Abstract: An anaerobic reactor (1) for treating waste water includes a reaction vessel (2) and a three phase separator (4) above the reaction vessel and arranged to receive effluent from the reaction vessel. The three phase separator includes an outer wall (10, 14) connected at its bottom to the top of the reaction vessel and a liquid outlet (42), a lid (16) closing the top of the outer wall. The lid has a gas outlet (17) above the level of the liquid outlet. The three phase separator also includes a funnel (18) arranged above the reaction vessel, a guide wall (30) spaced from and arranged radially outward of the funnel so to surround an upper aperture of the funnel and a baffle wall (36) spaced from and arranged between the guide wall and the liquid outlet.