PERIPHERAL FEED VS. CENTER FEED CLARIFIERS

You may know that I have been a big fan of peripheral feed clarifiers for many years.  Why?  Can you say 10 mg/L TSS at an SOR of 2000 gpd/sf?  Thats double the 10 states standards flow limits.

How do peripheral feed and center feed differ?   Let’s look at a cutaway of a center feed clarifier.  

 (Image from Westech)

At peak flows under 10 states standards, the MLSS and RAS flows upward in the column at abougt 3 fr/sec (fps).   It exits the ports at about the same velocity.  The velocity in the flocculation chamber is about 0.19 fps, and the detention time is about 1.1 minutes.

When the flow exits the center column, it hits the flocculation basin walls and has nowhere to go but DOWN, creating a current called the waterfall effect.   The waterfall can disrupt the sludge blanket.  The current then flows along the bottom till it gets close to the weirs, where it rises,  This current will pull solids along with it and degrade clarifier performance.

Now lets look at a peripheral feed clarifier 

(Figure taken from SIEMENS, thanks)

Mixed lliquor enters the tank through an outer raceway (1), which circles the tank.  Flow drops through the floor through a tube .  The velocity through this tube at peak 10 state standards flow is 0.50 fps.  The area just below the raceways has a target baffle (8) and a skirt baffle (9).  The target baffle breaks up the current from the drop pipe.  The skirt baffle provides a large flocculation zone that is an analogue to the center feed flocculation basin.  The sludge velocity behind the skirt baffle is 0.018 fps, or 1/10 that of the center feed tank. The detention time is 83 minutes as opposed to 1.1 minute

After leaving the flocculation zone, the current pushes the sludge to the center of the tank, the clarified water loops back and leave the tank by the effluent raceway adjacent the influent raceway

There are 3 keys as to why peripheral feed tanks are better:

1. They provide less turbulent conditions.  Good gravity settling means getting rid of turbulence, and allowing the flow to form.  Violent conditions shear the floc and slow down settling
2. The long flow path essentially doubles the water’s travel path from inlet to outlet, reducing short circuiting and providing more settling time.
3. The flow path takes the sludge blanket away from the effluent launder and towards the center of the tank.  This is the reason why these tanks perform s well at high flows; the blanket is being pushed AWAY from the launder, not TOWARDS it.

OTHER GOOD THINGS ABOUT PFPO CLARIFIERS

Head loss and flow splitting

PFPO clarifiers have a significant advantage in headloss due to the lack of the center feed pipe.  The reduction in head loss has advantages in elevation. flow capacity and gives youhead to burn for good flow splitting.

Next, flow splitting.  With the peripheral feed, mixed liquor can flow from the aeration tank to the clarifier through an open channel, wheich can be an advantage.  If you are using a diamond shaped flow splitter for 4 tanks, the splitter can be made simpler and closer to the tanks.  Another advantage is with tanks in a line.  I remember she Ohio EPA "SWAT" team using their fancy fluorometer (which my employers would never buy me) to demonstrate problems with flow spitting in recgtagular tanks in a line.  They were able to improve flow splitting by using  (I swear to God) pieces of shingles to add headloss to the clarifie inlets to improve flow splitting,  With a PFPO clarifier, you can put in a weir or a flume and measure the flow, and add an up-to-close weir gates to adjust flow to each tank.  WARNING! Do not allow too many people the ability to adjust the flow to individual clarifiers, or write a computer program to do it, because overadjustment leads to instability and poor performance.  The ability to adjust the gates can lead to "fingeritis", a sin of which the Author (that's me) has been guilty in the past.  It's best to adjust the gates so the flow splits are pretty good most of the time rather than seek perfection.

OTHER ADVANTAGES

Other advantages of the PFPO clarifier are reduced construction costs, and the sheer elegance of the design.  Whats's wrong with elegance?

I have never figured out why more engineers don’t build these tanks.  The Chinese have standardized on them.  After all, one billion chinese can't be wrong.

I want to thank ED ROGOZINSKI for the data on center feed clarifiers.  May god bless you and your sons.  and now, for Ed  a little musical interlude.