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Material selection and prediction of solar irradiance in plastic
devices for application of solar water disinfection (SODIS) to
inactivate viruses, bacteria and protozoa
Ángela García-Gil, Cristina Pablos, Rafael A. García-Muñoz,
Kevin G. McGuigan, Javier Marugán
PII: S0048-9697(20)32643-7
DOI: https://doi.org/10.1016/j.scitotenv.2020.139126
Reference: STOTEN 139126
To appear in: Science of the Total Environment
Received date: 12 February 2020
Revised date: 10 April 2020
Accepted date: 28 April 2020
Please cite this article as: Á. García-Gil, C. Pablos, R.A. García-Muñoz, et al., Material
selection and prediction of solar irradiance in plastic devices for application of solar water
disinfection (SODIS) to inactivate viruses, bacteria and protozoa, Science of the Total
Environment (2020), https://doi.org/10.1016/j.scitotenv.2020.139126
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MATERIAL SELECTION AND PREDICTION OF SOLAR IRRADIANCE IN PLASTIC
DEVICES FOR APPLICATION OF SOLAR WATER DISINFECTION (SODIS) TO
INACTIVATE VIRUSES, BACTERIA AND PROTOZOA
Ángela García-Gil1, Cristina Pablos1, Rafael A. García-Muñoz1, Kevin G. McGuigan2, Javier
Marugán1*
1 Department of Chemical and Environmental Technology (ESCET), Universidad Rey Juan Carlos,
C/ Tulipán s/n, 28933 Móstoles, Madrid, Spain.
2 Department of Physiology & Medical Physics, Royal College of Surgeons in Ireland (RCSI), Dublin 2, Ireland.
*Corresponding author email: javier.marugan@urjc.es
Abstract
Solar Water Disinfection (SODIS) is a simple, inexpensive and sustainable Household Water
Treatment (HWT) that is appropriate for low-income countries or emergency situations. Usually,
SODIS involves solar exposure of water contained in transparent polyethylene terephthalate (PET)
bottles for a minimum of 6h. Sunlight, especially UVB radiation, has been demonstrated to
photoinactivate bacteria, viruses and protozoa. In this work, an in-depth study of the optical and
mechanical properties, weathering and production prices of polymeric materials has been carried
out to identify potential candidate plastic materials for manufacturing SODIS device. Three
materials were ruled out (polystyrene (PS), polyvinyl chloride (PVC) and polyethylene (PE)) and four
materials were initially selected for study: polymethylmethacrylate (PMMA), polypropylene (PP),
polycarbonate (PC) and polyethylene terephthalate (PET). These plastics transmit sufficient solar
radiation to kill waterborne pathogens with production costs compensated by their durability under
solar exposure.
A predictive model has been developed to quantitatively estimate the radiation available for SODIS
inside the device as a function of the material and thickness. This tool has two applications: to
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evaluate design parameters such as thickness, and to estimate experimental requirements such as
solar exposure time. In this work, this model evaluated scenarios involving different plastic
materials, device thicknesses, and pathogens (Escherichia coli bacterium, MS2 virus and
Cryptosporidium parvum protozoon). The developed Solar UV Calculator model is freely available
and can be also applied to other customized materials and conditions.
Keywords
Mechanical properties, optical properties, UV Calculator, Photoinactivation, Household Water
Treatments, Sustainable Development Goals.
1. Introduction
In 2015, the General Assembly of United Nations announced the Sustainable Development Goals
(SDGs) to “end poverty in all its forms” (United Nations (UN) 2015). That same year, there were
almost 900 million people without access to safe drinking water (United Nations (UN) 2018) which
is one of the priority themes of the SDGs. SDG 6 “Clean water and sanitation” aims to “ensure
availability and sustainable management of water for all”. Provision of drinking water is the first
challenge that must be approached and is considered the first target of the goal titled “Achieve
access to safe and affordable drinking water” . Most people without access to safe drinking water
live in low-income countries with few financial resources, thus, they require low-cost, sustainable
and simple systems for Household Water Treatment (HWT). Moreover, these countries are located
in regions where solar irradiance levels are very high throughout the year. Therefore, a suitable
HWT alternative for them is the use of solar water disinfection (SODIS), commonly carried out in 1-
2L polyethylene terephthalate (PET) bottles exposed to solar radiation for a minimum of 6 h
(McGuigan et al. 2012). This process is based on the harmful effect of UV radiation and its synergy
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with temperature on pathogens (Joyce et al. 1996; McGuigan et al. 1998; Romero et al. 2011;
Gómez-Couso et al. 2012; Castro-Alférez et al. 2017).
SODIS photoinactivation processes can follow direct and indirect mechanisms and are strongly
affected by the different UV spectral ranges and specific microbiological features of the pathogens
(bacteria, viruses or protozoa).
i) Direct photoinactivation is an endogenous process that takes place when a constituent of the
microorganism itself (e.g. nucleic acids, proteins or other macromolecules) absorbs photons,
inducing changes to their chemical structure. For example, nucleic acids absorb radiation of
wavelengths below 320 nm. As a result, many viruses can be photoinactivated with the UVB
component of the solar spectrum (Lytle and Sagripanti 2005; Mattle et al. 2015; Nelson et al. 2018).
