RAS RATES

I have been thinking about how to determine the optimum RAS rate for years.  Here's what I think:

The optimum RAS  rate is the one that maintains the system in equilibrium with a minimal blanket.

What RAS control methods are there?

PROPORTIONAL RAS

The proportional RAS scheme is where you select the RAS rate to be a percentage of the influent flow, and to change the RAS flow with the influent flow.  This is supposed to make sure there is sudffdicient RAS to treat the wastewater as the flow changes.

There are two big problems with this method
1.  It assumes that there ios a big inventory to be returned to the aeration tank at a moments notice. Your mixed liquor belongs in the aeration tank, not the clarifier blanket.
2   It will overload the clarifier at high flow.

RAS SS = CONCENTRATION OF SLUDGE IN THE 30 MINUTE SETTLOMETER (AKA RSC=SSC30)

In this scheme the RAS flow is adjusted to maintain the RAS SS at the same concentration as the solids in the bottom of your 30 minute settleometer.  This means that if you have a higher SVI, your RAS flow will be higher, and if your SVI is lower your RAS flow will be lower.  This temds to work pretty well, as long as you pay attention to the blankets at high RAS flow.

CONSTANT RAS FLOW

In this scheme you set the RAS flow and leave it there

This scheme can work well, although it means that you store some of your MLSS in the clarifier blanket at high flow and then return it at low flow.  Still, this works well as long as you pay attention to the blanket

CONSTANT BLANKET LEVEL

In this scheme, you adjust the RAS flow to maintain a level blanket.

This works well as long as you are willing to measure the blanket level several times a day and to keep a LOW blanket level - 2 ft or less.  a 6-inch blanket is not too low.