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  1. 1. University of Bristol 1 Jamie Lowe Using a parabolic dish to disinfect water in the 3rd world Aim The problem of dirty water is huge in third world countries. 748 million people in the world don’t have access to clean drinking water. The issue is that rural communities are difficult to help because they are generally hard to access and are off the electricity grid. If we could devise a low tech systemwhich disinfects water without the need for consumable chemicals or electricity then it would go a long way to improving the lives of those with poor water. It should be able to be created and used by fairly unskilled people with local tools and materials. Ideally, a device would be designed which would enable the user to fill a bottle of water and know the exposure time required to purify that volume of water, given a known solar irradiance in their country. This leaves minimal skill and knowledge required by the user. Literature review This work builds upon the interesting work of Kevin McGuigan. His work centres on using the sun’s energy to disinfect water in plastic bottles. E. coli is one of the most common and harmful bacteria in drinking water. Whilst most strains of the virus are harmless, some strains, such as serotype O157:H7 can cause illness, in extreme can cause bloody diahorea which results in fluid loss (1). Several strains can also cause Anaemia and Kidney failure (2), which can lead to death. As such the amount of it in drinking water must be minimised. E. coli is used as an indicator of bacterial contamination of the water. E. coli is the easiest to measure given that it is in the highest concentrations. Additionally, for various reasons, it has been the bacterial indicator of choice for many decades so the method is very common and well used. As such the E. coli concentration is a good measure of the total number of bacteria in the water. E. coli is found in the lower intestine of warm blooded animals. As such it is present in both faeces and dead animals. From these sources, the E. coli can then be leached into the water system, be it groundwater or a river. E. coli and other bacteria are killed by UV as it damages their DNA. The UV initiates a reaction between two thymine molecules within the DNA of the bacteria. This damage inhibits the cell from carrying out it’s normal functions. When extensive exposure to UV occurs, the damage is sufficient to kill the cell. (3) Thermal killing of E. coli occurs when the bacteria is heated up. The high temperatures cause the enzymes in the bacteria to be denatured, meaning they change shape and are ineffective. Heat also damages the cell envelope. Proteins and fatty acids which keep the
  2. 2. University of Bristol 2 Jamie Lowe structure of the envelope are weakened by heat. Additionally, the interior of the cell expands with heat so at some point this pressure will break the cell envelope, killing the cell (4). In solar treatment of water both UV and thermal killing of E. coli occur. Thermal inactivation is only found to be relevant above 45°C, as below that is fine living conditions for E. coli. As such, before the water reaches 45°C, the killing is driven by UV light. Above 45°C, the killing becomes thermally driven (5). Figure 1 (6) shows the temperature range at which E. coli can survive. It shows that it grows in the 10 - 48°C range, and 48+°C is the killing region of the curve. As stated by McGuigan (5), solar thermal killing starts at an average temperature of 45°C. This is due to part of the water mass, generally the point where the radiation hits first, is at 48°C or above. Hence if you hold the temperature above 45°C, you can kill all of the E. coli Previous papers(7) have shown that disinfection can be achieved within 7 hours in hot climates, assuming the water temperature reached 55°C, by merely setting the plastic bottles in the sun in Kenya. New Idea Clearly the 7 hours timescale is a long time, and the people may not have sufficient time to perform this sterilisation. Additionally, you need those 7 hours to be cloudless. Both of these reasons mean that this method of solar disinfection is far from ideal. Reducing the exposure time required for sterilisation would greatly enhance the technique’s practicality and convenience. As a result, it would be much more widespread as a technique and would result in much cleaner water for those in problem areas. This would result in reduced illness and death. Inactivation of E. coli is a function of both temperature and time. As the temperature increases, the thermal decay constant increases, so you achieve the same disinfection in the same amount of time (5). As such, to reduce the treatment time, we must increase the temperature. Increasing the temperature requires a higher effective amount of sunlight hitting the bottle. As such, my new concept is to construct a parabolic mirror which focuses light from a larger Figure 1 – E. coli growth– temperature relationship
