Ninety-2 % of the literature reviewed was quite identical in terms of paper content arrangement, methodology, and gist. Twenty-two of reviewed studies conducted a bacteriological and physicochemical analysis on water in storage tanks/vessels whereas 2 studies were specifically physicochemical analyses of tank/vessels water quality.
User practices like tank cleaning frequencies were investigated in a few of the reviewed research works. The studies reviewed were quantitative and qualitative from disciplines such as environmental engineering, water quality, and public health. To elaborate on how features and practices affect water quality, discussions of these phenomena were discussed in relation to the water quality indicators used in the studies. The results were presented under the following themes: Stored water contamination indicators; Water contamination in tanks/ vessels; and, Effects of user practices on water quality.
The contamination indicators that were used to assess the quality of stored water in the reviewed research studies can be broadly categorized as biological and physicochemical.
The biological contamination indicators also known as the microbial or bacteriological indicators are widely used in the analysis of water quality in storage vessels. The use of bacteria as indicators of the sanitary quality of water, probably dates back to 1880 when Von Fritsch described Klebsiella pneumoniae and K. rhinoscleromatis as microorganisms characteristically found in human faeces (Geldreich 1978). In the publications reviewed in this study, multiple parameters were used to indicate microbial contamination, as shown in Fig. 4.
Total coliforms are a group of related bacteria that are often useful indicators of other pathogens in drinking water. They were the most used microbiological contamination indicator, as it was considered in several studies (n = 17) of those reviewed. This was followed by E. coli and faecal coliforms that were considered in (n = 12) and (n = 8) studies, respectively. These are low numbers, considering the important role E. coli and faecal coliforms play in confirmation of faecal contamination.
Heterotrophic Aerobic Bacteria was another indicator considered by most of the reviewed studies. Some studies considered Klebsiella, Enterobacter, Serratia, Citrobacter, Tatumella, Escherichia vulneris (Al-Bahry et al. 2013), Salmonella spp., Legionella spp., Yersinia spp., Aeromonas spp., Pseudomonas, Pasteurella spp. (Duru et al. 2013), Slime Forming Bacteria, Iron Related Bacteria (Schafer and Mihelcic 2012), and phytoplankton species (Duru et al. 2013) as the other indicators of microbial contamination.
Fig. 4Microbial indicators used in the reviewed studies
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Overall, the choices of indicators of microbial contamination appeared to be in line with WHO’s guidelines for water sampling and analysis, which require testing for indicators of faecal contamination as a minimum requirement. E. coli and faecal coliforms are the best indicators of faecal contamination because they confirm the presence of faecal matter, which are considered to pose the greatest risk to human health. Salmonella typhi., a bacterium that causes typhoid fever; a very common infection in developing countries was not investigated by the reviewed studies.
These are physical and chemical aspects of water used in determining whether its quality is acceptable or not. Some of the common physicochemical contamination indicators that were used in characterising the quality of water in the studies reviewed included: pH, Electrical Conductivity, Total Dissolved Solids, Total Suspended Solids, Turbidity, Temperature, Dissolved Oxygen, Iron Fe+, Cu, NO3−, PO4, Zn, Cr, Pb, Zn, K, Mn, Cl (free and residual). pH was the most widely used parameter for physiochemical characterisation of stored water in the studies included in this review (n = 12; 50 %).
pH is paramount in checking the corrosiveness of water and the lower the pH the more corrosive the water, because of its acidic tendencies at low pH values (World Health Organization 2007). The water source, the material of the water storage tank or vessel, the temperature, mineral absorption, dust, the level of bacterial activity in a vessel, and the duration of water storage before use, affect the pH of water (Duru et al. 2013; Packiyam et al. 2016).
Electrical conductivity (EC) came second in frequency of use as a water quality indicator among the studies included in the review. 10 of the 24 (42 %) studies considered EC as a dependent variable, as there was a correlation between the level of Total Dissolved solids (TDS) and EC (see Fig. 5). This is depicted in the study conducted by Akuffo et al. (2013), where the EC value increased with an increase in the TDS value. About 38 % of the reviewed 24 studies used TDS to describe the physicochemical nature of water stored (see Fig. 5). The esthetic quality of water in terms of colour is affected by the level of TDS (Oram 2020). The age and material of the tanks were found to affect the TDS of stored water (Nunes et al. 2018). However, no study of those included in this review undertook to determine the degree of correlation between the TDS and the EC. TDS has also been criticized as a poor parameter for measuring water quality as it does not detail the contents of the dissolved solids (Magnus 2019).
