Sustainable Water Purification for the Developing World Intermittent Slow Sand Filter (BioSand)Ross GordonFebruary 5th, 2007
Overview of Challenge • A serious problem facing those who wish to assist the rural communities in obtaining suitable drinking water is the selection of the technology to be used. The technology chosen must not only perform its intended purpose, it must be affordable, culturally acceptable and very well understood by the people who will be using it. In short, the technology must be appropriate and sustainable. Despite the existence of a vast knowledge base on water supply development and treatment, only a few solutions may be recommended for use in rural communities. • Dr David Manz (creator of BioSand filter)
Performance Criteria • Effectiveness in improving and maintaining microbial water quality • Reducing waterborne infectious disease • Technical difficulty or simplicity • Accessibility • Cost • Socio-cultural acceptability • Sustainability • Potential for dissemination.
What is BioSand? Intermittent…Household level … Slow sand filtration
General Performance • To fill a bucket with purified water: • Water gently passes through diffusion plate (does not disturb sand filter) • Passes through biologically active top layer of sand • Passes through remaining filter media • Enters standpipe and exits filter at a peak flow rate of 1 liter/min • Suspended particles, bacteria, parasites, viruses, and cysts are all removed at high percentages
Effect of BioSand • BioSand can filter out or destroy up to 100 % of… • E. coli and other fecal coliforms • Suspended particles (turbidity) • Vibrio cholerae (Cholera) • Shigella flexneri (Shigellosis dysentery) • Salmonella typhii (Typhoid Fever) • Viruses • Giardia • Cryptosporidium • Schistosome cercariae (schistosomiasis) • Iron • Arsenic (with modifications)
What is BioSand? • BioSand filtration works by: • Combination of several mechanisms • Mechanical trapping (sieving) • Adsorption of suspended materials • Biologically active layer • BioSand is a water treatment method that: • Improves the microbiological quality of drinking water • Reduces the turbidity of water • Is applicable at household level • Is simple in application • Can be built locally • Is easy to maintain • Does not require electricity • Relatively inexpensive ($15-$45) – no reoccurring costs
History of BioSand • Research and Development • Sand filtration has been used for over 150 years in water treatment processes • 1991 – research on Intermittent slow sand filtration initiated by Dr. David Manz at the University of Calgary • First field trial in 1993 - Nicaragua • BioSand Use Today • Over 65 countries • Over 200,000 filters in use • Over 1,000,000 people are drinking water treated by a BioSand filter.
Slow Sand Filtration • Very slow flow rate (0.3m/hr) • Requires continuous flow of water to aerate biological layer • Lengthy cleaned process: removing upper 5 cm of sand (biological layer) +back-wash. • Recovery time to reestablish biological layer • Suspended solids +parasites, bacteria, and viruses are captured on the filter surface. • 60% Initial bacteria capture. Parasite removal is near 100%. Virus removal similar to bacteria. • As biological layer develops, rate of bacterial and viral removal can approach 100%
Slow Sand vs. BioSand Develop effective, inexpensive, small-scale water treatment for disadvantaged communities in developing countries. • Operates at double the flow rate of traditional slow sand (up to 60 liters/hour • Sized for household use • Demand Operated: • Filter drains until 5 cm of water are present on top of filter media • Oxygen can diffuse through 5cm of water to aerate the biological layer • Allows for intermittent use • Easy Maintenance: • Do not need to remove top 5 cm of filter media or backwash the filter • Do not need to replace filter media • Quick recovery time – ability of filter to remove bacteria is not significantly affected by the cleaning process • Do not need to use treated water for cleaning
How BioSand Works • Mechanical Trapping (sieving) • Larger particles, silt, parasites, and organic material become trapped in the biological layer and between the sand grains • Adsorption • Micro-organisms attach to each other, sediment and sand grains. • *Biologically Active Layer* • Aerobic micro-organisms consume nutrient sources including diseases causing organisms in the top inch or two of the filter media.
