Chemical Treatment and in particular chlorine, which is the most widely used disinfectant globally,We’ll explore a bit the chemistry of chlorine and chlorine dosing We’ll look at different forms of chlorine, gas, liquid, and solid.Review the effectiveness of chlorine in reducing pathogens.Finally we will give an example of chlorine use in Haiti. Chlorine has been used for over a century for the treatment of drinking water.
Beginning in the UK and USA in the early 20th century Today, chlorine is used in 98% of US water utilities and in similarly high numbers across the world Chlorine is also widely used at the household level, Of course, many households use bleach for cleaning and laundry.But in surveys conducted in low and medium income countries 56% of respondents reported adding chlorine to their drinking water to make it safer to drink. This was the second most common household water treatment practice mentioned, after boiling, and was particularly high in Latin American and Caribbean countries Chlorine is a strong oxidant.
And it reacts rapidly with different kinds of organic material, including cell wall, DNA, and enzymes. That’s also why it’s good at cleaning things, it really destroys organic compounds But actually, in part because it’s so reactive, the key mechanisms in pathogen inactivation are not well understood. Especially for viruses and protozoa When we talk about chlorine, we can refer to many different things, Large drinking water utilities typically use elemental chlorine, present as chlorine gas Which is cost effective, and includes, by definition, 100% chlorine The next most concentrated form, also used in many utilities, is high test hypochlorite, or HTH Which contains about 60 to 70% chlorine and comes in powders or tablets Bleaching powder or chlorinated lime or calcium hypochlorite is another powdered form.
And is often marketed as around 35% chlorine But the chlorine isn’t stable and actual concentrations vary widely Sometimes as low as 15%. It makes calculating doses difficult Household bleach is a solution of sodium hypochlorite, usually around 5% chlorine and water is a similar product, typically a little bit less strong Finally, it’s possible to make chlorine locally by passing in an electrical current through simple salt solution This usually results in something less than 1% chlorine Whatever the chlorine source, when it is added to water, it becomes hypochlorous acid, or HOCl And hypochlorous acid loses a proton at higher pH and becomes hypochlorite or OCl, with a negative charge.
These two species together are called Free Chlorine At pH 754, the two species are present in equal amount At higher pH, hypochlorite dominates While at lower pH, hypochlorous acid is the main species. This is important because hypochlorous acid, the one with the proton attached, is a much stronger disinfectant than hypochlorite. For this reason it’s recommended that water should always have a pH of less than eight when chlorination is being applied. So that there’s a reasonable amount of hypochlorous acid present Chlorine is highly reactive, and when added to water, a lot of the chlorine will react with natural chemicals in the water So, the dose has to big enough to meet both the chlorine demand of the water.
And to produce a residual that is strong enough to kill the target pathogens Chlorine demand can be caused by organic carbon, which is naturally present in surface waters But also by iron and manganese, or ammonia, which tends to be more present in ground water It’s normally recommended that to have a residual concentration of at least 05 milligrams per liter.
And this can require a dose of any where from one to five milligrams per liter with two milligrams per liter being fairly typical. Turbidity reduces the effectiveness of chlorine both by shielding pathogens directly and by consuming chlorine. So standard doses are normally doubled when the water is turbid Ideally, the right dose will be determined through trial and error. By adding different amounts of chlorine to the water to be treated to see what yields the desired residual.
