The ambitious United Nations Sustainable Development Goal for 2030 to “leave no one behind” concerning safe drinking water calls for the development of universally applicable and affordable decentralized treatment systems to provide safe drinking water. Published results suggest that well-designed biological sand filters (BSFs) amended with metallic iron (Fe⁰-BSFs) have the potential to achieve this goal. Fe⁰-BSFs quantitatively remove pathogens and a myriad of chemical pollutants. The available data were achieved under various operating conditions. A comparison of independent research results is almost impossible, especially because the used Fe⁰ materials are not characterized for their intrinsic reactivity. This communication summarizes the state-of-the-art knowledge on designing Fe⁰-BSFs for households and small communities. The results show that significant research progress has been made on Fe⁰-BSFs. However, well-designed laboratory and field experiments are required to improve the available knowledge in order to develop the next generation of adaptable and scalable designs of Fe⁰-BSFs in only two years. Tools to alleviate the permeability loss, the preferential flow, and the use of exhausted filters are presented.

This study characterizes the decrease of the hydraulic conductivity (permeability loss) of a metallic iron-based household water filter (Fe⁰ filter) for a duration of 12 months. A commercial steel wool (SW) is used as Fe⁰ source. The Fe⁰ unit containing 300 g of SW was sandwiched between two conventional biological sand filters (BSFs). The working solution was slightly turbid natural well water polluted with pathogens (total coliform = 1950 UFC mL⁻¹) and contaminated with nitrate ([NO₃⁻] = 24.0 mg L⁻¹). The system was monitored twice per month for pH value, removal of nitrate, coliforms, and turbidity, the iron concentration, as well as the permeability loss. Results revealed a quantitative removal of coliform (>99%), nitrate (>99%) and turbidity (>96%). The whole column effluent depicted drinking water quality. The permeability loss after one year of operation was about 40%, and the filter was still producing 200 L of drinking water per day at a flow velocity of 12.5 L h⁻¹. A progressive increase of the effluent pH value was also recorded from about 5.0 (influent) to 8.4 at the end of the experiment. The effluent iron concentration was constantly lower than 0.2 mg L⁻¹, which is within the drinking-water quality standards. This study presents an affordable design that can be one-to-one translated into the real world to accelerate the achievement of the UN Sustainable Development Goals for safe drinking water.

There are many factors to consider for the design of appropriate water treatment systems including: cost, the concentration and type of biological and/or chemical contamination, concentration limits at which contaminant(s) are required to be removed, required flow rate, level of local expertise for on-going maintenance, and social acceptance. An ideal technology should be effective at producing clean, potable water; however it must also be low-cost, low-energy (ideally energy-free) and require low-maintenance. The use of packed beds containing metallic iron (Fe⁰ filters) has the potential to become a cheap widespread technology for both safe drinking water provision and wastewater treatment. Fe⁰ filters have been intensively investigated over the past two decades, however, sound design criteria are still lacking. This article presents an overview of the design of Fe⁰ filters for decentralized water treatment particularly in the developing world. A design for safe drinking water to a community of 100 people is also discussed as starting module. It is suggested that Fe⁰ filters have the potential for significant worldwide applicability, but particularly in the developing world. The appropriate design of Fe⁰ filters, however, is site-specific and dependent upon the availability of local expertise/materials.

Metallic iron (Fe⁰) corrosion under immersed conditions (Fe⁰/H₂O system) has been used for water treatment for the past 170 years. Fe⁰ generates solid iron corrosion products (FeCPs) which are known to in situ coat the surface of aggregates, including granular activated carbon (GAC), gravel, lapillus, manganese oxide (MnO₂), pyrite (FeS₂), and sand. While admixing Fe⁰ and reactive aggregates to build hybrid systems (e.g., Fe⁰/FeS₂, Fe⁰/MnO₂, Fe⁰/sand) for water treatment, it has been largely overlooked that these materials would experience reactivity loss upon coating. This communication clarifies the relationships between aggregate addition and the sustainability of Fe⁰/H₂O filtration systems. It is shown that any enhanced contaminant removal efficiency in Fe⁰/aggregate/H₂O systems relative to the Fe⁰/H₂O system is related to the avoidance/delay of particle cementation by virtue of the non-expansive nature of the aggregates. The argument that aggregate addition sustains any reductive transformation of contaminants mediated by electrons from Fe⁰ is disproved by the evidence that Fe⁰/sand systems are equally more efficient than pure Fe⁰ systems. This demonstration corroborates the concept that aqueous contaminant removal in iron/water systems is not a process mediated by electrons from Fe⁰. This communication reiterates that only hybrid Fe⁰/H₂O filtration systems are sustainable.

Quiescent batch experiments were conducted to evaluate the influences of Cl^-, F^-, HCO₃^-, HPO₄^2-, and SO₄^2- on the reactivity of metallic iron (Fe⁰) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe⁰ and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe⁰, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO₃^- enhanced the extent of MB discoloration, while Cl^-, F^-, HPO₄^2-, and SO₄^2- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO₃^- inhibited O-II discoloration. The increased MB discoloration in the HCO₃^- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe⁰-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Permeable reactive barriers (PRBs) containing metallic iron (Fe⁰) as reactive materials are currently considered as an established technology for groundwater remediation. Fe⁰ PRBs have been introduced by a field demonstration based on the fortuitous observation that aqueous trichloroethylenes are eliminated in Fe⁰-based sampling vessels. Since then, Fe⁰ has been tested and used for treating various biological (e.g. bacteria, viruses) and chemical (organic and inorganic) contaminants from polluted waters. There is a broad consensus on the view that “reactivity loss” and “permeability loss” are the two main problems hampering the design of sustainable Fe⁰ systems. However, the view that Fe⁰ is a reducing agent (electron donor) under environmental conditions should be regarded as a distortion of Corrosion Science. This is because it has been long established that aqueous iron corrosion is a spontaneous process and results in the Fe⁰ surface being shielded by an oxide scale. The multi-layered oxide scale acts as a conduction barrier for electrons from Fe⁰. Accordingly, “reactivity loss”, defined as reduced electron transfer to contaminants, must be revisited. On the other hand, because “stoichiometric” ratios were considered while designing the first generation of Fe⁰ PRBs (Fe⁰ as reductant), “permeability loss” should also be revisited. The aim of this communication is to clarify this issue and reconcile a proven efficient technology with its scientific roots (i.e. corrosion science).