Perspective Chapter: Solar Disinfection – Managing Waterborne Salmonella Outbreaks in Resource-Poor Communities

Abstract

Salmonella outbreaks remain a significant problem in many resource-poor communities globally, especially in low and middle-income countries (LMICs). These communities cannot reliably access treated piped water, thus reverting to the use of environmental water for domestic and agricultural purposes. In most LMICs, the maintenance and expansion of the existing wastewater and water treatment infrastructure to meet the growing population are not considered. This results in regular wastewater and water treatment failures causing an increase in an assortment of waterborne pathogens, including Salmonella. Solving these problems would require the maintenance, expansion and construction of new wastewater and water treatment infrastructure. The implementation of such interventions would only occur over a long period. Unfortunately, time is not a luxury in communities experiencing the effects of such problems. However, highly disruptive household interventions such as solar disinfection (SODIS) could be implemented in communities experiencing endemic Salmonella outbreaks. SODIS has been shown to inactivate a variety of water-related pathogens. SODIS requires significantly less financial input to implement in comparison to other household-level interventions. Various studies have shown better health outcomes due to SODIS in communities that previously struggled with waterborne diseases, including Salmonella. The aim of this chapter is to share a perspective on the continued reliance on SODIS as for the control waterborne Salmonella in LMICs.

1. Introduction

The genus Salmonella consists of two species with over 2500 serovars. The serovars within the species Salmonella enterica are classified as either typhoidal or non-typhoidal. Although genetically similar these serovars elicit significantly different diseases. Typhoidal Salmonella serovars such as Typhi and Paratyphi A are human restricted and cause an invasive systemic typhoid fever that is life threating in both healthy and immune compromised individuals [1]. Non typhoidal Salmonella (NTS) serovars such as Typhimurium and Enteritidis cause self-limiting gastroenteritis in either humans or animals. The gastroenteritis caused by NTS is often mild in healthy adults but severe in immune compromised individuals [2, 3].

S. enterica infections primarily those associated with serovars Typhimurium and Enteritidis [4, 5] remain a global burden especially in low and middle income countries in Africa and Asia affecting more than 93 million people and causing the deaths of over 1.2 million people globally [6, 7]. Most of these infections and deaths occurred in people living in resource-poor communities, especially those in Low- and Middle-Income Countries (LMICs) in Africa [4, 8]. This could be attributed to the high prevalence of malnutrition and immune compromising diseases such as malaria and AIDS. Infections due to Salmonella are not exclusive to LMICs. According to the Centre for Disease Control (CDC), more than 1 million people in the United States of America (USA) experience a Salmonella infection. This costs the USA more than $ 3.7 billion US in medical costs [9]. Recently, the World Health Organisation (WHO) was alerted to a Salmonella outbreak associated with European food products for the European and Global markets [10]. The CDC estimates that 46% of foodborne diseases and 23% of deaths are linked to produce consumption [9]. In High-Income Countries (HIC), there is a more likelihood of acquiring a Salmonella infection through the consumption of food products as opposed to water [11, 12].

This chapter highlights the role played by water in the transmission of Salmonella especially in resource poor LMICs. Thereafter, an overview of how water and sanitation infrastructure is prioritised in Africa is provided. This is followed by an evaluation of water treatment approaches that could be used at a household level to reduce the burden of Salmonella. The chapter ends by providing reasons why SODIS is an ideal water treatment intervention at a household level.

2. Water and Salmonella infections

Environmental water resources play a critical role in food crop produce linked to Salmonella infections occurring in HICs [12, 13]. Environmental water bodies can harbour Salmonella for several months [13, 14]. This makes Salmonella a waterborne pathogen that could be introduced into a susceptible animal host when untreated environmental water is consumed or used for domestic and agricultural purposes. Previously, Salmonella infections were mainly associated with consuming contaminated animal products. However, in recent years, Salmonella outbreaks associated with consuming contaminated food crops such as fresh fruits, vegetables, spices, and nuts have increased [15, 16]. This is probably driven by the increased adoption of a vegetarian or vegan lifestyle [17]. The presence of Salmonella on food crops has been attributed to the microbiological quality of water used for irrigation [12, 16].

