INTRODUCTION
Water is a chemical substance that is essential to all forms of life. It is the most important natural resources on our planet. Water is the medium in which the reactions necessary for living functions take place. It is an active participant in many biological processes; water acts as the carrier of nutrients in the bodies of living organisms and it serves as temperature regulator.
Water is employed by man for several purposes such as in the industry as solvent,
in agriculture for irrigation, for recreation, travel, commerce and as regards
this study-domestic uses. To meet the above needs, it must satisfy certain requirements.
For instance, there are stipulated standards for drinking water by several organizations
such as WHO (2004) EU, UK and US EPA as shown in Table
1. The major sources of water in developing countries are rain, streams,
well, boreholes etc. Even in Canada, over four million people depend on private
wells for their drinking water. In addition, lakes, rivers and other sources
of surface water often serve as the sole water supply for cottagers, campers,
boaters and hikers (Health Canada, 2008).
Sources of water contamination could be from natural and/or anthropogenic sources. Private wells can become contaminated if they have been poorly constructed or improperly sited or if they have been infiltrated by contaminated surface water. In fact, the aquifer itself can even be the source of contamination. Surface waters and unprotected groundwaters are susceptible to faecal contamination from humans, livestock, wild animals and even house pets.
Water quality is the composition of water as affected by natural processes and human activities. Many different diseases are spread by contaminated drinking water, including Campylobacter, Cholera, Amoebic dysentery, Giardia (beaver fever) and Cryptosporidia. Other pathogenic microorganisms that can be found in drinking water are Caliciviruses, Heliobacter bacteria, Mycobacteria and Giardia Lambia. In the future more pathogenic microorganisms will emerge and spread through water, because of agricultural magnification, increased population growth, increased migration and climate change. Pathogenic microorganisms can also emerge because they built up resistance to disinfectants. These organisms usually get into drinking water supplies when source waters such as lakes or streams, community water supply pipes or storage reservoirs are contaminated by animal wastes or human sewage.
Water disinfection means the removal, deactivation or killing of pathogenic
microorganisms. Microorganisms are destroyed or deactivated, resulting in termination
of growth and reproduction. When microorganisms are not removed from drinking
water, drinking water usage will cause people to fall ill (Lenntech,
1998-2009).
Infectious diseases caused by pathogenic bacteria, viruses and protozoan parasites
are among the most common and widespread health risk of drinking water. Drinking
water standards shown in Table 1. People are introduced to
these microorganisms through contaminated drinking water, water drops, aerosols
and washing or bathing. Some waterborne pathogenic microorganisms spread by
water can cause severe, life-threatening diseases. Examples are typhoid fever,
cholera and hepatitis A or E. Other microorganisms induce less dangerous diseases.
Often, diarrhea is the main symptom. People with low resistance, mainly
elderly people and young children are vulnerable to these diseases as well (Lenntech,
1998-2009).
The lack of money needed to develop the elaborate drinking water infrastructure
favored in the developed world in addition to the difficulty or impossibility
associated with importing materials and expertise necessary for sustainable
operation of such facilities demand another solution. Different approaches must
therefore be developed and undertaken in the developing world if safe drinking
water is to be supplied indefinitely into the future. The following review will
provide an overview of techniques capable of eliminating or neutralizing water-borne
pathogens using little or no external input (capital, material, expertise, etc.).
For instance, studies on the reduction of diarrhea among Solar Disinfection
(SODIS) users show reduction values of 30-80% (Rose et
al., 2006; Hobbins, 2003).
NEED FOR DISINFECTION
It is estimated that in Latin America more than 40% of the population is utilizing
water of dubious quality for human consumption. This value is probably even
higher in Africa and areas of Southeast Asia. The cholera pandemic which struck
Latin America in January 1991 and has become endemic in many of the countries,
continues to exemplify the public health significance of contaminated drinking
water (Reiff et al., 1996). Worldwide, 1.2 billion
people do not have access to clean and safe drinking water and 2.4 billion people
lack sanitation. Every year, 5 million people die of waterborne diseases (Lenntech,
1998-2009). According to an assessment commissioned by the United Nations,
4,000 children die each day as a result of diseases caused by ingestion of filthy
water.
