Effectiveness of solar disinfection (SODIS) in rural coastal Bangladesh

Access to safe drinking water is a basic need for humans. Unsafe drinking water along with poor sanitation and hygiene are the main contributors to an estimated 4 billion cases (attributes to 88%) of diarrheal diseases annually, causing 1.8 million deaths, mostly children below 5 years of age (WHO 2005). Unfortunately, almost 84% of the world's population who inhabit rural areas are currently using unimproved sources of drinking water, viz., surface water, unprotected spring and well water, and water from tanker trucks, etc. (WHO & UNICEF 2008). To overcome the challenge of providing safe drinking water to the rural poor in developing countries, point of use (POU) water treatment is being extensively promoted (WHO 2007; Sobsey et al. 2008; Hunter 2009; Meierhofer & Landolt 2009). However, the prime challenges in disseminating such technologies are dependent on performance efficacy and sustained use (Sobsey et al. 2008).

Sobsey et al. (2008) documented a variety of POU technologies that basically work through either a process of disinfection (chlorination with safe storage, combined coagulant, and solar disinfection) or filtration (ceramic filter and bio sand filter). Solar water disinfection (SODIS) is a low-cost technology with a high efficacy in eliminating waterborne pathogens and reducing diarrheal diseases (Rainey & Harding 2005a, b; Amin & Han 2009; Mäusezahl et al. 2009; Meierhofer & Landolt 2009; du Preez et al. 2010; Lewis et al. 2011; McGuigan et al. 2012). More than 5 million people spread over more than 50 countries in Asia, Latin America and Africa use this method (McGuigan et al. 2012). However, the main obstacle in effective implementation of SODIS is it demands some behavioral changes for the user, such as to prepare bottles by cleaning and fill them with water and put them in the sun and collect them afterwards, cleaning the glass used for drinking before consumption of water (Rufener et al. 2010; McGuigan et al. 2012). Moreover, the effectiveness of this technology in reducing diarrheal disease risk will be hindered if used intermittently, not used for long periods, treated some of the water of daily use, or provided to only some of the household members (Hunter 2009). Although a higher compliance rate is observed during study, this may drop profoundly in post-study (Rainey & Harding 2005a; Rose et al. 2006; Sobsey et al. 2008).

In Bangladesh, 73% of the population live in rural areas that uses tubewell water as the primary source of drinking water (WHO & UNICEF 2010). These tubewells are installed at various depths depending on the availability and the level of groundwater. However, it is difficult to find a dependable water supply system in the coastal area of Bangladesh because fresh water aquifers are unavailable at a reasonable depth (Kamruzzaman & Ahmed 2006; Islam et al. 2010). Besides, many places in the coastal districts are not suitable for even deep tubewells because of the high salinity in groundwater. As a result, harvesting rainwater becomes the only source of drinking water for the inhabitants of these areas and they preserve rainwater in natural or man-made ponds (Kamruzzaman & Ahmed 2006; Alam et al. 2011). Therefore, an alternative water supply option such as pond sand filter (PSF), rainwater harvesting system (RWHS) and rain-feed ponds became the main source of drinking water supply for the coastal population of Bangladesh.

