the BPWTP for 1996 and 1997.

Fig. 4 Seasonal variations of raw water particulate aluminium at

the BPWTP for 1996 and 1997.

Fig. 5 Seasonal variations of treated water

total and dissolved aluminium at the

BPWTP for 1996 and 1997.

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# 1999 IWSA, J Water SRTÐAqua 48, 167±175

2. This indicates that much of the raw water total aluminium

was in particulate form for both the years.

Figure 5 shows the seasonal variations in treated water total

and dissolved aluminium concentrations at the BPWTP for

1996 and 1997. Comparing Fig. 2 and Fig. 5, it is seen that the

treated water dissolved aluminium concentrations for both the

years were higher than raw water dissolved aluminium concen-

trations. Alum changes the distribution between total and

dissolved aluminium with alum generally increasing free

(aquo) aluminium Al3+ (which is also accounted in dissolved

aluminium). It can be seen from Fig. 6 that the treated water

total aluminium concentrations closely followed the trend of

treated water particulate aluminium for 1996 and 1997. This

implies that much of the treated water total aluminium was also

in particulate form.

A simple linear regression analysis of the monthly average

data for raw water suspended solids (total in mg/L) and

particulate aluminium (in mg/L) in 1996 and 1997 shows that

they are well correlated [Fig. 7]. The regression equation

(Y=particulate aluminium=0.0436 total suspended

solids7 33) explained 91% of the total variance in the particu-

late aluminium data (r2=0.91; the correlation coe�cient was

found to be statistically signi®cant (t-test) at 99% con®dence

level). Such a high correlation coe�cient implies that particu-

late aluminium is mostly derived from the suspended solids

present in raw water. A similar analysis of the monthly average

data of the treated water suspended solids total (in mg/L) andparticulate aluminium (in mg/L) for 1996 and 1997 (Fig. 8)

shows that they are correlated well (r2=0.67; the correlation

coe�cient was found to be statistically signi®cant (t-test) at

99% con®dence level). Regression analysis for the both the

years indicated that in order to keep treated water particulate

aluminium levels below 50 (mg/L), an average suspended solids(total) removal of 90±95% would be needed.

FACTORS INFLUENCING RESIDUAL

ALUMINIUM AT BPWTP

Data on seasonal variations in raw water dissolved organic

carbon, alum dosage, clear water turbidity and clear water

(total) aluminium for 364 weeks (1 January 1991 to December

1997) are shown in Fig. 9. It can be seen from the ®gure that

alum dosage generally followed the trend of raw water DOC;

whereas wide ¯uctuations can be seen in both clear water

turbidity and treated water (total) aluminium levels. It can be

seen (shown as Y in Fig. 9) that when raw water dissolved

Fig. 6 Seasonal variations of treated water particulate aluminium

at the BPWTP for 1996 and 1997.

Fig. 7 Monthly average raw water total suspended solids (mg/L) vs.raw water particulate aluminium (mg/L) for 1996 and 1997.

Fig. 8 Monthly average treated water total suspended solids (mg/L)vs. treated water particulate aluminium (mg/L) for 1996 and 1997.

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organic carbon was the highest (13 mg/L; corresponding alum

dose is 95 p.p.m.), increase in clear water turbidity (shown as X

in Fig. 9) was associated with an increase in clear water total

aluminium. This con®rms that turbidity increased when alumi-

nium levels increased. Afterwards (beyond Y in Fig. 9) dis-

solved organic carbon decreased and so did the alum dose but

turbidity ¯uctuated between 0.1 and 0.2 NTU associated with a

sharp decrease in clear water total aluminium. When the raw

water dissolved organic carbon started ¯uctuating (as shown by

A in Fig. 9), the alum dosage remained constant (shown as Z in

Fig. 9); in the same region (i.e. Z) clear water total aluminium

peaked (up to 400 mg/L); but such a peak was not observed in

turbidity (i.e. turbidity ¯uctuated very little, only in the range of

0.05±0.1 NTU).

Monthly average turbidity and total/particulate aluminium

of raw and treated waters for the periods in which treated water

particulate aluminium exceeded the raw water particulate

aluminium levels is shown in Table 1. It can be seen from the

Table 1 that when particulate aluminium after treatment

increased turbidity of water after treatment decreased. This

Fig. 9 Seasonal variations of raw water DOC, alum dosage, clear-well turbidity, and clear-well aluminium from 1991 to 1997.

Table 1 Selected raw and treated water

parameters of BPWTP Raw water Treated water Turbidity (NTU)

Particulate Total Particulate Total Raw Treated

Time period Al (mg/L) Al (mg/L) Al (mg/L) Al (mg/L) water water

1±31 Jan 1996 26 31 27 55 0.9 0.13

1±29 Feb 1996 37 38 47 86 1.1 0.13

1±31 Dec 1996 44 54 277 318 1.2 0.2

1±31 Jan 1997 28 37 326 360 0.8 0.22

1±28 Feb 1997 31 43 270 326 0.9 0.18

1±31 May 1997 33 38 332 372 0.9 0.22

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phenomenon is not consistent with the generally perceived

concept that an increase in particulate aluminium levels in

®nished water will also give rise to an increase in treated water

turbidity. This phenomenon at BPWTP needs to be investi-

gated in detail.

