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WELL FACTSHEETHousehold Water Treatment Author: Thomas Clasen, January 2005 Quality assurance: Sandy Cairncross Household-based approaches to water treatment may be more effective and cost-effective means of preventing diarrhoeal disease than conventional treatment at the source. This fact sheet summarizes the available evidence and some of the leading approaches. Diarrhoeal diseases kill an estimated 2.5 million
people each year, the majority children under five.
An estimated 4 billion cases annually account for
5.7% of the global burden of disease and place
diarrhoeal disease as the third highest cause of
morbidity and sixth highest cause of mortality.
Among children under 5 years in developing
countries, diarrhoeal disease accounts for 21% of
all deaths. Health authorities generally accept that safe water
plays an important role in preventing outbreaks of
diarrhoeal disease. Accordingly, World Health
Organization (WHO) guidelines for water quality
allow no detectable level of harmful pathogens at
the point of distribution. However, in those
settings in which diarrhoeal disease is endemic,
much of the epidemiological evidence for increased
health benefits following improvements in the
quality of drinking water has been equivocal (Cairncross,
1989). Since many of these same waterborne pathogens
are also transmitted via ingestion of contaminated
food and other beverages, by person-to-person
contact, and by direct or indirect contact with
infected faeces, improvements in water quality alone
may not necessarily interrupt transmission.
Water Quality: Refining the dominant paradigm Two decades ago, Esrey and colleagues reviewed
previous studies on the impact of environmental
interventions on diarrhoea, and found improvements
in water quality to be considerably less effective
than those aimed at water quantity, water
availability and sanitation.
The review was subsequently updated and
expanded to include hygiene interventions (Esrey et
al., 1991). Ubiquitously
cited in both professional journals and practical
guides, the reviews have led to the dominant
paradigm respecting water supply and sanitation
interventions: that to achieve broad health impact, greater attention should
be given to safe excreta disposal and proper use of
water for personal and domestic hygiene rather than
to drinking-water quality.
The corollary has become equally established:
that interventions aimed solely at improving
drinking water quality would have relatively little
impact in reducing diarrhoeal disease. Recently, however, an increasing body of evidence
has suggested the need to refine the dominant
paradigm (Clasen & Cairncross, 2004).
Much of this evidence arises from a
relatively new approach to enhancing water quality
as part of a public health initiative: improved
household water management and storage.
Esrey’s conclusions that water quality
improvements could reduce diarrhoeal disease by
15%-17% were based exclusively on studies involving
interventions at the point of distribution, such as
protected wells and springs.
However, it is well known that even safe
water becomes faecally contaminated during
collection, transport, storage and drawing in the
home (Wright et al., 2004).
Accordingly, improving water quality at the
point of distribution only may not secure the full
health gains that are possible by ensuring that
drinking water is microbiologically safe through the
point of use. This
distinction was suggested in a recent systematic
review that demonstrated a 34% reduction in
diarrhoea from higher quality studies of
interventions at the point of use, fully twice the
impact reported by Esrey of improvements at the
source (Fewtrell et al., 2005).
The health impact of treating water at the point of
consumption, however, is not universal or absolute. Except in the case of Vibrio
cholerae, a reduction in waterborne pathogens is
not clearly associated with a corresponding
reduction in diarrhoea (Gundry et al., 2004).
Moreover, while more than two dozen studies
have shown household water treatment to be
protective, the range of effects is quite broad.
And a few studies, including one of the only
blinded trials, have not demonstrated any
statistically significant reduction in diarrhoea (Kirchoff
et al., 1985).
Such heterogeneous results can perhaps be
anticipated, given the variety of interventions
employed (some of which included hygiene instruction
and other components), the diverse risk settings in
which they are introduced, and the different
methodological rigour of the studies themselves.
A pending Cochrane review should clarify the
apparent difference in health impact between
interventions at point of distribution versus those
at the point of use, and help explain the
heterogeneous results observed for interventions at
the household level. Household water treatment and the WHO As part of its Millennium Development Goals, the
United Nations expressed its commitment by 2015 to
reduce by one half the 1.1 billion people without
sustainable access to improved water supply.
Providing safe piped in, disinfected water,
to each household may be the best solution to
waterborne disease. The
WHO acknowledges, however, that such a solution
would entail an investment of tens of billions of
dollars each year.
