Efficacy of solar water disinfection treatment system in improving rural and peri-urban household drinking water quality and reducing waterborne diarrhoeal diseases

In the natural environment, sources of contamination that end up in drinking water are ubiquitous (Lawrence et al. 2001). The leading and widely recognized health risk associated with drinking water is contamination with pathogenic micro-organisms (bacteria, viruses, protozoa, and helminths) transmitted through faeces (WHO et al. 2022). The drinking of untreated faeces-contaminated water leads to transfer of these pathogenic micro-organisms from the environment into humans (Gwenzi et al. 2017). Once ingested, these pathogens can lead to outbreaks of waterborne diseases such as cholera, giardia, dysentery, and typhoid fever. The alarming reality is that 25% of the global population lacks access to safely managed drinking water at home (WHO & UNICEF 2021), and approximately 2 billion individuals depend on water sources contaminated with faecal matter (WHO et al. 2022). This means that the world is still off track to meet the first target of sustainable development goal number 6, which aims to ensure universal and equitable access to safe and affordable drinking water for all by 2030. Deplorably, least developed nations have the furthest to go, and it will be especially challenging for them to accelerate progress especially in fragile contexts (WHO & UNICEF 2021). For example, at current rates, it is projected that just 37% of the population of Sub-Saharan Africa will be using safely managed drinking water by 2030.

Many homes in rural areas of the developing world are too few and/or highly dispersed to justify the cost of installing reticulated systems from a high-quality water supply source or a centralized drinking water treatment facility (REAL-Water 2022). As a result, the majority resorts unsafe drinking water sources. Besides concerns of contamination at source, subsequent microbiological contamination of previously safe water also occurs due to unhygienic handling of water during collection, transportation, and storage within the home (Sobsey 2002; World Health Organization 2007; Yefanova et al. 2022). Several studies have observed significant deterioration in microbiological quality of drinking water between water source and point-of-use (POU) (Lindskog & Lindskog 1988; Mølbak et al. 1989; Roberts et al. 2001; Moyo et al. 2004; Wright et al. 2004; Cronin et al. 2006; Gundry et al. 2006; Psutka et al. 2011; Kanyerere et al. 2012). To protect the vulnerable population from consuming microbiologically contaminated water, innovative household water treatment (HWT) techniques have emerged as practical and cost-effective solutions. These techniques specifically target and enhance the microbiological quality of drinking water at the point of use, thereby providing immediate relief in areas lacking centralized water treatment infrastructure (Clasen 2009). The combination of these HWT interventions together with safe storage, collectively called household water treatment and storage, provides interim measures to communities to take charge of improving their own microbiological water quality at homes until they can be reached with safe, reliable, household piped-water connections (Sobsey 2002; Clasen 2009; Clasen et al. 2015).

Solar water disinfection (SODIS) is one such HWT method that has gained recognition and popularity over the years. The method utilizes sunlight to inactivate microbial pathogens in water, offering a simple, cost-effective, and environmentally friendly water treatment solution that does not produce toxic disinfection by-products. Its effectiveness and potential have earned it official recognition and recommendation by the World Health Organization (WHO). By exposing transparent containers containing contaminated water to sunlight for a specific duration, the method harnesses ultraviolet (UV) radiation and increases temperature to kill or inactivate a wide range of pathogens in drinking water. Pioneering research conducted in the Maasai community in Kenya by Conroy et al. (1996, 1999, 2001) validated the effectiveness of SODIS in enhancing water quality and reducing waterborne illnesses. Subsequent studies and field trials in different parts of the world have consistently shown significant reductions in microbial contamination and improvements in health outcomes through SODIS implementation.

Despite its beneficial features, the SODIS method has limitations that impede its effectiveness under certain circumstances. Conventional SODIS often requires prolonged exposure times ranging from 6 to 48 hours to achieve effective disinfection. This limitation is attributed to the reliance on sunlight availability and the requirement for water temperatures of 50 °C or higher (Wegelin et al. 1994; Sommer et al. 1997). Insufficient sunlight or cloudy weather can impede the disinfection process, while the need for high temperatures restricts the treatment volume to small quantities, limiting its scalability and practicality (Pulgarin 2015; Luzi et al. 2016). Extended exposure times and inadequate disinfection may lead to bacterial re-growth in treated water. Various factors including water turbidity and climatic conditions can influence the disinfection process and heighten the risk of bacterial re-growth, compromising the safety of the water. In addition, challenges such as limited volume, the unavailability of PET bottles, and user behaviour contribute to the social acceptance of SODIS.

This review critically examines the effectiveness of SODIS as an HWT technique by analysing scientifically robust evidence documenting its microbiological efficacy and the positive health impacts reported among SODIS users. It investigates the limitations that affect its pathogen inactivation efficiency and the challenges that influence its scalability and sustainability in real-life applications. The review also highlights the importance of behaviour change interventions to ensure consistent and correct application of SODIS. The novelty of this review lies in its expanded exploration of available strategies and innovative approaches to overcome the issues associated with SODIS disinfection efficiency and implementation. By providing a comprehensive analysis of the existing scientific evidence, this review presents compelling reasons for the implementation and scalability of SODIS in the developing world. The novel perspective presented here contributes to the existing literature on improving access to safe drinking water in underserved communities, offering valuable insights into the advancement of SODIS as a practical and sustainable HWT solution.

SODIS is the water disinfection method that relies on effects of solar radiation to destroy or inactivate pathogenic micro-organisms in microbiologically contaminated drinking water. The method was endorsed by WHO in 2001 and has been recommended for low-income countries and in the aftermath of natural disasters or humanitarian crises to improve access to safe water (Rainey & Harding 2005; Meierhofer 2006). Since then, the method is being promoted in developing countries through information and awareness campaigns, training and advising of the public sector (government institutions), networking activities, as well as user training at the grassroot level (Meierhofer & Landolt 2009). Being a fundamentally behaviour change strategy than a product, its adoption by the corporate world remains unattractive and many are reluctant to support its promotion campaigns (Lantagne et al. 2006; Clasen 2009; McGuigan et al. 2012). Nevertheless, the technology has gained the largest traction and acceptability among low-income populations, with more than 2.1 million reported users by the end of 2007 (Clasen 2009). For the past years, the method has achieved some coverage in areas where it has been promoted, although the coverage figures varied widely among settings (between 9 and 66%, although persistent use is less clear) (Schmidt & Cairncross 2009). However, there is limited information regarding recent practical application of SODIS, including its application scale and geographical coverage. As noted by Ballesteros et al. (2021), there is limited attention given to the real-world application of solar-driven methods, as well as other crucial aspects such as testing of regulated microbial indicators and emerging pathogens. By 2012, it was estimated that more than 5 million people in more than 50 countries across the developing world (Latin America, Asia, and Africa) were regular users of SODIS at that time (McGuigan et al. 2012).