Mechanical Trapping • Foreign particles are trapped on top of the filter bed because the particles are too large to pass through the pores • Tightly packed bed of sand grains can capture particles about 5% of the grain diameter • Sand with a diameter of 0.1 mm will strain out particles that are 5 μm or larger. This is substantially larger than many particles to be removed from surface water such as cysts (1-20 μm) and bacteria (0.1 to 10 μm) • Viruses are much less than 1μm and must, therefore, be removed by other means, such as biological mechanisms
Adsorption • Through inter-particle attractions, foreign particles absorb to the surface of the filter media or to other particles already attached to the filter media. • Adsorption is a function of brownian diffusion, electrostatic attraction, Van der Waals forces, and adherence. • Mechanical trapping and adsorption remove the majority of particles with diameter’s less than 2 micrometers. • Even without the development of a complete biofilm, the BioSand filter is able to remove upwards of 80 % of the total bacterial population
Schmutzdecke (Biofilm) • Although there is some filtration deep in the sand bed the most vital process of filtration occurs at the biologically active surface, the top 2 to 10 centimetres of the filter media. • During the initial stage of a BioSand filter, with little or no biofilm present, organic particles (algae, bacteria, protozoa, and small invertebrates) are deposited on the filter surface. These deposits are used by bacteria and other microorganisms as food. The bacteria oxidize part of the food to provide the energy needed for metabolism and convert the rest into cell material and growth • This biologically active layer develops in the micro-environment near the interface of the sand and standing water where nutrients are plentiful and dissolved oxygen is available
Schmutzdecke (Biofilm) • This layer, known as the schmutzdecke, (German, meaning "dirty blanket") is populated by an array of microbes which act as predators, consuming or inactivating pathogens introduced in the source water. • Very efficient at destroying bacteria, viruses, and other pathogens • Takes from one to three weeks for the schmutzdecke to mature and function properly. • This diverse ecosystem consists of algae, bacteria, protozoa, and small invertebrates, which are both free and attached to biofilm communities that form on the surfaces of the schmutzdecke and sand grains.
Filter Ripening When newly installed, or when the biofilm layer is damaged (during filter cleaning) time is needed for the biofilm to grow to maturity • Initial ripening normally takes a period of one to three weeks • After cleaning, reestablishment of biolayer takes one to two days • During that time, the removal efficiency of the filter increases as the biological layer grows • During ripening period, filter does not remove bacteria effectively - only physical-chemical mechanisms are at work • New sand bed removes 85% of the coliform bacteria • Mature sand bed removes upwards of 99% of coliform bacteria
Design Characteristics • Concrete basin or plastic bucket • Internal plumbing vs. external plumbing • Diffuser basin – 250 cm • 100 holes, no larger than 1/8” diameter, on a 1” x 1” grid • Filter basin – 650 cm • 7 cm for underdrain layer • 3 cm for separation layer • 50 cm for filtration media • 5 cm for standing water • Tightly fitting lid Concrete basin design and construction instructions are on the Owlspace course website.
Aeration + Static Water Level Key operating parameter – always ensure 5 cm of standing water above the fine sand layer at all times • 5 cm height determined to be optimum height for pathogen removal • Too shallow - the biofilm layer can be easily disturbed and subsequently damaged by the force of the incoming water • Too deep - insufficient amount of oxygen diffuses to the biofilm, resulting in suffocation of the microorganisms in the biofilm layer • Diffuser box above the fine sand layer serves an important purpose to reduce the force of input water from disturbing the top layer of sand
Underdrain Layer Allows filtered water unrestricted access to filter standpipe. • The thickness must be sufficient to cover the inlet to the filter standpipe with 2 cm of material • Composed of particles ranging in size from 6.25mm to 12.5mm (1/4 to 1/2 inch) in diameter • The total depth of the underdrain layer is usually between 5 and 8 cm
Separation Layer Prevents filter media from entering the underdrain layer and the filtered water standpipe • Should be between 3 and 5 cm in thickness • Composed of particles ranging in size from 3.125 mm to 6.25 mm (1/8 to 1/4 inch) in • diameter
Filter Media Responsible for removal of particles and microorganisms including viruses, bacteria, and parasites. • Flow of water through the filter is controlled by the selection and preparation of the filter media • Largest particles comprising the filter media must be less than 3.125 mm or 1/8 inch in diameter • Thickness of the filter media should be no less than 40 cm • MUST contain particles varying in size from very fine, almost dust size, to 1/8 inch • Filter media that is all dust will not work because the flow rate will be too slow • Filter media that does not contain any fines will not work because the flow rate will be too fast
Media Preparation • Wash underdrain and separation media thoroughly • Place media in large bucket – add water • Stir repeatedly with hands or a stick • Pour off our scoop out turbid water • Repeat until water is clear • If needed, wash filter media as well in the same fashion • Washing filter media will remove the fine particles • Will increase flow rate through filter
