Improved Field Methods for Construction of Concrete

International Journal for Service Learning in Engineering,

Humanitarian Engineering and Social Entrepreneurship

Vol. 11, No. 1, pp. 32-50, Spring 2016

ISSN 1555-9033

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Improved Field Methods for Construction of Concrete

Biosand Water Filter Housings

Timothy J. Bowser, P.E. Department of Biosystems & Ag Engineering

Oklahoma State University

[email protected]

Paul R. Weckler, P.E. Department of Biosystems & Ag Engineering


[email protected]

Marquette Bugg Masters of International Ag Program


[email protected]

Eric Lam Consulting Engineer

CIMMYT

Texcoco, CP 56237 Mexico

[email protected]

Jim F. Chamberlain, P.E., BCEE Water Technologies for Emerging Regions (WaTER) Center

University of Oklahoma

[email protected]

Abstract – This paper discusses two alternative solutions to remedy the difficulties and

challenges associated with low-cost field methods for construction of concrete biosand

water filter housings. Outer and inner forms are required to retain the concrete in the

desired shape for the filter housing. Removal of the inner form can be a challenging

process. The first method outlines a procedure for the use of an inflatable boat fender that

is used for the inner form. The boat fender can be deflated and easily removed after the

concrete is set. The second method uses stuffed feed sacks for the inner form. The feed

sacks are stuffed with a granular material and stacked vertically. After the concrete is set,

the granular material is removed and the feed sacks are readily separated from the

concrete. Both methods of construction are described in detail. Findings include two rapid

field methods for constructing inexpensive concrete biosand water filter housings.

Index Terms - Biosand, water, filter, concrete, housing, household, field, method

INTRODUCTION

A Biosand Filter (BSF) is a home-use, slow-sand water filter that was developed by Dr. David

Manz of the University of Calgary, Canada.1 Concrete is the material of choice for BSF housings

in many parts of the world because of its durability and low cost. The Centre for Affordable

Water and Sanitation Technology2 (CAWST) has led efforts in the design and application of the

concrete BSF housing, along with a similar model designed by BushProof.3 The concrete BSF

can be entirely poured on-site, with a reusable mold, simple tools and few supplies, making it

very effective at meeting sustained water needs for communities in developing countries.4

The CAWST and BushProof models can be mass-produced using steel molds for the

concrete casting. The steel molds are relatively expensive and not practical to build on-site for

casting of smaller numbers of filter housings. As an alternative, Lam4 encouraged the use of

mailto:[email protected]







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native wood materials to build forms. Lam suggested plywood or dimensional timbers, held

together with screws or nails. Lam acknowledged that the wooden forms were not as durable as

the steel versions, but were low-cost and more feasible for small-scale production centers located

in remote areas.

An example of a cast concrete BSF and the wooden forms used to make it is shown in Fig. 1.

Regardless of the existing design of the form, or the type of materials used to build the form, one

of the most difficult steps in the fabrication process has been the removal of the internal form (an

internal form is shown in Fig. 2). There are several reasons for this difficulty: 1. Adhesion

and/or cohesion of the concrete to the surface of the form; 2. Incorrect geometry of the form

(manufacturing error) that causes binding between the form and concrete; 3. Swelling of porous

forms (e.g. wood); 4. Damage to the form that caused it to be misshapen (often happens with

repeated use); 4. Rust, dirt, or foreign material on the form; and, 5. Improper use of the form

(e.g. placement and anchoring).

FIGURE 1 EXAMPLE OF A COMPLETED CAST CONCRETE BSF (LEFT) AND WOODEN FORMS USED IN THE FABRICATION

PROCESS (RIGHT)



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FIGURE 2 INNER FORM FOR CONCRETE BSF HOUSING WITH TAPER AND PLASTIC COVERING TO IMPROVE RELEASE

FROM CONCRETE

Evidence of the difficulty of removing the inner form can be found in the literature. The

BushProof instructions describe coating the inner form surfaces with food grease3 to help

promote release of the concrete. This procedure alone suggests that the form may be difficult to

remove. Beyond the grease coating, a mechanical “puller”, fabricated of steel, was required5 to

remove the BushProof inner form. The BushProof puller resembled a gear remover commonly

used in power transmission applications. The puller was anchored to the outside mold and

attached to the inner mold. A threaded rod was turned with a wrench to pull up on the inner

mold. The CAWST instructions for using the biosand filter mold described a device that was

similar to the BushProof puller that they called an “extractor”.2 Tapping the mold with a hammer

or piece of wood was also encouraged to help release the concrete from the form.1, 2

