Arsenic, drinking-water and health risk substitution in arsenic mitigation: A discussion paper
Annex 1: Water safety issues and examples of "model" Water Safety Plans
This annex provides an overview of how the microbiological quality of drinking water may be controlled through protection of water sources, control of treatment processes and management of distribution and handling of water. It uses the principles of water safety plans and provides guidance on how WSPS and codes of practice can be defined for a range of water supply technologies and for household water handling and storage. For a range of technologies, ‘model’ water safety plans are defined.
WSPs should be subject to approval by the regulatory body who should have access to a range of statutory tools to impose penalties for non-compliance. This may be in response to failure to prepare an adequate management plan or failure to comply with it once established. However, as with all regulatory regimes, flexibility will be required and a range of other tools (relaxations, exemptions etc) may also be needed.
In some circumstances national or regional authorities may wish to establish a suite of basic management plans to be used by local suppliers either directly or with limited adaptation. This may be of particular importance when the supplies are community-managed. For community managed supplies, an approach focusing on ensuring operators received adequate training and support to overcome management weaknesses will be more effective than enforcement of compliance.
Hygiene codes are also presented for household treatment of water and water hygiene. These should by used in conjunction with education programmes as a way of promoting good hygiene. However, there should also be enforcement of minimum design criteria by manufacturers of water treatment technologies.
The following sections provide examples of outline management plans for some of the more frequent types of supply. In many cases several components may be needed to prepare an overall management plan. Thus for piped supplies it may be appropriate to link the management plan components of source protection and treatment with those for distribution. Where water supplies are not continuous, then household management of water will be an important additional component to be included.
The hygiene codes that follow are indicative and should be modified to meet local needs and to suit local conditions. Hygiene codes are presented for the following types of water supply and household management of water:
- Tubewell from which water is collected by hand
- Spring from which water is collected by hand
- Simple protected well
- Rainwater catchment
- Storage and distribution through community managed piped systems
- Groundwater from protected boreholes/wells with mechanised pumping
- Household handling and storage of water
- Household disinfection
- Household filtration systems
In each section there is an initial introduction to provide an overview of the situations when the type of supply may be found and the evidence of health risks derived from the use of the technology. Each section then addresses four groups of issues.
Selection of reference pathogens and assumptions made. These sections provide an outline for the basis of identifying key challenges to health and therefore the water resource, design and control measures required to minimise the risk to public health. The reference pathogens relate to those discussed in a report of a WHO meeting on regulation of microbiological quality held in Adelaide, Australia, 2002, but it should be noted that not all these pathogens are applied to all technologies.
Hazard assessments. These sections review the process of conducting a qualitative assessment of hazards that may cause contamination of the water supply. For surface waters this means an assessment of the catchment and for groundwater an evaluation of the recharge area. In groundwater hazard assessments, the type of aquifer must be taken into account.
IWRM and regulatory issues. This section outlines the major controls that should be in place at national and regional level in order to enable local action to be effective. These are aspects which may be outside the direct control of the supply agency itself but which are important to the management plan. These primarily concern national legislative frameworks, local laws and integrated water resource management. They also cover basic issues of importance such as training needs.
Design Issues. This section looks at the basic design criteria required to ensure the adequacy of the installation to provide water reaching water quality targets. Many design issues are also control measures in a water safety plan.
Small, community-managed point source groundwater supplies
The following drinking water quality management deal with a series of small, usually community-managed water supplies that use shallow groundwater. These supplies are mainly ‘point’ supplies – i.e. water must be collected from the source by hand.
The majority of these supplies are to be found in low and middle income countries, although occasional examples may be found in wealthier countries. Whilst such supplies are generally considered to be found primarily in rural areas, there are very large numbers of such supplies in poor urban and peri-urban settlements throughout the developing world. This includes small towns as well as some of the World’s largest cities such as Dhaka. The use of such supplies may not be the preferred water supply solution in such situations, however, the reality is that millions of people in cities worldwide have little prospect of access to treated piped water in the short term. This emphasises the need to address the quality of all such sources whether urban or rural.
