Summary of findings

Access to safe drinking water remains a challenge in many rural and suburban areas of developing countries (RWSN 2010; Treacy 2019). Mortality rates from contaminated water are correspondingly high, with communicable diseases a serious threat (Demena et al. 2003; Eitner and Kondruweit-Reinema 2019). Governments in developing countries worldwide struggle with inadequate resources and infrastructure to meet drinking water needs for all citizens (Savage 2018). PoU water treatment has been shown by various authors (CAWST 2011; Kausley et al. 2018; Lantagne and Yates 2018; Pandit and Kumar 2019; Treacy 2019) to improve drinking water safety and reduce the burden of waterborne diseases. It is an interim measure to drinking water provision and sometimes the only option in many rural and suburban areas of developing countries with little or no access to formal drinking water supplies. Although efforts to solve drinking water problems in poor communities are underway globally, challenges still exist (Lantagne and Yates 2018; Pandit and Kumar 2019; Treacy 2019).

The aim of this research was to develop and optimize combined small-scale low-cost gravity driven PoU systems for water treatment (able to provide bacteriologically safe and aesthetically acceptable drinking water) in rural and suburban areas of Southern Africa. A final optimized novel combined small scale low- cost (about 25US$) PoU system for drinking water treatment in the rural and suburban areas of Southern Africa has been proposed in the final Chapters. The final system was developed after development, evaluation and optimization of a range of PoU system configurations. The final system offers a promising and viable method for safe drinking water provision in poor communities of the Southern African region. Additional to the final optimized combined PoU system, during the course of the research, a range of low- cost treatment methods and technologies for application in low-cost PoU systems were investigated specifically for application in the Southern African region in Chapters 3, 4 and 5. Local materials were sourced and different combined PoU system configurations were designed, constructed and experimented on. Knowledge gained from the experimentations in Chapters 3, 4 and 5 alongside that gained from Chapter 2 was further used in Chapter 6 to develop a specialized comparison framework to aid in the design and choice of materials and systems to use depending on the application. The experimental work further led to the design, optimization and modelling of the final developed system.

The experimental investigation on locally available materials and locally applicable processes in Chapters 3, 4 and 5 initially resulted in the development of three novel and simple, yet innovative water treatment systems namely the: (i) Modified intermittently operated slow sand filtration system incorporating geotextile and GAC (ISSFGeoGAC) for removal of bacteria, particles, color, taste, odor and selected heavy metals, (ii) eight-layer four-pot bidim sequential filtration (BidimSEQFIL) system for bacteria and particle removal and, (iii) indigenous wood filtration combined with GAC (WFSGAC) for removal of bacteria, color, taste, odor, particles and selected heavy metals; as presented in Chapters 3, 4 and 5. These were then comparatively evaluated alongside two commercially available PoU systems (investigated in Chapter 2) using the novel comparison framework developed in Chapter 6 as mentioned above.

This further led to the design, optimization and modelling of a novel combined PoU system as presented in Chapters 7 and 8. Thus, the knowledge gained in the experimental investigations of Chapters 2, 3, 4 and 5 and the comparison framework results in Chapter 6 was further applied in developing the proposed novel

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combined PoU system presented in Chapter 7. It has been developed and optimized to produce bacteriologically safe water at an optimized filtration rate of 2 L/h. This system was subsequently chosen for optimization and the detailed modelling which was done in Chapter 8.

Based on the study in Chapter 6 and various issues identified in Chapters 2, 3, 4 and 5, it was decided to incorporate the following in the design of the final product as initiatives towards possible alleviation of certain identified issues: (i) safe storage compartment to minimize recontamination, (ii) GAC filtration for aesthetic improvement and possible enhancement of additional removal of other contaminants such as iron and manganese which impart color and taste to water, (iii) inbuilt disinfection provided by the silver in the SCCGM to avoid further treatment by disinfection (e.g. chlorination which imparts smell and taste to water) and thereby enhance acceptability of the treated water. The more so that, silver concentrations needed for bacterial inactivation in water do not impart color, taste or odor to water (Pandit and Kumar 2019). This is also expected to avoid DBPs by chlorination such as trihalomethanes and Halo acetic acids (HAAs) which are suspected carcinogens, and (iv) pre-filtration by the nonwoven bidim geotextile as a form of pretreatment to remove debris and larger microbes (e.g. helminths and protozoa) and reduce particulate loads in the water before it passes through the disinfection step. This makes the raw water fit for silver disinfection and reduces clogging especially in that bidim geotextile is a robust fabric and can be easily washed by hand. Thus, pre-filtration removes most of the suspended particles and enables silver disinfection to be more effective.

