Flocculation

[AH2410]

In a 35000 M3/day water treatment plant, Alum solution is added to raw water in the distribution chamber and a vertical turbine rotating at 6 rpm is employed for rapid mixing. Water is then conveyed to two circular flocculators each has a gentle mixing system at the inner circle which rotates at 12 rpm. I cannt understand why is the flash mixer slower than the gentle mixing system ( vertical turbine also). Does anyone have an explaination for thais.

 

[bimr]

You have not provided enough information for someone to comment on your application.

You are correct in thinking that the flash mixer should generally be operating faster than a flocculator. The rpm that you mentioned appears to be appropriate for a flocculator.

The rapid mixing rpm appears to be too low if you are describing a standard rapid mix system with 30 seconds detention.

However, you have not mentioned the piping layout. It is possible that the designer may be accomplishing mixing through changes in pipe direction.

You have not mentioned operating results either. If the plant is operating satisfactorily, why change? If not, then the rapid mix system may be due for an upgrade.

 

[AH2410]

bimr
I do not think there is another ways for mixing rather than the mechanical mixer. here you are further info;
the distribution chamber is a cylinderical shaft 3m diameter and 2.5m height. water enters from the bottom and flows upward where alum is added and mixed by the rapid mixer(6 rpm).flow is then distributed radially over four weirs separated by walls ( 2 weirs in operation and the other two for future extention ). the intent is for the flow to be evenly split to the four flocculators. After passing over the weirs, Water is conveyed through a 800mm pipleline to the flocculators * 2 in operation). The 800mm pipelines extend vertically downwards (4.5m) and runs horizontally (10m) to feed int the inner circle of the flocculator whwere the slow mixing system is situated.

 

[bimr]

From what you have submitted, it seems that the rapid mix process does not have adequate mixing energy.

The velocity gradient or G value is the standard method used to compare mixing systems.

G = sq rt [ P/uV ]

where P = powr, ft-lb/sec
u = dynamic viscosity, lb-sec/ft2
V= volume, ft3

This is discussed in Metcalf & Eddy's Wastewater Engineering and you should look there for further information.

What you can do is calculate the g values and see what you actually have.

This lecture is a brief overview and will give appropriate G values:

http://www.ees.ufl.edu/homepp/mazyck/webpage/egn1002/Lectures/LectureII.ppt

http://www.usace.army.mil/usace-docs/eng-manuals/em1110-3-162/c-3.pdf

However, if the plant is operating satisfactorily, why change?