  3. 3. University of Bristol 3 Jamie Lowe area onto the water vessel. This effectively increases the solar irradiation hitting the bottle, so increases the temperature at a much higher rate to a greater maximum. The conditions I put on my dish were as follows: Portable – Both for testing and in real world usage, the dish must be lightweight and small enough for one person to carry. Easy to use – The mirror should have a relatively short focal length so that it is easy to suspend a bottle at that distance away from it. A short focal length is also much safer as it is very difficult to accidently focus the light in someone’s eyes. Easy to reproduce – The dish should be able to be constructed from local materials with local skills. Mirror Design For ease of use and safety, I chose a focal length of 180mm from trough of the curve. The equation for the focal length of a parabola is: With this as the equation for the curve of my mirror, and the maximum dimensions of 400mm * 400mm, I knew that the depth of the mirror would be y=55.56mm, by setting x=200. X=200 because the origin is at the midpoint of the parabolic bowl. Rotating this curve about the y axis gives the shape of the bowl. This bowl was then made on AutoCAD, as can be seen in figure 3. 10mm width was added to the rim of the dish to improve the quality of the 3D print. 5mm was also added to the bottom of the bowl to allow it to be printed (otherwise the trough would be a hole). 20*20*10 blocks were put on the structure to which the superstructure could be attached. These blocks are on the edge of the dish, 130mm(c/c) apart – 65 from the centre. 5mm holes were made in the centre of these blocks to allow struts to be bolted on. 4𝑝( 𝑦 − 𝑘) = ( 𝑥 − ℎ)2 𝑤𝑖𝑡ℎ: 𝑝 = 180𝑚𝑚 𝑇ℎ𝑒𝑟𝑒𝑓𝑜𝑟𝑒: 𝑦 = 𝑥2 720 Figure 2 - Definitionsof equationvariables (5) Figure 3
  4. 4. University of Bristol 4 Jamie Lowe The 3D printer at the university workshop has a maximum print size of 130*140*250mm, which the mirror is too large for. As a result, it was printed in 6 pieces, of dimension 60.56*140*210. The printing process can be seen in figure 4. The mirror was printed on the lowest quality option. This was to acheive fast printing and a light structure, a result of having minimal infill. After the six pieces had been printed, they were glued together and the bowl surface sanded to achieve continuity, seen in figure 5. The 2D design for the superstructure to hold the bottle at 180mm from the trough can be seen in figure 6. All values are from bolt hole centre to centre in mm. Structurally, this design is a mechanism as it has been drawn, assuming the bolts are pinned connections. However, the bolts were done up tightly and the bottle is fairly light, so there is little chance of the connections rotating, so can be considered fixed. A lip on one of the supports was also added to prevent the bottle from sliding off the structure when it was tilted during operation. Additionally, two bolts were added to each support beam to prevent rolling of the bottle from side to side. These additions can be seen in figure 7. Figure 6 Figure 4 – Printingof one of the segments, the printer broke in the late stages of printing,so this pink segment wasn’t used Figure 5 – The glued together sections prior to surfacing Figure 7