Similarly, Cryptosporidium parvum protozoon has been also proved to be inactivated under solar
light, predominantly by the direct genome damage produced by the UVB radiation (Liu et al. 2015;
Nelson et al. 2018). Bacterial pathogens are also sensitive to direct photoinactivation by solar UVB
radiation (Jagger 1985).
ii) Indirect photoinactivation occurs when a photo-produced reactive intermediate (PPRI) (e.g.
hydrogen peroxide, hydroxyl radical, singlet oxygen, carbonate radical and superoxide) damages
microorganism components. Endogenous indirect inactivation takes place when PPRI are generated
from internal sensitizers. For viruses, insufficient generation of PPRI produced by a few internal
sensitizers makes endogenous indirect damage negligible and slower than direct damage (Love et
al. 2010). When only UVA and visible radiation is available, indirect damage produced by internal
sensitizers can slowly damage protozoon components (Liu et al. 2015; Nelson et al. 2018). Several
studies confirm the importance of indirect endogenous mechanisms for bacteria when internal
sensitizers (catalase, alkyl hydroperoxide reductase (Ahp) and peroxidase enzymes) are illuminated
with wavelengths across the solar UV (UVA and UVB radiation) and visible ranges, promoting the
formation of PPRIs such as reactive oxygen species (ROS) (Nelson et al. 2018). For example, in
Escherichia coli endogenous damage decreases as wavelength increases, with negligible effects for
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wavelengths above 400 nm. In contrast, Enterococcus faecalis extends its susceptibility up to 500
nm (Silverman and Nelson 2016). In such cases, direct and indirect endogenous damage can
interact and, for this reason, both are often studied simultaneously.
If the sensitizer is external, then the indirect photoinactivation is exogenous. This mechanism is
only possible if the water contains these sensitizers. For example, as external PPRI are not present,
indirect exogenous photoinactivation does not happen in pure water (Nelson et al. 2018). In other
water matrices, the photoinactivation of viruses and bacteria can be enhanced under UVA and
visible radiation (Kohn and Nelson 2007; Romero et al. 2011; Mattle et al. 2015). However, it is a
complex mechanism that depends on many factors (pathogen species, physiological state, radiation
wavelength, type of sensitizers and its relationship with the pathogen) (Maraccini et al. 2016). For
protozoa, exogenous indirect damage is generally negligible due to the thick resistant oocyst-wall
(Liu et al. 2015; Nelson et al. 2018).
Summarizing, protozoa and viruses are mainly photoinactivated by direct endogenous mechanisms
caused through the action of UVB radiation while bacteria are damaged by direct and indirect
endogenous processes through the action of UVA and UVB radiation. In addition, if the water is not
pure, exogenous indirect damage can enhance solar disinfection for bacteria and viruses but not for
protozoa.
Consequently, the efficiency of SODIS processes for the different types of pathogens is strongly
affected by the transmission spectra of the materials used for the devices. Usually, SODIS treatment
is carried out using PET bottles due to their low-cost and accessibility. Concerns about chemical
contaminants from plastic migration have been addressed by previous studies (Wegelin et al. 2001;
Ubomba-Jaswa, Fernández-Ibáñez, and McGuigan 2010). Alternative materials have been
successfully evaluated, such as polypropylene copolymer (PPCO), polycarbonate (PC) and
polystyrene (PS) 1 L bottles (Fisher et al. 2012), polyethylene (PE) bags (Lawrie et al. 2015),
polymethylmethacrylate (PMMA) (Ubomba-Jaswa, Fernández-Ibáñez, Navntoft, et al. 2010;
Reyneke et al. 2020) and glass reactors (Kalt et al. 2014; Mac Mahon and Gill 2018; García-Gil et al.
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2018) fitted with compound parabolic collectors (CPC). The design of SODIS systems requires the
selection of optimal materials for the manufacturing of the devices considering the UV transmission
spectra but also the mechanical properties, resistance to weathering and production cost.
Moreover, the predictive estimation of the SODIS efficacy requires accurate calculation of the
radiation intensity and spectrum inside the devices. For these reasons, the goals of this work are
the selection of the optimal candidate materials for SODIS device manufacturing and the estimation
of solar available radiation and its spectral distribution as a function of the material and wall
thickness. For the latter, a tool called the Solar UV Calculator has been developed and evaluated to
predict SODIS under different scenarios of plastic materials, device thicknesses and target
pathogens (E. coli bacterium, MS2 virus and C. parvum protozoon). This Solar UV Calculator, was
developed as part of the WATERSPOUTT H2020 project which aims to develop HWT technologies
based on the SODIS process for African settlements in rural areas of Malawi, Ethiopia, Uganda and
South Africa. The Solar UV Calculator is freely available to any potential user interested in the
evaluation of SODIS devices with customized materials and conditions or even for other solar
applications based on spectral transmission calculations.
