Decentralized Urban Groundwater Nitrate Remediation Using Low-Cost ZVI Reactors: Laboratory Validation, Colony-Scale Simulation, Techno-Economic Assessment, and Deployment Feasibility

Elevated nitrate levels in groundwater across India's urban peripheries present a pressing public health challenge, especially in unplanned settlements where formal remediation infrastructure is lacking. This study proposes a scientifically validated, cost-effective nitrate mitigation approach based on modular zero-valent iron (ZVI) reactors designed for decentralized installation and passive operation. Using detailed laboratory experimentation and field sampling across 30 stations in Bellandur (Bengaluru, India), a deployment simulation was created to model the reactor's real-world scalability. The gravity-fed column reactor constructed using locally available media—sand, charcoal, brickbats, and red soil, which contain ZVI capable of absorbing nitrate—demonstrated 85%–90% nitrate removal efficiency across varied synthetic influent levels, achieving compliance with the Bureau of Indian Standards (BIS) nitrate permissible standard of 45 mg/L. The deployment simulated reactor incorporating field data, population exposure indexing, and reactor allocation was developed, achieving an average nitrate removal efficiency of 82.4% across high-risk zones, with post-treatment concentrations reduced below the BIS permissible standard. Based on the above functions, the simulation recommended the installation of 3335 ZVI reactors across 17 high-risk colonies, directly serving over 28,500 households and addressing a cumulative nitrate load of 2984 mg/L. Comparative cost analysis revealed that this system reaches breakeven against reverse osmosis (RO) within 4 years and against ion exchange (IX) in 7 years, offering a practical alternative for corporate social responsibility-aligned groundwater rehabilitation programs. This is one of the first studies to integrate laboratory validation, colony-scale deployment simulation, and techno-economic assessment into a single nitrate remediation framework for peri-urban India and similar underserved regions, providing both technical validation and a replicable policy blueprint, providing a validated deployment framework rooted in field data, cost realism, and performance metrics.