Water remains a key factor in the transmission of S. enterica in LMICs [14] and HICs [12, 13, 18]. Access to clean water is a fundamental human right. However, many resource-poor communities worldwide, especially LMICs, struggle to access clean water [14]. Currently, more than 2 billion people lack access to safely managed water, of which more than 700 million live without basic drinking water. Most of these live in Africa [19]. The current paradigm of Salmonella infection places poor sanitary habits and practices as critical contributors toward the reintroduction of S. enterica into the environmental water resources. Although an increase in global sanitation has been reported, more than 3.6 billion people lack access to well-managed sanitation, of which 1.7 billion still lack basic sanitation [19]. Therefore, people living in resource-poor communities in LMICs contract Salmonella infections by consuming contaminated food and water [14]. But, if these communities had access to treated water, a reduction in waterborne Salmonella would occur as observed in HICs.

Furthermore, practicing proper sanitation and hygiene in tandem with the availability of treated water would reduce the prevalence of S. enterica in the environmental water resources. This would improve the microbiological quality of natural water resources for agricultural purposes. Providing resource-poor communities with clean water would require establishing effective sanitary and water treatment infrastructure.

3. Investment in water and sanitation infrastructure with focus on Africa

Sanitary and water treatment infrastructure availability is a major driver of economic development because it curbs health risks, enables education and other productive activities, and enhances the labour force’s productivity [20, 21]. For instance, the lack of proper sanitation in South Asia results in financial and economic losses of up to 2 and 9 billion dollars, respectively, while adequate sanitation infrastructure in France enables tourism and sustains the jobs of more than 2000 people in the tourism sector [20].

The African continent consists of 53 member states with a combined population of more than 1.4 billion people [22]. Currently, the African continent has the highest population below the age of 15 [23]. By 2050 Africa will be home to more than half of the world’s population, and 1.3 billion people will live in urbanised areas [24, 25]. About 56% of people living in urban areas in Africa have access to piped water compared to 67% in 2003, and just 11% can access a sewer connection [26]. This observation implies that the current infrastructure cannot support the increasing population and hence threatens social stability and may act as a driver of migration within and out of Africa [26]. Given the growing population, it is logical to prioritise the expansion of existing as well as the construction of new sanitary and water treatment infrastructure in Africa.

However, this is not reflected in the African infrastructural commitments. In 2017 the transport infrastructure sector received the highest commitment ($ 34 billion, 41.7% of the total obligations) in comparison to the water infrastructure sector ($ 13.2 billion, 16.2% of the total commitments) [27]. In the same year, the funding gap for transport infrastructure (8%) was lower than that of the water infrastructure (84%) sector [27]. Previous reports showed that the transport and water sectorial infrastructural commitments had increased by 30 and 8% between 2016 and 2017 [27]. Nevertheless, African states’ water and sanitation infrastructure financing declined by 3% between 2016 and 2017 [27]. During that period, foreign aid commitments were made to finance water and sanitation infrastructural projects in many Low-Income Countries (LIC) in Africa. For instance, Italy committed $ 69 million to a Mozambique water and sanitation project. China committed $ 1.5 billion to construct the Gerbi Dam in Ethiopia to provide water to Addis Ababa [27].

Investment in sanitation and water treatment infrastructure should be prioritised because water remains a critical link between agriculture and energy. Therefore, robust sanitation and water treatment infrastructure provides and supports opportunities in the agriculture, manufacturing and energy sectors but to mention a few [21]. The African agriculture sector offers and supports the highest number of jobs compared to any other sector [17]. Therefore, there is a need to assess and invest in agricultural water needs. The link between Salmonella infections and crop produce in HIC has been established [12]. This may make the export of African crop produce challenging based on the quality of water used for agricultural purposes. This justifies prioritising wastewater and sanitation infrastructure because of their positive impact on the quality of water used for agricultural purposes. Improving the quality of water used for agricultural purposes will enable the export of better-quality produce.

Investment in sanitation and water treatment infrastructure offers social and economic benefits. But why is it that African governments do not prioritise such critical infrastructure? Water infrastructure financing would require loans and hence a well-managed system of offering a paid water service [26]. Currently, the provision of paid water services remains a challenge. Thus, the servicing of water, wastewater, and sanitation infrastructure loans is associated with a high financial risk to the lender. Perhaps this is one of the reasons why social impact research and interventions have focused on point-of-use systems to ensure the availability of treated water for consumption at a household level. The only challenge with this approach is that the sanitation aspect may not receive the attention it deserves.

4. Water treatment at the household level

The chronic lack of sanitation and water treatment infrastructure in resource-poor communities, especially those in LMICs, makes the people living in these communities vulnerable to Salmonella infections. Point of Use (POU) water treatment systems have been suggested as a short to mid-term intervention to protect human health. The currently available POU water treatment systems work based on coagulation-flocculation, filtration, and disinfection [28].