2. Materials and methods
2.1. Mechanical properties
Selection of candidate plastics for the manufacturing of SODIS devices has been carried out through
an exhaustive review of literature data. Only transparent plastics and without any kind of UV-
stabilizer were considered in the study. Three sets of plastics were studied depending on their
photostability:
Poorly photostable plastics (Polystyrene (PS), polyvinyl chloride (PVC), polypropylene (PP)
and polyethylene (PE)).
Moderately photostable plastics (Polyethylene Terephthalate (PET) and polycarbonate
(PC)).
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Highly photostable plastics (polymethylmethacrylate (PMMA)).
Mechanical properties such us tensile strength, elasticity, impact resistance and weathering
resistance (humidity, temperature and solar radiation) as well as degradation mechanisms of
photooxidation, were analysed. Finally, manufacturing costs were also considered using Ecoinvent
and Plastic Europe Databases (Frischknecht et al. 2005; PlasticEurope 2012).
2.2. Optical properties
The optical properties of candidate plastic materials were experimentally evaluated by recording
their transmission spectra with a UV-Vis-NIR spectrophotometer (Varian Cary 500, Palo Alto,
California, USA). Three independent and replicated measurements for different thicknesses of the
plastic samples were considered.
2.3. Solar UV Calculator tool
A predictive tool called the Solar UV Calculator has been developed to predict the total radiation
available and its spectral distribution inside the plastic device as a function of the thickness and
type of plastic. For this purpose, maximum solar radiation inside the device has been calculated by
applying Eq. 1 in the solar UV-Visible range from 290 nm to 800 nm:
(Eq. 1)
where is the monochromatic radiation intensity (W·m-2·nm-1) inside the device,
the monochromatic solar radiation intensity (W·m-2·nm-1) and is the transmittance of the
plastic material at each wavelength (dimensionless). The transmittance ( is directly related to the
absorbance of the material (dimensionless) by the well-known Eq 2:
(Eq. 2)
whereas the absorbance is related to optical path length in (m) by the Beer-Lambert law (Eq. 3):
(Eq. 3)
where is the monochromatic extinction coefficient (m-1) for wavelength , which depends on
the material. For each type of plastic, the absorption spectrum was measured for three different
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thicknesses and the monochromatic extinction coefficients were obtained from the slope of the
linear regression of absorbance vs thickness plot (for each wavelength).
Based on Eq. 1, Eq. 2 and Eq.3, the radiation intensity inside the device can be expressed as:
(Eq. 4)
where the optical path length for the light transmission is the wall thickness of the device in
(m).
2.4. Application of the Solar UV Calculator tool to different SODIS scenarios
The Solar UV Calculator tool was tested for the design of SODIS devices. For that, the influence of
the thickness on the available radiation and its spectral distribution inside device was assessed.
Evaluations were performed with selected plastic devices with a typical thickness of 2 mm for high-
capacity devices (20 L) and 0.5 mm for low-capacity bottles (2 L). The ASTM G-173 reference AM 1.5
solar spectrum was considered for the incident solar radiation calculations. For the estimation of
the UVB, UVA and Visible radiation, the integrals in the wavelength ranges of 290 - 320 nm, 320 -
400 nm and 400 - 800 nm were calculated, respectively.
The Solar UV Calculator tool was also tested for estimating the experimental requirements to
ensure a specific pathogen inactivation. For that, a pseudo first-order kinetic model that relates the
inactivation with the radiation intensity was used (Eq. 5):
(Eq. 5)
By integrating Eq. 5, the expression for the logarithmic inactivation of the pathogen with time is
obtained:
(Eq. 6)
Where is the final concentration of the pathogen (microorganisms·mL-1), is the initial
concentration of the pathogen (microorganisms·mL-1), is the kinetic constant (m2·W-1·h-1), is the
incident radiation and is the reaction time (h). For decimal logarithmic reductions, the napierian
reduction has to be divided by 2.303.
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To calculate the kinetic constant, the lethal doses (D) for a specific inactivation logarithmic
reduction of three model pathogens (E. coli bacterium, MS2 virus and C. parvum protozoon) were
found in the literature (Fisher et al. 2012; Lawrie et al. 2015; Abeledo-Lameiro et al. 2017). The
three disinfection experiments were carried out under natural sunlight, were performed at low
water temperature and in clear water to avoid inactivation speeding up due to temperature or
exogenous damage and ensure that the damage is only product of solar radiation. Moreover, no
attenuation coefficient is required in clear water as many authors describe a diffuse attenuation
coefficient related to the water composition (Kirk 1994; Craggs et al. 2003).
Finally, with Eq. 7 (combination of Eq. 4 and Eq. 6) the minimum exposure time for a specific
thickness or the maximum thickness for a specific time reaction can be calculated for a set
logarithmic reduction.
