Performance Evaluation of a Modified SODIS under Sub-tropical Climate Conditions in Bangladesh

Abstract

Solar disinfection (SODIS) is a low-cost, effective, sustainable, and easy-to-use method recognized by the World Health Organization (WHO) as an HWT option for eradicating different microorganisms in drinking water. Studies have shown that the main limitations of SODIS are prolonged exposure time (>6h), ineffective during the monsoon and winter seasons and the regrowth of microorganisms after treatment. To overcome these limitations, the performance of a modified SODIS with a photocatalyst (H2O2) was evaluated in this study by the photo-Fenton process using test water and drinking water collected from restaurants, slums, and household areas following the WHO protocol during the monsoon and winter seasons in sub-tropical climatic conditions of Bangladesh. In addition, to predict the bacterial disinfection rate using the modified SODIS with H2O2, regression analysis was performed. Two types of test waters were prepared according to the WHO protocol. The SODIS experiment was conducted using reactors (polyethylene terephthalate (PET) bottles and plastic bags) of 500 ml capacity. 5 ml of H2O2 were added to each PET or plastic bag (PB). In each batch, six PET or PB with test water or collected drinking water samples were used and exposed to sunlight using a fabricated SODIS chamber for 6 h in both the monsoon (June-October, 2022) and winter (November-February, 2023) seasons. Before the SODIS experiment, the physicochemical and bacteriological water quality parameters were measured. In every test, physicochemical parameters such as dissolved oxygen (DO), electrical conductivity (EC), pH, turbidity, and water temperature were assessed along with bacteriological parameters such as Escherichia coli (E. coli) testing. During each hour of the SODIS experiment, one sample was taken for physicochemical and bacteriological testing along with the measurement of solar irradiance at an interval of 1 min. After 6 h, the SODIS-treated water was kept in the dark at room temperature for 12 and 24 h to check the regrowth potential of xv | P a g e the microorganisms. Drinking water samples were also collected from restaurants, slums, and household establishments in Dhaka City to evaluate the modified SODIS. The physicochemical parameter variations before and after SODIS illustrated that there were no significant changes except in the EC values. The efficacy of modified SODIS with H2O2 illustrates only 2 h was required by the PET bottle to inactivate bacteria, and 1 h was required for PB in the monsoon season, where a 6.7 log reduction value (LRV) was achieved. On the other hand, in the winter season, 2 h was required to inactivate bacteria in a PET bottle and PB, where a 5.49 LRV was achieved. There was no regrowth after the 12 and 24 h post-SODIS periods in the monsoon and winter seasons, respectively. The performance of SODIS with H2O2 was termed “Highly Protective” based on microbial inactivation (LRV >4). The Weibull bacterial inactivation model fits well with the data of PET bottles and PB in the monsoon and winter seasons, with an R2 value of 0.95-0.98. The safe exposure time for achieving the four LRV was 1 h as the minimum and 2 h as the maximum. In terms of regression analysis, the maximum accuracy was illustrated by PB (TW-1) with an R2 value of 0.79 (79%), where the equation coefficients are turbidity, water temperature, solar irradiance, and DO. Regression analysis showed that the disinfection rate increased when the water temperature, solar irradiance and DO increase and decreased when the turbidity increased. The statistical analysis results from the regression analysis also illustrated the fit of the model to the data obtained in this study. Drinking water samples from restaurants, slums, and household areas water parameter results illustrate that most of the water was microbially contaminated and that iron was present in the water. The application of the modified SODIS with H2O2 and conventional SODIS illustrates that the modified SODIS performs better, and there was no regrowth in the modified SODIS. This study’s outcome shows data similar to the literature available on SODIS for inactivating bacteria. SODIS, if promoted properly, can be a potential xvi | P a g e method for drinking safe water and providing access to water in the water stressed areas of Bangladesh and other developing countries. The results of this study will help people acknowledge the efficacy of SODIS in Bangladesh and other developing countries, and use SODIS for potable water