The report says four out of every 10 people in the world, particularly those
in Africa and Asia, do not have clean water to drink. Villagers collect water
from boreholes and consume without any treatment mostly which are highly contaminated
by more than one pollutant (Okoye and Okpara, 2010). Most
waterborne diseases occur worldwide. In developed (Western) countries, contagion
is prevented by drinking water purification and by hygienic measurements. But
even in developed countries, people can fall ill from waterborne diseases. This
is caused by using insufficiently disinfected water, by implementing non-hygienic
food preparation and by insufficient personal hygiene. Water disinfection is
necessary to eliminate pathogens, as many horticultural products are to be consumed
raw and in regions with high values of solar radiation it can be used for this
purpose (Tripanagnostopoulos and Rocamora, 2007).
In developing countries, waterborne diseases are a major problem which contributes
to the vicious circle that people are in. In many developing countries, there
is a lack of medicine to treat ill people. Vaccination is usually very scarce
as well. Many people weaken because of waterborne disease and, as a result,
are more susceptive to other infections. Their physical capacity decreases and
they cannot work and provide their families with money and food. A lack of sufficient
nutritional food weakens people, especially children, even further. They become
even more susceptible to diseases. Children run behind at school, because they
cannot be educated when they are ill. Waterborne diseases frustrate the economic
development of many people. During wars and natural disasters (floods) many
people are infected with waterborne diseases. Diseases are easily spread because
water treatment and sewage no longer function or are lacking completely (Lenntech,
1998-2009).
To improve the economical progress of developing countries, water contamination and spread of infectious diseases must be handled. This is achieved through (drinking) water treatment, sewage, waste and sewage water treatment and education on personal and food hygiene. Analysts say eliminating disease and death due to unclean water and poor sanitation would reap billions of dollars in health and productivity gains. They estimate that for every dollar spent, there would be an economic return of between $3 and 34, depending upon the country.
ROLE OF RENEWABLE ENERGY
| • |
Minimise long term environmental consequences (global warming
of CO2 emissions) |
| • |
Minimise near-time health impact of fossil fuel emissions |
| • |
Create new economic opportunities for companies and workers |
| • |
Job creation in the industrial world |
| • |
Potential for growth in the developing countries |
| • |
Spend less on energy, more on community |
DISINFECTION METHODS
Water can be disinfected with heat, chemicals, or light. For chemical disinfection
of water the following disinfectants can be used: chlorine (Cl2);
chlorine dioxide (ClO2); hypo chlorite (OCl¯); ozone (O3);
halogens: bromine (Br2), iodine (I), bromine chloride (BrCl); metals:
copper (Cu2+), silver (Ag+); kaliumpermanganate (KMnO4);
phenols; alcohols; kwartair ammonium salts; hydrogen peroxide; several acids
and bases. Chlorine also oxidizes iron and manganese so they can be filtered
out. Ozone may be used to disinfect public water supplies, but is rarely used
for private supplies (Liukkonen, 2006).
The factors that determine the selection of disinfectants are mainly availability, cost factors, logistics, cost of equipment and safety factor.
UV DISINFECTION OF WATER
Solar radiation is a form of renewable energy that is abundant and accessible
in most Southern countries. Sunlight with wavelengths of 315-400 nm on the ultraviolet
(UV) range of the electromagnetic spectrum is most effective at destroying bacteria.
Since colourless glass or plastic can transmit light in this near ultraviolet
range, they are the best materials for disinfection. Visible light (400-750
nm) next in terms of efficiency, with the visible band of violet and blue light
(400-490 nm) is the most useful within this range. As a result, violet, blue
and very light green-tinted glass follow colourless glass or plastic in order
of suitability (Lawand et al., 1997).
As noted by Wolfe (1990), the disinfection of drinking
water with ultraviolet light first took place in the early 1900s. However, the
systems were not highly successful for a number of reasons, including high operating
costs, poor equipment reliability, maintenance problems and the advent of chlorination,
which was found to be more efficient and reliable. The UV technology has improved
and become less expensive since the turn of the century. In Europe, UV has been
used for the bacteriological disinfection of water for a number of years (Taylor,
2003). Currently, approximately 2000 water treatment plants in Europe use
UV disinfection systems (Wolfe, 1990).
