Abstract: A modular capping arrangement for an anaerobic slurry digester comprising a basin which may conveniently be dug out of the earth and a plurality of elongate capping members of inverted channel form comprising wall members which, when the channel members are placed in side-by-side arrangement, define therebetween channels which are upwardly open. The capping members are formed with outlets in upper regions thereof for removal of gas produced during slurry digestion and are weighted down on the slurry by ballast means positioned in the upwardly open channel. The capping members are disposed in parallel arrangement and held together at the ballast means and are so formed that the capping arrangement is capable of floating on a body of slurry while being held thereon by the ballast means and being lifted therefrom by gas under pressure thereunder.