La prensa para los “sectores sociales organizados”

It is advisable to visit a fully operational factory before starting production. During production start-up, factories should work with an experienced ceramic filter technician to aid in

determining the appropriate filter mixture ratio for raw materials. This will also help new factories start reliable, quality-controlled production. A current list of experienced filter technicians who can help set up factories and a list of factories that strive to apply best

practice guidelines and accept visitors will be posted at www.ceramicwaterfilter.org in the near future. It is also important to work with local potters who are familiar with the local clays and who will be employed at the filter factory on an ongoing basis.

The appropriate ratio of clay to burn-out material is determined empirically when the filter factory is established and will be specific to the characteristics of the materials used. The flow rate of finished filters is measured as an indicator of consistency of production and filter quality, both when determining filter mixture ratio and throughout production. Too much burn- out material can lead to fragile filters that do not adequately treat water and not enough burn- out leads to slow filtration rates. An appropriate filter mixture ratio, when other processes are carried out carefully, will produce filters that:

1) Press without tearing or slumping. 2) Dry without warping or cracking. 3) Fire without deforming or cracking. 4) Have sufficient strength once fired.

Indo-1 implemented Statistical Process Control procedures whereby filter weight, drying time,

kiln temperatures (6 positions), visual faults, and filtration rates are documented. Firing curves, filtration rate distributions, and diagrams are created for data

5) Have consistent and appropriate flow rates. 6) Pass microbiological testing.

The filter mixture ratio is not the only variable that influences the flow rate. Clay characteristics, the type and size of the burn-out material, the humidity of the burn-out material, amount of water included in the mixture, the type and particle size of additional materials included in the filter mixture, the method of shaping, the amount of pressure applied during pressing, the location of the filter in the kiln, and the firing profile and peak temperature reached will also influence the flow rate; therefore, the materials must be consistent and processes must be carried out carefully and consistently when producing prototype filters and during normal production. This will aid in achieving consistent results and increase the likelihood that filters that fall within the specified flow rate range are representative of the prototype filters that passed microbiological testing.

To determine the appropriate clay:burn-out ratio, batches of a minimum of ten filters are made from different clay:burn-out ratios. Depending on the local clay, initial trials should start with 50% dry, powdered, sieved clay and 50% dry, sieved burn-out, by volume. Generally, for sawdust, a ratio between 40:60 and 60:40 clay:burn-out by volume is an appropriate range (see Section 3.5, Figure 3-7 and Figure 3-8 for existing factory ratios). Once an equal volume of clay and burn-out is measured, they should be weighed to determine the relative proportions of each (approximately 5:1 clay to sawdust by weight). Materials for subsequent tests should be measured by weight. Measuring materials for production by weight is more accurate than by volume since low-density burn-out material can settle. Burn-out material is typically dried in the sun; however depending on the season and weather conditions, it can retain enough moisture from the atmosphere to affect its weight; therefore, factories should dehydrate the burn-out material before measuring it. This can be done in a pan over a small wood fire, in a drying oven, or by another dehydration method. If laterite, sand, or grog is added to the filter mixture, the amount of burn-out material required may need to be reduced, since these additives tend to increase the flow rate. Instructions for line and triaxial blending are provided in Annex D. It is important to label prototype filters and to thoroughly document materials characteristics, mixture ratios, and procedures so that suitable ratios and methods can be repeated.

Once dry materials have been mixed thoroughly, a measured amount of water is added gradually. Thirty percent (30%) water, by weight, is an appropriate amount to begin testing. The wet filter mixture should be moist, but on the dry end. It should feel a little drier than a typical cement mix or a typical clay mix for throwing on the wheel. Enough water has been added when the filter mixture can hold its shape when squeezed out through the hands or when it begins to compact into a ball. If the filter mixture is too wet, the pressed filter will “slump” or fall over when removed from the press. The amount of water added to the filter mixture might vary slightly depending on the climatic conditions and should be documented. Masks should be worn during mixing and anytime there is risk of inhaling silica dust or other airborne particles, as exposure can have serious long-term health consequences.

After pressing, drying, and firing the prototype filters, flow rates should be measured (Section 8.3.4) and the flow rate range for each ratio test batch should be evaluated and documented. Filters made from the mixture with flow rates closest to the target flow rate should be tested for

microbiological efficacy (Section 8.3.5). It is critically important that filters made from the selected mixture ratio consistently demonstrate microbiological efficacy in laboratory testing. A minimum of three batches of ten filters must be manufactured with the proposed ratio and a minimum of three filters from each batch should be tested with contaminated water in a laboratory to ensure there is a minimum 99% (2-log) reduction in microbiological indicators. This is necessary to demonstrate that the results of the first successfully fired batch can be consistently repeated in normal production. If filters with both high and low flow rates are tested, the results can provide information regarding the flow rate limit. Effluent water from prototype filters should also be tested for heavy metals and inorganic chemicals to ensure the selected materials and processes do not contribute towards leaching contaminants into filtered water. Once a materials ratio has been established and verified, production can begin.