Turbidity removal by the up-flow pelletisation process for low turbidity water

A fluidised-bed pellet reactor was successfully used to remove turbidity from synthetic high-turbidity (2000 NTU) raw water, as is done in China. In this study, an up-flow pelletisation process, after a certain start-up process, removed the turbidity present in the synthetic low-turbidity (50 NTU) feed water. The process consisted of a rapid-mixing unit and an agitated fluidised-bed pelletiser with an overflow effluent weir. Poly aluminium chloride and anionic polymer were used as the coagulant and coagulant aid, respectively. The pellets thus formed were very stable and were of a very high density. A high up-flow velocity of 30–40 cm/min. (18–24 m/h) in the reactor resulted in a shorter retention time of only 4–5 min in the pelletiser, while still producing a very clear effluent of better than 3 NTU quality. This retention period is much less than the 2–3 h retention time normally employed in conventional sedimentation tanks, and can therefore drastically reduce the construction cost.