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© 2006 International Centre for
Diarrhoeal Disease Research, Bangladesh
J HEALTH POPUL NUTR 2006 Sep;24(3):267-272
ISSN 1606-0997
$ 5.00+0.20
REVIEW ARTICLE
Review of Coagulation Technology for
Removal of Arsenic: Case of Chile
Ana María Sancha
Department of Civil Engineering, Division of Water Resources and Environment,
University of Chile, Blanco Encalada 2002, Santiago, Chile
ABSTRACT
Coagulation technology has been used since 1970 in northern Chile for removing arsenic from drinkingwater. This experience suggests that coagulation is an effective technology for the removal of arsenic.
It is currently possible to reduce arsenic from 400 µg/L to 10 µg/L at a rate of 500 L/sec, assuming pH,
oxidizing and coagulation agents are strictly controlled. The Chilean experience with the removal of arsenic demonstrates that the water matrix dictates the selection of the arsenic-removal process. This paper
presents a summary of the process, concepts, and operational considerations for the use of coagulation
technology for removal of arsenic in Chile.
Key words: Arsenic; Arsenic removal; Coagulation technology; Drinking-water; Chile
INTRODUCTION
The presence of hazardous concentrations of arsenic in
drinking-water and the serious health effects this situation is causing to untold hundreds of millions of people
across the planet, have led the World Health Organization (WHO) to recommend that the maximum concentration of arsenic in human drinking-water not exceed
10 µg/L. Researchers in many countries are studying to
identify the most feasible technologies for the removal
of arsenic in their particular situations. Some removal
systems recommended in the international water market involve advanced or emerging technologies which
generally require extensive pre-treatment processes
and/or very high construction, operation and maintenance costs. For many affected populations, neither
they nor their governments are able to afford such expensive investments in infrastructure.
Chile, a small but emerging nation with significant
arsenic exposure through water, has faced the challenge
of removal of arsenic with large-scale water-treatment
Correspondence and reprint requests should be addressed to:
Prof. Ana María Sancha
Department of Civil Engineering
University of Chile
Blanco Encalada 2002
Santiago
Chile
Email: [email protected]
Fax: 56 2 6894171
plants since the 1970s and has developed a strategy using the conventional technology which is both very effective and relatively inexpensive to build, operate,
and maintain.
This study presents a summary of the Chilean experience in removal of arsenic from water, including an
overview of the problems and variables involved and a
discussion of our investigations and results at the level
of large-scale water-treatment facilities.
BACKGROUND
Chile, located along a 4,320-km strip in southwestern
South America (Fig. 1), with a population of approximately 15,400,000, has extensive experience in the
removal of arsenic from drinking-water supplies (1,2).
Due to the particular geological characteristics of Chile
and its intensive mining activity, many water sources in
the northernmost area and central zones of the country
are contaminated with arsenic (3).
In the late 1960s, it became evident that the consumption of water from the Toconce River—with
concentrations of arsenic in the range of 600-900 µg/
L—was causing serious problems for residents of the
northern zone (4). The Chilean Government commissioned a study by German researchers from Berkefeld
Filter concerning the removal of this contaminant from
drinking-water. Working together with Chilean colleagues, these researchers ascertained the parameters
required to remove arsenic from water by means of
268
J Health Popul Nutr Sep 2006
Sancha AM
Fig 1. Name and location of water-treatment plants for removal of arsenic, Chile
Chuquicamata
Chuquicamata
Cerro
Topater
Calama
Chile
Salar Del
Carmen
Antofogasta
coagulation (5). The first plant was built and began operations in 1970. Four arsenic-removal plants (Table 1)
rently investigating the coagulation process, and their
results indicate that further improvements are viable
Table 1. Water-treatment utilities for removal of arsenic in Chile. Chuquicamata is owned by a mining
company and has limited availability of public data
Arsenic range (µg/L)
Water source
Utility
Capacity (L/sec)
Salardel Carmen Complex*
Old plant (1970)
New plant (1978)
500
520
Cerro Topater* (1978)
500
Chuquicamata* (1989)
210
Taltal** (1998)
32
Toconce
Lequena
Quinchamale
Siloli Polapi
Toconce
Lequena
Quinchamale
Colana
Inacaliri
Agua Verde
600-900
150-350
100-250
<50
600-900
150-350
140-250
70-90
80-90
60-80
*Surface water; **Groundwater
that use the same process have been built in the northern zone in subsequent years (6-9).
Experience has confirmed that the coagulation
process is a good device for both quality of water
(Table 2) and volumes of water to be treated. Recently, the WHO identified the coagulation process as being the most appropriate technology to remove arsenic
in large volumes (10). Numerous researchers are cur-
(11-16). Some new knowledge became apparent during
Chile’s long experience with full-scale arsenic-removal
treatment plants.