Delivery of safe drinking water continues to be a significant priority in the coastal area of Bangladesh because of higher incidence and prevalence of waterborne diseases. People living here drink water mostly from PSF, RWHS or rain-feed pond without any in-house treatment. Several recent studies (Ahmed et al. 2005; Karim 2010; Islam et al. 2011) showed that water from PSF, RWHS and rain-feed pond are microbiologically contaminated and not safe. The fecal coliform (FC) counts in water from PSF were found to vary from nil to over 4,000 cfu/100 mL (Kamruzzaman & Ahmed 2006; Islam et al. 2011). Some studies (Howard et al. 2006; Karim 2010; Islam et al. 2011) in Bangladesh showed that the rooftop harvested rainwater satisfies the national and WHO physical and chemical water quality standard, although microbiological contamination was found to occur to a great extent. This indicates the need for a proper treatment technology to use rainwater for drinking. Besides, rain-feed pond waters become dangerously polluted due to unhygienic sanitation, indiscriminant uses and non-protection. Here, the FC and Escherichia coli counts were found to vary from 12 to 10,000 cfu/100 mL and from nil to 3,000 cfu/100 mL, respectively (Islam et al. 2011) along with several pathogenic bacteria. Therefore, household-based water quality intervention, such as SODIS, may be used to reduce microbial contamination to provide safe drinking water. However, the effectiveness of SODIS in reducing microbial indicators under realistic household use conditions has not been previously investigated in the coastal area of Bangladesh. This study aims to evaluate the effectiveness of SODIS to remove microbial contamination and the acceptability of this technology in rural coastal Bangladesh.

A reconnaissance survey was conducted in the study area before household selection. During the reconnaissance survey we chose two households randomly from each of the seven villages selected for the study and interviewed them to collect information regarding their drinking water sources, water collection and treatment method. Discussions were also held with the officials of the Department of Public Health and Engineering (DPHE) and non-governmental organizations (NGOs) regarding the existing water supply system. After the reconnaissance survey we conducted a field visit for household selection and baseline data collection. A total of 50 households were selected from seven villages of the study area by systematic random sampling. In selecting the households we took every 10th household starting from any side of the village. In case of any household refusing to participate, we considered the nearest one. Since the female-head of every household is primarily associated with collection and treatment of drinking water (WHO & UNICEF 2010), hence only the female-head from each of the selected households was interviewed and trained to apply SODIS. The respondents were questioned regarding water supply, water treatment, sanitation, and hygiene behavior. The effectiveness of the SODIS method was assessed by laboratory analysis of water samples collected from the trained households.

For each of the selected households we provided in-house training on the application of SODIS. The training sessions were conducted by enumerators who were also well trained about the aspects of community work and data collection. The participants received information about: the sources of water pollution, the ways of contamination of water, health risks of consuming contaminated water, and implementation of SODIS method for drinking water. They were instructed to fill PET bottles and place it in direct sunlight for a minimum of 6 hours. However, to reduce the burden of maintaining time, they were instructed to place the bottle outdoors from early morning to the end of the day. In addition, a poster containing a detailed pictorial demonstration of the SODIS method was fixed at a suitable place in each of the households, so that they can recall the process. Since it is recommended for SODIS water to be consumed within 48 hours of treatment (McGuigan et al. 2012), participants were instructed to store the water for not more than 48 hours. All the participating households were provided with two clear PET bottles (each of 2 L capacity) for every person in the household for a period of 2 months. This allowed the rotation of two sets of bottles so that half of the set of bottles were exposed on day 1 and consumed on day 2 while the second set was being exposed on day 2 and consumed on day 3 in an ongoing cycle. The average daily water consumption in the study area is about 3.35 L/d (Islam et al. 2013). Although, the number of bottles provided per person may not supply the daily drinking water demand of the households, the objective was to encourage them to apply SODIS and gradually adopt the method.

To evaluate the performance of SODIS, water sampling was carried out in two rounds consecutively in April and May 2013. It was dry season and the reason behind sampling at this time was two-fold: (i) abundance of high sunlight and (ii) higher dependence on pond water for drinking purposes. In each of the rounds, water samples from all the households were collected in a single visit. In addition, in each of the sampling rounds, enumerators visited a subset of the participating households over the course of the following 2 months for short spot checks to objectively verify the usage of the method and to sample water for bacteriological analysis. Here, each of the subsets represented 50% of the selected households. Households not using the method at the day of sampling were excluded. None of the households were found to use PSF water during the sampling time. After 2 months of the study we conducted a post-survey to assess the intervention use and preference of the SODIS method.