The nature of community-managed supplies also suggests that while engineering interventions may do much to reduce risks, training and support to communities in water supply management is likely to be more critical and this should not be neglected by the water supply and surveillance bodies.
The collection of water from such sources by hand implies that controlling the quality of the water at these sources will not be sufficient on its own to reduce water-related health risks to an acceptable level. Additional interventions are also likely to be required in water handling and potentially household water treatment as discussed further below.
Selection of reference pathogens and assumptions made
The selection of reference pathogens and key assumptions do not significantly differ between the different types of technology and are therefore presented here. The comments will therefore apply to the next three drinking water management plans.
Critical to the establishment of water quality targets for point source groundwater supplies is an understanding of the movement, survival and attenuation of different pathogens within the sub-surface environment. For a full review of this please consult Chapter 3 of the monograph Protecting groundwater for health.
Evidence suggests that control of the risk posed by viruses in groundwater is difficult to achieve solely through land-use control and wellhead protection measures. There is good evidence of greatly extended survival and travel of viruses within the sub-surface and attenuation processes may only retard and not remove viral pathogens. One consequence of this is that elution of viral pathogens may occur due to changes in environmental conditions caused by recharge. Therefore, for greater confidence that risks from viruses have been controlled, contact disinfection is likely to be required. Whilst the land-use control measures outlined below provide some confidence in reducing viral risks (particularly at the longer travel times) this may not reduce levels to those deemed acceptable. Furthermore, such controls may not be feasible even when using vertical as well as horizontal flow in many settings and therefore an elevated residual risk from viral pathogens may need to be tolerated (ARGOSS, 2001). In most cases, as first exposure to viral pathogens is likely to occur during childhood rather than adulthood, control of virological quality may be less urgent. In most cases, unless disinfection is practised this would be difficult to achieve. The principal basis for the control of microbiological quality of these supplies is in relation to risks posed by bacterial and protozoan pathogens. For bacterial pathogens, E.coli 0157 is used. The measures put in place to reduce risks from E.coli 0157 would be adequate to deal with other bacterial pathogens. Cryptosporidium parvum is used as the reference pathogen for protozoan agents as it has been shown to be present in some groundwater supplies.
Hazard assessments for point water supplies should, like for most water sources, be undertaken prior to construction and commissioning of the source and on periodic visits to the source. This will usually be undertaken through a sanitary inspection. Initial hazard assessments should be used to plan and design the water supply. It may be that hazards exist that are associated with a significant risk due to distance from proposed water source or because of flow rates. In such cases, risk management strategies may require careful thought such as deepening the intake.
IWRM and regulatory issues
The IWRM and regulatory issues for all three principal forms of point water supply from groundwater sources are covered in a single section here as they are all basically the same. The linkages between some key IWRM issues for point groundwater sources and those identified for deep boreholes with mechanised pumping should be noted. Groundwater management and protection strategies should cover all forms of groundwater abstraction found within the country and it is important that shallow point sources of groundwater are not disadvantaged by measures to protect deeper abstraction.
Tubewell or borehole from which water is collected by hand
Design issues Shallow tubewells or boreholes are used in many developing countries and are often the preferred method of water provision in rural communities. Many different techniques exist for drilling tubewells and some of these, particularly some of the very low-cost methods themselves raise the risks of contamination. Tubewells are usually fitted with handpumps, although some designs of windlass have been used. A variety of types of handpump are available and again these may themselves represent water quality risks, particularly where water is need for priming. Sustaining handpump-based water supplies is often difficult because of associated costs and this should be borne in mind when promoting their use.
Protected spring from which water is collected by hand
Springs serve a significant proportion of rural populations in many countries and have lower capital investment costs and usually lower maintenance requirements. Springs located uphill of communities are often linked to simple community-managed gravity flow pipe systems which provide greater convenience and may improve hygiene through greater water use. A water quality management plan for such supplies has been previously outlined. In this section, only springs that have been protected are covered as unprotected springs are open to contamination and their use may represent a significant health risk.