  5. 5. University of Bristol 5 Jamie Lowe The reflective coating of the prototype mirror can be changed and replaced as required. Shop bought Mylar based birthday banner was used originally, this was used as it is a material that is freely available. Aluminium foil and high quality mirrored back plastic should also be tested to see how much of a heating change you get with different reflective materials. 3rd World Application For the real use device that would be used in Africa. The body of the mirror would be constructed out of a setting material, such as clay or cement. This would be put into a locally made wooden box and a reverse mould of the mirror shape would be pressed into it. The material would be left to set and the mould removed after for reuse. The mirror coating would then be surfaced onto the parabolic bowl. The treatment bottle can then be suspended on string from stakes in the ground. In this way, these dishes can be created and reproduced with only the inverse bowl mould from the developed world. These can be created anywhere, as you can make a mouldable material that sets out of almost anything. Treatment Bottle Figure 8 shows the bottle that was used for treatment. It is a regular 500ml beer bottle that is widely available. It had to be a glass bottle as the temperatures reached at the mirror’s focus point are sufficiently high to deform the plastic. The treatment bottle was painted black for a section of the bottle where the light would be focused. This increased the absorption of the light and so increased heating power of the device. Clearly this would greatly reduce the UV rays going through the water. However, with the mirror designed as it is, it is very difficult to achieve killing by both optical and thermal ways. Temperature killing distributes itself throughout the water whereas UV wouldn’t in the same way. Therefore, with the light concentrated as with the parabolic design, temperature is a much more suitable method of killing. Additionally, thermal inactivation of E. coli is independent of water turbidity, whereas UV is absorbed by the opaque water and so does not penetrate to the centre of the bottle. It was known that the mirror could take the water to a sufficiently high temperature for killing E. coli so the addition of the black spot did more to improve the treatment than diminish it. E. coli testing procedure In order to test the effectiveness of the solar treatment, there must be a method for testing the concentration of E. coli in the water both in the treated water and also the water which hasn’t been treated. Figure 8
  6. 6. University of Bristol 6 Jamie Lowe You can’t test directly under a microscope for the E. coli bacteria as you can’t tell the difference between a bacterium which is alive and one which is dead. As a result you would get the same E. coli count from the treated and untreated water as all the dead E. coli will still be present in the treated water. Consequently, after the treatment has been conducted, the E. coli must be allowed to reproduce so that the active E. coli colonies are visible to the naked eye. In this way you only count the active E. coli as the dead E. coli wouldn’t have multiplied and formed a colony. The way you culture the E. coli is in on filter paper in a petri dish. First you suction filtrate the water sample through 0.45μm filter paper, this is sufficiently small so that the E. coli bacteria can’t get through and are trapped on the filter paper. The more water you filtrate, the more E. coli will be trapped on your filter paper. To culture the E. coli trapped on the filter paper, you must prepare petri dishes to put them in. The petri dishes are thinly glazed with the blue agar. The agar was M-FC agar, using a method set out by Geldreich et al. (11), and recommended by the American Public Health Association (12) for the detection of E. coli bacteria. This medium promotes the growth of E. coli by providing it with nutrients. It also inhibits the growth of any other bacteria, ensuring that the colonies that are counted are solely from E. coli. When the water has been filtered, the E. coli covered filter paper is placed on the agar in the petri dish. The petri dish is sealed shut with its lid and tape and is placed in an incubator at 37°C, the ideal temperature for its growth and reproduction. After 24 hours of incubation, the dishes are removed from the incubator and the colonies are counted and a photograph taken of the plates. The number of colonies on the incubated filter paper represents the number of bacteria that were originally in the sample of water that was filtered. Comparing the number of colonies on the filter papers from both the treated and untreated waters will indicate do what degree the treatment is effective. Preliminary investigation In order to conduct a test on the effectiveness of the treatment, we must have a source of water contaminated in E. coli. Water was sourced from the Bristol harbour; there is generally a high E. coli count there. This is thought to be due to the discharge of river boats, which contain traces of faeces, which contain E. coli.
