Water Fluoridation Legislation
Introduction
The author analyzes water legislation and measurement of the fluoride level in water. The study was carried out in South Africa. The dosing tolerances established by the water fluoridation legislation seem to have been violated at some instances by water providers. This is the reason why the author used inter-laboratory data to compare the level of fluoride in water that is manageable. The following observations were made by the researcher. First, to measure the natural sample of water was more difficult as compared to synthetic samples. Therefore, water that is extracted from natural sources had more fluoride level than water sourced out from artificial sources. Second, errors in measuring natural water have not been minimized by the advancement in technology over the last 25 years. Thus, this article is concerned with the issues of measurement of fluoride level in water, as well as the regulations and strict measures that would ensure the reduction of fluoride level in natural water.
The Author’s Intended Audience
The author audiences are the water providers and analytical chemists who are yet to comply with South African Regulations. Thus, water distributed to the population contains fluoride that exceeds the 0.2 mg/l mark. This poses danger to the lives of people that in turn may result in sanctions by the regulatory body. At the same time, the operators are liable to the public through provision of clean and healthy water. If this is overlooked, then, the population will suffer from teeth decay, bone weakness and other forms of diseases connected with high intake of fluoride.
Relevance to the chosen legislation. The article is relevant to Water Fluoridation Legislation which denotes that water providers should ensure that the fluoride level is 0.2mg/l or less. There are errors in measuring the concentration of fluoride. The tools used have not shown any improvement in reducing the errors. Therefore, volume of water distributed to the community fails to meet the set standards and this poses health risk to the society. Data recorded by South African bureau of standard (SABS), Division of water technology (DWT) and Medical University of South Africa (MEDUNSA) were extremely helpful in understanding the processes through which water is measured. However, the institutions expressed some alterations or errors when it came to the measuring of fluoride concentration. At the same time, the implementation process fails to mitigate the risks associated with the measuring processes. Mostly, other chemicals interfere with the measurement of fluoride. This means that before measuring samples, an analysis should be conducted to ensure that the sample contains only fluoride. By doing this, the process will be free of errors.
Medical
University of Southern Africa (MEDUNSA)
Comment on author’s use of theories, models and references. The author utilizes colorimetric methods and ion chromatography (IC) for the measurement of fluoride in water samples. These methods are widely used in the fluoridation process all over the world. However, they have loopholes in terms of chemical interferences that affect fluoride concentration. This means that new and destructive methods need to be put into place to oversee the measuring of water. Information is derived from credible sources that deal with the regulation of water. They include the Data recorded by South African bureau of standard (SABS) and Division of water technology (DWT).
Implications and recommendations of the article. The author recommends that sophisticated monitoring and dosing equipment tools be applied. At the same time, a set of practical guidelines should be established by the South African water industry in order to reduce errors and keep fluoride level adequate. The implication of the article is that the regulation body needs to re-evaluate its stand and regulations that control the water industry.
References
Haarhoff, J. The accuracy of fluoride measurement in water and its implications for water fluoridation: rapid communication. Water SA. 2003, 29(2):219-224. Retrieved from: http://www.ajol.info/index.php/wsa/article/download/4859/12595
The accuracy of fluoride measurement in water and its
implications for water fluoridation
Johannes Haarhoff
Rand Afrikaans University, PO Box 524, Auckland Park 2006, South Africa
Abstract
The accurate measurement of the fluoride concentration in water is an essential prerequisite to stay within the allowable dosing
tolerances required by the South African water fluoridation legislation. In the absence of reliable error estimates for fluoride
measurement in natural water samples, a study was conducted utilising data from interlaboratory comparison studies conducted
by the CSIR, the SABS and MEDUNSA. This study shows that:
- natural samples are more difficult to measure than synthetic samples;
- technology advances over the last 25 years did not reduce the measurement error significantly;
- all analytical methods suffer to some extent when natural samples are analysed; and
- the measurement error will have to be appreciably reduced if the legal requirements of the pending water fluoridation are
to be met.