Accordingly, it has called for other
approaches while progress is made in improving
infrastructure. Interventions to treat and maintain the microbial
quality of water at the point of use are among the
most promising of these alternatives.
In many settings, both rural and urban,
populations have access to sufficient quantities of
water, but that water is microbiologically unsafe.
The up-front cost of treating such water at
the point of use can be dramatically less than the
cost of conventional water treatment and
distribution systems.
According to the 2002 World Health Report,
point-of-use water treatment, such as
household-based chlorination, is the most
cost-effective environmental intervention to prevent
diarrhoeal disease across a wide range of countries
and settings (WHO, 2002).
In 2003, the WHO helped organize the
International Network for the Promotion of Safe
Household Water Treatment and Storage, a global
collaboration of UN and bilateral agencies, NGO’s,
research institutions and the private sector
committed to improved household water management as
a component in water, sanitation and hygiene
programs. The
Network’s website, hosted by WHO, contains a
considerable amount of information on household
water management: www.who.int/household_water/en/ The WHO also commissioned a comprehensive review to
identify the most promising POU treatment
technologies based on selected technical
characteristics and performance criteria, including
effectiveness in improving and maintaining microbial
water quality, health impact, technical difficulty
or simplicity, accessibility, cost, acceptability,
sustainability and potential for dissemination (Sobsey,
2002). After
evaluating at least 37 different technologies,
Sobsey concluded that 5 were the most promising:
filtration with ceramic filters, chlorination
with storage in an improved vessel, solar
disinfection in clear bottles, thermal disinfection
(pasteurization) in solar cookers or reflectors, and
combination systems employing chemical flocculation
and chlorination.
While this Fact Sheet will focus on these
technology groups, readers are urged to explore
other options which may be more suitable for a
particular setting.
Moreover, the potential commercial market for
household-based water treatment has attracted
private sector participants who attempting to adapt
or develop new technologies.
Accordingly, readers are encouraged to
investigate these emerging technologies. 1.
Chemical
Disinfection. Chemical disinfection is the most widely-practised means of treating water at the community level. It is also the method used most broadly in the home. While a wide range of oxidants are used in treating water, most household-based interventions employ free chlorine derived from liquid sodium hypochlorite or solid calcium hypochlorite which are usually available and affordable. Tablets formed from chlorinated isocyanurates (e.g., NaDCC), a leading emergency treatment of drinking water, and novel systems for on-site generation of oxidants such as chlorine dioxide, may also have a role in household water treatment in the future. At doses of a few mg/l and contact time of about 30 minutes, free chlorine inactivates more than 4 logs of enteric pathogens, the notable exceptions being Cryptosporidium and Mycobacterium species. The “Safe Water System”, a programmatic intervention developed by the US Centers for Disease Control and Prevention that combines chlorination of water in the home with safe storage and hygiene instruction, has an estimated 5 million users in 19 countries (www.cdc.gov/safewater/default.htm) Its impact in reducing diarrhoeal diseases has been documented (Quick et al., 2002). Like most other household-based water interventions, however, the hardware must be accompanied by an extensive behavioural change program to stimulate adoption and continued utilization by householders.
2.
Filtration. Household filters potentially present certain
advantages over other technologies.
They operate under a variety of conditions
(temperature, pH, turbidity), introduce no chemicals
into the water that may affect use due to objections
about taste and odour, are easy to use, and improve
the water aesthetically, thus potentially
encouraging routine use without extensive
intervention to promote behavioural change. Higher
quality ceramic filters treated with bacteriostatic
silver have been shown effective in the lab at
reducing waterborne protozoa by more than 3 logs and
bacteria by more than 6 logs.
Their potential usefulness as a public health
intervention has been suggested in a recent field
trial (Clasen et al., 2004).
While the up-front cost of gravity systems
employing such commercial ceramics is high (US$10 to
US$25), their long life (up to 50,000L per ceramic
“candle element”) renders such systems
comparable to chlorination on a per litre treated
basis. The
improving quality of locally-fabricated silver
coated ceramics is particularly promising as a
sustainable and low-cost alternative. Slow-sand filters, which remove suspended solids and microbes
by means of a slime layer (schmutzdecke)
that develops within the top few centimetres of
sand, are capable of removing 2 logs or more of
enteric pathogens if properly constructed, operated
and maintained (Hijnen
et al, 2004). A simpler but more advanced
version, known as the “bio-sand” filter, was
specifically designed for intermittent use and is
more suitable for household applications.
