Flow-through hydraulic flocculator

Abstract

Slow mixing is a basic and important process in physico-chemical water treatment plants, for induction of tapered orthokinetic flocculation in the formation of flocs that would be suitable for sedimentation. Hydraulic mixing gives certain advantages in terms of maintenance and energy saving, a fact that would make them preferable in many instances in water and wastewater treatment plants particularly in the developing countries. However field experiences with hydraulic flocculators have been far from satisfactory. Where an increase in the sizes of the flocs has been expected, floc break up has been observed instead. This study therefore was used to document and confirm the problems associated with hydraulic flocculators as a starting point in addressing these shortcomings. A 17.2-m3/day pilot plant with velocity gradient induced by round-the-end baffle mixing was used to investigate flow through a hydraulically mixed flocculator. Sodium chloride tracer was introduced instantaneously at the inlet of flocculator and the slug of the tracer detected on-line using a conductivity meter. Visual monitoring of tracer dye was used to complement and corroborate the tracer salt measurements. The results obtained showed that hydraulic mixing achieves reasonable dissipation. However the flow regime was not stable and resulted in the formation of dead zones in the centre and corners of the flocculator, short-circuiting and excessively high velocities at the 180-degree bend that disintegrated the flocs formed.


(Journal of Civil Engineering, JKUAT: 2002 7: 89-104)