Keywords: Water fluoridation, fluoride measurement, accuracy, measurement error, natural samples, synthetic
samples
Introduction
South Africa is poised for the mandatory fluoridation of drinking
water up to a general target concentration of 0.7 mg F/l. This often
emotional issue had been debated during years of preparation of
draft legislation, until the approval of the final regulations by
Parliament and their promulgation in September 2001. The atten-
tion of the water industry has therefore moved to the many smaller
but important technical issues of practical implementation. One of
the obvious keystones of water fluoridation is the ability to measure
the fluoride concentration in water – firstly for determining how
much supplementation is required, and secondly for checking that
the target is reached within the allowable tolerances. This paper
thus directs its attention to the measurement of fluoride in water; a
vital, difficult aspect of water fluoridation which has seemingly
been overlooked.
Dosing accuracy legally required in SA
The SA Regulations (Government Gazette, 2001) are explicit in
their requirements for dosing accuracy, thereby closely following
similar UK regulations. The first stipulation is:
- Instantaneously measured fluoride concentration must be within
0.2 mg/l of the target.
The fluoridation plant operator therefore has to measure the fluo-
ride concentration in the raw water; measure the water flow rate;
calculate the fluoride dosing rate to make up the shortfall towards
the target concentration; and check the procedure by measuring the
fluoride concentration in the fluoridated water. For the purposes of
this paper, assume that there are no calculation errors, that the water
flow rate and the fluoride solution strength in the day tank are
exactly known, and that the fluoride dosing pump can be set with
absolute accuracy. The only error to consider is therefore the error
due to the measurement of the fluoride concentration before and
after dosing.
It is obvious that any measurement error will be compounded
during the required feedback loop. This is illustrated by the
following example, where a constant under-measurement error of
30% is assumed for illustration:
- The raw water fluoride concentration is actually 0.20 mg/l, but
measured as 0.14 mg/l. The shortfall is therefore actually 0.50
mg/l below the target level of 0.70 mg/l, but perceived by the
operator to be 0.56 mg/l.
- The dosing pump is set to add 0.56 mg/l, resulting in an actual
concentration of 0.76 mg/l or 0.06 mg/l above the target. The
operator measures this as 0.53 mg/l and still perceives a short-
fall of 0.17 mg/l.
- The operator, correctly, increases the dosing rate by 0.17 mg/l
to get an actual concentration of 0.93 mg/l, which is still
perceived as 0.65 mg/l or 0.05 mg/l short of the target.
- This cycle continues until the concentration stabilises at the
perceived target concentration. By this time, the actual concen-
tration is about 1.00 mg/l, or about a 40% overdose.
As an apparent independent check on the dosing accuracy, the SA
Regulations further stipulate that:
- Average dosage over a week must be within 0.1 mg/l of the
target.
The fluoridation operator should check this requirement by a
careful site inventory of on-site fluoridation chemicals and the
volume of water fluoridated since the previous check. (It should
!+27 11 489 2148; fax: +27 11 489 2148; e-mail: jh@ing.rau.ac.za
Received 3 March 2003.
ISSN 0378-4738 = Water SA Vol. 29 No. 2 April 2003220 Available on website http://www.wrc.org.za
certainly not be checked by mathematical averaging of the spot
measurements taken during the week, as this would not detect an
error such as demonstrated above!). An inventory check will
provide an external check on how much fluoride was dosed. To
then obtain the average concentration in the fluoridated water, the
fluoride dosed has to be added to the concentration originally
present, which can be obtained in no other way than taking the
volume-weighted average of the raw water concentrations meas-
ured during the week. It is clear that an inventory check is still
dependent on the accuracy of measuring the fluoride concentration
in the raw water. Using the same example as above where the raw
water concentration is undermeasured by 0.60 mg/l, the target will
therefore also be missed by 0.60 mg/l without knowing.
This line of reasoning underscores the vital importance of
measuring the fluoride concentration accurately. For the proper
estimation of risk, it is essential to get a quantitative grip on the
practical limits of the error of fluoride measurement. Despite
numerous enquiries by the author, no comprehensive or convinc-
ing information could be readily obtained. As the author is neither
an analytical chemist nor a statistician, it is hoped that this cursory
analysis will stimulate further, more comprehensive consideration
of this question by others.
