Water Disinfection

A Simplified Overview of Water Disinfection

By: Duvon McGuire
New Life International, Inc.

While most people take for granted the need for clean water, and they have a basic understanding that bad water with bacteria in it can make one sick, few people give much thought as to how one can disinfect water to make it drinkable.

There are several methods available to help cleanup and disinfect water, but when you have to answer the question: “If I had to drink the water that the poorest of the poor have to drink; what treatment method would I choose?” … the issues and methods become much clearer. We will first focus upon some better known methods such as boiling, ultraviolet, ozonation, reverse osmosis, filtration, and chlorination. We will use this background to then move in the direction of giving the layman a simplified working knowledge of the energy efficient technology of the McGuire Purifier we have developed, and why it is the most appropriate technology for those who still lack access to safe water.

A well-established common method of water disinfection is to boil water for ten to fifteen minutes in order to inactivate bacteria and other waterborne parasites. While this method is generally effective and simple, it can be a tremendous energy burden upon the needy that need the most help. Also, once the water cools there is nothing to resist the water being recontaminated by pathogen laden dust, or the water storage containers themselves. In other words, there is no “residual disinfection ability”. In the developing world this is extremely important particularly where sanitation is inadequate and people do not have safe water to wash their hands before eating.

From a practical sustainability standpoint, there are more than one billion people without access to safe water, and the cost of boiling water to meet their basic needs is unimaginable. Additionally, boiling and cooking using wood, coal, or charcoal has a hidden side effect: SMOKE — Approximately 1.6 million children under the age of five die each year of respiratory illnesses directly attributable to cooking smoke! In an age in which the world is concerned about carbon dioxide emissions, deforestation, and loss of wildlife habitat - boiling water for the more than one billion people in the world who lack safe drinking water is simply not sustainable!

Ozonation and ultraviolet radiation, these two methods are lumped together because they have many similarities. The biggest advantage is that under the right conditions they kill pathogens very well. There is one protozoa cryptosporidium that tends to be chlorine resistant and these two methods are effective at neutralizing it. Fortunately these protozoa can be effectively eliminated by filtration down to about one micron absolute. The downside to ozonation and ultraviolet is that the methods are costly, and just like boiling, shortly after the water leaves the active zone; there is no disinfectant residue to prevent recontamination. Also, by the time one adequately filters the water to make the methods effective, one could get to the same place and beyond by simply filtering and chlorinating. Why is that? First, there is no good way to field test or accurately predict if ozonation or ultraviolet radiation has in fact adequately treated the given water. Secondly, in both of these methods there is extensive filtration in these systems, and if the water is to be stored for a reasonable length of time, it has to be chlorinated anyway to keep bacteria from infesting the storage tanks.

However, when it comes to the need of extensive prefiltration reverse osmosis requires the most. The front end costs of such systems tend to be high, and the maintenance is also very high. Additionally a large proportion of the water is “rejected” and has to be discarded. Sometimes the discarded water can be as much as four times the saved process water. The single biggest advantage of reverse osmosis is that it has the ability of reducing chemical contaminants. If chemical contaminates are the primary issue this technology has utility, but at a price. Filter replacement in RO is a high cost and again the water has to be chlorinated if it is to be stored.

Filtration is nearly always a good first step towards providing safe water. However, with the exception of rare specialty filters, normally available filtration can NEVER do the complete job by itself! The primary benefit of filtration is to help reduce cloudiness in the water and to filter out suspended solids and cysts. Filtration available to the developing world is NOT capable of filtering out viruses! Filtration by itself has NOT been able to reduce diarrhoeal disease in children. Boiling water or chlorination if done properly can eliminate the incidence of diarrhoeal disease attributed to drinking water; but filtration alone can not. An example of this is gastroenteritis that causes many of the diarrhoeal deaths in children - it is viral; filtration alone does not have the ability of eliminating viruses.

The most effective system for the developing world is chlorination combined with appropriate filtration. What do we mean by appropriate filtration? How do we appropriately combine these two technologies? If you start with nothing, how do you prioritize the sequence of interventions? I strongly dislike most statement that start with the premise that something is better than nothing. But what if you in fact have nothing, and we have people that are drinking biologically contaminated water?

