Arsenic Contamination: Searching for Solutions - 1997
by Sylvia Mortoza
Introduction
In Bangladesh today, more than 90 per cent of the population get their
drinking water from the million plus tube-wells that were installed for
the anti-diarrhoeal campaign. This was one of the few success stories in
the public health sector for it helped reduce water-borne diseases. But
tubewells are now dispensing arsenic-laced water, with the result that
many people are showing arsenical skin lesions in the late stages of manifestation
of arsenic toxicity. This came as a shock for no one was prepared for what
could be a disaster - arsenic contamination. The source of arsenic is believed
to be geological and most water samples show a mixture of arsenic and arsenate.
None showed any methylarsonic or dimethylarsenic acid.
Water is one of the major means of transport of arsenic in the environment.
Arsenic in the aquatic environment is predominant in places with high geo-thermal
activity. Soil erosion and agricultural run-offs are also large contributors
to the arsenic concentration in sediments. High arsenic levels have been
reported to be associated with sediments and a potential exists that it
may be released in hazardous amounts to the overlying waters. Industrial
effluents are also a major source of arsenic to the environment. Arsenic
and arsenical compounds are found in effluent from metallurgical industry;
glassware and ceramic industry; dye and pesticide manufacturing industry;
petroleum refining; rare earth industry and other organic and inorganic
chemical industries. It finds an application in the manufacture of herbicides
and pesticides. Other industries using arsenic include wood and hide preservatives;
lead shot manufacture; phosphate detergent builder; and pre-soaks used
in many fertilisers. A chemical factory manufacturing several chemicals
including the insecticide Paris-Green (acetocopper arsenic) was responsible
for the contamination of wells in the southern part of Calcutta, India.
Over 7000 people were consuming the arsenic contaminated water for several
years but this fact remained unnoticed until September 1989. A few died
and some of the victims were hospitalised while symptoms of arsenic poisoning
were evident in many families living in the area.
As arsenic is a cause for skin, liver, lung and kidney or bladder cancer,
it is a big headache for the nation. Due to this carcinogenicity of some
arsenic compounds, the objective should now be to reduce exposure to arsenic
contaminated water to a level as close to zero as possible, taking into
consideration its health effects and toxicology, occurrence and human exposure,
availability and cost of treatment technology, the practical quantitation
limit of arsenic normally found in drinking water.
Addressing the problem
Various treatment methods have been adopted to remove arsenic from drinking
water under both laboratory and field conditions. The major mode of removing
arsenic from water by physical-chemical treatment methods. Various treatment
methods include:
- adsorption-co-precipitation using iron and aluminium salts
- adsorption on activated alumina/activated carbon/activated bauxite
- reverse osmosis
- ion exchange
- oxidation followed by filtration
Scientists and health and water experts have intensified their search
for treatment methodologies and reliable alternative sources of water supply.
The United States Environmental Protection Agency (USEPA) have summarised
coagulation with iron and aluminium salts and lime softening as the most
effective treatment process for removing arsenic from water to meet the
interim primary drinking water regulations standard of 0.05 mg/L [30].
That regulatory agencies are reviewing maximum allowable concentration
levels in drinking water is comforting but, as arsenic contamination of
ground water and the possibility of poisoning is too alarming to wait for
solutions, it is essential to locate alternative sources.
The most logical answer would be to go back to using surface water sources.
But as most of these sources are heavily polluted with bacteria, some simple
method of disinfection is required. Boiling is an effective method of disinfection
but, in Bangladesh, poverty, the difficulty with which fuel can be obtained
as well as the current rate of illiteracy, prevents people from boiling
water.
Solar decontamination
One alternative may possibly be solar decontamination. During the eighties,
Professor Acra and his colleagues at the University of Beirut demonstrated
how a wide range of microbes, including pathogenic bacteria and viruses,
can be inactivated by exposing the contaminated water to sunlight. The
process that was adopted was simple. All that was needed to make the contaminated
water drinkable was to place the water inside a transparent glass container
or a transparent plastic container and place it in the direct sunlight
for two to three hours before drinking.
Although some doubt has been cast on the effectiveness of solar disinfection,
the anti-microbial properties of sunlight have been known for a long time
but it is only recently that solar radiation has been seriously proposed
as a means for decontaminating water. All that is really needed is a reasonably
constant and reliable source of sunlight. Several researchers have successfully
demonstrated that sunlight will destroy much of the faecal bacteria present
in contaminated drinking water but this process is especially effective
if the water contains a sufficient amount of oxygen. An easy way to ensure
this is to oxygenate the water before hand by mixing it with air.
