A box-type solar heater was designed, constructed, and used to determine the effect of solar heating on quality of domestic roof-harvested rainwater (DRHRW). During testing, naturally contaminated DRHRW was harvested in Ibadan, Nigeria, and released into the system at 93.96 Lh−1 (
Good quality water is gradually getting beyond the reach of average households in developing countries [
Most rural and semiurban settlements do not have the “privilege” of being connected to national electricity grid either. Where they are connected, the erratic supply makes it imperative for cheaper and more reliable source of energy that could be used especially for water pasteurization to be sourced. Solar energy is a free, inexhaustible, and environment-friendly resource [
Solar water heater can be used to pasteurize water thereby destroying harmful food-water microbes including bacteria and viruses when heated to temperatures of about 65°C [
Contamination of surface and ground waters by pathogens and chemicals tends to make domestic roof-harvested rainwater an alternative. In the study environment, most households are in the habit of harvesting and storing rainwater in jars and cisterns to supplement other sources of potable water during the raining season. Previous studies have reported on the poor microbial quality of DRHRW [
The study was conducted at the University of Ibadan, Ibadan, Nigeria (latitude 7°261N; longitude 3°541E). Solar water heating system was designed to handle continuous flow of water with a mean design temperature of 65°C. Experimental arrangement of the solar heater for temperature measurements is shown in Figure
Experimental arrangement for temperature measurements (collector area, 0.47 m2).
The solar water heating system was evaluated in an open field belonging to the Nigerian Micrometeorological Experiments (NIMEX) Research Group, Department of Physics, University of Ibadan, Nigeria, for eleven days. The meteorological instruments used comprised both slow and fast responses. These measured mean and turbulent parameters in the surface layers simultaneously. A 15 m mast was set up to measure the profiles of the mean wind speed at 0.7, 1.2, 2.2, 3.3, 5.2, 7.2, 10.2, and 14.8 m (the mean wind direction is inclusive only at the 14.8 m height) and air temperature (wet and dry bulb) at 0.9, 4.9, and 10.0 m. The same mast also supported radiation sensors for both global and net radiation at 1.5 m. The slow measurements were controlled by the use of two Campbell CR10X data loggers which sampled the data every 1 second and subsequently stored them as 1-minute averaged value. A list of all the meteorological equipment used in this study is contained in Table
List of meteorological instruments used in this study.
Parameter | Device and model | Manufacturer | Accuracy | Number |
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Wind speed | Cup anemometer A101ML/A100L2 | Vector Instruments | Distance const. 2.3 m | 10 |
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Wind direction | Wind vane W200P | Vector Instruments | Distance const. 2.3 m | 2 |
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Air temperature (wet and dry bulb) | Frankenberger psychrometer | Theodor Friedrichs | ±0.05°C | 5 |
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Surface temperature | Infrared pyrometer KT1582D | Heitronics | ±0.05°C | 1 |
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Global radiation | Pyranometer SP-LITE | Kipp & Zonen | 80 |
1 |
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Net radiation | Net radiometer (REBS) Q7, NR-LITE | Campbell/Kip & Zonen | +9.6 (−11.9) |
2 |
Source: NIMEX Research Group, Department of Physics, University of Ibadan, Nigeria.
From standard procedure, thermal efficiency of a solar water heater or cooker is determined from water heating test using the relationship
A typical box-type heater requires adjustment every 15 to 30 min or when shadow appears on the absorber plate. The solar water heater was designed such that the orientation of the reflector lid (when operated) is facing the sunset (west). This was found to be representative of local conditions, since the users will not likely have the time to stay with the system and be turning it every 15 to 30 min. The limitation of this fixed condition is that the system will only operate effectively between the hours of 11:30 a.m. and 4:30 p.m. on clear/sunny days. However, manual azimuth adjustment will increase its efficiency. Readings were monitored and recorded at 10 min interval, similar to the method used by Mahavar et al. [
Most of the studies reported in the literature on solar disinfection used water with laboratory grown organisms subjected to simulated solar irradiation [
Samples were also collected into McCartney bottles at both the inlet to and the outlet from the heating chamber at 10 min interval for microbiological analysis. All the samples were analyzed immediately.