  7. 7. University of Bristol 7 Jamie Lowe We must also learn how much of the sample water should be filtered to get a number of colonies on the filter paper that is able to be counted. If the paper is saturated in colonies you can’t count them so you can’t compare how effective the treatment was. The sample was collected in a 1 litre Perspex bottle which had been put in the furnace overnight to ensure it was sterile. The collection site was the jetty circled in figure 9, and is photographed in figure 10. It filled two times and emptied back into the dock on the other side of the jetty. The bottle was filled once more and the top was put on. This was done to ensure that the water sample represented the dock water well. The bottle was then taken back to the lab and tested with the method detailed above, with 25ml, 50ml and 100ml of the water sample. Both 50ml and 100ml of sample resulted in an entirely saturated filter paper, so much so that colonies couldn’t be counted. On the 25ml filter paper, there was a high count of E. coli colonies but they could be counted, these results can be seen in the results section along with the photos of the filter papers. As even the 25ml was slightly difficult to count due to the high level of colony saturation, 20ml was chosen as the amount of water to be filtered when testing the effectiveness of the water treatment method. These results mentioned can be seen in the results section of this report. Preliminary tests were also done using the mirror device itself. Due to funding issues and limited lab availability, the biological aspect of the test wasn’t available. As a result, tests were conducted with just the device, the temperature logger and the solarimeter. These results can be seen in the results section of this report. These tests were beneficial as showed the rate at which the temperature would rise given the solar insolation and the water volume. It was known that the water temperature would Figure 9 – Sampling location Figure 10 – Sampling location
  8. 8. University of Bristol 8 Jamie Lowe need to reach at least 45°C to get much killing effect due to the temperature. As such, these preliminary tests suggested, given the solar insolation, how much time the test should be conducted for to reach a certain temperature. Treatment procedure First, bottles of dock water were collected from the same site as where the preliminary testing was done, in the same way as was outlined above. They were put straight into a cool box and were transported in a car to the Bristol downs where the equipment was set up. The mirror device was angled towards the sun and tilted up using a steel rod. In this way, the mirror was pointed directly at the sun, resulting in all the light being focused on one point, 180mm above the trough of the mirror and at the midpoint of the two support beams. The treatment bottle was washed out 3 times with Milli- Q water and then 500ml the sample water was measured into the bottle using a measuring cylinder. The Tiny-Tag temperature probe was washed with the Milli-Q water as well before it was pushed into the treatment bottle through a rubber bung. This was to prevent loss of water and temperature through evaporation. The Tiny-Tag logger was set to record the temperature every minute. The set up can be seen in figure 11. The solarimeter was levelled using the devices spirit level and was plugged into a multimeter on its mV setting. This can been seen in figure 12 The testing vessel was then placed on the mirror device and the mirror was adjusted to ensure the focus of the mirror was on the black spot on the bottle, this is shown in figure CCC. The reading on the multimeter was then noted. Both these tasks were completed every 10 minutes until the completion of the experiment. Upon completion of the treatment, the water was poured into a storage bottle to cool. Once at ambient temperature it was put in a cool box to be transported back to the lab for testing. Results Figure 11 Figure 12
  9. 9. University of Bristol 9 Jamie Lowe Preliminary dock water testing Count of 25ml filters FilterA FilterB FilterC Average Colony count 492 540 724 585.3 Heating testing Figure 13 – Preliminary testing results for 100, 50 and 25ml of filtered dock water Graph 1 – All temperature regimes recorded 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Watertemperature(°C) Time (minutes) Temperature change through treatment 0.617 kW/m^2 0.518 kW/m^2 0.507 kW/m^2 0.645 kW/m^2 0.785 kW/m^2