A detailed conventional SODIS application procedure is described in the SODIS manual by Luzi et al. (2016). In short, it involves filling potentially microbiologically contaminated water into transparent PET bottles of up to 2L volume (or smaller) and exposes it to full sunlight for at least 6 h on a sunny day or for 2 consecutive days (48 h) if cloudiness is greater than 50% (McGuigan et al. 2012; Luzi et al. 2016). The treated water is then stored in the bottles until consumption in order to avoid re-contamination. It is also recommended to not drink within 24 hours to avoid the possibility of post-exposure re-growth (McGuigan et al. 2012).

SODIS operates based on two fundamental principles: utilizing sunlight for water disinfection and adhering to recommended exposure times. When water is exposed to sunlight, it undergoes two simultaneous processes that contribute to its safety for consumption. The first process involves solar ultraviolet (UV) radiation, specifically UV-A light, which exhibits germicidal properties. This radiation penetrates the water, causing disruption of the DNA of pathogens. The second process involves the infrared heat from the sunlight, which raises the temperature of the water inside the bottles. This elevated temperature further enhances the inactivation of micro-organisms, complementing the UV disinfection process. The combined effect of both stresses produces synergistic action that makes SODIS an effective and accessible method for treating household water in resource-constrained settings (Meierhofer & Wegelin 2002; Borde et al. 2016).

Optical radiation within the electromagnetic spectrum includes ultraviolet radiation, visible light, and infrared radiation (IR) (Gallagher et al. 2010). According to WHO, ultraviolet (UV) radiation is the radiation energy that comes naturally from the sun as a major source, and it covers the wavelength range of 100–400 nm, which is a higher frequency and lower wavelength than visible light (WHO 1994). It is divided into three bands, namely, UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (100–280 nm) (WHO 1994). Solar UV radiation causes various biological impacts such as changes in the composition of proteins, DNA, and other important biological compounds. It also has long-term effects on vital physiological processes, resulting in decreased growth and cell division, loss of pigmentation, and hindered energy production, and induces acute physiological stress in different organisms (Rastogi et al. 2010). The main mechanism of pathogen inactivation during solar disinfection is direct or mediated damage to proteins and the DNA of the organisms, induced by radiation in the UV-B, UV-A, and possibly the lower visible range (Luzi et al. 2016). When the pathogens are exposed to sunlight, UV-A radiation directly interacts with the DNA, nucleic acids, and enzymes of their living cells, changes the molecular structure and eventually leads to cell death (Meierhofer & Wegelin 2002). However, UV-B plays a minor role in SODIS inactivation of micro-organisms because most (95%) of it (and also UV-C) is absorbed by the stratospheric ozone (O₃) layer which protects the earth from radiation coming from space, water vapour, oxygen, and carbon dioxide (WHO 1994; Meierhofer & Wegelin 2002; Luzi et al. 2016). In addition, materials of the vessels used (PET) absorb most of the UV-B radiation (Luzi et al. 2016).

Only a higher fraction of UV-A radiation, in the wavelength range of 320 nm to 400 nm, near the visible violet light, reaches the surface of the earth (Meierhofer & Wegelin 2002). Therefore, the UV-A portion of sunlight is the main inactivation factor of SODIS (Moreno-SanSegundo et al. 2021). When water is exposed to UV-A light, the combined effects of UV-A absorption and photochemical reactions contribute to the inactivation of pathogens during solar disinfection. Clear plastic or glass bottles used in SODIS allow UV-A radiation to pass through and be absorbed by the water. This absorbed UV-A radiation interacts with the micro-organisms present in the water, damaging their DNA, nucleic acids, and enzymes present in their cells. This interaction alters the molecular structure, eventually causing the cells to die (Meierhofer & Wegelin 2002). However, according to Luzi et al. (2016), the absorbed UV-A radiation does not directly harm the genetic material of pathogens. Instead, it triggers photochemical reactions within the micro-organisms. During these reactions, the UV-A radiation interacts with oxygen molecules in the water, resulting in the generation of highly reactive oxygen species (ROS) such as singlet oxygen, superoxide, hydrogen peroxide, and hydroxyl radical. These ROS subsequently interact with and damage DNA or proteins (and other cellular components), impeding the pathogens' ability to reproduce and survive (García-Gil et al. 2021). It is worth noting that the ROS can be mediated by organic photosensitizers dissolved in the water, such as organic molecules or iron (exogenous mechanism), or by molecules of the pathogenic organisms themselves (endogenic mechanism) (Luzi et al. 2016).

In addition to UV-A radiation, sunlight also contains IR, which contributes to the heating of the water in SODIS bottles. The IR absorbed by the water is responsible for heating it up. Elevated temperatures can enhance the inactivation of pathogens through thermal effects, as many micro-organisms are sensitive to higher temperatures. Higher temperatures can also accelerate the biochemical reactions occurring within the micro-organisms, further compromising their viability (Meierhofer & Wegelin 2002; Luzi et al. 2016). In actual fact, heat damages the molecular structure of pathogenic organisms, a mechanism known as thermal inactivation or pasteurization when the water temperature is raised to 70 °C–75 °C (Meierhofer & Wegelin 2002; Luzi et al. 2016). During solar exposure, the water is typically heated up by solar IR depending on the irradiation intensity, ambient temperature, and location (wind cooling, heat absorbing background) (Luzi et al. 2016). As the water is heating up, bottle water temperatures rise above the metabolism temperatures of the micro-organisms (Luzi et al. 2016).