Data sources and exclusion criteria
The Division of Water Technology (DWT) of the CSIR ran an
interlaboratory comparison programme from the middle of the
1970s to 1998. A wide variety of sample types and analytical
methods were covered for water, sewage, sediments and sludges.
As part of this programme, samples for fluoride analysis were
distributed on eight occasions, comprising both synthetic samples
(made up of distilled water with reagent-grade chemicals) and
natural samples (surface water, effluent and groundwater). In total,
21 samples were sent out between 1977 and 1998, with an average
of 35 laboratories participating in each survey. A total of 725 data
points were collected. The DWT followed a consistent approach
of removing all the outliers with a statistical procedure before
reporting. For the study reported here, all the original data points,
outliers included, were retrieved from old files and recaptured
electronically.
The South African Bureau of Standards (SABS) Test House
runs an ongoing WATERCHECK programme which covers more
than 70 analytical laboratories in Southern Africa. Fluoride is part
of a group of determinants which is measured every third month on
three samples (one synthetic and two natural samples). The latest
annual data set for 2002 was retrieved, during which a total of 631
fluoride values were returned from 55 laboratories. No outliers are
removed in the reporting process adopted by the SABS, but a
socalled robust statistical evaluation is used to reduce the effect of
outliers. For this study, all the original values, outliers included,
were recaptured electronically.
A small once-off survey was conducted by the Medical
University of Southern Africa (MEDUNSA) during 2002. Five
samples of tap water, spiked to varying degrees with fluoride
compounds, were sent to 10 commercial laboratories in the Gauteng
area for fluoride analysis, with a clear note that it formed part of an
interlaboratory comparison. This data set, in its entirety, was
included in this study.
In an operational environment, the operator has to react to each
measurement as it is taken. Outliers, on the other hand, can only be
detected and rejected after some time, when a retrospective statis-
tical analysis becomes possible. For this reason, all data points were
critically considered for this analysis. Six complete sample sets
were excluded on the basis of:
- Two samples were spiked to more than 20 mg/l, which is way
outside the range where water fluoridation plants will ever
operate.
- One laboratory returned values for a set of four samples where
every single value was deemed to be an outlier by the DWT
procedure, which indicated a gross, systematic error which
warranted exclusion from the data set.
A number of individual data points were removed next, as they
were obviously extreme outliers:
- Individual measurements which were less than 20% of the
average concentration were excluded. This eliminated 12 of the
data points.
- Individual measurements which were more than five times the
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9
measured F / true F
fraction of data points
synthetic samples
natural samples
Figure 1
Histogram of data
points for synthetic
and natural samples
respectively
ISSN 0378-4738 = Water SA Vol. 29 No. 2 April 2003 221
Available on website http://www.wrc.org.za
average concentration were excluded. This eliminated 31 of the
data points.
This left a total of 1 278 data points, including 36 samples over
25 years, covering a range of average concentrations between
0.07 mg/l and 4.37 mg/l. Eighty per cent of the samples had
average concentrations of between 0.2 mg/l and 2.1 mg/l.
In synthetic samples, the “true” fluoride concentration is
known from the way the samples are made up. In some cases, they
were reported, but they were not used for this study. For the
purposes of this study, the “true” fluoride concentration of each
sample was taken as the arithmetic mean of all the measurements
reported for that sample, after removal of the outliers as described
above. All the data points were then finally normalised by dividing
each value by the “true” value for that sample.
Data analysis
Natural vs. synthetic samples
As with most analytical procedures, care has to be taken to
eliminate interferences by background chemical compounds. Fluo-
ride, to name but one example, will form complexes with alu-
minium which will affect both aluminium and fluoride measure-
ments if no precautions such as preliminary distillation or digestion
are taken. During the DWT programme, the consistency of fluoride
measurements generally suffered from a high percentage of outliers
attributed to not adequately pretreating the samples.
