Mixing of Sludges with Liquid Jets

Sotiriadis, T., Firmansyah, T. and Dawson, M., BHR Group Limited


Mixing is one of the commonest operations in sludge processing and yet to date has received comparatively limited research
and development attention. Existing activities such as blending of different sludges, chemical conditioning, thickening, dewatering, heating and cooling, pasteurization and digestion require varying degrees of homogenization as an essential part
of process optimisation. Increasing energy costs are driving the industry towards mixing choices that favor lower power input
whilst the demand to mix more challenging thickened sludges has pushed power requirements in the opposite direction. This
has resulted in the need for better specified and designed mixing systems to ensure the process result is achieved with
optimum efficiency.

Sludge mixing is mainly being achieved using gas mixers, mechanical mixers and fluid jet mixers. This paper is concerned
with research into fluid jet mixers for sludge blending applications. Jet mixing does not require any rotating parts within the
vessel and the cost of purchase and maintenance is therefore perceived as being less than competing equipment designs.
The problems associated with jet mixer design for sludge applications are sizing the devices (nozzle number, orientation, jet
velocity & flows) for achieving quantified mixing performance in sludges of different rheological properties.
This paper discusses the recent advances in jet mixing modeling, research and design for sludge blending applications at
BHR Group Limited and elsewhere. The existing published jet blending correlations and their underlying assumptions as well
as numerical modeling efforts using Computational Fluid Dynamics (CFD) are put under the microscope and their usefulness
for sludge applications is thoroughly assessed.

Fluid Mechanics; Mixing; Sludge Blending; Jet mixer; Sludge tanks; Rheology; CFD.

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