The invention concerns an apparatus for filtering and separating flow media by reverse osmosis or ultrafiltration, in particular for water desalination and water purification, wherein there are provided a housing (10) having at least one feed (14a) for the flow medium to be filtered or separated and at least one respective discharge (14b, 20a) for the permeate and the concentrate, and a filter device (30). The filter device is formed by a flat filter element (30) which is arranged in the housing (10) and which is disposed in a back and forth meander configuration in the housing (10) forming at least one flow channel (K1, K2) for the flow medium to be filtered or separated.