Abstract

Rainwater harvesting (RWH) provides household water supply to rural and peri-urban populations that do not have access to centralised water services. The climate in south-west Uganda is particularly well suited to RWH due to bi-annual rainy seasons, which allows for the collection of rainwater to occur over two periods throughout the year. Water quality from RWH, however, depends on how well risk of contamination (ROC), from catchment to consumption, is managed. Using a mixed-methods approach, we assessed the maintenance practices and water quality of 20 RWH installations in Uganda in the dry and rainy seasons. Both domestic and institutional RWH systems were assessed. Sanitary surveys, site inspections, key informant interviews and physiochemical and bacteriological tests were conducted to identify the factors that have an impact on water quality. Water quality test results were compared to guidelines recommended by the World Health Organisation (WHO). We looked at measures that can reduce the ROC across the RWH system. We found that water quality at 75% of the sites met WHO standards. At these sites, end-users reported that they cleaned systems at least twice a year. Where training on system maintenance had been carried out, end-users reported more regular cleaning and maintenance of systems. Sanitary surveys highlighted an absence of first-flush or pre-filtration as the most prevalent ROC. Overall, we found that both access to technical specialists and capacity-building activities led to well-maintained RWH systems that provide acceptable water quality at both a household and community level.

HIGHLIGHTS

  • This research identified which practices have the potential to improve domestic water quality from rainwater harvesting.

  • The aim of the research is to create knowledge that can contribute to the better uptake of rainwater harvesting in water-stressed regions of the world.

  • The research identifies several low-cost interventions that can improve the quality of rainwater so that the consumption of this water is safer.

INTRODUCTION

Only 14% of Uganda's population has access to potable water managed by the National Water and Sewerage Corporation (NWSC), the majority of which live in urban environments (WHO/UNICEF 2015). In the absence of NWSC-provided water in rural environments, hand-dug shallow wells, groundwater collection schemes, gravity-flow schemes and rainwater harvesting are all practised by rural and peri-urban communities in Uganda (Parker et al. 2010). Rainwater harvesting (RWH) supplies water directly to homeowners, reducing the labour time required to collect and transport water. Western Uganda's climatic conditions are well suited to domestic rainwater harvesting as the bi-annual rainy seasons allow for well-distributed collection of rainwater throughout the year. However, RWH is under-utilised with only 1% of the rural population using it. Barriers to the adoption of RWH in Uganda include challenges with asset maintenance and water quality management (Staddon et al. 2018).

The management of water quality from RWH is a global challenge. Kohlitz & Smith (2015) find that users in Fiji would benefit from improved education and awareness on identifying risks and learning techniques to manage them. In a survey of RWH in rural Nepal, Domènech Heijnen & Saurí (2012) find that poor performance of rainwater harvesting systems is attributed to lack of curing during tank construction, lack of maintenance and deficiencies in technical design. Few studies on RWH assess users' practices and how they respond to the responsibility of operation and maintenance of RWH. Identifying practices that can improve water quality from RWH may help to improve user confidence in the technology.

RWH, an improved water source, uses the roof of a building to collect rainwater for a variety of domestic or productive purposes including cooking, cleaning, washing and drinking (Sturm et al. 2009). Typically, existing household rooftops are used to capture rainfall, which is then directed into gutters that carry the rainwater into either cement or plastic storage tanks. When implemented correctly, RWH can provide good quality water in terms of microbial and physiochemical quality (WHO 2008; Parker et al. 2010). There are, however, strong links between the quality of water provided by RWH systems and whether the system has been maintained (Rahman 2017). To date, numerous studies have found that the quality of rainwater is directly related to the cleanliness of catchments, gutter and storage tanks (Abdulla & Al-Shareef 2009; Baguma et al. 2010; Campisano et al. 2017; Misati et al. 2017). This cleanliness can be ensured by regular maintenance tasks, such as cleaning gutters, emptying filters and bi-annual cleaning of the inside of water storage tanks (Kohlitz & Smith 2015). The overall upkeep of the system such as repairing external cracks in tanks and guttering has also been found to be critical for the collection of good quality water (Domènech Heijnen & Saurí 2012).