In this case, water does not have to be boiled to kill 99.9% of the micro-organisms. For instance, in their experiment to investigate efficacy of SODIS for enteric pathogens and to test applicability of the reciprocity law, Berney et al. (2006) found out that Vibrio cholerae was more susceptible to mild heat at temperatures as low as 40 °C and Escherichia coli, Salmonella typhimurium, and Shigella flexneri were sensitive at temperature above 45 °C. Previously, World Health Organization (2015) presented results of various investigations which have shown that waterborne pathogens are sensitive to heat and are killed or inactivated even at temperatures below boiling (less than 100 °C). According to Clasen (2009), heating water to even 55 °C has been shown to kill or inactivate most enteric pathogens. As Table 1 shows, heating the water to 50–60 °C for 1 h has the same effect as boiling (Meierhofer & Wegelin 2002). However, it is still argued that bringing water to a rolling boil is the only way to ensure high enough temperatures to eliminate the risk of pathogenic bacteria, viruses, and protozoa (Sobsey 2002; Clasen 2009; Rosa et al. 2010; Juran & MacDonald 2014).

Table 1 shows temperatures and exposure times required to eliminate micro-organisms.

It has been established in the scientific literature that combined exposure to the sun's heat and the UV light in the SODIS process has a lethal synergistic effect that produces enhanced inactivation efficiency far greater than the cumulative effect that a single agent can produce when used individually (Wegelin et al. 1994; McGuigan et al. 1998; Meierhofer & Wegelin 2002; Sobsey 2002; Mtapuri-Zinyowera et al. 2009; Graf et al. 2010; Castro-Alférez et al. 2017). At temperatures above 45–50 °C, a synergistic effect of thermal inactivation and UV-A radiation occurs which strongly enhances the inactivation efficiency of SODIS (Luzi et al. 2016). According to Sobsey (2002), combined effects of UV radiation in the UV-A ranging from 320 to 400 nm and heating to temperature of 50–60 °C, respectively, have germicidal effects and are high enough to extensively (99.9%) inactivate many enteric micro-organisms. This same synergistic effect was also observed in a study by Mtapuri-Zinyowera et al. (2009) where cysts of Giardia duodenalis and Entamoeba histolytica or Entamoeba dispar were destroyed when temperatures increased above 50 °C, with complete death at 56 °C.

One of the primary limitations of the conventional SODIS method is the prolonged exposure time required to achieve effective disinfection. Exposure times typically range from 6 to 48 hours, depending on factors such as sunlight intensity and water turbidity. Insufficient sunlight or cloudy weather conditions can significantly impede the disinfection process, prolonging the time required to treat water effectively. This limitation poses challenges in regions with inconsistent sunlight patterns, particularly during certain seasons or in areas with high cloud cover.

SODIS relies on the energy from sunlight to inactivate pathogens in water. Therefore, factors such as cloud cover, rainfall, and seasonal variations in sunlight intensity can affect the efficiency of the process. The process requires favourable climatic conditions, i.e., sunlight radiation and ambient temperatures preferably not under 500 W/m² and 20 °C, respectively (EAWAG/SANDEC 1991). The amount of solar irradiation can significantly fluctuate throughout the day due to factors like cloud cover (Berney et al. 2006). An increase in cloud cover leads to reduced sunlight and lower temperatures, which logically means a slower rate of inactivation (Sommer et al. 1997). During periods of cloudy weather or continuous rainfall, it is advisable to increase the exposure time of SODIS by keeping the bottles exposed for two consecutive days. However, in cases of prolonged cloudiness or persistent rainfall, it is recommended to consume boiled water or previously treated SODIS water that has been stored (Wegelin et al. 1994; Meierhofer 2006; Luzi et al. 2016).

SODIS is associated with potential difficulties in treating highly turbid water (Sobsey 2002). This is because high turbidity hinders penetration of sunlight into the water, reducing its effectiveness as a disinfection method (Borde et al. 2016). To yield more effective results, it is recommended that SODIS should be applied to relatively clear water, < 30 NTU (World Health Organization 2007). This means that other simple methods for reducing the turbidity of water before solar treatment should be applied. While the importance of low-turbidity water has been emphasized (Wegelin et al. 1994; Meierhofer & Wegelin 2002; Meierhofer 2006; Dessie et al. 2014), studies conducted by McGuigan et al. (1998) and Joyce et al. (1996) demonstrated that SODIS can effectively inactivate certain pathogenic bacteria even in highly turbid water (200 NTU), provided that the water temperature exceeds 55 °C.

Microorganisms, including pathogenic ones, have evolved mechanisms to repair DNA damage caused by heat and other stressors. This allows them to regain their infectivity and survive adverse conditions (Linden et al. 2002). One important repair mechanism in various organisms is photoreactivation, which involves the enzyme photolyase. Photolyase uses light energy to repair UV-induced DNA lesion (Sinha & Häder, 2002). This repair process is crucial for maintaining DNA integrity and preventing the accumulation of genetic damage. In addition to photoreactivation, excision repair is another significant mechanism for DNA repair. Excision repair, categorized as base excision repair (BER) and nucleotide excision repair (NER), also plays a significant role in DNA repair. BER involves glycosylases, while NER involves polymerases (Sinha & Häder, 2002). Due to some limitations, SODIS may not completely inactivate all pathogens in water. Due to their self-repair abilities, the pathogens may re-emerge in SODIS-treated water. Understanding the repair mechanisms employed by micro-organisms and the limitations of SODIS in completely eliminating all pathogens can guide the development of more effective water treatment strategies. In addition, unhygienic handling and poor storage practices can introduce new contaminants, including pathogenic micro-organisms, into the treated water. This highlights the importance of proper water storage and handling procedures to minimize the risk of re-contamination and ensure the safety of SODIS-treated water. It is recommended to consume the treated water directly from the original small, narrow-necked bottles with caps within a 24-h period (Lantagne et al. 2006).

