Module 2
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Module
2.1 2.1 BioSand Filter Purpose and Design
BioSand Filters Ready for Schools in Haiti The purpose of constructing the BioSand filter is to enable the user to enjoy good quality water which should lead to better health. To provide the filter constructor with an appreciation of the need for good quality water, it is important to understand the relationship between good health and good water supply, treatment, sanitation and hygiene. This module is a brief explanation of these relationships. This module also gives the design basis for the concrete BioSand filter and why the filter is built as it is.
Module 2.1 BioSand Filter Purpose and Design
2.1 BIOSAND FILTER PURPOSE AND DESIGN........................................................1 WATER TREATMENT..............................................................................................................2 Sedimentation..............................................................................................................2 Filtration......................................................................................................................3 Disinfection..................................................................................................................3 BIOSAND FILTER..................................................................................................................3 THE FILTRATION PROCESS......................................................................................................6 The Pause Period ........................................................................................................6 Oxygen Gradient..........................................................................................................6 The Start of the Run.....................................................................................................8 Halfway Through the Run ...........................................................................................9 The End of the Run....................................................................................................10 ARSENIC REMOVAL.............................................................................................................11 FILTER OPERATION..............................................................................................................11 Flow Rates.................................................................................................................12 Pause Periods............................................................................................................12 Water Depths.............................................................................................................12 Influent Water Quality...............................................................................................13 Filtered water quality................................................................................................13 Maintenance .............................................................................................................13 Time...........................................................................................................................14 Conclusion.................................................................................................................14 THE BIOSAND FILTER ADVANTAGES .....................................................................................16 THE BIOSAND FILTER LIMITATIONS......................................................................................16 COMPARING TECHNOLOGIES.................................................................................................16 Functionality..............................................................................................................17 Capital Cost...............................................................................................................17 Operating Costs.........................................................................................................17 APPENDICES.......................................................................................................................18 Appendix A: BioSand Filter – Specification Sheet....................................................18 Appendix B: BioSand Filter – Summary of all Lab and Field Tests..........................18
Water Treatment Raw water is treated in 3 main ways: sedimentation, filtration and disinfection. Sedimentation
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Module 2.1 BioSand Filter Purpose and Design If the water contains particles of sand, grit and dirt, it can be left in a container for some time to allow the particles to settle. Bacteria often grow attached to particle surfaces. Removal of particles by sedimentation will produce a marked reduction in bacterial concentrations. This reduction is achieved by allowing a container of water to settle for 2 – 12 hours. Filtration Filters remove pathogens in several ways. These include straining, where the particles or larger pathogens such as worms become trapped in the small spaces between the grains; adsorption, where pathogens become attached to the filter media; and biologic processes, where pathogens die naturally or the micro organisms which live in the filter consume the bacteria and pathogens. Examples of filtration systems: rapid sand filter, ceramic filter, slow sand filters and the biosand filter. Disinfection Disinfection comes about primarily through the destruction of the organism cell walls by oxidation. This oxidation is normally a result of the addition of chemicals such as chlorine. It can also be induced by ultraviolet radiation. Pathogens can hide from disinfecting agents in organic and inorganic residue in the water. Removal of suspended materials by sedimentation and filtration greatly improve the performance of chemical disinfection agents. Examples of disinfection are the addition of chlorine or sodium hypochlorite to water, solar disinfection (SODIS) or solar pasteurization.
BioSand Filter The major benefits of slow sand filtration are due to the microbiology of the filter. The microbiological community must be kept alive for the filter to be effective. In a conventional slow sand filter, oxygen is supplied to the organisms through dissolved oxygen in the water. Consequently, they are designed to be operated continuously. Also, because the water moves through at a slow rate, the filter beds tend to be very large. The biosand water filter is an invention that modifies the traditional slow sand filter in such a way that the filters can be built on a smaller scale and can be operated intermittently. These modifications make the filter suitable for use at the household or small group level. Household use would simply not be possible with conventional slow sand filtration because of the size requirements and the mode of operation.
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Module 2.1 BioSand Filter Purpose and Design A bucket of contaminated water can be poured into the top of the BioSand filter as necessary. The water simply flows through the filter and is collected in another bucket or container at the base of the spout. It normally takes a few minutes for the entire bucket to make its way through the filter. There are no valves or moving parts and the design of the outlet pipe ensures that a minimum water depth of five centimetres is maintained over the sand when the filter is not in use. When water is flowing through the filter, oxygen is supplied to the biologic layer at the top of the sand by the dissolved oxygen in the water. During pause times, when the water is not flowing, the oxygen is obtained by diffusion from the air and by slow convective mixing of the layer of water above the sand. If this layer is kept shallow, enough oxygen is able to pass through to the micro organisms to keep them alive and thus effective. The filter is made up of five distinct regions; the influent reservoir, the supernatant, the schmutzdecke, the biologically active zone, and the sand support and underdrain, as shown below.
