Two is Better than One: A More Efficient Drying System

Browne, D.W., Kurvink, M.R. and Fernando, T.
Flo-Dry Engineering Ltd


The use of Rotary Drum Dryers for drying dewatered sewage sludge has become a conventional process. There are
numerous operating installations in the USA, Europe and Asia, including Australia and New Zealand.
In the drying process a “sticky phase” is encountered at dry solids levels of around 60-65%. In this phase the drying
sludge becomes difficult to convey, thus increasing the risk of hold-up and over-drying. Moisture removal from wet sludge
particles becomes difficult, with the result that drying rates are lowered. 

To avert this “sticky phase”, conventional drum dryer designs use a recycle of dried granular product. The recycle stream is
mixed with incoming wet sludge in an appropriate ratio (up to 3:1) to give a feed to the dryer of dry solids content above
70%. The mixing process results in the dry granules being coated with wet sludge, in such a way that only surface drying
is needed.

This recycle system adds complexity and results in relatively high energy use because of the energy used in the
conveying, screening and crushing operations. The recycling of dry product also demands specialised equipment and
instruments in order to overcome the hazards associated with dust.

In an attempt to find an improved drying process a research team was formed. The team considered the limitations
outlined above and in particular how to remove the complexity of the recycle stream without losing the ability to
produce a good quality dried product. One important area of consideration included how to feed wet product into the
drying system in the absence of recycle.

The result was a design that combines two drying technologies in a unique way to produce a consistently better
outcome in terms of fuel and electrical energy efficiency. The Two-Stage Dryer is designed to dry dewatered sludge only,
without recycle, using two small dryers of different types in series.

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