Cuatro Communidades Flocculator

Abstract

The flocculator at the Cuatro Communidades plant uses a vertical set of baffles to provide energy dissipation to mix alum and contaminants. The Cuatro Communidades plant has three flocculator channels. The baffles are arranged in a tapered configuration so that flocs are exposed to higher energy dissipation rates initially and lower rates as they grow. In this study, The tube settlers were used to measure residual turbidity at different points along the flocculator. Tube settlers were placed along the flocculator to determine how much flocculation was occurring in the last flocculator channel. The results were compared at different raw water turbidities.

Introduction and Objectives

During flocculation alum is mixed with raw water and in the process aggregates of contaminants called flocs are formed. Flocs settle out of water more easily than the contaminants themselves because they are heavier. Alum is a coagulant used to help floc formation. Conventional flocculators typically use a long slow mechanical stirring process to mix alum and contaminants together. This long process slowly builds large flocs. To avoid the use of electricity, flocculation occurs as water snakes through a vertical set of baffles. The energy dissipation rate through these baffles is set by the distance between the baffles. This rate is a measure of how much mixing is occurring.

The Cuatro Communidades plant has a tapered set of baffles. The baffles are close together and the energy dissipation rates are high early in the flocculator. When the flocs have grown and are larger and more fragile the energy dissipation is lower. It was unclear at what point in the flocculator the flocs were large enough that they would settle out in the sedimentation tank. Tube settlers were used to measure residual turbidity along the flocculation channels. Tube settlers were placed at different locations in the flocculator to determine if adequate flocculation had occurred before the end of the flocculator and if the last channel in the flocculator was improving residual turbidity.

Methods

Each tube settler functioned in the same way as isolated lamella in the sedimentation tanks. In order to mimic the design of the lamella in the Cuatro Communidades plant the tube settlers were designed to rest at a sixty degree angle, alpha, and with a critical velocity, V _alpha, of 10 m/day. The critical velocity of the tube settler determines the size of the smallest floc that is able to settle out in the tube settler.
The critical velocity is calculated by the equation:
The tube settlers were designed to siphon water from the flocculator. The flow rate was controlled by the difference between the tube settler outlet and the water level in the flocculator using the equation:

The tube settlers were placed at various locations in the last two flocculator channels and the effluent turbidity from the tube settlers was measured.

DIAGRAM OF FLOCCULATOR and locations measured

Results and Discussion

graphs

Conclusions

decreasing by 1-3 NTU in 3rd channel