The Chilean drinking-water standard permitted a
maximum arsenic concentration of 50 µg/L until 2004.
Currently, the Chilean drinking-water standard has been
modified to reach a goal of 30 µg/L in 2010 and 10 µg/
L in 2015 (17). The WHO recommended that water for
269
Coagulation technology for arsenic removal
Table 2. Quality of water in the northern zone of Chile. The process only removes arsenic. Other
parameters are essentially the same in the effluent
Surface water (range) Groundwater (range)
Unit
Parameter
PH
Total disolved solids
Arsenic
Sulphate
Chloride
Alkalinity
Hardness
Silica
Boron
Disolved organic carbon
8.0-8.4
700-800
400-600
80-100
120-140
100-120
130-150
20-30
3-4
Negligible
mg/L
µg/L
mg/L
mg/L
mg/L CaCO3
mg/L CaCO3
mg/L SiO2
mg/L
mg/L
human consumption should not contain more than 10
µg/L (18). Currently, 99.98% of the Chilean population
have access to potable water with arsenic <50 µg/L, but
only 52.69% have access to potable water with arsenic
<10 µg/L. To meet the new Chilean standard and the
WHO guidelines, Chile will need to treat significantly
more water for removal of arsenic in other zones of the
country.
REMOVAL OF ARSENIC
The coagulation process consists of the addition of
metal-based coagulant, such as ferric chloride (FeCl3),
to arsenic-contaminated water. FeCl3 hydrolyzes in
water to form positively-charged ferric hydroxide
[Fe(OH)3]. Arsenic must be in oxidized form [As(V)]
for effective removal. Thus, if any arsenite [As(III)] is
present, it may be necessary to oxidize it to As(V) using
chlorine as a pre-treatment process. Arsenate [As(V)] is
a negatively-charged anion and sorbs to the positivelycharged Fe(OH)3 particles or flocs. The sedimentation
and filtration processes then remove arsenic particulate.
A general schematic diagram of the arsenic-removal
treatment process is given in Fig. 2.
7.0-8.0
730-790
60-80
50-60
350-400
20-30
2-5
Negligible
The arsenic-removal system by means of coagulation in
the 1970s delivered water with a residual arsenic concentration of 120 µg/L to the population of the northern Chile. The delivery concentration decreased to 50
µg/L in the 1980s and to as low as 10 µg/L in the 2000s.
This increase in efficiency in the removal of arsenic
has been achieved by improving the treatment-system
follow-up, including control of pH and adjustment of
reagent doses (19). Table 3 lists some principal arsenicremoval conditions at Salar del Carmen, Cerro Topater,
and Taltal.
The coagulation processes are typically used for
removing turbidity. When the same processes are used
for removing arsenic from surface water, the design of
the treatment system should maximize the formation of
a floc with characteristics of size, cohesion, and sedimentation speed that favour stable arsenic adsorption
onto it. In this way, arsenic changes from a dissolved
species into a particulate species that can be separated
or removed from water by means of sedimentation
and filtration. In the case of groundwater, the removal
process often includes only oxidation, coagulation, adsorption, and filtration. Regardless of the method of re-
Fig 2. General schematic Chillean arsenic-removal treatment process: (a) Surface water and (b) Groundwater
(a)
H2SO4
Cl2
Raw
water
Pre-oxidation
Arsenic
400 µg/L
(b)
H2SO4
Cl2
Raw
water
Pre-oxidation
Arsenic
70 µg/L
FeCl3
Coagulation
absorption
Cl2
Sedimentation
FeCl3
Coagulation
absorption
Filtration
Cl2
Filtration
Finished
Post-oxidation water
disinfection Arsenic
10 µg/L
Finished
Post-oxidation water
disinfection Arsenic
10 µg/L
270
J Health Popul Nutr Sep 2006
Sancha AM
Table 3. Arsenic-removal conditions: Salar Del Carmen, Cerro Topater, and Taltal, Chile
Arsenic-removal conditions
Arsenic in raw water (µg/L)
Chemical dosage
Oxidant (mg/L Cl2)
Coagulant (mg/L FeCl3)
Decantation rate (m3/m2/day)
Filtration rate (m3/m2/day)
Sludge generation (kg/day)
Arsenic in finished water (µg/l)
Salar del Carmen
400
Cerro Topater
400
Taltal
70
1.0
56.1*
70-75
143
25-30
10
1.0
40.5*
70-75
143
20-30
10
1.0
8.0
150
10
*H2SO4 for adjustment of pH
moval, the arsenic-removal process becomes a simple
device for removal of suspended material. Arsenic speciation, pH, coagulant doses, and agitation speed are
important parameters in this process (7,19). Any problems that may originate in the process of floc formation
and separation by sedimentation and/or filtration may
limit the efficiency of removal of arsenic from water.