Protected dug well
The key design issues for dug wells that should provide basic protection against most pathogens are outlined below. However, although exclusion of protozoan pathogens should relatively easy to ensure, controlling bacterial and viral pathogens is often more problematic as ensuring impermeability of lining material is difficult. Disinfection is possible using low-cost techniques and is included here as an option that should be considered. However, it should be borne in mind that sustaining disinfection may be difficult in low-income communities and a balance should be maintained between water quality targets desired and practical implementation.
Rainwater collection is widely used throughout the developing and developed world. In low-income countries, collection is typically practised at the household level with roof collection being the most common approach used. In many cases, low volumes of rainwater are collected using makeshift gutters and open containers for use the same day or to provide a limited reserve lasting 2-3 days. The methods used in such cases are not protected and contamination is difficult to prevent. Simple improvements in the collection, guttering and storage containers can significantly increase efficiency and provide water reserves that can last several weeks. Simple improvements can also greatly improve the control of water quality and significantly reduce contamination risks. Where water is stored for longer periods (several weeks or more) then increasing problems may be found with vector-borne disease and sometimes taste and odour problems. Some designs are available to reduce such problems, although these typically increase costs.
Highly sophisticated forms of rainwater collection are used in developed countries, often using specially prepared impermeable ground catchments, where rainwater feeds a treatment plant and distribution system. Such catchments need basic maintenance and protection to prevent unacceptable build-up of pollution. A further refinement of rainwater collection that is included for completeness is fog collection. This is applied in only a limited number of countries (notably Chile and Peru) but is attracting increasing attention in other dry areas of the world.
Selection of reference pathogens and assumptions made
Where rainwater is collected from large ground catchments, then it is assumed that this will be part of a public water supply supplying water via treatment works an distribution systems. This water therefore is essentially a surface water source and should meet the criteria outlined above for water treatment. Source protection will be important and should exclude human activity. However, wild animals and in particular birds may represent a particular hazard, although these may be difficult to control. A hazard assessment for such systems would include periodic surveys to ensure that:
- human activity has not encroached into the catchment or controlled areas;
- no discharges of human waste occur upstream of the catchment;
- solid or hazardous waste has not been dumped in the catchment or so that its leachate can run-off into the catchment; ;
- type and numbers of animals likely to be found in the catchment
For large ground catchment rainwater collection systems, the reference pathogens are the same as those for any other surface water source and the control measures will be the same as noted previously for treatment processes.
Household rainwater collection
As it is generally assumed that rainwater is not microbiologically contaminated to a significant degree, most household rainwater collection system will not undergo treatment, although some designs include filtration units (of generally unproven efficacy) or periodic disinfection may be practised. The presence of animal and bird faeces represents a risk of bacterial and protozoan pathogen presence. Human faeces would be unlikely to be a significant hazard, although it is possible that this could occur where excreta disposal is poor and the ‘wrapper’ or ‘flying’ latrine method is used or where contaminated water sprays can reach the catchment (for instance see Simmons et al).
Roofing material may exert a significant influence of water quality, with hard impermeable surfaces preferred to grass thatch as the latter may harbour significant microbial ecosystems (Uba and Aghogho, 2000). The first rains are likely to represent a time of elevated risk as contamination on the roof and gutters that has built up over the dry period are washed into the collection tank (Gould et al, 1999). Therefore the diversion of water derived from the first rains is an important control measure for microbiological quality.
The cleanliness of the roof will be critical to avoid contamination in the rainwater tank and this should be the primary focus of a hazard assessment. Hazard assessments will typically be regular visual assessment of cleanliness of the roof and gutters (WHO, 1997).
The principal reference pathogen of interest is E.coli 0157, as the majority of data available on pathogen presence has suggested that bacterial pathogens (and in particular those with animal as well as human hosts) are of greatest concern. E.coli 0157 will clearly provide a good reference pathogen in these cases. Viral risks are less certain (few studies have been undertaken) and it would be likely that there was commonly childhood exposure to viral risks where widespread use of unchlorinated rainwater is practised. Furthermore, without disinfection it unlikely that viral risks could be minimised in any case.
