This study evaluates the implementation and effectiveness of in-line chlorination (ILC) for improving drinking water quality in rural Nigeria. Unsafe drinking water remains a major contributor to diarrheal disease, particularly among children under five. Inline chlorination is a passive water treatment approach that automatically doses chlorine at community water systems without requiring electricity or daily user action. Two cluster randomized controlled trials will be conducted in Kano State (North-West Nigeria) and Cross River State (South-South Nigeria). Communities will be randomized to either receive in-line chlorination installed at eligible communal water systems or serve as controls with no chlorination. The unit of randomization is a community or a cluster of communities that share water system for drinking water. The primary objective is to estimate the causal impact of in-line chlorination on household drinking water quality. Outcomes include the prevalence of Escherichia coli contamination in tap water and stored household water as well as the presence of free chlorine residual. Secondary objectives assess water source usage and adoption of chlorinated sources, as well as reduction in diarrheal disease. Implementation fidelity and operational performance of chlorination devices will also be monitored.