Kanchan arsenic filters in the lowlands of Nepal: mode of operation, arsenic removal, and future improvements

Abstract

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.

Results of the First Improvement Step Regarding Removal Efficiency of Kanchan Arsenic Filters in the Lowlands of Nepal—A Case Study

“…Inspection of the other two removal rates (39.4% of the nail bed; 95.1% of the sand layer) clearly indicated the importance concerning the capacity of the fine-grained sand (grain size < 2 mm) to remove exfoliated particles from the nails above. Inspection of the calculated contact time with the nails revealed a time of 31.55 min for the SN4A filter, which was the highest of all, and it was determined so that the extremely high influent As concentration of 735.6 µg/L could be lowered to 21.65 µg/L, despite the low concentration of Fe (1.27 mg/L) [9,11]. In the second-best performer (i.e., SN63), with an overall removing efficiency of 93.2%, the nails were lightly covered with water, the filter was used regularly keeping the nails wet, and the sand was fresh.…”

“…Commonly elevated concentrations of As are typically observed in areas with fine-grained sediments [6][7][8][9]. Arsenic is particularly incorporated in finer particles such as clay minerals [9][10][11].…”

Fenton pre-oxidation of natural organic matter in drinking water treatment through the application of iron nails

“…Considering this, some researchers have applied commercial iron-nails in different water/ wastewater treatments e.g. in sand filters during the filtration process for arsenic removal [38][39][40][41], in Electrocoagulation process for removal of mercury (II) [42], and in ultrasound-assisted Fenton process for wastewater treatment [43]. However, the application of commercial iron-nails for NOM removal in the drinking water treatment plants has not been reported.…”

Universal Access to Safe Drinking Water: Escaping the Traps of Non-Frugal Technologies

“…The current presentation focuses on decentralized or point-of-use (POU) systems for safe drinking water provision at a community-scale. Household systems are excluded because monitoring their operation is very difficult [5,[60][61][62][63]. However, the generic approach for the development of modular water treatment systems discussed here can be downscaled to the household level.…”