Chemical Disinfection In Household Water Treatment

Automatic Dosing Device

Under this topic we shall consider chlorine disinfection, 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. And review the effectiveness of chlorine in reducing pathogens. 

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 U.S. 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, 5.6% 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.

Calcium Hypochlorite(HTH)

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. Which is cost effective, and includes, by definition, 100% chlorine. The next most concentrated form, also used in many utilities, is high test hypochlorites (HTH) Which contains about 60 to 70% chlorine and comes in powders or tablets. Bleaching powder or chlorinated lime or calcium hypochlorite is another powder 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 Javel 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 7.54, 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 be 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 0.5 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. 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 milligrammes per liter. But this is conservative and would probably result in unacceptable tastes or odour at this level in any case.

The use of chlorine in household water treatment(HWTS) 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 behaviour 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 0.75% 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 0.75% free chlorine added to a 20 liter jerry can gives a dose of 1.9 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,  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 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. 

So on balance, chlorine can achieve from two to five LRV for protozoa though not for

cryptosporidium. Nowacteria 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 til now we have been talking about free chlorine which is that combination of hypochlorous acid and hypochlorite. 

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 odour. If you ever smell a strong indoor swimming pool, it's not the bleach itself for the hypochlorite. But, 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 government agencies 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.

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 out breaks. 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.

 

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