(Eq. 7)
Using Eq. 7, the minimum exposure time was calculated for a specific thickness of 0.5 mm and the
minimum thickness was obtained when the exposure time is 6 h, the reaction time recommended
in the SODIS manual with cloudless sky (Luzi et al. 2016).
3. Results and discussion
3.1. Selection of candidate plastic materials based on mechanical properties
The determination of the tensile stiffness of a plastic prior to breaking or permanently deforming
and the energy required to fracture a plastic (toughness), ascertained by the measurement of the
tensile modulus of elasticity and impact resistance, respectively, are critical mechanical properties
for any plastics to be used as devices material. However, in addition to them, resistance to
weathering is also critical. The harmful effects of weather exposure on plastics has been attributed
to photo-degradation or photo-oxidation processes by UV light and the action of oxygen (Gillen and
Celina 2017). In general, polymer degradation is usually controlled using UV-stabilizers that block
UV-transmission through plastic or avoiding the exposure to high temperatures, water or oxygen.
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However, in the case of SODIS processes, blocking UV is not desirable, while oxygen is dissolved in
the water and present in the surrounding air. Consequently, the plastic materials candidate for
SODIS devices should be pure plastics, without any kind of UV stabilizer. In this sense, poorly
photostable plastics are generally dismissed and moderately photostable polymers will require
regular replacement (Feldman 2002).
Polystyrene (PS), polyvinyl chloride (PVC) and polyolefins such as polypropylene (PP) and
polyethylene (PE) are poorly photostable polymers. General Purpose Polystyrene (GPPS), the only
transparent PS, exhibits high stiffness, but brittle behaviour (Nexant’s ChemSystems Solutions
2006). PS becomes brittle when chain scissions are produced by radiation action and O2 addition to
chain radicals (Wypych 2015a). PVC can be either flexible and malleable or rigid and brittle (Titow
1984). When PVC is exposed to natural weathering, mechanical properties such as tensile strength,
elasticity, and impact resistance decline, and the material becomes increasingly discoloured,
indicating lower transparency (Feldman and Barbalata 1996). In the case of polyolefins, degradation
by photooxidation proceeds by a free radical chain mechanism in four steps: initiation, propagation,
chain branching, and termination. This is initiated by the presence of hydroperoxides and their
decomposition leads to the formation of other radicals, chain scissions and crosslinking. Although
the degradation mechanism of PE and PP by photooxidation is similar, the initiation, termination,
and chain-branching reaction rates are different. Thus, PP chains contain tertiary hydrogens that
make the formation of intermediate hydroperoxides much easier and faster. In addition, the
reaction rates of the termination step for PE are 100-1000 times higher than for PP (Wypych
2015b). Overall, these polymers have a lifetime less than one year when exposed to weathering
without any stabilizer. Non-stabilized PE becomes exceedingly brittle within a few weeks (Rånby
1993; Feldman 2002). PP and HDPE are used in the production of many products (estimated at 10
and 6.3 million tonnes in 2017 by Plastics Europe Market Research Group, respectively) due to their
low production prices valued at 8.25€/kg and 7.77 €/kg (data from Database Plastic Europe
(PlasticEurope 2012)).
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Polyethylene Terephthalate (PET) and polycarbonate (PC) are considered to be moderately
photostable polymers. These plastics have considerable lifetimes in the outdoors without
stabilizers. PC is a tough and transparent plastic material with excellent strength, stiffness, impact
resistance, and resistant to high temperature, that suffers a slow attenuation of these properties
under photodegradation by UV radiation. Humidity does not affect PC ageing, however, it produces
a synergistic effect when UV radiation initiates photooxidation. At lower humidity, the reaction
happens at a slower rate but UV penetrates deeper into the polymer, resulting in a thick layer of
degraded material. At higher humidity, a thin photooxidized layer is quickly formed which shields
the core of the polymer from further damage by UV radiation (McKeen 2019). Turton and White
(2014) confirmed experimentally that photodegradation decreases with the depth of unstabilized
PC (Turton and White 2014). PET is a strong, stiff engineering plastic with excellent machining
characteristics, and chemical resistance. PET is often used for food applications where low moisture
absorption, low thermal expansion, resistance to staining, or resistance to cleaning chemicals is
required. Most PET photodegradation occurs at the surface. Photooxidation produces
hydroperoxides that lead to chain scission causing the ageing of the material (Wypych 2015b). PET
can be transparent in the amorphous state with physical and chemical resistance and good
hardness although it degrades upon weathering exposure (McKeen 2019). Photodegradation of PET
and PC leads to more brittle materials with eroded surfaces and lower transmittance. Wall
thickness is obviously an important factor for both polymers. Some authors have studied the
suitable lifetime of polymer ageing taking into account the distribution of the reaction products
across the width of the material and the problems related with the use of Fick’s law for estimating
this goal (Audouin et al. 1994). Wall thickness has to be sufficiently high to ensure that
photodegradation reactions do not affect the core of the plastic piece but as thin as possible to
allow maximum UV transmission to the interior. The production prices for PET and PC are
estimated at 13.9 €/kg and 19.6 €/kg respectively (data from Database 2.0 Ecoinvent (Frischknecht
et al. 2005)).
