According to the recent World Health Organization (WHO) report [1
], the countries which still have limited access to water for drinking purposes are mainly those in the Sub-Saharan region.
In the case of small rural communities, it may be very expensive to guarantee safe water by means of small decentralized water treatment plants (including investment, and operational and maintenance costs) fed by surface water or groundwater. Pollution of source water may be due to many different natural and anthropic causes, including: geochemical processes, heavy rain, flooding, release of untreated wastewater (generated within the rural communities) and industrial effluents, manure spreading on soil, land runoff, acid mine drainage, and infiltration-percolation of water from polluted sites [2
In addition, in order to produce
a safe water, the (decentralized) treatment train must be able to face sudden changes in the feeding water and in order to always guarantee
a safe water, the personnel managing the treatment system must know the contamination risks occurring from the water source to the different users and also evaluate which actions should be taken to face them [6
In the case of centralized water treatment plants, namely water works in urbanized areas, characterized by a high flow rate to treat, these issues have been deeply investigated over the years [7
]. For them, the designing phase (i) takes into considerations the expected variability of the water sources, (ii) selects treatment technologies resulting adequate from a technical and economic view point, (iii) includes a monitoring system able to control the quality of the water under the whole treatment. Once completed the waterworks, the remote monitoring system is able to provide information about the status of the operations and in case of malfunctions, the personnel is immediately informed and may face and solve the problem.
In the case of decentralized water treatments, less is known from many points of view. Referring the attention to the rural and peri-urban areas of the Sub-Saharan region, the qualitative and quantitative characteristics of the potential water sources are often not well defined, access to safe water is still limited [1
], existing small treatment plants frequently present technical and management problems due to scarce and inadequate maintenance, and security culture is missing [10
]. In this context, the European project H2020 SafeWaterAfrica (https://safewaterafrica.eu/en/home
) contributed to filling some knowledge gaps. The aim of this project is to investigate the possibility of developing a water treatment plant for small communities in the rural and peri-urban areas of the Republic of South Africa (RSA) and Mozambique (MZ). In order to support the choices for the treatment train to adopt, it is necessary to know the quality of surface water and groundwater in the study area and of its variability around the year.
Bearing this in mind, the current study provides an overview of the surface water and groundwater quality in rural and peri-urban areas in terms of macropollutants (namely conventional parameters), inorganic chemicals, and micropollutants as well as microorganisms in RSA and MZ and discusses the possibility of withdrawing the water for drinking purposes. In order to evaluate how polluted these waters are, the water quality standards for potable use established in RSA and MZ are provided and compared with the observed variability ranges of concentrations. In this way, it is possible to identify the most critical pollutants in surface water and groundwater and relate them to their potential origin. Finally, the paper highlights interventions which could improve the quality of source water for drinking needs and provides recommendations on water treatment selection.
2. The Area under Study
The area under study is RSA and MZ (approximately 1.2 million km2
and 800,000 km2
respectively). In RSA, the resident population in 2019 is estimated at around 58.8 million [11
]. According to statistical data from the last formal census in 2018, 80.1% of the population are living in formal settlements, 13.1% in traditional settlements, and 5% in informal housing (corresponding to 2,940,000 people) [12
]. Based on the World Bank data referring to 2018, (https://data.worldbank.org/indicator/SP.RUR.TOTL.ZS?locations=ZA
), the rural population in RSA corresponds to 34% of the whole inhabitants. In MZ, the population in 2017 was around 28 million inhabitants [13
], with 69% living in rural areas [14
Access to (safe) drinking water is limited in some regions of RSA and in most of MZ. Figure 1
shows the percentage of the rural population using untreated
surface water or groundwater for potable needs all over the world. The data is taken from 2017. A look at the online map of Figure 1
] shows that, in MZ, rural population using untreated surface or groundwater corresponds to 60% and the remaining 40% of the rural population uses improved water which is available at a distance from their households. In RSA, the corresponding percentages are 19% and 81%, respectively. Safely managed water in these rural areas is not available.