INFLUENT RESERVOIR – Volume above the filter media which allows for a full pail of water
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Module 2.1 BioSand Filter Purpose and Design
SUPERNATANT – The standing water layer present during pause periods - Oxygen can diffuse into the water SCHMUTZDECKE – Layer of slime, mud and micro-organisms which develops at the sand surface
BIOLOGICAL ZONE – 510 cm deep – living layer of micro-organisms
SAND BED – Contains virtually no living microorganisms
GRAVEL – No living mirco-organisms present
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Module 2.1 BioSand Filter Purpose and Design
The Filtration Process The Pause Period (When No Water Is Flowing) Oxygen in the air diffuses through the supernatant to the biologic layer. Since the oxygen demand is constant, the diffusion is steady state. Nutrients are consumed over the entire pause period reducing the concentration of suspended contaminants in the supernatant.
Slow convective mixing of the supernatant increases oxygen available to the biologic layer.
Organisms die off due to lack of substrate food and oxygen or they move toward the hospitable environment at the sand surface. Therefore the level of contaminants in this area and below is small.
Micro organisms in the biologic layer use diffused oxygen and consume pathogens. This results in growth of the organisms. Organic material is reduced or stabilized and converted into energy. Insoluble organics are converted to soluble salts. This results in partial unplugging of the filter pores during the pause period. Oxygen Gradient Oxygen gradient is lower in the supernatant because the total area is available for oxygen transfer. It is higher in the sand bed because the area available for oxygen transfer is partially blocked by the sand grains.
Air Water Biologic Layer
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Module 2.1 BioSand Filter Purpose and Design
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Module 2.1 BioSand Filter Purpose and Design The Start of the Run
The high water level provides the hydrostatic head to push the water through the filter.
The dissolved oxygen in the water provides the oxygen required by the microbes in the biologic layer.
Organic and inorganic material is strained out at the top of the sand.
Influent Water, high in oxygen, nutrients and microbiology, mixes with the supernatant. The low volume of supernatant compared to influent water results in little change to the characteristics of the influent Supernatant is lower in oxygen, nutrients and microbiology than influent water because nutrients were consumed during the pause phase.
Initial water exiting the filter is of very good quality. It has spent the pause time in the under drain and biologically inactive lower sand layers where the level of contamination is small.
Page 8 of 18
Module 2.1 BioSand Filter Purpose and Design Halfway Through the Run
The water level decreases as the water flows downward through the sand resulting in a decreasing pressure The larger particles are strained out, partially plugging the pore openings. This causes an increase in the resistance of the sand to the flow of water.
Mobile predators travel upwards towards the influent because of the more abundant food source. Many pathogens are consumed here.
Water that has spent the pause period in close association with the biologically active layer contains some contaminants which were not consumed by the micro organisms. These are initially swept through the pore openings which have been partially opened up during the pause period. This water has reduced oxygen concentration because of consumption by the organisms in the biologic layer.
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Module 2.1 BioSand Filter Purpose and Design
The End of the Run
Flow rate decreases with time due to declining head and increased resistance of sand. Water flow slows and finally stops. Diffusion from higher oxygen content water replenishes oxygen in zoogeal; a slimy substance excreted by the bacteria.
Contaminant removal increases with time because of decreasing flow rate and the decreased size of pore openings.
Contaminants that are easily consumed are oxidized. Material from the previous run which has been partially oxidized is more completely broken down.
Better quality water is produced at the end of the run because of increasing contaminant removal during the run time.
Page 10 of 18
Module 2.1 BioSand Filter Purpose and Design
Arsenic Removal The BioSand filter can be easily modified to remove arsenic from water If unsafe level of arsenic is found in the raw water, the regular biosand filter can be easily modified for effective arsenic removal. This is accomplished by replacing the diffuser plate with a deep diffuser basin. The basin will be filled with 5 kg of small, non-galvanized iron nail, covered by a layer of brick chips. Diffuser Basin Lid Container
Brick chips Iron Nails Water
Fine Sand Pipe Coarse Sand Gravel
The iron nails in the diffuser basin, after contact with water and air, will quickly rust. Iron rust (ferric hydroxide) is an excellent adsorbent for arsenic. When arsenic-contaminated water is poured into the filter, arsenic may stay in the diffuser box (i.e. adsorbed to the surface of the rusted nails in the box), or the arsenic-loaded iron particles can be flushed down and trapped on top of fine sand. The purpose of the brick chips is to protect the underlying iron nails from dispersing due to the force of the incoming water.