Ultraviolet disinfection of water consists of a purely physical, chemical-free
process. The UVC radiation in particular, with a wavelength in the 240 to 280
nanometers range, attacks the vital DNA of the bacteria directly. The radiation
initiates a photochemical reaction that destroys the genetic information contained
in the DNA. The bacteria lose their reproductive capability and are destroyed.
Even parasites such as Cryptosporidia or Giardia, which are extremely resistant
to chemical disinfectants, are efficiently reduced. The sterilized microorganisms
are not removed from the water. UV disinfection does not remove dissolved organics,
inorganic compounds or particles in the water (Harm, 1980).
However, UV-oxidation processes can be used to simultaneously destroy trace
chemical contaminants and provide high-level disinfection, such as the world's
largest Indirect Potable Reuse plant in Orange County, California (Wikipedia,
2009a). To assure thorough treatment, the water must be free of turbidity
and color. Otherwise some bacteria will be protected from the germ-killing ultraviolet
rays. Since ultraviolet light adds nothing to the water, there is little possibility
of its creating taste or odor problems.
Common UV applications: One of the most common uses of ultraviolet sterilization is the disinfection of domestic water supplies due to contaminated wells. Coupled with appropriate pre-treatment equipment, UV provides an economical, efficient and user-friendly means of producing potable water. The following list shows a few more areas where ultraviolet technology is currently in use: surface water, groundwater, cisterns, breweries, hospitals, restaurants, vending, cosmetics, bakeries, schools, boiler feed water, laboratories, wineries, dairies, farms, hydroponics, spas, canneries, food products, distilleries, fish hatcheries, water softeners, bottled water plants, pharmaceuticals, mortgage approvals, electronics, aquaria, boats and RV's, printing, buffer processing, petro-chemical, photography and pre- and post-reverse osmosis.
Factors affecting the effectiveness of UV disinfection:
| • |
Because UV does not leave any measurable residual in the water,
it is recommended that the UV sterilizer be installed as the final step
of treatment and located as close as possible to the final distribution
system. Once the quality of your water source has been determined, you will
need to look at things that will inhibit the UV from functioning properly
(e.g., iron manganese, TDS, turbidity and suspended solids) |
| • |
Iron and manganese will cause staining on the quartz sleeve and prevent
the UV energy from transmitting into the water at levels as low as 0.03
ppm of iron and 0.05 ppm of manganese. Proper pre-treatment with a sediment
filter and Triangular Wave Deposit Control System is required to eliminate
this staining problem |
| • |
Total Dissolved Solids (TDS) should not exceed approximately 500 ppm (about
8 grains of hardness). There are many factors that make up this equation
such as the particular make-up of the dissolved solids and how fast they
absorb the available UV energy. Calcium and magnesium, in high amounts,
have a tendency to build up on the quartz sleeve, again impeding the UV
energy from penetrating the water. A triangular wave deposit control system
will handle TDS before it becomes a problem for the UV system |
| • |
Turbidity is the inability of light to travel through water. Turbidity
makes water cloudy and aesthetically unpleasant. In the case of UV, levels
over 1 NTU can shield microorganisms from the UV energy, making the process
ineffective. Suspended Solids need to be reduced to a maximum of 5 μ
in size. Larger solids have the potential of harboring or encompassing the
microorganisms and preventing the necessary UV exposure. Pre-filtration
is a must on all UV applications to effectively destroy microorganisms to
a 99.9% kill rate |
| • |
An additional factor affecting UV is temperature. The optimal operating
temperature of a UV lamp must be near 40°C (104°F). The UV levels
fluctuate with temperature levels. Typically a quartz sleeve is installed
to buffer direct lamp-water contact thereby reducing any temperature fluctuations |
Advantages of ultraviolet light: Automatic, no taste or odor and low
contact time. A major advantage of UV treatment is that it is capable of disinfecting
water faster than chlorine without cumbersome retention tanks and harmful chemicals.
