Ultra Violet Lamp
We'll look at both artificial and natural radiations, which needs electricity and lamps to generate the energy, and natural radiation, making use of the sun's energy for solar disinfection. Ultraviolet radiation is one type of energy along the electromagnetic spectrum, where different kinds of radiation are defined by their wavelength. Ultraviolet has wavelengths in the 100 to 400 nano meter range, whereas the visible light is in the 400 to 700 nano meter range.
If we zoom in on this range we can define some subcategories.
1.The lowest wavelength we call vacuum UV.
It only exists in the vacuum, but then we have UV-C, or deep ultraviolet, where the germicidal peak at 260 nano meters is found. Then you've got UV-B and UV-A shading into the visible light spectrum from 400-700 nano meters.
Now in order to describe the efficacy of UV light against different types of pathogens, weneeds to first define the dose. And just as for heat, it's important both, the intensity of the disinfectant and the time. So for UV radiation the units used, in the US they use mill joules per centimeters squared, whereas in Europe and other parts of the world the preferred units are joules per meter squared.
Now DNA absorbs ultraviolet light in the 200-300 nano meter range, and it has a large peak right at 260 nano meters in the UV-C range. So most of the commercial ultraviolet systems target this wavelength then. And UV-C has been shown to be most effective against bacteria followed by protozoa and cysts.
And then, finally, against viruses and bacterial spores, where you need a bigger dose to inactivate those kinds of pathogens. Typically, a dose might be 400 joules per meter squared, or 40 mill joules per centimeter squared to get good removal of even viruses and bacterial spores. From experiment to determine the efficacy of UV-C against different kinds of pathogens. Having define the dose, and the log inactivation, or LRV.
The first thing we can see, is that E. Coli is very readily killed by ultraviolet.
And total coliforms, a related bacteria are also fairly quick to die exposed to UV. Rotavirus takes a little bit more time to kill, but it's the Bacillus subtilis spores that takes the longest time.
Different methods of generating UV-C radiation
There are a number of different ways to generate UV-C radiation using lamps, which of course require electricity.
1. Low-pressure mercury lamps emit light in a very narrow wavelength band of
around 254 nano meters, which is close to that DNA absorption peak at 260.
So, this is a very effective way to target DNA.
2. Medium pressure lamps generate a broader spectrum of UV-C, so not only the
DNA could be targeted, but proteins and other, other kinds of inactivation could take place.
3. And then finally LED lamps are currently under development.
These would also be very focused at a particular wave length but hold out
the possibility generating of UV radiation at a lower cost, than mercury lamps. UV-C is very effective in water, it's not impacted by changes in
pH, though turbidity can shield microbes from the radiation. There's a modest effect of temperature where a higher dose is needed at low temperatures, but it's not a major effect.
There are many commercially available UV-C systems used primarily in middle or high income countries, kitchen scale systems with a small UV lamp. And there's some interesting research into developing versions of UV-C treatment that would be suitable for low to middle income households.
Here's one example, the Mesita Azul, which was developed in laboratory at Berkeley University and tried in Baja California, Mexico. It consists of a small UV-C lamp that does need electricity, that outputs 1200 Joules per meter squared, so three times that conventional range of 400. And there's a bucket reservoir, where water drains by gravity through the UV tube into a receptacle. And along the way it passes in a channel here, underneath the UV lamp, that's not in direct contact with the glass but it's a thin layer so that the UV can come in contact with all the pathogens in the water.
The flow rate is pretty high, about five liters per minute in the system. A recent evaluation found that it was very effective in the field, water quality was better than the same household before the intervention, that are also then control household, and better than household that were using other kinds of disinfection like commercial filters.
However, the exclusive use is pretty low, that only about 40% of households. While the Mesita Azul system is still under development, there is another UV disinfection process that has been researched extensively over the past 30 years, both in laboratory and field settings. And this is called solar disinfection.