Filter Operation Consistent operations are very important in a BioSand filter
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Module 2.1 BioSand Filter Purpose and Design Flow Rates The micro-organisms are more closely confined near the surface of the sand bed in a biosand water filter than in a continuously operated slow sand filter. This is because in the biosand water filter, the oxygen supply is limited by diffusion from the surface. Because of the thin biologic zone, there is a shorter contact time between the bio film and water during filter runs. Slower filtration rates are therefore required in a biosand filter to produce water of similar bacteriological quality as a continuously operated filter. The percentage removal of contaminants is inversely proportional to the flow rate through the filter because the biologic reduction of contaminants takes time. Each biosand filter has been designed to allow for a filter loading rate (the flow rate per square metre of filter area) which has proven to be effective in laboratory and field tests. The amount of water that flows through the biosand filter is controlled by the size of sand media contained within the filter. If the rate is too fast, the efficiency of bacterial removal may be reduced. If the flow rate is too slow, there will be an insufficient amount of treated water, and the users will become impatient and may use contaminated sources of water. Pause Periods Having pause periods is very important because it allows time for the micro-organisms in the biologic layer to consume the pathogens captured from the water, thereby increasing the hydraulic conductivity of the filter. As the pathogens and substrate are consumed in the biological zone, the flow rate through the filter is restored. There is an exponential increase in the hydraulic conductivity of the filter as the pause length is increased. Consequently, the BioSand filter is most effective and efficient when operated intermittently. Flow rates may increase as the length of pause period is increased. However, if the pause period is extended for too long, the micro organisms will eventually consume all of the food supply and then die off. This will result in a marked reduction in removal efficiency of the filter when it is used again. The BioSand filter is most effective and efficient when operated intermittently and consistently. A pause period of 6-12 hours is a suggested time with a minimum of 1 hour and a maximum of 48 hours. Water Depths Changes in the depth of water above the sand surface during the pause period will change the depth of the biological zone, disrupting the efficiency of the filter. A water depth of greater than 5-8 cm results in lower oxygen diffusion and consequently a thinner biological zone. With increasing water depth, the bio-layer moves upwards in the sand bed and thus oxidation and metabolism by the micro organisms decrease. Eventually, the layer dies off, and the filter becomes a non living system which is ineffective in pathogen removal.
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Module 2.1 BioSand Filter Purpose and Design If the water depth during the pause period is increased suddenly and the water level is too deep to allow any oxygen to reach the biologic layer, the entire biologically active zone becomes anaerobic. A high water level can be caused by a blocked outlet spout or by an insufficient amount of sand media. Correct installation and operation of the biosand filter results in the required constant water level of approximately 5 cm (2”) above the sand during pause periods. Influent Water Quality Over time, the micro-organisms in the biologic layer become adapted to conditions where a certain amount of food is available. When an influent spike occurs, whereby an increased level of contamination in the water is present, the micro-organisms are unable to consume or destroy all the contaminants. The spike event may provide enough contaminants that the filter will not degrade and destroy them for several days. Previous experiments have indicated that the largest portion of bacteria from a spike show up in the effluent water the next day (Buzunis, 95). The water supplied to the filter can be from rain water, deep wells, shallow wells, rivers, lakes, reservoirs or surface water. It should be consistently taken from the same source because the biological layer cannot quickly adapt to different influent water quality. The turbidity or amount of suspended particles in the water is also a key factor in the operation of the filter. The influent water should be relatively free of suspended particles. If the turbidity is greater than 100 NTU, the water should be pre filtered before it goes though the biosand filter. A simple test to measure the turbidity is to use a 2 Litre clear plastic soft drink bottle filled with water. Place this on top of large print such as the CAWST logo on this manual. If you can see this logo, the water probably has a turbidity of less than 50 NTU. Filtered water quality To ensure that all bacteria are killed and to provide the highest possible quality of water for the users, it is recommended that a disinfection process such as chlorine addition or SODIS be used in conjunction with the BioSand filter. Disinfection with chlorine bleach is accomplished by adding 4 drops of bleach to one Litre of filtered water (1 teaspoon to 5 gallons). Mix the water well and let sit for 30 minutes before drinking. The water should have a slight bleach odour. If it does not, then add a similar quantity of bleach and wait 15 minutes. The residual free chlorine should be approximately 0.4 to 0.5 mg/l using a comparator or test strip method of analysis. Maintenance Over time, continued use of the filter causes the pore opening between the sand grains to become clogged with debris. As a result, the flow rate of water through the filter decreases.