UV treatment systems are also extremely cost efficient.
| • |
Environmentally friendly, no dangerous chemicals to handle
or store, no problems of overdosing |
| • |
Universally accepted disinfection system for potable and non-potable water
systems |
| • |
Low initial capital cost as well as reduced operating expenses when compared
with similar technologies such as ozone, chlorine, etc. |
| • |
Immediate treatment process, no need for holding tanks, long retention
times, etc. |
| • |
Extremely economical, hundreds of gallons may be treated for each penny
of operating cost. |
| • |
Low power consumption |
| • |
No chemicals added to the water supply-no by-products (i.e., chlorine+organics
= trihalomethanes) |
| • |
Safe to use |
| • |
No removal of beneficial minerals |
| • |
No change in taste, odor, pH or conductivity nor the general chemistry
of the water |
| • |
Automatic operation without special attention or measurement, operator
friendly |
| • |
Simplicity and ease of maintenance, TWT deposit control system prevents
scale formation of quartz sleeve, annual lamp replacement, no moving parts
to wear out |
| • |
No handling of toxic chemicals, no need for specialized storage requirements,
no OHSA requirements |
| • |
Easy installation, only two water connections and a power connection |
| • |
More effective against viruses than chlorine |
| • |
Compatible with all other water processes (i.e., RO, filtration, ion exchange,
etc.) |
Disadvantages of ultraviolet light: Low penetration power, shielding
by turbidity, slime layer develops on tube, no simple test of results, no residual
effect and ultraviolet tube gradually loses power.
The cost of ultraviolet disinfection: The estimated one-time capital
cost of an ultraviolet system is $500, including valve, fittings and labor.
The life of the stainless-steel chamber is expected to be approximately 40 years;
the UV lamp requires replacement annually. At 12% discount rate, the annualized
capital cost of the UV system is approximately $60 year-1. Assuming
that the system is operational for 12 h day-1 and that the price
of electricity is 8 cents kW h-1, the annual operating cost of a
UV system is approximately $44 (including the replacement UV lamp and the cost
of electricity). Thus, the total annual cost is approximately $104. It is assumed
that the villagers provide their own storage tanks and sand filter; the raw
materials for these components are readily available and inexpensive. These
are not included in the present cost calculations. Operating for 12 h per day,
the system will disinfect 7884 tonnes (7.9 million liters) of water annually.
The cost of disinfecting water is thus about I per ton. Based on a per capita
drinking water requirement of 10 liters per day, a single system can provide
enough water for approximately 2200 villagers. Accordingly, a UV system could
ensure potable water year-round for a community of 2200 people at a cost of
about 5 cents per villager per year (Gadgil and Shown, 1995).
Guidelines for the application of Solar Disinfection (SODIS) at household
level
| • |
Water from contaminated sources are filled into transparent
water bottles. For oxygen saturation, bottles can be filled three quarters,
then shaken for 20 sec (with the cap on), then filled completely. Highly
turbid water (turbidity higher than 30 NTU) must be filtered prior to exposure
to the sunlight |
| Table 2: |
Suggested treatment schedule |
 |
| • |
Filled bottles are then exposed to the sun. The effective
duration of exposure to the depends on the weather condition as shown in
Table 2. Better temperature effects can be achieved if
bottles are placed on a corrugated roof as compared to thatched roofs |
| • |
The treated water can be consumed. The risk of re-contamination can be
minimized if water is stored in the bottles. The water should be consumed
directly from the bottle or poured into clean drinking cups. Re-filling
and storage in other containers increases the risk of contamination |
The following issues should also be considered:
| • |
Bottle material: Some glass or PVC materials may prevent
ultraviolet light from reaching the water (Wikipedia,
2009b). Commercially available bottles made of PET are recommended.