Risks of infection by cysts are also uncertain given limited data. It is likely that there is potential for cysts derived from wild animals to be present in rainwater. However, it is not clear in what numbers cysts may be present and therefore a true estimation of risk may be difficult. Furthermore, in many areas where untreated rainwater is widely collected, the level of risk posed by drinking water would almost be certainly far lower than those posed by direct human-animal contact. Some rainwater collection systems use sand filters on the inlet. The efficacy of these filters in removing microbiological contamination is far from certain and it is not clear that they could be relied upon to remove cysts. However, they do remove larger debris and so will also remove pathogens adsorbed onto particulate matter.
The combination of the above factors suggests that in most cases establishing drinking water quality management plans for viral and protozoan risks will have limited effectiveness and may be counter-productive by increasing costs.
The regrowth of pathogens within rainwater tanks again is not well researched but could be projected to be significant. It is certainly possible that biofilms could be developed within a rainwater tank and that this could harbour pathogens introduced through poor tank maintenance or poor catchment hygiene. This area requires further work in order to establish whether this is a real risk, or simply a theoretical problem.
The water quality management plan outlined below assumes that the system of rainwater collection follows some form of improved systematic design – i.e. a tank or other container is linked to a system of gutters. It is not designed where rainwater is occasionally collected in a bucket. As rainwater collection in most countries is a household activity, it is implicit that the process of monitoring of quality requires support from local health bodies, although the cost implications of such an approach are significant (Simmons et al, 2001).
Fog collection is a relatively new technology and is not widely practised. The risks associated with this are not widely reported but it can be assumed that they potentially exist primarily from contamination by birds or animals. Direct control may be difficult and disinfection is likely to be the principal control measure available. However, pathogen loads would not be expected to be high. Furthermore, in areas where fog collection is practised tend to have quantity problems in water supply and therefore undue attention on controlling drinking-water quality may be counter-productive as the primary risk may result from poor hygiene caused by inadequate volumes of water.
IWRM and regulatory issues
Note that many design issues are also control measures and are repeated on the table of verifications
Storage and distribution through community managed piped systems
In many parts of the world, simple piped water systems are managed by communities. Such facilities are typically fed by gravity and are often drawn from groundwater sources such as springs. In these cases, treatment or disinfection of drinking water is rarely undertaken. Some supplies are also drawn from upland streams where again no treatment or disinfection is performed. In some communities, a mechanised borehole feed a small tank and distribution systems are used. In some cases community managed treatment plants linked to the distribution system are used. Many control measures and management actions are similar to those in the previous section, but are covered here as the absence of disinfection may increase risks.
Selection of reference pathogens and assumptions made
The hygiene code outlined below is based on an assumption that the source is protected in some form and that there is no disinfection of the water prior to distribution. The selection of reference pathogens reflects the likely socio-economic conditions within communities, which are primarily small, rural communities in developing countries. In these communities, first exposure to viruses may be expected to be more likely to occur in childhood rather than adulthood and therefore whilst viral risks should be controlled as far possible, without disinfection this will not be fully effective. Cryptosporidium control will be focused primarily at the source and would not be expected to be of great importance during distribution. Furthermore, exposure is likely to occur through other routes and this should be borne in mind. Re-growth may be controlled through pipe materials, but there will be little or no alternative control measures available and th
The hazard assessment for small community-managed systems will have many of the characteristics of those for utility-managed distribution systems. However, the focus will be on the above ground sources of faecal matter in the environment and the physical state of the infrastructure rather than estimating biological stability. Such an approach requires regular sanitary inspection by ‘walking of the line’.
IWRM and regulatory issues
Many factors will influence the design of a piped water system, including ensuring the resulting cost of water remains affordable, that demand can be met and losses are minimised. The control of water quality must be set against decisions relating to affordability and improvement in access. However, designs to improve water quality and in particular those that relate to ingress of contamination water are all likely to also have a positive impact on reducing losses and improving user perceptions of the service. The latter may be important when trying to improve overall access.
Groundwater from boreholes with mechanised pumping linked to a distribution system
It is generally assumed that such facilities will be operated by a public entity/utility charged with the supply of drinking water and will therefore have sufficient operational capacity to undertake proper design, construction, operation and maintenance. It is expected that such supplies will be regulated and compared to enforceable water quality targets/standards. It should be noted that the recommendations here regarding disinfection relate solely to the production stage of water taken from groundwater and not to distribution.