Heavy metals such as Fe+, Cu, and Mn were also used in the assessment of water quality in (n = 6), (n = 6), and (n = 5) studies, respectively (see Fig. 5). Heavy metals have an adverse effect when they accumulate in the human body as they can cause damage or reduce the mental central nervous function, lower energy levels, and damage body organs (Verma and Dwivedi 2013). Fe+ was seen predominantly in tanks that were made of steel and it was in high concentration where the tanks were relatively old (Al-Ghanim et al. 2014; Chia et al. 2013; Chalchisa et al. 2017; Schafer and Mihelcic 2012; Nunes et al. 2018). The cleaning of the tanks also affects the concentration and accumulation of heavy metals. In a study by Ziadat (2005), it was noted that the level of heavy metals in water stored in tanks was elevated because the tanks were old and worn out, and had not been cleaned in a long time.
A study in Venezuela found temperature to be an important parameter of water quality because it affects the rate of microbial growth (Schafer and Mihelcic 2012). According to the same study, temperatures of 15 °C and/or higher inside water storage tanks can cause significant bacterial growth. Other physicochemical indicators that were included in some of the of the reviewed studies were odour and taste (Duru et al. 2013; Varghese and Jaya 2008). Figure 5 shows the frequency of the key physicochemical indicators of water quality used in the studies reviewed.
Fig. 5Physicochemical indicators used in the studies reviewed
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Due to intermittent water supply problems in many parts of the world, especially developing countries, water storage using tanks as well as small containers such as jerry-cans are commonly used by households to reserve water for use throughout the day. Rural communities use small containers that can easily be transported from the water sources to homes, while urban communities have piped water, therefore use water storage tanks to reserve water. There is a great deal of concern regarding in-house microbial contamination during handling and storage of water in developing countries (Akuffo et al. 2013).
Ziadat (2005) evaluated the impact of residential storage tanks on drinking water quality in comparison to its drinking water source, through analysis of major anions, cations, and heavy metals. It was found that the water in storage tanks had higher ionic concentrations compared to the sources. Rusting was suggested as a possible cause since most of the tanks had rusted. However, according to the WHO, most chemicals arising in drinking water are of health concern only after extended years of exposure rather than months. The study by Graham and VanDerslice (2007) investigated the effectiveness of large household water storage tanks for protecting the quality of drinking water in El Paso County, Texas, and found that the water from the tanks was generally of poor quality. Longer storage periods of household water were noticed for households with large water tanks, which may have potentially increased the risk of contamination, and also led to chlorine volatilisation.
The study by Schafer and Mihelcic (2012) found that water storage impacts on water quality through storage tank material, which is most likely because of different water temperatures inside the tank. It was further found that storage tank designs can affect water quality if they do not allow the tanks to be completely emptied during use or cleaning. This may however not be an issue of tank design, but rather, the workmanship of the plumbers, because provisions for the outlets and washouts are usually made by manufacturers and the plumbers use these provisions to install the washouts and outlets depending on the size of pipes to be connected. Interestingly, the age of water storage tanks was found not to have any significant impact on water quality. However, a study by Al-Ghanim et al. (2014) contradicted this and suggested that the high levels of TDS in some tanks were as a result of the tanks being old.
A study conducted in Pakistan by Al-Ghanim et al. (2014) revealed that all tanks were contaminated with heterotrophic bacteria: 80% contained coliforms, 30 % contained fecal coliforms, but E. coli was not detected. It was also found that 60 % of the tanks contained algal counts exceeding 103 unit/l. The study further revealed that different types of tank surfaces encouraged microbial growth differently. The quality of water in the different types of storage tanks was also investigated by Al-Bahry et al. (2013). Three types of water tanks were examined: glass-reinforced-plastic (GRP), polyethylene (PE), and galvanized iron (GI). Results showed that all water storage tanks supported microbial regrowth with high values of the microbial total count. Microbial regrowth varied with the type of the water storage tanks. Coliforms were isolated from all tanks but were abundantly found in GRP.
