Household Unit for SOLAR WATER DISINFECTION

Solar water disinfection has been explored and many types of small and big gadgets have been developed. It is the UV-A ray component that kills bacteria of different types. Although glass is considered to be impervious to the UV rays, but it was confirmed that ordinary soda-lime–silica type (window or plate glass) can transmit more than 90 per cent of the incident radiation in the UV-A component and visible regions of the sunlight. It was also noted that even the visible spectrum of the light also contributes to disinfection. A UNESCO report suggests that if water filled in PET bottle is subjected to sunlight for 16–20 hours, it will disinfect water. However, the temperature reached on exposure is of the order of 40–42°C depending on the ambient temperature. It does not specify the extent of disinfection. It was noted that exposure to sunlight at temperatures above 50°C for about 2 hours results in disinfection. However, data on extent of disinfection was not available. Other set ups include a continuous flow type unit where water flows through a serpentine path in sunlight which results in disinfection. A large scale pilot plant can disinfect about 500 L water per day. However, there is no unit which will meet daily requirement of disinfected water for a family of 4–5 members, i.e. about 8–10 L water per day.

Design of Experiments

As the additives in PET bottles leach into water at higher temperature causing harm, it was found that prolonged use of hot water in PET bottle would be harmful. PET bottles were used as container was not considered for experimental purpose. Based on this information that UV-A component penetrates ordinary glass, it was decided to use the glass bottles of different sizes with small narrow neck for storing water. In one UNESCO report it was pointed out that large mouth vessels are susceptible for spoiling water as the kids and even elders too are inclined to insert dirty hands from the large open top. In order to achieve higher temperature above 50°C it was decided to use box made of acrylic transparent sheet of 3 mm thickness as acrylic transmits visible, UV as well as infrared components of the sunlight. A fish tank-type acrylic box of base 22 cm X 44 cm and 38 cm height was fabricated for initial trials. It had a lid of acrylic plate fitting tight on the top. Provision was made for inserting thermometer inside the box. This box was accommodating three bottles of about 2 L volume each.

Initially, thermal treatment experiments were conducted. All the three bottles were filled with ordinary untreated water and were inserted in the acrylic box in the morning around 8.30–9.00 a.m. For the first few trials hourly check of the temperature rise was measured. It was found that on an average it took about 2–2.30 hours to reach temperature above 50°C. The box was allowed to remain till 3.30 p.m. in the sunlight. It ensured exposure to the sunlight above 50°C for more than 2 hours. By 2.30–3.00 p.m., temperature of water in bottles would rise to as high as 55–60°C. It was like a glass house effect.

In order to confirm consistency in temperature rise, several experiments were conducted in summer and winter months. It was found that except on days when it rained, temperatures higher than 50–55°C were invariably achieved. In order to simulate the cold weather conditions, freezed water at 0°C was used in for a few batches. It was found that in all these cases temperatures above 50°C was attained.

In order to achieve higher temperature a double-walled box was fabricated. With inner box dimensions same as above. The double-walled box gives water temperature of the order of 7–10 degrees higher than the single-walled box. It would rise to 65–70°C.

It was observed that within the same acrylic box, temperature of water inside the bottle changed with colour of the bottle. This prompted the author to design a simple experiment to verify and quantify the colour and energy absorption correlation for physics laboratory. Some experiments were carried out with copper jars suitable for refrigerators. Copper jars showed faster temperature rise and gave higher temperatures. Profile of the temperature for different coloured glass bottles and copper container is given in Table 1.

The overall pattern of temperature rise as it emerges is given in Table 2.
Coloured bottle temperature pattern: Blue > Green > Brown > transparent

Simulation of cold weather conditions is shown in Table 3.

Biological Testing

Once the temperature profile of water in bottles was confirmed that water temperature rises to higher than 50–55°C and remains there for more than two hours, the next step was to conduct biological testing. For this purpose, the usual method of biological testing was carried out with the E. coli strain. Freshly grown E. coli strain on the previous day was injected in the sterilized water in the morning and initial count was measured. The initial morning coliform count used to be in the range of 105–107 per cc. The bottles were put in the box in sun at 8.30–9.00 a.m. and removed from the box after 2.30 p.m. The final count of E. coli was measured. It was found that in all the cases where the temperature of water in bottle rose to higher than 50°C the bacterial count dropped down to below detection level (BDL) indicating that highly infected water turned into potable water. These experiments were carried out in single-wall box and the doublewalled boxes with bottles of different colours. In all the cases, initial E. coli concentration in the range of 105–107 dropped down to BDL. Table 4 gives the biological testing data.

The above type of experiments were performed many times to confirm consistency of results and total disinfection. It was noted that in some cases the green coloured bottles were not that effective. So, the green coloured bottles should not be used.

Application of the Device

This is a simple device. It can be manufactured locally. The fabricator for all the different designs was a mini-scale entrepreneur. Bottles were purchased from the local market. The locally fabricated acrylic boxes worked well for more than two years and the set-up cost was in the range of `1,200–1,500. These develop loosening of joints but the joints can be easily repaired locally. However, the boxes are moulded, these will be single piece moulds. Their life will be longer. Also, the production in bulk can reduce the price. It is also possible to design the foldable boxes to be assembled at the user end to reduce the transport volume. The acrylic boxes are easy to fabricate and repair. These are simple to handle. If the casting of acrylic boxes is done and these are moulded as single piece box, then these would have longer life and will be sturdy to handle. Some major observations about this renewable energy product are as follows:

• An acrylic box can be used for speedier solar water disinfection
• Glass bottles with narrow neck can be used for storing water for disinfection
• The device can be used in the cold climate such that water at near 0°C could be disinfected.
• An optimum size box with 4–5 glass bottles with 2 L capacity can be placed at a time in the box to get 8–10 L of disinfected water enough for a family with 4–5 members so that 2 L water per head could be provided.
• Even if different coloured bottles or copper containers are used, the end result is total disinfection of water.
• The device is simple to operate and runs on solar energy only.

Dr Pramod V Pathak, Member Secretary, Goa Energy Development Agency, Ds&T Compound, Saligaon, Goa, India.