Mr David de Hoxer from Southern Water in Hampshire gave a presentation at the Lodge discussing new developments in separation devices in Water Treatment plants.

Sewage Treatment is usually carried out in two stages: primary settlement tasks, in which most of the solids settle and a secondary treatment stage, in which other parts of the solids are recovered. The spiro separators, invented by David de Hoxer, are used in the primary settlement stage and are based on the slow movement of a series of concentric helix shaped plates. It is a novel gravity separation device which deals with sludge in a more efficient way allowing the use of smaller areas for treatment.

The device consists of eight different helical plates which when rotating, allow the solids to slide downwards, raising the water as in an Archimedes Screw. Their dimension varies from over a metre to nearly ten metres, which are the dimensions of the new spiro recently designed by David.

The new device is an improvement on the classical Lamelle's separators which consist of a series of fixed individual plates inside the tanks. These separators require more maintenance and are less efficient.

The advantages of the spiro separators are :

  • Radial symmetry which give good inherent flow distribution
  • Rotation improves separation
  • Cross flow design
  • The plate pack depth is not limited by the level of sludge.
  • Produces a thickened sludge

The new invention has been employed in four different plants and the system is working well. They can achieve up to 99% of solids after secondary treatment or up to 85% in just the primary settlement stage.

His solution is also environmentally friendly as it allows for the design of smaller plates and ensures that the sewage has secondary treatment, which is becoming essential in view of more stringent current standards.

Another advantage of the spiral systems, in addition to the smaller site required, is that they can be easily covered to control odours.

The Seminar was followed by a lively discussion, further demonstrating the interest of WIT researchers in practical engineering problems.