Electricity free operating water purification installation with a membrane filter and a system of automatic backwashing of that filter characterized in that during the operation a fixed percentage is diversed from the outflow of filtered water and is stored in a container under progressive pressure, which to a certain maximum pressure is cut of from a feedback to the membrane filter, so that consequently after filtration of a certain volume of polluted water through the membrane filter the pressure in the container will exceed the maximum resistible pressure by the one way closure, which then will open so that the contents of the container will be forced back through the membrane filter under higher pressure than the original filtration pressure, whereby is provided in automatic closure after the container is emptied.