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.Key words: Purification, pollution, Hooghly, Calcutta, Water, Pulta, sewage 1 At the heart of this paper is the colonial project of purifying the water of river Hooghly for the domestic supply of Calcutta. The first water purification system for the city was built by the British 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 about not just whether the river was polluted or suitable for the supply of drinking water to the city but whether the piped water itself was pure. At one level, these were based on two different notions of purity and pollution. On the one hand, in the Victorian sanitarian regime that imposed itself upon Calcutta and its river, impurity had a clear physical dimension in terms of visible filth on the surface of the water, microscopic germs, or silt. On the other, within Hindu ideas pollution and impurity had a more intangible and ritualistic meaning. The river itself, the object of the colonial project of purification, was venerated by Hindus as sacred and was thus considered inherently pure and incorruptible.In this narrative thus, the modern sanitarian ideas confronted the Hindu notions of ritual purity.Yet, this was more than a debate between orthodox Hindus and the colonial regime, as for both the parties the question of filth had deep moral connotations. Modern practices and habits of cleanliness have often been seen to resemble non-modern rituals of warding off impurity.1 For both the Hindus and the colonial sanitarians, the impurity and purity of the river had layers of visible and invisible meanings. While the natural and visible muddiness of the Hooghly was perceived to be an impurity by the colonial officials, it was accepted as an essential part of the presumably pure and sacre...