It has been tested (Palmeteer et al., 1999)
and is being deployed widely in development settings
with the help of CAWST, a Canadian NGO (www.cawst.org).
Other filtration media, ranging from simple
folded sari cloth for the removal of
cholera-associated zooplankton (Colwell et al.,
2003) to advanced but inexpensive carbon nanofibre
membranes capable of removing even viruses at
gravity pressure, demonstrate the broad range of
opportunities for filtering water at the household
level.
3.
Thermal
and Solar Disinfection. Boiling or heat treatment of water with fuel is
effective against the full range of microbial
pathogens and can be employed regardless of the
turbidity or dissolved constituents of water. While the WHO and others recommend bringing water to a
rolling boil, this is mainly intended as a visual
indication that a high temperature has been
achieved. In
fact, studies have demonstrated that heating to
pasteurization temperatures (60º C) for 10
minutes will kill or deactivate most pathogens.
However, the cost and time used in procuring
fuel, and the environmental issues around denuding
forests and adding especially to poor indoor air
quality, have led to other alternatives.
Solar disinfection, which combines thermal
and UV radiation, has been repeatedly shown to be
effective for eliminating microbial pathogens (Reed,
2004) and reduce diarrhoeal morbidity (Conroy et
al., 1999). Among
the most practical and economical is the “Sodis”
system, developed and promoted by the Swiss
Federal Institute for Environmental Science and
Technology (EAWAG) (www.sodis.ch). It
consists of placing low turbidity (<30NTU) water
in clear plastic bottles (normally discarded 2L
beverage bottles, preferably PET) after aerating it
to increase oxygenation and exposing the bottles to
the sun, usually by placing them on corrugated metal
roofs. Exposure
times vary from 6 to 48 hours depending on the
intensity of sunlight.
Like filters, thermal and solar disinfection
do not provide residual protection against
recontamination.
Accordingly, householders must have a
sufficient number of bottles to allow them to cool
and maintain treated water in the bottles until it
is actually consumed.
4.
Combination
Flocculation and Disinfection. A particular challenge for most household-based water treatment technologies is high turbidity. Solids can use up free chlorine and other chemical disinfectants, cause premature clogging of filters, and block UV radiation essential in solar disinfection. While turbidity can often be managed by pre-treatment or even simple sedimentation, flocculation/coagulation using additives such as alum can be an effective and relatively low-cost option. Such forms of assisted sedimentation have been shown to reduce the levels of certain microbial pathogens, especially protozoa which may otherwise present a challenge to chemical disinfectants. However, disinfection is still required in most cases for complete microbial protection. Certain manufacturers have combined flocculation and time-released disinfection in a single product that is sold in sachets for household use. Proctor & Gamble’s PUR® product, the most extensively tested, has been shown to reduce waterborne cysts by more than 3 logs, viruses by more than 4 logs and bacteria by more than 7 logs. Unlike the other methods of household water treatment discussed above, it has also been shown effective in reducing arsenic, an important non-microbial contaminant in certain settings, by more than 2 logs. Field studies have demonstrated such flocculation-disinfection products effective in preventing diarrhoeal diseases (Reller et al., 2003). While these products are relatively expensive on a per litre treated basis, they may have application in certain emergency and other settings. It may also be possible to achieve similar results by combining conventional and lower cost approaches to assisted sedimentation and subsequent disinfection at the expense of convenience. Acceptability, Affordability and Sustainability Household water treatment as an intervention
against diarrhoeal disease is still at a nascent
stage in its development.
While there is considerable research to
support the microbiological effectiveness of certain
approaches, and a growing body of promising though
not definitive research about its health impact,
there is relatively little evidence about the
potential uptake of such interventions.
Questions about acceptability, affordability,
long-term utilization and sustainability must still
be addressed, particularly in programmatic settings.
These issues will ultimately help determine
the potential role of household water treatment in
preventing diarrhoeal disease among vulnerable
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