Health-risk assessment for roof-harvested rainwater via QMRA in Ikorodu area, Lagos, Nigeria

This paper presents a study to assess the roof-harvested rainwater (RHRW) in the Ikorodu area of Lagos state, Nigeria, and recommends guidance to minimise the health risk for its households. The types, design and use of rainwater harvesting systems have been evaluated in the study area to inspect the human risk of exposure to Escherichia coli (E. coli). To achieve these objectives, a detailed survey involving 125 households has been conducted which showed that 25% of them drink RHRW. Quantitative microbial risk assessment (QMRA) analysis has been used to quantify the risk of exposure to harmful E. coli from RHRW utilised as potable water, based on the ingestion of 2 L of rainwater per day per capita. Results have revealed that the maximum E. coli exposure risk from the consumption of RHRW, without application of any household water treatment technique (HWTTs) and with application of alum only, were 100 and 96 respectively, for the estimated number of infection risk per 10,000 exposed households per year. This estimation has been done based on 7% of E. coli as viable and harmful. Conclusively, it is necessary that a form of disinfectant be applied to the RHRW before use.


INTRODUCTION
In the 2012 millennium development goals report, Nigeria was listed as one of the nations with lack of access to a drinkable water source. It has been stated by WHO/ UNICEF JMP () that Nigeria has limited or no progress for its sanitation facilities. Despite Nigeria narrowly meeting the target for improved drinking water, the report showed that most of these improvements were recorded in urban areas. Access to clean drinkable water in Lagos state is low, with most residents depending on individual harvesting sources such as well, rainwater, borehole and river (Balogun et al. ). Harvested rainwater has the potential to improve access to a water source, therefore it is imperative to investigate its impact, especially where there is limited access to pipe-borne treated water. Sediment, pollutant and saline from natural and manmade terrains or channels (Pu  There is a lack of access to clean water in Lagos, including Ikorodu, and the dependency on rainwater is high (Longe et al. ; Balogun et al. ). Therefore, it is important to assess the health risk of drinking RHRW. In this paper a pilot study was performed through QMRA in the Ikorodu area in order to assess the pathogenic bacteria risk from consuming roof-harvested rainwater, which is one of the area's common practices. By doing this, this paper aims to conduct a useful study to alert the authorities and community about the risks and safety practices for the sustainable and secure consumption of rainwater. Pathogenic strains of E. coli have been used to develop the QMRA as suggested in previous studies (Soller et al. ; Abia et al. ). Furthermore, in this study, the quality of roof-harvested rainwater (RHRW) and different scenarios obtained from the field-work survey have been used to estimate the risks posed by exposure to E. coli. A survey of structured questionnaires was delivered to 125 households to collect the relevant data, and the households were chosen to cover the whole study area. This study analysed the results from the questionnaires and expanded the analysis into the whole population of the study area. The information from the survey and enumerated bacteria were coherently applied to assess the health of the population who consumed untreated harvested rainwater. These will be discussed in the following two main sections: Materials and method, and Results analysis.

MATERIALS AND METHODS
The study area and administration of questionnaire The study area is the Ikorodu Local Government Area of Lagos, Nigeria. It is situated in the northern part of Lagos, approximately located in longitudes 3 30 0 E and latitude 6 36 0 (Umunnakwe et al. ). This area has been chosen because of the inhabitants' dependency on rainwater, especially during the rainy season; and it is also one of the fastest developing areas in Lagos (Longe et al. ). This area also has various commercial and retail institutions, residential buildings and public and private institutions, hence rainwater collection can impact those different communities. This fact makes the proposed research both challenging and important for the study area.
A house-to-house surveying method was used to administer the household questionnaires in a mixture of good and poor sanitation areas. This method was chosen because of the availability and ease of access to each household in the area, and the difficulty of obtaining detailed information through the internet. The sampling error was statistically calculated to be around 0.5%. The map and population status of the area were analysed, and a visual inspection was conducted before classifying the regions. The questionnaires were administered to a person between the age of 25 and 50 in each of the interrogated households with support from the authors to explain the questionnaires to ensure accurate response from the households. The questionnaire household's distribution is presented in Figure 1.
A total of 125 questionnaires were collected, and on average there are about 4.9 persons in each household.
Among a total of 613 people, 11% were younger than five years old; while only 1% were older than 60 years of age.
This information is important as ingestion of pathogens may more severely impact these groups. These demographic results, together with the amount of pathogenic strains of E. coli ingested from the rainwater storage tank, were used to develop the QMRA.
In this study, each household was interrogated once, and each questionnaire was monitored to be fully answered.
The survey results were analysed in two stages: (1) strategy of water use, and (2) water and sanitation infrastructure. A sample of the questionnaire can be found in Appendix A.

