Biosand FiltrationBiosand Filtration(BSF)(BSF)

FluiDivas: Lindsey Ehinger, Laura FluiDivas: Lindsey Ehinger, Laura Mar, Stephanie Wedekind, Kim Mar, Stephanie Wedekind, Kim

WilsonWilson

How Biosand Filters WorkHow Biosand Filters Work

• Bio-layer Bio-layer – Organic Organic

material, living material, living and dead, silt, and dead, silt, clay, and clay, and particulates on particulates on filter bed. filter bed.

– The thicker the The thicker the layer, the more layer, the more effective the effective the filter.filter.

Source: Water Tiger, 2004

SchmutzdeckeSchmutzdecke

• Filter cakeFilter cake – a layer of particles deposited on top of the filter bed a layer of particles deposited on top of the filter bed – biological growth on top of the filter bedbiological growth on top of the filter bed

• Biologically Active ZoneBiologically Active Zone

Bio Activity Removes Bio Activity Removes ParticlesParticles

• Biodegredation Biodegredation – Deposited organics being used as food (oxidized to provide Deposited organics being used as food (oxidized to provide

metabolic energy)metabolic energy)– Not a large role (only small % of organic carbon is Not a large role (only small % of organic carbon is

biodegradable) biodegradable)

• BacterivoryBacterivory– Consumption of bacteria by predators such as protozoa and Consumption of bacteria by predators such as protozoa and

rotifersrotifers• predators graze on bacteria and debris attached to sand grains predators graze on bacteria and debris attached to sand grains • predators remove suspended particles as the particles flow predators remove suspended particles as the particles flow

through the filterthrough the filter– NOT the mechanism for removal of larger (>2µm) NOT the mechanism for removal of larger (>2µm)

pathogenic protozoa such as Giardia lamblia and pathogenic protozoa such as Giardia lamblia and Cryptosporidium oocystsCryptosporidium oocysts

Physical Removal Physical Removal MechanismsMechanisms

• Surface strainingSurface straining– Captures larger particles (algae and vegetative debris) Captures larger particles (algae and vegetative debris) – Filter cakeFilter cake

• Accumulation of particles capturedAccumulation of particles captured• Living organisms and other debris from the waterLiving organisms and other debris from the water• The more capture, the smaller the pores and the The more capture, the smaller the pores and the

better the straining better the straining • Particle AttachmentParticle Attachment

– Particles follow streamlines, if intercepted and interaction Particles follow streamlines, if intercepted and interaction favorable, attachment will occur. favorable, attachment will occur.

– Viscous forces hinder attachment or cause detachment Viscous forces hinder attachment or cause detachment by shearing by shearing

– Shearing forces increase as medium pore size decreases Shearing forces increase as medium pore size decreases • Net attachment may be less efficient in filter cake Net attachment may be less efficient in filter cake

than underlying filter bedthan underlying filter bed

Biosand Filter DesignBiosand Filter Design

• DiffuserDiffuser – prevents disturbance of prevents disturbance of

surfacesurface– blocks large particles blocks large particles – distributes water evenly distributes water evenly – held down (rock/wood held down (rock/wood

wedge) to prevent wedge) to prevent dislodgingdislodging

– handle for removalhandle for removal– Metal/wood/plasticMetal/wood/plastic

• FilterFilter– water flows through water flows through

various layers various layers – removes particles removes particles

Plastic Diffuser Plate

Source: Lee, 2000

Design ParametersDesign 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

AdvantagesAdvantages

•High Flow Rate (60 Liters/hour)High Flow Rate (60 Liters/hour)

•High Removal EfficiencyHigh Removal Efficiency

•Simple to Maintain and CleanSimple to Maintain and Clean

•Easily Constructed with LocalEasily Constructed with Local

Materials fabricated within the Materials fabricated within the

Local communityLocal community•CementCement•Wood for lid and diffuserWood for lid and diffuser•Fine sandFine sand•Coarse SandCoarse Sand•GravelGravel•Plastic tubingPlastic tubing