The handling is much more convenient in the case of PET bottles. Polycarbonate
blocks all UVA and UVB rays and therefore should not be used |
| • |
Aging of plastic bottles: The SODIS efficiency depends on the physical
condition of the plastic bottles, with scratches and other signs of wear
reducing the efficiency of SODIS. Heavily scratched or old, blind bottles
should be replaced |
| • |
Shape of containers: The intensity of the UV radiation decreases
rapidly with increasing water depth. At a water depth of 10 cm and moderate
turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles
are often easily available and thus most practical for the SODIS application |
| • |
Oxygen: Sunlight produces highly reactive forms of oxygen (oxygen
free radicals and hydrogen peroxides) in the water. These reactive molecules
contribute in the destruction process of the microorganisms. Under normal
conditions (rivers, creeks, wells, ponds, tap) water contains sufficient
oxygen (more than 3 mg oxygen per litre) and does not have to be aerated
before the application of SODIS |
| • |
Leaching of bottle material: There has been some concern over the
question whether plastic drinking containers can release chemicals or toxic
components into water, a process possibly accelerated by heat. The Swiss
Federal Laboratories for Materials Testing and Research have examined the
diffusion of adipates and phthalates from new and reused PET-bottles in
the water during solar exposure. The levels of concentrations found in the
water after a solar exposure of 17 h in 60°C water were far below WHO
guidelines for drinking water and in the same magnitude as the concentrations
of phthalate and adipate generally found in high quality tap water |
BOILING
In terms of limiting the need for external inputs, it would be difficult to
imagine a technique simpler and more sustainable than boiling water. Water is
simply placed in a clean container and brought to a full boil for at least three
minutes. This will eliminate all pathogenic activity, including giardia (Extension
Bulletin 795, 2003). The best means of water disinfection is boiling (for
about 1 h) which destroyed all the coliform in the water (Ibeto
et al., 2010).
This conventional method can be time-consuming and expensive. In many areas,
there is little fuel available for boiling water. Although, very few Indian
villagers disinfect their drinking water by boiling it over a cook stove, this
is common practice among rural families in some developing countries (e.g.,
China). The burning of biomass fuels for water disinfection increases the pressure
on the forests. In many areas of India as well as other developing countries,
deforestation is extensive, wood fuel supplies have dwindled and families are
forced to depend on residue fuels such as crop residues, cattle dung and twigs.
Collection of fuel wood is an increasingly difficult chore for rural women in
India. In addition, there is serious health risks associated with smoke inhalation
from biomass-fueled traditional cook stoves (Gadgil and Shown,
1995).
In order to eliminate biological activity, pasteurization temperature (150°F
or 65°C) must be achieved. This is difficult to do with plastic bottles
alone. The simplest solutions to this problem are the solar box and the solar
pond. A solar box consists of an insulated box constructed from wood or cardboard
with a glass or plastic lid. The inside surfaces should be painted black. A
covered vessel with water (ideally, also black) is placed inside. The pot needs
to remain in the box until pasteurization temperature is achieved for a few
minutes. On average, a solar box can pasteurize about 1 gallon of water in 3
h on a very sunny day (Rolla, 1998).
Advantages
| • |
Readily available |
| • |
Well-suited for emergency and temporary disinfection |
| • |
Will drive volatile organic chemicals out of water |
| • |
Extremely effective disinfectant that will kill even giardia cysts |
Disadvantages
| • |
Requires a great deal of heat |
| • |
Time to bring water to boil and cool before use |
| • |
Can give water stale taste |
| • |
Typically limited capacity |
| • |
Not an in-line treatment system |
| • |
Requires separate storage of treated water |
The cost of boiling water: The estimated cost of wood used to boil water
was US$ 0.272 per month for wood collectors and US$ 1.68 per month for wood
purchasers, representing approximately 0.48 to 1.04%, respectively, of the average
monthly income of participating households (Clasen et
al., 2008) (Table 3).
CHLORINE DISINFECTION
Chlorine disinfection kills all pathogens, including giardia. In addition,
chlorine has a residual effect; that is, if bacteria are reintroduced into a
chlorinated water supply, the new bacteria will die. Although, chlorine disinfection
is a well-proven technique, it has a few disadvantages. Often, people dislike
the taste and smell of chlorinated water. In some regions, it is difficult to
ensure a reliable supply of chlorine. In addition, because it is easy to overdose
water with chlorine, it is necessary for a trained person to test chlorine levels
before water is consumed. Most importantly, it is necessary to maintain a steady
supply of chlorine bleach; the current cholera outbreaks in India are largely
attributed to a breakdown in its supply chain (Times of India,
1994).
