Sludge from a waste water treatment plant is de-watered within a single- or two-stage eccentric worm pump (10) which receives dilute sludge through an inlet (28). The helical eccentric pump (10) feeds a further worm pump (14) driven by the same shaft (18), and which has a sludge-separator within the same housing (16). Filtrate is discharged to an outlet (26) on the downstream side of the assembly, while thickened sludge is discharged at the other end of the worm (14). A flocculant agent is continually fed (30,32) to the sludge pump (10) inlet zone.
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