slow sand filtration workshop

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Slow Sand Filtration Workshop

Saturday 31 January 2009Tufts University

Anderson Hall, Nelson Auditorium200 College Ave

Medford, MA 02155

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

• Presentation– Workshop objectives– Introduction to Tufts EWB– Global role of sand filtration – Slow sand filter technology – Filter construction and maintenance

• Break• Prototype build session• Discussion

– Successes and obstacles with the technology

Agenda

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Workshop Objectives

• Learn about slow sand filtration technology

• Learn how to design, construct and maintain slow sand filters

• Collaborate on future project work and workshops

• Discuss ideas and share helpful tips

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Tufts University Engineers Without Borders

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

History

• Four projects– Tibet (2005)

• Latrines

– Ecuador (2006-2008)• Green House, BioGas Systems, Water Quality

– El Salvador (2006-Present)• Water Distribution and Quality

– Haiti (2008-Present)• Assessing (Latrines, Water Accessibility, Market

Construction)

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

El Salvador

• Arada Vieja– Located south of capital

of San Salvador

– Population: ~120

– Assessment trip in July of 2006 resulted in a need for filtered water

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Slow Sand Filtration• Successive trips

dedicated to building filtration system– January 2007: Began

construction– July 2007 and January

2008: Finished construction and conducted water quality tests

– January 2009: Follow up trip

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

System Profile125’

100’

75’

50’

25’

Intermediate Tank

Spring Box

Hydraulic Ram Pump

Sand Filtration System

Ele

vati

on

300’

150’

600’

450’

900’

750’

~3000’

1050’Distance

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Water Quality Tests – Jan. 2008

•Tested for bacteria and E. coli at key locations

Spring box

Intermediate tank

Filters

Dirty and clean spigots

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Sand Filter Prototypes

• Started after Summer 2006 assessment trip

•Provides hands on experience with the technology

•Gives insight into the construction process

• Assesses the effectiveness of the filters

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Prototype Water Quality Testing

• Test water before and after filtration

•Test water for

Turbidity

Total coliform

E. Coli

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Prototype Test Results

Full removal of E. coli bacteria

E. coli Results

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

El Salvador Project Future

• Arada Vieja Project – Expect 1-2 more trips for further water quality

testing and community health surveys

• Porvenir Project – Assessment trip January 2009– Small community with water access and quality

problems– Assessing feasibility of slow sand filtration

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

TuftsEWB@gmail.com

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

The Global Role of Sand Filtration

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

The Global Role of Sand Filtration• Small scale micro-enterprise projects

– Africa, Latin America, Asia

• Large scale– Springfield, MA – Paris, France – London, England– Antwerp, Belgium

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

• Emergency and disaster relief– Pilot project by Medair in Darfur starting in 2003

• Provided clean water to 7,500-10,000 people in the region

The Global Role of Sand Filtration

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

• Utah State University in Mexico

• San Francisco Professionals Chapter in Fiji

• University of Maryland, College Park in Ecuador

• Lafayette College in Honduras

• New Jersey Institute of Technology in Milot, Haiti

• Hope College in Nkuv, Cameroon

• University of Colorado at Boulder in Muramba, Rwanda

Past, Present and Future EWB Projects

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Slow Sand Filter Technology

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

• Oldest form of water treatment

• Used to treat surface and ground water

• Improves physical, chemical and bacteriological quality of water– Transportation, attachment and

purification

Slow Sand Filtration: The Basics

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Source: Oasis Design (2006)

Slow Sand Filter: Physical Setup

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

How It Works• Standing water

– Heavy particles settle– Light particles coalesce – Algae grows – release of oxygen

• Biofilm (Schmutzdecke)– Forms within 10-20 days of consistent flow– Made up of algae, bacteria, fungi and other microorganisms– Feeds on dead algae and live bacteria in the water– Strains suspended particles– Color removal– Breakdown of nitrogen compounds

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

How It Works• Filter bed

– Straining of particles– Adsorption– Biological activity

• Most activity takes place closer to the top of the layer• Formation of microorganism layer

• Gravel– Support filter bed– Prevent sand particle flow through outlet

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Advantages of Slow Sand Filtration