Risks to the contamination of rainwater arise between the point of collection (roof catchment) and consumption. Quality of rainwater can deteriorate at any point in the water chain. Wind-blown dirt, leaves and animal faeces can contaminate the catchment area (rooftop) and guttering. Unprotected cisterns and contaminated storage tanks can also impact water quality (Despins Farahbakhsh & Leidl 2009). Sanitary surveys are recommended by the JMP as party of a comprehensive Water Safety Plan (WSP), providing a simple, risk-based assessment of water sources and are structured to provide clear guidance for remedial action to improve water systems (Luby et al. 2008). Misati et al. (2017) assess whether sanitary surveys can replace water quality testing but find that a more suitable approach is for sanitary surveys to help users identify potential hazards.

The objective of this research was (a) to identify the most common risks associated with the contamination of rainwater, (b) to identify the relationship between the results of sanitary surveys and the quality of water provided by RWH systems and (c) to identify the RWH system management practices that have the potential to reduce the risk of contamination (ROC) in RWH systems. Twenty sites for assessment were selected by local partners. Sites that had RWH tanks installed between 2009 and 2016, and where users were willing to participate in the study, were selected. In the urban Kakoba division of Mbarara, 10 RWH systems, installed by a private plumber, were selected. In rural Keeru, Rubanda District, two sites were installed by end-users themselves and eight RWH systems were installed in parallel with end-user training by the Kigezi Diocese Water and Sanitation Project (KDWSP).

METHODS

A mixed-methods evaluation including site inspections, key informant interviews, sanitary surveys and water quality testing, was used to assess the sites. Sanitary surveys, site inspections and water quality testing were carried out twice, in September 2018 (rainy season) and July 2019 (dry season).

Site inspections

Inspections were carried out at each site and included a checklist of eight questions on the physical attributes, such as roof material and size, number of roofs in the catchment, type of guttering, tank material and volume and whether the site was a community institution or a private household.

Sanitary surveys

Sanitary surveys for RWH consist of 10 questions with ‘yes’ or ‘no’ answers. ‘Yes’ answers indicate that there is a ROC. Each ‘yes’ is assigned 1 point. Each ‘no’ answer scores 0 points. The maximum ROC score for RWH is 10. A higher score corresponds to more hazards present during the survey and thus a greater risk that drinking water is contaminated by faecal pollution (Mushi et al. 2012). A list of the sanitary survey questions can be found in the appendix.

A Shapiro–Wilk's test (p > 0.05) showed that the sanitary survey results for the dry season were normally distributed (p = 0.085). The results for the wet season, however, were not normally distributed (p = 0.03) (Razali & Wah 2011). As a result, and due to the small sample size (n < 20), the non-parametric, Wilcoxon signed-rank test was used to compare the results between the two seasons (McDonald 2014b).

Water quality testing

Samples were collected in 300 ml sterile plastic sample bottles and were tested for pH, turbidity, colour and thermotolerant coliforms (TTC). Water quality tests were done using the Oxfam-DelAgua testing kit (Wiltshire, UK), which uses the membrane filtration method to determine the number of TTCs. Samples were passed through a sterile filter, which was then placed on a pad soaked in the liquid growth medium. Samples were then incubated at 44 °C for 16 h, which ensured only thermotolerant bacteria grew. All samples were tested at both time points, in duplicate, within 4 h of collection to ensure accuracy of results. pH was measured using phenol red. Turbidity was measured in situ using a turbidity tube. Positive and negative controls were included in the analysis. Results were included only if negative controls were blank.

Data were manually recorded using Microsoft Excel 2011 Version 16.37 (Redmond, WA, USA) and analysed using IBM Statistical Package for Social Sciences Version 26.0.0.0 (SPSS Inc., Chicago, IL, USA). Microbiological water quality results were compared to WHO's risk classification levels. Fisher's exact test, appropriate for a small sample size, was used to assess statistical significance between sites that did not meet WHO criteria for either turbidity or TTC presence (McDonald 2014a). The significance level was set at p < 0.05.

Key informant interviews

Interviews were carried out with 20 end-users and five stakeholders involved in the management of RWH systems. A semi-structured interview approach was adopted as recommended by Bryman (2008). An interview guide can be found in the appendix. The interview guide included 18 questions on demographics, RWH system cost, functionality and management practices, alternative sources of water and comparative cost, availability of water and perception of water quality. A translator was used where English was not spoken. Interviews were recorded using a Dictaphone, then transcribed and coded using Nvivo (QSR Int. Doncaster, Victoria, Australia). Key quotations were extracted for common themes. The grouped themes were then assessed in order to link them to the findings from the quantitative data. The study was approved by the University of Cambridge Department of Engineering Ethics Review Board and informed consent was obtained before each interview.

