Intro: Modern Survival Method of Purifying Water With Sunlight:
-Solar water disinfection is simple and low cost household method for removing pathogenic microrganisms from contaminated water to improve the quality of water.
- This method uses solar energy to disinfect various pathogenic organisms like Ecoli,vibrio cholera,salmonella which causes water borne diseases.
-2 diffrent components of sunlight are used in water disinfection
(UV-A) and (infrared radiation)
-UV-A has germicidal effect and infrared rays increase the temperature of water to 70-75 degrees(pasteurisation)
-combined effects of both will increase the effeciency of the process.
SOLAR RADIATION is divided in to diffrent wavelength ranges of light ultravilolet,visible and infrared.
1)UV-B and UV-C- wavelength range 200- 320 nm which are harmful radiations which damage skin cells therefore protected by our ozonelayer in reaching atmosphere.
2)UV-A- wavelength range 320- 400 nm nearer to visible violet reaches earths surface and is known to disinfect various pathogens in water improving the quality of water. UV-A directly interacts with DNA and enzymes changing the molecular structure of the cells and also interacts with oxygen forming high reactive oxygen radicles hydrogen peroxide killing the pathogens.
Coming to INFRARED radiations:
These radiations cannot be seen by human eye but the heat produced by these rays of wavelength range above 700nm shows lethal effect on many water pathogenic organisms as they are sensitive to heat.This method shows same effect as of boiling the water to 90 degrees.
Step 1: Procedure:
- Plastic bottles made of PET ( polyetheylene tetraphthalate) are usually recommended as they are very light in weight,transparent, durable .
- UV-A light does not penetrate when depth of water exceeds 10 cm. Using PET bottles solves this problem because depth of water does not exceed 10 cm when horizontally exposed to sun.
1)Check if the climatic and weather conditions are proper for water disinfection treatment.know your water source as best as possible.if it is chemically toxic eliminate it.
2) clean the bottle , fill the bottle 3/4 with water and shake it properly and now fill water completely in plastic PET bottles and close the screw cap tightly.
3) Aeration can be acheived by shaking the bottle filled with 3/4 water. Aeration is required to kill the pathogenic microorganisms. After proper shaking Place the bottle on a aluminium sheet.
4) check the turbidity of water. It should be less than 30 NTU. Then only water can be exposed for disinfection treatment or else it should be filterd first before pouring in to bottles.
5) If the sky is 50 percnt cloudy expose PET bottles for 6 hrs and if is 100 percnt cloudy expose for 2 consecutive days.
6) water is ready for consumption.
Step 2: How to Check Water Turbidity:
- place the PET bottle on a news paper.
- Look through the opening of the bottle to the bottom.
- If u can read the letters at the bottom then the turbidity is 30 NU. If letters are not visible then turbidity is more than 30NU and hence water must be filtered to remove any suspended particles before disinfection of microorganisms by using a cloth or by sedimentation process.
- suspended particles prevent solar radiation penetration and protects the organisms.
- This solar water disinfection method was sucessful in china,bolivia, indonesia ,ecquador and many african countries.
- If your highly concerned about using plastic bottles world health organisation has approved the usage of PET bottles.
Step 3: Thermoresistance of Microorganisms:
Microorganisms Temperature for 100percent destruction
1) Enteroviruses 62 degree centigrad for 60mins
2)feacal coliforms at 80degrees centigrade complete destruction for 1to 6 mins
3)vibrio cholera 45 degree centigrade for 60 mins
4)salmonella 62 degrees for 6mins and 58 degree centigrade for 60mins
5)ascaris eggs 68 degrees for 1min , 62 degrees for 6mins and 57 degree centigrade for 60mins.
Synergetic effect of UV-A radiation and temperature:
At water temp of 30 degrees centigrade fluence of 555wh/m2( 450nm,dose of solar radiation corresponding to 6 hours of mid day summer sunshine)is required to acheive a 3 log reduction of feacal coliforms.under these conditions only effect of UV-A radiation is present.
however the death rate of coliforms increased when synergetic effect of UV-A radiation and increased temperature are present.At water temp 50degree centigrade a 3log reduction of coliforms requires a fluence of 40
wh/m2.this is equivalent to exposure time of 1hr.