Safe
Water System Manual
ALTERNATIVE
WATER TREATMENT TECHNOLOGIES
A
number of water treatment methods that employ simple, low cost technology are
available. These methods include straining; aeration; storage and settlement;
disinfection by boiling, chemicals, solar radiation; and filtration; coagulation
and flocculation; and desalination. The following classification is based on Skinner
and Shaw.29 The different methods are presented alphabetically.
Aeration
can be accomplished by vigorous shaking in a vessel part full of water or allowing
water to trickle down through one or more perforated trays containing small stones.
Aeration increases the air content of the water, removes volatile substances such
as hydrogen sulfide, which affect odor and taste, and oxidizes iron or manganese
so that they form precipitates which can be removed by settlement or filtration.
Coagulation
and flocculation. If water contains fine suspended solids, coagulation and
flocculation can be used for removal of much of the material. In coagulation,
a substance is added to the water to change the behavior of the suspended particles.
It causes the particles, which previously tended to repel each other, to be attracted
towards each other, or towards the added material. Coagulation takes place during
a rapid mixing or stirring process that immediately follows the addition of the
coagulant.
The
flocculation process, which follows coagulation, usually consists of slow gentle
stirring. During flocculation, as the particles come into contact with each other,
they cling together to form larger particles which can be removed by settlement
or filtration. Alum (aluminum sulfate) is a coagulant used both at the household
level and in water treatment plants.31, 32 Natural coagulants include
powdered seeds of the Moringa olifeira tree and types of clay such as bentonite.
Desalination.
Excessive chemical salts in water make it unpalatable. Desalination by distillation
produces water without chemical salts and various methods can be used at household
level, for example to treat seawater. Desalination is also effective in removing
other chemicals like fluoride, arsenic and iron.
Disinfection
is a way of ensuring that drinking water is free from pathogens. The effectiveness
of chemical and solar disinfection, and to a lesser extent boiling, is reduced
by the presence of organic matter and suspended solids.
Disinfection
by boiling. A typical recommendation for disinfecting water by boiling is
to bring the water to a rolling boil for 10-12 minutes. In fact, one minute at
100ºC. will kill most pathogens including cholera and many are killed at
70ºC. The main disadvantages of boiling water are that it uses up fuel and
it is time- consuming.
Chemical
disinfection. Chlorination is the most widely used method for disinfecting
drinking water. The source of chlorine can be sodium hypochlorite (such as household
bleach or electrolytically generated from a solution of salt and water), chlorinated
lime, or high test hypochlorite (chlorine tablets). Iodine is another excellent
chemical disinfectant that is sometimes used. Iodine should not be used for extended
periods (longer than a few weeks). Both chlorine and iodine must be added in sufficient
quantities to destroy all pathogens but not so much that taste is adversely affected.
Deciding on the right amount can be difficult because substances in the water
will react with the disinfectant, and the strength of the disinfectant may decline
with time depending on how it is stored.
Solar
disinfection uses solar radiation to inactivate and destroy pathogens present
in water. Treatment consists of filling transparent containers with water and
exposing them to full sunlight for about five hours (or two consecutive days under
100 percent cloudy sky). Disinfection occurs by a combination of radiation and
thermal treatment (the temperature of the water does not need to rise much above
50ºC). Solar disinfection requires relatively clear water (turbidity less
than 30NTU). More information on solar disinfection is available on the website
www.sodis.ch.
Filtration
includes mechanical straining, absorption and adsorption, and, particularly in
slow sand filters, biochemical processes. Depending on the size, type and depth
of filter media, and the flow rate and physical characteristics of the raw water,
filters can remove suspended solids, pathogens, and certain chemicals, tastes
and odors. Straining and settlement are treatment methods that usefully precede
filtration to reduce the amount of suspended solids that enter the filtration
stage. This increases the period for which a filter can operate before it needs
cleaning or replacing. Coagulation and flocculation are also useful treatments
to precede settlement and improve still further the removal of solids before filtration.