• Unskilled labor to install and maintain• Inexpensive• Does not require mechanical power • Low flow rates

– Increased time for pathogen removal in biofilm

– Pathogens not transmitted deeply into layers

– Biofilm can become better developed

• Can produce potable water in accordance with EPA standards

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Disadvantages of Slow Sand Filtration

• Pretreatment required for highly turbid raw water

• May need disinfection after filtration to meet superior water quality standards

• Requires large surface area• Cold temperatures lower

filtration efficiency• Algae may interfere with

operation

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Design, Construction and Maintenance

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Basic Design

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Filter Types• Concrete

• Box or cylinder with outlet embedded in wall

• Generally cheap

• Widely used in developing countries

• Need metal mold for construction- decreases mass production

• Plastic• Simple design

• Mass producible

• Relatively cheap

• Durable

• Lightweight, easy to transport

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Filter Types• Drum Filter

• Good when cement is difficult to obtain and oil/fuel drums are readily available

– Will need cleaning equipment

Drum Filter Concrete Filter

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Considerations for Filter Design

• High surface area/volume ratio• Slow inflow rate

– 120 L/m2-h or 100-400 L/m2-h

• Constant or intermittent flow– No significant effect on bacterial

removal – Lower flow rates improve turbidity

and color

• 5 cm head above fine sand layer• Clean sand of specific grain sizes

Variable Speed Pump

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Suggested Grain Sizes

Gravel ~ 5mm diameter

Std. pea gravel, or # 3 sieve

Coarse Sand

~ 1 – 1.2 mm

~#12 - #16 sieve

Fine Sand

0.25 – 0.35 mm

~#56 - #70 sieve

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Cleaning the Sand

Method One:– Sieve sand– Put sand in bucket and add clean water– Swirl bucket and decant dirtied water– Let sand dry

• Method Two:– Buy pre-cleaned/pre-sieved sand from distributor

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

ConstructionSand Filter Depths• Fine sand (.25 - .35 mm effective)

• 20.5 – 24 inch depth

• Coarse sand (1.2 mm effective)• At least 2 inch depth

• Gravel (5 - 15 mm effective)• 3 inch depth

• Outlet Pipe• 1 inch pipe diameter

• 5 cm above fine sand layer

• Perforated, covered by mesh

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Prototype Construction• General Design

- 4” diameter PVC casing- Clear acrylic tube- Piezometers- Outflow tubing- Overflows- Gravel- Coarse Sand- Fine Sand

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Importance of Backfilling

Air bubbles!

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Maintenance Performed Every 3-5 Months

1) Remove the lid and the colander/diffuser basin.

2) Lower the water level inside the filter by using a small cup to scoop out the water that has not drained through the outlet pipe.

3) Make a small hole in the sand with the cup. Scoop out the water that accumulates in it until only wet sand remains.

4) Remove 3 to 5cm of the fine sand layer and set it aside. (After washing and drying in the sun, this sand may be reused next time maintenance is performed.)

5) Add clean, fine sand from previous maintenance. Level the surface of the sand.

6) Reinstall the colander/diffuser basin.

7) Slowly add water to the filter until water begins to flow through the outlet pipe again and water is 5 cm above fine sand layer.

8) Again remove the lid and colander/diffuser basin

9) Level the surface of the sand again

10) Reinstall the colander/diffuser basin

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Schmutzdecke Scraping

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Questions

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Break

Enjoy refreshments!

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Discussion

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Successes and Obstacles:Arada Vieja as a Case Study

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Successes• System requires no electricity

– Pump is gravity fed

• Materials can all be found locally except for pump

• Filters succeed in removing E. coli

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Obstacles to Overcome

• How long can flow be cut off to filters?– Currently 24 hours off,

48 hours on

• Bacterial growth in clean water storage tank from surviving bacteria after filtration

• No residual disinfection

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Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

Thank You!

• EWB - USA

• Tufts EWB Executive Board

• Workshop participants

Tufts Engineers Without Borders

Tufts EWB Slow Sand Filtration Workshop, 31 January 2009

References

•WHO•Biosandfilter.org•Medair•EWB-USA