The total viable count was carried out by means of the standard plate count technique using plate count agar. Dilutions of water samples in buffered peptone water were inoculated by putting 1 mL into each 10 mL molten standard plate count agar in McCartney bottles. After thorough mixing, these were poured into sterile Petri dishes and incubated for 48 hours at 22°C. Petri dishes from dilutions counting 50 discrete colonies were counted and the results expressed as the number of bacteria colonies per millilitre. The isolates were further identified using their macroscopic, cultural, physiological, and biochemical characteristics. Presumptive coliform test for the detection of coliform was done after the methods stated in [
The schematic drawing for the design and fabrication of a flat plate collector, box solar water heater is presented in Figure
Schematic drawing of the solar heating system.
Design conditions are as follows: transparent surface area = 0.84 m × 0.56 m = 0.47 m2; absorber plate = 0.3 m × 0.6 m black coated (front and back); aluminium sheet 1.4 mm gauge was used as the absorber plate; absorber pipe = 4.34 m length and 0.0064 m diameter, black coated, was used as the absorber/conveyance pipe.
Aluminium plate and copper pipe were used because of their high thermal conductivity, low weight per unit area, availability, affordability, workability, and good resistance to corrosion. Aluminium is nontoxic and hence it is used in cooking ware. Corrosion of copper is most often associated with soft, acidic waters with pH below 6.5 [
Aremu [
Dimensions of the inner box are as follows: surface area = 0.84 m × 0.56 m; base area = 0.6 m × 0.3 m; sides = 0.56 m × 0.30 m × 0.17 m and 0.84 m × 0.60 m × 0.17 m (in pairs).
The inner box was lagged 0.10 m on all sides.
For a flat plate collector, applying the First Law of Thermodynamics (conservation of energy),
For a double cover arrangement, Stout [
Total heat losses from the system are the sum total of heat losses from the bottom (
where
From Duffie and Beckman [
Total heat loss was calculated to be 77.04 W.
Hence, output energy
From Figure
Neglecting form losses,
The effect of temperature on microbial load of treated water with solar water heater is presented in Table
Effect of temperature on microbial load of treated water with solar water heater.
Sample | Inlet water temperature (°C) | Outlet water temperature (°C) | Microbial load cfu/mL × 104 | |||||||
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Total viable count | Total coliform count |
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Inlet | Outlet | Inlet | Outlet | Inlet | Outlet | Inlet | Outlet | |||
A1 | 32.0 | 75.0 | 48 | 5 | 18 | 1 | 10 | nd | 9 | nd |
A2 | 34.0 | 63.0 | 33 | 10 | 10 | 3 | 5 | nd | 10 | nd |
A3 | 34.0 | 62.0 | 30 | 9 | 15 | nd | 9 | nd | 7 | nd |
A4 | 32.0 | 60.5 | 48 | 19 | 18 | 3 | 10 | nd | 9 | nd |
A5 | 33.0 | 58.0 | 63 | 12 | 20 | 10 | 13 | 6 | 10 | nd |
A6 | 32.0 | 52.0 | 48 | 28 | 18 | 8 | 10 | 2 | 9 | 4 |
A7 | 32.0 | 49.0 | 48 | 28 | 18 | 12 | 10 | 8 | 9 | 6 |
nd: not detected.
Variation in percentage reduction in microbial load with change in water temperature as a result of solar disinfection process. (1) The bar charts corresponded with the labels to the left, representing percentage reduction in microbial load. (2) The line corresponded with the labels to the right, representing water temperatures. (3) Stages of data collection refer to variation in water temperature (measured at the outlet to the heating chamber) ranging from 49 to 75°C and the corresponding percentage reduction in microbial load. Stages 2, 3, 4, 5, 6, 7, and 8 correspond with 75, 63, 62, 60.5, 58, 52, and 49°C, respectively.
The averagecost of fabricating a unit of the solar water heater at the time of this research was ₦32,161.76 ($201.01) at the rate of $1 = ₦160. This amount is equivalent to the cost of purchasing 402.02 L of bottled water in Ibadan, Nigeria, where the experiment was conducted. A bottle of 1.5 L of water is being sold for about ₦120 ($0.75) in the area at the time of the experimentation. It would take the developed solar heating system 2 clear/sunny days to treat 402.02 L. Hence, the cost of constructing this system would be recovered in only 2 clear/sunny days.
An appropriate low cost solar heating system was developed and evaluated as a way of reducing microbiological contamination of domestic roof-harvested rainwater. The solar water heater recorded a maximum operating temperature of 75°C with 89.6 and 94.4% reduction in total viable count and total coliform count, respectively, while
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge Nigerian Micrometeorological Experiments (NIMEX) Research Group, Department of Physics, University of Ibadan, Ibadan, Nigeria, for access to their meteorological facilities during evaluation of the developed solar heating system.