Figure 10 shows some salient aspects of alum coagulation at

BPWTP which may explain the reasons for the inconsistent

variations between clear water aluminium and turbidity (i.e.

clear water aluminium increases without much changes in

turbidity and vice versa). Black boxes (in Fig. 10) delineate

periods of high total aluminium residuals. It can be seen in all

four black boxes that raw water total aluminium minimums

coincided with treated water total aluminium maximums. This

indicates that some of the aluminium present in alum contrib-

uted to the treated water total aluminium in some form.

Analysis of the data within the black boxes of the Fig. 10

indicates that occurrence of high ®nished water total alumi-

nium coincided with low alum/DOC ratios (57). Monthly

average raw/treated water dissolved aluminium levels for 1996

and 1997 (Figs 2 and 5) clearly showed that dissolved alumi-

nium, after alum treatment slightly increased in treated water.

It is assumed that particulate aluminium levels constituted the

major species of total aluminium. This assumption is supported

by the data related to selected raw and treated water parameters

of BPWTP (Table 1) for 1996 and 1997. It can be seen from Fig.

11 that there was consistently good increase in treated water

particulate aluminium levels compared to raw water levels. It

can also be seen in Fig. 12 that there was an increase in treated

water total suspended solids but turbidity decreased after

treatment. Whenever the alum/DOC ratio decreased below 7,

treated water total aluminium levels increased compared to raw

water levels. Based on stoichiometry of alum-coagulation

mechanism it can be stated that when DOC increases (Alum/

DOC ratio 57) insu�cient alum addition leads to incomplete

coagulation resulting in colloidal material mostly consisting of

organic-aluminium complexes in particulate form. Hence par-

ticulate aluminium increased in treated water thereby increas-

ing total aluminium in treated water.

Temperature of the water also in¯uences this result. It can be

seen (from the four black boxes of Fig. 10) that the time frame

in which high ®nished water residual aluminium occurred was

late autumn and extended up to the end of winter (approxi-

Fig. 10 Seasonal variations of total aluminium (raw and ®nished water) and alum/DOC ratio from 1991 to 1998.

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mately) in a year. The average water temperature at plant for

this period was close to 4 8C. E�ective coagulation of alum is

retarded at low temperatures because of slower hydrolysis

reaction and consequently lighter ¯oc formation [5]. Light

alum ¯ocs would escape during ®ltration process eventually

giving rise to an increase in total aluminium in treated water.

Treated water particulate aluminium and treated water

turbidity for the year 1996 is presented for each season namely

winter, spring/summer and autumn in Fig. 13. It can be seen in

Fig. 13 that during winter 1996 treated water particulate

aluminium correlated very poorly (R2=0.35; the correlation

coe�cient was found to be statistically signi®cant (t-test) at

97% con®dence level) with treated water turbidity and the

correlation improved for the rest of the year (R2=0.60 for

spring/summer; the correlation coe�cient was found to be

statistically signi®cant (t-test) at 99% con®dence level and

R2=0.76 for autumn; the correlation coe�cient was found to

be statistically signi®cant (t-test) at 99% con®dence level). The

disproportionate increase in treated water total aluminium

compared to treated water turbidity during low (57) alum/

DOC ratios at colder months is re¯ected by a poor correlation

between particulate aluminium and turbidity of treated water.

Formation of organic aluminium complexes that may be a

reason for high residual aluminium in treated waters of

BPWTP during low alum dosages is supported in literature.

Jekel & Heinzmann (1989) [6] reported that dissolved organic

substances at moderate to elevated concentrations strongly

interfere with alum coagulation process and if the alum dosage

is low, as compared to the DOC value, very high residuals (in

the form of organic complexes) can be found in ®ltered water.

They found that the critical ratios of Al3+:DOC were below

20 mmol Al3+/g DOC for water treatment plant conditions

Fig. 11 Changes in alum/DOC, Al(P) ±

raw, Al(D) ± raw, Al(P) ± treated and Al(D)

treated (on logarithmic scale) for some

selected months of 1996 and 1997.

Fig. 12 Changes in raw water (turbidity),

raw water (total suspended solids), treated

water (turbidity) and treated water (total

suspended solids) for the conditions of

Fig. 11.

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encountered in their study. The critical value of Alum/DOC

ratio was 7 for the BPWTP; this value will vary from plant to

plant depending on raw water characteristics.

In spite of the fact that an increase in treated water total

aluminium was not re¯ected in turbidity increase for the

periods of low alum/DOC ratio (57); monthly average treated

water total aluminium correlated well with the percentage

turbidity removal for both 1996 and 1997, respectively.