Development of the QMRA
The development of QMRA involves a four-phase process to estimate the human health risk associated with exposure to the target pathogen. The utilised method is described by Gerba et al. () and Ahmed et al. (). The four phases include: (i) hazard identification; (ii) exposure assessment; (iii) dose-response assessment, and (iv) risk characterisation. These phases are described as follows.

Hazard identification
This phase was achieved by collating the presence of target pathogens in different household water treatment techniques (HWTTs). The enumeration of target pathogen in both the harvested rainwater tanks and the roof were assessed using the standard Colilert-18 method (i.e. APHA protocol number 9223 B. Enzyme substrate test). This phase represents an initial assessment of data, and it is evaluated more meticulously in the subsequent QMRA phases to fully identify and categorise all the risks involved in drinking rainwater. One of the main emphases of this phase is to decide if there is enough information to consider a

Studied pathogens
E. coli was used as the target pathogen for this study due to their significance in water-borne human health issues.

Sampling methods
The Colilert-18 method was used to enumerate the bacterial counts for all the investigated 49 different rain events in both rainy and dry seasons (the details of all the observed rain events can be found in Appendix B). This sampling exercise was used as it was proven by the manufacturer's analysis to be reliant, and it aimed to provide good statistical confidence in terms of the sample size. The Colilert-18 tests were executed in accordance with the manufacturer's guidelines. Initially, 100 mL of the sample was added into IDEXX's dehydrated media in the supplied sterile jars. The samples were shaken by hand 3-4 times over 6 minutes to dissolve the media. The contents of the jars were then emptied into sterile quanti-trays and heat sealed with a sealer.
The quanti-trays were then incubated at 35 C for 18 hours. Following incubation, the quanti-trays were compared to the supplied comparator. The quanti-trays were then placed in the fluorescing wells (366 nm), where the number of E. coli cells was measured.

Exposure assessment
In this second phase of QMRA, the number of the pathogenic strains of E. coli in the rainwater storage tank and the volume consumed by a person were estimated. The pathogens number was inserted into the dose-response models to estimate the possibility of infection. The exposure assessment also determined the magnitude and period of exposure by each pathway and estimated the number of people exposed as well as the categories of people affected (Petterson et al. ; Whelan et al. ). The pathway considered in this study included the enumerated microbes before and after application of different HWTTs.

Dose-response assessment
This phase was used to assess the risk of a response for a known dose (number of microbes) of target pathogen. The dose-response models are statistical equations that define the dose-response association to target pathogen, were used as the target pathogen in this study, the Beta-Poisson model was used as follows.
where P(i) in Equation (1) denotes the probability of infection per 10,000 persons in Ikorodu in the exposed population for a single event while d denotes the dose (i.e. number of infective units); u in Equation (1) denotes the percentage of E. coli strains that are viable and harmful (it is taken as 7% in this study). In Equation (2)  The E. coli exposure and risk scenarios were obtained from the well-structured questionnaires.