The authors have extensive experience using wooden forms to make concrete biosand filter

housings in local workshops held in Oklahoma and on site in Honduras, Guatemala and

Tanzania. Local workshops include the annual, International WaTER conference held at the

University of Oklahoma, Norman and led by author Chamberlain. This workshop focuses on

water needs in developing countries. The Oklahoma State University chapter of Engineers

without Borders has ongoing humanitarian aid projects involving BSF manufacturing and

application in Honduras and Guatemala. Authors Weckler and Lam have served as the club

advisor and president, respectively. Author Bowser has led a regional workshop in Tanzania in

cooperation with Pioneer Missions, Eads, Tennessee, to teach BSF construction to community

leaders from Kenya, Rwanda, Tanzania, and Uganda.



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On many occasions, the authors have witnessed the inner form binding for each of the

reasons listed above. Frequently, the inner form can be removed with patience and hard work;

however, the form may be destroyed in the process. On occasion, the concrete biosand filter

housing is damaged or broken beyond repair during the process of removing the inner form.

Improvements have been suggested to help achieve release of the inner form with mixed

success. Lubrication using oils and fats are among the most common suggestions. Lubrication

often achieves a better release, but there are concerns about the unassessed effects of the residual

oil on the biosand filter layer and water quality. Lam4 taught the use of plastic wrap as an

alternative release agent. He wrapped plastic sheeting around the inner mold prior to setting it in

place (Fig. 2). The sheeting did not adhere to the concrete and prevented the wood from

absorbing moisture and swelling.

Another suggestion4 to help achieve better inner form release was to taper the form (see Figs.

2 and 3). In theory, tapering from the top (open end of the biosand water filter) to a reduced

profile at the bottom would eliminate the chances of the inner form wedging inside the concrete

structure. The slight taper would have little effect on the capacity and function of the water filter.

Field experience taught that the taper was difficult to achieve with available hand tools (hand or

power saw and crooked or warped wood of varying thickness). If even slightly misshapen, the

tapered inner mold may solidly wedge in the concrete.

FIGURE 3 DRAWING OF INNER FORM FOR CONCRETE BSF HOUSING WITH TAPER AND SEPARATE PIECES FOR

INDIVIDUAL REMOVAL.4

Finally, Lam4 suggested making the inner form in separate pieces that could be removed

individually (see Fig. 3). A cleat and spacer were used to hold the lower portion of the form in

place during the concrete pouring and curing steps. The upper portion of the inner form (nearest

the opening of the biosand water filter housing) was held in place using temporary blocks.

Pressure from the concrete forced the pieces of the form together and could cause them to move

out of position or twist. Pressure from the concrete and small movements of the inner form

during pouring and curing has also caused problems with form removal.

This paper outlines two new methods for constructing concrete biosand water filter housings

that eliminate the problems associated with inner form removal. Both methods use materials that

are relatively inexpensive, convenient for travel, and readily available. The methods presented



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are general guidelines and not meant to be rigid instructions. The engaged reader will find many

ways to improve and modify the methods to achieve optimum results for their location and end

use requirements.

METHODS

The first novel method for constructing concrete BSF housings requires the use of an

inflatable inner form. The outer form can be fabricated of most any material that will hold the

concrete in place. The inflatable inner form can be deflated after the concrete sets and then

removed without difficulty. This is a vast improvement over the previously described, rigid inner

forms made of wood or metal. Basic requirements for the inflatable form include: rugged;

inflatable with a hand pump; inflation valve located on the distal end; means of anchoring the

inflatable in the form; low cost; and, availability. A search of the literature revealed numerous

inflatable cylinders that were available for use in applications such as vehicle suspension,

aviation, marine, and recreation.

Boat fenders are an example of an inflatable cylinder that meets all of the basic requirements

listed above. They are used in marinas worldwide to protect boat hulls from wear and collision

with docks or other watercraft. Boat fenders come in a range of sizes that are ideal for the inner

form of a BSF. An example of an inflatable, cylindrical boat fender is shown in Fig. 4. The boat

fender shown in Fig. 4 has a hole through the middle that can accommodate a rope that may be

used to anchor the fender to the form.

