Quantitative risk assessment of norovirus and adenovirus for the use of reclaimed water to irrigate lettuce in Catalonia
Graphical abstract
Introduction
Reuse of wastewater for agricultural irrigation is being implemented widely because water scarcity is reported in nearly all river basins in the Mediterranean area. Wastewater is often a reliable year-round source of water, and it contains necessary nutrients for plant growth. For example, Spain uses 71% of its total volume of reclaimed water for agricultural irrigation (Iglesias et al., 2010). Reclaimed water is also used for urban, industrial, recreational and environmental activities. Wastewater needs to be treated to produce reclaimed water to be used for irrigation (EU, 2016; Sanz and Gawlik, 2014). The use of reclaimed water in Spain is regulated under the Real Decreto 1620/2007. This regulation sets the minimum acceptable safety limits for each type of use in Spain, including agricultural irrigation. These limits include the levels of intestinal nematode eggs, Escherichia coli, suspended solids and turbidity (Boletin Oficial del Estado, 2007), but this regulation does not include addressing the acceptable levels of viruses. Food crops irrigated with untreated or poorly treated water are a main source of viruses in outbreaks associated to fresh vegetables (Gerba et al., 2018).
The control of the microbiological quality of reclaimed water in wastewater treatment plants (WWTPs) is currently based on the levels of fecal indicator bacteria (FIB), which include fecal coliforms, Escherichia coli and enterococci. However, bacterial indicators are poorly related to the presence of human enteric viruses (Petterson et al., 2001). FIB behave differently than enteric viruses in wastewater and aquatic environments, where these bacteria are more susceptible to water treatments and environmental conditions than enteric viruses (McMinn et al., 2017). Among the pathogen groups found in wastewater, viruses present the greatest risk because they generally occur in much greater concentrations and have a much greater infectivity (i.e. higher probability of infection with a given exposure), than bacteria and parasitic protozoa (Gerba et al., 2018). Viruses have been associated with outbreaks via irrigated fresh produce (Chatziprodromidou et al., 2018) and the risk of illness from viruses is 10–10000 times greater than that from bacteria at a similar level of exposure (Haas et al., 1993). For that reason, the evaluation of reclaimed water systems with only FIB underestimates the public health risk of enteric viruses.
The most effective means of consistently ensuring safety in the agricultural application of wastewater is through the use of a comprehensive risk assessment and risk management approach that encompasses all steps in the process from waste generation to the treatment and use of wastewater to product use or consumption (WHO, 2006). Quantitative Microbial Risk Assessments (QMRAs) generate an understanding of the risks associated with water reclamation, by characterizing the pathogen occurrence in wastewater and evaluate how well these pathogens are controlled by the wastewater treatment system (and follow-up control measures in irrigation, farming and food processing practice). The pathogen dose that consumers are exposed to in a particular scenario is translated into probabilities of infection and illness. These can be compared against a tolerable disease burden. Disability-Adjusted Life Years (DALYs) are the recommended metric in the WHO guidelines for the overall community health burden, and the tolerable recommended value is 10−6 DALY loss per person per year (pppy) (WHO, 2006).
Among the viruses of fecal origin that are present in reclaimed water, norovirus (NoV) is the main cause of viral gastroenteritis in people of all ages worldwide and is replacing rotavirus as the predominant gastrointestinal pathogen in children. This virus is often found in wastewater and selected as reference virus in QMRAs in a broad variety of scenarios, including exposure to irrigated crops (Allende and Monaghan, 2015; Barker, 2014; Mara and Sleigh, 2010; Mok et al., 2014; Owusu-Ansah et al., 2017; Sales-Ortells et al., 2015). Previous epidemiological studies have demonstrated that NoV genogroup II (NoV GII), including the genotypes GII.2, GII.3, GII.4, and GII.6, is the main cause of endemic persistence and recent large outbreaks of gastroenteritis. Furthermore, another genotype, the GII.P17-GII.17 virus, emerged in 2013 and is spreading as fast as GII.4 (Kobayashi et al., 2016).
