Pathogen Control:  Chlorine V. UV

For potable water the most dangerous possible contaminant is biological.  Waterborne pathogens include viruses, bacteria, and protozoa, most originating in the gastrointestinal tract of a warm-blooded animal.  In areas that are subject to human contamination, the list of pathogens and resultant infirmities is legion, including such terrible conditions as:  cholera, hepatitis, typhus, typhoid fever, amoebic and bacterial dysentery, diarrhea, shistosomiasis, and others.  In native waters originating in streams and springs in the Costa Rican rain forest, the only pathogens are those that circulate through animals.  In practice, this gets down to one bad actor, Giardia lamblia, a protozoan that is responsible for beaver fever, or giardiasis.

In order to protect municipal water supplies against the large number of potentially deadly waterborne diseases, water engineers use chlorine as a disinfectant.  Conventional practice is to dissolve enough chlorine in water so that a chlorine “residual”  remains in the water to actively attack pathogens while the water remains in the distribution system.  Chlorine is so toxic that if it is present in even a small amount, no microbe can survive its oxidative toxicity.  Chlorine can be detected by very simple colorimetric means, unlike the detection of microbes, which is very complicated and imprecise.  This means that it is possible to have a 100% guarantee of safe drinking water based on so simple a process as a chlorine check. 

Chlorine is itself a carcinogen and its reaction with some organic compounds produces even more carcinogenic metabolites.  So, chlorine is not something that you really want in your water.  However, the lives saved by its use as a disinfectant outweigh the lives lost through unintended cancers by many orders of magnitude, so chlorination of public water supplies is a globally standard engineering practice to ensure water safety.  All Costa Rican public water supply systems are chlorinated and potable.  However, homeowners can install a granular activated carbon filtration system in their homes to remove chlorine prior to use.  This way the water is protected by chlorine until its delivery to the home and the danger of carcinogenicity is eliminated by removing chlorine at its point of use, representing the best available water quality management practice.  

For individual homes, ultraviolet disinfection is the best way to disinfect water.  It deactivates microorganisms during passage through a reaction chamber illuminated with ultraviolet light.  UV disinfection does not leave a residual like chlorine and is most appropriate for point of use applications and is ideal for residential applications.   


Your home water supply should be particulate free due to upstream sediment removal operations, from the intake itself to storage tanks plus any large-scale filtration steps in between.  But suspended sediment intrudes from all water supply sources except for springs.  A cartridge-based filtration system is usually ideal for individual homes.  For community water supplies, an automated backwash mixed media filtration system is required for particulate control at commercial levels.

Activated Carbon

Activated carbon removes dissolved organic compounds and chlorine from water.  Natural organic compounds contribute to color and odor and can be particularly abundant from ground water with abundant organic material and from rainfall capture storage tanks when the organic matter from leaves is not adequately screened.  Rarely do naturally occurring dissolved organics concentrate enough to reach the taste threshold.  These compounds are innocuous health-wise.  Native chlorine and its metabolites on the other hand are proven carcinogens and teratogens.  Activated carbon removes the free chlorine residual of municipal water supplies.

Activated carbon also removes synthetic organic compounds (industrial pollutants) from water as well as heavy metals (either from pollution or native).  SOC and heavy-metal contamination of water supply is unusual in Costa Rica.

Water hardness.

Water hardness is a problem that is widespread for deep wells that typically penetrate aquifers in volcanic rocks with an abundance of calcium and magnesium.  There is nothing in itself intrinsically wrong with hard water.  It is certainly not toxic, but objectionable.  It causes scale to precipitate in pipes and valves and particularly inside domestic appliances that change the temperature of water, most notably ice makers, espresso machines, boilers, irons, and similar devices.  Hard water requires a greater amount of detergent to achieve comparable “sudsiness,” and it precipitates objectionable rings around drains and toilet bowls.  While softening cartridges are available and can be used inside filter housings for small applications, these do not work as reliably as ion exchange water softeners.  If hardness is high enough to require treatment at all, it is best to not mess around with cartridges and to opt instead for a full water softener. 

Iron, Manganese, and Sulfur.

For high iron, manganese, and sulfur, chlorination systems tend to be the only viable option other than development of a different water supply altogether.  These come from deep wells when the homeowner is unlucky and are often the source of continual discontent until the investment is made in a robust water treatment system capable of removing the objectionable contaminants.

The Pura-III Home Purification System

The Pura-III is a 15-gpm rated home purification system that includes two big blue filter cartridges followed with an ultraviolet disinfection module.  Although most commonly the first two housings include a particulate filter followed by a block activated carbon filter, the two housings can also be used for a prefilter cartridge and a polishing particulate filter to optimize the system for particulate removal for those users that don’t require activated carbon.  The UV disinfection module is rated for deactivation of viruses at 15 gpm.  Viruses are much more hearty than protozoans and bacteria.  So for the actual Costa Rican target, Giardia lamblia, the effective rating of these units is significantly higher than 15 gpd.

These units are $1250 cash and carry, with a $250 installation fee plus parts, which vary according to location but usually run $150.

Work Plan

The first step in evaluating water treatment needs is representative sampling and chemical and biological analysis to determine the basic mineral makeup of the water and any contaminants that may be present.  Generally speaking, springs are the highest water quality source for rural homeowners.  Typically, a well installed spring water supply requires no water treatment of any kind, not even a sediment filter.  All rainfall capture and stream water systems require particulate, GAC, and ultraviolet disinfection, making the Pura-III system ideal for these cases.  Near surface wells often have bacterial contamination and higher natural organics but rarely have mineral problems like water hardness, iron, manganese, and sulfur.  Deep wells are nearly always free of biologic contaminants but commonly have mineral water quality issues that may or may not require treatment for optimal home use.  And all municipal water supply comes with chlorine, which the homeowner may prefer to remove prior to ingestion.  So, while some generalizations are possible, nothing will ultimately take the place of a chemical analysis of your water to know first what’s in it.

For question on water quality, you can contact me by filling out the questionnaire or  just write me an email.  It is usually possible for you to collect samples and submit them to a laboratory and gather all the information necessary to make excellent water treatment decisions without incurring any consulting costs so that the equipment and its installation is often the only actual expense required to ensure that your water supply provides you with the degree of water quality that you decide is appropriate.