FIGURE 4 INFLATABLE, CYLINDRICAL BOAT FENDER (343 MM DIAMETER X 884 MM LONG) MADE BY POLYFORM,

USA (MODEL HTM-4) WITH A HOLE THROUGH THE MIDDLE FOR A ROPE ANCHOR

The next paragraphs describe a method of constructing a concrete BSF using a boat fender.

This method requires pouring the concrete filter housing in the inverted position. Inverting the

form permits anchoring the boat fender to a base using a rope that is passed through the hole in



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the middle of the boat fender. Better control of the drain tube placement for the BSF is one

advantage of having the form in the inverted position. A list of required materials and tools is

given in Table 1.

TABLE 1 LIST OF REQUIRED MATERIALS AND TOOLS NEEDED TO POUR A CONCRETE BIOSAND WATER FILTER

HOUSING USING AN INFLATABLE INNER FORM

Materials Hand tools

1. Inflatable inner form

2. Flexible sheet for outer form

3. Flexible sheet for inner form trim

4. Boards or plywood for base

5. Boards for feet on base

6. Concrete mix

7. Screws or nails

8. Rope

9. Duct tape

10. PVC tube and fittings for drain and spout

1. Hammer or screwdriver

2. Bucket for water

3. Shovel for mixing concrete

4. Drill and 10 mm wood bit

5. Saw

6. Hand pump for inflatable

The first step of construction is to fabricate a base to secure and support the boat fender. The

base should be able to support the full weight of the concrete BSF (about 150 to 175 kg). A piece

of 450 x 450 x 19 mm plywood was used for this example (see Fig. 5). A 10 mm hole was drilled

in the center of the base to tie off the boat fender. A spacer ring (Fig. 6) was cut from plywood

and attached to the top of the base using screws. The ring was centered on the base. The purpose

of the spacer ring was to help establish the annular space between the inner form (boat fender)

and the outer form.

FIGURE 5 PLYWOOD BASE USED TO SUPPORT THE CONCRETE BSF MADE WITH A BOAT FENDER SERVING AS THE

INNER FORM (BOTTOM VIEW)



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19

.1 m

m

Ø 443 mm

Ø 343 mm

FIGURE 6 DIAGRAM OF SPACER RING THAT WAS USED TO OFFSET THE OUTER FORM FROM THE INNER FORM (LEFT)

AND PLYWOOD SPACER RING ATTACHED TO PLYWOOD BASE USED TO SUPPORT THE CONCRETE BSF

(RIGHT)

The plywood base was supported by two 51 x 102 x 450 mm (nominal) boards that served as

“feet” to elevate the base above the ground (see Fig 5). The feet were attached to the base using

screws. The base was elevated on feet to aid in management of the rope anchor for the boat

fender. An overhand knot was tied in the end of a rope to keep it from completely pulling

through the boat fender. The free end of the rope was passed through the boat fender and the hole

in the base (Fig. 7, left). Then the rope was stretched tight on the lower side of the base (Fig. 7,

middle) and tied off. Fig. 7, right, shows the boat fender snugly anchored to the base. Fig. 8

shows how several pieces of duct tape were placed over the knot in the rope to prevent the knot

from embedding in the concrete. A piece of flexible sheet material was used to form a collar to

cover the gap between the boat fender and the base. In the example shown in Fig. 9, Formica®

(approximately 250 x 1.100 mm), a common veneer used on tables and countertops, was taped it

to the boat fender with duct tape. The outer form was made using a piece of scrap sheet metal

that was about 1.400 x 1.000 mm).



