Centrifugal Pump De-rating in Sludge Applications

Holm, R., ITT Flygt, Sweden


Wastewater treatment plants (WWTP) require a wide range of fluid transports of suspensions with different flow
characteristics. The aqueous suspensions of various concentrations of suspended particles and dissolved
substances result in non-Newtonian behaviour. The non Newtonian characteristics of sewage sludge also vary
depending on its origin and the history. Sludge shows different characteristics for household sewage sludge or for
industry sludge. The treatment process itself alters the sludge non-Newtonian characteristics such as: polymer additives or
mechanical steps like drainage. However, in a recent study1 it is shown that sludge shows essential shear-thinning

Pumping sludge in WWTP has traditionally been performed by progressive cavity pumps, (PC-pumps). However, they are
associated with high maintenance costs. An introduction of centrifugal pumps in these applications is of great interest.
The knowledge of centrifugal pumps in water applications is deep, but there are still some open issues in pumping sludge
that requires further study.

The purpose is to study the pump performance of N-impeller pumps by ITT Flygt impact of non-Newtonian flows. This also
includes improved knowledge about the sludge treatment process and the rheological characterisation of sludge.
Furthermore, it is part of our business marketing plan for Npumps in municipal waste-water treatment plants.
The objectives for this project is to provide tools and recommendations for dry-mounted N-impellor centrifugal
pumps in sludge application in order to obtain more appropriate custom solution in a energy saving manner.

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