We examined FC and E. coli contamination of drinking water in the first round and FC contamination in the second round of analysis. Water samples were collected from household water storage containers and SODIS bottles to determine the effectiveness of SODIS in reducing levels of fecal contamination from household containers. Water samples were collected following the standard procedures (APHA 1998). For microbiological analysis, 500-mL water samples were aseptically collected in sterile Nalgene plastic bottles. All samples were placed in an insulated box filled with ice packs (Johnny Plastic Ice; Pelton Shepherd, Stockton, CA, USA) and transported to the Environmental Microbiology Laboratory of the Environmental Science Discipline, Khulna University for bacteriological analysis immediately after collection. We assessed the concentration of FC and E. coli using the membrane filtration technique. For enumeration of FC and E. coli, 100-mL of water samples were filtered through a 0.45 μm pore-size membrane filter (Millipore Corp., Bedford, MA, USA), and the filters were placed on mFC and mTEC agar plates, respectively, following standard procedures (APHA 1998; Islam et al. 2001). The mFC plates were incubated at 44 °C for 18–24 hours for enumeration of FC, and mTEC agar plates were incubated at 35 ± 0.5 °C for 2 hours followed by further incubation at 44.5 ± 0.2 °C for 22–24 hours for enumeration of E. coli. After incubation, characteristic blue colored colonies were counted as FC and red or magenta colored colonies were counted as E. coli and expressed as colony forming units (cfu) per 100 mL. All water samples were analyzed for pH, electrical conductivity (EC), turbidity and salinity. Physico-chemical analyses were performed according to APHA (1998).

A quantitative health risk assessment (QHRA) model was used to quantify the likely disease risk associated with the water supply options. QHRA is a simple deterministic spreadsheet model developed by the Arsenic Policy Support Unit of the Government of Bangladesh. The model estimates microbial disability-adjusted life years (DALYs) for three reference pathogens, rotavirus, cryptosporidium and E. coli for viral, protozoal and bacterial diseases, respectively, to determine the total disease risk. The output of the model is expressed as μDALY/person-yr, which is a globally accepted parameter to measure health risk. The details of the model assumptions regarding pathogen and indicator organisms and the dose-response relationship are beyond the scope of this paper, but can be found in other literature (Ahmed et al. 2005; Howard et al. 2006, 2007). The E. coli data were used to estimate the likely disease risk associated with each water supply option in DALYs, a globally applied matrix as recommended by WHO (2004) to compare different disorders and diseases with different health outcomes.

Statistical analysis was performed by SPSS V.16 statistical software. Since the data failed to meet the assumption of normal distribution, hence parametric tests cannot be used to compare samples. Therefore, non-parametric tests were used, which work with ranks rather than absolute numbers. Medians, averages and ranges were used for descriptive purposes. The Mann–Whitney U test was employed to analyze the differences in bacterial concentrations between storage water and SODIS treated water.

Table 1 shows characteristics of the participants. The age of the participants varied between 15 and 57 years. Around 20% of the participants were illiterate and 40% passed secondary level. About 65% of the households had a family size of between 3 and 5 members. Approximately 58% of the participants reported their annual family income to be between 36,000 and 60,000 BDT (Bangladeshi taka). The majority of the participants (74%) use alum (locally known as phitkari) for water treatment, and about 24% of them do not use any in-house treatment for drinking water.

Table 2 shows range, mean and median values for FC and E. coli concentrations in the water of household storage containers and of SODIS bottles, with respect to the sources (both rain-feed pond and RWHS) of water. The household storage water of both ponds and RWHSs showed higher indicator bacterial contamination compared with SODIS treated pond and RWHS water. Median FC concentration of water from household storage containers for rain-feed pond and for RWHS were 900 cfu/100 mL and 270 cfu/100 mL, respectively, in the first round of water sample analysis. However, after SODIS treatment, the concentrations were found to be 13.5 cfu/100 mL and 26 cfu/100 mL, respectively. Maximum indicator bacterial concentrations of pond water were very high compared with RWHSs.