RESULTS AND DISCUSSION

Site inspection

Twelve of the 20 sites were domestic, and 8 were community institutions including churches, hospitals, schools and community centers (Table 1). Gutters were either made from HDPE or galvanised sheet metal. 75% (n = 20) of sites used galvanised corrugated metal for roof catchment, 15% used concrete tile and 10% used standing seam metal panels. Catchment areas ranged from 60 to 900 m2 for institutions and 24 to 150 m2 for domestic installations. 25% (n = 20) of tanks were plastic, 65% ferrocement and 10% were informal mortar jars, a low-cost, low-volume alternative to formal RWH which are typically constructed by end-users.

Table 1

Physical properties and demographics of users at domestic and institutional rainwater harvesting sites in south-west Uganda (n = 20)

Site #Urban/ruralSystem installed byInstitution/householdRoof typeTotal catchment area (m2)Tank typeTotal tank volume (m3)Number of people served by RWH systemFrequency of RWH system cleaningTrained in RWH maintenanceWater treatment type
Urban Local Plumber Institution Corrugated metal 250 Plastic 68 20 Twice a year No Candle filter 
Urban Local Plumber Institution Corrugated metal 750 Ferrocement 360 85 Never No Boiling 
Urban Local Plumber Institution Corrugated metal 150 Ferrocement 40 40 Twice a year Yes UV 
Urban Local Plumber Institution Concrete tile 900 Ferrocement 180 500 Weekly No Chlorine 
Urban Local Plumber Household Concrete tile 100 Plastic 10 Twice a year No Candle Filter 
Urban Local Plumber Household Corrugated metal 150 Ferrocement 180 Twice a year No Boiling 
Urban Local Plumber Household Concrete tile 150 Ferrocement Twice a year No Boiling 
Urban Local Plumber Household Corrugated metal 75 Plastic Weekly Yes Boiling 
Urban Local Plumber Household Corrugated metal 40 Plastic Twice a year No Boiling 
10 Urban Local Plumber Household Standing seam metal 480 Ferrocement 10 35 Twice a year No Boiling 
11 Rural KDWSP Institution Corrugated metal 100 Ferrocement 10 150 Weekly Yes Boiling 
12 Rural KDWSP Institution Corrugated metal 264 Plastic 40 700 Monthly Yes Boiling 
13 Rural KDWSP Institution Corrugated metal 120 Ferrocement 10 100 Monthly Yes Boiling 
14 Rural KDWSP Institution Corrugated metal 60 Ferrocement 20 150 Monthly Yes Boiling 
15 Rural KDWSP Household Corrugated metal 30 Ferrocement Monthly Yes Boiling 
16 Rural KDWSP Household Corrugated metal 60 Ferrocement Monthly Yes Boiling 
17 Rural KDWSP Household Standing seam metal 60 Ferrocement Weekly Yes Boiling 
18 Rural KDWSP Household Corrugated metal 32 Ferrocement Monthly Yes Boiling 
19 Rural Home-owner Household Corrugated metal 24 Informal Jar Never No Boiling 
20 Rural Home-owner Household Corrugated metal 48 Informal Jar Never No Boiling 
Site #Urban/ruralSystem installed byInstitution/householdRoof typeTotal catchment area (m2)Tank typeTotal tank volume (m3)Number of people served by RWH systemFrequency of RWH system cleaningTrained in RWH maintenanceWater treatment type
Urban Local Plumber Institution Corrugated metal 250 Plastic 68 20 Twice a year No Candle filter 
Urban Local Plumber Institution Corrugated metal 750 Ferrocement 360 85 Never No Boiling 
Urban Local Plumber Institution Corrugated metal 150 Ferrocement 40 40 Twice a year Yes UV 
Urban Local Plumber Institution Concrete tile 900 Ferrocement 180 500 Weekly No Chlorine 
Urban Local Plumber Household Concrete tile 100 Plastic 10 Twice a year No Candle Filter 
Urban Local Plumber Household Corrugated metal 150 Ferrocement 180 Twice a year No Boiling 
Urban Local Plumber Household Concrete tile 150 Ferrocement Twice a year No Boiling 
Urban Local Plumber Household Corrugated metal 75 Plastic Weekly Yes Boiling 
Urban Local Plumber Household Corrugated metal 40 Plastic Twice a year No Boiling 
10 Urban Local Plumber Household Standing seam metal 480 Ferrocement 10 35 Twice a year No Boiling 
11 Rural KDWSP Institution Corrugated metal 100 Ferrocement 10 150 Weekly Yes Boiling 
12 Rural KDWSP Institution Corrugated metal 264 Plastic 40 700 Monthly Yes Boiling 
13 Rural KDWSP Institution Corrugated metal 120 Ferrocement 10 100 Monthly Yes Boiling 
14 Rural KDWSP Institution Corrugated metal 60 Ferrocement 20 150 Monthly Yes Boiling 
15 Rural KDWSP Household Corrugated metal 30 Ferrocement Monthly Yes Boiling 
16 Rural KDWSP Household Corrugated metal 60 Ferrocement Monthly Yes Boiling 
17 Rural KDWSP Household Standing seam metal 60 Ferrocement Weekly Yes Boiling 
18 Rural KDWSP Household Corrugated metal 32 Ferrocement Monthly Yes Boiling 
19 Rural Home-owner Household Corrugated metal 24 Informal Jar Never No Boiling 
20 Rural Home-owner Household Corrugated metal 48 Informal Jar Never No Boiling 

