Control and Elimination of Scale

Formation of scale deposits within sewage sludge dewatering systems can be a source of much costly downtime and maintenance. The Yorkshire Water Waste Water Treatment Plant at Naburn provides facilities for the South York area and had been suffering from this problem for some time. SNF was approached to explore solutions to the problem. Following a site visit and discussions, proposals were put forward to solve the problem by installation of a small dosing system for use with Flosperse 3000 W10.

What causes scale?

  • Precipitation of insoluble phosphate salts
  • Struvite: magnesium ammonium phosphate
  • Vivianite: calcium ammonium phosphate
  • Caused by a rise in pH via digestion and loss of CO2 through air expulsion during centrifugation

Consequences of scaling:

  • Scale deposits are found on the centrifuge scroll and bowl
  • Can unbalance machine leading to expensive damage
  • Also found in centrate lines leading to restriction (centrate backs up and foams) or ultimately complete blockage needing specialist descaling or line replacement

During the site discussions, it was found that the centrate pipework was completely blocked with hard scale.

Before dosing with Floperse 3000 W10, it was necessary to dissolve and flush out the scale with a dilute acid solution furnished by SNF.

Dosing commenced in July 2007. It was noted at a routine centrifuge service visit a few weeks later that the scale build up was almost non-existent. The project was reviewed after 6 months and it was estimated that, after chemicals costs, a net annual saving of £3,700 on maintenance costs alone was being achieved. There will be associated cost benefits from reduced wear and tear on the centrifuges, pipework and downtime for maintenance.



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