A sand filter works in a way that the water flows from top to bottom through a sand bed, sand of a certain particle size is used. The particle size of the sand depends on what people want to remove from the water, it is often seen that different layers are used with different particle size to remove various floating particles. In the sand bed is a porous layer that ensures the operation of the filter. Various physical and biological process can also take place in a sand filter that causes removal of even more components. The removed particles will accumulate in the upper layer of a sand filter and after expiration of time, more pressure will be needed to get the water through the sand filter. In order to prevent a filter from becoming clogged, one will after a while backwash the sand filter, whereby the removed particles are flushed out and the filter can be used again. This back-washing is done by reversing the flow direction of the water.