Different micro-organisms have varying levels of heat resistance, and while technologies like SODIS primarily target the vegetative forms of micro-organisms, certain pathogens can exist in more resilient stages such as cysts or spores, which are not effectively deactivated by solar radiation (Den Besten et al. 2018). SODIS has been successful in eliminating common waterborne pathogenic bacteria after a 6-h exposure under suitable conditions. However, there are cases where faecal coliforms show slower rates of inactivation, and certain subpopulations of E. coli have been found to be more resistant to light and show reduced rates of inactivation (Wegelin et al. 1994). This highlights the diverse responses of bacterial strains to solar disinfection, suggesting the presence of factors that influence their susceptibility. While spore-forming bacteria like Bacillus cereus and Clostridium perfringens can withstand SODIS, they are not typically associated with waterborne transmission or considered pathogens, but studying their behaviour provides insights into the effectiveness of SODIS against more complex micro-organisms (McGuigan et al. 2012).

Protozoan pathogens, such as Giardia lamblia and Cryptosporidium spp, exhibit resistance to SODIS due to their infective stages being enclosed in protective structures, such as cysts or oocysts (McGuigan et al. 2012). These structures make them more resilient to environmental stress, including solar UV radiation, making it more difficult to efficiently remove or inactivate them using SODIS (Luzi et al. 2016). Even other disinfection methods like boiling and chlorination struggle to fully inactivate cysts, requiring higher heat intensity and longer exposure durations (Den Besten et al. 2018). Studies have shown that SODIS can effectively inactivate poliovirus but may not significantly reduce the viability of Acanthamoeba polyphaga cysts at lower temperatures (Heaselgrave et al. 2006). Similarly, SODIS and other disinfection methods have limited effectiveness against the cyst stage of A. polyphaga (Lonnen et al. 2005). These findings emphasize the challenge of dealing with the resistant stages of protozoan pathogens in the context of SODIS, necessitating alternative approaches or higher UV doses for effective inactivation.

On the other hand, different viral species exhibit varying degrees of resistance to solar disinfection. Some viruses such as somatic phage, bacteriophage f2, bovine rotavirus, and polio virus have been completely inactivated within a few hours of exposure to sunlight. However, picornavirus (encephalomyocarditis) required a longer exposure time of 12.5 h for complete inactivation (Wegelin et al. 1994). In a study conducted by Harding & Schwab (2012), it was found that MNV (murine norovirus) displayed exceptional resistance to SODIS. Despite a 6-h exposure, SODIS only achieved a 1.4 log reduction in MNV. This raises concerns about the efficacy of SODIS in preventing viral gastroenteritis caused by human norovirus. Further investigation is needed to gain a deeper understanding of this issue. Parsa et al. 2021 contend that SODIS has limited effectiveness in eradicating the SARS-CoV-2 virus. The reason for this lies in the specific requirements of the virus, which necessitates higher temperatures (>56 °C) and UV-C wavelengths (100–280 nm) for successful inactivation. In contrast, conventional SODIS systems typically operate at lower temperatures (<45 °C) and utilize UV-A radiation (315–400 nm). As a result, relying solely on SODIS as a method for wastewater treatment or ensuring the safety of drinking water during the pandemic may not be a dependable approach.

Disinfection efficacy of the SODIS method depends on a number of parameters including type of pathogenic organism, irradiation intensity, material and size of bottles, place and position of bottle exposure, turbidity and dissolved organic matter, oxygen content, water temperature, and re-growth of pathogens after treatment. Each of them affects pathogen removal in one way or another. A detailed discourse of each of these factors is discussed in the SODIS manual (Luzi et al. 2016).

To address some of the limitations of the conventional SODIS method, some innovative strategies aimed to enhance the disinfection process and maximize the benefits of SODIS have been explored. They include approaches such as painting the bottom of plastic bottles black to enhance thermal disinfection (Wegelin & Sommer 1998), using solar concentrators/reflectors to amplify radiation exposure, and incorporating chemical additives like citric acid or riboflavin that can be activated by sunlight (McGuigan et al. 2012; Cowie et al. 2020). Chemical additives like citrus-based compounds (Harding & Schwab 2012) and riboflavin (Heaselgrave & Kilvington 2010) are also known to enhance solar disinfection. For instance, lime juice/pulp reduced E. coli levels within a short duration of 30 min in a study by Harding & Schwab (2012). Heaselgrave & Kilvington (2010) used riboflavin (vitamin B2) in combination with UV-A radiation as a potential therapeutic application for treating ocular bacterial and fungal pathogen. The results demonstrate that the inclusion of riboflavin in SODIS (SODIS-R) significantly improved the eradication of micro-organisms compared to SODIS alone. The addition of riboflavin reduced the time needed for complete inactivation, allowing SODIS-R to achieve a faster and more effective elimination of organisms than SODIS alone. However, the highly resistant A. polyphaga cysts and Bacillus subtilis spores were unaffected by both SODIS and SODIS-R at an optical irradiance of 150 W m⁻².

Further, a study by Rodríguez-Chueca et al. (2019) shows the feasibility of enhancing solar disinfection (SODIS) treatment by incorporating chemical additives such as peroxymonosulfate (PMS) and peroxydisulfate (PDS) to generate sulphate and hydroxyl radicals through various activation routes. The study examined the efficacy of using different promoters, including sunlight irradiation, mild heat (40 °C), and micromolar amounts of Fe²⁺ (iron) as activation factors. The results showed that PMS had higher efficiency compared to PDS in removing E. coli, requiring lower concentrations and shorter reaction times for complete bacterial inactivation. When all the promoters were combined (oxidant/Fe2 + /sunlight/40 °C), total bacterial inactivation was achieved within 30 min using PMS, whereas it took twice as long with PDS. Moreover, the addition of hydroxyl radicals alongside PMS further enhances the disinfection process, achieving complete bacterial removal in just 20 min. In addition, the combined processes were effective in eliminating contaminants of emerging concern, such as drugs and pesticides.

Efforts have also been made to concentrate solar radiation inside SODIS bottles by using solar mirrors. For instance, E. coli inactivation efficiency was significantly improved in aluminium foil backed bottles in a study by Kehoe et al. (2001). Bacterial populations in bottles with foil backing exhibited a mean decay constant that was 1.85 times higher than those without foil backing, indicating a more rapid inactivation. According to McGuigan et al. (2012), several flow reactor designs have been investigated to improve solar disinfection. Some designs concentrate on increasing optical inactivation by utilizing solar collectors and reflectors, while others aim to enhance thermal inactivation by incorporating black plates or TiO₂ photocatalyst.