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Module 2.1 BioSand Filter Purpose and Design To clean the filter, the surface of the sand must be agitated, thereby suspending captured material in the standing layer of water. The dirty water can then be simply removed using a small container. The process can be repeated as many times as necessary to regain the desired flow rate. After cleaning, a re-establishment of the biological zone takes places quickly, returning the removal efficiency to the previous level. Time It normally takes a period of three weeks for the biologic layer to develop to maturity in a new filter. During that time, both the removal efficiency and the oxygen demand of the filter increase as the biologic layer grows. BioSand filters are cleaned by stirring up the thin layer of sand at the surface and scooping out the resulting dirty water. After cleaning, the removal efficiency declines somewhat, but increases very quickly to its previous level as the bio-layer is re-established. This effect is illustrated in the following figure. Filter Effectiveness Over Time - After Maintenance
Removal Efficiency Filter Cleaning
%
Time Weeks
As for arsenic, effective removal will occur as soon as the iron nails are rusted. Therefore, non-galvanized iron nails are highly preferred because they can be rusted within hours. Small nails are desirable because of their large surface areas, thus more adsorption sites for arsenic. Conclusion
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Module 2.1 BioSand Filter Purpose and Design In conclusion, although the BioSand filter is simple in appearance and low in cost, it has been carefully engineered. Feedback on potential improvements is strongly encouraged so that the filter design can be continuously improved. However, independent field modifications of the design, installation or operation of the filter may result in incorrect functioning and sub optimal performance.
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Module 2.1 BioSand Filter Purpose and Design
The BioSand Filter Advantages can remove bacteria
90 - 99.99 % of the time
can remove viruses
~ 99 %
can remove protozoa
~ 99 %
can remove helminths
100%
can remove iron
Can remove arsenic
• • • • • • • • • •
by physical straining predation natural die - off by physical straining predation natural die - off by physical straining predation natural die - off physical straining
•
oxidizes into particles which are strained physical straining by adding 5 kg of non-galvanized iron nails to the diffuser basin
• •
85 – 95%
The BioSand Filter Limitations cannot handle high turbidity continuously cannot remove some dissolved compounds e.g. salt, hardness cannot guarantee pathogen free
cannot remove all organic chemicals (i.e. pesticides, fertilizers) cannot remove all colour from the water
(lab 97 - 99.9 %) (field 90 - 99 %)
•
plugs prematurely
•
molecular sized particles
• • • •
small bacteria risk of poor operation recontamination if required, use disinfection (bleach) small size
• •
sometimes brown, organic water may not be removed
Comparing Technologies The following notes apply to the biosand filter and can be used when comparing it to other technologies.
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Module 2.1 BioSand Filter Purpose and Design Functionality The term functionality considers how appropriate the technology is for the user. The BioSand filter is a ‘point of use’ or household treatment device. The water to be filtered can be obtained from the closest water supply point, whether river, stream or well, carried to the filter, and used immediately after filtering. The water supply, water treatment and water distribution are therefore all within the control of the individual householder. Effective use of the technology does not require the formation of user groups or other community support which are sometimes difficult to develop. The independence of the household makes this technology extremely suitable for use in developing countries which often lack the governance and regulatory processes needed for effective and efficient multi-family systems. The operation and maintenance of the filter are simple. There are no moving parts that require skill to operate. When the flow through the filter becomes too low, the maintenance consists simply of washing the top few centimetres of sand. The operation and maintenance of the filter are therefore well within the capacity of women in the household, who are normally responsible for food preparation and care of the children. The filter takes up very little space and can easily fit into most rooms. In fact, previous experience has shown that because it is so important to the individual household, it normally occupies a place of significance in the living room. Capital Cost The cost of providing potable water to the household is simply the cost of the filter since the cost of water supply and distribution can be provided by the individual householder’s labour. The cost of a concrete BioSand water filter is approximately $15 US. This cost does not change dramatically from country to country, because its principal components, concrete and sand and gravel are readily available in all developing countries. The manufacture of the filters also involves a significant labour component to mix the concrete and place it in the filter mold. The skills required to do this are again readily available in developing countries at a very low cost, and, in fact, can be provided by the individual home owner himself. Operating Costs The cost of operating the filter is negligible. There are no consumables involved in the filtration process. There are no moving parts which require replacement on an ongoing basis. Maintenance costs may include the occasional replacement of iron nails (for arsenic removal) and any wooden components (lid ) that may deteriorate over time. This cost may amount to $1 to 2 US over a 3 year span. Consequently, once installed, the filter can be used almost indefinitely with negligible cost to the owner.
