Putting the Squeeze on Sludge – The Adoption of Screw Press Technology for Municipal Sludge Dewatering At New Plymouth WWTP

New Plymouth District Council have undertaken a major upgrade of their sludge thickening and dewatering facilities to substantially improve the dry solids content of dewatered sludge. The key drivers of the upgrades are; to reduce energy use in the existing downstream thermal dryer process, improve the operational reliability of the existing dewatering process by replacing the ageing direct coupled Gravity Belt Thickener (GBT) and Belt Press process; and increase redundancy by producing a dewatered sludge that can be landfilled directly if required (>20%DS). Screw presses were selected as the preferred technology based on early trials and evaluation of a range of technologies.

A screw press essentially squeezes the feed material (in this case thickened waste activated sludge) against a screen, utilising a slowly rotating screw. The “pressate” (liquid stream) is drained by gravity from the underside of the machine, and the dewatered sludge is conveyed out of the end of the machine. The “squeezing process” is regulated by an automatically adjusting backing plate which moves back and forth to maintain a constant “squeeze” pressure inside the machine.

This paper presents the project from master planning to commissioning, including the outcomes of early operation. A number of considerations and lessons learnt that influenced the design, construction and early operation of this project, which have also been discussed in this paper.