deHoxar Spiral Separator

Very compact gravity settlement device, maximising settled water quality whilst minimising wasted space

Compact Gravity Settlement

The de Hoxar Spiral Separator is an ultra-compact gravity settlement device invented by PWS Chief Engineer, David de Hoxar, and developed in conjunction with Southern Water.  The conventional means of separating out waste sludge using large settlement tanks takes up huge areas of land.  Once the sludge has been settled, it then has to be thickened for transport to the anaerobic digester sites. This requires additional equipment and buildings.

Our technology performs the same task as traditional settlement tanks, but requires as little as 3% of the space. It also has the added advantage of producing thickened sludge which saves the requirement for additional thickening equipment.

Our Spiral technology is now in operational use on twelve wastewater and potable water treatment sites around the UK and has potential for many other applications, including use on a range of industrial processes.

Further design updates are in progress to reduce the cost of production and improve the accessibility for maintenance and repair. Mobile units have also been designed to enable efficient installation and significantly reduce the time spent on site during the construction period.


Municipal wastewater treatment

  • Primary settlement of normal domestic sewage
  • Primary settlement with co-settlement of secondary (or other recycled) sludge
  • Activated sludge final tank
  • Humus tank
  • Other secondary settlement duty
  • Sludge liquor settlement

Potable water treatment – as a clarifier or for wash water recovery

Non-municipal wastewater treatment (or municipal with a high content of trade waste)



  • 3% of the site area is required when compared to conventional tanks and only about 25% when compared to lamella separators.
  • Thicker sludge is produced – typically 6% dry solids (thicker can be produced if required). In many cases, this saves one stage of sludge treatment.
  • Low cost to cover for odour control – once covered the volume of air needed to be treated is typically 1%  compared to conventional tanks.
  • Reduced visual impact makes it suitable for urban sites and sites next to areas with public access.
  • Anaerobic Digestion – ideal solution for efficient extraction of high calorific primary sludge for Anaerobic Digestion as part of utilities CHP programs.