The integration of SODIS with more advanced approaches such as heterogeneous photocatalysis and solar photothermal disinfection systems holds promise in addressing limitations of traditional SODIS methods. These methods have been suggested to be more efficient, and their integration with SODIS has been reported to yield improved outcomes. For instance, according to Cowie et al. (2020), incorporating a highly efficient photocatalyst has the capacity to enhance SODIS to a much greater extent compared to other enhancement approaches. However, their costs are prohibitively high to be considered as a part of the SODIS system for use in the developing world where the majority are poor and vulnerable (McGuigan et al. 2012). Other affordable techniques to enhance the treatment efficiency of the standard procedure include the placing of filled plastic bottles on sunlight-reflective surfaces such as aluminium or corrugated iron sheets to boost the amount of sunlight absorbed by the bottle, shaking a two-thirds filled bottle vigorously for 30 s before topping up and sealing, to increase initial levels of dissolved oxygen for solar induced oxidative inactivation processes, and filtering the water before filling (McGuigan et al. 2012).

Studies have shown that incorporating TiO₂ into SODIS significantly enhances its effectiveness compared to traditional SODIS alone. Duffy et al. (2004) demonstrated that the inclusion of flexible plastic inserts coated with TiO₂ powder in PET plastic SODIS reactors resulted in an improvement of 25% in their effectiveness compared to standard SODIS reactors, in terms of inactivating E. coli K12. In another study, TiO₂-equipped SODIS reactors achieved faster inactivation of micro-organisms compared to the standard SODIS, even under partial cloud cover (Lonnen et al. 2005). The presence of TiO₂ in SODIS treatments led to faster and more efficient disinfection, achieving non-detectable levels of coliform bacteria within a shorter exposure time. In a study by (Gelover et al. 2006), a minimum average global radiation of 800 W/m² was found to be necessary for effective disinfection, and TiO₂ photocatalysis prevented bacterial re-growth observed in standard SODIS treatments. The photocatalytic properties of TiO₂ facilitated irreversible damage to bacterial cells, even in UV-resistant micro-organisms, preventing re-growth after disinfection.

Solar photothermal disinfection systems utilize solar energy to generate heat, which is crucial for killing pathogens. Noble metal-based plasmonic nanoparticles like gold, silver, and platinum have been extensively studied for their photothermal properties. However, their practical use is limited due to high costs and potential toxicity (Tang et al. 2022). To overcome these challenges, researchers have focused on developing alternative and cost-effective photothermal catalysts using carbon-based and phosphorus-based nanoparticles derived from readily available resources. Incorporating light-harvesting materials such as carbon nanotubes (CNTs) or metallic nanoparticles into photothermal disinfection systems enhances heat absorption and water temperature, leading to pathogen inactivation (Loeb et al. 2018). Solar photothermal disinfection combines thermal effects with other mechanisms to destroy pathogens by disrupting cellular processes, damaging cell structures, denaturing proteins, and disrupting cell membranes. Studies have demonstrated that solar radiation increases water temperature, accelerating bacterial inactivation. Advanced SODIS systems with photothermal conversion materials achieve even higher temperatures. The synergy between solar radiation and photo-induced heat results in the accumulation of ROS, further enhancing bacterial inactivation (Hong et al. 2022).

The recent study by Hong et al. (2022) investigated the use of a photothermal film reactor made of CNTs, which efficiently transferred heat to water, and rapidly heated it to temperatures above 45 °C, leading to significant bacterial inactivation within just 20 min. The accumulated ROS played a crucial role in destroying bacterial cell membranes and RNA, ultimately leading to bacterial cell death. Transcriptomic analysis revealed changes in gene expression during solar photothermal disinfection, with specific genes showing heightened activity under heat and photothermal stress. SafA, for example, played a role in suppressing ROS accumulation and oxidative stress. Solar photothermal disinfection also affected metabolic pathways, ribosome-related pathways, and nucleic acid synthesis, contributing to bacterial deactivation.

To overcome the limitations of bulk water boiling for water sterilization, Loeb et al. (2018) explored nanomaterials capable of converting solar energy into heat for direct bacterial and viral inactivation. To facilitate practical implementation, Loeb et al. developed a method to immobilize these light-absorbing nanomaterials onto films, creating functional devices for water treatment. Through their experiments, they demonstrated that these specially designed films, when exposed to simulated solar light, possessed the capability to thermally deactivate bacteria and viruses. The efficacy of the approach was verified through successful inactivation of surrogate organisms, including E. coli K-12, as well as bacteriophages MS2 and PR772. In a subsequent study (Loeb et al. 2019), it was reported that using light-harvesting nanoparticles in the form of photothermal films allowed for the achievement of functional disinfection temperatures using unconcentrated sunlight. The films containing nanoparticles, carbon black, and gold nanorods in a polymer, inactivated pathogens even at higher flow rates. Lower light intensity reduced inactivation, but Sun still achieved significant reduction. Carbon black alone showed similar efficacy to composite films with gold nanorods. Simulations indicated a compact reactor footprint (45 cm × 45 cm) could produce 8 l of safe water per day with 8 h of sunlight exposure, highlighting scalability for community needs.

SODIS has been subject to rigorous testing, both in the laboratory and under field conditions, and has been proven to improve microbial quality of drinking water due to its biocidal effects against all waterborne pathogenic micro-organisms (Sommer et al. 1997; Wegelin & Sommer 1998; Sobsey et al. 2008; Clasen 2009). Results by McGuigan et al. (2006) have shown that cysts of Giardia muris and oocysts of Cryptosporidium parvum are rendered completely non-infective after batch SODIS exposures of 4 and 10 h, respectively. Likewise Joyce et al. (1996) reported complete disinfection of heavily contaminated water with a wild-type strain of E. coli within 7 h. There were no viable E. coli organisms detected either at the end of the experiment or 12 h later, indicating that no bacterial recovery occurred after disinfection. A study by Dessie et al. (2014) in Ethiopia also demonstrated complete and irreversible faecal coliform inactivation within 4 h of exposure in areas with ample sunshine (solar irradiance of about 3.99 kW/m² and above). Several other laboratory experiments and field studies have demonstrated solar disinfection efficacy in destroying diarrhoea-causing enteric pathogens (Wegelin et al. 1994; McGuigan et al. 1998, 2006; Smith et al. 2000; Kehoe et al. 2004; Lonnen et al. 2005; Berney et al. 2006; Méndez-Hermida et al. 2007; Boyle et al. 2008; Mtapuri-Zinyowera et al. 2009; Beattie et al. 2019; Chidya et al. 2021; Karim et al. 2021). Table 2 presents some of enteric pathogens commonly known to be inactivated by SODIS.

