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



• 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



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 University Engineers Without Borders



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)



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



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



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



Water Quality Tests – Jan. 2008

•Tested for bacteria and E. coli at key locations

Spring box

Intermediate tank

Filters

Dirty and clean spigots



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



Prototype Water Quality Testing

• Test water before and after filtration

•Test water for

Turbidity

Total coliform

E. Coli



Prototype Test Results

Full removal of E. coli bacteria

E. coli Results



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



[email protected]



The Global Role of Sand Filtration



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



• 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



• 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



Slow Sand Filter Technology



• 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



Source: Oasis Design (2006)

Slow Sand Filter: Physical Setup



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



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



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



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



Design, Construction and Maintenance



Basic Design



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



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



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



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



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



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



Prototype Construction• General Design

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



Importance of Backfilling

Air bubbles!



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



Schmutzdecke Scraping



Questions



Break

Enjoy refreshments!



Discussion



Successes and Obstacles:Arada Vieja as a Case Study



Successes• System requires no electricity

– Pump is gravity fed

• Materials can all be found locally except for pump

• Filters succeed in removing E. coli



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



Thank You!

• EWB - USA

• Tufts EWB Executive Board

• Workshop participants



References

•WHO•Biosandfilter.org•Medair•EWB-USA

slow sand filtration workshop

Documents