The Multi-Barrier Approach for Household Water Treatment Systems (HWTS)
Using the multi-barrier approach is the best way to reduce the risk of drinking unsafe water. Each step in the process, from source protection, to water treatment and safe storage, provides an incremental health risk reduction. The household water treatment process includes: sedimentation, filtration and disinfection.
More often than not, people focus on a particular technology that is directed towards one step rather than considering
the water treatment process as a whole. While individual technologies, like the biosand filter, can incrementally improve drinking water quality, the entire process is essential in providing the best water quality possible.
· Sedimentation to remove larger particles and often > 50% of pathogens
· Filtration to remove smaller particles and often > 90% of pathogens
· Disinfection to remove, deactivate or kill any remaining pathogens
The household water treatment process is primarily focused on removing pathogens from drinking water – the biggest water quality issue around the world. While improving the microbiological quality, there are some technologies that may also be able to remove certain chemicals as a secondary benefit, such as arsenic and iron.
Although all five components of the multi-barrier approach greatly help to improve the quality of drinking water, this manual focuses primarily on filtration, which should be used in combination with the other components to ensure healthy, uncontaminated water..
Biosand Filter Overview What is the Biosand Filter?
The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community water treatment for almost 200 hundred years. The biosand filter is smaller and adapted for intermittent use, making it suitable for households. The filter container can be made of concrete or plastic and is filled with layers of specially selected and prepared sand and gravel.
History of the Biosand Filter
Dr. David Manz developed the household biosand filter in the 1990s at the University of Calgary, Canada. Dr Manz has trained many organizations on the design, construction, installation, operation and maintenance of the biosand filter. He also co-founded CAWST in 2001 to provide the professional services needed for the humanitarian distribution of the filter in developing countries. As of September 2011, CAWST estimates that over 300,000 biosand filters have been implemented in more than 50 countries around the world.
Biosand Filter Components.
Lid – Tightly fitting lid prevents contamination and unwanted pests.
Diffuser – Prevents disturbing the filtration sand layer and protects the biolayer when water is poured into the filter.
Filtration Sand Layer – Removes pathogens and suspended solids.
Outlet Tube – Required to conduct water from the base to the outside of the filter.
Filter Body – Holds the sand and gravel layers.
Separating Gravel Layer – Supports the filtration sand and prevents it from going into the drainage layer and outlet tube.
Drainage Gravel Layer – Supports the separating gravel layer and helps water to flow into the outlet tube.
How Does the Biosand Filter Work?
The biosand filter has five distinct zones: 1) inlet reservoir zone, 2) standing water zone, 3) biological zone, 4) non-biological zone, and 5) gravel zone.
1. Inlet Reservoir Zone - Where water is poured into the filter.
2. Standing Water Zone – This water keeps the sand wet while letting oxygen pass to the biolayer.
3. Biological Zone – Develops at the top 5-10 cm (2-4”) of the sand surface. The filtration sand removes pathogens, suspended particles and other contaminants. As in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the top 1-2 cm (0.4-0.8”) of the sand surface.
4. Non-Biological Zone – Contains virtually no
living microorganisms due to the lack of nutrients and oxygen.
5. Gravel Zone – Holds the sand in place and protects the outlet tube from clogging.
Pathogens and suspended solids are removed through a combination of biological and physical
processes that take place in the biolayer and within the sand layer. These processes include:
mechanical trapping, predation, adsorption, and natural death.
• Mechanical trapping. Suspended solids and pathogens are physically trapped in the spaces between the sand grains.
• Predation. Pathogens are consumed by other microorganisms in the biolayer.
• Adsorption. Pathogens become attached to each other, suspended solids in the water, and the sand grains.
• Natural death. Pathogens finish their life cycle or die because there is not enough food or oxygen for them to survive.
Contaminated water is poured into the reservoir on an intermittent basis. The water slowly passes through the diffuser and percolates down through the biolayer, sand and gravel. Treated water naturally flows from the outlet tube.
During the Run
The high water level pushes the water through the diffuser and filter (also called the hydraulic head). The water level in the reservoir goes down as it flows evenly through the sand. The flow rate will slow down over time because there is less pressure to force the water through the filter. The inlet water contains dissolved oxygen, nutrients and contaminants. It provides the oxygen required by the microorganisms in the biolayer. Larger suspended particles and pathogens are trapped in the top of the sand and they partially plug the pore spaces between the sand grains. This also causes the flow rate to slow down.
The water finally stops flowing. The standing water layer will be at the same height as the end of the outlet tube. Some oxygen from the air diffuses through the standing water to the biolayer. The pause period allows time for microorganisms in the biolayer to consume the pathogens and nutrients in the water. The flow rate through the filter is restored as they are consumed. If the pause period is too long, the biolayer will eventually consume all of the pathogens and nutrients and eventually die off. This will reduce the removal efficiency of the filter when it is used again. The pause period should be a minimum of 1 hour after the water has stopped flowing up to a maximum of 48 hours. Pathogens in the non-biological zone die off due to the lack of nutrients and oxygen.
How Well Does the Biosand Filter Work?
Water naturally contains many living things. Some are harmless and others can make people sick. Living things that cause disease are also known as pathogens. They are sometimes called other names, such as microorganisms, microbes or bugs, depending on the local language and country.
The physical characteristics of drinking water are usually things that we can measure with our senses: turbidity, colour, taste, smell and temperature. Turbid water looks cloudy, dirty or muddy. Turbidity is caused by sand, silt and clay that are floating in the water. Drinking turbid water will not make people sick by itself. However, viruses, parasites and some bacteria can sometimes attach themselves to the suspended solids in water. This means that turbid water usually has more pathogens so drinking it increases the chances of becoming sick.