Page 17 of 18
Module 2.1 BioSand Filter Purpose and Design
Appendices Appendix A: BioSand Filter – Specification Sheet Appendix B: BioSand Filter – Summary of all Lab and Field Tests
Page 18 of 18
2.1 2.1 BioSand Filter Purpose and Design
BioSand Filters Ready for Schools in Haiti The purpose of constructing the BioSand filter is to enable the user to enjoy good quality water which should lead to better health. To provide the filter constructor with an appreciation of the need for good quality water, it is important to understand the relationship between good health and good water supply, treatment, sanitation and hygiene. This module is a brief explanation of these relationships. This module also gives the design basis for the concrete BioSand filter and why the filter is built as it is.
Module 2.1 BioSand Filter Purpose and Design
2.1 BIOSAND FILTER PURPOSE AND DESIGN........................................................1 WATER TREATMENT..............................................................................................................2 Sedimentation..............................................................................................................2 Filtration......................................................................................................................3 Disinfection..................................................................................................................3 BIOSAND FILTER..................................................................................................................3 THE FILTRATION PROCESS......................................................................................................6 The Pause Period ........................................................................................................6 Oxygen Gradient..........................................................................................................6 The Start of the Run.....................................................................................................8 Halfway Through the Run ...........................................................................................9 The End of the Run....................................................................................................10 ARSENIC REMOVAL.............................................................................................................11 FILTER OPERATION..............................................................................................................11 Flow Rates.................................................................................................................12 Pause Periods............................................................................................................12 Water Depths.............................................................................................................12 Influent Water Quality...............................................................................................13 Filtered water quality................................................................................................13 Maintenance .............................................................................................................13 Time...........................................................................................................................14 Conclusion.................................................................................................................14 THE BIOSAND FILTER ADVANTAGES .....................................................................................16 THE BIOSAND FILTER LIMITATIONS......................................................................................16 COMPARING TECHNOLOGIES.................................................................................................16 Functionality..............................................................................................................17 Capital Cost...............................................................................................................17 Operating Costs.........................................................................................................17 APPENDICES.......................................................................................................................18 Appendix A: BioSand Filter – Specification Sheet....................................................18 Appendix B: BioSand Filter – Summary of all Lab and Field Tests..........................18
Water Treatment Raw water is treated in 3 main ways: sedimentation, filtration and disinfection. Sedimentation
Page 2 of 18
Module 2.1 BioSand Filter Purpose and Design If the water contains particles of sand, grit and dirt, it can be left in a container for some time to allow the particles to settle. Bacteria often grow attached to particle surfaces. Removal of particles by sedimentation will produce a marked reduction in bacterial concentrations. This reduction is achieved by allowing a container of water to settle for 2 – 12 hours. Filtration Filters remove pathogens in several ways. These include straining, where the particles or larger pathogens such as worms become trapped in the small spaces between the grains; adsorption, where pathogens become attached to the filter media; and biologic processes, where pathogens die naturally or the micro organisms which live in the filter consume the bacteria and pathogens. Examples of filtration systems: rapid sand filter, ceramic filter, slow sand filters and the biosand filter. Disinfection Disinfection comes about primarily through the destruction of the organism cell walls by oxidation. This oxidation is normally a result of the addition of chemicals such as chlorine. It can also be induced by ultraviolet radiation. Pathogens can hide from disinfecting agents in organic and inorganic residue in the water. Removal of suspended materials by sedimentation and filtration greatly improve the performance of chemical disinfection agents. Examples of disinfection are the addition of chlorine or sodium hypochlorite to water, solar disinfection (SODIS) or solar pasteurization.
BioSand Filter The major benefits of slow sand filtration are due to the microbiology of the filter. The microbiological community must be kept alive for the filter to be effective. In a conventional slow sand filter, oxygen is supplied to the organisms through dissolved oxygen in the water. Consequently, they are designed to be operated continuously. Also, because the water moves through at a slow rate, the filter beds tend to be very large. The biosand water filter is an invention that modifies the traditional slow sand filter in such a way that the filters can be built on a smaller scale and can be operated intermittently. These modifications make the filter suitable for use at the household or small group level. Household use would simply not be possible with conventional slow sand filtration because of the size requirements and the mode of operation.
Page 3 of 18
Module 2.1 BioSand Filter Purpose and Design A bucket of contaminated water can be poured into the top of the BioSand filter as necessary. The water simply flows through the filter and is collected in another bucket or container at the base of the spout. It normally takes a few minutes for the entire bucket to make its way through the filter. There are no valves or moving parts and the design of the outlet pipe ensures that a minimum water depth of five centimetres is maintained over the sand when the filter is not in use. When water is flowing through the filter, oxygen is supplied to the biologic layer at the top of the sand by the dissolved oxygen in the water. During pause times, when the water is not flowing, the oxygen is obtained by diffusion from the air and by slow convective mixing of the layer of water above the sand. If this layer is kept shallow, enough oxygen is able to pass through to the micro organisms to keep them alive and thus effective. The filter is made up of five distinct regions; the influent reservoir, the supernatant, the schmutzdecke, the biologically active zone, and the sand support and underdrain, as shown below.
