A study of field performance and sustainability of Fluoride Nilogon in defluoridation in Yemen.

1. Introduction

Groundwater constitutes a vital renewable natural resource integral to the hydrological cycle. Concomitant with population growth, dependence on groundwater is increasing exponentially, particularly in developing nations where it represents the primary source of potable water (Costantini et al., 2023, Xu et al., 2021). Over 2.5 billion individuals rely on groundwater for safe drinking water, out of which, more than 0.5 billion across 106 countries are affected by groundwater quality issues, including fluoride contamination (Shaji et al., 2021). Fluoride, a prevalent anion in groundwater worldwide, is essential for dental health when present in the optimal concentration of 0.5–1.0 mgL^-1, which also helps prevent tooth decay. However, it possesses a significant challenge to the provision of safe drinking water when present in excess concentration with potential adverse health effect such as skeletal and dental fluorosis (Susheela, 2007). The World Health Organization (WHO) has prescribed an upper guideline value of 1.5 mgL^-1 for fluoride in drinking water (WHO, 2017). Instances of elevated fluoride concentrations exceeding its guideline value have been documented in numerous countries across Americas, Africa, and Asia including the Middle East, rendering groundwater unsuitable for human consumption (Ahmad et al., 2022, Amini et al., 2008, Shaji et al., 2024).

Access to safe drinking water remains tragically out of reach for millions in conflict-ridden regions. Yemen, situated in a semi-arid to arid zone, already faced significant water scarcity challenges. The ongoing conflict has precipitated a catastrophic humanitarian crisis, with access to safe drinking water drastically curtailed, and exacerbating existing vulnerabilities. In this context, access to safe drinking water is not just a challenge, but also a desperate struggle for survival, further compounding the already dire humanitarian situation in Yemen (UNICEF, 2025, Varisco, 2019, Weiss, 2015). Furthermore, many groundwater sources in Yemen contain fluoride concentrations above WHO guidelines, posing an additional health risk (Aqeel et al., 2017, Al-Hmani et al., 2024). In Yemen, bone disease and dental fluorosis are observed in many areas because of the excessive intake of fluoride (Baghel, 2015). Reports from the government indicate high fluoride content in groundwater in governorates such as Sana’a, Ibb, Dhamar, Taiz, Lahj, Al Dhale’a and Raimah, with concentrations far exceeding the guideline value prescribed by WHO (Al-Amry et al., 2020). While both the government and the Yemen WASH (Water, Sanitation and Hygiene) sector acknowledge the presence and risk of fluoride in drinking water, hardly any cost-effective interventions to reduce fluoride levels have taken place so far. This is mainly due to the lack of low-cost treatment options and the nature of short-term humanitarian interventions that focus on access and quantity of water supply rather than on quality.

While providing fluoride-free surface water is the ideal solution to this problem, delivering this water via pipelines to remote and sparsely populated rural areas may be impractical due to logistical and economic constraints. Several defluoridation techniques based on adsorption, ion-exchange, precipitation, reverse osmosis, electrocoagulation, photocatalysis, membrane filtration and electrodialysis have been developed in the recent time (Arab et al., 2024, Gogoi and Dutta, 2016, Gourai et al., 2023; Nath and Dutta, 2010; Pan et al., 2019; Rathi et al., 2024).

Selection of an appropriate defluoridation method remains a complex task, in humanitarian crisis settings like Yemen, where high fluoride concentration in groundwater demands a solution that is simultaneously highly efficient, cost-effective, and low energy. Conventional approaches such as adsorption are generally more cost-effective and simpler to implement but they are often limited by lower lifespan and require timely regeneration. Advanced technologies like membrane filtration and electrocoagulation offer superior performance but at the expense of higher operational costs (Crini and Lichtfouse, 2019). Reverse osmosis (RO) and electrodialysis, are constrained by their high energy demand and high capital cost making them unsuitable for remote, decentralized community use especially for treating water with high fluoride concentrations. A huge quantity of reject water is also another disadvantage of RO. Thus, there is a need for a robust, simple, and locally deployable technology that can achieve compliance with WHO standards without relying on a consistent power supply.

The phosphoric acid–crushed limestone treatment (PACLT) method, popularly known as Fluoride Nilogon, is an effective hybrid adsorption and precipitation-based technology that has been increasingly implemented in several fluoride-affected regions of India with high fluoride levels (up to 20mgL^-1), including Assam, Chhattisgarh, Odisha, Karnataka, and Rajasthan (Gogoi et al., 2015, Gupta et al., 2021, Mohan et al., 2020). The method was developed by a research group at Tezpur University in Assam, India and was implemented in some rural areas of Assam in the beginning, and so the Assamese word, Nilogon for removal, came in the name. Several studies have affirmed the environmental and economic sustainability of this approach, emphasizing its operational simplicity, minimal maintenance requirements, low implementation costs, and the notable advantage of functioning without the need for electricity (Das et al., 2025, Gogoi and Dutta, 2016, Gogoi et al., 2015, Gupta et al., 2021, Mohan and Dutta, 2020b, Mohan and Dutta, 2020a; Nath and Dutta, 2010). However, the efficiency of fluoride removal using the PACLT method is significantly influenced by various operational parameters, including the particle size of the crushed limestone, contact time, pH control, and the stoichiometric ratio between phosphoric acid and limestone. Furthermore, the quality parameters of the limestone, such as density, porosity, hardness, and presence of iron and aluminium impurities, determine suitability of the limestone for Fluoride Nilogon, though almost 60 % of the limestone samples from different mines or sources across India and the rift valley of Africa have been reported to be suitable (Mohan and Dutta, 2020a). Recent studies have demonstrated its potential for the simultaneous removal of other pollutants, such as hexavalent chromium, thereby extending the applicability of PACLT beyond fluoride mitigation (Das et al., 2025).

