Recently we were presented with a great opportunity to make a proposal for a new $100 million dollar Design-Build water treatment plant. This plant consisted of a Preliminary Treatment System, Flocculation System, and Residuals Treatment Systems all of which can be expanded for larger capacities in the future.
JMS would participate in the Flocculation System which was intended to produce a floc optimized for direct filtration. The city specified vertical shaft pitched-blade impeller mixers for each of the first and second stage flocculation cells. JMS also proposed Vertical Paddle Wheel Flocculators as an alternative to the city’s preferred impeller mixers.
Mixer specifications called for flocculator with impellers, shaft, drive unit, accessories and the installation hardware for a complete installation. Drive units had a motor for VFD drive and a gear reducer and mounting plate. JMS proposed our recommended Vertical Paddle Wheel Flocculator as an alternative to the Impeller Mixer.
Costs for both of the mixing systems were priced as follows for comparison:
- Impeller mixers $37.2k
- Vertical paddle wheel flocculator $36k
Each was a complete system for the application, and were approximately equal in price. There are, however, other differences that can influence results, and the service lifetime of the equipment, some are:
- There is a larger paddle surface area contacting the water phase on the Vertical Paddle Wheel flocculator. The blades of the VPWF extend to within 1’ from the walls and the top water level. This means that adequate mixing can be achieved at lower speeds. Lower speed means fewer RPM from the gear box and better longevity.
- The Vertical Paddle Wheel Flocculator can run at a lower tip speed so it meets the 10 States Standards for tip speed. The higher impeller speed of the Impeller mixer does not meet 10 States Standards and can result in floc shear, and reduced performance. Poor flocculation can cause issues in the sedimentation process which can send floc downstream to filters or membranes causing damage to these high cost components.
- Since the Impeller Mixer is supported by the gear box, a special agitator gear box is used which contains more bearings. The Vertical Paddle Wheel Floccuator uses a special radial bearing to extend the bearing’s life.
- The Impeller Mixer does not have any bottom steady guide. The Vertical Paddle Wheel Flocculator uses a steady guide to prolong the life of the system. In this case JMS was also proposing the Lamella Plate Settlers and sludge removal, so the choice of the Vertical Paddle wheel flocculators would result in a single point of responsibility for the flocculation basin.
So, all in all, the two types of Flocculation Basin mixers are approximately equal in price. The Vertical Paddle Wheel Flocculator provides the advantage of meeting the 10 States Standards, provides for a better floc that results in better performance, and offers greater longevity.
Customers sometimes make decisions on equipment based on preferences from past applications. In this case, however, with the two types of equipment at the same price, we feel the Vertical Paddle Wheel Flocculator is the best choice.