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Related Articles


Activated Sludge Process

Activated Sludge Process

Activated Sludge Process

We have a lightly loaded (BOD5 -100 mg/L) municipal extended sludge plant (1500 m3/d) that sees almost zero flows during the off peak times. (3- 5 hrs/day ). As a practice we run the RAS pumps continuously throughout the day. In order to save energy is it okay to shut off the pumps when there is no flow to the plant. Would this impact the bugs?

Thanks in advance.

RE: Activated Sludge Process

You will probably save more money if you can reduce the aeration rate.

When RAS flow rates are too low, thick sludge blankets in the final clarifier can result. The operator will see gas bubbles (from nitrogen gas) and rising/floating sludge clumps on the clarifier surface. The clarifier may go anaerobic.

RE: Activated Sludge Process

If your RAS pumps are on variable speed drives you could slow them down somewhat and reduce your energy costs a little.
Some plants flow pace the RAS pumps against the inlet flow so that when the flow reduces so does the RAS rates. However they probably do not go to zero.
Some SBR designs have unaerated periods of 2-4 hours without any problems. Sludge sitting in a clarifier for several hours wont affect the bugs. But if it compacts and thickens and is a long time before it is collected then it may go anaerobic as bimr suggests and then you will have problems.

"Any water can be made potable if you filter it through enough money"

RE: Activated Sludge Process

I know that this is a little late based on the date of your post, but, you might find this interesting. I work with an extended air plant and we recently changed out the blowers and changed to fine bubble diffusers. The plant operates at approximately 40 to 45% of capacity (based on flow) and has average BOD's (<250 mg/l). When we brought the new equipment on line, the blowers could not be turned down low enough, so, the DO was too high (over 6.0), the sludge would not settle, there was excessive foam and the pH was running too low. The engineers who designed plant upgrades sized the blower to provide for a minimum mixing flow rate.

We were able to develop a control system that allowed the blowers to shut down for 15 min intervals. We had aeration/no aeration cycles. After some experimentation, the operators settled on 1/2 hr on, 1/2 hour off through most of the day with 15 min intervals during normally high flow levels. The foam disappeared. The pH came up. The D.O. floated around 2.0 during aeration cycles. The sludge began to settle better and the hypochlorite use dropped. The BOD removal rates remained above 90%. There is no significant accumulation of solids in the aeration basins.

The high efficiency blowers were already reducing the energy bill and this dropped the energy use by about 35% more.

You didn't say what type of blower system you are using, but, if they could handle on/off cycles, you might look into this type of control.

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