Storage
and settlement. Storing water in safe conditions for one day can result in
the die-off of more than 50 percent of most bacteria. Longer periods of storage
will lead to further reductions. During storage the suspended solids and some
of the pathogens will settle to the bottom of the container. Water removed from
the top of the container will be relatively clear (unless the solids are very
small such as clay particles) and contain fewer pathogens. The three-pot treatment
system where raw water is added to the first pot, decanted into the second pot
after 24 hours and into the third pot after a further 24 hours, exploits the benefits
of storage and settlement.
Straining.
Pouring water through a clean cotton cloth will remove a certain amount of the
suspended solids or turbidity. Special monofilament filter cloths have been developed
for use in areas where Guinea-worm disease is prevalent. The cloths filter out
the copepods which are intermediate hosts for the Guinea-worm larvae
The following tables
(Figures 19 and 20) describe the systems currently promoted for household treatment
in developing countries, the advantages and constraints of each system, and costs.
Figure 19 also indicates whether published reports of lab tests or field trials
of household applications are published in the epidemiologic or environmental
literature. Promotion and education are essential elements for the successful
implementation of any of these systems. The costs given in Figure 20 do not include
the costs of promotion and education leading to behavior change because the major
determinant of these costs is likely to be the context or setting in which the
treatment systems are being promoted. Promoting household treatment in a setting
where there are trained extension agents and community health promoters is very
different from working in communities and neighborhoods where there is no institutional
capacity.
19.
Household treatment systems advantages and constraints
| System |
Process |
Removal |
Lab
tests | Field
testsin developing countries | Advantages |
Constraints |
| Aeration |
Shaking part-full
container or some form of cascade that exposes water to air |
Some taste and
odor removal, oxidizes iron and manganese facilitating removal by filtration |
Yes |
Yes |
Low-cost component
of iron and manganese removal |
Limited removal,
normally used in combination with other treatment methods |
|
Boiling |
Bring water to
rolling boil for 10-12 minutes |
Kills nearly all
waterborne pathogens | Yes |
Yes |
Materials available
in most households | Time
taken to gather firewood. Increased demand for firewood leading to deforestation |
| Ceramic
filters | Water
passes (by gravity or siphon) from outside to inside of unglazed, ceramic cylinder
(often called a candle). Good quality ceramic has a pore size of 0.2 microns.
Some candles are impregnated with silver to kill pathogens. In some systems, candle
filter is preceded by a polypropylene rope filter to remove suspended particles
or packed with activated carbon to remove organic chemicals and tastes. |
Suspended solids
and pathogenic organisms. In theory viruses can pass through 0.2 micron pore but
they are normally attached to other material and are prevented from passing. |
Yes |
No |
Simple and robust. |
Blind quickly if
water contains suspended solids. Suspended solids are removed by scrubbing candle
and scrubbing wears away ceramic material. Candles are relatively expensive. |
|
Chlorine
tablets | Disinfection
with calcium hypochlorite or trichloroisocyanuric acid tablets |
Inactivates
or destroys nearly all waterborne pathogens, oxidizes organic substances |
Yes |
Yes |
Relatively
easy to distribute and use, particularly in emergencies. Residual effect. |
Not
locally available in many developing countries, have to be imported. Expensive
for long term use. Dose depends on organic material, etc in water. Available chlorine
in tablet can decline with age. Adequate contact time required. |
|
Rapid
sand filters | Use
coarser sand and higher flow rate than slow sand filters to remove impurities
by sedimentation, adsorption, straining, chemical and microbiological processes.
