Proceedings

The use of lime for sludge conditioning before dewatering in filter presses (pre-liming) is a widely used
concept. Lime (usually as milk of lime) is added to liquid sludge as a chemical conditioner together with a metal salt
such as iron chloride or aluminium sulphate to coagulate and flocculate fine particles, enabling improved
dewatering. Besides improving dewatering, lime sanitises sludge through pH elevation > 12, by destroying pathogens
(bacteria, viruses and parasites) and by transforming sulphides (H2S) and mercaptans (RSH) into their
non-volatile, ionic, non-odorous (HS-, S-- and RS-) forms.

The filtrate (at ph > 12 and containing metal salts) is usually recycled at the head of the waste water plant
and contributes to phosphate precipitation which in many cases avoids a separate de-phosphatising step.
In centrifuge dewatering, lime is usually not used for pre-liming, as this technology needs organic
cationic conditioners (usually cationic polymers) which are destroyed (hydrolysed) by the high pH that lime
brings.

This is the reason why in many countries where sludge hygienisation is compulsory for agricultural
recycling, an additional post liming step is commonly practiced, mixing lime with the centrifuged sludge.
However, this has the disadvantage of de-structuring sludge making it more “pasty” and adding an
additional step into the process.

Although dewatering efficiency is usually lower in centrifuges than in plate filter presses (typical 18-30%
DS for centrifuges versus 30-40% for plate filter presses), municipal waste water plants operators prefer
centrifuges which work continuously and need little space whereas plate filter presses work in batch
operation requiring more supervision and more maintenance.

Delayed reactivity lime (DRL), developed and patented by CARMEUSE, enables the use of quicklime
for pre-liming (sludge conditioning) in centrifuges, together with cationic or anionic polymers, combining
the advantages of the filter press process (conditioning, sanitisation, phosphates precipitation) with those
of the centrifuge.

DRL is quicklime that increases pH of the sludge to > 12 in the first step after mixing, without any negative
effect on most dedicated polymers, enhancing flocculation, and then reacts exothermically with the water
of the sludge only after dewatering. The relatively coarse particle size (typically x50 = ~80µ) compared to
hydrated lime (¯x50 = ~5µ) mineralises sludge, enhancing the dewatering to an even greater degree.
This technique which has been successfully tested in France and in Benelux, represents a real
breakthrough in sludge conditioning and sanitisation for agricultural recycling or further processing.

KEY WORDS
Delayed reactivity lime (DRL), Sludge conditioning, Sludge sanitisation (also called hygienisation), Dephosphatising, Waste water sewage sludge (WWSS)