Risk characterisation and management
The where P N in Equation (3)

Experiments on different scenarios
In this field study, rainwater was collected directly into a 255 L tank via a 2.5 m gutter, and 5 L of the water samples were collected from the top of the tank on cessation of the rainfall event. This level was selected because residents from the study area collect water from the top of the tank via cups/jugs, and do not store the harvested rainwater for long periods. The storage tank and water sample container were washed with sterilised water and emptied after the harvest of each rain event to prevent contamination. This was carried out to ensure that the storage tanks were bacteriafree before the harvest of the next rainfall.
The experiment was performed to enumerate the amount of E. coli both before and after the application of different HWTTs. The HWTTs considered in this study include alum, chlorination, boiling and the combination of chlorination and alum, and these are the HWTTs used by the residents in the study area. The pH range of the harvested rainwater was between 5.5 and 6.5. Also, the raw sample was analysed for E. coli before the application of each HWTTs for each rainfall event. Details for each HWTT are discussed below: • Alum: 28 g of the powdered alum was weighed, mixed and dissolved into 1 L of raw water sample in accordance with the manufacturer's instruction. Rapid (110 rpm) and slow (40 rpm) mixing was applied for 3 and 25 minutes respectively, and the water sample was allowed to settle for 1 hour. The water sample was then filtered and analysed for E. coli.
• Chlorination: Sodium hypochlorite was used as a source of chlorination. Fifteen grams of powdered sodium hypochlorite was weighed and dissolved into 1 L of the raw water sample following manufacturer's guidance. The water was allowed to settle for 1 hour before the process to analyse E. coli.
• Boiling: One litre of the rainwater was boiled for 10 minutes to 100 C. The water sample was allowed to cool for 30 minutes and then analysed for E. coli.
• Chlorination and alum: A combination of alum and sodium hypochlorite was applied to the water to investigate its impact. Fifteen grams of powdered sodium hypochlorite and 28 g of the powdered alum were used.
Rapid and slow mixing took place, and the treated water sample was allowed to settle for 1 hour before being analysed for E. coli.
• No HWTT: This scenario involves analysing the raw water sample for E. coli using the Colilert-18 technique.

Analysis of the questionnaire
The survey results were analysed in two stages: (1) strategy of water use, and (2) water and sanitation infrastructure.
Their analyses are presented in the following sections.

Water use strategy
Results from the analysis of questionnaires showed that boreholes are the major source of drinking water throughout the year. However, the proportion of respondents that use boreholes reduced from 82% in the dry season to 55% in the rainy season (see Figure 3)

Water and sanitation infrastructure
The sizes of storage tanks are shown in Figure 5. Among smaller rainwater tank (20-225 L tank) users, 77 households (about 76%) believed that there will be more contamination if the harvested rainwater is stored for  Figure 4). These data were linearly extrapolated to the overall population of Ikorodu to estimate the percentage of total population that is exposed to harmful strains of E. coli.
The roof types on the buildings of the inspected households and people who drink rainwater are presented in Figure 6, while the rainwater harvesting techniques employed by all the people who harvest, and drink rainwater are illustrated in Figure 7. The results showed that 83% of the rainwater harvesters gather it from the roof-top into a collection vessel (and later transfer it into the storage tank), while the remaining 17% collect the rainwater directly into the storage tanks. Further analysis showed that 88% of all the roofs were corroded, of which 84% of those belonged to rainwater drinkers. This fact is important because previous studies have shown that contamination of RHRW can be caused by aged roof materials (Uba & Aghogho ). The studies also stated that the amount of a roof's reactive materials with rainwater could be dependent on the air quality of the harvest area.
The statistics of the first flush practice by the rainwater harvesters is shown in Figure 8. It is crucial as several     Furthermore, for most households, the distance between where they keep their storage vessel and their toilets was generally less than 10 m, thereby suggesting the probability of transmittance of microorganisms. Also, it was observed that boreholes used for potable supply were often dug at places close to septic tanks, which has a risk of contaminating the borehole especially for the shallow water table.

