Question

if humans disappear will the water not get purified in nature

Answer 1

The same thing would happen to water purification plants as would happen to all technology if humans on Earth vanished - they would break down.

The more sophisticated the plant, the quicker it would break down - because it would be reliant on other technology for continuing operation - such as power & chemicals.

Less sophisticated forms of treatment, such as slow-sand filters - which treat the water by filtration and biologically by the ‘bugs’ that live in the sand media, would take longer to break down, but would eventually clog-up & be over-grown.