  10. 10. University of Bristol 10 Jamie Lowe This graph shows the change in water temperature of the water in the bottle during treatment. It is always 500ml of water and is always mounted in the same way during treatment. It is noticeable that the 0.645 kW/m^2 and the 0.507 kW/m^2 data sets are not very consistent. This is because there was a lot of cloud cover during those tests so the irradiation level recorded is not hugely reliable and this is why the water didn’t heat up as expected. Hence, in the following graph, these data sets have been removed. In this graph the killing region for E. coli (45+°C) is shaded in red. In order for all the E. coli in the bottle to be killed, the temperature of the water must be sustained above 45°C for a certain amount of time. Part of the aim of this project was to find this amount of time but insufficient time and resources were available to achieve this, this will be mentioned in the future work section of this report. Graph 2 – Cloudless temperature regimes
  11. 11. University of Bristol 11 Jamie Lowe Main treatment test This data set is the one which shows how the water sample from the docks was treated. It can been seen from the graph that the water temperature is above E. coli killing temperature (45°C) for about 90 minutes. This is a long amount of time so should result in total eradication of all the E. coli bacteria. This theory was reflected in the microbiological lab tests, results shown below: Filter1 Filter2 Filter3 Average Colony count Treated 0 0 0 0 Untreated 354 270 282 302 This shows an eradication of the E. coli in the sample, making this water safe to drink, photos of “Filter 1” of both treated and untreated samples are shown below: Graph 3 – Main testing temperature regime
  12. 12. University of Bristol 12 Jamie Lowe The contrast is very visible. The untreated sample on the left is covered in E. coli colonies, to such a degree that the most of the colour in the agar has been taken up by the bacteria. In contrast, there are no colonies on the treated sample on the right. This shows that the treatment has worked. Extrapolation of temperature data to hotter countries Using the data of the temperature increase with time along with the measurement of the incoming solar irradiation, the proportion of incoming energy transferred to the water can be calculated. The calculation is shown below for the 0.617 kW/m^2 regime. Vol 500ml Mass 0.5 kg Time 60 mins SpecificHeatCapacity 4182 J/kg'C 3600 secs Av.Power 0.617 kW/m^2 Change inTemp 39.101 °C Areaof dish 0.1256 m^2 Irradiationon mirror 0.077539 kW Energyused 81759.8 J 81.8 kJ Energy incoming 279.1403 kJ % incomingenergytransferred 29.3 Figure 14 – Post incubation untreated and treated filter papers
  13. 13. University of Bristol 13 Jamie Lowe Performing this calculation for all the data sets acquired gives the energy transferred proportion as 15% - 30%. This calculation makes the assumption that no thermal energy is lost from the water, which clearly it would be as the water heats up. This factor amongst others explains the variation in calculated energy transfer. Using this value for energy transferred you can estimate how fast water would heat up in hotter countries. Taking the solar irradiation level in Kenya as 1.1 kW/m^2 (6), this should take a 2 litre bottle to 51°C in 1.5 hours, a sufficiently high temperature to kill the E. coli if held there long enough. This assumes the starting water temperature is 25°C or higher (7). Vol/Mass 2 kg start temp 25 C Power 1.1 kW/m^2 Time 1.5 hours Powerondish 0.14 Kw/m^2 Energyincoming 746.06 Kw/m^2 Energytransferred 218.52 Kw/m^2 tempchange 26.13 °C End temp 51.13 °C Discussion These results show that in this scenario the device works perfectly. In 2 hours, possibly far less, it achieves total disinfection of 500ml of contaminated water. Extrapolation of the data has shown that it could have a similar effect on larger volumes, due to the higher insolation level in hotter countries. As can be seen in graph 2, the temperature regimes are fairly different and in my test it is not hugely correlated to the average insolation level. This is probably due to a variety of factors. One main one is that the regimes showing slower heating were done a few weeks after those showing fastest heating. It is visible on the mirror itself that the reflective material has deteriorated, probably from the heat and the UV. This has happened because there was a pattern printed on the reverse side of the material so this has begun to show through as it has deteriorated. This would not happen a dedicated reflective material had been used. As a result, a lower proportion of the energy hitting the dish is transferred to the water, therefore, the water heats up more slowly. Potential problems There are a few problems which may affect whether or not the device can be implemented into the 3rd world.