Risks of rainwater contamination

The most common risks across the test sites were dirty guttering (dry = 10, wet = 5, n = 20), a lack of pre-filtration (dry = 11, wet = 12, n = 20), first-flush (dry = 20, wet = 20, n = 20) and inadequate drainage (dry = 10, wet = 10, n = 20). All of the RWH systems installed by KDWSP had a basic large particle filter: a low-cost, mesh coarse leaf filter at the storage tank opening. No other sites had a pre-filter installed.

Quality of construction for the ferrocement tanks was high. Less than 35% (n = 20) of all sites presented with defects (wet season). Drainage at water access points was inadequate at 50% (wet and dry seasons, n = 20) of sites (Figure 1). Typically, more frequent rainfall in the wet season washes away debris which can account for the better condition of RWH systems in the wet season; however, for this set of results, the difference in overall sanitary survey results between the wet and dry seasons was not statistically significant (p = 1.00). Nonetheless, more sites presented with contaminated roofs and dirty guttering during the dry season than wet.

Figure 1

Dry and wet season sanitary survey results for rainwater harvesting sites in south-west Uganda (n = 20).

Figure 1

Dry and wet season sanitary survey results for rainwater harvesting sites in south-west Uganda (n = 20).

Rainwater quality

In total, 80 water samples were tested. The WHO's guidelines on drinking water quality classify TTCs into four risk categories: conforms to guidelines (<1 CFU/100 ml), low risk (1–10 CFU/100 ml), medium risk (>10–100 CFU/100 ml, and high risk (>100 CFU/100 ml) and state that TTC count per 100 ml should be zero (WHO 2008). We found that 25% of sites tested positive for TTCs in the WHO medium risk category: 10–100 CFU/100 ml (Table 2). There was a statistically significant correlation between median turbidity and the presence of TTCs (dry: p = 0.031, wet: p = 0.033). At all sites where TTCs were identified, the water had a turbidity above 5 NTU and so failed to meet WHO standards (WHO 2008).

Table 2

Physiochemical and bacteriological results for rainwater harvesting sites in south-west Uganda (n = 20)