Advantages
| • |
Provides residual disinfectant |
| • |
Residual easy to measure |
| • |
Chlorine readily available at reasonable cost |
| • |
Low electrical requirement |
| • |
Can be used for multiple water problems (bacteria, iron, manganese, hydrogen
sulfide) |
| • |
Can treat large volumes of water |
Disadvantages
| • |
Requires contact time of 30 min for simple chlorination |
| • |
Turbidity (cloudy water) can reduce the effectiveness of chlorine |
| • |
Gives water a chlorine taste |
| • |
May combine with organic contaminants to form cancer-causing compounds |
| • |
Does not kill giardia cysts at low levels |
| • |
Careful storage and handling of chlorine is required (Curators
of the University of Missouri, 1995) |
Disadvantages of Chemical method of disinfection: Many disinfection
byproducts are bioaccumulative. They are not destroyed by the body and can accumulate
in body tissues. Some disinfection byproducts are considered harmful for public
health (chloroform, dibromochloromethane and bromoform are probably carcinogenic
and dichlorobromomethane, dichloroacetonitrile and chloral hydrates are possibly
carcinogenic).
| • |
Scientists fear that 14-16% of all bladder cancer cases can
be attributed to exposure to disinfection byproducts (Lenntech,
1998-2009) |
| • |
Another study proved that people who were exposed to concentrations of
50 μg L-1 or more had 1.5 times bigger risk developing intestinal
cancer (Lenntech, 1998-2009) |
| • |
The number of epidemiological studies on exposure to disinfection byproducts
and the influence on reproduction and birth defects is small. However, these
studies show there is a connection between exposure to trihalomethanes and
spontaneous abortion, birth defects and growth delay (Lenntech,
1998-2009) |
| • |
The risk on abdominal wall defects increases significantly after higher
exposure (Bing-Fang, 2002 in Lenntech, 1998-2009) |
| • |
A research in Sweden showed that trihalomethane concentrations lower than
the standard levels still have effects on reproduction (Lenntech,
1998-2009) |
| • |
Immunity of women exposed to chlorine dioxide is decreased |
DEEP TUBEWELLS
In India, many rural families obtain drinking water from deep tubewells. Because
the wells are more than 200" deep, the water has been sealed beneath an impermeable
layer of earth for a long time and is commonly bacteria-free. One disadvantage
of obtaining water from a deep tubewell is that many people dislike the taste.
Because the water is old, it has a high dissolved salt content and many people
prefer the taste of fresher, surface water. Additionally, deep tubewells can
be expensive and time-consuming to construct because of the specialized deep-drilling
equipment that is required. Tubewell cannot be easily moved from one place to
another as needed (Gadgil and Shown, 1995).
FILTRATION
Filters work by physically removing infectious agents from the water. The organisms vary tremendously in size, from large parasitic cysts (Giardia and Entamoeba histolytica 5-30 μm), to smaller bacteria (E. coli 0.5x3 μm, Campylobacter 0.2x2 μm), to the smallest viruses (0.03 μm). Thus, how well filters work depends to a great extent on the physical size of the pores in the filter medium. Hence, Table 4 shows microorganism size and susceptibility to filtration. Reverse osmosis filtration can both remove microbiologic contamination and desalinate water. The high price and slow output of small hand-pump reverse-osmosis units currently prohibit use by rural dwellers; however, they are important survival aids for ocean voyagers.
Filters have the advantage of providing immediate access to drinking water without adding an unpleasant taste. However, they suffer from several disadvantages: micro cracks or eroded channels within the filter may allow passage of unfiltered water, they can become contaminated and no filters sold for field use are fine enough to remove virus particles (Hepatitis A, rotavirus, Norwalk virus, poliovirus and others). In addition, they are expensive and bulky compared to iodine.