Another virus transmitted by contaminated food and water is human adenovirus (HAdV), which is highly prevalent and resistant to sewage treatment (Adefisoye et al., 2016; Calgua et al., 2013b; Grøndahl-Rosado et al., 2014). This virus has been recommended as an indicator for human fecal contamination in water (Albinana-Gimenez et al., 2009; Pina et al., 1998; Rusiñol et al., 2015; Wyn-Jones et al., 2011). However, little scientific information is available about the transmission of HAdV through vegetables. HAdVs can cause an array of clinical diseases, including conjunctivitis, gastroenteritis, myocarditis, and pneumonia (Ghebremedhin, 2014). However, HAdVs and NoV rarely cause serious illness or death although infants and people with weakened immune systems or existing respiratory or cardiac disease are at higher risk of developing severe disease. Nevertheless, the high prevalence of both viruses could make them suitable ‘indicator viruses’; adequate control of these viruses in a water reclamation system implies that other enteric viruses are also controlled.
This study characterizes the HAdV and NoV GII viral concentrations in reclaimed water based on q(RT)PCRs and removal by tertiary wastewater treatment. The main objective of this study was to develop a Quantitative Microbial Risk Assessment for viral gastroenteritis caused by norovirus GII and adenovirus, associated with the ingestion of lettuce irrigated with reclaimed water. We use a mathematical approach that models the variability of the viral load before and after treatment and its reduction in WWTPs. Moreover, we assess the health risk associated with the consumption of lettuce irrigated with reclaimed water from two WWTPs with different tertiary treatments: conventional with flocculation, UV, chlorine, and a constructed wetland. We also evaluate the use of these viruses as indicators of virus control in reclamation systems.
Section snippets
Study site description
Two WWTPs located in the northeast of Spain were selected. WWTP 1 was designed to treat wastewater from two million inhabitants with a flow capacity of 420,000 m3/day. WWTP 2 was designed to treat wastewater from 112,000 inhabitants with a flow capacity of 30,000 m3/day. Both WWTPs have conventional primary and secondary treatments that consist of sedimentation and activated sludge. WWTP 1 has a tertiary treatment, with a design capacity of 3.25 m3/s, that consists of chlorination, flocculation
Results
The results of the measured concentrations of HAdV and NoV GII in raw sewage and secondary and tertiary effluent, including the number of positive samples, are described in Table 2. The virus concentrations in the tertiary effluent of each WWTP and in a joint model were estimated using the measured virus concentrations in the raw sewage, and an assessment of the reduction due to secondary treatment, and of the total reduction (secondary and tertiary treatments together) using the differences
Discussion
In this study, the concentrations of HAdV and NoV GII were quantified monthly for one year in two WWTPs and analyzed to characterize the viral concentrations in raw sewage and treated effluents. The changes in viral concentrations by the two WWTPs, both with conventional secondary treatments but different tertiary treatments, were compared. The virus concentrations found in raw sewage were similar to those of other raw sewage in Mediterranean areas (Calgua et al., 2013b; Iaconelli et al., 2017)
Conclusions
To assess the health risk associated with reclaimed water, we used a stochastic QMRA model to estimate the annual disease burden from the consumption of lettuce irrigated with tertiary-treated water from two different WWTPs. Major findings are:
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High concentrations of NoVGII and HAdV were present in sewage.
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The virus removal from two WWTPs that applied either wetland or conventional tertiary treatment with UV, chlorination and Actiflo® differed, with the wetland treatment giving better reductions
Acknowledgements
This work was supported by the projects JPI Water METAWATER [4193- 00001B]; the Spanish Ministry of Economy, Industry and Competitiveness [AGL2014-55081-R and AGL2017-86797-C2-1-R]; and the EU Seventh Framework Program under the VIROCLIME project [Contract No. 243923]. During the development of this study, Eloy Gonzales-Gustavson was a fellow of the Peruvian Government. We acknowledge the Ajuntament de Granollers, and especially X. Hidalgo, for their helpful collaboration.
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