Site #Site descriptionRisk of contamination score (max = 10)
Ph median (n = 2)
Turbidity (NTU) median (n = 2)
CFU/100 ml Meana (range) (n = 4)
Dry seasonWet SeasonDry seasonWet SeasonDry seasonWet seasonDry seasonWet season
Urban 6.8 6.9 10 <1 <1 
Urban 8.2 8.2 20 10 23.2 (12–50) 9.9 (6–20) 
Urban 6.8 6.8 10 <1 <1 
Urban 6.8 <1 <1 
Urban 8.2 8.1 <1 <1 
Urban 8.2 20 21.6 (12–40) 17.1 (8–30) 
Urban 6.8 6.8 <1 <1 
Urban 6.9 6.8 2.5 10 <1 <1 
Urban 10 15 53.5 (36–70) 29.1 (15–50) 
10 Urban 7.2 7.2 <1 <1 
11 Rural 6.8 <1 <1 
12 Rural 6.9 <1 <1 
13 Rural 7.2 7.4 <1 <1 
14 Rural 8.1 8.2 <1 <1 
15 Rural 6.9 6.9 <1 <1 
16 Rural 8.1 <1 <1 
17 Rural 6.8 6.9 <1 <1 
18 Rural 8.1 8.2 <1 <1 
19 Rural 6.9 6.9 20 20 25.2 (16–40) 39.8 (28–50) 
20 Rural 6.9 6.9 20 <1 64.67 (36–120) 
Site #Site descriptionRisk of contamination score (max = 10)
Ph median (n = 2)
Turbidity (NTU) median (n = 2)
CFU/100 ml Meana (range) (n = 4)
Dry seasonWet SeasonDry seasonWet SeasonDry seasonWet seasonDry seasonWet season
Urban 6.8 6.9 10 <1 <1 
Urban 8.2 8.2 20 10 23.2 (12–50) 9.9 (6–20) 
Urban 6.8 6.8 10 <1 <1 
Urban 6.8 <1 <1 
Urban 8.2 8.1 <1 <1 
Urban 8.2 20 21.6 (12–40) 17.1 (8–30) 
Urban 6.8 6.8 <1 <1 
Urban 6.9 6.8 2.5 10 <1 <1 
Urban 10 15 53.5 (36–70) 29.1 (15–50) 
10 Urban 7.2 7.2 <1 <1 
11 Rural 6.8 <1 <1 
12 Rural 6.9 <1 <1 
13 Rural 7.2 7.4 <1 <1 
14 Rural 8.1 8.2 <1 <1 
15 Rural 6.9 6.9 <1 <1 
16 Rural 8.1 <1 <1 
17 Rural 6.8 6.9 <1 <1 
18 Rural 8.1 8.2 <1 <1 
19 Rural 6.9 6.9 20 20 25.2 (16–40) 39.8 (28–50) 
20 Rural 6.9 6.9 20 <1 64.67 (36–120) 

aGeometric mean.

Presence of TTCs is often related to seasonal influences such as rainfall (WHO 2008). Nonetheless, four of the five sites that tested positive for TTCs in the 2018 visit (wet), did so also in the 2019 visit (dry). The pH recorded at each test site lay in the WHO's acceptable range of 6.5–8.5. All of the RWH systems installed by KDWSP conformed to WHO guidelines. The two informal RWH rural sites with mortar jars both tested positive for TTCs and had high ROC scores.

Sites that presented positive for significant turbidity (above 10 NTU) and TTCs all had ROC scores above 6. This suggests that, as was found by Misati et al. (2017), sanitary surveys could provide a good indicator of potential risks to good water quality; however, further research and water quality testing should be carried out to establish how sanitary surveys can contribute to the comprehensive risk management of rainwater.

Rainwater harvesting management and maintenance practices

Institutions vs household

Trends in the frequency of tank cleaning were similar at both institutions and households. At institutional sites, more people were served by each RWH systems; however, responsibility for maintenance fell on individuals charged with general building maintenance. A lack of clearly defined roles of responsibility at institutional sites was cited as a barrier to maintenance. In contrast, ‘pride of tank’ was cited as an incentive to regularly maintain tanks at the household sites.

Maintenance practices

85% of interviewees stated that either they themselves or a technician maintains and checks on their RWH tanks at least bi-annually. Users of the eight RWH systems installed by the KDWSP reported that they felt well informed on how to maintain and clean their systems. KDWSP asserted that all installations were carried out in parallel with user training on tank construction and system maintenance. One interviewee summarised that ‘if a repair is needed on a tank, I do it myself. I can even offer my services to my neighbour because I have been trained’. Maintenance practices cited by interviewees included ‘washing leaves away’, ‘cleaning the inside of the tank’ and ‘cleaning around the tap’.

