Transcript FluiDivas

Biosand Filtration
(BSF)
FluiDivas: Lindsey Ehinger, Laura Mar,
Stephanie Wedekind, Kim Wilson
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
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
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
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
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
Plastic Diffuser Plate
Source: Lee, 2000
– water flows through
various layers
– removes particles
Design Parameters
• Moderate Sand
Porosity
Filter Design Parameters for Household BSF in Nepal
– small enough to
trap particles in
the water
– large enough to
let the water
through and
allow biological
growth
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
Contaminant
Percent
Removal
Protozoa
100
Helminthes
100
Organic and Inorganic
Toxicants
50-90
Viruses
99.9
Fecal Coliform
90
Zinc, Copper, Cadmium,
Lead
95-99
Iron, Manganese
<67
Arsenic
<47
Suspended Solids
100
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
Material
Cost
1 Bag of
Cement
$6.21
Sand and
Gravel
$2.88
Wood
$3.67
PVC Elbows
$1.04
PVC Pipe
$0.39
Metal Sheet
$0.39
Vegetal Oil
$1.97
Total
$16.55
Disadvantages
• Requires intermittent use on a regular basis
• Removal efficiency decreases with increased flow rate
• High turbidity
Giardia
Cysts
% Removal
100
(>100 NTU) will
99.5
clog filter and
require more
99
maintenance
98.5
• Does not remove
98
color or dissolved
0
compounds
• Filter is bulky and
heavy to transport (300 lbs)
Total
Coliform
0.2
0.4
Flow Rate (m /hr)
0.6
Fecal
Coliform
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
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
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
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
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
Mean
82.08%
Mean
89.17%
Median
91.67%
Median
95.15%
Maximum
100.00%
Maximum
99.96%
* Minimum
33.33%
* Minimum
37.05%
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)
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
Honduras
100 %
Nicaragua
99 %
Mozambique
98 %
Kenya
94 %
Cambodia
83 %
Vietnam
81 %
World Average (sample size = 577)
93%
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!
References
• http://www.fbfinternational.org/davnor/tripre
•
•
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port.htm
http://www.geocities.com/nica_can/
http://www.cawst.org/technology/watertreat
ment/summaryoflabandfield.php
http://www.cawst.org/technology/watertreat
ment/filtration-biosand.php
http://waterloo.ewb.ca/Research/results/Wate
r%20Treatment%20Modelling.pdf
http://web.mit.edu/11.479/www/Lukacs.doc