Development of the QMRA model
The information needed to develop QMRA in order to characterise the E. coli exposure risk includes the average pathogen densities, average water ingestion for exposure scenario, dose-response relationships for pathogens, and  The probabilities (P(i)) and numbers (N) for different scenarios were calculated using Equations (1) and (2) respectively and are presented in Table 2. In Figure 9, In the risk characterisation of the final phrase, the numbers exposed to pathogenic E. coli per 10,000 persons in Ikorodu was estimated. Figure 10 shows the employed methodology to estimate the proportion of persons who were affected by harmful E. coli. The figure presents each  and who did or did not drink rainwater) from the total population. From the total population of 685,045 persons, which averaged to 137,009 households, the survey estimated that 91% of the total households harvested rainwater. Furthermore, Figure 10 also illustrates the adjusted percentage of households that applied different forms of HWTT. The results from Table 3 suggest that 34,252 households in Ikorodu harvested and drank rainwater during the rainy season, while almost 5% of the total households do not apply any form of HWTT. This implies that a significant number of households may be at exposure risk to the target pathogen found in the rainwater storage tank.
In a worst-case scenario, E. coli can present in the rainwater storage tank for 281 days in the rainy days of a year. It is found that this was an overestimation since it was very    (3) and (4), the results in Table 4 can be estimated, which shows the maximum infection risk for individuals exposed to contaminated tank water for all investigated HWTT scenarios. The results showed that for 10,000 households, the maximum risk is 96 and 100 for those who apply alum and no HWTT respectively. This further reinforces the need for a disinfectant.

Public health impact of different scenarios in the Ikorodu area of Lagos
The analysis of the results in Table 3 showed that almost 25% of total households drink rainwater in Ikorodu, and 10.5% of total households are at risk of exposure due to E. coli. This estimate includes 7,741 households that apply only alum and the 6,645 households that apply no form of treatment before drinking the harvested rainwater (  Alum þ chlorine -----Note: Ps/Rs and Py/Ry denote the probability/range of infection risk per 10,000 exposed households with rainwater tanks from A single event and probability/range of infection risk per 10,000 exposed households per year in Ikorodu respectively.
HWTT respectively. The burden levels from the different scenarios in Table 2   can be regarded as a prime option for estimating the worst-case scenario due to it presenting higher probability to give a larger risk number.
Furthermore, this pilot QMRA study in Ikorodu recommends the use of RHRW with the application of at least one form of disinfectant. The methods suggested in this study include boiling and chlorination-based approaches. The cost of applying chlorination and boiling could be high in the studied rural area, but nonetheless the methods are imperative to reduce the risk of exposure.
Also, it is important that more research on the long-term public health risk associated with rainwater consumption is conducted in this study area.

CONCLUSION
In this paper, the health risk from the uses of RHRW in the Ikorodu area of Lagos, Nigeria using the QMRA process was investigated. The study also involved the use of results from survey questionnaires conducted in the study area.
Data from the surveying of 125 households showed that rainwater was one of the major sources of drinking water in the rainy season, even though it may be exposed to pollutants, microbes, heavy metals, or toxic matter.
Approximately 25% of the total respondents consumed rainwater. Most of the people who drank rainwater applied one form of the discussed HWTT treatments (including chlorine, boiling or the combination of chlorine and alum treatments); however, some still drank rainwater without any treatment. The risk caused by the consumption of RHRW was further evaluated using the guidelines for drinking water quality. Based on 2 L of ingestion a day and an average of 281 rainy days, the results revealed that the pathogenic E. coli exposure risk from the consumption of RHRW without application of any HWTT and with application of alum only were 100 and 96 respectively per 10,000 exposed households per year, showing the importance of applying disinfectant to the harvested rainwater before use.
This QMRA research represents a pilot study at Ikorodu to evaluate the potential and scale of pathogen exposure risk, and to raise awareness within the affected community.
The results from this study can provide a useful insight into the possibility of QMRA utilisation at any other similar under-developed community who suffer from rainwater contamination.

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