Selection of reference pathogens and assumptions made
A full discussion of the survival, transport and attenuation of pathogens in groundwater is given the background monograph. Hepatitis viruses and Cryptosporidium parvum are of particular importance as the control of risks from these pathogens would be likely to resolve the problems of bacterial pathogens. However, E.coli 0157 is retained as a reference pathogen specifically because it inclusion allows a greater flexibility in defining levels of tolerable risk and in land-use control.
The principal challenges in groundwater posed by viruses relate to extended potential survival and more limited potential for attenuation. Attenuation is highly dependent on environmental factors in the sub-surface and often only retards, rather than eliminates viruses. Retardation may be reversible. In most situations where the water supply from the groundwater source undergoes at least disinfection and subsequent distribution, overall socio-economic development and environmental hygiene are often also good. This suggests that first exposure to viruses in adulthood may be more likely.
Whilst control of viral hazards through land-use control is desirable for all groundwater supplies, it is also important to recognise that this may not be adequate to reduce risks. Chapter 3 in the background monograph indicates that viral survival may be greatly extended in comparison to other pathogens. The evidence suggests that once travel times from point of entry into the water body to the point of abstraction exceeds 50-60 days, then the processes of attenuation and die-off result in significant reductions in pathogen densities and therefore the probability of exposure through ingestion of water are greatly reduced. However, a residual risk is retained and in countries with limited alternative childhood exposure routes may be greater than acceptable. In these circumstances, reductions in viral risks can only be achieved through contact disinfection prior to distribution.
With the exceptions of karstic or other fracture dominated aquifers, removal of cysts during recharge is likely to be rapid and primarily a function of filtration. In such cases, the principal means of control will be to ensure that direct entry into the borehole caused by poor completion of surface headworks and the first few metres underground is prevented.
In aquifers dominated by fracture flow, detailed knowledge of the hydrogeological regime are required to estimate risks. Cysts have relatively long survival times (see microbial quality review) and in groundwater systems that have limited filtration capacity it is likely that protozoan cysts will be able to travel extended distances in an infective state. However, for karstic systems, there is a rational for considering these to be surface waters that require full treatment (see groundwater monograph).
In general the measures that are adopted to prevent protozoan and virus contamination of drinking water should be adequate to reduce risks from bacteria to an acceptable level. Survival of bacteria in groundwater is significantly lower than for viruses (see microbial review) and attenuation is generally more effective given the greater size of bacteria and increased potential for mechanisms such as microbial predation (see Chapter 3, groundwater monograph).
Reductions in bacterial density occur relatively rapidly and thus the probability of exposure through water to numbers of bacterial pathogens likely to result in infection is reduced rapidly. The application of protection zones geared towards reducing bacterial risks are likely to be effective. Travel times of 50 days would usually be more than adequate and shorter travel times (for instance 25-30 days) may be adequate (Groundwater monograph, Chapter 3). This suggests that land-use control measures may be sufficient to reduce risks to an acceptable level and that contact disinfection designed to inactivate bacterial pathogens should not be required.
There is good evidence that bacterial contamination occurs due to poor wellhead completion. However, the controls put in place to prevent direct ingress designed to control cysts would be expected to reduce bacterial pathogens to an acceptable level, particularly where these increase vertical movement to the point of intake.
However, measures specific to bacterial pathogens are included here for two specific reasons. Firstly, for many countries, control of epidemics remains the primary goal of water quality management and therefore control of bacterial pathogens is an important goal. Secondly, in setting water quality targets in relation to endemic disease which are based on design measures, the use of bacteria (particularly where protection zones are a key control point) could be used when setting a lower (but still acceptable) water quality. This approach supports the principle of local decision-making based on a tolerable disease burdens, available resources and targets for health. The reference bacterial pathogen used is E.coli 0157 as there is strong evidence of link to outbreaks.
