The processing of burn-out material is important since the pores created by the burn-out material form the path water travels through the filter walls. The burn-out material should be inspected visually after receipt from the supplier and regularly during production to ensure the material has consistent characteristics and consistent particle size distribution. There should also be no foreign material (rocks or pieces of wood) present in the burn-out.
The amount of burn-out material required per filter will vary depending on the established ratio. In Nicaragua, approximately 27 kg (59 pounds) of sawdust per day will be used to produce 50 filters. Prepared burn-out material can be stored in 25-kilo (50-pound) sacks in a dry, raised, covered location, protected from moisture.
Indo-2 found that using sawdust as a burn-out
material increased shrinkage too much yet
Cam-1 expressed a preference for sawdust since the filters appear smoother and are nicer
looking.
The Dominican Republic factory found that sawdust
from oak left an oily residue on the fired filters and resulted in a reduced
flow rate. The Nigeria factory, however, uses oak sawdust successfully.
Equipment needed for processing burn-out material will depend on the characteristics of the material received and the processing required, but in general includes an area that is protected from rain, scoops, sieve(s), sacks, and a grinder, blender, or
hammermill with sharpened blades. It is important that the equipment is inspected regularly and the sieves be inspected daily. Resources for repairing or replacing damaged parts or equipment should be readily available. Worker safety materials needed for burn-out preparation include face masks, goggles, gloves, earplugs, and closed shoes.
At a minimum, burn-out material should be sun-dried and sieved with a 30 mesh screen. If the best available burn-out is coarse or is not predominately of the desired mesh size it must be milled or ground before sieving. Some factories have two hammermills, but a hammermill with a 5 horse-power reversible motor can be adapted for burn-out processing by sharpening the reverse side of
the hammers to a sharp point (at about a 60 degree angle) and then tempered by heating them in the embers of the firebox; once they reach the color of the embers, they can be removed and quenched in water or oil. The same hammermill can be used with the motor rotating in one direction to mill the clay, and in the opposite direction to mill the burn-out. By reversing the direction of the motor, the sharp ends of the hammers will mill the burn-out material to a finer particle size. The hammermill should be running before adding material, and only a small amount of material should be added at a time to prevent the machine from
jamming. Alternatively, a chili grinder or kitchen blender can be used to mill or grind burn-out material.
Sieving burn-out with only one screen will not create a uniform burn-out particle size as there is still no control of particles smaller than the sieve used. Although defining an appropriate particle size distribution for the burn-out material requires further research, the exclusion of very fine burn-out particles might be helpful as these particles could have a negative effect on the plasticity and thus the workability of some clays. Sifters, like the ones used in Myanmar (Figure 4-4), can be manufactured locally and used to efficiently sift using two or more mesh sizes. As the handle is turned, the sloping screens vibrate. The coarser particles stay on the top screen and the very fine particles fall through the bottom screen, leaving the burn-
out material for the filter mixture on top of the bottom screen. As with clay, the coarser material can be remilled and resifted.
Ideally, a custom, consistent mixture of burn-out particle sizes would be formulated by
combining appropriate amounts of representative particle size ranges. Particle sizes would be classified using sieves of different sizes (perhaps 16, 30, and 60 mesh screens). Although this
The Nigeria factory has successfully used a
kitchen blender to increase yield from sawdust and as an alternative, SL-2 uses a chili grinder for milling rice
husks.
At Indo-2, two sieve sizes are used to eliminate both
the larger and finer particles.
Figure 4-4: Burn-out Sifter (Thirst-Aid 2010)
could achieve a consistently high control of the particle size distribution of the burn-out
material, it could increase labor requirements. Further research is needed to determine if there is an ideal particle size range as well as the appropriate quantities for each particle size. In addition to following national or local health and safety guidelines, workers should wear face masks for protection against inhaling airborne particles and goggles to protect their eyes from dust and material that spits from the hammermill when processing burn-out material. Earplugs should be worn when using a mill or other loud machinery, long hair should be tied back, and loose clothing, which can get caught in machinery, should not be worn.