Clarification for sludge transport

[MirJev]

Hi everybody. This question might seem stupid, but I just can't figure it out.
I am participating in a design of a municipal WWTP. We have two secondary clarifiers and one sludge pumping station with submersible pumps, for the sludge from the clarifiers. The sludge flows by hydrostatic pressure from the bottom of the clarifier to the sludge pumping station. Concerning the fact that these two objects (clarifier and pump station)are communicating vessels, the liquid level in both of them should be the same. So, when I start the pump in order to recirculate the sludge, the liquid level is going to drop, both in pump station and in clarifier, going below the weir level in clarifier.

My colleague suggests that the sludge shouldn't be discharged from the clarifier to the pump station on a continuous basis, but I am quite sure that this should be a continuous process.

So, my question, or better be said a plea is: Could somebody clarify my the whole process control of secondary sludge, as detailed as possible.

Thank you very much in advance,
Mirko

 

[QualityTime]

You are confusing sewage flow, RAS rates and WAS rates. The final clarifier will never drop in level

 

[bimr]

Some of the flows are recirculate so there is no drop in level.

 

[QualityTime]

For a normal BOD removal plant the RAS return flow rate is 34%. It just recirculates. Therefore there is no drop in water level.


You will have to calculate the WAS rate. By the time you thicken an dewater it most of the water will be returned to the process. Therefore it ia almost all recirculated water and it is not a factor in dropping the water level in the final clarifier

 

[ashtree]

If you are wasting "hydraulically" as is often done to control based on a constant SRT, then WAS will probably come from the aeration anyway. But i agree nearly everything ends up back in the system anyway.

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

 

[MirJev]

Thank you all for the answers, I get it now!
So, for example, if I have a RAS rate 100%, my RAS pump has to be connected (on-line) to the flow meter at the WWTP inlet and than it regulates its flow according to the current inflow of wastewater? Am I right? It should work on a continuous basis, right?
One more question: the pumps at the inlet pumping station of the WWTP are designed to manage peak hourly flow (wet weather). Do my sludge recirculation pumps also have to be of same capacity, concerning the fact that I need RAS of 100%, or I can put smaller recirculation pumps, designed to manage only daily average. As far as I see it, that excess "wet weather" flow will not have an impact on the process itself, so my RAS rate shouldn't follow those peaks?

It is very important for me to fully understand the process control of the WWTP. You don't get to learn this stuff at school.
Concerning the fact that I have just started working as a junior proceess designer, I greatly appreciate your help.

Thank you again,
Mirko

 

[ashtree]

There are a range of design philosophies on the RAS flow rates and it will depend on the style of process, the overall complexity of the plant, the design specification, and the effluent quality.

Normally if the RAS is flow paced its all done in relation to raw sewerage flow and set as a percentage of ADWF(average dry weather flow). It would definitely not be normal to pace the RAS pumps against the wet weather flow peak or perhaps even the daily diurnal flow peak. A minimum value of RAS flow is typically about 15% of ADWF and a maximum is usually somewhere between 100 and 200% of ADWF. Plant that are nitrify/denitrifying typically go for the higher end of the range.

Flow pacing puts a significant hydraulic and solids loads onto sedimentation tanks during high flow conditions so this should be considered in the design. Many plants particularly smaller and older operate at a fixed RAS flow and do okay but this is less common in plants doing high level removal of nitrogen and phosphorus.

Conventional waste water plants (as compared to intermittently aerated or batch plants) typically run the RAS all the time.

I assume you are working for a firm that has experienced designers that you are working with. They should be able to talk you through this. If they can't do this then maybe you need to think about working somewhere else. What is important for you to learn is why these things need to be as they are, not just the numbers. This will help you understand the process principles and under what circumstances to use a certain flow or whatever.

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

 

[bimr]

The peak hourly flow is stormwater which generally contains no organic contaminants. As ashtree stated, this flow just passes through the WWTP.