Replacement Aeration System Design

[sjohns4]

I'm working with a client who has an existing WWTP with damaged air diffusers, old blowers and drives, and no auto DO control system.

The plant is an 8MGD ADF activated sludge plant w/ full nitrification. The aeration basin is two parallel process trains with three cells each. The first cell is approximately 65% the volume of either of the second two. The second two are equal volume. All three contain air diffusers, the first with fewer diffusers than the other two. The plant was designed to operate with DO levels in each cell at: 0,1,2 mg/l respectively. Operators normally maintain:0.2,1.3-2.4,2.1-2.8 mg/l DO respectively.

I've looked through text references for calculating the amount of air required, however the variables are influent loadings and flow rates to get a single volume or mass flow rate of air required.

My basic question is how do you determine the volume of air required to each cell? Particularly the first cell since, according to a fairly recent re-rate study the diffusers are supposed to be utilized only to prevent septicity in the first cell?


Thanks!

 

[ch81pc]

It will be very difficult to calculate. Don't forget that in your first cell you will need air for mixing as well as to prevent scepticity. If you are using your first cell as an anoxic zone then 0.1mgl is fine and can be acheived through sumbmerged mechanical mixing or through the use of a surface mounted pump with sumberged discharge (slightly better as easier to derag/maintain and lift)leaving you more air for stage 2 & 3.
What are you retention times in each of the stages? As it sounds as though cell 1 is too big to be run anoxic.

A simple way of doing things is to have DO probes at the end of each stage linked via a PLC to an actuator on the respective leg of the air main which will open or close to obtain the desired DO in that cell. With trial and error a very robust system can be set up especially if you put invertor drives on you blowers. You'll save on power too by only runnign what is needed and by getting rid of any power factor loses.

 

[sjohns4]

Yes, first cell is anoxic and is equiped with submersible mixers, and I am tieing a PLC to the DO monitors and VFD's. My thought is to have the PLC monitor the DO in one (operator selected) cells and adjust blower speed to match. The air flow to the other cells would be manually controlled by valves on the header pipe. Now all control is manual - total air flow rate is changed by throttling the blower discharge valve, but the valves to each cell are set and left alone.

Detention times in cells 1, 2, and 3 are 125 min, 192 min, and 192 min, respectively (not including RAS or recycle flow).

I'm more woried about putting too many diffusers / too large of a blower in, which although is better than the alternate could end up being very costly in initial capital and power if the blower doesnt like its actual normal operating point.

Thanks,

Mike

 

[bimr]

Metcalf & Eddy's Wastewater Engineering textbook has methods to calculate the air volume.

You must be able to provide the minimum mixing energy to maintain the particles in suspension, in addition to the aeration requirements.

As to the blower, you should use a positive displacement blower like the Kaeser Blowers or equivalent. Centrifugal blowers will not work well in this application since the discharge pressure will change with speed. The use of valves to control air flow is very difficult because air is a compressible fluid.

 

[sjohns4]

I have the Metcalf & Eddie text book and have used that example, but how do you know how much air goes to each cell, since each is operating at a different DO?

Soon as I came up with a max and normal blower operating point I was going to evaluate blowers. I know centrifugals dont like to turn down much...but as I understand it PD's typically operate at much less efficiency than a centrifigual? I was considering even doing a mix. A centrifigual operating at full speed and best efficiency for normal plant flows and have larger PD's to cover the areas outside of the norm?

 

[bimr]

Centrifugal blowers do have limited turndown and are not suited for application where you have to turn down the output. Centrifugals are also limited in applications such as digesters where you turn off the blowers and want the blowers to force the water out of the aeration system on restart.

When you try to control the air flow on the centrifugals, the dicharge pressure changes on the pump curve, similiar to what happens with a centrifugal water pump. If you throttle back slightly too much, the blower will go into surge and shut off.

The same thing will happen if you use a VFD on the centrifugal. The discharge pressure is going to change.

Centrifugals are suited to applications with constant discharge pressures.

It is also difficult to throttle the air flow with valves. It is almost impossible

I have a client with a system like this that does not work, if you want to see one.

It is much simpler to use a VFD on a PD blower. The blower efficiency is really that much different.

You also can not mix and match the different types of blowers in the same basin.

 

[Watthour]

If the aeration grid is damaged, you might consider a submersible aerator/mixer. ABS has one that is decent to a point, but Aquatech has a really good one in multiple sizes. You only have to buy it once instead of replacing/repairing diffusers all the time. They can mix without air, too.

 

[FiltrationEngineer]

Metcalf and Eddy method in the textbook is different than the method used by fine bubble diffuser manufacturers, which is the EPA method.

In any case, you need the following information:
-Basin dimensions
-AOR (oxygen demand) in this case you have what is called a step aeration process I think. Ususally you'd add a peaking factor to the calcuated value.
-DO concentration you want in each zone
-SOTE of the diffusers
-Temp of the WW (25C)
-elevation of the site
-Alpha value (based on MLSS concentration, etc.)
-Beta and Theta value - usually .95 and 1.024