Chris Salter, October 2010

Everything in water, that is not water, is a contaminant.  Not all contaminants are harmful: indeed, some may be considered to be beneficial.  Unfortunately, many of these "beneficial" contaminants may not be welcome visitors to your expensive appliances such as hot water heaters or boilers, washing machines, dishwashers, or household plumbing and bathroom, kitchen and swimming pool tiles etc., or even your hair and skin after having a shower.


La Paz Municipal delivered water is usually somewhere between 650 - 850 ppm TDS (parts per million Total Dissolved Solids) which includes 20 - 25 grains per gallon of hardness, mostly calcium and magnesium (5 grains per gallon is considered to be hard water), so the water is extremely hard, and can contain elevated levels of arsenic and nitrates.  If metered, this water costs around 22 Pesos per cu. M.: If not metered, it may be somewhat less.

In addition, the water contains a significant amount of suspended solids which may from time-to-time include the pathogen family of bacteria, virus, algae and possibly occultist cysts (cryptosporidium etc.).  The turbidity is probably in excess of 5 Nephelometric Turbidity Unit's (NTU's).  The US EPA and the WHO recommend a level of less than 1 NTU in potable drinking water.

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.

Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.

LOCAL WATER in La Ventenas/El Sergento

La Ventenas/El Sergento Municipal delivered water is around 3,200 ppm TDS. The high level of total dissolved solids is a result of sea water intrusion into the underground water supplies.  In addition the water has the same hardness and turbidity characteristics as La Paz Municipal water, but is also known to contain elevated levels of arsenic, nitrates and cyanide.  Pretty grim stuff.

Trucked-In Water.

Trucked-in water, from a different source to the Municipal delivered water, is normally around 550 - 650 ppm TDS with about 20 grains per gallon of hardness.  This water will cost around 55 to 65 Pesos per cu. M.  Again, this water will also contain a considerable element of suspended solids.

Treatment & Conditioning Options

Technology exists, by which we can remove ALL of the suspended solids by direct filtration.  The removal of the smallest suspended solids, including the pathogen family, and, at the same time, to provide an adequate flow of water, will result in a higher capital cost for the filters, and the subsequent replacement elements.

A professional water treatment and conditioning specialist has to determine the maximum amount of water you will probably consume on any given day together with the maximum amount of water you will be consuming at any given moment in time…and with that information they have to design accordingly.
Traditionally, in the USA, the hardness would be treated with salt water softener systems. These are currently being gradually banned in many US States and are not a good solution for B.C.S. as you are just adding more salt to the water.  In addition, a POU (Point-of-Use) under-sink Reverse Osmosis system will be essential for the salt removal of cooking and drinking water.

ANY specific dissolved contaminant can be selectively removed by ion exchange resins or selective treatment media (there are far two many contaminants in local water to even contemplate this solution as this would be totally cost prohibitive) OR around 99.5%+ of dissolved contaminants can be totally removed by either Distillation (which is both messy and again cost-prohibitive), or hyper-filtration membrane treatment (Reverse Osmosis) which will incur an element of waste water and an energy cost.

Whatever solution or combination of solutions that is recommended or selected, there has to be a trade-off ….there is no universal panacea, although some salespeople will tell you otherwise. These people are nothing more than Charlatans or Alchemists.


In La Paz Municipal water or trucked-in water this would involve the installation of a Reverse Osmosis (R.O.) treatment system installed at the POE (Point-of-Entry) to the property.

Assuming you are living in La Paz and your maximum demand is 300 gallons per day (gpd), the trade-off cost in reject water is another 150 gpd (i.e. 66.66% recovery) this will total 450 gpd or around 1.70 cu. M per day.  At maximum occupancy and usage this will cost around 37 Pesos per day for the Municipal water or around 102 Pesos per Day if you are using trucked-in water. 

If you live La Ventenas/El Sergento you will need a membrane system with a slightly different design operating at higher pressure that will waste 300 gpd (i.e. 50% recovery). This will total 600 gpd or around 2.27 cu. M per day.  At maximum occupancy and usage this will cost around 50 Pesos per day for the Municipal water or around 120 Pesos per Day for trucked-in water. 