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Finally, polymethylmethacrylate (PMMA) is a highly photostable polymer typically used without any
stabilizer for extended (many years) outdoor applications (McKeen 2019). PMMA has a number of
advantages over other candidate materials including outstanding weathering, resistance to UV
radiation, transparency, hardness, rigidity and dimensionally stable. In addition, PMMA weakly
absorbs UV radiation producing some chain scission and the formation of methyl methacrylate
monomer. However, these reaction products hardly impair the stability of PMMA due to the fact
that the monomers can combine again (Rånby 1993). PMMA also has a good impact strength. Its
main disadvantage is its high price: 28.9€/kg (data from Database 2.0 Ecoinvent (Frischknecht et al.
2005)). A summary of the mechanical properties for PS, PVC, PE, PP, PC, PET and PMMA are
provided in Table 1.
Summarising, PS, PVC, PE and PP are poorly photostable and non-durable polymers that makes
photostabilization necessary for outdoors. However, PP with 1% of UV stabilizers remains stable for
long periods, consequently maintains transparency and can also be easily and cheaply replaced.
For this reason, PP has been considered as a possible plastic material for SODIS devices while PS,
PVC and PE were excluded as candidate materials for the manufacturing. The production prices for
PET and PC are higher than that of the PP, however it is compensated by durability. Finally,
considering weathering resistance and UV radiation transmission PMMA could be the optimal
material for manufacturing SODIS devices. Furthermore, although its production price exceeds that
of the rests of polymers described, again, this is compensated for by durability. Therefore, on the
basis of mechanical properties, PP, PET, PC and PMMA were the four candidate materials
considered for manufacturing SODIS devices.
3.2. Optical analysis of candidate plastic materials
SODIS relies on the damage caused by solar UV radiation to pathogens. However, pathogen
susceptibility varies with the wavelength and microbiological features. Thus, the wavelengths
transmitted inside the SODIS device is an important factor for disinfection performance.
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Consequently, the transmission spectra of the four materials selected on the basis of their
mechanical properties were experimentally analysed and are shown in Figure 1. PP transmits
radiation from 235 nm, PMMA from 250 nm and PC from 290 nm. At first sight, PP is more
transparent than PMMA and both of them are more transparent than PC. However, the solar
radiation reaching the Earth’s surface does not include wavelengths below 290 nm, and therefore
this wavelength range can be overlooked in the analysis. For wavelengths of solar spectra,
transmittance is higher in the case of PMMA and therefore better for manufacturing SODIS devices
than PP, followed by PC. Nevertheless, all three polymers were found to be suitable plastics for
SODIS process considering optical properties, since they transmit UVB and consequently, cause the
inactivation of protozoa, viruses and bacteria. PET blocks UVB radiation transmission and, as a
result, viruses and protozoa inactivation is difficult (Busse et al. 2019).
3.3. Solar UV Calculator tool
Both mechanical and optical properties, critical for manufacturing SODIS devices, are directly
related to the wall thickness: the higher thickness, the higher resistance to deformation or
breakage and the lower transmission. Consequently, it is necessary to achieve a compromise
between both of them to estimate the optimal thickness for the walls of the device. The developed
Solar UV Calculator allows the assessment of the impact of thickness on the transmission of
radiation. The tool estimates the available radiation inside device using Eq. 4, where is the
thickness of the device and is the extinction coefficients as a function of wavelengths for every
specific plastic. The extinction coefficients () for the candidate plastics were obtained using Eq.
3 and are shown in Figure 2.
PET only allows the transmission of visible and UVA radiation due to the high values of extinction
coefficient for wavelengths between 290-320 nm (UVB radiation). PC, PP and PMMA all allow
transmission of UVB, UVA and visible radiation. For wavelengths lower than 290 nm, the dramatic
increase of the extinction coefficients indicates that this radiation is completely absorbed.
However, this fact is hardly significant since the solar spectrum reaching the Earth surface does not
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include wavelengths below 290 nm. In contrast, for wavelengths higher than 290 nm, the values of
PP and PC extinction coefficients are higher than that of PMMA. Therefore, the PMMA will transmit
more solar radiation than PP and PC plastics for equal thickness.
The Solar UV Calculator has been evaluated and tested for two different purposes: i) to evaluate
design parameters such as the thickness; and, ii) to estimate experimental requirements of the
SODIS process, such as the solar exposure time.
