Water disinfection methods

In the last decades, governments in developing countries have invested large amounts in the installation of water treatment plants and public water supply systems, especially in urban areas. However, these plants often fail to produce water safe for consumption due to several issues (non-reliable operation and maintenance of the systems, the lack of chemicals and spare parts, or financial constraints, among other problems). Water shortages frequently lead to water supply interruptions and leaky distribution systems worsen the situation. In addition, the rapid population growth in urban areas puts an excessive stress on the existing infrastructures and creates enormous problems in planning and constructing new water and sanitation facilities.

Therefore, inhabitants of many urban centres in developing countries, as well as the rural population, only have access to water of poor quality. Thus, the treatment of water to be safe for consumption remains under the responsibility of each individual household. Options that rely solely on time- and resource-intensive centralised solutions would leave hundreds of millions of people without access to safe water, and other approaches to support the households should be promoted. The following domestic treatment methods are usually recommended to reduce the faecal contamination in drinking water.

Boiling

Boiling drinking water with fuel is the oldest and most commonly practiced household water treatment method. According to WHO, water needs to be heated until the appearance of the first big bubbles to ensure that it is pathogen free. Many organizations recommend boiling both for water treatment in developing countries and to provide safe drinking water in emergency situations throughout the world – though it is quite laborious and uses a lot of energy. Boiling only kills pathogens and does not remove turbidity or chemical pollution from drinking water. So prior to boiling, water can be purified by settling or filtration.

Effectiveness and Health Impact

If the boiling point is reached, boiling is effective in killing bacteria, viruses, protozoa, helminths, and most pathogens from drinking water. Incomplete inactivation of pathogens in boiled water is attributed to users not heating the water to the boiling point and/or recontamination of boiled water in storage. Boiling does not remove turbidity, chemicals (e.g. arsenic), taste, smell or colour from water. Therefore, settling or even filtration (by cloth or slow sand or biosand filter) is often needed prior to boiling.

Implementation Costs

Boiling is suitable where enough fuel sources (e.g., wood, kerosene, electricity, gas, charcoal etc.) are locally available all the time at affordable cost. Especially in densely populated areas, boiling with fuel wood is not appropriate due to the overexploitation of the wood resources and the subsequent environmental damage such as desertification and soil erosion. Boiled water may cause burn injuries if not handled properly. Long-term exposure to fire or stove smoke may cause associated respiratory diseases. For this indoor cooking, space should be made well ventilated.

Benefits

Effectively kills most pathogens
Easy, simple, and widely accepted method of disinfection
Biogas cooking stoves can be used

Drawbacks

Can be costly due to fuel consumptions
Use of traditional fuel (firewood, kerosene/gas) contributes to deforestation and indoor air pollution
Potential user taste objections
Risk of injuries, especially when children are around
Does not remove turbidity, chemicals, taste, smell, and colour
Time consuming
Water needs to cool down before use unless for hot drinks

Pasteurisation

Contrary to belief, it is not necessary to boil water to make it safe to drink. Heating water to 65°C (149°F) for 6 minutes, or to a higher temperature for a shorter time, will kill all germs, viruses, and parasites. This process is called pasteurisation.

A simple method of pasteurising water is to put blackened containers with water in a solar cooker. The solar cooker reflects sunlight onto the container, which heats up the content. The cooker may be an insulated box made of wood, cardboard, plastic, or woven straw, with reflective panels to concentrate sunlight onto the water container. The box cooker should be frequently repositioned to ensure it is catching all available sunlight and never in shade. A thermometer or indicator is needed to tell when the required temperature is reached for pasteurisation to monitor the required exposure time of six minutes. Common devices for monitoring the water temperature use either beeswax, which melts at 62°C (144°F), or soybean fat, which melts at 69°C (156°F).

Effectiveness and Health Impact

As the water heats due to solar radiation, the increased temperature between 65° and 75°C (149-167°F) will kill or inactivate an important part of commonly waterborne pathogenic bacteria, viruses, helminths, and protozoa. But spores are more resistant to thermal inactivation than vegetative cells; treatment to reduce spores requires a thermal treatment up to boiling point and must ensure sufficient temperature and time. Furthermore, solar pasteurisation does not improve turbidity, odour, taste, colour, or chemical contamination.

In rural Kenya, a simple thermo indicator which changes colour at 70°C was applied to show household members when pasteurisation temperature had been reached. This increased the number of households whose drinking water was free of coliforms from 10.7 to 43.1% and significantly reduced the incidence of severe diarrhoea compared to a control group.

