Biosand Filtration (BSF)

1. Biosand Filtration(BSF) FluiDivas: Lindsey Ehinger, Laura Mar, Stephanie Wedekind, Kim Wilson

2. How Biosand Filters Work • Bio-layer • Organic material, living and dead, silt, clay, and particulates on filter bed. • The thicker the layer, the more effective the filter. Source: Water Tiger, 2004

3. Schmutzdecke • Filter cake • a layer of particles deposited on top of the filter bed • biological growth on top of the filter bed • Biologically Active Zone

4. Bio Activity Removes Particles • Biodegredation • Deposited organics being used as food (oxidized to provide metabolic energy) • Not a large role (only small % of organic carbon is biodegradable) • Bacterivory • Consumption of bacteria by predators such as protozoa and rotifers • predators graze on bacteria and debris attached to sand grains • predators remove suspended particles as the particles flow through the filter • NOT the mechanism for removal of larger (>2µm) pathogenic protozoa such as Giardia lamblia and Cryptosporidium oocysts

5. Physical Removal Mechanisms • Surface straining • Captures larger particles (algae and vegetative debris) • Filter cake • Accumulation of particles captured • Living organisms and other debris from the water • The more capture, the smaller the pores and the better the straining • Particle Attachment • Particles follow streamlines, if intercepted and interaction favorable, attachment will occur. • Viscous forces hinder attachment or cause detachment by shearing • Shearing forces increase as medium pore size decreases • Net attachment may be less efficient in filter cake than underlying filter bed

6. Biosand Filter Design • Diffuser • prevents disturbance of surface • blocks large particles • distributes water evenly • held down (rock/wood wedge) to prevent dislodging • handle for removal • Metal/wood/plastic • Filter • water flows through various layers • removes particles Plastic Diffuser Plate Source: Lee, 2000

7. Design Parameters • Moderate Sand Porosity • small enough to trap particles in the water • large enough to let the water through and allow biological growth Filter Design Parameters for Household BSF in Nepal

8. Advantages High Flow Rate (60 Liters/hour) High Removal Efficiency Simple to Maintain and Clean Easily Constructed with Local Materials fabricated within the Local community Cement Wood for lid and diffuser Fine sand Coarse Sand Gravel Plastic tubing No chemical additives

9. Advantages • Cost Effective • One time setup cost • no continuing cost • $10-$30 US dollars for each filter • Opportunity for local businesses • Durable • Each filter lasts many years • Cement is very durable • No replaceable parts

10. Disadvantages • Requires intermittent use on a regular basis • Removal efficiency decreases with increased flow rate • High turbidity (>100 NTU) will clog filter and require more maintenance • Does not remove color or dissolved compounds • Filter is bulky and heavy to transport (300 lbs)

11. Disadvantages • Cultural Problems • People do not understand the need to clean the water filters • People may not trust it at first because they do not understand how it works • Remote areas may not have access to materials like sand, concrete, and plastics • Dependence on biological mechanisms for bacteria removal • Bacterial removal varies between 60-99% • Additional treatment may be necessary to ensure all bacteria are killed • Biological layer takes 1-2 weeks to develop to maturity

12. Is It Feasible To Use in LDC’s? • “Un Gran Cosa” (A Great Thing) • Economic Feasibility • Improves local economy by using local materials, businesses, labor • One time expense, no operating costs • The cost of the system is less than the amount of wages lost due to poor health

13. Continued Use • Kenyan study found all units well maintained and in use after one year • Flow rate decreased but still covered daily needs • Status symbol: water tastes, looks, smells better • “Quick and easy” – saves labor costs of boiling water

14. Obstacles to Continued Use • Disinfection is recommended before infants and elderly consume the water - obstacle to decreasing U5MR • Improper installation reduces filter effectiveness • Cleaning techniques are ambiguous • Still much to learn about removal mechanisms

15. Anguiatu, Guatemala • November, 2002--31 filters tested to measure CFUs/100mL • Found that there was no correlation between flow rate and success in filtration which is odd • Possibly installed incorrectly • Use of river sand, contains more pathogens than other sands, as the top filter media Total E. coli filtration rates Total Coliform Filtration Rates

16. Nicaragua • Tests done in July/August of 1999 • Coliform removal rates: 79.7% with a range from 64.4%-95% • Regression performed between initial coliform contamination and final contamination yielded no correlation (p>0.42 and R^2=0.0951)

17. 2002 BSF Evaluation Report • Most rigorous testing on Biosand Filters submitted to Samaritan’s Purse Canada • almost 600 filters located in 6 countries on 3 continents • 98.4 % filters used on a regular basis Average Fecal Coliform Removal Rates For the Individual Countries

18. Summary • Biosand filters are cheap, effective, and easy to construct and maintain • Field studies have shown positive results in underdeveloped countries • A great option for the global south!

19. References • http://www.fbfinternational.org/davnor/tripreport.htm • http://www.geocities.com/nica_can/ • http://www.cawst.org/technology/watertreatment/summaryoflabandfield.php • http://www.cawst.org/technology/watertreatment/filtration-biosand.php • http://waterloo.ewb.ca/Research/results/Water%20Treatment%20Modelling.pdf • http://web.mit.edu/11.479/www/Lukacs.doc