4.1 Coagulation: Flocculation

Coagulation – flocculation-based systems offer a reliable, low, energy means of reducing the particulate matter in water, leaving it clearer than before. This approach would require using coagulants such as aluminium sulphate, lime, polyelectrolyte and iron salts (ferric chloride and ferric sulphate). Also, biopolymers, especially natural gums and bio-flocculants, have been investigated for their ability to serve as effective coagulants and flocculants [29]. However, the coagulation-flocculation treatments reduce turbidity the offer the added advantage of reducing the microbial burden of turbid water [29, 30]. Flocculation follows the addition of a coagulant. The flocculation process is often facilitated by gentle mixing to enable the formation of flocs. Mixing increases the collisions and interaction between the flocs and coagulant, thus increasing the size of the flocs resulting in them settling at the bottom by sedimentation. Coagulation – flocculation can be accessed at the POU using either the PUR or Poly Glu sachet manufactured by Procter & Gamble Co or Poly Glu International Co, respectively [28]. Both PUR and Poly are accessible worldwide. Nonetheless, extra measures are needed to ensure that these approaches are supplemented with either a disinfection method (PUR sachet) or proper hygiene handling of treated water to avoid recontamination (Poly Glu sachet) [28]. It should be noted that coagulation-flocculation-based POU solutions are often single-use and hence may be costly for some communities in the long run.

4.2 Filtration

Filtration systems offer a simple means of removing colloids, suspended solids, and microorganisms from water. Size exclusion is the basic principle behind the filtration process. As such, a properly configured filtration system can remove not only Salmonella or related bacteria from water but also viruses, toxins, and chemicals. This depends on the size of the pores on the filter membrane or biosand configuration. Membrane filtration systems used at the POU would ideally require one to consider the quality of the influent water and an external driving force relative to the membrane’s pore size. These filters require maintenance because, with time, a foulant layer forms on them. This makes membrane-based filtration systems an option for communities that may have access to piped water that is not sufficiently treated. But inaccessible to those people in communities with no access to piped water.

Furthermore, membrane filtration is costly to maintain and would require technical skills to do so [28]. Sand-based filtration systems offer a more viable solution for those living in communities without access to treated piped water. Sand filters are easy to manufacture because the required raw materials are readily available. More than 500,000 people worldwide use biosand filters to meet their needs for potable water [28]. Biosand filters have been shown to reduce the turbidity of water. They have also been reported to reduce microbial contaminants but not to the level that meets the WHO water guidelines. Although the material to make biosand filters is easily accessible, the manufacturing process requires some technical skills. For instance, a correct balance between the flow rate and retention time is needed during manufacturing. These two variables have an inversely proportional relationship that influences the effectiveness of the removal of microbial contaminants such as Salmonella [28]. Also, the filter’s depth needs to be considered to remove viruses.

4.3 Disinfection

Disinfection is an approach to enable the availability of safe water that relies on the destruction of the water contaminating microorganisms. Currently, two methods have been used to achieve disinfection: nanotechnology and Solar Disinfection (SODIS). Nanotechnology-based POU water treatment approach uses either titanium dioxide (TiO₂) or silver (Ag) nanoparticles for disinfection. The TiO₂ method requires a source of UV–vis light which facilitates the generation of hydroxyl radicals and hydrogen peroxide [31] that oxidise the organics, thus inactivating the microorganisms. TiO₂ is not depleted during this process, so the reaction continues. TiO₂ has been used to reduce biofouling on membranes used for water treatment and enhance the SODIS process [32]. TiO₂ has been used to develop a POU product, the Solarbag produced by Paralytics.

Ag nanoparticles are toxic to microorganisms. They bring about the death of microorganisms by either permeabilising the cell membranes or bioaccumulating causing irreversible damage to the DNA [33]. Ag nanoparticles have been used to coat ceramic [34] and polyurethane filters [35] to improve microbial log reduction. Currently, Ag nanoparticles are used in POU products such as Tata Swach and Folia filters to disinfect water. Although TiO₂ and Ag nanoparticles improve water microbiological quality, the long-term effects of these nanoparticles are not understood. At elevated levels, these nanoparticles are harmful to aquatic life [36, 37].

Furthermore, a high concentration of Ag nanoparticles has been shown to reduce mammalian cell vitality and mitochondrial function and cause cell membrane leakages [38]. This means that the use of nanoparticles to improve the quality of the water needs to be supplemented with proper disposal of damaged, unusable systems and accumulated waste. Besides the use of nanoparticles, natural sunlight could be used to sterilise the water before its consumption.