Implementation Costs

Solar pasteurisation has proven to be a very low-cost disinfection method to produce drinking water out of non-turbid fresh water. However, solar pasteurisation is not that easy to implement and monitor, thus it is not a widespread method for point-of-use water treatment. Heating the water to the pasteurisation temperature rather than the boiling point reduces the energy required by at least 50%. But solar pasteurisation is only effective if treated water is protected from post-treatment contamination during storage.

The solar pasteurisation method seems more suitable on household level rather than for producing high quantities of drinking water. Because it relies on solar energy its effectiveness depends on the daily hours of sunshine in the area of application. Solar pasteurisation might be an option on both, the village and household level, but household usage is more competitive because village-scale alternatives have much lower treatment costs. Existing solar devices have water disinfection costs that are an order of magnitude lower than boiling.

Benefits

The system requires no additional inputs (electricity, chemicals or fossil fuels) after installation
Simple designs are available at very low cost, and this device may be built with parts available in most countries
Anyone can be trained to construct a solar cooker and there are no specific manufacturing hazards
Solar pasteurisation boxes can also be used as solar cookers for cooking meals
Compared to boiling, the pasteurisation process does not consume wood, charcoal, or other biomass as energy supply (environmentally more sustainable) and does not take time and energy for its procurement

Drawbacks

Requires sunny weather and does not work during continuous rainfall, on very cloudy days, or under freezing conditions
Recontamination is possible after the water has cooled because it contains no residual disinfectant; subsequent safe storage is essential
Does not reduce turbidity, odour, taste, or colour and does not remove chemical pollutants from water
Users require a thermometer or pasteurisation indicator device
Water needs to cool down before use
Cookers are made from lightweight and easily breakable materials

Chlorination

The Safe Water System (SWS) was developed in the 1990’s in response to epidemic cholera in South America by the Centers for Disease Control and Prevention (CDC) and the Pan American Health Organization. The treatment method for the SWS is point-of-use chlorination by consumers with a locally-manufactured dilute sodium hypochlorite (chlorine bleach) solution. To use the chlorination method, families add one full bottle cap of the sodium hypochlorite solution to clear water (or 2 caps to turbid water) in a standard sized container, agitate, and wait 30 minutes before drinking.

Effectiveness and Health Impact

At concentrations that are used for household water treatment programs, the hypochlorite solution is effective at inactivating most bacteria and viruses that cause diarrheal disease. However, it is not effective at inactivating some protozoa, such as Cryptosporidium. Numerous studies have shown complete removal of bacterial pathogens in SWS treated water in developing countries. In seven randomized, controlled trials, the SWS has resulted in reductions in diarrheal disease incidence in users ranging from 22-84%. These studies have been conducted in rural and urban areas, and include adults and children that are poor, living with HIV, or using highly turbid water.

Implementation Costs

A bottle of hypochlorite solution that treats 1,000 litres of water costs about 10 US cents using refillable bottles and 11-50 US cents using disposable bottles, for a cost of 0.01-0.05 cents per litre treated. SWS programs can be fully subsidized such as in emergency situations.

In a project in Zambia, the average cost per bottle (treating 1,000 litres) of production, marketing, and distribution at project initiation in 1999 was $1.88. This decreased by 82% to 0.033 US cents per litre treated in 2003, when 1.7 million bottles were sold, showing that significant cost efficiencies can be gained as programs grow to scale.

Benefits

Proven reduction of most bacteria and viruses in water
Residual protection against recontamination
Ease-of-use and acceptability
Proven reduction of diarrheal disease incidence
Scalability and low cost

Drawbacks

Relatively low protection against protozoa
Lower disinfection effectiveness in turbid waters
Potential taste and odour objections
Must ensure quality control of solution
Potential long-term effects of chlorination by-products

Flocculant/Disinfectant Powder

The Procter & Gamble Company developed P&G Purifier of Water™ in conjunction with the CDC. P&G™ sachets are now centrally produced in Pakistan and sold to NGOs worldwide. The product is a small sachet containing powdered ferric sulphate (flocculant) and calcium hypochlorite (disinfectant). To treat water, users open the sachet, add the contents to an open bucket containing 10 litres of water, stir for 5 minutes, let the solids settle to the bottom of the bucket, strain the water through a cotton cloth into a second container, and wait 20 minutes for the hypochlorite to inactivate the microorganisms.

