Effectiveness of solar disinfection for household water treatment: an experimental and modeling study

Solar disinfection (SODIS) is a simple and low-cost household water treatment (HWT) option used for disinfection of drinking water. In this study, the bacterial inactivation potential of SODIS was evaluated under the solar irradiance observed in different seasons in Bangladesh according to WHO evaluation protocol of HWT, and the SODIS experiments were conducted for both transmissive and re ﬂ ective reactors using PET bottles and plastic bags. In summer, log reduction value (LRV) more than 5 was observed for the transmissive PET reactors for 6 to 8 hr exposure to sunlight and the treated water complied with the microbial standard of zero colony forming units/100 mL in drinking water. In monsoon and winter, LRV > 4 can be achieved for 16 hr and 8 hr exposure to sunlight, respectively, using re ﬂ ective reactors. The plastic bag was found to be more effective than PET. A safe exposure time was estimated from the Weibull model to be maintained for SODIS application to achieve 4.0 LRV and also to prevent the re-growth of microorganisms in the treated water. A signi ﬁ cant re-growth of microorganisms was observed in the treated water, thus SODIS with other HWT processes can be recommended for use in communities with an unsafe drinking water supply. This study conducted to evaluate the bacterial inactivation potential using Escherichia coli as an indicator organism under the solar irradiance observed in in The experimental setup test for the experiments by Using experimental results, a developed estimate


INTRODUCTION
The Sustainable Development Goal 6.1 of the United Nations aims to achieve universal and equitable access to safe and affordable drinking water for all by 2030.
Access to safe and clean water is the keystone of sustainable development. In 2017, about 2.2 billion people globally lack access to safely managed drinking water services, among them, 435 million people taking water from unprotected wells and springs and 144 million people This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). collecting untreated surface water from lakes, ponds, rivers, and streams (WHO ). Furthermore, a minimum of 2 billion people use a drinking water source contaminated with feces (WHO ).  WHO (), the disinfection process is termed as highly protective if the LRV is !4 and protective if the LRV is !2. Currently, there is no standard of LRV for defining safe water; LRV > 3 is usually recommended to determine the effectiveness of the treatment process.
Although Bangladesh has made considerable progress in water supply coverage, improving health and sanitation over the last decade, there are still about 20 million people who lack access to safe drinking water. The poor segment of the urban people living in the slums is still lacking access to safe and reliable water supply. Moreover, about 28% of the country's total population living in the coastal areas of Bangladesh drink water mainly from rain-fed ponds, pond sand filters (PSF), and rainwater harvesting (RWH). Studies have shown that water from these sources is highly microbiologically contaminated (Karim ; Islam et al. ) and not safe for drinking.
As a tropical country, plenty of solar irradiation is available throughout the year and thus SODIS may be used in both urban and rural communities in Bangladesh as a low-cost HWT for the inactivation of microbial pollutants to make drinking water safe, which may have a positive health impact on the vast rural and urban communities. . The regrowth of the microorganisms into the photo-treated water was also evaluated, which determined the safe storage time before drinking. The study findings may be helpful in advancing SODIS as an HWT option among the coastal and low-income urban communities lacking access to safe drinking water supply, which is necessary to achieve SDG 6.1 in Bangladesh and other south-east Asian countries.

MATERIALS AND METHODS
The performance of SODIS was tested with two types of test water and the evaluation protocol of HWT options by the WHO () was followed in preparing the test waters and spiking with E. coli, and conducting the SODIS exper-

E. coli culture and spiking
The E. coli (ATCC 25922) used in the experiment was obtained from the International Centre for Diarrheal Disease Research, Bangladesh (icddr,b), which was cultured on mTEC medium by streak plate procedure. A few loops of E. coli were mixed in sterilized 0.85% normal saline (pH: 7.8-8.0) of 500 mL in order to obtain the initial concentration that was spiked into the sample water. E. coli concentration was maintained at greater than or equal to 10 5 colony forming units (CFU)/100 mL in the sample water. Spiking was done 1 hr before exposing the containers/bags to the sunlight so that bacteria could adjust with the new environment.

Test water 1
Groundwater used in the IUT water supply was collected in a 10 L plastic container. The turbidity of the water was <5 NTU and pH was 7.0-9.0 (WHO ). The water was then poured into eight PET bottles and eight plastic bags with an air space of about 15% by volume to allow air circulation for aeration (Reed ). Each bottle and plastic bag was then spiked with E. coli to ensure an initial count of 5 × 10 5 CFU/100 mL.

Test water 2
The same groundwater (10 L) was mixed with 1% by volume of autoclaved untreated sewage water collected from IUT sewage line and sterilized in an autoclave at 121 C for 24 hr. Test water 2 requires turbidity of more than 30 NTU, which was incorporated by adding clay passing through a 200 mm sieve. This clay was taken from an undisturbed soil sample collected at a depth of 30 m below the ground surface. This sample was then sieved with a 200 mm sieve to obtain the clay. The turbidity of the water was >30 NTU and pH was 6.0-10.0. This water was then poured into eight PET bottles and eight plastic bags with a 15% air space. Each bottle and bag was then spiked with E. coli to obtain a coliform count of 5 × 10 5 CFU/100 mL.
The average physicochemical and microbial characteristics of both test waters are shown in Table 1.  Table 2.