Treated water (%) turbidity removal and total aluminium

(treated water) are correlated by a second order polynomial

equation for the year 1996 (y=2.25x27 420x+19657; where

y=treated water total aluminium (mg/L); and x=% treated

water turbidity removal NTU with R2=0.61; the correlation

coe�cient was found to be statistically signi®cant (t-test) at

99% con®dence level) and for 1997, the data ®t a second order

polynomial equation (y=70.3x2+41x7 938; where

y=treated water total aluminium (mg/L); and x=% treated

water turbidity removal NTU with R2=0.91; the correlation

coe�cient was found to be statistically signi®cant (t-test) at

99% con®dence level; Fig. 14). It can be seen from Fig. 14 that

to limit treated water total aluminium at 50 (mg/L), at least 90±95% treated water turbidity removal is required. This implies

that in addition to matched alum dose in response to changes in

dissolved organic carbon, turbidity control is also essential for

minimising residual aluminium at BPWTP.

Short-lived aluminium peaks during the start of GAC units

When freshly regenerated activated carbon contactors were

placed in service at BPWTP, short-lived periods of high

dissolved aluminium occurred in ®ltered (treated) water (Fig.

10). Elevated pH levels (48.5) were recorded by plant opera-

tors during the times of high dissolved aluminium levels. It can

be hypothesized that the presence of alkaline metal (calcium

and magnesium) oxides in regenerated GAC shifts the pH of

®ltered water to alkaline range and it is known that aluminium

is highly soluble in alkaline pHs due to the formation of

aluminate Al(OH)47 anion.

Fig. 13 Treated water particulate

aluminium vs. treated water turbidity

(NTU) for the winter, spring/summer and

autumn of 1996.

Fig. 14 Percentage treated water turbidity vs. treated water total

aluminium (mg/L) for 1996 and 1997.

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# 1999 IWSA, J Water SRTÐAqua 48, 167±175

CONCLUSIONS

Analysis of BPWTP data showed that aluminium concentra-

tion is a dynamic parameter in treated drinking water and can

change rapidly with changes in raw water quality or with plant

upsets or operational changes. Analysis of BPWTP data for

two years showed that much of the raw water total aluminium

was in particulate form. Raw water particulate aluminium

correlated well (r2=91) with raw water total suspended solids

indicating that much of the raw water particulate aluminium

may be derived from the suspended solids present in raw water.

A similar analysis showed that much of the treated water

aluminium was also in particulate form. Regression analysis

indicated that to keep treated water total aluminium levels

below 50 (mg/L), an average turbidity removal of 90±95%

would be needed. One of the reasons for low dissolved (raw

and treated water) aluminium concentrations was that both

raw and treated water pHs were in the range of 7.2±8.0, at

which the solubility of aluminium is also a minimum.

Analysis of eight-year data showed that dissolved organic

carbon present in the raw water played a major role in

controlling e�cacy of alum coagulation at BPWTP. The data

showed that when alum/DOC was less than 7, insu�cient alum

addition led to incomplete coagulation resulting in colloidal

material mostly organic-aluminium in particulate form in the

treated water. Hence particulate aluminium increased in treated

water after treatment in the treated water. But this increase in

particulate aluminium did not increase the turbidity of treated

water.

The plant data also showed that when freshly regenerated

GAC contactors were used, peaks of dissolved aluminium

occurred as a result of alkaline metal (calcium and magnesium)

oxides present in the regenerated GAC that shifted the pH of

®ltered water to alkaline range with consequent formation of

soluble aluminium species Al(OH)47. Overall the analysis of

operating data of BPWTP showed that: (i) monthly average

dissolved aluminium (treated water) levels were less than 50 mg/L; and (ii) elevated treated water particulate aluminium levels

occurred when alum/DOC was less than 7. An adequate alum

dose in response to dissolved organic carbon change was

important in minimising residual aluminium in treated water

of BPWTP.

ACKNOWLEDGEMENTS

The ®rst author thanks the Faculty of Graduate Studies &

Research for partial ®nancial support. The authors would like

to thank Mr B. Kardash, Senior Laboratory Chemist of

BPWTP, for his help in data analysis.

BIBLIOGRAPHY

1 Barnett P, Skonstad MW,Miller KJ. Chemical characteristics of

a public water supply. JAWWA 1968; 61(1): 61±67.

2 Miller RG, Kop¯er FC, Ketly KC, Stober JA, Ulmer NS. The

occurrence of aluminium in drinking water. JAWWA 1984;

76(1): 84±91.

3 Driscoll CT, Letterman RD. Chemistry and fate of Al III in

treated drinking water. J Envir Engrg 1988; 114(1): 21±37.

4 Sollars CJ, Bragg S, Simpson AM, Perry R. Aluminium in

European drinking water. Environ Techn Lett 1989; 10: 131±150.

5 Ruehl KE. Cold water coagulation case studiesÐfull scale

evaluations. Proceedings of the Water Quality Technology Con-

ference, AWWA, Denver, CO, 9±12 November 1997.

6 Jekel MR, Heinzmann B. Residual aluminium in drinking water

treatment. J Water SRTÐAqua 1989; 38: 282±288.

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