Table 2 also shows the efficiency of SODIS in removing indicator bacteria. FC and E. coli concentrations reduced significantly in both rain-feed pond and RWHS water samples in the first round of sampling (p < 0.05). Here, the overall reductions of E. coli were respectively 89.7% and 95% and for FC were, respectively, 96.1% and 98.2% for pond water and RWHS. Again, in the water samples of the second round of sampling, FC concentrations reduced significantly by 99.7% and 74.6% for both pond and RWHS water, respectively.

A summary of the physico-chemical parameters of the water samples is shown in Table 3. The pH value of most of the water samples remained within the Bangladesh drinking water quality standard (BDS) and WHO guideline values of 6.5–8.5. In the second round of analysis, only two of the RWHS samples were found to have a pH value of more than 8.5. For pond water, higher EC was observed which indicates a higher amount of dissolved substances in the water. Except for a few pond water samples, the turbidity of all treated water samples was well below the BDS acceptable value of 5 NTU. However, the turbidity of pond water was reduced significantly by applying SODIS. Here, the average removal of turbidity was more than 53% and 61%, respectively, in the first and second round of analysis.

To assess the social preference and adaptation of SODIS, we conducted a post-survey among the selected households after 2 months of the study and we found only 4% of the participants had been using SODIS. However, during the study period about 34% used SODIS regularly, 36% used it for 4 days in a week, 18% used it 3 days in a week, and the remainder were more irregular (Table 4). The respondents who were irregular in using SODIS were asked to identify the reasons for not using it and they reported several reasons, viz., reluctant about treatment (50%), difficult to treat the large amount of water required (39%) and difficult to manage time for the treatment process (11%). All of the respondents agreed that SODIS is easy to apply and almost all of them (98%) reported that drinking water from SODIS bottles can reduce diarrhea. However, only 52% of them thought SODIS would be socially acceptable and 60% believed it was cheap. Among the respondents who thought SODIS would be socially unacceptable (48%), 12% of them believed that SODIS is expensive, while 36% said it is difficult to treat the large amounts of water required for family members. Only 22% of the respondents agreed to buy bottles for SODIS.

The research was conducted under daily household use condition in the study area which represents varied water quality and hygiene practices. Results indicate that SODIS can reduce the indicator bacterial levels of the water supply options significantly, although the performance of SODIS did not meet the WHO protective level (WHO 2011). Previous studies (Rainey & Harding 2005b; du Preez et al. 2011) also designate that SODIS can reduce indicator bacterial contamination significantly under daily household use conditions.

Water supply options evaluated in this study indicate a significantly higher disease risk compared to the recommended disease risk of 1 × 10^–3 DALYs/1000 person-yr (WHO 2004). The disease risk was primarily dominated by bacterial and viral pathogens. As indicated by this study, the median health risk was found to significantly reduce by the introduction of SODIS at the household level. Although a significant microbial health risk reduction was observed, intervention at the household level by the SODIS alone cannot ensure complete microbial safety of drinking water.

In the first round of analysis, there were three SODIS samples where the level of contamination increased compared to the respective control samples; it is possible only if the bottles were not exposed to sunlight, or kept in a place where sunlight was not abundant at the required level, or recontamination occurred due to improper handling. This raises the important issues of correct placement of SODIS bottles to ensure maximum exposure to ultraviolet rays and proper handling of the bottles to avoid recontamination. The motivation to adhere to the protocols in a sustainable way set for water quality interventions is greatly influenced by human behavior and has been problematic in previous SODIS studies (Mäusezahl et al. 2009; Rai et al. 2010). In the second round of analysis, only two RWHS samples were found to have a pH value of more than 8.5. These were the harvested rainwater samples collected from a reinforced cement concrete (RCC) tank. The pH value of stored rainwater in an RCC tank is normally found to be higher than the recommended maximum value due to leaching of calcium-oxide from the cement used for construction of the tank (Karim 2010). As turbidity of the SODIS treated pond water reduces, the water becomes very clear and transparent, which is more aesthetically acceptable for drinking. It is necessary to mention that using alum for rain-feed pond water is a common practice in the study area and most of the households use alum in the household storage containers, which leads to a good decrease in turbidity, however, it did not affect the results of this study because rain-feed pond water used for SODIS was collected from the household storage containers and water samples were collected from both the household storage containers and the SODIS treated water.