3.3.1. Evaluation of design parameters
The thickness of the plastic material is determined primarily by mechanical resistance
considerations but also by the device size/volume. The thickness of the plastic material should
provide enough resistance to carry the water inside. However, the thickness of the plastic material
has a critical effect on the optical properties of the material and therefore on the solar radiation
available inside the device for SODIS purposes. When thickness increased, the transmittance
decreased for all polymers. However, due to the differences in the extinction coefficients, the
influence of the thickness on the available radiation inside device is very dependent on the
material. In order to show the importance of estimating the transmitted radiation before
manufacturing, predictions were performed with devices with a typical thickness of 2 mm for high-
capacity devices (20 L) and 0.5 mm for low-capacity bottles (2 L). For a thickness of 0.5 mm the
incident radiation (transmitted radiation) seems to be very similar for each polymer (Figure 3).
However, from the numerical data shown in Table 2 it follows that PET does not transmit UVB
radiation and this is a critical factor for protozoa and viruses inactivation. PMMA transmits a high
percentage of the solar radiation UVB (97%) and UVA (97.9%), while the behavior of PP and PC is
very similar transmitting around 65% of UVB radiation and 70-80% of UVA solar radiation. However,
when the thickness is increased up to 2 mm, transmitted UVB radiation decreases strongly to 17%
in the case of PP and PC. Even though only 0.26-0.29 W/m2 of the UVB is available inside the device,
it is still a useful result due to its potential to inactivate protozoa and viruses. The transmitted UVA
radiation also decreases, reaching values similar to the transmitted UVA radiation by PET with 2
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mm of thickness. In contrast, PMMA behavior hardly changes due to its low extinction coefficient
spectra maintaining high levels of UVB and UVA incident radiation (88.5% and 92.0% of the solar
irradiance).
To sum up, PMMA, PP and PC transmit UVB radiation and a higher percent of UVA; however, the
thickness must be taken into account due to the fact that a small increase in the thickness can
cause a disproportionately large decrease of the radiation available for SODIS inside the device.
3.3.2. Estimation of experimental requirements for SODIS applications
A lethal UV dose must be attained inside the device during SODIS in order to achieve complete
disinfection. Moreover, the susceptibility of pathogens to be photoinactivated is wavelength-
dependent; therefore, the lethal UV dose required is different for each pathogen (Mattle et al.
2015; Silverman and Nelson 2016; Busse et al. 2019).
3.3.2.1. MS2 virus
As it was previously explained, in general, viruses only can be photo-damaged by UVB radiation.
Fisher et al. (2012) found that MS2 needed a lethal UVB lethal dose of 12.83 kJ/m2 (0.31W/m2
during 11.5 h) to achieve three-log inactivation under sunlight conditions using polypropylene
copolymer (PCCO) bags transparent to the complete UV region. Taking into account Eq. 6, kinetic
constant of the virus inactivation was estimated to be 5.38·10-3 cm2/mJ.
Using the results of the Solar UV Calculator, the kinetic model was employed to estimate the
required solar exposure time to ensure 3-log inactivation of the virus for a minimum thickness of
0.5 mm. In the case of PP and PC, the exposure time was very similar (3.36 and 3.40 h, respectively)
due to its similar transmission in the UVB region. For PMMA, disinfection is achieved in only 2.24 h
as a result of its low extinction coefficient and its high transmission in the UVB range. Finally, since
PET does not transmit UVB radiation, the viruses cannot be inactivated by solely photoactivated
processes in PET devices (Table 3). The maximum thickness of the devices to guarantee virus
inactivation for an exposure time of 6 h was also calculated. As expected, in the case of PP and PC
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devices the thickness was almost the same (1.17 mm and 1.16 mm). For PMMA devices, the
thickness reached a value of 16.8 mm, a value extremely high that assures resistance and durability.
Again, the opacity of PET to UVB radiation range makes us conclude that PET cannot be considered
a good candidate for device manufacture if only photoinactivation is considered (Table 4). The
available radiation inside each device for this scenario is shown in Figure 4.
3.3.2.2. Cryptosporidium parvum protozoon
When UVB is present in the solar spectrum, protozoan photoinactivation is mainly mediated by
these wavelengths. Abeledo-Lameiro et al. (2017) found that under sunlight conditions, C. parvum
viability decreases to 35% of its original value with a UVB dose of 32.29 kJ/m2 (5 h at an average
UVB radiation of 0.78 W/m2). Fitting to a first order kinetic model (Eq. 6), the kinetic constant was
3.25·10-4 cm2/mJ. The thickness and the exposure time effect were also evaluated for C. parvum
photoinactivation. PET was not evaluated due to the opacity to the UVB radiation. Although UVA
radiation can also inactivate the protozoa slowly, a synergistic temperature contribution (above
37°C) is then required (Gómez-Couso et al. 2010), since the temperature of the experiments of
Abeledo-Lameiro et al. (2017) was always below 35°C. For a reduction in C. parvum oocyst global
viability of up to 50% with a thickness of 0.5 mm, the exposure time was determined, resulting in
5.58, 5.64 and 3.72 h for the PP, PC and PMMA, respectively (Table 3). The required solar exposure
time of PP and PC devices is very close to the “standard” SODIS time of 6 h. The thickness required
to achieve a 50% the oocyst viability in 6 h was determined. The maximum thicknesses of devices
were 0.58 mm for PP, 0.57 mm for PC and 8.24 mm for PMMA (Table 4). Again, as a result of the
high UVB transmission of PMMA, C. parvum is inactivated quicker when the water is exposed in
PMMA than in PP and PC devices, which have similar transmittances and, therefore, similar
thickness to achieve the same oocyst viabilities. Available radiation inside each device for this
scenario is shown in Figure 5.
