Lignite conditioned sewage sludge has better dewaterability and better properties as a solid by-product. This study investigates the optimisation of the flocculation process, which is used to combine the lignite and the sludge. An insitu laser probe using focused beam reflectance measurements (FBRM) has been used to characterise the floc number and size distribution. The efficiency of the polymer and lignite additives are determined from these distributions.

Two quite different cationic flocculants are tested. The optimum dose and flocculation efficiency is higher for the polyelectrolyte with low molecular weight and high charge density. The FBRM study also show that the lignite particles agglomerate with the flocculated sludge during conditioning process which improves the dewatering properties of the flocculated sludge. The lignite particles are strongly agglomerated with the flocculation sludge for the high molecular weight and low charge density polyelectrolyte whereas; the agglomeration is weaker and shear sensitive for the low molecular weight and high charge density polyelectrolyte. The amount of lignite for optimum water removal is lower for the low molecular weight and high charge density polyelectrolyte compared to the high molecular weight and low charge density polyelectrolyte.