Disinfection by Chlorine Addition
Chlorine doesn’t pose any health risks itself though the concentrated forms can be dangerous to handle WHO does list a guideline value of five mg per liter But this is conservative and would probably result in unacceptable tastes or odors at this level in any case.The use of chlorine in household water treatment was first developed by the US Centers for Disease Control, and the Pan American Health Organization They proposed an intervention, including three elements
Chlorine disinfection, and this was later expanded to include other HWTS processes, Safe storage, and behavior change communication. This model has been widely taken up, especially through Latin America and parts of Africa And typically involves production of a dilute sodium hypochlorite solution around 075% free chlorine Now, because hypochlorous acid is much more reactive and also more volatile than hypochlorite, the shelf life is much longer at high pH
So these solutions are typically stabilized at pH 11 or greater For similar reasons household bleach has a pH around 12 And it’s slightly irritating to the skin In different countries, different brands of liquid chlorine are promoted Some of the common brand names are Water Guard, Clorin, Claro and in francophone countries, Sur Eau
The bottles are normally designed so that one capful is a good dose for the locally used storage containers For example, a five milliliter capful of 075% free chlorine added to a 20 liter jerry can gives a dose of 19 milligrams per liter And, again, this dose should be doubled if the raw water is turbid
There is another form of chlorine which isn’t used in conventional water treatment, but is popular in swimming pools and increasingly in household water treatment.This is a chemical with a long name It’s called sodium dichloroisocyanurate. So you can see why people call it NaDCC instead And it’s about 60% free chlorine by weight
The other 40% is a chemical, cyanuric acid, which bonds with free chlorine and provides a reservoir Thus, as free chlorine is consumed through chlorine demand or inactivation of pathogens, the reservoir can release more free chlorine into the water Cyanuric acid itself doesn’t pose a health risk The guideline value of 40 milligrams per liter is well above levels that would be reached in drinking water treatment Because of this reservoir effect, any DCC allows a more steady and consistent free chlorine level
Which may lead to fewer reports of taste and odor problems NADCC is produced in the form of effervescent tablets or fizzy tablets, which are individually wrapped and have a long shelf life They come in a wide range of concentrations The most popular are 33 and 67 milligram versions Which are good for 10 or 20 liters of water, respectively
Aquatabs, shown here, is one of the leading brands globally, but there are others as well NaDCC tablets are easy to use and have proven particularly popular in emergency settings, but now are increasingly used routinely at the household level Chlorine efficacy, like other disinfectants is a combination of the concentration and the time of exposure For chlorine the dose can be expressed as a CT value in units of minute, milligrams per liter For conventional drinking water treatment, a common goal is to have a residual of 0
5 milligrams per liter free chlorine In contact with water for at least 30 minutes to ensure a good pathogen kill Multiplying these together gives a Ct value of 15 minute milligrams per liter Which is also equivalent to 15 minutes at one milligram per liter In HWTS, a typical dose is around 1
9 milligrams per liter, which gives a larger Ct of 56 minute milligrams per liter How effective is this dose? Well this chart shows the Ct99 That’s the Ct dose necessary to achieve 99% kill And you all know instantly by now that that means a two log reduction value, right? Well bacteria are very sensitive to chlorine And a Ct of less than 0
08 is necessary to achieve 2 LRV But viruses are significantly tougher It takes a Ct of from 2 to 30 to get our two log reductions And protozoa are tougher still, with Ct99s from 25 to 245 And that doesn’t include cryptosporidium cysts, which are basically unaffected by chlorine
One study even reported a Ct value of over 15,000, just to achieve 3 log reductions of cryptosporidium Now recall, our typical dose was in the range of 15 to maybe 60 milligrams per liter So you can see that chlorine is not very effective against some protozoa Now to be fair, not all protozoa are as resistant as cryptosporidium Giardia, for example, has a Ct 99 of about 15
So on balance, chlorine can achieve from two to five LRV for protozoa though not for cryptosporidium bacteria are more sensitive to chlorine and removal can easily exceed six LRV Viruses have intermediate resistance Disinfection might take a bit longer, but still good removal can be achieved Up to now we have been talking about free chlorine which is that combination of hypochlorous acid and hypochlorite But when free chlorine reacts with ammonia, it creates a different form called chloramines
And there is a whole family of chloramines with monochloramine, dichloramine, and trichloramine These are called combined chlorine, and they’re a less efficient disinfectant But they’re still used in some countries because they’re long lasting and can be useful, say, in a distribution network However, the chloramines have a stronger odor, especially Trichloramine In fact, trichloramine and a bit of dichloramine is what’s responsible for that swimming pool odor