At sites where users were not trained on maintenance, if blockages occurred, either they would contact the installing plumber to do repairs, or some institutional sites had on-site staff to maintain tanks. The most common repairs required by interviewees were ‘replacing guttering’ (30%, n = 20) and ‘replacing filters’ (25%, n = 20). The two users of the RWH mortar jar sites in the rural setting had received no capacity-building support and reported never cleaning their systems. At both sites, users cited ‘lack of awareness’ and ‘high cost’ as deterrents to maintaining and repairing their RWH systems.

Users at three sites that presented with TTCs reported that they never cleaned their RWH systems and none had been trained in RWH maintenance. No pre-filtration was used at any of these sites and all catchment areas presented with debris. Homeowners/site managers had not been trained on RWH system maintenance and did not have any support from an intermediary organisation with specialism in rainwater harvesting. This suggests that a lack of training can lead to poor maintenance of systems which in turn can increase the likelihood that rainwater is contaminated. In accordance with findings by Domènech Heijnen & Saurí (2012), we found that where poor quality rainwater was detected, it was associated with poor cleaning and management of RWH systems.

At all sites where end-users reported either weekly or monthly cleaning of RWH systems and where systematic capacity-building practices had been implemented in parallel with the construction of RWH systems, tests indicated acceptable water quality. This suggests that capacity-building activities have a positive impact on how well systems are maintained and can reduce the ROC. Additionally, all of these sites had a basic pre-filter installed, which was often made by end-users themselves, constructed from locally available buckets, pierced with holes and covered with a woven steel mesh with 5 mm spacing. The results suggest that simple interventions, such as installing a pre-filter and regular cleaning, can improve the likelihood that water meets WHO standards.

Representativeness

In Rubanda, the Ministry of Water and Environment estimates that only 1908 people or 1% of the population is served by RWH (MWE 2019). In this region, the KDWSP is the most prominent supporter of RWH, having installed over 800 tanks since the mid-1990s, suggesting that many of the RWH users in the region have been installed in line with the KDWSP approach (Danert & Motts 2009). In Mbarara, it is estimated that 10,275 people are served by RWH, about 4% of the population (MWE 2020). These users can be differentiated into formal and opportunistic users, where water is diverted into whatever container is available (MWE 2020). The urban users involved in this study are only representative of formal RWH users. Given that the participants of the study self-selected, this data set may not provide an accurate representation of the range of quality of RWH systems in the community. It is likely that the RWH systems in the study were of better quality and better maintained than the typical RWH system from the communities in question.

Limitations of study

The results of this study are limited as surveys and test results were only conducted at two points across an 11-month period in time. The small sample size limits the applicability of the findings. Water was sourced from the tap of the RWH tank and not at the point of consumption, where further contamination could occur. The sanitary survey used in this study weighted all risks equally, but there is research to suggest that some risks are more likely to cause contamination than others (Misati et al. 2017). Further research could identify which risks have the strongest correlation to poor water quality and could look at developing weighted ROC scores. Additional research could analyse further the effectiveness of the interventions highlighted in this study in ensuring good water quality from RWH.

CONCLUSIONS

We carried out a mixed-methods evaluation of RWH system monitoring and maintenance practices at 20 sites in south-west Uganda to understand the ROC of rainwater and to identify measures that can reduce these risks. The most prevalent risks included a lack of pre-filter or first-flush, dirty guttering and inadequate drainage. Overall, the presence of TTCs was low and most sites had low ROC scores. Where high turbidity and presence of TTCs were detected, all sites had high ROC scores.

The sanitary survey, microbiological and physiochemical test results indicate that the presence of an intermediary organisation (KDWSP) that guides installation, while carrying out capacity building on RWH system maintenance, can mitigate the ROC to rainwater. It is not clear whether the good water quality exhibited by the systems installed by the KDWSP was due to the presence of a pre-filter, good maintenance or training of end-users, but it does appear that overall, the proactive role of the intermediary organisation reduced the ROC of the collected rainwater.

FUNDING

This research was funded by the Engineering and Physical Sciences Research Council (EPSRC grant reference number EP/L016095/1) as part of the Future Infrastructure & Built Environment Centre for Doctoral Training at the University of Cambridge and by National Geographic as part of the National Geographic Explorer's Award.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of both Afrinspire (UK Charity Number 1163774) and the Kigezi Diocese Water and Sanitation Project in carrying out this research. Ethical Approval was provided by the University of Cambridge Department of Engineering ethics committee.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

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

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