Flocculation Performance of Mixing Impellers

Authors:

J. Y. Oldshue

O. B. Mady

Mixing Equipment Co., Inc.

This study investigates the performance of various mixing impellers in flocculation processes for municipal water treatment, comparing full-scale and pilot-scale results. Key factors analyzed include shear rate, shear stress, D/T ratios, impeller type, and mixing time, with turbidity and color measurements used to evaluate effectiveness. Results show that impeller design, tank geometry, and operating conditions significantly influence flocculation efficiency, power consumption, and overall water quality.

Key Learnings

• Different impeller types (A212, A200, R100, rake, and dangling plate) significantly impact flocculation performance in terms of turbidity and color removal.
• Shear rate and shear stress are critical factors: adequate shear is necessary for particle contact, but excessive shear can break up flocs.
• The D/T (impeller diameter to tank diameter) ratio affects both flocculation efficiency and power consumption; larger D/T ratios generally improve performance but require more energy.
• Batch tests with 60-minute settling times provide results comparable to full-scale plant performance, making them useful for pilot studies.
• Tip speed is not a universal measure of shear across different impeller types; each impeller has its own relationship between speed, shear rate, and performance.
• Flocculation efficiency is influenced by tank size and geometry, highlighting challenges in scaling up from pilot to full-scale systems.
• Power and capital costs can be optimized by selecting the appropriate impeller type and size for the required water quality.
• Turbidity and color measurements are reliable indicators for evaluating flocculation effectiveness when suspended solids are difficult to measure.
• Filterability of flocs produced by different impellers can be preliminarily assessed using small-scale vacuum filtration methods.
• Overall, proper matching of impeller design, operating speed, and tank configuration is essential for efficient and cost-effective flocculation.