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FIGURE 7 ROPE THREADED THROUGH THE BOAT FENDER AND THEN THROUGH THE TOP OF THE PLYWOOD BASE

WITH SPACER RING (LEFT), BOAT FENDER PULLED FLUSH TO THE PLYWOOD BASE (MIDDLE), AND THE

BOAT FENDER SHOWN UPRIGHT SECURED TO THE BASE USING ROPE (RIGHT)

FIGURE 8 DUCT TAPE APPLIED TO THE ROPE KNOT TO PREVENT IT FROM EMBEDDING INTO THE CONCRETE



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FIGURE 9 COLLAR THAT COVERS THE GAP BETWEEN THE BOAT FENDER AND THE BASE. THE COLLAR IS A PIECE OF

FLEXIBLE SHEET MATERIAL THAT WAS TAPED TO THE BOAT FENDER

The outer form was made using a piece of scrap sheet metal that was about 1.400 x 1.000

mm). The outer form was hand-rolled into a cylinder and placed over the boat fender. The

overlapping end of the sheet metal was pulled tight so the outer form fit snugly around the spacer

ring on the plywood base. Screws were used to anchor the outer form to the spacer ring as shown

in Fig. 10. Ropes were looped around the outer form and tied off to keep it in place (see Fig. 11).

FIGURE 10 A SHEET STEEL OUTER FORM WAS ATTACHED TO THE SPACER RING ON THE PLYWOOD BASE USING

SCREWS (LEFT) TO FORM AN ANNULAR CAVITY FOR THE BSF (RIGHT)



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FIGURE 11 DRAIN TUBE PLACED ON THE BOAT FENDER AND COVERED WITH DUCT TAPE TO PREVENT CONCRETE

FROM CLOGGING THE TUBE INLET (LEFT) AND THE EXTERIOR VIEW OF DRAIN TUBE HELD IN PLACE WITH

CARDBOARD (RIGHT). THE CARDBOARD WAS ALSO USED TO CLOSE THE GAP IN THE STEEL SHEET IN

ORDER TO CONTAIN THE POURED CONCRETE

Concrete ready mix (Quikcrete, Quikcrete companies, Atlanta, GA, USA) was thoroughly

combined with water to a dry consistency according to the instructions on the package. The

concrete was slowly transferred into the space between the inner and outer forms while being

constantly tamped with a wooden stick to remove voids. Just prior to covering the entire boat

fender with concrete, the drain tube was placed on the boat fender and covered with more tape

(optional) to keep the inside opening of the tube clear of concrete (see Fig. 11, left). Once the

tube was in place, the boat fender was covered with about 50 mm of concrete (Fig 11, right). The

concrete was cured for about 48 hours.

The next step was to remove the outer and inner forms. The outer form sprang off the

concrete after the ropes and screws were removed (Fig. 12). The BSF housing was carefully

tipped onto its side to expose the bottom of the base and the rope. The rope was untied and the

base removed. The inflation valve cap was unscrewed from the boat fender and a screwdriver

was inserted into the valve to permit some of the air to escape (Fig. 13). Deflation of the boat

fender allowed it to be easily removed. Concrete did not stick to the poly material of the boat

fender. Fig. 14 shows the interior of the completed concrete BSF housing after the boat fender

was removed. To cure the concrete more thoroughly, the BSF housing may be immersed in water

or covered with wet straw or rags for several days.



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FIGURE 12 REMOVAL OF THE OUTER FORM. IT DID NOT STICK TO THE CONCRETE. CARDBOARD THAT WAS USED TO

HELP SEAL THE SEAM OF THE OUTER FORM IS ALSO VISIBLE

FIGURE 13 DEFLATION OF THE BOAT FENDER USING A SCREWDRIVER TO HOLD OPEN THE VALVE



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FIGURE 14 INTERIOR OF THE COMPLETED CONCRETE BSF HOUSING AFTER REMOVAL OF THE BOAT FENDER

The second novel method for construction of concrete BSF housings requires the use of

plastic bags to make the inner form. Basic requirements for the plastic bags include: low cost

(preferably free); flat width of about 480 mm; plastic fabric or sheet; and, strong construction.

Feed sacks are one example of plastic bags that are available virtually around the world at very

low cost or free. In this example, dubbed the “feed-sack” construction method, feed sacks were

used in place of the inflatable inner form described above. The outer form can be a flexible sheet,

such as steel (used in the previous construction method), plywood or boards.

Feed sacks were obtained at a market in Mwanza, Tanzania, Africa. A quick search through

the shops revealed that a variety of sizes were available at low cost. The sacks selected were

about 460 mm wide and are shown in Fig. 15. The feed-sack construction method required

pouring the concrete filter housing in the upright position. A three-level, plywood outer form was

used which made placement of the drain tube in the lower level form a relatively simple task. A

list of required materials and tools is given in Table 2.