SODIS experiment
The SODIS experiment was conducted by exposing the reactors (PET and plastic) directly into sunlight by placing them on the corrugated tin sheet roof of the parking shed of IUT, which is inclined by a 60 angle to the south. All reactors were shaken before exposure to sunlight and left undisturbed during exposure, typically from 9.00 a.m. (±30 min) to 5.00 p.m. to maintain a total exposure of 8 hr in a day.
During the rainy season, the total exposure of 16 hr was done on 2 consecutive days (8 hr in each day). In each hour during the SODIS experiment, one sample (bottle and bag) from each batch was withdrawn from the roof for subsequent physicochemical and microbial analysis. The last water samples of each batch (which were withdrawn after 8 or 16 hr exposure to sunlight) after physicochemical analysis, were kept in the room environment for 24 hr to check the treatment efficacy and monitor the re-growth of microorganisms into the photo-treated water by measuring E. coli after 12 and 24 hr of exposure (Giannakis et al. ). Solar irradiance and air temperature were measured at an interval of 1 min throughout the exposure period using a Solar Survey 200R Pyranometer (Seward Group, UK) with a data logger.
A total of 14 sets of experiments (7 sets each with TW1 and TW2) were conducted under three climate conditions (summer, monsoon, and winter) within an annual cycle.

Evaluation of physicochemical and microbial parameters
Physicochemical and microbial parameters, turbidity, dissolved oxygen (DO), pH, electrical conductivity (EC), and

Bacterial inactivation and modeling
The inactivation of bacteria (E. coli) was measured by log reduction value (LRV), which was calculated by: where C1 where N is the (

Solar radiation and temperature
The variation of solar radiation and water temperature during the SODIS experiments under the three climatic conditions is shown in Figure 2. During summer, higher solar radiation under the strong sunlight condition was observed, reaching a maximum within 3 hr of exposure (strong sunlight). In monsoon, a scatter pattern of solar irradiance was observed as the sky remains cloudy for most of the time (moderate sunlight). In winter, relatively lower but almost continuous solar irradiance was observed with a maximum peak after 3 hrs of exposure (weak sunlight).
The inactivation of bacteria in the bottles and plastic bags occurred by the combined effect of the upcoming UV radiation and the heat energy provided by the heated corrugated tin roof, and the temperature was also found to reach the maximum level at midday after 3 to 4 hr of exposure.

Bacteriological inactivation
The bacterial inactivation of test waters in the two types of reactors under the three climatic conditions is shown in Figure 3. The initial E. coli count was 5 × 10 6 CFU/100 mL in both test waters and a sharp decline in microbial level during the first 2 hr of the experiment was observed and more than 99% of E. coli was found to be inactive during the first 4 hr of exposure. In summer (strong sunlight con-  The performance of SODIS for bacterial inactivation in terms of LRV is shown in Table 3. In summer, bacterial inactivation of more than 5 LRV was observed for both test waters using transmissive reactors (PET and plastic bag), indicating a highly protective level performance of SODIS. However, in monsoon, protective performance level with an exposure of 8 hr was observed for both test waters using both transmissive and reflective reactors, and highly protective level performance for TW1 was achieved using reflective reactors (PET and plastic bag) for 16 hr exposure to sunlight on 2 consecutive days. In winter, a highly protective level performance can be obtained using reflective reactors for both test waters. As mentioned by bacterial inactivation of SODIS in TW1 was found to be higher than TW2 (Table 3) because of lower turbidity, which allows more transmissibility of UV irradiation into the water. Moreover, the bacterial inactivation efficiency of plastic bags was found to be higher than PET bottles in monsoon and winter (Table 3) Modeling bacterial inactivation  LRV as obtained from this study is much higher that other   Table 4.

Re-growth of microorganisms
The re-growth of the microorganisms in the treated water was examined by placing the photo-treated water in a dark room for a subsequent period of 24 hr. Re-growth of microorganisms was found to occur in both test waters, and It is thus essential to maintain sufficient exposure time to control the microbial re-growth in the photo-treated water while storing the water in-house before drinking. More experiments will be necessary to explore the safe storage time of water treated by SODIS before microbial re-growth.  LRV and also to prevent the re-growth of microorganisms during the post-irradiation period. No significant change in physicochemical water quality of the test waters during SODIS experiments was observed and a higher level of turbidity in water was found to decrease the SODIS performance.

CONCLUSIONS AND RECOMMENDATION
The study results showed that the microbial standard of The turbidity of the urban supply water, PSF water, and harvested rainwater was found to be less than 5 NTU (Karim ); thus, the water is very transparent and mostly suitable for SODIS. However, the turbidity of rain-fed pond water used for drinking water in the coastal area in Bangladesh was found to be very high (>30 NTU); this water needs pretreatment like filtration and sedimentation to reduce turbidity before disinfection by SODIS. SODIS application in the urban and rural context in Bangladesh needs more study into its potential for producing safe water from the available unsafe water supply sources currently being used for drinking water.