That sufficient oxygen is needed to be effective has also been confirmed
by other researchers in the UK who have shown that the effectiveness of
solar decontamination of water is strongly dependent upon its oxygen status.
Under controlled conditions, tests using water contaminated with either
pure cultures of faecal bacteria, freshly voided faeces or raw sewage,
were exposed to full strength, natural sunlight for several hours causing
a substantial decrease in the bacterial count when the water was fully
oxygenated. De-oxygenated water gave a far slower rate of decrease.
This means that over a period of several hours, sunlight and oxygen
can act together to inactivate faecal bacteria. But the most significant
practical aspect of this kind of solar "photo-oxidative" disinfection
is that the oxygen level of the water must be kept close to maximum value
during the exposure of water to full strength sunlight. Simple experiments
undertaken by the team showed that a clear plastic or glass bottle, three-quarters
filled with de-oxygenated water and capped, shaken vigorously for a couple
of minutes will create air bubbles which will restore the oxygen level
of the water to near saturation point. As microbes present in the water
may consume the dissolved oxygen, this will reduce the effectiveness of
this disinfection process. Therefore it is essential to shake each bottle
a few times during the period of exposure to sunlight to ensure the oxygen
level is kept close to maximum.
If the above routine can be followed to the letter, solar photo-oxidative
disinfection will give consistently effective results provided the bottles
are illuminated by sunlight of a sufficient intensity to give clearly defined
shadows. Professor Acra's pioneering work has shown that the most favourable
solar conditions are obtained between the latitudes of 15" and 35"
North and South of the equator where sunlight is both consistent and predictable.
With slightly less favourable conditions such as in equatorial and tropical
regions (between latitudes of 0" and 15") it may not be as effective,
due to the higher cloud cover, for this process is slowed down under cloudy
conditions.
Nevertheless this technology has important implications for us for small-scale
water treatment. Clear glass bottles however, restrict the penetration
of short-wave ultra-violet component of sunlight which may lead to a slower
rate of microbial inactivation, in which case clear plastic containers
may be a better option. Glass containers may. however, be more durable
for as the temperature rises during illumination, some plastic bottles
may leach small amounts of plasticisers. Also they have a tendency to becoming
increasing opaque to short-wave-length light from any prolonged exposure
to the sun.
Water turbidity and colour may reduce the rate of bacterial inactivation.
Such negative effects are insignificant except when the water is highly
turbid or coloured when light transmission is reduced to less than half
the surface value. Solar photo-oxidative disinfection works better if the
water source is relatively clear or when turbidity or colour does not substantially
restrict the penetration of sunlight. This could be a problem in time of
flood when water turbidity and colour normally increase, in which case
the water will need some initial processing before being placed in sunlight.
This could have an advantage for a proportion of the contaminating microbes
would be removed prior to solar photo-oxidation.
Solar photo-disinfection can be carried out in the home by individuals,
families or small communities without any need for a significant financial
investment or external agency support - (the only requirement being a sufficient
number of bottles to provide enough drinking water to meet each person's
daily needs). Therefore, solar water treatment might be the answer to the
problem of arsenic contamination for any low-income community where the
cost of engineering solutions may prove too high.
In places where there is already a water supply system, it may make
greater sense to install large solar powered water purification systems
which can treat large quantities of water. Ultraviolet disinfection uses
an ultraviolet (UV) light source enclosed in a transparent protective sleeve,
mounted in such a manner that the water that passes through the flow chamber
admit the UV rays and absorb them into the stream. These rays are able
to destroy bacteria and inactivate many viruses. This kind of system disinfects
the water without the need for adding chemicals and as a result, possesses
some of the benefits of distillation. It neither creates new chemical complexes
nor changes the taste or odor of the water - and - it does not remove any
beneficial minerals that may be in the water. However if the water is partially
treated by the sediment process and carbon filter prior to passing through
the UV flow chamber, it is more effective. Some of the bacteria that can
be rendered ineffective through this process include - Leptospira interrogans
(Infective Jaundice); Salmonella paratyphi (enteric fever); Salmonella
typhosa (typhoid fever) Shigella dysenterai (dysentery); Shigella Flexneri
(dysentery) Vibrio cholerae; Streptococcus; Staphylococcus; Escherichia
coil; Hepatitis virus; Influenza virus; Poliovirus (poliomyelitis); Rotavirus
and Bacteriophage (E.col) and yeast and fungi, all common in Bangladesh.
No. of words: 1857
- Acknowledgements:
- ITDG UK
- AIT, Bangkok, Thailand
- Disaster Forum, Bangladesh
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