Fortunately, most local based homes probably only use around 150 gallons on an average day, so the above cost analysis can be cut in half.

Some of the reject water can be used for toilet-flushing (if there is a separate water line to the toilets) and irrigation, although it is not suitable for irrigation in La Ventenas/El Sergento due to the high salt-content.  However, it can be used for drive-watering as the salt content tends to compact the sand and dirt.

The product water will be around 35-40 ppm TDS (i.e. high quality potable water)…most bottled waters in La Paz are in the 20 -60 ppm TDS range.

This system will remove ALL suspended contaminants and 99%+ of all dissolved contaminants.

If any specific property in the development requires PRISTINE QUALITY Water, they can always install an additional POU (Point-of-Use) under-sink low-pressure R.O. system for their Ice- Makers etc. but this is really luxury personified.


We can address the vast majority of the suspended contaminants, including ALL of the pathogen family at POE by direct filtration, and change the chemical constituency of the hard water by converting the damaging calcium and magnesium ions into a crystal form (Template Assisted Crystallization). 

In this format, the calcium and magnesium remain in the water and, as such, are beneficial to human consumption, but will prevent all of the lime-scale damage to appliances, plumbing, water heaters etc.  Indeed, over a period of time they will gradually remove existing lime-scale. 

These systems require no electricity, no salt, no energy, have no waste water and, as such, it is a totally GREEN solution and are normally installed at POE after the hydro-electric delivery pump.

However, this will not address the high level of dissolved contaminants in the water, other than the crystallization of the hardness ions.  Consequently, some people with highly sensitive palettes, and everybody in La Ventenas/El Sergento, may detect a slight "salty taste" to the water for drinking and cooking, and the ice will still contain these dissolved contaminants.

Again, this can be addressed by installing a low-pressure POU under-sink R.O. system which will provide up to 50 gpd of good-quality WATER per residence for drinking, cooking, ice-making etc.


For some quite unaccountable reason these systems are quite popular in BCS.  Unfortunately, very few are initially correctly specified and even less of them are adequately maintained.  There is no doubt that when correctly specified and regularly maintained these systems can address most of the pathogen concerns in Municipal and Well water.

Ultra-violet (UV) treatment is the disinfection process of passing water by a special light source immersed in the water in a protective transparent sleeve: the special light source emits UV waves that can inactivate harmful microorganisms. The ultra-violet rays, similar to the sun's UV but stronger, alter the nucleic acid (DNA) of viruses, bacteria, molds or parasites, so that they cannot reproduce and are considered inactivated.

UV systems alone are neither intended to treat water that is visually contaminated nor intended to convert wastewater to safe, microbiologically potable water.

UV treatment does not alter the water chemically as nothing is added except energy. It should be noted that inactivated microorganisms are not removed from the water. UV treatment does not remove dirt and particles, metals such as lead or iron, or hard minerals such as calcium, and consequently do not address any hardness and scale issues that are prevalent in BCS water supplies. Other devices are required to remove these particles, metals and minerals.

UV systems exist to treat all possible flow ranges, from small POU applications to entire municipalities. For household applications, a POU or POE UV system can be used. A POU system is a small, portable device that attaches to a faucet and rests on the counter: it can also be mounted under a counter. Larger POE systems are also available which are installed where the water supply enters the home, disinfecting the entire household water supply.

Class A systems can be both POE and POU (large or small); and, are designed to inactivate and/or remove microorganisms including bacteria, viruses, Cryptosporidium oocyst and Giardia cysts from contaminated water. However, they are intended to be installed on visually clear water (not colored, cloudy or turbid water) and not for converting wastewater or raw sewage to drinking water.  Class B systems are intended to reduce nuisance microorganisms and are not intended for disinfection.

Class A systems deliver a dose of 30 to 40 mj/cm2, enough to be used on water supplies which are not considered safe. A dose of 40/cm2 is recognized by Health Canada, US EPA and the WHO as sufficient for this type of application.