We communicated SODIS methods and health risk messages to rural women, who conventionally had limited access to information. According to Solvic (1999), men tend to judge risk as being less problematic than women. In addition, women are more health conscious than men with respect to consumptive health behavior (Severtson et al. 2006; Burger et al. 2008). Therefore, the technique used to disseminate information in this study not only has the potential to reduce gender inequality in water consumption knowledge, but can also reduce the consumption of contaminated water since water collection and treatment for drinking purposes is generally done by the woman-head of the household. Thus, by communicating the SODIS method to women, we reached the target audiences because they can play a vital role in changing the consumption behavior of the household members.

In this study, only 34% of the selected households used SODIS regularly, which indicates a lack of consciousness regarding safe drinking water. To reduce the health risk associated with consumption of contaminated water it is necessary to inform coastal communities about possible risks from different drinking water sources besides introducing methods that can minimize these risks. Providing risk messages is not expected to be a magic bullet, but proper risk messages will make people aware of their own risk which will lead to positive behavioral changes. Studies (Caldwell et al. 2003; Burger 2004; Keraita et al. 2008) have also shown that knowledge does not always necessarily transmit into practice; however, it is generally assumed that knowledge of risk encourages people to take precautions to reduce such risk (Brewer et al. 2004). In the dry season (November–May), the households mainly drink rain-feed pond water because rainwater is rarely available in the rainwater tanks. Conversely, in the wet season (June–October), they depend on harvested rainwater. This may have been the reason why only 4% of the selected households used SODIS during the post-survey as it was conducted in the wet season. However, during the second round of sampling, we found the majority of the households using rainwater due to early rainfall. Therefore, it is presumable that the SODIS method communicated with a proper risk message may lead to an increased use of SODIS for pond water, especially in the dry season.

During the questionnaire survey only a few participants reported that they will buy PET bottles for SODIS, whereas approximately half of the respondents thought that SODIS may not be socially acceptable. Therefore, changes in consumption behavior are partly dependent on a proper awareness program and availability of PET bottles within short distances of the households. It was also observed that the households were not properly cleaning the storage container and were collecting the water from the container in an unhygienic way, such as dipping dirty hands in it. Thus, the storage container becomes a reservoir for microbial growth. Therefore, safe in-house water storage and good hygienic practices are important issues that need to be addressed by a water safety plan (WSP) for long-term sustainability of SODIS in delivering safe water to the users.

It is obvious from the data presented in this study and in previous studies that the water supply options currently used in the southwest coastal areas of Bangladesh pose a significant health risk. Rain-feed pond water has higher health risks compared with other options. The study findings showed that water treatment using SODIS can remove microbial contamination of the household storage water significantly. Although a significant microbial removal and health risk reduction was observed, intervention by SODIS alone cannot ensure complete microbial safety of drinking water. About 34% of the households in the study adopted SODIS on a routine basis during the study period, whereas others used it either intermittently or only when they had time. Despite the fact that the SODIS method was not widely adopted, the experience from this study indicates the future efforts to improve water quality and reduce diarrhea in the coastal areas of Bangladesh. Proper education and training of the rural community about the sources of water contamination, health risks of consuming contaminated drinking water, and benefits and barriers of SODIS should be conducted. Good hygienic practices and WSPs should be implemented for all water supply options to ensure the supply of safe drinking water in the long term. The use of WSPs should also support greater community participation because of its emphasis on working with communities to monitor and manage their water safety effectively.

We would like to thank the women who participated in this research and the officials of the DPHE and NGOs of the study area who provided relevant information and support. We also express our sincere appreciation to the Ministry of Science and Technology Bangladesh for funding support.