  14. 14. University of Bristol 14 Jamie Lowe One of these problems is in the construction of these dishes. Whilst the shape of the mirror should be able to be achievable, using a mould, there may be problems with the reflective surface. In my experiment I have used a reflective material based on Mylar. However, whilst this is very available in the UK, it’s availability in 3rd world countries is questionable. For fast implementation, all the materials should be available locally. You could use aluminium foil to do this, as this is available in these countries. If tests prove foil is insufficiently effective then implementation would require material sourcing from the western world which would vastly increase costs and complications for such a project. Another problem is that as the sun moves across the sky, the dish needs constant adjustment to keep the focus of the dish on the bottle. To reduce the frequency with which this needs to be done, you could suspend the bottle transversely across the mirror. This means as the sun moves across the sky, the focus point stays on the bottle despite its movement. Even with this change, the dish would still need changing orientation about every 40-60 minutes. This frequent adjustment is a limitation of the system. As you need someone to be monitoring them almost constantly, this compromises the efficiency of the device as it requires a time commitment. Another problem with the dish is that it needs total sunlight. This means there can’t be any clouds in the sky. If there is the heating is very intermittent and the water never really heats up very much. Whilst this is generally not a problem in central Africa, where it is designed for, there is little opportunity for it to be rolled out to higher latitudes. With all these problems, they can all be solved. However, there are more reliable methods of water purification such as chlorine tablets. As a result, for the device to be viable, it must be cheap and simple to both build and run. Consequently, when solving all these problems, it is crucial to keep an awareness of how it will impact the build-ability of the device and how easily it is to run. Future work As this is a new concept and research area, and my project has been very short, there is extensive further work that needs to be done on the device before it could be implemented. One problem that needs to be researched is the reflective surface of the mirror. As mentioned above, if aluminium foil could be used then implementation would be far easier. Tests need to be conducted with foil as the reflective material. If it can heat to water to a similar temperature as in these tests and kill E. coli in lab tests then it will be perfect for roll out to the 3rd world as it would be very cheap to make and effective. The practicality of production of the mirror in the 3rd world using a reverse mould system should also be investigated. For the mirror to be effective the shape needs to be fairly accurate. A number of mirrors should be constructed in this way and water should be heated using them to see if there is a drop in performance from the prototype mirror.
  15. 15. University of Bristol 15 Jamie Lowe If the foil and new moulding method result in a drop in performance, then the size of the dish could be increased. This would have more solar light hitting it so would increase the heating power of the dish. This will offset the reduction in the performance of the dish due to build quality. To improve the efficiency of the device, the minimum amount of treatment time should be found. The tests conducted have achieved 100% killing of the E. coli but the temperature was higher that the killing temperature (45°C) for a long time, probably much longer than required. If the time needed in the ‘killing zone’ can be defined then the treatment times can be reduced and so more water can be decontaminated and at a faster rate. This will result in an increase in viability of the device. In order to fully understand and predict how the device will perform, a plumb line and protractor should be added to the device. Depending on the sun’s height in the sky the dish is orientated differently. As the solarimeter measures the irradiation on a horizontal surface, it does not directly indicate the irradiation level hitting the mirror. The irradiation level hitting the mirror can be worked out with basic trigonometry given the orientation of the mirror. As such, with a plumb line and protractor, you can measure the angle that the mirror is inclined so can therefore calculate the irradiation amount hitting the device. This will result in a higher correlation between dish performance and irradiation level. In the tests already conducted, the correlation was minimal; this was because tests were conducted at different times of day, so the stated irradiation level wasn’t entirely representative. In order to