The study by Chalchisa et al. (2017) assessed the quality of drinking water in storage tanks in Ethiopia and found that water samples collected from drinking water storage tanks were positive for total coliforms and faecal coliforms. The result of this study showed that the drinking water was microbiologically contaminated in all sampling points. It was discussed that the high temperature after storage (up to 23.1 °C) increased the number of faecal coliforms in storage tanks. All these studies confirm that water storage impacts the quality of water in many ways.
Various tank material types were found to be used in different regions from the literature reviewed as shown in Table 4. It is important to note that plastic tanks were widely used in the different regions compared to the rest of the tank materials. However, no explanation was given in the literature studied to justify the usage of the different tank materials, whether it was the cost, convenience, availability of the tank material, or climatic conditions of the regions.
Table 4 Tank used in different regionsFull size table
Tank material was found to be a leading cause of water contamination. There was a variation in the frequency of microbial contamination relative to each type of water tank (Al-Bahry et al. 2013). Water storage tank materials, which are in direct contact with water can contribute contaminants from either the material used for tank construction/ production or from internal coatings used to protect the tank materials from contact with the water (Akuffo et al. 2013).
The studies conducted by Akuffo et al. (2013), Al-Bahry et al. (2013), Schafer and Mihelcic (2012) and Schafer (2010) all agreed that tank material affects water quality through temperature. This could be because different materials have different thermal conductivity, for instance, steel have a higher thermal conductivity and cools faster than plastic under the same weather condition. Additionally, different materials have different heat capacities. A steel tank of a given size absorbs more heat than a plastic tank of the same size. A comparison between fiberglass, fiber cement and black polyethylene tank materials, showed that temperatures are generally higher in the polyethylene tanks throughout the day than in fiberglass, and fiber cement tank materials (Schafer and Mihelcic 2012) (see Fig. 6). As such microbial activity is expected to be higher in the polyethylene tanks than in other tank materials.
Fig. 6Temperature variation of stored water in tanks of different materials (Schafer and Mihelcic 2012)
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When the temperature of the water reaches above 15 °C, the occurrence of coliforms and heterotrophic bacteria is significantly higher (Khan and AlMadani 2016). An investigation conducted by Ogbozige et al. (2018) revealed that steel metal tanks have more EC than the plastic tanks, suggesting less mineral concentration in the steel metal tanks.
Different tank materials were also found to affect water quality because of the various unique features they possess (Table 5). For instance, Ziadat (2005) and Akuffo et al. (2013) found that heavy metals dissolve in water because of rusting. Plastic tanks allow certain types of bacteria to stick to the plastic surface and enable growth (Al-Ghanim et al. 2014). In the same vein, Jagals et al. (2003) and Al-Bahry et al. (2013) found that tank surfaces allow the growth of biofilm. Biofilms provide a variety of microenvironments for microbial regrowth (Al-Bahry et al. 2013). These films break loose from the sides especially during filling with no subsequent rinsing and form particulate suspensions in water which harbour significant numbers of viable bacteria (Jagals et al. 2003). This could be a major cause of water contamination particularly in developing countries because of the rampant intermittent water supply issues, which result into frequent emptying and refilling of water storage tanks, thus causing dislodging of biofilm into stored water. However, a comparison of the levels of biofilm formation on the different water storage tank materials has not been investigated to-date.
In a study by Mohanan, et al. (2017) it was concluded that conventional water storage vessels such as copper, brass, and clay possessed antimicrobial activity and were highly efficient against pathogenic bacteria than vessels made up of plastic, steel, and aluminium. In some other reviewed studies, there were contradictory findings on the contamination levels of the different water storage tank/vessel materials. For instance, Al-Bahry et al. (2013) found that glass-reinforced plastic (GRP) tanks contained the most contaminated water, and polyethylene (PE) tanks contained the least contaminated water. However, Schafer and Mihelcic (2012) found that PE tanks contained the most contaminated water while GRP tanks contained the least contaminated water. These results could, however, be attributed to other variables that may not have been investigated in these studies. These discrepancies demand further research to determine which materials are best suitable for household water storage. Table 4 shows the studies that focused on the different tank/vessel material and the corresponding values of water quality indicators as per the studies.