Slow Sand Filtration (SSF) is the world’s oldest known water treatment system. It emulates nature’s purification process when rainwater seeps through the layers of the earth’s crust and forms aquifers or underground rivers. Slow filtration is used mainly to eliminate water turbidity, but can be considered a water disinfection system if it is properly designed and operated. Unlike rapid sand filtration, in which the microorganisms are stored in the filter interstices until they are returned to the source water through backwashing, SSF consists of a group of physical and biological processes that destroy waterborne pathogens. It is a clean technology that purifies water without creating any additional source of environmental contamination.
A slow filter is basically a box or tank containing a floating layer of the water to be disinfected, a sand filter bed, drains and a set of regulating and control devices.
Slow f iltration is a simple, clean, yet efficient water treatment system. It needs larger areas than a rapid filtration system to treat the same water flow. Therefore, its initial cost is higher. Its simplicity and low operating and maintenance costs, however, make it an ideal system for rural areas and small communities, considering also that the land in those areas is relatively less expensive.
| Table 4: |
Microorganism size and susceptibility to filtration |
 |
| Source: Backer (1995) |
Factors that influence water disinfection
| • |
Contact time (CT): Contact time between disinfectant
and microorganism and the concentration of disinfectant, CT is used to calculate
how much disinfectant is required to adequately disinfect water. C refers
to the final residual concentration of a particular chemical disinfectant
in mg L-1. T refers to the minimum contact time (minutes) of
material that is disinfected with the disinfectant : |
CT = disinfectant concentration x Contact time = C (mg L-1)xT
(min)
When a particular disinfectant is added to water, it does not only react with
pathogenic microorganisms, but also with other impurities, such as soluble metals,
particles of organic matter and other microorganisms. The utilization of a disinfectant
for reactions with these substances makes up the disinfection demand of the
water
| • |
The type of microorganism: Disinfectants can effectively
kill pathogenic microorganisms (bacteria, viruses and parasites). Some microorganisms
can be resistant. E. coli bacteria, for example, are more resistant
to disinfectants than other bacteria and are therefore used as indicator
organisms. Several viruses are even more resistant than E. coli.
The absence of E. coli bacteria does not mean that the water is safe.
Protozoan parasites like Cryptosporidium and Giardia are very resistant
to chlorine |
| • |
The age of the microorganism: The affectivity of a particular disinfectant
also depends upon the age of the microorganism. Young bacteria are easier
to kill than older bacteria. When bacteria grow older, they develop a polysaccharide
shell over their cell wall, which makes them more resistant to disinfectants.
When 2.0 mg L-1 chlorine is used, the required contact time to
deactivate bacteria that are 10 days old is 30 min. For bacteria of the
same species and of the age of 1 day 1 min, contact time is sufficient.
Bacterial spores can be very resistant. Most disinfectants are not effective
against bacterial spores |
| • |
Water that requires treatment: The nature of the water that requires
treatment has its influence on the disinfection. Materials in the water,
for example iron, manganese, hydrogen sulphide and nitrates often react
with disinfectants, which disturb disinfection. Turbidity of the water also
reduces the affectivity of disinfection. Microorganisms are protected against
disinfection by turbidity |
| • |
Temperature: The temperature also influences the affectivity of
disinfection. Increasing temperatures usually increases the speed of reactions
and of disinfection. Increasing temperatures can also decrease disinfection,
because the disinfectant falls apart or is volatized |
A summary of the degree of cost, required expertise, required external inputs
and Adverse health effects of the different disinfection methods is shown in
Table 5.
| Table 5: |
Treatment summary |
 |
CONCLUSION
The recent controversy about the safety of chlorine and its by products has renewed interest in other forms of disinfection. From the health point of view, achieving the water and sanitation target by simple technologies would lead to global average reduction of 10% of episodes of diarrhea. The burden of disease associated with lack of assess to safe water supply, adequate sanitation and lack of hygiene is concentrated on children under five in developing countries. Accordingly, emphasis should be placed on interventions likely to yield an accelerated, affordable and sustainable health gains. This review points to household water treatment and safe storage as an option of particular potential with high health improvements and low costs. It is recommended that developing countries inculcate the various outlined disinfection methods adapting it to their local conditions inorder to abate health diseases; thereby ensuring the health safety of the populace.
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