If you ever smell a strong indoor swimming pool, it’s not the bleach itself for the hypochlorine It’s the combined chlorine Combined chlorine isn’t typically used in household water treatment Though it can be formed unintentionally if the water has high ammonia levels There’s another important potential, unintended reaction that can happen with free chlorine
If there are high levels of dissolved organic carbon in the water being treated, then the free chlorine will react with the carbon and produce what are called disinfection byproducts Such as trihalomethanes or haloacetic acids And some of these compounds have been shown in laboratory studies to cause cancer in laboratory animals when applied at high doses So accordingly, WHO and many governments have derived health based drinking water guideline values for these compounds in drinking water And these are typically set at a level which would be expected to cause one additional case of bladder or kidney cancer for every 100,000 people, who drink two liters of water per day at that guideline value for a 70 year lifetime period
And while that is a real health risk, it’s still very small compared to the mortality rates caused by diarrhea and other waterborne diseases Here’s what the World Health Organization has to say about disinfection by-products [BLANK_AUDIO] >> The Jolivert Safe Water for Families Program has sold sodium hypochlorite to households in rural Haiti since 2002 The program began at the Missions of Love health clinic, in Jolivert northern Haiti And since 2008, it has been managed by the NGO, Deep Springs International
The program makes its own chlorine through electrolysis of salt water Which is branded as Gadyen Dlo, meaning water guardian in Haitian Creole Project team members enroll families by selling safe storage containers or Bokit Espesyal with lids and taps The team also sells bottles of Gadyen Dlo keeping sales records for each family And conducts regular household visits to monitor use and provide ongoing education
One bottles costs about $125 and contains enough chlorine to treat an average family’s drinking water for about a month and a half By May 2010, the program had reached over 4,000 participants and had sold enough Gadyen Dlo to treat 12 million liters of drinking water In 2010, an impact evaluation was done in northern Haiti and reported a few years later in a scientific journal The team found that 3/4ths of the randomly selected households in the program area reported that they were currently using Gadyen Dlo
And 56% of these households compared to just 10% in controlled areas had free chlorine residuals between 02 and 2 milligrams per liter Indicating that they were applying chlorine correctly This evaluation was made after the earthquake in January 2010, but focused on areas not affected by the quake The Gadyen Dlo program was extended to the earthquake zone
And accelerated greatly after the onset of cholera in the region in October 2010 Half a million bottles of Gadyen Dlo were distributed in the affected areas for free Use of Gadyen Dlo in the earthquake effected zone has continued but this may be due to continued distribution of free product It’s an open question whether people will be willing to pay for Gadyen Dlo once the free distribution period ends While chlorine is by far the most commonly applied chemical disinfectant
There are other ones, especially in commercial treatment, chloramines or chlorine dioxide in some cases And ozone is gaining in popularity But these are not typically applied, at a household level, in lower and middle income countries. There are some emerging products that use bromine as an alternative to chlorine Some filters have this
And silver has at least a bacteriostatic and maybe a bactericidal effect And is often combined with another treatment, such as in ceramic filtration So certainly one of the main advantages of chlorination is it’s highly effective against bacteria, including cholera and has been demonstrated to really stop cholera outbreaks However, it’s ineffective against at least some protozoa and requires longer contact times for viruses Chlorine does give a residual protection which can help minimize recontamination or regrowth
But it has a characteristic taste and odor which people may find unpleasant if they are not accustomed to it It’s fairly simple to apply chlorine but it does require that the water has low turbidity Chlorine application is low cost, but does require supply chains to replace consumables. And then finally, there’s often misunderstanding about disinfection by-products And the relative importance of these Which can cause resistance to chlorination programs So, in summary, chlorine is a widely used disinfectant in both conventional treatment and household water treatment Chlorine can be obtained from a wide variety of sources, but all of these produce free chlorine in water, which is the active disinfectant Chlorine is highly effective against bacteria including vibrio cholera and also is effective against viruses and some protozoa, but is not effective against cryptosporidium which is a major cause of disease Cryptosporidium cysts are larger and should be removed through filtration processes especially following coagulation and flocculation