Solar Disinfection
Solar disinfection works simply by taking a clean plastic bottle, a PET bottle, and filling it with water and then lying it on a flat service, maybe a rooftop, and exposing it to natural sunlight for at least six hours, or if it's a cloudy day for two days. After the sunlight exposure, the water is safe to drink. This SODIS technique then has been applied in more than 20 countries and more than five million people are using it today.
Now, solar disinfection doesn't make use of UV-C radiation, and that's because as sunlight passes through the atmosphere, all of the UV-C and even about 90% of the UV-B is absorbed by ozone, water vapor, oxygen, and carbon dioxide in the atmosphere. UV-A radiation is less affected by the atmosphere.
Therefore, the UV radiation reaching the earth's surface is largely composed of UV-A with a small UV-B component. Clearly then SODIS has to work differently than UV-C disinfection since UV-C isn't reaching the earth's surface, thankfully. And it does so with three main mechanisms. First is, UV-B radiation can still directly inactivate cells. It won't be absorbed so much by the DNA but other components within the cells, perhaps proteins, enzymes, can be affected.
However, the plastic bottles used for SODIS, PET, remove much of the UV-B. So unless special bottles are used, this is probably a minor effect. A more important effect is thought to be indirect inactivation through UV-A radiation. Where the UV-A is absorbed by some intermediate species, and then goes on to create reactive oxygen species like hydroxyl radical or hydrogen peroxide, which cause the damage to the microbes.
Those, reactive oxygen species could be generated inside the cell if the cell contains chromophores, which are typically present in bacteria cells. Or it could be external through, dissolved organic matter or dissolved iron. And then finally, heat can play an important role. Of course, if the temperature gets up to 60 or 70 degrees, we're entering the realm of pasteurization. But even at 50 degrees, there seems to be some synergy along with the effects of radiation.
Factors Influence The Effectiveness Of SODIS
There are a number of factors that influence how effective SODIS can be.
1. First, around the radiation itself the sun height is important, and the higher the sun is the higher the radiation level and this varies with the time of day of course but also the time of year. So, the maximum levels occur at noon during the summer.
2. The latitude is also important radiation does not fall equally on the earth. And there are two belts from 15 to 35 degrees in the Northern Hemisphere, and in the Southern Hemisphere, where radiation is at a maximum. Also, the belt between 15 degrees north and 15 degrees south has quite high solar irradiance, and it's a good zone for practicing SODIS.
3. Altitude. At higher altitudes there's more radiation, UV levels increase by about 10% for every 1,000 meters in altitude.
4. Ozone in the atmosphere does reduce UV and ozone can vary spacially it can vary over the year and even across the day. The ground can also reflect ozone, this is more important maybe for sunburns.
5. But think about a rooftop reflecting radiation back into a SODIS bottle.
6. And then finally the depth of water in a water bottle is important, because UV radiation is attenuated by water. So, if the bottle were too large and the water column were too thick, then UV wouldn't penetrate to the center of the bottle.
7. Also characteristics of the water are important. When the water temperature reaches 50 degrees or so, there's a synergy and only about one hour of UV exposure is needed.
8. However, turbidity can negatively impact SODIS and if the turbidity is higher than about 30 NTU, it's advised to pretreat the water with filtration or perhaps coagulation and sedimentation.
With all of the factors that can effect SODIS disinfection, it's not surprising that evaluations have shown a range of removal values. Most of the studies have been on bacteria, especially E. Coli and these tend to find three to five log removal units.
Sequence studies have shown that Salmonella is more resistant than E. Coli, and that Shigella and Vibrio cholera are relatively susceptible to ultraviolet. Studies on protozoa have shown slightly lower removal from two to four log units, and both Giardia cysts and Cryptosporidium spores have been shown to be inactivated.
There are relatively fewer studies of viruses. The methods are more difficult, and they show a greater range of one to more than five log removal values. Where we see that certain viruses like Adenovirus and the indicator virus phi are quite resistant to ultraviolet. Adenovirus is also resistant to UV-C treatment. And other viruses, the human echovirus and the indicator virus MS2 phage are relatively easier to disinfect with SODIS.