The biosand filter can be used with any water source such as rainwater, deep groundwater, shallow groundwater, rivers, lakes or other surface water. The source should be the cleanest available since the filter is not able to remove 100% of the pathogens and turbidity. If the source water is very contaminated, the filtered water may still have some contaminants.
Over time, the biolayer becomes adapted to a certain amount of contamination from the source water. If source water with a different level and type of contamination is used, the biolayer may not be able to consume all of the pathogens. It may take the biolayer several days to adapt to the new source water, level of contamination, and nutrients. It is recommended to consistently use the same source water to ensure the highest treatment efficiency.
The turbidity of the source water is also a key factor in the operation of the filter. Higher turbidity levels will plug the filtration sand layer more quickly. As such, maintenance will be required more often to ensure a convenient flow rate for the user. It is recommended to use a sedimentation method if the source water turbidity is greater than 50 NTU. A simple test to measure the turbidity is to use a 2 litre clear plastic bottle filled with the source water. Place this on top of large print such as this manual. If you can see this logo looking down through the top of the bottle, the water probably has a turbidity of less than 50 NTU.
The biolayer is the key component of the filter that removes pathogens. Without it, the filter removes about 30-70% of the pathogens through mechanical trapping and adsorption. The ideal biolayer will increase the treatment efficiency up to 99% removal of pathogen
It may take up to 30 days for the biolayer to fully form. During that time, both the removal efficiency and the oxygen demand will increase as the biolayer grows. The biolayer is NOT visible – it is NOT a green slimy coating on top of the sand. The filtration sand may turn a darker colour, but this is due to the suspended solids that have become trapped.
The water from the filter can be used during the first few weeks while the biolayer is being established, but disinfection, as always, is recommended during this time.
The biosand filter has been designed to allow for a filter loading rate (flow rate per square metre of sand surface area) which has proven to be effective in laboratory and field tests. There is a recommended filter loading rate for each biosand filter design. For the concrete Version 10 biosand filter, it has been determined to be not more than 400 litres/hour/square metre.
The biosand filter is most effective and efficient when operated intermittently and consistently. The pause period should be a minimum of 1 hour after the water has stopped flowing up to a maximum of 48 hours. The pause period is important because it allows time for the microorganisms in the biolayer to consume the pathogens in the water. As the pathogens are consumed, the flow rate through the filter may be restored. If the pause period is extended for too long, the microorganisms will eventually consume all of the nutrients and pathogens and then eventually die off. This will reduce the removal efficiency of the filter when it is used again.
Standing Water Layer
Correct installation and operation of the biosand filter requires a standing water depth of approximately 5 cm (2”) above the sand during the pause period. The standing water depth can be 4-6 cm, but ideally it should be at 5 cm (2”).
A water depth of greater than 5 cm (2”) results in lower oxygen diffusion and consequently a thinner biolayer. A high water level can be caused by a blocked outlet tube, an insufficient amount of sand installed in the filter or the sand settling in the first few weeks of use.
The treatment efficiency declines somewhat after maintenance, but returns to its previous level as the biolayer is re-established.
A water depth less than 5 cm (2”) may evaporate quickly in hot climates and cause the biolayer to dry out. A low water level may be caused by too much sand being put into the filter during installation.
The spaces between the sand grains will become plugged with suspended solids over time. As a result, the flow rate will slow down. A slower flow rate is not an issue in terms of water quality. In fact, the slower the flow rate, the better the water quality. However, it may become slow enough that it is inconvenient for the user and they may choose to not use the filter at all. When the flow becomes much slower than the recommended rate, the user will need to do basic maintenance (called the “swirl and dump”) to restore it. As well, users will need to clean the outlet tube, safe storage container, diffuser, lid, and outside surfaces of the filter on a regular basis.
Although the water may look clear after filtration, it is still necessary to disinfect it to ensure the best water quality possible. The biosand filter removes most, but not all of the bacteria and viruses. The most common methods used around the world to disinfect drinking water are:
Solar disinfection (SODIS)
Ultraviolet (UV) disinfection
When water has high levels of turbidity, pathogens “hide” behind the suspended solids and are difficult to kill using chemical, SODIS and UV disinfection. The biosand filter reduces the turbidity and is a necessary step to improve the effectiveness of these disinfection methods.
Safe Water Storage
People do a lot of work to collect, transport and treat their drinking water. Now that the water is safe to drink, it should be handled and stored properly to keep it safe. If it’s not stored safely, the treated water quality could become worse than the source water and may cause people to get sick. Recontamination of safe drinking water is a common issue around the world and has been documented in several cases.
Safe storage means keeping treated water away from sources of contamination, and using a clean and covered container. It also means drinking water from the container in a way so that people don’t make each other sick. The container should prevent hands, cups and dippers from touching the water, so that the water isn’t recontaminated.
There are many designs for water containers around the world. A safe water storage container should have the following qualities:
Strong and tightly fitting lid or cover
Tap or narrow opening at the outlet
Stable base so it doesn’t tip over
Durable and strong
Should not be transparent (see-through)
Easy to clean
Other safe water handling practices include:
Using a container to collect and store untreated water and using it only for untreated water
Using a different container to store treated water - never use this container for untreated water
Frequently cleaning out the storage container with soap or chlorine
Storing treated water off the ground in a shady place in the home
Storing treated water away from small children and animals
Pouring treated water from the container instead of scooping the water out of it
Drinking treated water as soon as possible, preferably the same day
Sometimes it is difficult for rural and poor households to find or buy a good storage container. The most important things are to make sure that it is covered and only used for treated water.