A sludge digester comprises a vertical silo internally partitioned into several sections by spaced, horizontal shelves which are permeable to the passage of air and water, each shelf being made up or hinged shutters which can pivot to deliver slude to the shelf below, the silo being top-loaded with raw sludge which is dropped, shelf-by-shelf, over a suitable digestion period, to be delivered from the bottom section as compost. Every other section has an inlet above sludge level into which air is blown to pass down through sludge in that section, into the top space of an intermediate section below, and up through sludge on the shelf above, into the top space of the intermediate section above, each intermediate section top space being provided with an exhaust outlet for incoming air. Used for disposal of sludge from waste water treatment plants by a digestion process producing a fertilising compost. Air circulation is controlled without employing secondary, impermeable shelves which are not only expensive, when adapted to drop the sludge periodically, but also create a problem by trapping pollution water draining from the sludge; in the new design, these drainings assist digestion and are finally sterilised by the heat in the last section at the bottom of the silo.
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