The key to good wet weather operation is simple. YOU NEED TO KEEP THE SURFACE RISE RATE (in feet per hour) SLOWER THAN THE SLUDGE SETTLING RATE. What is the surface rise rate? Its the rate, in feet/hr that water fills the tank if it were empty and the flow is x MGD If the flow doubles, the rise rate doubles.
What is the sludge settling rate? Take you r settleomeeter and run a settling test. Take the readings EVERY 30 SECONDS for 10 minutes or so. Plot the readings on graph paper or on your computer if you can stop watching that internet porn. The part of the graph that is a straight line is you sludge settling rate. This is the settling rate when the sludge is not hindered by other particles.
To keep your sludge in the plant durng a storm, you have to either decrease the sludge rise rate (put more tanks on line), or make the sludge settle faaster (yes you can)
I will divide this post into 2 sections:
- Physical plant additions and alterations, and
- Things the plant staff can do themselves, instead of whining about it
The keys to good settling at high flows is, in addition to a good settling sludge, are QUIESCENCE, DILUTION OF THE MIXED LIQUOR, AND REDUCING CURRENTS/SHORT CIRCUITING
What is quiescence?
Quiescence is quietness, of a non turbulent area in the clarifier where the activated sludge floc can coagulate and begin to settle. The best way to improve quiescence is to have better clarifiers. The best clarifiers are peripheral feed-peripheral overflow (PFPO) clarifers. I discussed this in a previous posting. PFPO clarifiers have 10 times the quiescent zone that a center feed clarifier does. It is possible to convert center feed clarifiers to PFPO clarifiers. I'd be glad to help you.
Rectangularr clarifiers also perform well at high flows, according to John Esler, clarifier expert. The clarifers have to be built a certain way with 2 effluent launders that are parallel to the long sides, influent baffling, and co-current sludge removal (Gould Types 1 and 2).
Extra clarifiers are good insurance for that rainy day. Why is that? BECAUSE YOU NEED TO KEEP THE SURFACE RISE RATE (in feet per hour) SLOWER THAN THE SLUDGE SETTLING RATE.
Diluting the Mixed Liquor
Why is diluting the mixed liquor good?
Because it lets you improve the settling rate
Secondary clarifiers can become solids limited - that is, you can push more solids into the clarifier than can settle. There are various ways of diluting the mixed liquor, including step-feed, contct stabilization and RAS adjustment
If you have multiple aeration tanks you can convert your system to step feed or contact stabilization. Since most plants have the influent agt one end and the clarifier at the other end, your system can be converted to 3-pass, 5-pass or 7-pass , etc.
Going to for example, a 3-pass step feed parks some the the mixed liquor in tanks one and 2, for instance and feeds some of the influent into pass 3, diluting the mixed liquor and lowering the solids loading rate to the clarifier
Step feed conversion can be done by your engineer, or you can do it yourself. If you want some help, call me. n Carey, OH, Doug Keller, Roy Johnson and staff did this as a DIY after I suggested it. They had 5 tanks of 2 different sized and a problem with short circuiting and flow spliting. They rented a concrete say and cut holes in the tanks to make them plug flow. The also had to instaqll a submersible pump and a float switch because 2 of their tanks were detache3d from the other 3. Total cost? $9000 plus their labor. The results? Well worth the money.
Another good example of how step feed can be effective is Bowling Green. I redesigned their system from 4, 1-pass tanks to 2. 3-pass tanks. After the conversion, they were still having problems with clarifier performance at high flow. They were finally persuaded to try step feed. The results? They went from a secondary efflulent TSS of 200 mg/L at 20 MGD to 20 mg/L at 24 MGD. I know some of you out ther are not impressed and will keep doing it the same old way.
Here are some things that operators can do without much effort and keep playing internet poker.
Adjust the RAS Rate
Adjusting the RAS rate will change the solids loading rate and reduce clarifioer turbulence. I leaned this lesson long ago as a Chief Operator at Bloomington IN, a 15 MGD AWT plant. (See I did have a real supervisors job at one time). We began there when it was a brand new plant. Control the RAS as a fixed percentage of gthe influent flow. When we had our firstg real big rain, we kept turning the RAS up until we turned the clarifiers into mixed liquor. My good friend and co-worker, Bill Bardes, suggested that we reduce gthe RAS frlow since turning it up didn't work so well. We figured out what to do by trial and error, and kept the sludge in the clarifier even at maximum flow. This trick has worked for me at several places.
I know there are some famous consultants who poo-poo this, and say that RAS has nothng to do with the solids loading rate, but I say I'm right, you're wrong and that's that.
Put Extra Tanks on Line
Putting extra tanks on line will give you some breathing space whille they are filling up and will dilute the MLSS
Reducing Aeration Tank Airflow
Reducing aeration tank air flow will promote good flocculation and rapid settling. You dont need to have a DO of 10 mg/L. Cut your air flow to the recommended minimum.
Follow these suggestions and your plant will work better.
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