ZOA (‘Zuidoost Azië’, Dutch words for ‘Southern Asia’), an international relief and recovery organization, has been actively involved in addressing the drinking water crisis in Yemen since 2012. Given the remote nature of the locations of Yemen, where ZOA encountered water sources with high fluoride levels, low-tech and low-cost options have been preferred to assess. In 2021, ZOA collaborated with Tezpur University and together conducted a pilot to introduce and test the Fluoride Nilogon filter among 300 households at three villages in Al Musaymir district of Lahj governorate. Following the successful implementation, demonstrated efficacy and wider adoption of the filter in this initial phase, ZOA expanded the project to the Al Dhale’a governorate in a second phase with 400 household filters and 1 community filter at 2 villages in Al Azariq and Al Hussein districts. The intervention areas are shown in Fig. 1.

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Fig. 1. Map of intervention areas in Yemen and the source locations of limestone samples used in the present study: A – from Al Melah district, B, C & D – from Al Musaymir district.

Here we report suitability study of four limestone samples, two each for phase I and phase II including detail characterization of the limestone, laboratory testing of suitability of the limestone to select the better ones in each phase complemented by performance in fluoride removal by the limestone samples in Fluoride Nilogon in series of laboratory defluoridation experiments. The evaluation of key physicochemical properties, including porosity, density, and chemical impurities, is crucial in determining the suitability of limestone for the Fluoride Nilogon method of defluoridation as these properties are anticipated to significantly influence the efficiency of the method (Mohan and Dutta, 2020b, Mohan and Dutta, 2020a). The resulting pH of the treated water, a critical parameter for water quality, was analysed.

Results and discussion

4. Conclusions

A total of 700 household (HH) and a community Fluoride Nilogon filters, based on phosphoric acid-crushed limestone treatment (PACLT), were installed in water-strained Yemen as a low-cost intervention to the severe fluoride contamination of drinking groundwater by ZOA, an international NGO with technology and technical support from Tezpur University (TU). It was implemented in two phases, with 300 HH filters at Farás, Al Lojmah, and Al Oqma villages in Al Musaymir district of Lahj governorate in 2021 catering safe water for 300 families in Phase I. Encouraged by the success of Phase I, 325 HH filters to cater for an equal number of families and a community Fluoride Nilogon filter of 7000 L water capacity to cater for another 100 families at Khawber village in Al Hussein district and another 75 HH filters to cater for another 75 families were installed at Habeel Ghadas village in Al Azariq district, all in Al Dhale’a governorate in Yemen in Phase II in 2024. Suitability of two limestone samples, for each Phases, from sources from Musaymir district in Lahj governorate in Yemen were examined at TU. Based on physicochemical properties of the limestone complemented by the fluoride removal abilities by the limestones, one sample was selected for using in each phase though both samples examined for the Phase II were found to be almost equally suitable. The limestone samples examined for Phase II had more favourable porosity and Fe-Al impurity, along with much better fluoride removal ability compared to those for Phase I.

The average initial fluoride concentrations were 3.2mgL^-1 and 10.5mgL^-1 in Phase I and Phase II, respectively. The fluoride concentrations of treated water samples from all 700 HH and one community Fluoride Nilogon filters conformed to the WHO guideline of 1.5mgL^-1 for fluoride. In addition to that, Fluoride Nilogon filters were also found to improve TDS, electrical conduction, and pH to make the water more suitable for drinking. The Fluoride Nilogon proved technically feasible and well-accepted by users by consistently reducing fluoride concentrations from as high as 11.19mgL^-1 to below the WHO guideline of 1.5mgL^-1 in both household and community levels. Initial challenges, including fluoride leaching from sand and occasional phosphoric acid shortages, were resolved without major impact on system performance. High daily usage and positive user feedback indicate strong engagement, driven by effective training and awareness programs, highlighting its potential for wider deployment in fluoride-affected areas of Yemen to ensure safe, sustainable access to drinking water.

CRediT authorship contribution statement

Bereket Godifay Kahsay: Visualization, Investigation. Saranga Baishya: Formal analysis, Data curation. Gerrianne Pennings: Visualization, Investigation. Tushmita Das: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Harm Bouta: Investigation, Funding acquisition, Conceptualization. Anwesha Chaliha: Data curation. Matthijs T. Wessels: Writing – review & editing, Visualization, Project administration, Investigation. Priya Devi: Data curation. Sara Bazarah: Visualization, Investigation. Dutta Robin Kumar: Writing – review & editing, Validation, Supervision, Resources, Funding acquisition, Conceptualization. Amal Hasan: Visualization, Investigation. Wasim Al Shehab: Visualization, Investigation. Hammam Mukred: Visualization, Investigation. Saleh Radhwan Mohammed: Visualization, Investigation. Melhani Akram Al: Visualization, Investigation. Mohammed Riyadh: Visualization, Investigation.