Strike the rock, and water will come out of it for the people to drink. - Exodus 17:6


Water-borne disease can often be correlated to the presence of fecal coliform in drinking waters (think E. coli).  Other bacteria, viruses, protozoa, and helminthes are the microbial culprits of water-borne disease.  Illness can also be caused by organic and inorganic compounds. Though spring water is often safe to drink, we believe that adding water purification will greatly reduce the risk of water-borne disease. Our water purification recommendation includes both filtration and disinfection. 



Water filtration is important for removing larger pathogens that are more resilient to disinfection (e.g. cryptosporidium), as well as for the reduction of total suspended solids or turbidity.  Filtration works on the mechanics of straining, adsorption (via advection, diffusion and sedimentation), consumption (via microbes formed in the schmutzdecke), and termination/inactivation (due to lack of food or oxygen).

We recommend using the CAWST biosand filter. This is a concrete filter that costs about $20 US for materials, is easily reproducible with the steel mold, lasts almost indefinitely with proper maintenance, and is easy to understand. A section view of the filter can be seen in the following figure.

Biosand Filter - Section View


Also, a virtually unlimited number of CAWST concrete filters can be made from a given mold – meaning the medical missions could eventually provide filters (or lend out the mold) to every family in the community for a relatively low price. Our team has constructed the steel mold (approx. $200 for materials and 40 hours of labor) and will donate that to the medical missions. The steel mold can be seen in the following figure.

Steel Filter Mold


CAWST (Center for Affordable Water and Sanitation Technologies) is a non-profit organization located in Calgary, Alberta, Canada.  They have developed a manual in Spanish and English with step by step instructions on the construction and maintenance of the biosand filter.



After filtration, we are recommending disinfection using granular calcium hypochlorite. Chlorine based disinfectants are great oxidizing agents; meaning the cellular membrance of pathogenic cells are stripped of their protons, rendering these pathogens incapacitated.

Granular calcium hypochlorite only loses 1% of its disinfecting potential over 1 year (when properly stored in its solid form).  Because Cuchiverachi is located in the mountains, about a day’s walk to the nearest town, we thought it appropriate to select a disinfectant that required minimal transportation and resupply. Contacts at Better Water Industried and PPG Industries have made the acquisition of granular calcium hypochlorite possible.

The disinfected product goes through three different phases: granular calcium hypochlorite, hypochlorite bleach solution, and finally the disinfected drinking water. The granules are stored in a cool, dry location until the bleach solution has been exhausted .  A bleach solution is made by adding a small (specified) amount of the hypochlorite granules to a container of filtered water of specified volume. Once the bleach solution is made, it can be added to a container of filtered water in the ratio of 1 part bleach : 100 parts water. The design procedure for the disinfection process is detailed in the final design report.