•No chemical additivesNo chemical additives

ContaminantContaminant Percent Percent RemovalRemoval

ProtozoaProtozoa 100100

HelminthesHelminthes 100100

Organic and Inorganic Organic and Inorganic ToxicantsToxicants

50-9050-90

VirusesViruses 99.999.9

Fecal ColiformFecal Coliform 9090

Zinc, Copper, Cadmium, Zinc, Copper, Cadmium, LeadLead

95-9995-99

Iron, ManganeseIron, Manganese <67<67

ArsenicArsenic <47<47

Suspended SolidsSuspended Solids 100100

AdvantagesAdvantages

• Cost EffectiveCost Effective– One time setup cost One time setup cost – no continuing costno continuing cost– $10-$30 US dollars for each filter$10-$30 US dollars for each filter

• Opportunity for local businessesOpportunity for local businesses

• DurableDurable– Each filter lasts many yearsEach filter lasts many years– Cement is very durableCement is very durable– No replaceable partsNo replaceable parts

MaterialMaterial CostCost

1 Bag of 1 Bag of CementCement

$6.21$6.21

Sand and Sand and GravelGravel

$2.88$2.88

WoodWood $3.67$3.67

PVC ElbowsPVC Elbows $1.04$1.04

PVC PipePVC Pipe $0.39$0.39

Metal SheetMetal Sheet $0.39$0.39

Vegetal OilVegetal Oil $1.97$1.97

TotalTotal $16.5$16.555

DisadvantagesDisadvantages• Requires intermittent use on a regular basisRequires intermittent use on a regular basis• Removal efficiency decreases with increased flow rateRemoval efficiency decreases with increased flow rate• High turbidity High turbidity

(>100 NTU) will (>100 NTU) will clog filter and clog filter and

require more require more maintenancemaintenance

• Does not remove Does not remove color or dissolved color or dissolved compoundscompounds

• Filter is bulky and Filter is bulky and heavy to transport (300 lbs)heavy to transport (300 lbs)

98

98.5

99

99.5

100

0 0.2 0.4 0.6

Flow Rate (m/hr)

% R

emo

val

GiardiaCysts

TotalColiform

FecalColiform

DisadvantagesDisadvantages

• Cultural ProblemsCultural Problems– People do not understand the need to clean the water filtersPeople do not understand the need to clean the water filters– People may not trust it at first because they do not understand People may not trust it at first because they do not understand

how it workshow it works

• Remote areas may not have access to materials like sand, Remote areas may not have access to materials like sand, concrete, and plasticsconcrete, and plastics

• Dependence on biological mechanisms for bacteria removalDependence on biological mechanisms for bacteria removal– Bacterial removal varies between 60-99%Bacterial removal varies between 60-99%– Additional treatment may be necessary to ensure all Additional treatment may be necessary to ensure all

bacteria are killedbacteria are killed

• Biological layer takes 1-2 weeks to develop to maturityBiological layer takes 1-2 weeks to develop to maturity

Is It Feasible To Use in Is It Feasible To Use in LDC’s?LDC’s?

• ““Un Gran Cosa” (A Great Thing)Un Gran Cosa” (A Great Thing)• Economic FeasibilityEconomic Feasibility

– Improves local Improves local economy by using economy by using local materials, local materials, businesses, laborbusinesses, labor

– One time expense, no operating costsOne time expense, no operating costs– The cost of the system is less than the The cost of the system is less than the

amount of wages lost due to poor healthamount of wages lost due to poor health

Continued UseContinued Use

• Kenyan study found all units well Kenyan study found all units well maintained and in use after one yearmaintained and in use after one year

• Flow rate decreased but still covered Flow rate decreased but still covered daily needsdaily needs

• Status symbol: water tastes, looks, Status symbol: water tastes, looks, smells bettersmells better

• ““Quick and easy” – saves labor costs Quick and easy” – saves labor costs of boiling waterof boiling water