Recent studies demonstrated that the presence of
hardness in water to be treated could favour removal of
arsenic, but that some anions, especially phosphate,
carbonate, and silicate, may compete with arsenic for
the sorption sites, thus interfering with removal of arsenic (20,21). Chilean water has both hardness and
these competitive anions. The efficiency of the process
is, thus, sensitive to the water matrix in this condition.
Quick and accurate measurement of concentrations
of arsenic in water has also been a fundamental factor
in improving the efficiency of the removal process. It
has always been important to use the analytical method
which has best responded to the requirements of the
control process, to the country’s economic situation,
and to the abilities of its technicians. Initially, the Gutzeit method was used (22), but later, in the 1980s, the
silver diethyldithiocarbamate colorimetric method was
used (23), since the 1990s, hydride generation-atomic
absorption spectrometry has been used (24). This has
allowed more frequent adjustment of the process and
the reduction of the detection limits.
Achieving higher efficiency in the removal of
arsenic from water has involved greater expenditure for
increasing coagulant dosage and dose automation, continuous control of pH, and more frequent monitoring
of concentrations of arsenic. The FeCl3 dose may be
reduced if pre-treatment of pH is applied (19). Greater
investment, operations and maintenance costs also require additional resources to train and maintain teams
of highly-qualified technicians to operate the plants.
Finally, the disposal of arsenical sludge generated during treatment of water has always been and will always
be difficult due to its dangerous characteristics. In the
case of Chile, the problem has been solved by carrying out this process in the desert. During the first years,
this sludge was disposed of without taking any special
precautions. In recent years, the process has taken place
in specially-engineered sites using a geotextil as component of a landfill bottom and capping barrier
systems (25)
The price of drinking-water in Chile is determined,
in part, by the cost of infrastructure for treatment of
water, the chemical reagents used, and the system’s operation and maintenance costs. In Antofagasta, where
arsenic is removed from water, the cost for a family that
consumes 20 m3 of water per month is currently US$
46.48, or 2.3¢ per litre. The cost for a family in Santiago, where arsenic is not removed, amounts to US$
16.18, or about a third of the cost in Antofagasta, for the
same volume of water. These figures reflect partially
the relatively high cost of removal of arsenic in Northern Chile.
FUTURE OF ARSENIC REMOVAL
Chile is making efforts to find cost-effective solutions
to achieve lower levels of residual arsenic concentration
in drinking-water from surface and underground water
supplies. In this new scenario, another possibility that
has been considered, for Northern Chile, is to replace
some current sources of surface water with de-salinized
sea water (26,27). This option would only be applicable
in coastal cities, but not in plants located farther from
the ocean, i.e. in Salar del Carmen Plant (Antofagasta),
but not in Cerro Topater (Calama).
In the central zone of Chile, where some surface
waters have concentrations of arsenic in the range of
14 to 16 µg/L, modifications to the current coagulation
process used for removing turbidity could meet a 10µg/L standard. In the case of groundwater with concen-
Coagulation technology for arsenic removal
trations of arsenic in the range of 20 to 80 µg/L, coagulation-filtration also would be the selected process to
remove arsenic (28). Because of the afore-mentioned
characteristics of water quality, adsorption processes
are inefficient in the removal of arsenic. In addition,
most manufacturers of sorbents do not provide regeneration instructions.
CONCLUSION
The Chilean experience in removal of arsenic demonstrates that the water matrix dictates the selection of the
arsenic-removal process. The coagulation process has
been proven to be an effective arsenic-removal process
for surface and groundwater. Moreover, it has the advantage that it does not typically require excessive pretreatment or conditioning of influents and chemicals
used that are not made in Chile.
The Chilean water industry has gained significant
operational experience in the removal of arsenic by
coagulation and will rely on achieving a residual arsenic concentration of 10 µg/L by coagulation technology through adjustment of pH and control of coagulant
dose.
The Chilean experience in the removal of arsenic at
water-treatment plants demonstrates that the processes
of coagulation/adsorption-sedimentation-filtration can
remove arsenic up to the WHO-recommended standards for drinking-water. These processes do not require
complex pre-treatment of water, but only pre-oxidation
and pH adjustment. This technology for the removal of
arsenic can be simplified by eliminating the sedimentation process if the conditions of water permit.
The inputs of this technology—oxidizing agent, coagulant, filtering medium—can be of local origin, and
the operation of the removal system requires personnel
with an intermediate level of training. The handling and
disposing of the sludge generated must always be done
with special precautions.
ACKNOWLEDGEMENTS
This paper is based on studies supported by Universidad de Chile and FONDEF-CONICYT through Project
FONDEF 2-24. The author is grateful for the many
contributions of her students, whose cooperation made
these studies possible.
271
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