In natural samples, as opposed to synthetic samples, there are
many more unknowns that could affect the fluoride measurements.
The hypothesis then has to be that accuracy of fluoride measure-
ments in natural samples has to be poorer than in synthetic samples.
The data set was therefore split into a ”natural” group (747 data
points from 21 samples) and a “synthetic” group (525 data points
from 15 samples). Figure 1 shows the distribution of the data in
histogram form. Two points emerge from Fig. 1:
- The synthetic samples are more narrowly distributed, as hy-
pothesised above. The remainder of this analysis will thus be
carried through by keeping natural and synthetic data separate.
- There is a definite asymmetry in the distribution, especially
evident for the natural samples. This is due to the fact that the
distribution is constrained on the left by a zero concentration
limit. The normal distribution is therefore not adequate for
describing fluoridation accuracy; the log-normal distribution
would be a more suitable candidate should someone wish to
pursue the modelling of the measurement error.
“Old” vs. “new” technology
The DWT data set spans the period 1977 to 1998, therefore
including data collected by what many would consider to be “old”
technology. The SABS and MEDUNSA data sets were collected
during 2002 and are a reflection of the current state of technology
used in South African laboratories. An obvious question is whether
the much-touted advances in instrument technology led to a signifi-
cant, if any reduction in measurement error.
The data set was next split by both sample type (natural and
synthetic) and data set (DWT, SABS and MEDUNSA), yielding
the following number of samples per subset:
- DWT synthetic 314 data points from 11 samples
- DWT natural 297 data points from 8 samples
- SABS synthetic 209 data points from 4 samples
- SABS natural 402 data points from 8 samples
- MEDUNSA natural 50 data points from 5 samples
These data were used to construct cumulative distribution lines,
which are shown in Fig. 2. The following points emerge from
Fig. 2:
- The difference between natural and synthetic samples observed
earlier, remains strongly evident for the individual data sets.
- There is very little, if any, difference between the “old” data set
of the DWT and the “new” data set of the SABS. This is evident
from a comparison of both the synthetic and natural sample
lines. Advances in instrument technology have made no
appreciable difference to the empirically observed measure-
ment accuracy.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
measured F / true F
fraction less than
DWT synthetic
DWT natural
SABS synthetic
SABS natural
MEDUNSA natural
Figure 2
Cumulative distribution
plot of fluoride
measurement error,
showing difference
between data sets and
sample types
ISSN 0378-4738 = Water SA Vol. 29 No. 2 April 2003222 Available on website http://www.wrc.org.za
- The MEDUNSA data set does not clearly follow either the
natural or the synthetic data. This is understood if the nature of
the MEDUNSA samples is explained. One sample was a tap
water sample with an average concentration of 0.07 mg/l. The
other four samples of the set were made up of the same tap
water, progressively spiked to concentrations of 0.19 mg/l,
0.45 mg/l, 0.87 mg/l and 1.18 mg/l respectively. The samples
themselves could therefore be considered to be progressively
more “synthetic” than natural.
Comparison of analytical methods
Fluoride in water can be measured in a number of ways. There are
colorimetric methods (of which the SPADNS method is the most
commonly used), the ion-selective electrode (ISE) method, and ion
chromatography (IC), all comprehensively described in Standard
Methods (1985). For routine measurement on site, the practical
choice is between SPADNS (only an option when spot analysis is
opted for rather than continuous monitoring) or ISE (suitable for
both spot and continuous measurement). During the course of the
DWT study, it seemed that the SPADNS method was more suscep-
tible to these interferences and it was suggested to SPADNS users
that they should consider switching to the ISE technique.
The SABS and MEDUNSA data sets do not have information
on the methods used by the participating laboratories, but the DWT
data set included such information between 1977 and 1990, which
includes 35% of all the data considered in this study. The method-
specific data points break down to:
- Natural samples measured colorimetrically 47 data points
- Synthetic samples measured colorimetrically 76 data points
- Natural samples measured by ISE 95 data points
- Synthetic samples measured by ISE 148 data points
- Natural samples measured by IC 33 data points
- Synthetic samples measured by IC 44 data points
The cumulative distribution plots for these six groups are shown in
Fig. 3. The following points emerge from Fig. 3:
- For synthetic samples, there is little difference amongst the
three methods. All follow the same narrow distribution seen in
the earlier analyses.
