Abstract: Thermal oxidiser (1) for the oxidation of a feed liquid (W) comprising an elongated retort (3) provided with a combustion gas input lance (5), a feed liquid input lance (7), one or more flow straighteners (11) and an exhaust gas outlet (13) wherein said retort (3) comprises an elongated wall (15) with a first end plate (17) fastened to a first end (19) thereof and a second end plate (21) fastened to the second, opposite end (23) thereof, combustion gas input lance (5) projects substantially horizontally through first end plate (17) into said retort (3) and feed liquid input lance (7) projects downwards though the retort wall into said retort (3).