Optimization of solar disinfection system for household water treatment

Abstract

Waterborne diseases are still common in developing countries as drinking water sources are contaminated and feasible means to reliably treat and disinfect these water sources are not available. Many of these developing countries are in the tropical regions of the world where sunlight is plentiful. The objective of this study was to optimize solar disinfection system for household water treatment. An experimental study was carried out from April1 to June7, 2011 at the laboratory of Environmental Health Sciences and Technology, Jimma University. Inactivation of microbes was tested at different water and environmental conditions (turbidity, pH, water depth, dissolved oxygen concentration, water temperature, container type, color of container, solar intensity) using fecal coliform as test organism. Optimization of solar disinfection (SODIS) system was done by testing the efficiency of SODIS at optimized conditions (at turbidity of 2NTU, pH 7, dissolved oxygen concentration of 6.52mg/L, half-surfaced black colored PET bottle, and water depth of 10 cm ). Fecal coliform enumeration was performed by pour plate method. The results showed that complete fecal coliform inactivation was found on clear water samples having 2nephlometric turbidity unit (NTU) with six hour exposure time. On the contrary, complete inactivation was not found for a water sample having turbidity of 13NTU. Statistically significant difference on the rate of fecal coliform inactivation was not found on water samples having different pH value in the range of 5.5 to 9 (p=0.05). Depth of water has shown significant impact on inactivation of fecal coliform (p=0.015). After 3 hour of exposure time, higher log inactivation (2.91±0.001) was found on water depth of 5.5 cm and the least log inactivation (0.474±0.044) was found on water depth of 10 cm. Higher log inactivation (0.79±0.03) was found on colorless Polyethylene terephthalate (PET) bottle and the least log inactivation (0.16±0.03) was found on black colored PET bottles. Aerating raw water samples prior to exposure enhanced the inactivation rate by a factor of 2.2 even if the difference was not statistically significant (p=0.05). Exposing raw water under half-surfaced black colored PET bottle has shown significant increment on the rate of fecal coliform inactivation. Place of bottle exposure didn’t show statistically significant effect on SODIS. Intensity of light has also shown significant effect on the rate fecal coliform inactivation. After 3 hour of exposure, 1.65±0.05, 0.95±0.03 and 0.2±0.01 log inactivation was observed on raw water II samples exposed to sunlight having a cumulative solar irradiance of 3.99kWh/m2 , 2.77kWh/m2 and 0.6026kWh/m2 respectively. Complete microbial inactivation was observed within exposure time of 3 to 4.5 hour, on raw water samples having 820CFU/mL under optimized conditions for water disinfection. Bacterial re-growth was not observed after solar disinfection, confirming that the inactivation was irreversible. The results demonstrated that under optimized conditions for water (at turbidity of 2NTU, pH 7, dissolved oxygen concentration of 6.52mg/L, half-surfaced black colored PET bottle, and water depth of 10 cm ). Complete fecal coliform inactivation can be achieved within an exposure time of less than four hour in all parts of the world which are receiving solar irradiance of 3.99kWh/m2 and above