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Sludge Conditioning / Thickening and Dewatering

Liquid/solid separation of bio-sludge occurs most effectively with optimal chemical preconditioning.  Most importantly, ferric sulfate or ferric chloride satisfies the high cationic demand of bacterially activated sludge.  It also produces moderately sized, but densely formed flocs from synthetic polymer flocculants.  Though dependent on individual sludge characteristics, adjusting sludge pH to the 4-7 range with a ferric salt typically provides optimum dewatering performance.  Sometimes an alkali, such as CAL~FLO® slurry, will be required to improve sludge dewatering and improve the solids content of the final sludge.

Sludge Thickening

Many water and wastewater plants use various mechanical means to thicken dilute sludge from the clarification process.  Employing ferric sulfate or ferric chloride to pre-condition sludge before a thickener will lower the polymer flocculant dosage required for thickening.  Sludge treated with a ferric salt will also have improved effluent (filtrate) clarity.


In belt presses, sludge conditioning with ferric salt and CAL~FLO® slurry has the potential to reduce polymer dosage and achieve higher cake solids concentration.  The operator will have the option to increase applied belt pressure or slow the belt speed without fouling the belts.  These options exist because ferric sulfate and ferric chloride help the polymer produce a denser, more freely draining floc.  In many cases a ferric salt will help lower filtrate solids because fewer solids will adhere to the belts.  Not only are there fewer solids to wash from the belts, but shower water consumption may be reduced.

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