Universal Self-reliance in Water Supply
Beyond Appropriateness and Sustainability
- Fig. 1: Flow chart of water treatment involving metallic iron (Fe0). Water is first filtered through a conventional filter, e.g. biosand with sand of various particle sizes. The filtrate is affined in two Fe0-based beds. The ideal vol. Fe0 ratio is 25% .
- Fig. 2: Flow chart of an alternative water treatment using Fe0 mostly as Fe2+ source. The O2 level of raw water is reduced in a biosand. The filtrate leaches Fe2+ from the Fe0 bed (Fe2+ is labile under anoxic conditions). Fe2+ is oxidized by aeration to enhance contaminant removal in the subsequent sand filter.
The world is on track to achieve the millennium development goals for safe drinking water. However, the current paradigm for water supply in low-income communities is not satisfying. This article presents a universal solution for self-reliance in safe drinking water provision by filtration on packed beds containing metallic iron.
Worldwide industrialization and urbanization result in increased water pollution. Sources of chemical contamination include agricultural, domestic, industrial, mining activities as well as medical and municipal wastes. Chemical contamination is leached from various solid wastes and transported into aquifers and rivers. Non-protected surface water could be regarded as a cocktail of pollutants, which should be treated to meet relevant standards.
The number of groups of chemical species that are potential contaminants is huge: chlorinated organic compounds, dyes, heavy metals, nitroaromatic compounds, pharmaceuticals, phenols etc. [1-3]. Each class of compound is made up of individual substances with different chemical and physical properties. For example, dyes are of various molecular sizes and solubilities, they are either anionic, cationic or neutral. Some of them are redox active. In other words, treating dyes as a class of substances with regard to remediation is not appropriate. In fact, the remediation technologies rely on specific interactions with the contaminants: adsorption, co-precipitation, coagulation, ion-exchange, oxidation, reduction, size-exclusion. From these processes, adsorption, co-precipitation, and size-exclusion are the methods of choice to remove aqueous microbial contamination.
Appropriate Technology for Safe Drinking Water
Considering natural waters as a cocktail of chemical and microbial contaminants implies that appropriate technologies for their treatment should address several types of contaminants.
This is conventionally achieved through a combination of processes including screening, coagulation, filtration, and disinfection. Such treatment chains are found in centralized waterworks, where raw water is collected, treated and distributed to the population by pipeline network. Centralized systems are expensive to install, operate and maintain, especially for low-income communities. Membrane technology combining ultra-filtration and reverse osmosis has been proven the sole compact method to free water from chemical (e.g. arsenic, pesticides), microbial (e.g. bacteria, viruses) and physical (e.g. colour, turbidity) contamination because it works on a pure size-exclusion basis. However, this technology needs high pressure, thus electricity, to operate.
The term "appropriate technology" emphasized that solutions in the developing world should be small-scale, affordable, energy efficient, environmentally sound, use locally available resources, and be capable of being controlled and maintained by the local community . These criteria make small-scale, decentralized membrane-based water treatment systems simply non appropriate. There are voices calling for a revision of basis criteria for an "appropriate technology". However, proponents of re-evaluation are mostly interested in money making as "the design of technology appropriate for developing countries is an increasingly profitable business for manufacturers and distributors" . The proponents of self-reliance regard membrane technology as a bridge or an emergency solution. This article presents the concept of filtration on packed beds of metallic iron (Fe0 filters) as a universally appropriate technology for safe drinking water provision.
Fe0 filters for safe drinking water
Fe0 as removal and recovery agent for dissolved metal is known to hydrometallurgists for more than 100 years. The concentrations of metal ions are in the range of some 100 mg/l and the pH of the solution is flexibly adjusted to pH ≤4.5. For natural waters, contamination levels are in the range of μg to a few mg/l and the pH of water is in the range 6.0 ≤ pH ≤ 9.5. Accordingly, water treatment with Fe0 occurs in a domain of low Fe solubility. In other words, Fe0 is corroded mostly by the solvent (H2O) and its surface is covered by layers of corrosion products. Corrosion products include two other reducing agents, Fe2+ and H/H2. The reductive properties of these reagents are enhanced by adsorbing onto nascent iron hydroxides . The major consequence is that contaminants, present in trace amounts (μg/l) must migrate through a multi-layer oxide scale to reach the Fe0 surface. Accordingly, although Fe0 corrosion by water is an electrochemical reaction, contaminant reduction (if applicable) is not the simultaneous cathodic process .
While regarding contaminant reduction as the cathodic reaction of iron corrosion, an abundant literature is available on the feasibility of using Fe0 filters as universal material for safe drinking water provision and proper sanitation in low-income communities [5,6]. The idea is to reproduce conditions available in subsurface permeable reactive barriers (PRBs) for sustainable Fe0 filters for safe drinking water provision in waterworks . Fe0 PRBs have been efficient for more than 15 years. The main parameter to be controlled is the dissolved O2 level which should be lowered to ≤ 1.5 mg/l. Basically, the O2 level can be controlled by biosand filters or by sacrificial Fe0 beds (e.g. containing just 5 to 10 vol% Fe0). In this case it could be necessary to introduce a flow equalizing bed (e.g. gravel, sand) between the sacrificial and the treatment beds (Figure 1 and 2).
Universality of Fe0 filters
The universality of Fe0 filters arises from the fact that they do not need electricity to operate. They can be coupled to all other existing devices as refinement stage. For example, if a charcoal filter after Kearns  is not efficient enough for the removal of micro-organisms or heavy metals, complementary Fe0 filters can be designed. They can be customized to meet the requirements of single households and small communities. They can also be tailored for seasonal use only. In the developing world, these filters give local researchers a unique opportunity to solve the long lasting problem of water supply on a self-reliant basis. Once a sustainable solution for safe drinking water is established, the won self-confidence will be the weapon to face remaining developmental challenges.
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