SODIS has proven to be a highly efficient approach for enhancing public health in areas where access to clean drinking water is limited. Various research and field trials have consistently confirmed the effectiveness of SODIS in enhancing water quality and reducing the occurrence of waterborne illnesses. Implementation of SODIS in various communities has resulted in substantial decreases in microbial contamination and notable improvements in health outcomes.

The pioneering research conducted in Kenya's Maasai community demonstrated the significant impact of SODIS on water disinfection and health outcomes (Conroy et al. 1996, 1999, 2001). In the initial trial, children who consumed water exposed to sunlight had a significantly lower risk of diarrhoeal episodes compared to a control group. A subsequent trial specifically targeting children younger than 6 years showed a sustained 16.0% risk reduction in diarrhoeal illness through SODIS. Finally, during a cholera outbreak, children younger than 6 years in households using solar disinfection had significantly lower cholera incidence compared to the control group. Although detailed information about the water quality in the control groups is not available, the results strongly indicate that SODIS played a crucial role in lowering illness rates within the community.

SODIS has achieved remarkable success globally, demonstrating its effectiveness in reducing diarrhoea and improving community health. These success stories, documented by Meierhofer (2006) and Meierhofer & Landolt (2009), provide compelling evidence of the positive impact of SODIS. For instance, in Bolivia, SODIS reduced diarrhoea incidence by over 35% among more than 200 children under 5 years old. Similarly, Uzbekistan saw a remarkable 53% reduction in diarrhoea cases among children younger than 5 years. These examples highlight the transformative power of SODIS in promoting the well-being of young children and securing a healthier future for them. In Pakistan's Rajoa and Chiniot, diarrhoea rates decreased from 26 to 13% and from 39 to 19% respectively, thanks to SODIS. In Nepal, East Lombok, and Assam, India, SODIS proved effective in reducing diarrhoea rates, benefiting over 970,000 users and preventing an estimated 2.4 million cases of diarrhoea annually in the project areas. The widespread adoption of this accessible and sustainable solution has positively impacted the lives of numerous individuals.

These findings are supported by several other successful controlled randomized trials under typical environmental and cultural conditions in different countries including Kenya, South African, Cameroon, India, and Cambodia (Table 3). As can be seen from Table 3, SODIS reduced diarrhoeal diseases by 36–75% in these countries. Apart from SODIS, other POU treatment techniques, such as chlorination, have shown almost similar diarrhoea reduction results, mostly in the range of 30–40% (Quick et al. 2002; Mengistie et al. 2013; Pickering et al. 2019; Solomon et al. 2020). However, the efficacy of boiling in reducing diarrhoea when applied in households has been poorly documented despite compelling evidence that it can completely eliminate all pathogens in drinking water. Nevertheless, according to a 2014 WHO report, effective household water treatment (taken as boiling or filtration and safe storage) has shown the greatest disease reduction of 45% (World Health Organization 2014).

While the efficacy of the SODIS method in reducing exposure to enteric pathogens and improving microbiological water quality has been manifested, the realization of the corresponding health benefits (diarrhoea reduction) can be possible only if considerable number of users accept (adopt) the technology and continue to use it correctly and consistently over long periods. Sustainability entails that people remain sufficiently motivated and committed to integrate the technique into their daily lives even long after intensive study interventions have ended (Sobsey et al. 2008). In this case, SODIS-treated water has to constitute a significantly large percentage of drinking water consumed by the people in the targeted community, and it should be consumed constantly, habitually, and without interruption to supply (McGuigan et al. 2012). Scalability is attained when the initial small-scale pilots that reached a small number of beneficiaries translate to widespread adoption of the intervention, eventually reaching millions, after implementation (World Health Organization 2007).

Usually, it is challenging to achieve continued high acceptance and long-term use for innovations that are associated with a higher user burden, high or recurrent costs, or those that demand significant behaviour change from users (Brown & Clasen 2012). In the following sections, we look at opportunities and challenges to widespread and sustained use of SODIS.

The cost and willingness to pay for HWT technologies are important considerations for their implementation (Sobsey 2002). High price is one of the most significant barriers to the adoption of any POU treatment option, and willingness to pay studies reveal that the demand for a POU system begins to decline when the expense reaches 0.3–0.4% of annual household income (Deng 2021). For instance, McGuigan et al. (2012) reported that the frequently cited reason for continued use of SODIS has been based on economic benefit first rather than improvement of health. With an estimated cost of only $0.63 per person per year (Clasen et al. 2007a; Clasen 2009), SODIS is one of the cheapest household-based interventions against waterborne disease when compared with other methods like boiling, chlorination, filtration, and flocculation, which have estimated costs of $10.56, $0.66, $3.03, and $4.95 per person per year, respectively (García-Gil et al. 2021, 2022). The method relies mainly on locally available resources (PET bottles and sunlight) and is replicable with low investment costs (Meierhofer & Wegelin 2002). Despite these low costs, the method has been demonstrated to have inactivation efficiency for bacteria in real field applications comparable to that of high-priced interventions such as chlorination and ceramic filtration (Luzi et al. 2016).