The hazard assessment would normally take the form of a sanitary survey of the catchment area and of the integrity of the infrastructure of the borehole, in particular at the wellhead. However, when translating the hazard assessment into a risk assessment, the hydrogeological environment and vulnerability of aquifers should also be taken into account to ensure that a realistic assessment can be made of the risk and its severity. This is of particular importance for groundwater as the nature of the aquifer will determine whether a hazard represent any risk to the water supply.
There are many potential sources of faeces within the environment that may represent a hazard. These include on-site sanitation (septic tanks, pit latrines), sewers, landfill sites, waste dumps and scattered waste, land applications of sewage sludge, animal husbandry and slurry pits. The hazard may be underground (e.g. on-site sanitation, sewers, landfill sites) or may be on the surface (e.g. waste dumps, animal husbandry and slurry pits). The nature of the hazard needs to be considered when undertaking a risk assessment and attention paid the likelihood of pathogen reductions through attenuation, die-off and dilution.
IWRM and Regulatory issues
The proper design of the facility is critical to protecting the borehole against ingress of pathogens. Wellhead completion and control measures in the immediate area are critical to reduce the risks of pathogens entering the supply. However, these measures should be supported by the development of a groundwater protection policy noted above.
Household handling storage and treatment of water
The safe handling and storage of water within the home is the final component of a safe water chain. Evidence from around the world suggests that this step is critical and that investments made in improving water source protection, treatment and distribution may not lead to significant improvements in health if household handling and storage is poor. In this section we deal both with storage of water in tanks within houses that are connected to a piped water supply and for storage in smaller containers when water is collected from a communal source of water.
Outbreaks of infectious diarrhoeal disease have occurred in both developed and developing countries resulting from contamination of plumbed in storage tanks in blocks of flats. Contamination of a storage tank by bird faeces has been a common problem. Poor storage and plumbing within buildings has also led to regrowth or bacteria and contamination with Legionella spp. remains a major problem in many countries. In these cases, interventions are primarily related to good operation and maintenance of water systems within buildings.
In addition to the problems noted above in relation to plumbing and large-scale storage within buildings, recontamination of water during collection, transport and storage when water is available only from a communal public water source are also widely reported (see background paper for more details). However, whilst clearly a significant problem, many of the studies indicating such problems have focused on indicator bacteria as opposed to pathogens. The relative importance of recontamination of water by pathogens has been questioned, given obvious greater potential for spread by other intra-familial routes (notably food) and likely acquired immunity (Vanderslice and Briscoe, 1995). However, control of recontamination is likely to be a key measure in reducing infectious disease transmission, although it should be integrated with a broader hygiene education programme dealing a variety of transmission routes.
Selection of reference pathogens and assumptions made
The scenarios outlined above relate to two very different aspects to poor water supply within households. To a certain extent, the measures put in place to control risks within piped distribution systems should also be adequate to deal with many of the problems related to poor in-building plumbing and storage. However, a few selected aspects are included within this section because of their particular importance. The rest of the section focuses directly on the safe handling of water and therefore may have greatest relevance to situations where the water supply is provided through a communal level of service.
For in-building plumbing, two principal areas of concern are noted for which reference pathogens should be selected. The first is regrowth within storage tanks and plumbing. As the principal risk will be relate to Legionella pneumophila this is taken as a key reference pathogen. The second area of concern is ingress into the storage or pipe work. The principal reference pathogen for this case is taken as being E.coli 0157. The control of all risks related to ingress are likely to be effective for all types of pathogen as they relate primarily to good sanitary integrity of the system as it can be expected that any residual disinfectant will disappear rapidly.
The hazard assessment would clearly need to look for likely sources of faeces within the building and is likely to primarily look at whether there is potential access into the storage tank for rodents and birds. Additional hazard factors to be considered will be the location of the tank and likely temperature and the materials used for storage. Location close to roofs may increase hazards as access for rodents and birds may be greater and may lead to increasing temperatures. Metal tanks may more readily support colonisation and exert a greater chlorine demand and may be likely to heat more quickly in hot weather than plastic tanks.
