1. Recent Advances in the BSF Technology
WET Centre LEx
2014

2. Reason and Basis for Improvements
Why change?
To make the filter more effective and to further improve sustained use.
Basis for change?
The advances were derived from:
Research to improve effectiveness
Learning how the filter is used
Understanding the challenges of implementing BSF projects
Determining the important parameters of intermittent slow sand filtration

3. Dimension Change – Version 10
What changed in the design?
Sand volume increased 22% while keeping the overall dimensions of the BSF the same.
This resulted in:
Pore volume = 100% reservoir volume
Maximum head decreased by 37%
Sand bed depth increased from 45 to 55 cm
Height of tubing outlet increased by 10 cm to 76 cm

4. Widths (internal) (mm)
Dimension Change
Research found that bacteria removal is significantly improved when water is retained in the sand during the pause period:
Removal with no retention: % Removal with retention: %
(Jill Baumgartner et al. 2007)
BSF Volume Comparison; v9 vs. v10
v9 v10
Pore volume sand = L L
Reservoir volume = L L
% water retained = 46% %
v10 BSF
Widths (internal) (mm)
Areas (m2)
Total Volume
Porosity*
Pore (void) Volumes
Depth (mm)
Top
Average
Bottom
(m3)
litres
(%)
Reservoir (Fill volume)
175
265
262
258
0.0120
12.0
100%
Supernatant (Standing Water) 50
245
244
243
0.0030
3.0
Fine Sand (Filter Media)
546
234
225
0.06
0.0300
30.0
40%
Coarse Sand (Separating Layer)
224
0.0025
2.5
33%
0.8
Gravel (Underdrain )
223
222
42%
1.0
Total
871
0.050
28.8

5. Dimension Change...continued
Version 10 concrete BSF cross-section
Research also determined that the maximum head, the height of water in the reservoir of the filter when it is filled, should be reduced to prevent high pore velocity at the beginning of each run.
Reduced head is necessary to achieve the target clean bed filtration rate* of 400 L/m2/h.
v9 v10
Head (cm) =
*The clean bed filtration rate is measured when the filter is first installed – when the sand bed is clean.

6. Filtration Rate Target filtration rate for the BSF is:
“The filtration rate, or hydraulic loading, is the key design parameter for all filtration processes… the heart of BSF design thus lies in the careful and appropriate selection of the media and sizing of the media bed.” Kubare & Haarhoff (2010)
Target filtration rate for the BSF is:
400 L /m2/ hour
Equivalent to flow rate:
0.4 L / minute*
*for CAWST concrete BSF with 0.06 m2 surface area
Higher rates cause high pore velocities which reduce effectiveness; while lower rates result in unacceptable time to filter water.
Measured when the filter is first installed;
This is a critical control parameter for the BSF.

7. Biosand Filter made in Peru with PVC pipe (Credit: DESEA Peru)
Sand Specifications
The filtration sand bed provides the resistance to flow (head loss) necessary to achieve the target filtration rate. Meeting the sand specifications means that the sand grains will be the correct size (effective size) and size distribution (uniformity coefficient) to provide the target filtration rate and effectively remove the pathogens in the water. Maximum silt content (<4%) and minimum sand bed depth (>50 cm) are also specified for the BSF.
Biosand Filter made in Peru with PVC pipe (Credit: DESEA Peru)
Community health worker shown measuring filter flow rate. Many of the CHWs don’t read numbers well, so colour zones are used to indicate acceptable ranges for flow rates.

8. Slow Sand Filter (SSF)** Recommendation by Others*
Sand Specifications
Parameter
Biosand Filter (BSF)
Slow Sand Filter (SSF)**
CAWST Recommendation
Recommendation by Others*
Filtration Rate/Hydraulic Loading Rate
m3/m2/hour (or m/hour)
0.4
0.16 – 1.1
0.1 – 0.3
Effective Size, mm (d10)
0.15 – 0.2
0.15 – 0.3
Uniformity Coefficient (d60/d10)
1.5 – 2.5
< 3
Maximum Grain Size, mm (dmax)
0.7 -
Silt content, %, (d < 0.1mm)
< 4%
Filtration Sand Bed Depth, m
> 0.5
> 0.4
0.6 – 1.0
* Lukacs (2002), Hillman (2007), Manz et al. (1993), Fewster et al. (2004), Baumgartner et al. (2007), Duke et al. (2006) ** Fox et al. (1994) and Campos et al. (2002).

