The daunting challenge
By now, most in the international drinking water industry are very familiar with the UN Millennium Development Goal to reduce by half the number of people lacking daily access to safe drinking water by 2015 and that the majority of the world’s 1.1 billion in greatest need are living in developing and under-developed countries.
In some of the developing world’s largest cities, water supply is erratic. Aging, leaky pipes and intermittent power supply can result in tap water flowing only a couple of times per day or even as little as merely two hours a day, even in middle- and upper-class neighborhoods. This is frequently the case, for example, in parts of major cities like Hanoi and throughout India. Common usage of rooftop cisterns provides continuous water supply to households and improves pressure, but often such water is shunned for drinking because it becomes fouled in the seldom-cleaned tanks.
Relying on local or national governments to solve these infrastructure challenges in the near term is not realistic. The connection between poor water quality and disease is proven. Emerging middle-class consumers are increasingly looking for simple, affordable means to take control of water quality in their own households. Clearly, with erratic supply, storing clean water safely is also a must.
Untrained consumer household use in developing world
There are many good technologies for treating water. However, when assessing those most practical for simple household use by an untrained consumer of limited financial means, the list grows short rather quickly. Residual protection after primary disinfection is of great concern when the requirement for safe storage is added, essentially necessitating the use of chemicals (i.e., halogens). Halogens are defined as the column of elements from fluorine to astatine. For the purposes of this article we will only concern ourselves with the most common three: chlorine, bromine and iodine.
Very few look to bromine among halogen options. In part, this is because until recently, the available chemical forms for bromine have not been practical or easily managed.
This article discusses how circumstances have changed and why brominated disinfection media is poised to bring new power and potency to bear on this important consumer need.
For the record, this author in no way wishes to challenge the benefits derived from the use of chlorine and iodine in a wide range of applications. In particular, chlorine is by far the most prevalent water treatment chemical used for disinfection and has arguably saved more lives than antibiotics. Keeping this fully in mind, let us turn our attention pragmatically toward constraints upon applications within impoverished households of the developing world struggling to achieve the UN goals.
Many people turn to chlorine or iodine in their quest for safe drinking water. These chemicals can certainly accomplish primary disinfection and provide residual protection for safe storage, but with some important limitations and trade-offs in the context of what will be readily accepted, properly used and achieve the desired positive health impact in the developing world.
Three major challenges
Three major obstacles stand in the way of finding ‘truly ideal’ means of household drinking water treatment and safe storage in developing nations:
1) Uncertainty of dosage and time required for adequate disinfection. In a well-equipped environment, trained personnel can accurately determine well-established dosage and time requirements for the halogen of choice and achieve the desired germicidal affect. However, a mother collecting water from a village well or an urban tap is not prepared to assess water quality variations of turbidity, organic load or temperature and the subsequent impact on chemical requirements.
2) Taste and odor criteria for consumer acceptance. Many would like to believe that people can become accustomed to the taste of halogens (most typically chlorine) added to water and that they may even be taught to appreciate the taste by associating it with the knowledge that it represents ‘safety’. The universal fact is that consumers globally prefer little or no chemical taste and odor in their drinking water. That is really the benchmark for a truly ideal solution.
3) Safety with long-term daily use/ingestion. There must be safety with daily ingestion over entire lives, inclusive of pregnant women and nursing mothers, infants and those with hyperthyroidism.
With these three obstacles in mind, let us consider the chemical options available for household water treatment and safe storage in the developing world.
The following dosing issue is not unique to chlorine, though it is the most common example encountered in household water treatment in the developing world. The core of the issue has to do with required manual intervention, which suffers from inadequate or improper user compliance.
The most common forms of chlorine for household use in the developing world are liquid chlorine bleach or chlorine tablets as additives to the raw water source. The problem lies in arriving at just the right dose for the needs of sanitation without compromising the qualities of the water that make it acceptable to the consumer.
Manufacturers may provide recommended dosage instructions, but widely varying water conditions and human error can confound reliability of results.
Consumers may assume if a few drops of bleach or a single tablet is good, then twice that much is bound to be better and more reliable. Such over-dosing can result in undrinkable water, reminiscent of aromas associated more with indoor swimming pools than with potable water. Perhaps more likely is the view that less is better in order to stretch the money spent on chlorine in order to treat more than the recommended volume of water. Under-dosing water, particularly when there is high chlorine demand, results in inferior disinfection and compromised safety. Consumers cannot see bacteria and viruses and they are not equipped to adjust chlorine dosing based on water conditions. For example, does the water temperature require a higher dose of chlorine? Another example is water containing high levels of organics (TOC) with oxidation demand that consumes chlorine before all disease-causing organisms are inactivated.
The foregoing is all too common and underscores the unpredictability of simply adding chlorine to achieve reliable results, while still maintaining acceptable taste and odor.
Iodine is another commonly used halogen for small-scale in-home or personal water treatment. It can also be administered in tablets or as a liquid from concentrate derived by soaking iodine crystals in water. In the last few decades, iodinated resins have been developed for passing water across beads to leach iodine residual in the range of 0.5 ppm to 2.5 ppm or higher.
Iodine left in the water is more foul-tasting and smelling than chlorine, often described as fishy or medicinal by consumers and it leaves an unpleasant, lingering metallic after-taste. Years ago, the US Marine Corps abandoned the use of iodine tablets as troops were dehydrating rather than drinking iodinated water.
Iodinated resins were developed in an effort to regulate dosage and increase performance reliability when used by consumers. They are typically used in drinking water treatment devices that include sediment and carbon prefiltration. The idea is to clean up incoming water quality in order to get more predictable water conditions prior to passing through a chamber filled with iodinated resin beads. A dwell chamber typically follows to give the iodine residual leaching from the beads adequate time to kill bacteria and virus. However, because iodinated resin relies on leaching and the iodine residual declines over the life of the cartridge, the dwell time (contact time) requirements increase over time, in order to maintain performance. One must, therefore, engineer an iodinated resin system with a dwell chamber of adequate size for the lowest level of residual delivered at the end of cartridge’s rated capacity.
Like chlorine, performance is affected by cold temperatures, variations in pH and/or salinity. Hot weather, high TOC levels, increased salinity or pH above 8.9 can cause increased spikes in iodine residual that can overwhelm polishing or scavenging carbon, raising the residual to highly undesirable levels.
Most important, certain people are cautioned against excessive iodine/iodide intake; these include individuals who are iodine-sensitive, have over-active thyroids or are pregnant or nursing mothers. As a result, iodine-based systems should have a good surplus of scavenger carbon to remove iodine and a strong base anion resin to remove iodine for long-term daily use. An alternate, though less desirable option is to use a good silvered carbon with a surplus of positively charged silver ions (not negatively charged silver oxide or ground-state silver) to precipitate insoluble silver-iodide to be filtered by the carbon. However, these scavenging measures eliminate the desired benefits of having residual disinfectant iodine in the water.