NOVELTY - All first permanent magnets (1) in a first array, present their north poles to the cavity (4), with their south poles directed away from it. The array surrounds the cavity. All second permanent magnets (2) in a second similar array, have the dispositions of their poles reversed. Field lines extend from north poles to south poles, passing through the cavity.    USE - A plant for magnetic treatment of sewage sludge. The method is used to treat any of the sludges or solid wastes (including detritor output) arising on waste water treatment plant (claimed).    ADVANTAGE - This method is an advance over that described in DE10118839-A1. Residual sludge arising from the treatment of sewage sludge is minimized. The magnetic field is claimed to open-up the cells in the sludge, and hence to accelerate its digestion. The quantity of sewage gas made available for heating, is claimed to rise by 20-50%. A 20-30% reduction in residual sludge at the end of the process is also claimed. 50% reduction in digestion time is to be expected.    DETAILED DESCRIPTION - First and second magnets (1, 2) are arranged in alternation in a first direction at right angles to their magnetic axis. In the first direction three permanent magnets of the first or second magnet array and two permanent magnets of the second other magnetic array are arranged in alternation. The first and second permanent magnets (1, 2) are arranged in alternation in a second direction at right angles to their magnetic axis and the first direction. In a second direction at right angles to the magnetic axis of the permanent magnets and the first direction respectively, only first or second permanent magnets are arranged in succession. Magnetic flux in the cavity amounts to at most 1800 Gauss, especially 1500 Gauss and preferably more than 800 Gauss. The first and/or second permanent magnets are strontium-ceramic magnets. The cavity is a supply pipe to a septic chamber. An INDEPENDENT CLAIM is included for the corresponding method of treatment.    DESCRIPTION OF DRAWING(S) - A longitudinal cross section of the arrangement is seen.    first permanent magnets (1)    second permanent magnets (2)    cavity (sludge line to digester) (4)