9. Diffuser Basin Injection-molded plastic diffuser basin
Standing water depth (5 cm) is insufficient to fully protect the biolayer so a diffuser is used
In 25 evaluations, 26% of diffuser plates were found to be damaged or incorrectly placed
Diffuser basins are far more effective, convenient and longer lasting than plates
DACAAR (2012) and Jones (2013) studied optimum diameter and number of holes
CAWST now recommends 64 to 144 holes of 1.5 to 3 mm diameter
Injection-molded plastic diffuser basin
(Credit: Clean Water for Haiti & Great Lakes Mold & Manufacturing)
Durable and robust - should last as long as the concrete BSF (15+ yr.)

10. Injection-molded Diffuser Basin

11. Pause Period
The pause period is the time when the water is retained in the sand pores spaces with zero pore velocity.
Three competing perspectives:
Longer pause periods generally improves filter effectiveness.
Shorter pause periods (more influent water) will provide more nutrients for the biolayer.
Shorter pause periods mean that more filtered water will be available for the family.
The pause period is ultimately at the discretion of the user.
CAWST recommendation:
6 to 12 hours (preferred)
minimum 1 hour
maximum 48 hours
A household typically needing 40 L/day is equivalent to approx. 6 hour pause period for the v10.
Research is underway at Lehigh University on the optimum pause period for the v10 BSF

12. Virus Removal in Biosand Filters, Tony Straub and Hanting Wang, 2012
Viruses cause 40% of diarrheal illness (Ramani and Kang, 2009)
Rotavirus alone kills 600,000 worldwide annually (Parashar et al., 2006)
Virus removal improves with:
Longer pause period
Increased ripening time
Lower filtration rate
Virus Removal in Biosand Filters, Tony Straub and Hanting Wang, 2012

13. Virus Removal Longer Pause Period
Jenkins et al. (2011): “Pause time may be particularly important for avoiding viral shedding by allowing sufficient time for adsorbed and attached viral particles to decay or be consumed before addition of the next batch.”
Elliot et al. (2011):
Virus removal occurs throughout the depth of the sand bed rather than only near the surface in the biolayer.

14. Virus Removal Increased ripening time Elliot et al. (2011):
The most probable mechanism of virus removal is predation: “production of microbial exoproducts such as proteolytic enzymes or grazing of bacteria and higher microorganisms on virus particles”
Bradley et al. (2011):
Initial 150 days: virus removal varied from zero to 99.9%, averaging about 99%
After 150 days: a steady increase in virus removal from 99% to over % (2log10 to >4log10) by 300 days
This efficacy was attributed to the significant biological ‘ripening’
[ten months of data is exceptional]

15. Virus Removal Port Filter -
Virus Removal in Biosand Filters, Tony Straub and Hanting Wang, University of Illinois at Urbana-Champaign, 2012
This data supports Elliot et al. (2011) that virus removal occurs in the depth of filtration sand bed.
Port Filter -
with 7 ports to measure virus removal in the BSF at various depths
Since viruses are believed to cause about 40% of diarrheal disease; the minimum sand bed depth of 50 cm is an important BSF design parameter.