INFLUENT RESERVOIR – Volume above the filter media which allows for a full pail of water
Page 4 of 18
Module 2.1 BioSand Filter Purpose and Design
SUPERNATANT – The standing water layer present during pause periods - Oxygen can diffuse into the water SCHMUTZDECKE – Layer of slime, mud and micro-organisms which develops at the sand surface
BIOLOGICAL ZONE – 510 cm deep – living layer of micro-organisms
SAND BED – Contains virtually no living microorganisms
GRAVEL – No living mirco-organisms present
Page 5 of 18
Module 2.1 BioSand Filter Purpose and Design
The Filtration Process The Pause Period (When No Water Is Flowing) Oxygen in the air diffuses through the supernatant to the biologic layer. Since the oxygen demand is constant, the diffusion is steady state. Nutrients are consumed over the entire pause period reducing the concentration of suspended contaminants in the supernatant.
Slow convective mixing of the supernatant increases oxygen available to the biologic layer.
Organisms die off due to lack of substrate food and oxygen or they move toward the hospitable environment at the sand surface. Therefore the level of contaminants in this area and below is small.
Micro organisms in the biologic layer use diffused oxygen and consume pathogens. This results in growth of the organisms. Organic material is reduced or stabilized and converted into energy. Insoluble organics are converted to soluble salts. This results in partial unplugging of the filter pores during the pause period. Oxygen Gradient Oxygen gradient is lower in the supernatant because the total area is available for oxygen transfer. It is higher in the sand bed because the area available for oxygen transfer is partially blocked by the sand grains.
Air Water Biologic Layer
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Module 2.1 BioSand Filter Purpose and Design
Page 7 of 18
Module 2.1 BioSand Filter Purpose and Design The Start of the Run
The high water level provides the hydrostatic head to push the water through the filter.
The dissolved oxygen in the water provides the oxygen required by the microbes in the biologic layer.
Organic and inorganic material is strained out at the top of the sand.
Influent Water, high in oxygen, nutrients and microbiology, mixes with the supernatant. The low volume of supernatant compared to influent water results in little change to the characteristics of the influent Supernatant is lower in oxygen, nutrients and microbiology than influent water because nutrients were consumed during the pause phase.
Initial water exiting the filter is of very good quality. It has spent the pause time in the under drain and biologically inactive lower sand layers where the level of contamination is small.
Page 8 of 18
Module 2.1 BioSand Filter Purpose and Design Halfway Through the Run
The water level decreases as the water flows downward through the sand resulting in a decreasing pressure The larger particles are strained out, partially plugging the pore openings. This causes an increase in the resistance of the sand to the flow of water.
Mobile predators travel upwards towards the influent because of the more abundant food source. Many pathogens are consumed here.
Water that has spent the pause period in close association with the biologically active layer contains some contaminants which were not consumed by the micro organisms. These are initially swept through the pore openings which have been partially opened up during the pause period. This water has reduced oxygen concentration because of consumption by the organisms in the biologic layer.
Page 9 of 18
Module 2.1 BioSand Filter Purpose and Design
The End of the Run
Flow rate decreases with time due to declining head and increased resistance of sand. Water flow slows and finally stops. Diffusion from higher oxygen content water replenishes oxygen in zoogeal; a slimy substance excreted by the bacteria.
Contaminant removal increases with time because of decreasing flow rate and the decreased size of pore openings.
Contaminants that are easily consumed are oxidized. Material from the previous run which has been partially oxidized is more completely broken down.
Better quality water is produced at the end of the run because of increasing contaminant removal during the run time.
Page 10 of 18
Module 2.1 BioSand Filter Purpose and Design
Arsenic Removal The BioSand filter can be easily modified to remove arsenic from water If unsafe level of arsenic is found in the raw water, the regular biosand filter can be easily modified for effective arsenic removal. This is accomplished by replacing the diffuser plate with a deep diffuser basin. The basin will be filled with 5 kg of small, non-galvanized iron nail, covered by a layer of brick chips. Diffuser Basin Lid Container
Brick chips Iron Nails Water
Fine Sand Pipe Coarse Sand Gravel
The iron nails in the diffuser basin, after contact with water and air, will quickly rust. Iron rust (ferric hydroxide) is an excellent adsorbent for arsenic. When arsenic-contaminated water is poured into the filter, arsenic may stay in the diffuser box (i.e. adsorbed to the surface of the rusted nails in the box), or the arsenic-loaded iron particles can be flushed down and trapped on top of fine sand. The purpose of the brick chips is to protect the underlying iron nails from dispersing due to the force of the incoming water.
