A water treatment device comprising a slow sand filter in fluid flow combination with a nanofilter for water pretreatment purposes. The nanofilter is surrounded by a housing spacedly arranged thereto which housing is capped at each end and axially aligned with and connected to a T-shaped pipe fitting at each end thereof. A pipe is arranged to extend axially through the T-shaped fittings, the caps and the nanofilter which pipe is apertured along that portion thereof in said nanofilter. Water fed from the slow sand filter into one T-shaped fitting is directed inwardly and laterally through the nanofilter with the purified water flowing through the apertures of the pipe with a discarded portion of the water being conducted through the second T-shaped pipe fittings out of the device for disposal purposes.