16. Iron-amended Biosand Filters
Removal mechanism is electrostatic attraction; the positively-charged iron oxide (rust) provide adsorption sites for negatively-charged arsenic and virus particles.
Adsorption of Rotavirus (IEP=4.5) onto hematite (iron) nanoparticle; (Gutierrez et al., Water Research, 2009)
Credit: Tony Straub and Hating Wang, (2012), Virus removal in Biosand Filters, UIUC

17. Iron-amended BSF – Virus Removal
You et al. (2005) found (in a flow-through column study):
removal of viruses was 99.99% to >99.999% (4log10 to >5log10) with an effective contact time with iron of 20 minutes
most of the viruses removed were either inactivated or irreversibly adsorbed.
as water flowed through the iron column, new iron oxides were formed continuously to serve as new adsorption sites
Virus Adsorption to Iron Oxides
Adsorption of MS2 virus (IEP = 3.6) on hematite nanoparticles;
(Gutierrez et al. Water Research, 2009);

18. Iron-amended Biosand Filters
Iron-amendment has been found to enhance virus and arsenic removal (versus the sand-only BSF).
An iron-amended BSF is made by adding common steel into the diffuser basin , mixed directly in the sand bed (or in a pre-filter container).
Studies to date have used:
common iron nails (non-galvanized),
steel wool,
iron particles, and
coated sand
Corrosion of the iron creates the iron oxides (rust).
MS2 Virus removal in v9 BSF
Bradley et al. (2011)

19. Iron-amended BSF – Virus Removal
Bradley et al. (2011), using full scale BSFs, indicated:
Virus removal can be as high as 3-4log10 for iron-amended BSF during the first week of operation, as opposed to less than 1log10 for the sand-only BSF.
Over time, microbial mediation (sand-only) processes begin to dominate and add to the surface charge (iron-amended) processes.
The BSF could potentially be modified to meet the USEPA drinking water standard for virus removal of >99.99% (4log10).

20. Iron-amended BSF – Arsenic Removal
Ngai et al. (2007) determined in a two-year evaluation of over 1000 Kanchan Arsenic Filters deployed in rural villages of Nepal, that the Kanchan Filter typically removes:
85–90% arsenic,
90–95% iron,
80–95% turbidity, and
85–99% total coliforms.
Kanchan Arsenic Filter
Components of the Kanchan Arsenic Filter (KAF - biosand filters amended with iron nails in the diffuser basin)

21. Kanchan Arsenic Filter
Uy et al. (2009) in a study in Cambodia of ten Kanchan Filters, in 5 configurations over 30 weeks found:
arsenic removal averaged 95-97% for all configurations
removal was unrelated to flow rate nor by filter cleaning to restore flow rate
no apparent depletion of adsorption capacity over 30 weeks (8400 L filtered)
Iron removal was also high (99%)

22. Iron-amended BSF & Water Chemistry
The chemistry of the influent water can affect removal:
Hard water can cause scaling which prevents rust from forming on the iron significantly affecting arsenic removal.
Naturally occurring iron in the source water will assist arsenic removal, but phosphates will compete with arsenic for adsorption sites on iron oxides.
“Ratio of iron to phosphates in the groundwater will affect iron-based arsenic removal systems” (Mahin et al., 2008)
It is important to understand the water chemistry of the source water to be treated by adsorption systems using iron-based media.

23. Alternate Versions of the BSF
Several different versions of the Biosand Filter body have been proposed to provide options for the numerous situations where the BSF is needed.
The versions suggested are intended to improve the filter in various ways such as:
lighter weight,
less expensive,
easier to transport,
possible to import,
simpler to make

24. Alternate Versions of the BSF
The suggested materials for the BSF body include:
cast concrete,
injection-molded plastic,
PVC pipe,
sheet metal,
concrete pipe,
molded plastic bag,
ceramic container,
large plastic drum, and
5 gallon (20 L) plastic bucket [not recommended].
The shape may be square or round or tapered for transport.

25. Alternate Versions of the BSF

26. Alternate Versions of the BSF

27. Summary and Conclusions
The simplicity, high user satisfaction, and sustainability of the biosand filter has led to substantial research and user feedback.
This learning has provided the evidence for the advances in the technology:
Dimension change in the BSF design – version 10
Filtration rate/ Flow rate recommendation
Sand specifications and sand bed depth
Diffuser basin design and preference over diffuser plate
New learning has improved understanding of :
Pause period
Ripening time
Virus removal, and
Potential for iron-amended BSF to improve virus and arsenic removal
These advances have led to further performance improvements and even greater potential for wide adoption of the biosand filter technology.