This paper explores the river as a site of urban modernity in India. At the heart of this paper is the colonial project of purifying the water of river Hooghly for the domestic supply of Calcutta. The British built the first water purification system for the city around the middle of the nineteenth century at Pulta. Around this history, the paper looks at the various discourses and practices of pollution, purity and purification. The debates were not just about whether the river was polluted or suitable for the supply of water to the city but whether piped water itself was pure. In this story, the science of purity confronted Hindu ritual purity. At another level, the very idea of purity itself was on trial. One of the main sites of examination for this paper is thus the various notions of purity at play in Calcutta at this time within both western science and Hindu scriptural deliberations. These were accentuated by the fact that in Calcutta and several other colonial cities, water was conceptualised through multiple semantic and spatial tropes. The paper situates the project of purification at the heart of this entangled reality and discourse of purity of water.