In the lowlands of Nepal (Terai), the WHO drinking water guideline concentration of 10 μg/L for arsenic (As) is frequently exceeded. Since their introduction in 2006, iron-assisted bio-sand filters (Kanchan filters) are widely used to treat well water in Nepal. The filters are constructed on the basis of As-removal with corroding zero-valent iron (ZVI), with water flowing through a filter bed of iron nails placed above a sand filter. According to several studies, the performance of Kanchan filters varies greatly and depends on the size of the iron nails, filter design, water composition and operating conditions, leading to concerns about their actual efficiency. This study examined 38 Kanchan household filters for which insufficient As-removal was reported, to evaluate the reasons for limited removal efficiency and to define measures for improved performance. The measured arsenic removal ranged from 6.3% to 98.5 %. The most relevant factors were the concentrations of As and Fe in the raw water, with the best removal efficiency observed for water with low As (124 µg/l) and high Fe (4.94 mg/l). Although the concentrations of other elements, pH, flow rates, and contact time with ZVI also played a role, the combined evidence indicated that the reactivity of the frequently drying nail beds between filtrations was insufficient for efficient Asremoval. Optimized filters with added top layers of sand and raised water outlets with flow restrictions to keep nails permanently immersed and to increase contact times, should be able to achieve higher and more consistent arsenic removal efficiencies.

As in several other countries of Southeast Asia (namely Bangladesh, India, Myanmar, China, Vietnam, and Cambodia) arsenic (As) concentrations in the groundwater of the lowlands of Nepal (the so called Terai) can reach concentrations that are unsafe to humans using the groundwater as drinking water. Whereas Bangladesh has received much international attention concerning the As crisis, Nepal was more or less neglected. The first report about As contamination of the groundwater above toxic levels in Nepal was published in 1999. Twenty-four percent of samples analyzed (n = 18 635) from the Terai Basin exceeded the WHO guideline of 10 μg/L. Since the first overall survey from 2001, only sporadic information on the situation has been published. The geological and geochemical conditions favour the release of the contaminant as As can be easily solubilized in groundwaters depending on pH, redox conditions, temperature, and solution composition. The thin alluvial aquifers of the Terai are some of the most severely As contaminated. These sediments constituting a hugh proportion of the Terai aquifers are derived from two main sources: (i) sediments deposited by large rivers that erode the upper Himalayan crystalline rocks, and (ii) weathered meta-sediments carried by smaller rivers originating in the Siwalik forehills. The generally low redox potential and low SO42− and high DOC, PO43−, and HCO3− concentrations in groundwater signify ongoing microbial-mediated redox processes favoring As mobilization in the aquifer. Other geochemical processes, e.g., Fe-oxyhydroxide reduction and carbonate dissolution, are also responsible for high As occurrence in groundwaters. Originally, gagri filters (a two-filter system with chemical powder) and later iron (Fe)-assisted biosand filters were commonly used to remove As and Fe from well water in Nepal—these two options were believed to be the best treatment option at household levels. This review focus on the description of the overall situation, including geogenic issues, occurrence of As in the sediments of the Terai, mechanisms for the release of As to the groundwater, and mitigation options.

In Nepal as well as in other countries in Southeast Asia, the World Health Organization drinking water guideline of 10 µg/L concerning arsenic concentrations in ground water hosted in Quaternary alluvial sediments is often regionally exceeded. The commonly accepted theories include that arsenic in ground water stems from reductive dissolution of As-rich Fe(III)hydr(oxides) including microbial degradation of sedimentary organic matter. On the contrary, the influence of clay minerals in the sediments as hosts for As was clearly underestimated, as geochemical analysis depicted that As was generally associated with specific elements such as Na, K, Al, and Li. Moreover, there was a very weak correlation or decoupling between As and Fe in the ground water in Nepal, and this fact points to consequences for water treatment. The so-called Kanchan filters, used for the removal of As, installed in the lowlands of Nepal often exhibited effluent As concentrations well above Nepal’s drinking water quality standard value (i.e., 50 μg/L). Ground water concentrations of Fe and As proved to be the most important geochemical factors regarding the performance of the filters. Moreover, the flow rate as well as the contact time to the rusty nails in the filter, intended to adsorb As on their surface, influenced the removal efficiency. The removal rate was severely influenced by the handling of the filters, too. This short communication provides an overview of the removal efficiency of 30 filters, their drawbacks, the influence of the aging material in the filters as well as measures of improvements to enhance the efficiency of the filters. Proper instruction for users of Kanchan filters is a major point that needs to be addressed in the future.

In the lowlands of Nepal, as well as in several other South Asian countries, the level of arsenic in groundwater extracted from Quaternary alluvial sediments frequently exceeds the World Health Organization (WHO) drinking water guideline of 10 μg/L. The widely accepted explanation refers to the reductive dissolution of Fe-bearing minerals releasing As-oxyanions from the soil minerals. However, this hypothesis is only to some extent applicable. Given that arsenic and iron in the groundwater are weakly correlated or decoupled and iron(III) (hydr)oxides are scarcely present, a substantial portion of As and Fe has to be retained in clay minerals. The low concentration of iron in groundwater can be explained by the origin of As and Fe in the rocks (leucogranites); in the High Himalayas, they are low in Fe but considerably enriched in Li, B, As, Se, Br, Sr, Mo, Cd, P, and U. This unique geochemical setting has a major impact on the performance of the Kanchan Arsenic Filters (KAFs) used to eliminate As from groundwater in Nepal. Lack of sufficient Fe in groundwater, combined with limited release of iron by corrosion of iron nails, call for an adapted version of these filters. The first results from 20 modified filters show a clear increase in As removal efficiency, achieved mainly by the replacement of old nails, an added upper sand layer over the nails, and elongation of the outlet tube to avoid wet–dry cycles and to keep the nail bed immersed. Proper instructions to the users on the operation and maintenance of filters, by replacing the reactive materials (iron nails) and the lower sand bed regularly, are imperative.

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