perform further testing, certain pieces of equipment will be necessary. One piece of equipment is a solar source. With the current testing technique, a lot of time is wasted waiting for a cloudless day, something which is not hugely common in the UK. This is generally the limiting step for the research, because without the right conditions, you can’t record any data. Additionally, solar irradiation in the UK is much less than in central Africa. With a solar source, you could accurately model the irradiation levels in the end user countries. This would eliminate the extrapolation element required to predict the effectiveness with increased irradiation and would result in much more understood and reliable device performance. Conclusion Ultimately, this project has shown that using this device can be effective and practical. It reduces the treatment time required for disinfection to less than two hours, whilst still killing 100% of E. coli. Further research and commitment is required to make this project a reality in the 3rd world. On a personal level, this has been a very interesting project. As a 2nd year undergraduate student, I don’t see a huge amount of the research side of the university. It has been great to be able to use some of the universities research facilities, particularly in the Microbiology
  16. 16. University of Bristol 16 Jamie Lowe lab in the Geography department, as it very different to anything I was used to. It was frustrating at times that I had no funding to buy things necessary for the research, which resulted in delays. I would recommend that summer research students in the future should be given an amount of money to spend on their research as it is sometimes difficult to work without it. Acknowledgement I would like to express my huge appreciation to a number of people who advised and assisted me with this project over the last 8 weeks. Firstly, I would like to thank my supervisor, Dawei Han. His enthusiasm and knowledge has been invaluable to me in completing my project. He has also been very interesting to talk to about the greater topics of research in Hydrology that are open to further exploration. I would also like to thank Simon Cobb, who runs the Microbiology lab in the Geographical Sciences department at the university. Sharing his expertise in a field in which I am largely ignorant has been hugely helpful and interesting. The commitment he has to students at the university is remarkable. Finally, I would also like to that the Civil Engineering department for providing the funding to undertake this project. I would also like to thank the staff in the department for giving me the skills and knowledge that have enabled me to complete this project. It has been a great experience and one that I’ve enjoyed greatly.
  17. 17. University of Bristol 17 Jamie Lowe Bibliography 1. Odonkor, S. and Ampofo,J. Escherichia coli asan indicatorof bacteriologicalquality of water:an overview,Microbiology Research 4. pp. 5-11 : s.n.,2013. 2. E. Coli InfectionFromFoodorWater. WebMD.[Online] [Cited:721, 2014.] http://www.webmd.com/a-to-z-guides/e-coli-infection-topic-overview. 3. Anne Rammelsberg.Howdoesultravioletlightkill cells? ScientificAmerican. [Online] 0817, 1998. [Cited:07 21, 2014.] http://www.scientificamerican.com/article/how-does-ultraviolet-ligh/. 4. White,Carol. HowPasteurizationWorks. HowStuffWorks. [Online] [Cited:0721, 2014.] http://science.howstuffworks.com/life/cellular-microscopic/pasteurization2.htm. 5. K.G. McGuigan,T.M. Joyce, R.M. Conroy,J.B. Gillespie,M.Elmore-Meegan.Solardisinfectionof drinkingwatercontainedintransparentplasticbottles:characterizingthe bacterial inactivation process.[Online] 1014, 1997. [Cited:07 21, 2014.] http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2672.1998.00455.x/pdf. 6. 5. EXPOSURE ASSESSMENT. fao.org. [Online][Cited:07 30, 2014.] http://www.fao.org/docrep/007/y5502e/y5502e0a.htm. 7. T.M. Joyce,K.G. McGuigan,R.M. Conroy,M.Elmore-Meegan.Inactivation of FecalBateria in Drinking Water by SolarHeating. s.l. : AppliedandEnvironmental Microbiology,1995. 8. Designand feasibilitystudyof PV systems. CHALMERSUNIVERSITYOFTECHNOLOGY. [Online] 2011. [Cited:07 28, 2014.] http://publications.lib.chalmers.se/records/fulltext/155055.pdf. 9. Average watertemperaturesinMombasa,Kenya. World weatherand climateinformation. [Online][Cited:07 28, 2014.] http://www.weather-and-climate.com/average-monthly-water- Temperature,Mombasa,Kenya. 10. Focusof a Parabola. www.mathwords.com. [Online][Cited:0723, 2014.] http://www.mathwords.com/f/focus_parabola.htm. 11. Geldreich,E.,Clark,H.,Huff,C. andBest,L. (1965) Fecal-coliform-organismmediumforthe membrane filtertechnique, Journalof theAmerican WaterWorks Association 57, pp.208-214. 12. AmericanPublicHealthAssociation(1976) Standard MethodsforExaminationof Waterand Wastewater,14th edition,CenveoPublisherServices:Richmond,VA,USA.