FIGURE 15 FEED SACKS USED AS THE INNER FORM FOR CONSTRUCTION OF A CONCRETE BIOSAND WATER FILTER



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TABLE 2 LIST OF REQUIRED MATERIALS AND TOOLS TO POUR A CONCRETE BIOSAND WATER FILTER HOUSING

USING FEED SACKS FOR THE INNER FORM

Materials Hand tools

1. Feed sacks (3 each)

2. Stuffing for sacks (e.g. sand)

3. Plywood, boards or sheet metal for outer form

4. Boards or plywood for base

5. Concrete mix

6. Screws or nails

7. Rope

8. PVC tube and fittings for drain and spout

1. Hammer or screwdriver

2. Bucket for water

3. Shovel for mixing concrete

4. Drill and 10 mm wood bit

5. Saw

6. Hand pump for inflatable

Fabricating a base to support the forms and concrete was the first step in the construction

process This step is optional if firm, level ground is available. The base must be solid enough to

support the full weight of the concrete BSF (about 100 to 125 kg). A plywood board, 450 x 450 x

19 mm was selected. Twelve plywood boards were cut for the outer forms. The height of the

boards was determined so that three levels would stack to make the full height of the BSF

housing (900 mm).

Six boards for the outer form were cut to 380 x 300 mm. The other six were cut to 480 x 300

mm. One of the boards had a hole, sized to accommodate the drain pipe, drilled midway on the

long side and 50 mm from the edge of the short side (see Fig 16). Twelve plywood strips were

cut from scraps left over after cutting the boards. The strips were about 40 x 300 mm. The strips

were tacked onto the short edges of the six, 480 mm wide boards, parallel and at a distance of

380 mm apart (see Figs. 16 and 17). Two boards with strips and two without were arranged on

the base as shown in figure 17 and held in place with a loop of rope.



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FIGURE 16 OUTER FORMS (480 X 300 MM, SIX PIECES) WITH 40 X 300 MM SPACING STRIPS TACKED ONTO THE SHORT

SIDES, AND CENTERED AT A DISTANCE OF 380 MM APART; 380 X 300 MM (SIX PIECES); BASE (ONE PIECE);

AND ROPE (LEFT). SPACING OF STRIPS (RIGHT)

FIGURE 17 ARRANGEMENT OF BOARDS FOR THE FIRST LEVEL OF THE OUTER FORM. THE CONCRETE PRESSES

OUTWARD FROM THE INSIDE OF THE FORM AND THE ROPE AND THE WOOD STRIPS HOLD THE BOARDS IN

PLACE WITHOUT NEED FOR FASTENERS

Concrete ready mix was thoroughly combined with water to a dry consistency according to

the instructions on the package. Concrete was transferred into the bottom of the outer form and

leveled to the bottom of the hole in the plywood board to make a base for the BSF housing. The



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PVC pipe was inserted through the hole in the plywood board and rested on the concrete.

Additional concrete was added to a level that was even with the lip of the pipe (see Fig. 18)

FIGURE 18 VIEW LOOKING DOWN INTO THE FIRST LEVEL OF THE OUTER FORM SHOWING THE CONCRETE BASE OF THE

BSF. THE OPENING OF THE DRAIN PIPE IS VISIBLE IN THE CENTER. THE DRAIN PIPE IS ALSO VISIBLE

PROTRUDING THROUGH THE OUTER FORM IN THE LOWER PORTION OF THE PHOTO

A feed sack was stuffed with a filling material. In this case, black mulch was used for the

filling material. Black mulch is a common and inexpensive yard decoration available in the US.

Sand, grain, small gravel, ground nut shells or similar filling material may be used. The sack was

stuffed firmly and evenly to form a cylindrical shape. The sack was filled to about 2/3 full. A

second feed sack was placed into the mouth of the first and it was stuffed with the granular

material (black mulch) to form an extended cylinder (see Fig. 19, left). Both sacks were lifted

together and placed into the form on top of the pipe and concrete and centered in the space (Fig.

19, right). Next, concrete was added between the bag and the form and tamped frequently to

remove voids. Tamping activity must not disturb the position of the sacks or the concrete base.



