An apparatus and a method for treatment of water is described. The method comprises the steps: - providing (401 ) untreated water 83) to a first compartment (12) of a container (2) - providing (403) a precipitation agent to the first compartment to precipitate a substance such as phosphate from the untreated water in the first compartment and providing a solid substance (6) having small particles to the first compartment, - aerating (405) the mixture in the first compartment (12) to let an precipitated substance bond to the small particles of the solid substance, - by aerating the layer allow small particles having the precipitated substance bonded thereto pass (407) under the force of gravity down through the layer to a collection container (8) located underneath the layer, - output (409) treated water (5) from a second compartment (13) of the container located 10 down streams the layer (4).