Furthermore, pre-filtration was included to enhance the system’s ability to treat a broader variety of raw water and extend filter runs and to cater for fluctuations in suspended particle concentrations in the surface waters of rural and suburban communities of Southern Africa. SCCGM is a replicable filter media produced by TAM ceramics (2019) based on “red firing clays” with many advantages, such as no need for pause (waiting) period or biolayer development which are required in ISSF systems and are difficult to manage by many users. Also, red firing clays can be found nearly anywhere in Southern Africa. Laboratory tests on the system in Chapter 7 showed high potential for significant E.coli and fecal coliforms removal (>99.99%) at an optimum flow of 2 L/h. The system exhibited substantial improvements of aesthetic aspects (color, odor and taste) with average turbidity removals of 99.2%.

The work in Chapter 8 presented mathematical modelling of E.coli removal of the developed multi-barrier PoU drinking water system. The system was modelled as a series of three compartments integrating the removals by geotextile filtration, SCCGM filtration and disinfection and GAC filtration. The individual models used accounted for removal mechanisms by each treatment stage. E.coli inactivation by SCCGM was modelled using the Chick’s, Chick-Watson, Collins-Selleck and complete mix system bacterial inactivation kinetic models, which were considered adequately representative for describing the removal. Geotextile removal was modelled using colloidal filtration theory (CFT) for hydrosol deposition in fibrous media. The filtration removal contributions by the SCCGM and GAC were modelled using CFT for removal of colloidal particles by granular media. In the CFT models E.coli was modelled as a microbial particle. Suitable parameter values were estimated and applied to the models. The theoretically combined models demonstrated that suitable removal mechanisms can be applied integrally to model a combined PoU system to predict overall effluent bacterial quality. This kind of modelling can be used to optimize the developed system and to design and optimize similarly combined PoU systems by allowing engineers to systematically vary design parameters until desired system effectiveness is attained. An attempt was also made to assess the effect of various factors that affect bacterial removal performance e.g. collector diameter, particle size, contact time, media depth and filtration rate.

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In addition, a novel specialized comparison framework was developed and demonstrated in Chapter 6 for evaluating low-cost PoU technologies as mentioned above. Although it is difficult to choose which type of PoU technology is best for all applications due to many factors required for different situations and resource availability, the comparison framework results in Chapter 6 showed that it is possible to qualitatively and quantitatively compare low-cost PoU technologies, thereby helping decision making. A recent study by Stubbe et al. (2016) concluded that insufficient reliable information is available for a straightforward recommendation for the most effective and affordable PoU system/device. Therefore, the specialized comparison framework finds possible application by engineers and implementers for comparatively assessing low-cost PoU systems. This can also assist engineers to improve and modify or innovate even further on low-cost PoU systems.

The study in Chapter 2 comparatively analyzed two commercially available low-cost PoU systems with similar process and material combination namely the GWS and DFS and assessed whether the quality of their treated water is sufficiently comparable to good quality tap water municipal supply. It showed that the treated water from the two systems compared well with good quality tap water supplied to Stellenbosch University with respect to bacterial, turbidity and suspended solids content. Both systems produced bacteriologically safe drinking water (with an apparent 100% removal for E.coli and fecal coliforms) due to chlorine disinfection in the gift of water system (GWS) and silver disinfection in the drip filter system (DFS) and are relatively affordable water treatment options, with their own benefits and drawbacks, most of which are highlighted in Chapter 2. The polypropylene string filter in the GWS was indicated to be able to pre-treat turbid water. Furthermore, the improvement of aesthetic aspects (turbidity, color, taste and odor) was generally good due to the presence of GAC in both systems. This may often enhance user acceptability of the two PoU systems. The main drawbacks with respect to the GWS are the need for regular filter replacement, and potential for production of DBPs-e.g., trihalomethanes-due to the use of NaDCC tablets in both the top and bottom buckets, especially if the GAC, which removes excess chlorine, fails during use. The major drawbacks with the DFS are the ceramic candle filter being fragile, slow filtration rate and regular filter cleaning to remove clogging. The findings on the investigated commercial PoU systems in Chapter 2 led to the further investigation into performance improvement of an ISSF system by incorporating geotextile fabrics for pre-filtration and GAC for aesthetic improvement which was done in Chapter 3. Overall, the study in Chapter 3 and Appendix A which resulted in the ISSFGeoGAC as mentioned earlier, demonstrated that modified ISSF systems incorporating pre-filtration by geotextile and further filtration by GAC can together with the other removal mechanisms in ISSF systems (predation, natural die-off, straining and adsorption) substantially enhance the removal effectiveness of multiple contaminants. Combined with a correct pause period, this can in turn enable the combined system to provide safe water of good aesthetic quality. After filter ripening, the E.coli removals by the ISSFGeoGAC recorded up to 99.9% E.coli and fecal coliforms removal. However, even before filter ripening, bacterial removal by ISSFGeoGAC was high, reporting fecal coliform removals of up to 94% and E.coli removals of up to 89%. In addition, the average E.coli removal rates (96%) by the ISSFGeoGAC were slightly higher than those typically reported for traditional ISSF systems, e.g. 90% (WHO 2017a). These findings could be attributed to the presence of the geotextile filter mats on its filter surface and enhanced adsorption by GAC presence in its system. This may considerably offer advantages over traditional ISSF systems which before filter ripening (full development of biolayer) only remove about 30-70% of bacteria through adsorption and mechanical trapping (CAWST 2010). It was noted, however, with the gained knowledge from the study in Appendix A that, physical removal alone is not adequate in ISSF systems. Table 3-3 indicated reasonable iron