Besides being cost-effective, the application procedure of SODIS is very simple. This is important for sustainability as it is argued that people are more willing to modify their behaviours towards a technology when its application is easy to perform and materials are locally sourced (Juran & MacDonald 2014). However, this simplicity may also present a disadvantage. The effectiveness of SODIS as a treatment method is frequently questioned and met with scepticism due to its simple procedure. As a result, it is often overshadowed by well-established methods such as chlorination (Morse et al. 2020). For instance, in their study to establish factors influencing the sustained use of SODIS from a global promotion and dissemination programme, Meierhofer & Landolt (2009) reported that educated people were more difficult to being convinced that SODIS can efficiently treat drinking water. Likewise, Rai et al. (2010) faced tough opposition to convince healthy people in the urban slum regarding the effectiveness of this simple cost-effective method.

SODIS is an environmentally sound technology that does not require the use of traditional energy sources such as firewood, charcoal, and kerosene/gas (EAWAG/SANDEC 1991; Davarcioglu 2015). Consequently, deforestation, which is a major environmental problem in most developing countries, and air pollution, created by burning conventional energy sources, are reduced (Davarcioglu 2015). For instance, it is estimated that 1 kg of wood is needed to boil 1 l of water (Clasen 2009). This is not only too far beyond the reach of most poor households but also contributes to deforestation. By using SODIS, it is speculated that a household can save approximately 3,650 kg of wood per year (Dessie et al. 2014). Besides, the time and energy that would have been otherwise spent on sourcing firewood, primarily for women and girls, is committed to other productive activities (Clasen 2009). Furthermore, the use of empty plastic bottles promotes the recycling of solid waste. However, frequent replacement of plastic bottles may lead to environmental issues (EAWAG/SANDEC 1991).

SODIS is independent of additional of chemicals. As a result, it is associated with minimal change in water taste and does not introduce chemicals or cause the production of harmful disinfection by-products in the drinking water (EAWAG/SANDEC 1991; Sobsey 2002). Besides, re-contamination is very unlikely if water is consumed directly from the small, narrow-necked bottles with caps in which it was treated (Lantagne et al. 2006).

Implementing SODIS entails additional efforts, such as proper water treatment, regular bottle cleaning and replacement to prevent biofilm growth, daily management of multiple bottles, post-treatment hygiene practices, and other activities that necessitate changes in both behaviour and lifestyle for SODIS users (Sobsey 2002; Clasen et al. 2007b; McGuigan et al. 2012). In this case, the target population is expected to be more willing to modify their routine behaviours and adopt new ones as demanded by the technology. Unfortunately, people seldom change and sustain new behaviours as intended (Figueroa & Kincaid 2010). This, coupled with factors such as cultural beliefs, lack of awareness, and competing priorities, contributes to inconsistent adherence and compliance among users. For example, individuals may be reluctant to adopt SODIS due to scepticism or cultural practices that favour alternative water treatment methods. In other cases, people may forget or neglect to expose the bottles to sunlight for the recommended time, compromising the effectiveness of the disinfection process. In addition, Figueroa & Kincaid (2010) pointed out the lack of clear and compelling benefits to convince the users to continue with the new behaviour. For example, McGuigan et al. (2012) noted the difficulties in persuading individuals who have grown accustomed to strong sunlight throughout their lives and have never considered the possibility that sunshine could disinfect water. To enhance the effectiveness of SODIS, it is suggested that the fundamental technical component be complemented with intensified efforts to promote behaviour change and motivation (Sobsey 2002). Meierhofer (2006) asserts that comprehensive community education, sensitization, and advocacy campaigns play a vital role in establishing SODIS at the grassroots level.

While the operation of SODIS requires simple steps, the system is suitable for treating small volumes of water and exposure to sunlight is typically carried out in small-volume plastic beverage bottles (Borde et al. 2016). Consequently, the safe drinking water demand is not always met unless a number of bottles are put to use at the same time. This can create a labour-intensive process, especially considering the considerable time required for water treatment, leading to decreased user acceptability (Meierhofer & Wegelin 2002; Lantagne et al. 2006; Borde et al. 2016). Nevertheless, as earlier discussed, some enhancements may be used to reduce treatment time. Besides, other studies have demonstrated the feasibility of increasing the volume PET containers. An experiment conducted in Spain by Keogh et al. (2015) confirmed that under conditions of strong sunlight and mild temperature, 19-l water dispenser containers provide bacterial inactivation similar to 2-l PET bottles. Results of a recent study conducted in four rural communities in the Tigray Region in Northern Ethiopia reported a similar decrease in cases of diarrhoeal diseases in children in the implementation (25 l PET TJC) and control (2 l PET bottles) groups (García-Gil et al. 2022). In another study, Polo-López et al. (2019) observed similar inactivation kinetics for E. coli, MS2-phage, and C. parvum using 5- and 20-l transparent polypropylene buckets fabricated locally at a low cost in Malawi. The larger 20-l bucket size resulted in a 10-fold expansion of the treatment batch volume, making it simpler to fulfil the drinking water needs of most households. These findings indicate that alternative container sizes and materials can offer potential solutions to the limitations of small bottle-based SODIS systems. Implementing larger containers or buckets may increase the water treatment capacity and efficiency, potentially enhancing the practicality and acceptability of SODIS in various settings.

The study further reported that UV transmission of the 20-l buckets remained stable and with physical integrity even after the longest ageing periods (9 months or 900 h of natural or artificial solar UV exposure, respectively). They also observed that the lesser thickness of the 5-l bucket resulted in easy physical degradation and loss of significant UV transmission than the 20-l bucket. However, the reason behind the 5-l bucket's greater loss of UV transmission compared to the thicker 20-l one is not clearly understood. This is surprising because thicker materials generally have a higher probability of scattering light compared to thinner materials. The increased thickness provides more opportunities for light to interact with the particles or structures within the material, increasing the likelihood of scattering events as the light passes through. Therefore, further investigation is needed to understand why the thinner bucket exhibited a higher loss of UV transmission despite the expected scattering behaviour associated with thicker materials.