According to local regulations in RSA, basic water services are defined as 25 L/day per person [16
], and, as suggested by the Department of Water Affairs, this is equal to 60 L/d per person [17
]. In MZ, this figure is less than 10 L/d per person, according to the United Nations Development Programme report [18
]. These values are much lower than those reported by [18
] for European countries (around 200–300 L/person day) and for the US (around 575 L/person day). Table S1
reports details of the average per capita water requirements for different categories of settlement according to the report by the Department of Water Affairs, South Africa [17
In RSA and MZ, the treatment of wastewater generated in rural and peri-urban areas (namely: rural settlement wastewaters) is absent or scarce [15
]. This practice leads to deterioration of the quality of the local surface water and also of the local groundwater resources due to percolation/infiltration. In addition, livestock farms, which represent an important activity for the rural communities, are not safely managed from an environmental view point and generally their household wastewaters, zootechnical effluents, and manure are directly released into surface water bodies or are directly spread on soil [19
2.1. Investigations Included in this Overview—Collected Parameters
This study collects quality data on the surface water and groundwater in RSA and MZ from 44 peer reviewed papers published between 2001 and 2019. Forty-two papers refer to RSA and only 3 to MZ (one paper [20
] presents data from both countries); 36 papers provide concentrations of pollutants in surface water bodies (20 for a first group of contaminants including: macropollutants, polycyclic aromatic hydrocarbons (PAHs), inorganic chemicals, and microorganisms, and 16 for a second group covering different classes of micropollutants) and 11 in groundwater (9 for the first group of contaminants and 2 for the second group).
The selection of papers to include in this review was based on these criteria. The study has to present investigations in rural or peri-urban area in RSA or MZ; it has to monitor surface water or groundwater in terms of common macropollutants (also called conventional pollutants), inorganic chemicals (namely heavy metals), polycyclic aromatic hydrocarbons (PAHs), microorganisms, as well as micropollutants (mainly pharmaceuticals, fragrances, parabens, X-ray contrast media). Values of concentration were included in this review, if a satisfactory description of the sampling mode and frequency was reported in the corresponding reference and analytical methods were well reported. In some cases, papers were included even if they investigated urban areas in addition to peri-urban or rural areas. This is the case of long rivers in RSA and MZ which cross urbanized areas and then peri-urban and rural areas before discharging into the ocean.
in the Supplementary Material
reports the main characteristics of the study areas in which sampling campaigns took place in each of the papers included in the review (namely: types of areas, place of investigations, monitored parameters and, where available, number of withdrawn samples). The selected parameters consist of 12 macropollutants, 25 inorganic chemicals (mainly heavy metals), 5 microorganisms, total PAHs, and 103 micropollutants (mainly pharmaceuticals, hormones, plasticizers, and pesticides), which are all reported in Table 1
. Three thousand five hundred and thirty-four (3534) pieces of data were collected for macropollutants, inorganic chemicals, PAHs, and microorganisms, and 1640 values for micropollutants. Their occurrence is reported in graphs and compared with the standards established in RSA and MZ for drinking purposes. The data are then discussed in order to identify the main critical pollutants in cases where the water containing such contaminants is used for drinking purposes.
The map in Figure 2
reports where the investigations included in this study took place, with the corresponding references. Due to lack of information, it is not possible to specify with a higher level of detail the place of the reviewed investigations. They generally refer to rural and peri-urban areas, in a few exceptions, they refer to urbanized areas, in case of monitoring of micropollutants in long rivers (see Table S2
). Looking at Figure 2
, it is easy to distinguish between the investigations dealing with macropollutants, PAHs, inorganic chemicals, and microorganisms or micropollutants, and surface water or groundwater.
2.2. National Standards for Potable Use in RSA and MZ
reports the legal limits for most of the selected macropollutants, inorganic chemicals, and microorganisms according to the regulations in Mozambique, Si_MZ
]), and the South African national standards Si_RSA
(SANS-241-1: 2015 [62
]). These values will be compared with the measured concentrations found in the two types of water sources (surface water and groundwater) in the areas under study.
Based on the comparison, the selected parameters will be divided into six groups according to the criteria defined in Table 3
called the Variability Range-Standards criteria. To complete the analysis of the collected data, for each pollutant, the percentage of the exceeding of the corresponding standard for the two different water supplies (surface water and groundwater) will be evaluated.
4. Discussion and Conclusions
Based on the collected data, it emerges that the release into surface water bodies of untreated or inadequately treated effluents from households, industries, and zootechnical farms are quite often one of the main causes of different contaminants occurring in the water. Once in the environment, such contaminants may be subjected to different natural removal mechanisms (hydrolysis, photodegradation, biodegradation, sorption, sedimentation, etc.), reducing their dissolved concentrations. For more persistent compounds, this attenuation could be quite modest and their concentrations may keep high for a long time and at great distances. If the release of untreated or scarcely-treated effluents is continuous or frequent, the quality of the receiving water body is destined to deteriorate over time.
