Performance improvement of a pelletisation system by pellet recirculation

A pelletiser is a reactor in which destabilised colloids are forced to be in contact with one another through a proper degree of agitation, resulting in small but very dense pellets. These pellets form a blanket through which the incoming turbidity is trapped. In this study, the performance of an upflow pelletisation process was improved by internal pellet recirculation. With the recirculation ratio of 0.1, local river water sampled during the rainy season (100–250 NTU) needed only 1.22 mg Al l⁻¹ and 0.1 mg l⁻¹ of polymer to achieve low turbidity of less than 5 NTU. In the summer, similar polymer dose and recirculation ratio were equally effective in treating 30–90 NTU water, even without an addition of alum, to the same level. An increase in the polymer dose could enhance the settling velocity of pellets, thereby considerably extending the feed velocity through the tested columns. Due to the higher washout effect from pellet recirculation, denser pellets of up to 1.05 or even 1.49 g cm⁻³ were found with the higher Q_R/Q ratios. With pellet formation, the recirculation system in conjunction with some polymer addition could perform very satisfactorily even in the absence of alum.