5. Solar disinfection of water

SODIS of water is an affordable and easy method of treating microbiologically contaminated water before its consumption. As such this section will focus on SODIS as opposed to the other approaches. During SODIS, microbiologically contaminated water in a transparent clear vessel is exposed to natural sunlight for approximately 6 hours on a sunny day with clear skies and on two rainy days overcast days. A detailed manual on the application of SODIS in the field has been developed and is accessible via the Swiss Federal Institute of Aquatic Science and Technology [39].

Effective bacterial inactivation is judged by the inability of the microorganisms to form colonies after SODIS treatment [40]. Downes and Blunt [41] were the first to present empirical evidence of the bactericidal effect of sunlight; however, its use to sanitise water can be traced as far back as 2000 BC. Presently, Downes and Blunts [41] observations on the bactericidal effect of natural sunshine are continuously confirmed by various research teams with consequent successful application in many countries globally. Studies by Acra et al. [42] and Conroy et al. [43] hypothesise that the observed bactericidal effect following sunshine exposure is due to the ultraviolet component of sunlight.

The harmful effects on the microbial population during SODIS are due to solar ultraviolet radiation (SUVR), which comprises wavelengths shorter than 400 nm. Natural sunlight reaching the earth’s surface contains 6% of SUVR [44]. The UV wavelength is subdivided into three wavebands categorised as UVA (400–320 nm), UVB (320–280 nm) and UVC (280–100 nm) [45]. Of these three wavebands, UVA is the most abundant (95%) form of SUVR reaching the earth’s surface, followed by UVB; UVC rarely reaches the earth’s surface because the stratospheric ozone layer absorbs it. The amount of Solar Ultra-Violet Radiation (SUVR) reaching a given location on the earth’s surface is influenced by geographical, meteorological and temporal factors such as the latitude, elevation, cloud cover, atmospheric conditions and ground reflection [45, 46]. The closer the exposure point is to the equator, the higher the levels of SUVR [46, 47]. However, due to the sun’s elevation in the sky, 75% of the daily SUVR is received between 0900 and 1500H, irrespective of the exposure point [45].

5.1 Factors influencing solar disinfection of water

Although SODIS may seem like an ideal means of sanitising microbiologically contaminated water, it is influenced by several factors. One key factor to consider when using SODIS is the weather conditions. Cloud cover affects the amount of SUVR received on the earth’s surface. It has been observed and reported that the amount of SUVR reaching the earth’s surface is less when the sky is cloudy than in a cloudless sky. However, the enhancement of SODIS technology by incorporating compound parabolic concentrators could provide efficient inactivation within a short time during cloudy days [48]. In the absence of SUVR enhancers, it is advisable to establish guidelines on the duration required to achieve the necessary solar radiation intensity (500 W/m²) [49].

Besides the weather conditions, water turbidity has a significant influence on SODIS. Turbid waters have been shown to reduce the efficacy of the SODIS process [50, 51] and thus protect microbes from inactivation. According to the recommendation by EAWAG, water turbidity higher than 30 Nephelometric turbidity units needs to be pretreated before SODIS treatment [52]. This could be achieved through filtration or simple settling. Turbidity can also be reduced by flocculation using minerals like Alum (potassium sulfate) and seeds of plants like Moringa oleifera. The ability of both these flocculants to clarify water before SODIS treatment has been tested and shown promising results [53]. However, consideration must be given to the fact that adding any form of pre-treatment step elongates the overall time required for disinfection and may have cost implications.

The amount of oxygen present in the water before SODIS significantly influences the outcome. Oxygen plays a key role in forming highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) during solar irradiation. These reactive molecules react with cell structures and kill pathogens [54]. SODIS is more effective in water containing high oxygen levels [52]. Therefore, the guidelines recommend vigorously hand-shaking the vessel to dissolve oxygen in the water [52].

The material from which the vessel to be solar irradiated is made significantly influences the outcome of SODIS. Different types of transparent plastic materials made from either polyethene terephthalate (PET) or polyvinylchloride (PVC) are good transmitters of light in the UV-A and visible range of the solar spectrum [55, 56]. Transparent clear bottles such as empty soda and water bottles made from PET and PVC could be used for SODIS. There have been some concerns regarding the leaching of chemicals from the plastic bottles used for SODIS, but this threat is negligible [57, 58].