Table 5 Different tank/vessel material and the corresponding values of water quality indicators as per the studiesFull size table
Tank size and capacity do have an effect on water quality. This is effect is realized through retention or residence time. Microbial growth increases as residence time increases (Schafer 2010). Previous studies such as Agensi et al. (2019) and Chia et al. (2013) found a significant associations between the duration of water storage and the level of contamination. For instance, Nnaji et al. (2019) found an average E. coli, total coliforms, and enterococci count of 3, 4, and 3 MPN/100ml on the first day and 8, 69, and 114 MPN/100ml respectively on the 35th day of water storage; heterotrophic plate count (HPC) of 5 CFU/ml on the 1st day and 31 CFU/ml on the 35th day of storage.
The study by Ogbozige et al. (2018) investigated the effect of storage duration on water quality in different material containers. The study revealed that the maximum retention period for storing water in all the container materials studied as inferred from the water quality was about 21 days, except for clay-pot material where the study showed that its retention period should not exceed 6 days; however, the uncoated steel metal tank was not recommended. It was concluded that black plastic containers preserved water quality better during storage, compared to coloured plastic containers, galvanized iron or coated steel containers, and clay pots.
Large storage tanks allow for longer water storage periods, which may potentially increase the risk of contamination and chlorine volatilisation (Graham and VanDerslice 2007). However, a factor that has not been well investigated by any of the reviewed studies is the fact that the residence time also depends on the household size and per capita water usage. A large tank serving a large household size or a small tank serving a small household size, both with high per capita water use, implies that the water residence time in the tank is very short, and thus it is less likely to get contaminated during storage. Conversely, a large tank serving a small household size with low per capita water use, may result into longer water residence time in the tank, and hence potentially more contamination during storage may be witnessed.
In the study by Al-Bahry et al. (2013) it was noted that the water distribution system started with low microbial contamination. However, when water was transferred to storage tanks, microbial contamination spread rapidly due to water stagnation. Static water is undesirable because this condition provides an opportunity for the suspended particles to settle in the tank as sediments and later stick on the sides of the tank. There is a need for further research on this phenomenon because it may also be argued that the biological and physical chemical contamination per unit of water in large tanks may be lower as compared to that in small tanks based of the amount of time the water is stagnant in the different tank sizes, given a constant number of users for all the tanks.
These four tank/vessel features were investigated by only a few studies; each was investigated by either one or two studies.
Tank age While Chia, et al. (2013) found a significant relationship between the age of the water storage tanks and the occurrence of a significant number of the pathogen species, Schafer and Mihelcic (2012) found no meaningful effect of tank age on water quality. The argument was that tanks that well-maintained tanks do not affect water quality even after many years of use. Proper maintenance ensures that undesirable conditions such as biofilm, rusts, broken covers, etc. are removed or restored to good conditions, enabling tanks to maintain good water quality.
Tank colour Chia et al. (2013) found that the colour of the tanks had a significant association with physicochemical parameters such as dissolved oxygen and biological oxygen demand, which also determined the occurrence and abundance of 9 (including 2 cyanobacteria) out of the 13 species reported in the study. Water storage tank colour may also affect water quality through temperature, since different colours absorb heat to varying extents, affecting the temperature of the water in the tanks differently. Darker colours absorb more heat than lighter colours. However, dark-coloured (plastic) tanks are more commonly used than light coloured tanks, especially in developing countries. Schafer (2010) found black polyethylene among the most common types of tanks in Bolivia. Chia et al. (2013) had a third of the water storage tanks investigated in Nigeria as black. In the same vein, the storage tanks provided by the government in El Paso County, Texas, USA and investigated by Graham and VanDerslice (2007) were also dark in colour. This could be further affecting water quality because high temperatures encourage bacteria growth as discussed above in the section of “Material of the water storage tank or vessel”.
Tank design This study review revealed that tank design affects water indirectly by affecting user practices. A study by Schafer (2010) reported that increased microbial growth in household storage tanks compared to water sources may be due to the design of household storage tanks, which sometimes complicates the complete emptying of the storage tank while in use or during washouts. However, the challenge of completely emptying the water storage tanks may also be attributed to the wrong pipe configuration of outlet and washout pipes on the tanks—as a result of poor workmanship of plumbers.
