Improved Field Methods for Construction of Concrete Biosand Water Filter Housings

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 Technology 2 (CAWST) has led efforts in the design and application of the concrete BSF housing, along with a similar model designed by BushProof. 3The 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. 4he 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, Lam 4 encouraged the use of 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).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 grease 3 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 required 5 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". 2Tapping the mold with a hammer or piece of wood was also encouraged to help release the concrete from the form. 1,2 e 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.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.Lam 4 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 suggestion 4 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.Finally, Lam 4 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 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.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 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.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.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).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).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.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 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.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

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

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

FIGURE 4 INFLATABLE
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

FIGURE 5 PLYWOOD
FIGURE 5 PLYWOOD BASE USED TO SUPPORT THE CONCRETE BSF MADE WITH A BOAT FENDER SERVING AS THE INNER FORM (BOTTOM VIEW)

FIGURE 7 ROPE
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 10 A
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)

FIGURE 12 REMOVAL
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 16 OUTER
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)

TABLE 1 LIST
OF REQUIRED MATERIALS AND TOOLS NEEDED TO POUR A CONCRETE BIOSAND WATER FILTER .
FEED SACKS USED AS THE INNER FORM FOR CONSTRUCTION OF A CONCRETE BIOSAND WATER FILTER