In designing a waterworks for rural areas in these regions, it is quite difficult to define the quality of the water source feeding into the potential water treatment plant, as the quality of this water is destined to worsen. The plant will in fact have some difficulties coping with the changes in the water being fed into the plant and in guaranteeing the production of safe water that is suitable for drinking. Variability in the main chemical and microbiological characteristics of the water fed into the plant is tolerated and is mainly due to natural events such as heavy rainfall, which leads to increased turbidity and resuspended contaminants due to induced turbulence. In this context, a preliminary study investigating the expected variability of the feeding is absolutely necessary [8
A waterworks consists of a multibarrier system, which is a sequence of steps where different removal mechanisms may retain, transform, and remove different pollutants from the water: raw materials, turbidity, colloidal substances, inorganic compounds, and microorganisms, etc. If well managed, the waterworks should guarantee continuous water purification in line with the legal requirements over the long term. Specific treatments could be added after the construction of the waterworks if revised local regulations lead to the definition of new standards for some pollutants (of emerging concern, or introduced for the first time in the specific regulation) or more restrictive standards for contaminants that are already regulated. With regard to the current situation in the rural areas of RSA and MZ, it emerges that the quality of surface water and groundwater will worsen over time due to the release of insufficiently treated or untreated wastewater linked to the existing anthropic activities (rural settlements, industries, and livestock farms) and their expected development.
In order to guarantee that the construction of waterworks will produce safe water in the long term in areas where access to drinking water is still modest, such as those under study, other actions must be planned and completed at the same time. This refers to the whole rural or peri-urban water cycle. A quick look at the graph of Figure 10
could help better understand this concept. The graph represents the water quality in a rural or peri-urban water cycle in terms of quality level (Y axis) at the different steps (i.e., withdrawal, potabilization, wastewater production by different users, wastewater treatment, release into the environment) versus time (X axis). It shows that in the case of improper treatment of wastewater released into the receiving body, the surface water quality will worsen and thus, over a short period of time, the local waterworks will require specific and further treatments to guarantee the production of safe water for the users over time.
In some areas, such as those under study, access to safe water could be increased for the local population if interventions are planned to increase the percentage of properly-treated wastewater (domestic, industrial, and zootechnical). These actions will guarantee that the surface water quality level will not worsen and the suggested waterworks will be able to function as planned without the need for any upgrading during its lifespan.
In Figure 10
, the blue dotted line refers to the water quality level in an urban water cycle, where treatments for drinking purposes and for wastewater are already present. The release of the treated effluent is regulated and would not lead to deterioration of the receiving water body. Sometimes, wastewater should be subjected to specific treatments that are able to produce “restored” water or “purified” water for specific needs (direct reuse, recycling in an activity). In this way, the reclaimed water is not discharged and directly destined to other functions and, at the same time, natural water is preserved and protected.
With regard to the characteristics of proper drinking water plants in rural areas, for small communities, based on the main results of the outlined overview, it emerges that great attention should be paid to the accurate selection of adequate pre-treatments that are able to face the expected wide variability ranges of concentrations for the main contaminants in the withdrawn water.
In addition to turbidity and microorganisms, which may reach very high values in the case of rain events [10
], it was found that the most critical compounds are metals. The coagulation-precipitation treatment is the recommended step able to reduce suspended solids and metals. In this context, the system proposed by [71
], known as PRE-Disinfection-Column PREDICO, could be a valuable solution as a pre-treatment: it combines coagulation–flocculation, lamellar sedimentation, and filtration into a single-column unit. In addition, it is able to treat highly polluted surface water which may occur quite often in South African and Mozambican rivers. It is also able to act as a reliable barrier for the subsequent disinfection steps which could be performed by conventional chemical systems (namely chlorination [69
]) as well as by the electrochemical disinfection steps investigated in [68
] by means of the CabECO© cell, specifically tested in surface water in RSA and MZ.
To conclude, efforts to increase the access to safe water in small rural communities in the sub-Saharan areas must include: in depth investigations on the source water quality; selection of reliable, flexible, and adequate water treatments on the basis of the expected feeding water quality and its variability; monitoring of the waterworks in order to control the status of the plant and to face any type of malfunctioning and to reduce the risk of producing unsafe water. These three points underline that the concept of water security must be taken into consideration in any step from the water treatment plant design to its operation.