Tank location This affects water quality through temperature. In the study by Schafer (2010), the temperature of water in a black polyethylene tank was high when the tank was positioned under direct sunlight, but the temperature of water dropped when the tank was covered by a shade of a wall; while the temperature of water in a fibreglass tank continued to rise because it remained under direct sunlight. Bacterial growth would therefore be expected to be higher in the fiberglass tank than the black polythene tank. However, as earlier discussed in this same section under ‘colour’, a black tank absorbs more heat than lightly coloured tanks. The same study found that the temperature of water in the black polyethylene tank remained higher than that of the other types of tanks throughout the day, including the period when it was under the shade.
Water storage tanks have an impact on the water quality if they are not handled in hygienic ways such as sealing or covering of the storage tanks (Chalchisa et al. 2017; Akuffo et al. 2013). Lack of tank covers, potentially increases the risk of contamination of stored water with animal and bird faeces, as well as dust and airborne particulates. This facilitates the growth of algae when the tanks are exposed to sunlight, and lead to undesirable changes in the taste, odor, and color of water (Al-Ghanim et al. 2014). Only a few of the reviewed studies investigated tank covering. There may be other implications of tank covering that were not investigated by the reviewed studies. For instance, if an elevated tank supplied by municipal water and located outside a house is not covered, rainwater my fall into the tank and thus increase the volume of water in the tank. Consequently, this would reduce the residual chlorine of the stored water.
Cleaning practices of water storage tank/vessel have impact on household water quality. Several studies did investigate tank cleaning (Rodrigo et al. 2010; Lévesque et al. 2008; Jagals et al. 2003; Schafer 2010; Nnaji et al. 2019). The study by Jagals et al. (2003) found that biofilm-like substances could build up in storage tanks or containers (especially in those not regularly cleaned), which could contribute to hazardous microbiological contamination of container-stored drinking water, especially if particles from the film become dislodged into, and ingested with the water. In a study by Lévesque et al. (2008), the effect of tank cleaning on water quality was investigated and the results showed that the contamination levels were almost the same for water tanks that had not been cleaned in a range of three years (2000 to 2002), but the contamination had strongly reduced in the year 2003 when the cleaning was carried out. Similarly, the study by Pesewu, et al. (2014) found that the recent cleaning of three (3) polyethylene tanks was responsible for lowering their total coliform and faecal coliform counts.
The study Schafer and Mihelcic (2012) found that the cleaning frequency of tanks impacts the quality of water in the storage tanks. The study found that storage tanks cleaned three (3) or more times per year had lower E. coli counts and turbidity than storage tanks cleaned less frequently (Table 6). The study by Chia et al. (2013) suggested that possible means of continuous contamination and recontamination that encourage the growth or re-growth of microalgae and cyanobacteria in the water storage tanks include inadequate periodic cleaning or scrubbing of the tanks. The study by Nnaji et al. (2019) found that total coliforms, enterococci, HPC, and E. coli counts increased with an increase in the intervals of regular cleaning as shown in Fig. 7; Table 6.
Fig. 7(Adopted after Nnaji et al. (2019))
Effect of tank cleaning on the bacteriological quality of water
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Table 6 Water quality at different cleaning frequencies as shown in 2 reviewed studiesFull size table
The study by Akuffo, et al.,(2013) also found that certain types of tanks (earthen) had less degree of contamination compared with other types (polyethylene and metal tanks) because among other reasons, they were cleaned more frequently. In Al-Ghanim, et al., (2014), it was found that 80 % of the tanks were not frequently cleaned, and therefore contained contaminated water. It can clearly be seen that results from the various studies do agree that cleaning practices of water storage tanks have a significant effect on the stored quality of water. However, the recommended cleaning frequency is still unclear.
This study only focused on studies written in the English language and there was no inclusion of studies or records made in other languages. As such, non-English studies that would provide knowledge on the subject studied may have been missed out. Also, the selection of grey literature was limited to theses and publications from accredited institutions and organizations, and thus there is a likelihood that some data may not have been captured from all the available grey literature.
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