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removals by ISSF-1, but the effluent iron concentrations were slightly above the criteria value of 300 μg/L given in Table 3-2. Possible explanation for this could be that at the time of sampling for metals the GAC which enhanced Iron removals had probably reached saturation as indicated by figures 3-4 and 3-5 for turbidity and TSS removals. Metal sampling was only done after 5 months of filter operation while GAC saturation was observed to have occurred after 4 months of operation during data analysis.

The study in Chapter 4 on “Low cost drinking water treatment using nonwoven engineered and woven cloth fabrics” resulted in an optimized simple, yet innovative low-cost PoU water treatment system namely BidimSEQFIL. BidimSEQFIL is a promising technology for low-cost water treatment in poor communities. In addition, numerical models for predicting turbidity removal (presented in Chapter 4) were developed for each fabric as support tools for selecting optimal process configuration. The bacterial removal numerical model for the BidimSEQFIL system was presented as additional content to Chapter 4. The optimized fabric filtration technique was constructed and tested. It was found that BidimSEQFIL can substantially remove indicator bacteria (E.coli and fecal coliforms) up to 3 log removal value (LRV). The finding is important because bacterial removal performance by BidimSEQFIL is much better than both ordinary fabric filtration and three-pot settling methods and has minimal recontamination potential. Additionally, bidim geotextile has comparative advantage for drinking water treatment over ordinary fabrics as it is stronger and can be reused more often with less cleaning needs. It is cost-effective and can be readily sold and easily transported in bulk to many parts of Southern Africa. Furthermore, bidim fabric is easy to wash without significant fabric loosening by normal hand wash as opposed to cloth fabrics which loosen significantly over time further reducing their contaminant removal. The fabric can also be disinfected in ordinary utility ovens at around 100 to 200o_{C and is structurally stable up to 200}o_{C (Kaytech Engineering 2018).}

The study in Chapter 5 on “Drinking water treatment using indigenous wood filters combined with granular activated carbon” resulted in a simple, yet innovative low-cost PoU water treatment system, namely the WFSGAC as mentioned above. The technology is appropriate for the rural poor. The study demonstrated the possibility of using Southern African indigenous wood filters under low water pressure for low-cost water treatment and use of wood filters in combination with GAC (and potentially charcoal). The indigenous wood species were found to be a valid technological research area for low cost water filtration in rural areas of Southern Africa and future research into this area is warranted. The case study also demonstrated simple but valid and novel possibilities of using and preserving the indigenous wood filters for drinking water treatment in the rural areas of Southern Africa using available resources. Although a few aspects remain to be investigated, some practicalities have been demonstrated such as gravity driven wood filtration, filtration rates by each wood species, effect of GAC incorporation on the quality of produced water, initial assessment of the period after which the filter elements should be replaced, significant bacterial removals and possible heavy metals removal.

Since the initial tests in Chapter 5 on using wood filters alone produced water with objectionable aesthetic aspects (color, odor and taste) which may discourage many potential users of the technology, it was decided to combine the wood filters with GAC to enhance aesthetic improvement. In areas where GAC is not available, normal charcoal may be used possibly with slightly deeper sections than GAC, however future investigation in this application is warranted. When tested using Combretum erythrophyllum and Salix mucronata tree species, the gravity driven WFSGAC recorded 100 % removal for indicator bacteria (E.coli and fecal coliforms). The combined system also significantly removed heavy metals: Fe, Pb, Ni, Al and Zn above 90%, and: Cu, As, Cr, Cd and Mn above 50%.

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The gravity driven wood filtration system was chosen for research over a mechanical pressure system because: (i) a gravity driven wood filter system does not require electricity or tap pressure for its operation and is expected to be easier to operate, appropriate and affordable to the rural poor, and (ii) to the author’s knowledge no gravity driven wood filtration has been presented in any published literature particularly using Southern African indigenous species. In other words - because pressure filtration requires pumps / high heads research into the possible use of an even simpler - gravity driven (low pressure) wood filter system was warranted to see if it could be at all feasible. The gravity driven WFSGAC technology finds possible application in PoU drinking water systems implemented by governmental or non-governmental organizations for the rural poor with little or no access to formal drinking water supplies.

The research done forms a strong basis for further studies to conduct field trials and optimize practical PoU water treatment systems for wider application, which could realize an important contribution to human health especially for those with little or no access to formal drinking water supplies. The study resulted in useful findings potentially beneficial to the water treatment sector. It provides significant contributions and insights towards decision making and application of low-cost PoU water treatment methods.