Filter Operation Consistent operations are very important in a BioSand filter
Page 11 of 18
Module 2.1 BioSand Filter Purpose and Design Flow Rates The micro-organisms are more closely confined near the surface of the sand bed in a biosand water filter than in a continuously operated slow sand filter. This is because in the biosand water filter, the oxygen supply is limited by diffusion from the surface. Because of the thin biologic zone, there is a shorter contact time between the bio film and water during filter runs. Slower filtration rates are therefore required in a biosand filter to produce water of similar bacteriological quality as a continuously operated filter. The percentage removal of contaminants is inversely proportional to the flow rate through the filter because the biologic reduction of contaminants takes time. Each biosand filter has been designed to allow for a filter loading rate (the flow rate per square metre of filter area) which has proven to be effective in laboratory and field tests. The amount of water that flows through the biosand filter is controlled by the size of sand media contained within the filter. If the rate is too fast, the efficiency of bacterial removal may be reduced. If the flow rate is too slow, there will be an insufficient amount of treated water, and the users will become impatient and may use contaminated sources of water. Pause Periods Having pause periods is very important because it allows time for the micro-organisms in the biologic layer to consume the pathogens captured from the water, thereby increasing the hydraulic conductivity of the filter. As the pathogens and substrate are consumed in the biological zone, the flow rate through the filter is restored. There is an exponential increase in the hydraulic conductivity of the filter as the pause length is increased. Consequently, the BioSand filter is most effective and efficient when operated intermittently. Flow rates may increase as the length of pause period is increased. However, if the pause period is extended for too long, the micro organisms will eventually consume all of the food supply and then die off. This will result in a marked reduction in removal efficiency of the filter when it is used again. The BioSand filter is most effective and efficient when operated intermittently and consistently. A pause period of 6-12 hours is a suggested time with a minimum of 1 hour and a maximum of 48 hours. Water Depths Changes in the depth of water above the sand surface during the pause period will change the depth of the biological zone, disrupting the efficiency of the filter. A water depth of greater than 5-8 cm results in lower oxygen diffusion and consequently a thinner biological zone. With increasing water depth, the bio-layer moves upwards in the sand bed and thus oxidation and metabolism by the micro organisms decrease. Eventually, the layer dies off, and the filter becomes a non living system which is ineffective in pathogen removal.
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Module 2.1 BioSand Filter Purpose and Design If the water depth during the pause period is increased suddenly and the water level is too deep to allow any oxygen to reach the biologic layer, the entire biologically active zone becomes anaerobic. A high water level can be caused by a blocked outlet spout or by an insufficient amount of sand media. Correct installation and operation of the biosand filter results in the required constant water level of approximately 5 cm (2”) above the sand during pause periods. Influent Water Quality Over time, the micro-organisms in the biologic layer become adapted to conditions where a certain amount of food is available. When an influent spike occurs, whereby an increased level of contamination in the water is present, the micro-organisms are unable to consume or destroy all the contaminants. The spike event may provide enough contaminants that the filter will not degrade and destroy them for several days. Previous experiments have indicated that the largest portion of bacteria from a spike show up in the effluent water the next day (Buzunis, 95). The water supplied to the filter can be from rain water, deep wells, shallow wells, rivers, lakes, reservoirs or surface water. It should be consistently taken from the same source because the biological layer cannot quickly adapt to different influent water quality. The turbidity or amount of suspended particles in the water is also a key factor in the operation of the filter. The influent water should be relatively free of suspended particles. If the turbidity is greater than 100 NTU, the water should be pre filtered before it goes though the biosand filter. A simple test to measure the turbidity is to use a 2 Litre clear plastic soft drink bottle filled with water. Place this on top of large print such as the CAWST logo on this manual. If you can see this logo, the water probably has a turbidity of less than 50 NTU. Filtered water quality To ensure that all bacteria are killed and to provide the highest possible quality of water for the users, it is recommended that a disinfection process such as chlorine addition or SODIS be used in conjunction with the BioSand filter. Disinfection with chlorine bleach is accomplished by adding 4 drops of bleach to one Litre of filtered water (1 teaspoon to 5 gallons). Mix the water well and let sit for 30 minutes before drinking. The water should have a slight bleach odour. If it does not, then add a similar quantity of bleach and wait 15 minutes. The residual free chlorine should be approximately 0.4 to 0.5 mg/l using a comparator or test strip method of analysis. Maintenance Over time, continued use of the filter causes the pore opening between the sand grains to become clogged with debris. As a result, the flow rate of water through the filter decreases.