- For natural samples, all the methods show significantly poorer
accuracy than for synthetic samples. The effect of interfering
compounds cannot be eliminated by simply switching the
analytical method.
- The colorimetric method returns higher concentration values
than the ISE and IC methods.
- The ISE and IC methods return comparable results for natural
samples.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
measured F / true F
fraction less than
colorimetric – syntheti c
colorimet ric – natural
ISE – synthetic
ISE – natural
IC – synthetic
IC – natural
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
measured F / true F
fraction less than
synthetic samples
natural samples
Figure 3
Cumulative distribution plot
of fluoride measurement
error, showing difference
between analytical methods
and sample types
Figure 4
Cumulative distribution plot
of fluoride measurement
error, showing difference
between natural and
synthetic samples
ISSN 0378-4738 = Water SA Vol. 29 No. 2 April 2003 223
Available on website http://www.wrc.org.za
Likely fluoride measurement accuracy
Analytical laboratories commonly use the “standard deviation
approach” as a scoring criterium during interlaboratory compari-
son studies. Both the DWT and the SABS programmes consider
results within 1 standard deviation from the mean as “good”,
between 1 and 2 deviations away as “average” and further than
2 standard deviations as “poor”. For normal distributions (nor-
mally implicitly or explicitly assumed), this translates into a
criterium that 68% of all measurements would be “good” and a
further 27% of all measurements would be “average”. A more
conservative, arbitrary assumption of only 50% “good” values is
adopted for the purposes of this paper – an assumption of the author.
Figure 4 shows the cumulative distribution plot of the entire
pooled data set, split between natural and synthetic samples. The
25th and 75th percentile lines from Fig. 4 provide error estimates of
the measurement error that could reasonably be expected from
“good” laboratories. These values are shown in Table 1.
Questions pertaining to pending water
fluoridation
Does the performance of analytical laboratories give a
reliable indication of measurement performance at an
operational water fluoridation plant?
This is a crucial question which cannot yet be answered. Intui-
tively, there are reasons why analytical laboratories should do
better than operational plants:
- Recognised analytical laboratories that care to participate in
nation-wide comparisons invariably have trained chemists
working under professional supervision and clean, controlled
laboratory conditions.
- It is only human that each participating laboratory will put its
best possible foot forward at the time of the comparative
analyses. It is difficult if not impossible to sustain the same
diligence over the many years of continuous operation of a
water fluoridation plant.
Conversely, it can be argued that operational plants should do
better:
- The interlaboratory comparisons normally request a suite of
analyses of which fluoride forms only a small part. A control
laboratory dedicated to a single compound may be better
focused and equipped to produce better results.
- Many of the participating laboratories do not specialise in
water analyses, but probably in some other areas, for example
air, food, coal, soils, etc..
Were the samples of this study representative of
general water treatment conditions?
Those samples with unrealistically high concentrations were ex-
cluded from the analysis. The data from synthetic samples were
analysed separately from the data from natural samples. A small
number of borehole and sewage effluent samples remained in the
data set, but in practice these sources could also be encountered. In
general, the synthetic samples were spiked to F concentrations
above 1 mg/l, while most natural samples had concentrations less
than 1 mg/l. The very low concentrations of say less than 0.3
mg/l were poorly represented in the data set and it seemed from the
few available data points as if the variability of their results was
greater.
In other words, the data sets were reasonably representative
of actual water sources, but further studies have to focus specifi-
cally on the measurement of concentrations at or below say 0.5
mg/l, as this will be the critical region for water fluoridation plant
operation.
Does the uncertainty around fluoride measurement
present an obstacle to water fluoridation?
