Influence of flocculator length and alum dosage on flocculation
Overview
Experimental Set-up
Abstract
Flocculation in water treatment plants is an essential step since it helps settle out colloidal particles with the use of a coagulant agent to facilitate particle collision and growth. For the AguaClara Tube Floc Team, the team's goal is to conduct experiments to further understand and optimize hydraulic flocculation. Specifically, the team will narrow down key design parameters such as the optimum length of the flocculator and optimum alum doses for each length and varying influent conditions tested. The experiments completed thus far include the replication of the Spring 2009 experiments and the study of the effects of alum dosage and length on flocculation.
Results from varying flow rate were consistent with the results from the previous team, which further confirmed that as energy dissipation rate increases the average floc size and the sedimentation velocity both decrease, while yielding higher residual turbidities. The success of replicating an experiment and obtaining similar data from Spring 2009 allowed the team to start conducting its own experiments to find the optimum alum dosages for different lengths of the flocculator. From these experiments so far, it's evident that as alum dosages increase, the residual turbidity decreases. However, at a certain point, a limit is reached where residual turbidity remains constant as alum dosage increases. Also these experiments have shown that with increased flocculator length, the behavior of the floc depends on whether the influent turbidity is high or low. At low influent turbidity, increasing the flocculator length improved the residual turbidity. At high influent turbidities in general, an equilbrium is being reached wherein the flocculator and particle size does not very. Thus, the same residual turbidities are found for all lengths of the flocculator at higher influent turbidities.
Introduction
The turbidity of water is caused by colloidal particles in suspension (and the presence of natural organic matter and other organic and inorganic contaminants). Colloidal particles are too small to settle and due to their negatively charged surfaces, electrostatically repel each other. Flocculation transforms colloidal particles into larger flocs that can settle out in the sedimentation tank. The probability (collision potential) that particles collide in a flocculator depends on energy dissipation rate and residence time in the flocculator. As flocs collide, they grow in size making it easier to remove them in subsequent processes.
There are different types of flocculation, such as charge neutralization and sweep flocculation. In AguaClara we utilize sweep flocculation methods, in which we need to add a coagulant agent, alum for our purpose, which forms a precipitate of aluminum hydroxide that covers particles and enables them to stick together when they collide in the flocculator. However, charge neutralization is also occuring in the flocculator and it is actually the predominant mecanism for high turbidity water.
Conventional design guidelines for a hydraulic flocculator are incomplete and the dynamics of how physical parameters affect flocculation are not well understood. The goal of the Tube Floc Team is to try to improve understanding of flocculation for a variety of influent water qualities and provide better guidelines in designing a flocculation system.
Conventional design characterizes a flocculator with a laminar velocity gradient, G, and residence time, θ (Tambo and Watanabe, 1979). Because these are known parameters in literature the team has adopted them into its research, which can be characterized using a laminar tube flocculator. Currently the Tube Floc team is studying the effects of length and alum dose on varying influent turbidities and will determine the optimal values for these two conditions.
Material and methods
For studying the function and development of the flocculator, there are many possible experimental setups and analysis methods. In the case of our experiments, we have been assigned to study the tube flocculator and have used several essential computer programs, such as Process Controller and Mathcad, to retrieve and analyze data. The following links will show more details on the specific apparatus and methods we have used:
*Experimental Apparatus
*Data Acquisition
*Data Analysis
Results and discussions
By the end of this project, we hope to fix any possible errors missed in the past and obtain enough information to aid in the improvement of the AguaClara Water Treatment plant.
During fall 2009, two challenges were tested with the completion of one. For the first challenge, the study of the effect of shear rate on tube flocculator performance, our goal was to replicate an experiment from spring 2009 for the team to familiarize itself to the experimental apparatus and analysis techniques. Though this challenge did not provide new data, it showed the overall functions of the tube floc experimental setup and the computer programs used for data analysis.
Participating teams
Fall 2009
*Fall 2009 Goals
*Fall 2009 Weekly minutes
*Future Challenges
Spring 2010
*Spring 2010 Goals
*Spring 2010 Weekly Minutes
*Future Challenges
References
Tambo, N. and Watanabe, Y. (1979) "Physical characteristics of flocs. I. The floc density function and aluminum floc", Water Research, 13(5), 409-19