Household handling and storage when communal sources of water are used
With respect to the handling and storage of water when the source is communal, the principal reference pathogen of concern is E.coli 0157. Whilst the recontamination by viruses and protozoa may occur, most of the basic measures to prevent contamination from these organisms do not significantly vary from those associated with bacterial pathogens. The only major difference will come when in-house water treatment processes are used, for which drinking water quality management plans are defined separately. Actual health risks from viruses and cysts in drinking water may in any case be lower in situations where communal source provision predominates. This is because childhood exposure to viruses is likely to occur from other means and because cyst exposure may be likely through direct human-animal contact. It is uncertain to what extent regrowth will be a problem, but certainly could occur if cleaning was not adequately performed.
The hazards relate to the quality of source water (which therefore should be dealt with under the appropriate hazard assessment by source type) and potential subsequent contamination. An additional hazard is the presence of animals within the home. The most important hazard will almost certainly be from contaminated hands and therefore wherever non-piped water storage is practised it is safe to assume that hazards always exist.
IWRM and regulatory issues
There is usually significant scope for improved designs of within-building storage tanks. Improved storage containers are available for use when water is collected from a communal source by hand, but uptake may be influenced by a number of factors.
Household disinfection has been used in a number of countries and proved effective in reducing risks of epidemics and in reducing endemic diarrhoeal disease burdens. Chemical disinfection methods include the use of chlorine, iodine or mixed oxidants. This are generally found in either tablet or liquid form. Physical disinfection includes boiling of water, UV radiation and low-cost solar disinfection techniques that work through a mixture of inactivation through temperature and exposure to UV radiation. Good evidence of efficacy is available for all these approaches, both in terms of epidemiological evidence and in pathogen inactivation rates during operational testing.
The use of household disinfection has until recently received far less attention that it deserves. Some studies have suggested that household treatment of water would have limited impact on health where environmental sanitation or hygiene remained poor (see for instance, Vanderslice and Briscoe, 1993; Moe et al, 1991). However, increasing evidence from a number of initiatives suggests that this is not the case and significant reductions in diarrhoeal disease have been noted (see background document for details).
Heat induced inactivation is very effective for bacteria and cysts and a rolling boil (>95oC) will inactivate all pathogens. Inactivation of all types of pathogen also occurs at lower temperatures, with viruses being most heat resistant, followed by cysts and bacteria. Of the chemical disinfectants, chlorine is highly effective against bacteria and viruses, but far less so against protozoa. It is unlikely that chlorination would be recommended alone for removing Cryptosporidium spp. cysts.
Iodine and the mixed oxidants both show greater effectiveness in cyst inactivation. However, as the long-term use of iodine not be acceptable to most users, control of protozoan pathogens may be more effective through filtration prior to disinfection. Polyiodide resins have proved effective disinfectants and release very little residual disinfectant as inactivation occurs on contact with bacteria. However, filtration prior to disinfection is usually essential in order to remove suspended solids from influent water. Commercial units have been produced that incorporate reverse osmosis and iodine resins and prototype units for ceramic filter/resin units are also available.
Solar disinfection has attracted increasing attention as a low-cost approach to producing water of very good microbiological quality. Low-cost solar disinfection systems have also been shown capable of reducing Cryptosporidium spp. and other pathogens in water, although this is likely to be primarily be a function of increasing temperature. Many of these techniques operate on a combined action of heat inactivation and UV disinfection. UV filters are also commercially available and known to be effective. These would tend to be larger-scale units and likely to be used for large buildings rather than individual households given the expense.
Selection of reference pathogens and assumptions made
The use of household disinfection is always promoted because of concerns over the quality of the water at sources or because of concerns regarding contamination during transport, handling and storage.
The principal reference pathogens are Hepatitis A virus and E.coli 0157 to ensure that efficacy was assured. This may be expanded to include Cryptosporidium parvum when the disinfectant used is expected to inactivate cysts during practical operation. As household water treatment imply hazards are available, there is little point in undertaking hazard assessments other than those related to risks of source contamination or recontamination during handling.
Colonisation of certain types of household treatment systems have been noted, but would not be expected to represent a major problem for systems that are only for disinfection as colonisation appears most marked in filter units. Furthermore, where disinfection would be expected to directly control re-growth and therefore it would not be expected that the use of Legionella pnuemophila would be necessary.
