A pressure-responsive switch senses the rising back pressure in a sand filter caused by debris collected in the filter, and at a predetermined pressure indicating the need for cleaning the filter, it triggers the reversal of valve means which reverses the flow in the filter and backwashes the debris into a separating tank where the debris is collected from the backwash flow which is then returned to the swimming pool. A timer controls the backwashing interval, at the end of which the valve is again reversed by a reversing mechanism, returning the system to normal filtering operation.