A modular water treatment apparatus is disclosed having a plurality of cells (12) each respectively arranged in use for housing a treatment bed and a plurality of spacers (14) located between respective adjacent cells (12). In use the spacers (14) are for providing an air gap between the cells (12). The cells (12) and spacers (14) are stackable to provide a multiple stage treatment apparatus in which water to be treated percolates through the respective treatment beds located in the cells (12).