Local availability of the sufficient number of the required bottles is crucial for consistent and widespread long-term use of SODIS application (Meierhofer 2006). In many areas, inaccessibility of PET bottles happens to be the limiting factor for the continued SODIS use. For instance, Murinda & Kraemer (2008) found out that unavailability of plastic PET bottles was the most important potential hindrance to the successful implementation of SODIS in Zimbabwe. In areas where plastic bottles are not easily accessible, it becomes necessary to establish a supply system to acquire and transport used plastic bottles from urban areas to rural communities (Meierhofer 2006). To address the scarcity of PET bottles in certain areas, Meierhofer & Landolt (2009) suggested the establishment of micro-enterprises. They highlighted the successful implementation of a bottle supply system in East Lombok, where the local health system initiated a program to purchase empty bottles from a PET-bottle producer for distribution to users through health posts. Currently, commercially produced PET bottles specifically designed for water disinfection purposes are scarce, prompting the use of standard PET bottles originally intended for other applications.

The inability of poorly educated villagers to comprehend the germ theory of disease also makes it difficult for them to appreciate the need to adopt water treatment practices (Gilman & Skillicorn 1985). As pointed out by Meierhofer (2006), it is difficult to create an understanding among illiterate people on the relation between the consumption of contaminated drinking water, hygiene practices, and the dangers of invisible pathogens on human health. The concept of invisible micro-organisms causing diseases is abstract and difficult to grasp without basic education on germ theory. According to Figueroa & Kincaid (2010), water treatment is not always perceived to have health benefits by many people, and diarrhoea is not always seen as a significant health threat. Rather, it is widely viewed as part of growing up and may even be seen as beneficial for children (Figueroa & Kincaid 2010). The observations made by Murinda & Kraemer (2008) in Zimbabwe further support the notion of limited knowledge about bacterial contamination of water as a major challenge for the implementation of SODIS. In their study, they reported that limited knowledge about bacterial contamination of water was one of the major challenges to the implementation of SODIS in the country.

Generally, the lack of awareness about the presence of harmful bacteria in water hinders the recognition of the need for water treatment methods like SODIS. Without a clear understanding of the risks associated with bacterial contamination, it becomes difficult to motivate individuals to adopt and sustain water treatment practices. Addressing this issue requires comprehensive education and awareness campaigns. These campaigns should aim to bridge the knowledge gap by providing accessible information about the connection between contaminated water, hygiene practices, and the impact of invisible pathogens on human health. By promoting understanding and raising awareness, it may become possible to overcome the barriers posed by limited knowledge on the germ theory of disease and foster the adoption of effective water treatment practices like SODIS.

While reusing plastic bottles over time makes SODIS cost-effective and a plus in solid waste recycling, a review in literature by Borde et al. (2016) revealed concerns about leaching of chemicals such as antimony and phthalates from plastic containers (with PET) into the food or water they contain. It is argued that exposing plastics made from PET to high temperatures results in leaching of some toxic additives such as acetaldehyde, antimony, and phthalates (Alabi et al. 2019). However, contrary to these fears, conclusions from studies by Wegelin et al. (2001); Schmid et al. (2008) and Ubomba-Jaswa et al. (2010) have refuted the migration of hazardous chemicals in critical levels into SODIS water.

Nevertheless, there are alternatives to plastic bottles for SODIS applications. For example, transparent pouches or bags made of UV-transmitting materials have been explored as alternative reactors. The study by Gutiérrez-Alfaro et al. (2017) revealed that certain types of transparent plastic bags, such as those made from polyethylene (PE), can yield better results compared to PET bottles traditionally used in SODIS. It was further found that PE bags demonstrated faster disinfection, achieving a 6-log reduction within 60 min. Other types of bags like bio-oriented polypropylene, polyamide, and PET also outperformed PET bottles in terms of disinfection efficacy. The orientation of the bags or bottles (vertical or horizontal) did not significantly impact the disinfection results. In addition, according to Wegelin & Sommer (1998), the use of flat plastic bags with a volume ranging from 2 to 6 cm was found to offer an improved area-to-volume ratio and higher water temperatures compared to traditional round bottles. Furthermore, glass reactors equipped with compound parabolic collectors have also been found to be effective and can accommodate larger volumes (García-Gil et al. 2019). Transparent 20L PE containers can also serve as durable and long-lasting alternatives to standard plastic bottles, reducing the need for frequent replacements (Polo-López et al. 2019).

A general overview of the effectiveness of the SODIS technique in both improving microbiological water quality and reduction of infectious diarrhoeal diseases was examined. The efficacy of SODIS in inactivating enteric pathogens and improving microbial water quality and reducing diarrhoeal disease incidences has been demonstrated in various laboratory works and field trials. Capitalizing mainly on locally available resources, it has been shown that SODIS has managed to reduce diarrhoeal disease incidences, as much as above 75%, in some developing countries. The synergistic effect that results as a consequence of the combined effect of UV light and rise in water temperature has been demonstrated to eliminate 99.9% of many enteric micro-organisms.

However, certain pathogens, particularly those with protective structures such as cysts, may be less susceptible to solar disinfection, requiring alternative approaches or higher UV doses for effective inactivation. Moreover, it is essential to carefully consider issues such as extended exposure time, pathogen re-growth, and resistance during the application of SODIS. To overcome these limitations, several innovative approaches have been proposed to enhance the effectiveness of SODIS and reduce the required exposure time. Technologies like TiO₂ photocatalysis and nanomaterial-enabled solar photothermal disinfection have shown promising results in terms of improved inactivation efficiency and prevention of pathogen re-growth. However, integrating these technologies with SODIS requires specialized equipment and materials, which negates the original concept of SODIS as a low-cost and simple water disinfection method. Utilizing inexpensive and sustainable materials would be a desirable option for scalability and sustainability. Besides, research efforts should focus on developing cost-effective and scalable production methods for catalysts and nanomaterials, making them more accessible to communities with limited resources. This may involve investigating alternative materials or modifying existing ones to enhance affordability and availability.

Comprehensive studies should also be conducted to assess the performance and limitations of the integrated system under different environmental conditions and water sources. Establishing practical guidelines for users is crucial to ensure optimal operation and reliable disinfection outcomes. Collaboration among researchers, engineers, and policymakers is vital to gather data, exchange knowledge, and develop standardized protocols for implementing the integrated approach. Moreover, raising public awareness and providing education about the integrated approach is essential. Communities should be informed about the advantages, limitations, and proper usage of the technology. Training programs and educational materials should be provided to empower individuals and communities to effectively adopt and utilize the integrated system, maximizing its benefits and ensuring long-term sustainability.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.