One of the countries with the longest history of SODIS is Bolivia, where since 2001, Eawag and the Fondacion SODIS have been promoting solar disinfection. First with pilots in Cochabamba, and then with broader uptake through NGOs, through health ministry, through schools, health professionals and women's groups.
However, a 2009 evaluation in Bolivia found that among those who had received SODIS training only about a third of them were routinely using SODIS treatment. Concerns have been raised about exposing plastic materials to the sun for water treatment because of the risk that some chemicals present in the plastic could leech into the water.
Now antimony, is a chemical that's used as a catalyst in the production of PET, but antimony only leeches into water at appreciable levels when the water is in contact with the plastic for many weeks or longer, or if the temperature exceeds 70 degrees Celsius.
Now this doesn't happen during SODIS treatment since treated water is only in the bottles for a few days and the temperatures rarely exceed 50 degrees. Adipates and phthalates are used as softeners in the production of certain types of plastics and packaging materials. For example PVC, which is used in plastic pipes. And although these softeners are not particularly toxic, they do represent a threat to health if they're consumed in large quantities.
However, adipates and phthalates are not needed in the production of PET. So they cannot leach from PET bottles into water. Aldehydes are formed when the plastic is heated in the manufacturing process for PET bottles.
A laboratory investigation of the diffusion of aldehydes from PET bottles into water found that exposure to the sun has no effect on the concentration of aldehydes that could be measured, especially for acetaldehydes. Though the concentration of formaldehydes does increase with the length of exposure.
However the concentrations of these aliphatic aldehydes remained far below the state regulatory limit for drinking water in all samples. Formaldehydes and acetaldehydes can be a problem for bottled carbonated water because the higher pressure and lower pH within those bottles can result in aldehydesin stored water.
But again, since the pressure and pH are more moderate in SODIS' treatment, these chemicals are not found at levels of concern in SODIS treated water. Then we have bisphenol A, a chemical present in some plastic bottles, that is suspected to cause breast cancer. But, bisphenol A is only an issue with polycarbonate bottles, polycarbonate plastic, and not in the PET bottles that are most commonly used for SODIS.
Another approach to assessing chemical safety of water is not to look for specific chemicals but to see if exposure to the water results in mutations in certain plants or protozoa, or bacteria cells. So this is a genotoxic approach to water safety assessment. And some of these genotoxic studies have found that when water is stored in plastic bottles for long periods of time, like several months, cell mutations could be observed.
But this happened with or without exposure to UV radiation. So while it might be an issue for long term storage in plastic bottles, it's not directly pertinent to SODIS. And this is similar to what we've seen in those studies looking at leeching of other chemicals, various chemicals.
And since SODIS involves neither long storage time nor high temperatures, studies at Eawag and other institutes have not found any evidence that SODIS introduces any
harm for chemicals to treated water. So, some considerations then for UV treatment are that, first, it's highly effective, both UV-C and SODIS.
Though UV-C being more powerful energy is more effective, especially against viruses and protozoa. And both UV-C and SODIS are limited to low turbidity waters because particles can shield pathogens from the radiation.
Furthermore, UV-C does require electricity to run the lamps. If we look at SODIS, some advantages are that it's very simple to operate. It's also quite inexpensive. You need plastic bottles but not much more. However, it's highly dependent on the climates, both in terms of daily and seasonal variations and different effects in different regions. And it does take a significant amount of time more in the treatment than in the actual work done to fill bottles.
An advantage of SODIS is that there's quite a low risk of recontamination because
Water is typically drunk from the same container where it's been treated, so less chance of recontamination. However, it's only small volumes of water that can be treated at a time in a batch of SODIS treatment.
In summary then, we've seen what ultraviolet radiation is today and how it works against different classes of pathogens. We've seen how UV-C intervention works and how they can be adapted for household water treatment. Currently, most of the experience there is in middle and high income setting but there's potential for application in low and middle income settings, especially as LED lamps come out. And we've seen the much more widespread practice of solar disinfection, SODIS, how that works and how it is being applied in many countries in the world today.
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