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Module 2.1 BioSand Filter Purpose and Design To clean the filter, the surface of the sand must be agitated, thereby suspending captured material in the standing layer of water. The dirty water can then be simply removed using a small container. The process can be repeated as many times as necessary to regain the desired flow rate. After cleaning, a re-establishment of the biological zone takes places quickly, returning the removal efficiency to the previous level. Time It normally takes a period of three weeks for the biologic layer to develop to maturity in a new filter. During that time, both the removal efficiency and the oxygen demand of the filter increase as the biologic layer grows. BioSand filters are cleaned by stirring up the thin layer of sand at the surface and scooping out the resulting dirty water. After cleaning, the removal efficiency declines somewhat, but increases very quickly to its previous level as the bio-layer is re-established. This effect is illustrated in the following figure. Filter Effectiveness Over Time - After Maintenance
Removal Efficiency Filter Cleaning
%
Time Weeks
As for arsenic, effective removal will occur as soon as the iron nails are rusted. Therefore, non-galvanized iron nails are highly preferred because they can be rusted within hours. Small nails are desirable because of their large surface areas, thus more adsorption sites for arsenic. Conclusion
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Module 2.1 BioSand Filter Purpose and Design In conclusion, although the BioSand filter is simple in appearance and low in cost, it has been carefully engineered. Feedback on potential improvements is strongly encouraged so that the filter design can be continuously improved. However, independent field modifications of the design, installation or operation of the filter may result in incorrect functioning and sub optimal performance.
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Module 2.1 BioSand Filter Purpose and Design
The BioSand Filter Advantages can remove bacteria
90 - 99.99 % of the time
can remove viruses
~ 99 %
can remove protozoa
~ 99 %
can remove helminths
100%
can remove iron
Can remove arsenic
• • • • • • • • • •
by physical straining predation natural die - off by physical straining predation natural die - off by physical straining predation natural die - off physical straining
•
oxidizes into particles which are strained physical straining by adding 5 kg of non-galvanized iron nails to the diffuser basin
• •
85 – 95%
The BioSand Filter Limitations cannot handle high turbidity continuously cannot remove some dissolved compounds e.g. salt, hardness cannot guarantee pathogen free
cannot remove all organic chemicals (i.e. pesticides, fertilizers) cannot remove all colour from the water
(lab 97 - 99.9 %) (field 90 - 99 %)
•
plugs prematurely
•
molecular sized particles
• • • •
small bacteria risk of poor operation recontamination if required, use disinfection (bleach) small size
• •
sometimes brown, organic water may not be removed
Comparing Technologies The following notes apply to the biosand filter and can be used when comparing it to other technologies.
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Module 2.1 BioSand Filter Purpose and Design Functionality The term functionality considers how appropriate the technology is for the user. The BioSand filter is a ‘point of use’ or household treatment device. The water to be filtered can be obtained from the closest water supply point, whether river, stream or well, carried to the filter, and used immediately after filtering. The water supply, water treatment and water distribution are therefore all within the control of the individual householder. Effective use of the technology does not require the formation of user groups or other community support which are sometimes difficult to develop. The independence of the household makes this technology extremely suitable for use in developing countries which often lack the governance and regulatory processes needed for effective and efficient multi-family systems. The operation and maintenance of the filter are simple. There are no moving parts that require skill to operate. When the flow through the filter becomes too low, the maintenance consists simply of washing the top few centimetres of sand. The operation and maintenance of the filter are therefore well within the capacity of women in the household, who are normally responsible for food preparation and care of the children. The filter takes up very little space and can easily fit into most rooms. In fact, previous experience has shown that because it is so important to the individual household, it normally occupies a place of significance in the living room. Capital Cost The cost of providing potable water to the household is simply the cost of the filter since the cost of water supply and distribution can be provided by the individual householder’s labour. The cost of a concrete BioSand water filter is approximately $15 US. This cost does not change dramatically from country to country, because its principal components, concrete and sand and gravel are readily available in all developing countries. The manufacture of the filters also involves a significant labour component to mix the concrete and place it in the filter mold. The skills required to do this are again readily available in developing countries at a very low cost, and, in fact, can be provided by the individual home owner himself. Operating Costs The cost of operating the filter is negligible. There are no consumables involved in the filtration process. There are no moving parts which require replacement on an ongoing basis. Maintenance costs may include the occasional replacement of iron nails (for arsenic removal) and any wooden components (lid ) that may deteriorate over time. This cost may amount to $1 to 2 US over a 3 year span. Consequently, once installed, the filter can be used almost indefinitely with negligible cost to the owner.
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Module 2.1 BioSand Filter Purpose and Design
Appendices Appendix A: BioSand Filter – Specification Sheet Appendix B: BioSand Filter – Summary of all Lab and Field Tests
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