Turbidity • To prevent clogging and premature fouling, do not put high turbidity water into filter • If turbidity is greater than 50-100 NTU, the water should be settled and then filtered before it is passed through the BioSand filter. • Solution: Lay a cloth in the diffuser basin • Biosand filter will remove turbidity very effectively, but doing so will shorten the period between cleanings
Flow Rate • Design flow rate: 600 liters / hour / m2 • For 0.1 m2area – 60 liters/hour • Main cause of reduction in flow rate… • Accumulation of silt and sediment in the biofilm (top 1-2 inches of filter) • After a period of use, the flow rate will become restricted and it becomes necessary to disturb the biofilm and remove silt and organic matter that has been deposited there in order to return the flow to a satisfactory rate
1 2 3 4 Maintenance • When flow rate slows: • 1) Remove diffuser basin • 2) Stir uppermost 1/2 inch of sand with your fingers • 3) Remove turbid water with a cup. Replace diffuser basin and Add more water • REPEAT TWO MORE TIMES • Filter is ready for use again. Biofilm will reform in approximately 1 day
Arsenic BioSand • Developed in 1999 by Massachusetts Institute of Technology (MIT) as part of the MIT Nepal Water Project • Improvement on the traditional Manz Biosand Filter
Arsenic BioSand • Arsenic Removal Mechanism: • Iron rust (ferric hydroxide) is an excellent absorbent for Arsenic • After contact with water and air, iron nails in the diffuser basin will quickly rust • Arsenic particles will absorb onto the ferric hydroxide particles • The relatively large Ferric Hydroxide particles and Ferric Hydroxide Arsenic particles are filtered out by the top layer of sand
Arsenic BioSand • Arsenic and Pathogen Removal: • Arsenic: 87-90 % • Iron: 90-97% • Total coliforms: mean of 58% • E. coli: mean of 64% • Arsenic BioSand Overview • Made with easily available materials • No chemical additives • Easy to operate and clean • Require 2 to 3 weeks to reach optimum removal of bacteria & viruses • Immediate arsenic removal after installation
Second Case Study • 187 households and 5000 water analyses, over a period of six months • Perceptions positive regarding ease of use, taste, smell and appearance • 95% believed their water was improved by the BioSand filter • 96% stated that their family’s health was ‘better’ or ‘about the same’ • Effectiveness inconsistent if the filter sand media is not properly selected • Long term filters had an average bacterial removal effectiveness of 98.5% after an average of 2.5 years
Second Case Study • New users’ filters removed only ~73% initially, rising to ~85% after 3 months. The sand media in these filters was not well graded flowing at 150% of the recommended maximum flow rate. • Turbidity removal was far better in the long term filters vs. the new users. • Longevity of the BioSand filter was found to exceed five years • Scale-up appears feasible as 95% of the households said they would recommend the BioSand filter. • Recontamination of the drinking water, post-filtering, was significant for both study groups reflecting the poor water storage, hygiene and sanitation practices observed.
Advantages of BioSand • Technical performance: • • 100% removal of protozoa, 99.9% removal of viruses • • 99.5% of bacterial removal in laboratory settings • • Simple, robust design (readily available materials, internal piping, heavy and durable) • • Substantial water provision (30-60 L/hr flow rate) • Can be adapted to treat arsenic contaminated water • Social acceptability: • • Opportunity for community participation in filter construction • • Easily maintained, cleaned by users • • Filtered water is cool and clear • • No chemicals added, no consumable parts • • Concrete is a common house or roofing material • Can serve as an entry point for health and hygiene education • • Provides users a simple method that can be applied at household level under their own control and responsibility • • Is easy to understand
Advantages of BioSand • Economical sustainability: • • Costs vary by region: US$27 in Nepal (Lee, 2001), $15 in Bangladesh, and $8 in Vietnam • • No costs associated with operation and maintenance • • 8+ year lifetime • • Good micro-enterprise for local artisans • Does not require a large and costly infrastructure and therefore easily is replicable in self-help projects • • Reduces the need for traditional energy sources such as firewood and kerosene/gas • • Financial advantages: less financial resources are needed for medical care when user’s family health is improved
Disadvantages of BioSand • BioSand Filter Limitations • • Cannot remove dissolved compounds (e.g. salt, hardness) • • Cannot guarantee bacteria free water (Lab tests shows removal efficiencies of 97 – 99.7 % ; Field tests show removal efficiencies of 90 – 97 %) • Bacterial removal varies from 60-99.9% based upon presence of biological layer • Time necessary for formation of biolayer during start-up and following cleaning • If implemented improperly, can actually worsen water quality • • Recommended to use disinfection (bleach) in filtered water • • Cannot remove all organic chemicals (e.g. pesticides, fertilizers) • Nearly impossible to tell if it is working properly
Performance Criteria • Effectiveness in improving and maintaining microbial water quality • Reducing waterborne infectious disease • Technical difficulty or simplicity • Accessibility • Cost • Socio-cultural acceptability • Sustainability • Potential for dissemination. • How Does BioSand Measure Up?