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FIGURE 19 FEED SACKS WERE STUFFED WITH GRANULAR MATERIAL AND STACKED ONE INSIDE THE OTHER TO FORM

A CYLINDER TO BECOME THE INNER FORM (LEFT) AND PLACED INTO THE WOODEN OUTER FORM (RIGHT)

Additional outer forms, bags and black mulch were added as each form was filled with concrete

and until all three levels of outer forms were in place. Fig. 20 shows the completed pour at full

height –about 1 meter. Fig. 21 is a view looking down into the completed pour. The concrete was

left to cure for about 48 hours.

FIGURE 20

COMPLETED CONCRETE POUR IN FORMS TO MAKE THE BSF HOUSING



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FIGURE 21

VIEW LOOKING DOWN INTO THE FORM AFTER IT HAS BEEN FILLED WITH CONCRETE TO SURROUND THE

FEED SACKS

Next the forms were removed. After the ropes were loosened, the outer forms came off easily

(see Fig. 22). The mulch was scooped out by hand and the feed sacks were loosened by gently

wiggling them free from the concrete. In some areas, small folds of the feed sack were trapped in

the concrete, but they were removed with light tugging. The final product is shown in Fig. 23,

and a view down into the filter housing cavity is shown in Fig. 24. To more fully cure the

concrete, the BSF housing can be immersed in water or covered with wet cloth or straw for

several days.

FIGURE 22

OUTER FORM PARTIALLY REMOVED TO EXPOSE THE CONCRETE BSF HOUSING



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FIGURE 23

CONCRETE BSF AFTER INNER (FEED SACKS) FORM AND OUTER (PLYWOOD) FORMS WERE REMOVED

FIGURE 24

VIEW INTO THE CAVITY OF THE BSF HOUSING THAT WAS FORMED USING FEED SACKS

RESULTS

Two new methods for constructing concrete biosand water filter housings were developed

and field tested. The resulting concrete BSF structures made as prototypes were structurally

sound and had proper dimensions. The new methods were successful because they were

inexpensive, simple to implement, and made use of materials that were field-available or readily

transportable. Historical issues relating to removal of the inner form were solved, making the



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new methods superior to the existing techniques for small-scale applications. The authors have

taught the boat fender method in Tanzania and it was received with enthusiasm and success.

Future work will continue to teach and refine construction methods for BSF housings and to

discover and test new materials.

ACKNOWLEDGMENT

The authors would like to thank the Department of Biosystems & Agricultural Engineering and

the Robert M. Kerr Food & Agricultural Products Center, both at the Oklahoma State University,

and the WaTER Center, at the University of Oklahoma, for providing facilities and funding for

activities related to the project; and, Pioneer Missions, Eads, Tennessee, for leadership and

funding for projects in Tanzania, Africa.

REFERENCES

1 Lee, T. 2001. Biosand household water filter project in Nepal. M.S. Thesis, Massachusetts Institute of Technology.

Available at: http://web.mit.edu/watsan/Docs/Student%20Theses/Nepal/Lee2001.pdf. Accessed on August 6, 2015.

2 CAWST. 2008. Biosand filter manual. Available for download at: http://resources.cawst.org/package/biosand-

filter-construction-manual_en. Accessed on May 20, 2014.

3Mol, A. and E. Fewster. 2012. Bio-sand filtration filter construction guidelines. Available at:

http://www.biosandfilter.org/biosandfilter/files/webfiles/BioSandFilter_Construction_Guidelines_180912.pdf.

Accessed on July 30, 2015.

4 Lam, E. 2014. Investigations in to the effect of influent total organic carbon concentrations on biosand water

filtration efficiencies and implementation implications in Honduras. M.S. Thesis, Oklahoma State University,

Stillwater. ISBN: 1321071302, 9781321071306

5 Mol, A. and E. Fewster. 2007. Bio-sand filtration mould construction guidelines. Available at:

http://www.biosandfilter.org/biosandfilter/files/webfiles/BioSandFilter_Mould_Construction_Guidelines.pdf.

Accessed on August 4, 2015.

http://web.mit.edu/watsan/Docs/Student%20Theses/Nepal/Lee2001.pdf

http://resources.cawst.org/package/biosand-filter-construction-manual_en

http://resources.cawst.org/package/biosand-filter-construction-manual_en

Documents

Improved Field Methods for Construction of Concrete