PCT No. PCT/NL96/00156 Sec. 371 Date Sep. 25, 1997 Sec. 102(e) Date Sep. 25, 1997 PCT Filed Apr. 10, 1996 PCT Pub. No. WO96/32177 PCT Pub. Date Oct. 17, 1996A settling device (2) for a fluid containing liquid, gas and particulate material comprises liquid-discharge elements (15) fitted close to the water level of the fluid and supply elements for supplying the fluid at the bottom of the settling chamber. In the settling chamber one or more caps (12) are disposed with their longitudinal axis at an angle to the water level, such that fluid supplied at the bottom of the settling chamber flows obliquely up, forced along the bottom of the at least one cap. Whilst gas bubbles contained therein collect in the ridge (17) of the at least one cap. The gas collected in the ridge of the at least one cap is trapped by elements (18) beneath the water level of the fluid. Preferably, use is made of a row containing a multiplicity of caps (12) which are disposed in oblique, overlapping and parallel arrangement. The caps (12) can be fitted at an angle of 60 DEG relative to the water level and preferably have a mutual interval of about 6 cm. The invention further relates to a purifier provided with a settling device of this kind and to a method for the anaerobic purification of waste water using a settling device or purifier according to the invention.