POU systems can be self- installed; however, it's important to know the condition of your water before use in the case you need pre-filters, which will be ABSOLUTELY ESSENTIAL in BCS. With POE systems, there are a number of aspects that need to be considered when installing a unit. These include assessing the condition of the incoming water, the need to install some pipes and the need to properly disinfect the system. You may want to consult a professional if a more complex system (one that uses filters) is required.

Water must be free of soil or sand particles (it should look clear and not cloudy). Such particles can block the UV rays and allow harmful particles to survive. Accordingly, a UV system normally has a five-micron filter installed upstream from the UV unit. A one-micron absolute filter should then be installed after the five-micron filter to remove cysts (small capsule-like sacs that enclose organisms).

The UV unit is installed after these filters. There are characteristics that can affect UV effectiveness such as water hardness, alkalinity, pH, and iron concentrations etc. Water should be therefore tested before installation to see if it will need additional treatment. This will assure proper UV disinfection. Contact a UV manufacturer or a water-testing laboratory to arrange a test.

For total UV disinfection suspended particles are a problem because microorganisms buried within particles are shielded from the UV light and pass through the unit unaffected. However, UV systems can be coupled with a pre-filter to remove those larger organisms that would otherwise pass through the UV system unaffected. The pre-filter also clarifies the water to improve light transmittance and therefore UV dose throughout the entire water column. Another key factor of UV water treatment is the flow rate: if the flow is too high, water will pass through without enough UV exposure. If the flow is too low, heat may build up and damage the UV lamp.

UV units operate at a low cost. The bulb gradually loses its disinfecting capabilities over time. It should be changed by you at least once a year - even if it is still operating. An annual filter/lamp replacement can be approximately $150. A lamp/bulb alone may cost from $40 to $100.00 depending on the wattage of the bulb.  Electricity costs are another consideration; however, the system is similar to running a 60W bulb. There is no additional water cost for running a UV system, as all of the treated water is available for consumption.

The quartz sleeve surrounding the bulb must be kept clean in order for the unit to function safely. It should be examined once a month; and if it becomes cloudy, it should be cleaned. Note: that no one system can treat water 100 per cent, and without proper maintenance it should not be considered 100 per cent reliable.

Other treatment devices may be required in addition to UV. Prolonged storage of water treated using UV, as the sole method of treatment, is not recommended.  If your drinking water comes from a private source, (such as a well), be sure to have your water tested periodically to ensure it safe to drink.

All products that come into contact with drinking water need to be certified to the appropriate health-based performance standard developed by NSF International. In the case of Ultra-Violet Light units, it is recommended that they be certified as meeting standard NSF/ANSI 55 for Class A devices. Components employed in conjunction with the UV system should also be certified to meet other applicable NSF/ANSI Standards.  These standards are widely accepted in North America, as they ensure the removal of specific contaminants, as well as the performance and mechanical integrity of the materials that come into contact with drinking water. Check the UV treatment unit's packaging or ask your dealer for a listing of the substances that the unit is certified to remove.

As a guideline, for a one-bathroom home or condo your UV system should be rated (and NSF certified) at a minimum of 3 gpm: a 5 gpm system for a two-bathroom residence: a three-bathroom residence should be 7 gpm: a four-bathroom residence should be 9 gpm, and for larger residences it should be 15 gpm+.  Restaurants and hotels may need systems up to 150 gpm.  It is extremely doubtful if any POE system in BCS can get by with standard 2.5" diameter x 10" long filter housings to provide for the adequate flow and treatment media contact time.  Housings that are 4.5" diameter x 20" long should be the minimum that are specified.

In summary, for the local waters of BCS, a RO Membrane system or a top-quality sub-micron filtration and conditioning system would be far better options than a under-specified UV system.
For operating cost comparison, Bottled Water in 17 liter/5 gallon Garrafones will cost between 600 - 850 Pesos per cu.M and in those small disposable bottles the cost is between 6,000 8,000 Pesos+ per cu. M.

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