Effectiveness and Health Impact

This flocculant/disinfectant powder has been proven to remove most bacteria, viruses, and protozoa, even in highly turbid waters. It has also been documented to reduce diarrheal disease from 90% to less than 16% incidence in five randomized, controlled health intervention studies. This powder also removes heavy metals—such as arsenic—and chemical contaminants—such as pesticides—from water. Studies showing its efficacy have been conducted for highly turbid water in the laboratory, in developing countries, in rural and urban areas, refugee camps, and include all age groups.

Implementation Costs

Each sachet of P&G™ is provided to global emergency relief organizations or non-governmental organizations at a cost of 3.5 US cents, not including shipping from Pakistan by ocean container. Sachets are generally free distributed in emergency situations.

Benefits

Proven reduction of bacteria, viruses, and protozoa in water
Removal of heavy metals and chemicals
Increased free chlorine protection against recontamination
Proven reduction of diarrheal disease
Visual improvement of water and acceptability
Transport of sachets easy
Long shelf life of sachets

Drawbacks

Multiple steps are necessary—requires training or demonstration
Requires a lot of equipment (2 buckets, cloth, and a stirrer)
The higher relative cost per litre of water treated

Other similar sachets:

Flo-Chlor

Watermaker

Solar Disinfection or SODIS Method

Solar disinfection (SODIS) was developed in the 1980’s to inexpensively disinfect water used for oral rehydration solutions. In 1991, the Swiss Federal Institute for Environmental Science and Technology began to investigate and implement SODIS as a household water treatment option to prevent diarrhoea in developing countries. Users of SODIS fill 0.3-2.0 litre plastic soda bottles with low-turbidity water, shake them to oxygenate, and place the bottles on a roof or rack for 6 hours (if sunny) or 2 days (if cloudy). The combined effects of ultra-violet light (UV)-induced DNA damage, thermal inactivation, and photo-oxidative destruction inactivate disease-causing organisms.

Effectiveness and Health Impact

In the laboratory, SODIS has been proven to inactivate the viruses, bacteria, and protozoa that cause diarrheal diseases. Field data have also shown reductions of viruses, bacteria, and protozoa in water from developing countries treated with SODIS. In four randomized, controlled trials, SODIS has resulted in reductions in diarrheal disease incidence ranging from 9-86%.

Implementation Costs

SODIS, as a virtually zero-cost technology, faces marketing constraints. Since 2001, local NGOs in 28 countries have disseminated SODIS through training of trainers, educating at the grassroots level, providing technical assistance to partner organizations, lobbying key players, and establishing information networks. The experiences gained have shown that SODIS is best promoted and disseminated by local institutions with experience in community health education. Both the Swiss Federal Institute of Aquatic Research and Technologyl and the SODIS Foundation provide technical assistance to NGOs implementing SODIS.

Benefits

Proven reduction of viruses, bacteria, and protozoa in water
Proven reduction of diarrheal disease incidence
Simplicity of use and acceptability
No cost if using recycled plastic bottles
Minimal change in taste of the water
Recontamination is low because water is served and stored in the small narrow necked bottles

Drawbacks

Need to pretreat water of higher turbidity with flocculation and/or filtration
Limited volume of water that can be treated all at once
Length of time required to treat water
Large supply of intact, clean, suitable plastic bottles required

Ceramic Filtration

Locally manufactured ceramic filters have traditionally been used throughout the world to treat household water. Currently, the most widely implemented ceramic filter is the Potters for Peace (link to https://www.pottersforpeace.org/) design. The filter is flowerpot shaped, holds about 8-10 litres of water, and sits inside a plastic or ceramic receptacle. To use the ceramic filters, families fill the top receptacle or the ceramic filter itself with water, which flows through the ceramic filter or filters into a storage receptacle. The treated water is then accessed via a spigot embedded within the water storage receptacle. The filters are produced locally at ceramics facilities, and then impregnated with colloidal silver to ensure complete removal of bacteria in treated water and to prevent growth of bacteria within the filter itself. Numerous other locally-made and commercial ceramic filters are widely available in developed and developing countries.

Effectiveness and Health Impact

The effectiveness of ceramic filters at removing bacteria, viruses, and protozoa depends on the production quality of the ceramic filter. Most ceramic filters are effective at removing bacteria and the larger protozoans, but not at removing viruses. Studies have shown adequate removal of bacterial pathogens in water filtered through high quality locally produced or imported ceramic filters in developing countries. A 60-70% reduction in diarrheal disease incidence has been documented in users of these filters. Studies have also shown significant bacterial contamination when poor-quality locally produced filters are used, or when the receptacle is contaminated at the household level. Because there is no chlorine residual protection, it is important that users be trained to properly care for and maintain the ceramic filter and receptacle.