If nothing is done to improve measurement accuracy, or to demon-
strate with a more comprehensive study that the problem is smaller
than it now seems, then it would. In the example at the beginning
of the paper, it was shown that undermeasurement of 30% would
make it extremely difficult to stay within the allowed dosing
tolerances. But the author has no doubt that the South African
water industry, internationally known for decades for its drive and
innovation, has adequate expertise and resources to successfully
resolve this problem, provided its leadership shows the necessary
will.
Is fluoride measurement error the only significant
source of error?
The only systematic study of total fluoride dosing error known to
the author has recently been published (Lalumandier et al., 2001).
Operators at 1 280 fluoridation plants in 12 US states responded,
amongst others, to a question of how close they could maintain the
fluoride dosing level to the optimal level. This effectively means
that operators had to assess themselves without any external
controls, which would probably lead to optimistic estimates. The
results, briefly summarised, were:
- A total of 25.9% responded that they could maintain the
concentration within 0.1 mg/l of the optimal level; 49.3%
could maintain it between 0.1 and 0.2 mg/l from the optimal
level and 19.5% could stay between 0.2 to 0.3 mg/l of the
optimal level.
- For the large plants (above a capacity of about 4 Ml/d) the
performance was significantly better, with 33.5% of the large
plants maintaining the dosage within 0.1 mg/l of the optimal,
as opposed to 21.3% for the small plants.
- The two main causes for the dosing error were reported to be
problems with feeding equipment (18.4%) and variations in
raw water flow (12.8%).
- Poor training of operators was blamed for the dosing error in
8.7% of the responses.
These survey results are interpreted as follows:
- Most US treatment plants would not be able to stay within the
tolerances allowed by the SA Regulations.
TABLE 1
Fluoride measurement limits based on
50% of all measurements being acceptable
Lower limit Upper limit
Synthetic samples -14% +11%
Natural samples -39% +17%
ISSN 0378-4738 = Water SA Vol. 29 No. 2 April 2003224 Available on website http://www.wrc.org.za
- Measurement error (more difficult to detect) is not amongst the
top two reasons for dosing error, indicating that the allowable
measurement error should be set at a significantly lower level
than the allowable total dosing error.
- Proper training of operators provides one way to improve
dosing performance.
Closure
This paper presents more questions than answers. It is primarily
intended to alert those water chemists and operators grappling with
the pending implementation of water fluoridation in South Africa
to the uncertainties surrounding the measurement of fluoride in
water. It was shown that very good accuracy for fluoride measure-
ment is a prerequisite for staying within the narrow dosing toler-
ances allowed by the SA Regulations.
Water providers are currently considering their technological
options for expensive, sophisticated dosing and monitoring equip-
ment for the pending fluoridation. Now is the time to undertake an
urgent, systematic study of the fluoridation measurement options,
pertaining to our typical source waters with their own unique
potential for analytical interference. In order to get water fluorida-
tion right, the SA water industry needs a set of practical, locally
relevant guidelines for fluoride measurement, which can only be
provided through the collaboration of the water chemists and
process controllers in water treatment practice.
Acknowledgments
The CSIR (Dr. Carlsson and Ms. Elizabeth Truter of Watertek), the
SABS (Mr. Chris Fouche and Ms. Marie McKay of the
WATERCHECK programme) and MEDUNSA (Prof. JB du Plessis)
generously shared the data reported on in this paper. Prof. Paul
Coetzee of the Department of Chemistry at the Rand Afrikaans
University provided better perspective on the analytical methods
and their susceptability to interferences.
References
DEPARTMENT OF HEALTH (2000) Regulation No. 873 Regulations
under the Health Act, 1977 (Act No. 63 of 1977) Regulations on
Fluoridating Water Supplies. Government Gazette, 8 September 2000,
Vol. 423, No. 21533, Pretoria.
LALUMANDIER JA, HERNANDEZ LC, LOCCI AB and REEVES TG
(2001) US drinking water: Fluoridation knowledge level of water
plant operators. J. Public Health Dent. 61 (2).
STANDARD METHODS (1985) Standard Methods for the Examination
of Water and Wastewater (16th edn.) Published jointly by the Ameri-
can Public Health Association, the American Water Works Associa-
tion and the Water Pollution Control Federation, Washington DC