A water treatment apparatus includes a housing comprising multiple housing portions and a plurality of treatment sections disposed within the housing, each treatment section comprising at least one distinct particulate layer. The apparatus is designed so that no water pressure need be applied to force the water through the apparatus, as gravity pulls the water down through the filtering layers. Water may also be force fed through the treatment apparatus; when the water is force fed, the housing portions need not be vertically stacked, but may be configured in any suitable arrangement. The treatment sections are distributed through the various housing portions in logical groups to perform various kinds of filtering. The housing portions are detachable and sealably stackable in multiple configurations, thus passing water through more or fewer treatment layers, depending on the quality of the incoming water.