3.3.2.3. Escherichia coli bacterium
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Bacteria can be inactivated with UVB and UVA solar radiation so both radiation ranges must be
considered for determining the SODIS inactivation time Lawrie et al. (2015) found diverse UV lethal
doses for the inactivation of E. coli, Enterococcus spp. and Clostridium perfringens in plastic bags (PE
and PET) with different transmission spectra. For this reason, it is necessary to know the sensitivity
coefficient of the pathogen for each wavelength for determining its wavelength contribution
(m2·W-1·h-1),. Silverman and Nelson (2016) developed a biological weighting function (BWF) to
estimate the sunlight inactivation rates of E. coli. Therefore, Eq. 6 is modified as follows:
(Eq. 8)
Considering the sensitivity coefficients from Silverman and Nelson (2016) and the developed tool,
Eq. 9 (derived from Eq. 6 and Eq. 8) was applied to the two scenarios to estimate the effect of the
thickness, plastic material and exposure time in the E. coli inactivation by SODIS technology.
(Eq. 9)
The solar exposure time required to ensure a 3-log inactivation for a thickness of 0.5 mm was very
similar for PP and PC (0.97 and 0.90 h, respectively) as a result of their similar transmission across
the whole UV region. Inactivation using PMMA was achieved in only 0.69 h due to its low extinction
coefficient and its high transmission in the UV range. Finally, PET cannot transmit UVB radiation,
and UVA slowly inactivates bacteria. In this case, 1.52 h were required to achieve a three-log
inactivation (Table 3). The maximum thickness of the devices has also been calculated for a three
log-inactivation in an exposure time of 6 h. Obviously, the widest thickness allowed is for PMMA
material (47.20 mm) due to its high UV transmission in the UVA and UVB range, followed by
thickness of PP and PC (3.10 mm and 4.22 mm) because of the combination of moderated UVA and
UVB transmission, and finally maximum thickness for PET was 2.47 mm as a result of only UVA
transmission (Table 4). Available radiation inside each device for this scenario is shown in Figure 4.
To sum up, considering the solar exposure time and plastic thickness, PMMA provides the best
results to be used as device for SODIS process. However, PC and PP also meet the requirements for
solar water disinfection due to the fact that they are able to transmit UVB radiation and therefore,
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bacteria, viruses and protozoa can be inactivated. Finally, if considering solely endogenous and non-
thermal processes, then PET devices are suitable only for bacterial photoinactivation.
4. Conclusions
PP, PC, PMMA and PET have been shown to be suitable candidate materials for solar water
disinfection devices: PP has low durability but can be economically replaced, PET and PC have
moderate durability and production costs, and PMMA is an expensive plastic but relatively
unaffected by weathering. From the optical point of view, with the exception of PET in the UV
range, they all achieve the required UV transmission.
The Solar UV Calculator presented in this work has been proved to be a useful tool to evaluate
design parameters and estimate experimental requirements for solar water disinfection in suitable
plastic devices. Taking into account the thickness and the spectral transmission of the materials, the
tool allows the estimation of the radiation available for SODIS inside the device and especially its
spectral distribution, since the sensitivity of pathogens are strongly dependent on the wavelength
range. The Solar UV Calculator was able to evaluate design parameters such as thickness, type of
plastic, and radiation spectra. For example, for a thickness of 0.5 mm, the transmitted solar UV-A
radiation was 97.9%, 70%, 72.8% and 81.0% for PMMA, PET, PP and PC, respectively, whereas the
transmitted solar UV-B radiation was 97%, 0%, 64.9% and 64.5% for the same plastics.
Furthermore, PMMA was found to be less sensitive to changes in thickness due to its low extinction
coefficients, thus, the radiation transmitted remains high. The Solar UV Calculator was an essential
tool for the estimation of the experimental requirements such as solar exposure time or
inactivation level for three model pathogens (E. coli bacterium, MS2 virus and C. parvum
protozoon). As an example, for a thickness of 0.5 mm and a 3-log inactivation of bacteria, the
required solar exposure time was determined at 0.69h for PMMA, 1.52h for PET, 0.97h for PP and
0.90h for PC.
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Finally, although the Solar UV Calculator has been validated for the specific materials and pathogen
scenarios used in this study, it is freely available as Supplemental Data to any potential user
interested in the evaluation of SODIS processes with other materials and conditions, or even to the
evaluation of other solar processes subjected to a strong spectral dependence on materials
transmission.
Supplemental data
The developed Solar UV Calculator is made freely available for anybody interested in performing
their calculations with user-defined materials defined by its transmission spectra.