It is important to note that when manufacturers or developers of household disinfection units are promoting their products that evidence is provided on pathogen inactivation and not simply on indicator bacteria reductions. This is essential as many of these units can be expected to be highly efficient with regard to coliform bacteria, but may have far less effectiveness against pathogens. It is also essential that data is presented on the basis of challenge experiments involving both batch and continuos run experiments. The latter should be designed to mimic real operating conditions and following the recommended cleaning procedures and frequencies. Failure to provide this type of data should suggest that licensing for widespread use is not justified, although this data could be collected through pilot field trials in the country.
IWRM and regulatory issues
Household filtration systems
Household filtration systems encompass a wide variety of technologies from sophisticated systems using reverse osmosis or micro-filtration, through less complex systems such as activated carbon cartridges, ceramic filters and combination units with disinfectants included, to very simple techniques using filtration based on sand or other granular media. The more complex systems tend to be those found in commercial units and which may be expensive to purchase. These may be point of entry units (i.e. plumbed into the piped water supply as it enters the home) or much smaller point of use systems. The very simple technologies are more typical of point of use units used at a household levels and less expensive to construct, although not necessarily lower maintenance.
Filtration devices remove particulate matter from water and as a result may lead to reductions in pathogen loads. Direct pathogen removal will primarily be a function of the pore size, although some adsorption onto the filter media may also occur. It is unlikely that either of these processes will be fully effective for viruses or bacteria, although in most fine filters cyst removal should be effective.
Particular attention should be paid to the development of cracks the filter media as this may allow rapid short-circuiting of the filter and increasing risks of pathogen breakthrough. Some ceramic filters are impregnated with silver which it is claimed will function as a disinfectant. There is little evidence of long-term bactericidal effect of silver and studies suggest that the bacteriostatic properties may be limited as silver tolerant bacteria can colonise filters. It is possible that that the limitations of the silver impregnation occur because whilst initial concentrations released are high, they rapidly decline to levels too low to be effective.
More expensive and very fine filters, for instance based on membrane filtration are likely to remove more pathogens. Micro-filtration will be effective against cysts, but would be not be effective against bacteria and viruses, although removal of particulate matter may reduce concentrations to a certain degree. Reverse osmosis would be expected to remove virtually all pathogens, but clogging may be problem. More widely available commercial units using ceramic or carbon filters will remove cysts and some bacteria and viruses. However, breakthrough by of viral or bacterial pathogens is common. Both types of filter may also be prone to colonisation, including by Legionella pneumophila. Water from such filters should normally be disinfected prior to consumption.
Simple filtration units are known to be effective for removing larger pathogens and may be useful in guinea worm eradication programmes. Their efficacy in bacterial, viral and protozoan pathogen removal is less certain. There are few available studies that evaluate the effectiveness of many of the much simpler filtration units in pathogen removal, largely because their application has been in very poor and often remote communities. The limited evidence available provides information on reductions in turbidity and indicator bacteria. These show that thermotolerant coliforms are rarely consistently absent, suggesting limited ability to remove pathogens.
Selection of reference pathogens and assumptions made
As filtration of water within the home may be carried out either because the water supply is of poor quality or because of concerns over chemical quality, the use of all four key reference pathogens could be justified to a certain extent. However, this may not be case in all circumstances and the reference pathogens selected may be somewhat dependent on the type of technology and the socio-economic conditions.
All filters would be expected to be effective at least against cysts and therefore Cryptosporidium parvum should be considered a reference pathogen against which performance is measured, even in situations where alternative routes of transmission may be more important. For simple granular filters and for ceramic candle filters and carbon filters without disinfectant impregnation this is will be the only valid reference pathogen. Disinfection of the water after filtration should be always recommended (unless specific evidence can be provided on bacterial and viral inactivation). Where the filter unit includes a disinfectant or uses reverse osmosis, then Hepatitis A, E.coli 0157 and Legionella pnuemophila should be considered as reference pathogens.
As the use of household filtration implies that source waters are contaminated, hazard assessments other than those related to the source are not necessary.
IWRM and regulatory issues
These are effectively the same as for household disinfection, but are repeated here for completeness.