| Suspended
solids especially after coagulation and flocculation. |
Yes |
Yes |
Relatively
small and compact. | Not
effective at removing pathogens. Needs system for backwashing. |
|
Safe
water system (sodium hypochlorite +
safe water container +
social marketing + education) |
Disinfection
with locally available chlorine source (sodium
hypochlorite solution generated from brine or purchased
as bleach), container
with faucet & narrow neck |
Inactivates
or destroys nearly all waterborne pathogens, oxidizes organic substances |
Yes |
Yes |
Complementary
package of disinfection, safe water container and hygiene promotion. |
Local
supply of hypochlorite must be continously available, strength of hypochlorite
solution and raw water quality must be relatively constant, otherwise dosing must
change. Adequate contact time required. |
|
Slow
sand filters | Use
a relatively fine sand and a low filtration rate to remove impurities by sedimentation,
adsorption, straining, chemical and microbiological processes. |
Substantially
reduces pathogens (microbiological is main mechanism for removal) |
Yes |
Yes |
Pathogen
reduction but not complete removal. Locally available materials. |
Only
suitable for raw water with a turbidity of less than 20 NTU. Requires careful
maintenance. |
|
SODIS (solar
disinfection +
social marketing + education) |
Disinfection
by UV radiation
& heat through exposure to full sunlight for 5 hours in transparent plastic
bottle | Inactivates
or destroys most waterborne pathogens |
Yes |
Yes |
Uses
plastic bottles which are easy to handle, convenient for storage and transportation,
and reduce risk of recontamination. Sustainable system that does require consumables
except for bottles. |
Does
not require chemical quality of water. Requires favorable climatic conditions.
Only suitable for water with turbidity of less than 30 NTU. |
|
"Sorption"
or "catalytic" filters |
Water
passes through a finely ground filter medium composed of zeolite or similar. Impurities
chemically bond with filter medium. Pore size in medium is about 2 micron. |
Taste,
odor, chlorine, and suspended solids, pathogens, volatile organic compounds, and
heavy metals. | Yes |
No |
Very
simple to use - small filters are attached to the cap of a water bottle. User
simply fills the bottle with raw water and sucks on a spout in the cap, drawing
the water through the filter. Removes nearly all impurities. |
Filters
are easily blinded by suspended solids. Small filters set in water bottle cap
have a limited life being capable of filtering a maximum of 750 liters of water
before media is used up. Filters specially formulated for arsenic removal have
an even shorter life: filtering about 100 liters. Sorption filters are relatively
expensive. |
|
Storage
& settlement | Raw
water is added to the 1st pot, poured or preferably siphoned into 2nd
pot after 24 hours, and into 3rd after further 24 hours |
About
50 percent of most bacteria die-off, Schistosomiasis cerceriae die-off, significant
removal of turbidity | ? |
Yes |
Pots
available in most households |
Only
partial removal of pathogenic organisms |
|
Straining |
Pour
water through monofilament cloth |
Copepods
(cyclops) containing Guinea-worm larvae, some turbidity |
Yes |
Yes |
Simple
method for prevention of Guinea-worm. In areas where copepods harbor V. cholerae,
can reduce, but not eliminate transmission. |
Cloth
must always be used with same surface uppermost. Limited removal of other pathogens.
|
20.
Household treatment systems Costs
| System |
Imported items (shipping
costs and customs duties add to cost) | Initial
per capita cost of
hardware (5
person household) | Annual
operating cost per capita (5
person household & 10 liters of treated water per day) |
|
Aeration |
None |
None |
None |
|
Boiling |
None |
None |
Time taken to gather firewood. Effects of deforestation. |
| Ceramic
filter | Filter
candles | $5
($20-25 per system) | $1
(replace $5 filter annually) |
|
Chlorine tablets |
Tablets |
None |
$6 |
|
Rapid sand filter |
None |
Bucket or other container for sand |
Time to gather and clean sand |
| Safe
Water System | Cells
for generating hypochlorite |
$1.60 (2 plastic 20 liter water
containers per household, $4.00 per container) |
$0.60 |
|
Slow sand filter |
None |
Bucket or other container for sand |
Time to gather and clean sand |
| SODIS |
None |
Cost of black paint for used
plastic bottles | None |
| Sorption
filter | Filter
media | $7.50
(one filter per person) | $37.50
(replace filter five times per year) |
|
Storage and settlement |
None |
Cost of three pots |
Cost of three pots (zero after
initial investment for every year that pots last) |
|
Straining |
Monofilament cloth |
Depends on location |
Depends on location |