The temperature has been reported synergies with SUVR to enhance the SODIS water process [50]. Giannakis et al. [59] showed that SODIS carried at temperatures between 50 and 60°C increased inactivation efficiency. Several approaches to enhance the thermal rate of microbial inactivation have been investigated, and these include (i) circulating water over a black surface in an enclosed casing that was transparent to UV-A light [60], (ii) painting sections of the bottles with black paint, and (iii) using a solar collector attached to a double glass envelope container [61]—increasing the temperature past the optimum growth temperature results in the destabilisation of the core structures of most proteins through denaturation. Denatured proteins cannot carry out their critical biological tasks, and as a result, the death of the affected microorganism may result. The increase in the water temperature has been attributed to infrared radiation from the sun.

5.2 The effect of SUVR on biological systems

UV’s bactericidal effect involves thermal and optical processes [62]. Exposure of biological systems to SUVR results in wavelength-dependent outcomes [47, 63]. The observed physical effects are based on the absorbing molecules’ action spectrum [47]. An action spectrum can be defined as a plot showing the relative effectiveness of radiations of different wavelengths to produce a given biological effect [47]. Therefore, the action spectrum leading to the formation of a particular photoproduct would be similar to the absorption spectrum of the molecules responsible for forming that photoproduct [47]. The damaging effects of SUVR on microorganisms are demonstrated by reduced exoenzymatic activity that often results in reduced DNA and protein synthesis, reduced amino acid uptake, reduced oxygen consumption and a decrease in bacterial abundance [64, 65]. Other biological entities, such as biofilms, greatly reduce the amount of SUVR absorption [66].

SUVR enables the formation of reactive oxygen species such as superoxide radicals, hydroxyl radicals, hydrogen peroxide and singlet oxygen. These reactive molecules, also known as photosensitisers, are formed through a process known as photo-oxidation [66, 67, 68, 69]. During SODIS, the interaction between the photosensitisers and the actively growing microorganism results in irreversible damage to the microbial catalase systems rendering them susceptible to damage from peroxide formation [64, 70]. Furthermore, UVA, through photo-oxidation, blocks the electron transport chain (responsible for energy production), induces damage to the cell membrane, thus inactivating transport systems, and interferes with metabolic energy production, causing single-strand breaks in DNA [65, 71, 72]. Overall, UVA confers indirect multi-target damage to the microbial cellular components such as DNA, protein and lipids through the formation of photosensitisers [63].

Even though SUVR-exposed biological systems result in reduced functionality and destruction, protective cellular mechanisms are capable of reversing some of this damage. Several DNA repair mechanisms relevant to SUVR damage have been established, including photo reactivation repair, nucleotide excision repair (NER), post replication repair and SOS repair [47, 63, 73]. But these all depend on the dose of SUVR [53] and the exposure environment.

5.3 Solar disinfection of water an ideal POU

The efficacy of SODIS to inactivate a variety of pathogens such as Vibrio cholera [74], Salmonella Typhimurium [40], and Shigella dysentriae [40] has been demonstrated by various research teams. Millions of people in more than 50 countries, especially resource-poor communities, rely on SODIS-treated water [75, 76]. Input costs for low volume (< 5 litres) vessels are less than the other alternative approaches discussed in Section 4 above. Communities scale the process through the exposure of multiple vessels. Containers that can disinfect more than 5 litres significantly would require financial input. The Sustainable Sanitation and Water Management (SSWM) toolbox [44] offers a one stop hub for knowledge on SODIS where the SODIS manual [39] can also be accessed. The SSWM toolbox is an invaluable resource for organisations promoting access to clean water through the adoption of low-cost technologies such as SODIS.

6. Conclusion

Salmonella remains a critical pathogen of concern globally. This pathogen is responsible for the deaths of many children below the age of 5 and the fragile and elderly. Overcoming infections due to Salmonella would require that sanitary and water treatment infrastructure is prioritised, especially in LMIC. Resource-poor communities without access to sanitary or water treatment infrastructure could use a combination of coagulation-flocculation, filtration, and disinfection methods to access treated water at the POU. However, these methods do not address the frequent reintroduction of pathogens such as Salmonella into environmental waters. This requires the adoption of sanitary measures at a household level. The water treatment at the POU may reduce the burden of Salmonella transmitted through the consumption of contaminated water. However, Salmonella can be transmitted through the consumption of food of either animal or crop origin. Therefore, it is important to consider using some of these methods to treat agricultural water before its use. High water capacity SODIS interventions should be developed and evaluated for the provision of water for agricultural purposes. Perhaps this would require the combination of SODIS with other low-cost treatment approaches such as coagulation-flocculation.

Acknowledgments

I want to acknowledge the financial support provided by the Vaal University of Technology to support SODIS research.

Conflict of interest

The author declares no conflict of interest.