There are no on-line methods of monitoring for bacteria in purified water that are rapid or sensitive enough. The usual procedure is to take samples and collect any bacteria on a filter which is subsequently incubated on a medium such as RA2 agar for several days before counting the colonies formed. The resultant value is the total viable count (TVC). As the bacterial count is not known when the water is used, it is essential to ensure that the water purification system is designed to minimise bacterial build-up and the systems are regularly monitored off-line.