Implementation Costs

Locally manufactured ceramic Potters for Peace-design filters range in cost from $7.50-$30. Distribution, education, and community motivation can add significantly to program costs. Ceramic filter programs can be fully subsidized such as in emergency situations. If a family treats 20 litres of water per day (running the filter continuously) and the filter lasts 3 years, then the cost per litre treated (including cost of filter only) is 0.034-0.14 US cents. Commercially available ceramic filter systems range in cost from tens to hundreds of US dollars, depending on where they are manufactured and purchased, and the quality of the ceramic filters.

Benefits

Proven reduction of bacteria and protozoa in water
Simplicity of use and acceptability
Proven reduction of diarrheal disease incidence for users
Long life if the filter remains unbroken
A low one-time cost

Drawbacks

Not as effective against viruses
No chlorine residual protection – can lead to recontamination
Variable quality control for locally produced filters
Filters can break over time – need for spare parts
A low flow rate of 1-3 litres per hour for non-turbid waters
Filters and receptacles must be cleaned regularly, especially after filtering turbid water

Slow Sand Filtration

A slow sand filter is a sand filter adapted for household use. Please note that although commonly referred to as the BioSand Filter, this terminology is trademarked to one particular design, and this page encompasses all slow sand filters. The version most widely implemented consists of layers of sand and gravel in a concrete or plastic container approximately 0.9 m tall and 0.3 m squared. The water level is maintained to 5-6 cm above the sand layer by setting the height of the outlet pipe. This shallow water layer allows a bioactive film to grow on top of the sand, which contributes to the reduction of disease-causing organisms. A diffuser plate is used to prevent disruption of the biolayer when water is added. To use the filter, users simply pour water into the top, and collect finished water out of the outlet pipe into a bucket. Over time, especially if source water is turbid, the flow rate can decrease. Users can maintain flow rate by cleaning the filter through agitating the top level of sand, or by pre-treating turbid water before filtration.

Effectiveness and Health Impact

Slow sand filter lab effectiveness studies with a mature biolayer have shown 99.98% protozoan, 90-99% bacterial, and variable viral reduction. Field effectiveness studies have documented E. coli removal rates of 80-98%. Two health impact studies report 44-47% reduction of diarrheal disease incidence in users. Experience has shown that appropriate filter maintenance is necessary for optimal performance, so proper user training and follow-up is critical to filter success. Since the filter is typically used without subsequent chlorination, training users to correctly care for and maintain a safe storage container is also necessary.

Implementation Costs

The average slow sand filter’s construction cost ranges from US $15-$60, depending on whether local or imported materials are used. Filter programs are either fully subsidized or operated at partial cost recovery using donor funds. Community motivation, distribution, education, and follow-up can add significantly to program costs. For the NGO Samaritan’s Purse, the average overall cost is about $100 USD per filter. Assuming it lasts 10 years and families filter 40 litres per day, the cost per litre of treated water is 0.068 US cents. Some NGOs have worked to train local entrepreneurs to manufacture, promote, and sell the filters within their communities, although this has met with limited success due to high initial filter expense and difficulty in identifying appropriate local entrepreneurs. Commercial implementation models are currently being explored.

Benefits

Proven reduction of protozoa and most bacteria
High flow rate of up to 0.6 litres per minute
Simplicity of use and acceptability
Visual improvement of the water
Production of sufficient quantities of water for all household uses
Local production (if clean, appropriate sand is available)
One-time installation with low maintenance requirements
Long life (estimated >10 years) with no recurrent expenses

Drawbacks

Not as effective against viruses
No chlorine residual protection – can lead to recontamination
Routine cleaning can harm the biolayer and decrease effectiveness
Difficult to transport due to weight – high initial cost

Links to find out more

Household Water Treatment and Safe Storage Systems (Sustainable Sanitation and Water Management)

The efficacy of chlorine-based disinfectants against planktonic and biofilm bacteria for decentralised point-of-use drinking water

Household Water Treatment and Safe Storage (Centre for Affordable Water and Sanitation Technology)

Potters for Peace

Evaluating household water treatment options (World Health Organization)

Household Water Treatment (Centers for Disease Control and Prevention)

Emergency Disinfection of Drinking Water (US Environmental Protection Agency)