The first step in the development phase is to start with chlorination, even if it means doing so with less than ideal filtration. In some cases, this may mean filtering silt laden water through a shirt, or through pillow cases, as was done in Mozambique following the two back to back cyclones that flooded the country in the year 2000. In this case the chlorine residual level is elevated and the contact time extended to help insure the destruction of giardia cysts. (This typically requires that the chlorine level be raised to at least 5 parts per million, and that its level remain above 2 parts per million for at least one hour). This should kill everything biological in the water with the possible exception of cryptosporidium cysts which are endemic across the planet, and most of us have already been exposed to it. [NOTE: if this set of conditions is strived for and at the end of one hour the chlorine residual is zero or close to it, the pipes and tanks in contact with the water need to be evaluated for overt biological contamination such as from a drowned animal. If such overt contamination is not present, the water should be evaluated to see if there is a chemical contamination component to the phenomenon.] In many cases the water is crystal clear, but yet teaming with invisible microorganisms. However, as the level of filtration improves, even this can be eliminated from the water as we move in the direction of providing biologically free potable water.

If filtration is added to the chlorination technology and we are progressively improving on the level of filtration, where do we stop? When do we say, a certain level is enough? The answer to these questions is largely dependent on what one decides to do with cryptosporidium and its perceived threat. If crypto is not present, the need for refined filtration beyond making the water aesthetically clear is probably not a big issue, and the above chlorination levels can be used as a general guideline. But this is certainly not an ideal long term concession that one would like to make. On the other end of the spectrum we have the issue of how fine do we go, and what are the practical limits in terms of the lower end of filtration size? Based upon the current state of the art in water treatment, prefiltering the water to one micron absolute and then chlorinating to rid the water of bacteria and viruses is probably more than adequate. In a fully developed system, this would be a good goal to strive for as we move in the direction of biologically free water. With appropriate chlorination, it is not likely that there will be much benefit to filtering smaller than one micron.

Chlorination is the only disinfection technology that effectively disinfects from the point of introduction all the way through the piping system to the tap as well as storage containers.

In most of the developed world, the modern water systems take advantage of the oxidizing capabilities of chlorine that is injected into the water for purposes of disinfection. Chlorination technology has been used for nearly 100 years and is still used in about 98% of the public water treatment plants in North America. This advanced disinfection technology has been unavailable to the neediest, until recently. We now have the ability to very simply and with comparatively little expense, help the needy of the world using the McGuire Water Purifier technology to move in the direction of a zero tolerance on bacteria in water.

A Primer on Chlorine Oxidation as a Means to Disinfect Water.

Lets start with the familiar and work from there: When we disinfect with boiling we have to have two main things present: 1) Adequate heat, (the water is in fact boiling), and 2) Time, or duration at the boiling temperature to insure that the pathogens have had adequate exposure to the temperature such that they are inactivated.

Two similar requirements are also necessary when it comes to disinfecting water with chlorine. In order to inactivate pathogens (disease causing organisms), we must have an adequately high chlorine concentration, and the pathogens must have adequate exposure time to that chlorine concentration in order to inactivate the pathogens. We measure the chlorine concentration in parts-per-million chlorine (ppm) or milligrams per liter. The parts per million chlorine concentration is analogous to “temperature” in our boiling illustration, - the higher the chlorine concentration in ppm, the “hotter” the oxidative environment is to “burn” the pathogens. And, just like in the boiling illustration, there must be adequate exposure time for the “heat” to kill the pathogens.

Let’s further explore and simplify this principle. For illustration purposes, let’s take the example of a burning candle. There is no question in anyone’s mind that the temperature of a candle’s flame would seriously burn a person’s hand if it were placed in the flame. However, if one quickly passed their hand through the flame fast enough, they would not get burned, and may not even notice much heat. If we tried to boil water for a few seconds, we would likely do little to kill the pathogens in the water. In a similar manner, if we exposed the pathogens to a few parts per million of chlorine for a few seconds, we have similarly done an inadequate job of disinfection. Once the water has been adequately chlorinated and passed the one hour test of being above 2 parts per million, the water can be safely transferred into a usage tank. Once in the usage tank the chlorine level and be safely allowed to drift downward to zero provided the tank is not recontaminated with untreated water or water that did not pass the one hour test.

Summary:

Filter the water and then chlorinate. When you chlorinate, insure that you have both adequate chlorine concentration and exposure time to insure successful water disinfection.