Obstacles to Continued UseObstacles to Continued Use

• Disinfection is recommended before Disinfection is recommended before infants and elderly consume the waterinfants and elderly consume the water- obstacle to decreasing U5MR- obstacle to decreasing U5MR

• Improper installation reduces filter Improper installation reduces filter effectivenesseffectiveness

• Cleaning techniques are ambiguousCleaning techniques are ambiguous• Still much to learn about removal Still much to learn about removal

mechanisms mechanisms

Anguiatu, GuatemalaAnguiatu, Guatemala

• November, 2002--31 filters tested to measure CFUs/100mLNovember, 2002--31 filters tested to measure CFUs/100mL

• Found that there was no correlation between flow rate and Found that there was no correlation between flow rate and success in filtration which is oddsuccess in filtration which is odd

• Possibly installed incorrectlyPossibly installed incorrectly

• Use of river sand, contains more pathogens than other Use of river sand, contains more pathogens than other sands, as the top filter mediasands, as the top filter media

Mean Mean 89.17% 89.17%

Median Median 95.15% 95.15%

Maximum Maximum 99.96% 99.96%

* Minimum * Minimum 37.05% 37.05%

Total Coliform Filtration RatesMeanMean 82.08% 82.08%

MedianMedian 91.67% 91.67%

MaximumMaximum 100.00% 100.00%

* Minimum* Minimum 33.33% 33.33%

Total E. coli filtration rates

NicaraguaNicaragua

• Tests done in July/August of 1999Tests done in July/August of 1999

• Coliform removal rates: 79.7% with a Coliform removal rates: 79.7% with a range from 64.4%-95% range from 64.4%-95%

• Regression performed between initial Regression performed between initial coliform contamination and final coliform contamination and final contamination yielded no correlation contamination yielded no correlation (p>0.42 and R^2=0.0951)(p>0.42 and R^2=0.0951)

2002 BSF Evaluation 2002 BSF Evaluation ReportReport

• Most rigorous testing on Biosand Filters Most rigorous testing on Biosand Filters submitted to submitted to Samaritan’s Purse CanadaSamaritan’s Purse Canada

• almost 600 filters located in 6 countries on 3 continents almost 600 filters located in 6 countries on 3 continents

• 98.4 % filters used on a regular basis 98.4 % filters used on a regular basis

Average Fecal Coliform Removal Rates For the Individual Countries

HondurasHonduras 100 % 100 %

Nicaragua Nicaragua 99 % 99 %

Mozambique Mozambique 98 % 98 %

Kenya Kenya 94 % 94 %

Cambodia Cambodia 83 % 83 %

Vietnam Vietnam 81 % 81 %

World Average (sample size = 577) World Average (sample size = 577) 93% 93%

SummarySummary

• Biosand filters are cheap, effective, Biosand filters are cheap, effective, and easy to construct and maintainand easy to construct and maintain

• Field studies have shown positive Field studies have shown positive results in underdeveloped countriesresults in underdeveloped countries

• A great option for the global south!A great option for the global south!

ReferencesReferences

• http://www.fbfinternational.org/davnor/http://www.fbfinternational.org/davnor/tripreport.htmtripreport.htm

• http://www.geocities.com/nica_can/http://www.geocities.com/nica_can/

• http://www.cawst.org/technology/http://www.cawst.org/technology/watertreatment/watertreatment/summaryoflabandfield.phpsummaryoflabandfield.php

• http://www.cawst.org/technology/http://www.cawst.org/technology/watertreatment/filtration-biosand.phpwatertreatment/filtration-biosand.php

• http://waterloo.ewb.ca/Research/http://waterloo.ewb.ca/Research/results/Water%20Treatmentresults/Water%20Treatment%20Modelling.pdf%20Modelling.pdf

• http://web.mit.edu/11.479/www/http://web.mit.edu/11.479/www/Lukacs.docLukacs.doc