The water in the Swami Pushkarini

 The Swami Pushkarini (The Sacred Temple Water Tank)

It is very usual and highly probable to find a water tank either just beside or nearby every Hindu temple constructed during the ancient and medieval times. The waters of the tank were strictly for temple purposes only - like for the God's "bath" that is a ritual/worship, cleaning and washing the temples insides and

outsides, for use inside the temples for drinking purposes (Not in Tirumala, though), kitchens (if any) and by large, by the folk that stay and live by the temple like the priests' families and workers, for their daily uses. The water's chief and famous uses are bathing and washing by the visiting pilgrims who take a dip or swim in it and, above all, for the temple's special occasions when the divine images are taken in the temple boat, in the tank waters. The water in the Swami Pushkarini is absolutely NOT stagnant OR infected. It has a state-of-the-art recycling facility and water is treated before it gets into and recycled continuously. Swami Pushkarini was a pleasure tank of Sri Maha Vishnu in Vaikuntham, and was brought and set on Earth by Garuda, for the sport of Sri Venkateshwara. It is adjacent to the Sri Venkateshwara temple. A bath in the Swami Pushkarini is believed to clean pilgrims of their sins and bestow temporal prosperity. Pilgrims bath here before entering the main temple. A bath in the holy tank purifies one's body and soul. 

Source : http://www.tirumala-tirupati.com