Acknowledgements
The authors declare no conflicts of interest. The authors gratefully acknowledge the financial
support of the European Union’s Horizon 2020 research and innovation programme under
WATERSPOUTT H2020-Water-5c-2015 project (GA 688928). Ángela Garcia Gil also acknowledges
Técnicas Reunidas for the economic support to finance her scholarship in Residencia de Estudiantes
and Spanish Ministry of Education for her FPU grant (FPU17/04333).
Figure Captions
Figure 1: Transmission spectra of PMMA, PP, PC and PET.
Figure 2: Spectral distribution of the experimental extinction coefficients for the studied
polymers.
Figure 3: Spectral incident/transmitted radiation for a thickness of 0.5 mm (left) and 2 mm
(right). The arrow indicates the order of the lines from the top to the bottom.
Figure 4: Spectral incident radiation required to reach three-logarithmic units inactivation
of MS2 virus and E. coli bacterium in 6 h for the maximum thickness values. The arrow
indicates the order of the lines from the top to the bottom.
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Figure 5: Spectral incident radiation required to reach 50% viability of C. parvum oocysts in
6 h for the maximum thickness values. The arrow indicates the order of the lines from the
top to the bottom.
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Table 1: Summary of mechanical properties and production costs for the selected plastic as
candidate material for manufacturing SODIS devices.
Resistance
Photostability
Durability
Production cost
PS
Low
Low
Very low
-
PVC
High
Low
Very low
-
PE
High
Low
Very Low
Very Low
PP
High
Low
Low
Very low
PC
High
Moderate
Reasonable
Moderate
PET
High
Moderate
Reasonable
Moderate
PMMA
High
High
High
High
Table 2: UVB, UVA, Visible and total incident/transmitted radiation for thickness values (Th) of 0.5
mm and 2 mm.
Incident/Transmitted Radiation
Solar
PP
PC
Th=0.5 mm
Th= 2 mm
Th=0.5 mm
Th= 2 mm
(W·m-2)
(W·m-2)
%
(W·m-2)
%
(W·m-2)
%
(W·m-2)
%
UVB
(280 - 320 nm)
1.63
1.06
64.9
0.26
17.7
1.05
64.5
0.29
17.5
UVA
(320 - 400 nm)
45.6
33.2
72.8
12.9
28.3
37.05
81.0
20.1
44.0
Visible
(400 - 800 nm)
543
462
85.2
288
53.0
480
88.3
333
61.3
UV-Visible
(280-800 nm)
590
497
84.1
301
51.0
518
87.7
353
59.8
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Incident Radiation
Solar
PMMA
PET
Th=0.5 mm
Th= 2 mm
Th=0.5 mm
Th= 2 mm
(W·m-2)
(W·m-2)
%
(W·m-2)
%
(W·m-2)
%
(W·m-2)
%
UVB
(280 - 320 nm)
1.63
1.59
97.0
1.45
88.5
0.00
0
0.00
0
UVA
(320 - 400 nm)
45.6
44.7
97.9
42.0
92.0
31.9
70.0
14.5
31.8
Visible
(400 - 800 nm)
543
534
98.3
506
93.3
484
89.1
342
62.9
UV-Visible
(280-800 nm)
590
580
98.2
550
93.2
516
87.3
356
60.3
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Table 3: Required solar exposure time (h) with a thickness of 0.5 mm to reach three-logarithmic
units inactivation of MS2 virus and E. coli bacterium and 50% viability of C. parvum oocysts.
PP
(h)
PC
(h)
PMMA
(h)
PET
(h)
MS2
3.36
3.40
2.24
-
C. parvum
5.58
5.64
3.72
-
E. coli
0.97
0.90
0.69
1.52
Table 4: Maximum thickness (mm) to reach three-logarithmic units inactivation of MS2 virus
and E. coli bacterium and 50% viability of C. parvum oocysts in 6 h.
PP
(mm)
PC
(mm)
PMMA
(mm)
PET
(mm)
MS2
1.17
1.16
16.8
-
C. parvum
0.58
0.57
8.24
-
E. coli
3.10
4.22
47.2
2.47
Author statement
Angela García-Gil: Investigation, Writing - Original draft.
Cristina Pablos: Investigation, Methodology, Writing – Review and Editing.
Rafael García-Muñoz: Methodology, Validation, Writing – Review and Editing.
Kevin G McGuigan: Funding acquisition, Validation, Writing – Review and Editing.
Javier Marugán: Conceptualization, Funding acquisition, Validation, Writing – Review and
Editing.
Declaration of interests
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☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Graphical abstract
Highlights:
- Revision of mechanical and optical properties, weathering and costs of plastics
- Selection of optimal materials for SODIS container manufacturing
- Development of a tool for estimating UV radiation available inside containers
- Predictive evaluation of bacteria, viruses and protozoa solar inactivation
- Freely available Solar UV Calculator for custom evaluation of solar irradiance
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