The plant according to the invention has a drying apparatus (9) which is provided with slow-running conveyor belts (8,11) disposed in each their separate compartments. In connection with the first belt (8) there is provided a sludge laying-out device (7) having a nozzle opening with a diameter in the order of 5-8 mm. The sludge laying-out device (7) has a nozzle which can be moved transversely to the belt (8). The drying apparatus (9) can be supplied with combustion gases which are developed in the burning of the dried sludge in a furnace. With the method according to the invention, the sludge laying-out device (7) lays dewatered sludge on the belt (8) in a first part-process where hot gases having a temperature between 150 and 250 C sweep the laid-out, string-formed sludge mass (74) on the belt (8), which ensures that the mass retains its string form and therewith its air permeability during the second part-process on an underlying belt (11).