Circular Clarifier Design Tips 1

[Erik_V]

I am developing a water treatment system for a operation that is not unlike a brewery, as we are settling, neutralizing, and discharging high solids fermentation effluent once it has been physically dewatered. I will likely be designing a circular cone-bottom clarifier specifically for this application, and would appreciate any guidance from operators or engineers with knowledge and/or experience on the draining of clarified effluent. Any feedback on the validity of the below statements or the scenario in the last paragraph would be appreciated. Thanks in advanced for any clarification (doh!).


Numbers and configuration matter here so here is the set up:

1. Effluent has 1 percent solids going into the clarifier.
2. After a 3-4 hour primary sedimentation, the contents of the clarifier are pumped to a continuous-flow neutralization system, adding hydrated lime at a minimum of 20 GPM.
3. A secondary clarifier will be paced after neutralization, as hydrated lime also has some coagulating properties and may facilitate further TSS and BOD removal.
4. Both clarifiers would have at least 45 degree slanted cone bottoms. They would be sized for roughly 800 gallons, with a height:diameter ratio between 1:1 and 1.5:1. I am basing this on the specs of industrial clarifiers that are available online.
5. The first clarifier would have a drainage port on the front of the tank, position near the bottom of the cylinder. This is distinct from the sludge port at the bottom of the cone, which is used to pump out sludge.

Specifically, I have been unable to find reliable design resources for the selection of hardware fittings and appropriate drain height that will remove clarified effluent without excluding a majority of the water below the drainage level. A drainage port that is connected to a vertical open pipe on the tanks interior may be the most obvious solution, but not necessarily the best. The ideal would be a drainage system that allows for the extraction of water from multiple heights, but can be configured on smaller industrial plastic tanks.

 

[ashtree]

A couple of questions.
Is this to be a batch process or continuous feed continuous withdrawal. You say "After a 3-4 hour primary sedimentation, the contents of the clarifier are pumped to a continuous-flow neutralization system". Is this everything in the clarifier although i assume you mean the supernatant only? Is the sludge dumped at the same time.
Why not neutralise before the clarifier that way you only need one unit?
Do you know how well your solids settle and compact after 3-4 hours? This will determine how much liquid you may be able to draw off.

I have done exactly what you are proposing with the multiple drainage heights. Generally for a particular operating condition you soon find which one works best but if things change you can always go higher or lower.

A sketch of what you are proposing with approximate dimensions would help

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

 

[Erik_V]

Ashtree, thanks for the reply. I have attached a diagram with a very generic design that I have in CAD so far. The labeled elements are either diameters, or Bulkheads. It is meant for a capacity of 800 gallons, with a 5.25' height and a 5' diameter. The cone bottom is cut off before its tip, 1.5' below the cylinders base.

Di - the inlet diameter is not yet defined, but it should transfer incoming water from the tanks top to the bottom. From the design guides I have read, this should reduce the incoming water velocity and mixing with the clarified fluid if the tanks contents are not fully empty when filled.
Bi - This is the bulkhead where liquid is introduced. I have not yet selected a fitting or method of creating this bulkhead.
Do - this is the outlet diameter. It's size is also not yet determined
Bo - outlet bulkhead
Deb - the diameter of the cone bottom, at the point where it is cut off. I am not sure if there are unforeseen constraints with regard to this diameter, but I would imagine that solids removal would be most effective if this diameter matches that of the pump inlet.
Bob - The bulkhead fitting used for the sludge discharge outlet.


To answer your questions in order:

The duration of treatment -

Yes you are right, it is only the Supernatant making it to a cotinuous flow neutralization system. The solids that have accumilated in the cone bottom are being planned for period flushing - once daily is a likely scenario.

Why not neutralize before the clarifier?

- This is a valid setup, there are a couple of technical reasons why I added a second tank afte nuetralization. We were able to get ahold of a reliable high flow rate system, complete with a pump. The pump should not be dipping below 20 GPM, so a large volume at the inlet of the pump would create the appropriate pressure head and batch size for several minutes of pumping. Otherwise we would be trying to match the rate of a physical dewatering process to the pump (which may vary quite a bit).

Additionally, an initial settling at the head of the nuetralization system may reduce the amount of coagulant needed. I'm basing this off of the assumption that the liquid will be less saturated with ions and particles and a slurry would incorporate itself and dissolve in greater quantities. This may be a case where theory is not reality so I'm curious if you have any experience or thoughts on that.

Tendency to settle -

With a small batch settling test, we were able to settle out 90 percent of the solids. This brought the mixture from 1 percent solids to .1 percent.

 

[ashtree]

How quickly does this settle out?
Is it a continuous flow into and out of this clarifier, or do you fill it up , let it settle , and then drain off the clear water and fill it up again?

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

 

[Erik_V]

Within 3-4 hours, 90 percent of the solids have a tendency to settle. It is possible though that batches will vary.

 

[ashtree]

Maybe i am not asking the question correctly

Here are some issues.

If the process is continuous flow you have a flow rate of 20GPM and a clarifier volume of 800gallons , and you require three to 4 hours to settle. At that flow rate you only have a detention time of 40 mins.

Therefore i assume its a batch process that you fill up , let settle for 4 hours then you drain off the supernatant and then refill. Once per day you drain off the sludge.

What you need to know is how much volume that settled sludge will take up once settled after 4 hours. Your draw off port has to be well above that. Forget the vertical pipe because sludge will settle in that when you fill up the clarifier.

Install a couple of ports down the side and draw off them sequentially top to bottom to limit the discharge velocity or else make them relatively large compared to the flow rate so entry velocity is quite low. This will limit the risk of picking up sludge.

Have you considered using a coagulant to increase the rate of settling and the density of the sludge in the bottom of the clarifier.

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

 

[Erik_V]

Ashtray, I owe you a coffee (or beer) after this since we are >3 replies deep here! I'll try to respond in order:


- I assume its a batch process that you fill up , let settle for 4 hours then you drain off the supernatant and then refill. Once per day you drain off the sludge.

yep, exactly right.

- Forget the vertical pipe because sludge will settle in that when you fill up the clarifier.

I planned on compensating for this by introducing the influent water within an internal pipe that enters from the top and spills into the bottom of the tank. This would reduce the incoming velocity and isolate the sludge from the drain port inlet. I am also considering stretching a fine mesh over the inlet of the drain port, to provide a secondary safeguard for solids infiltrating the pipe.

- Install a couple of ports down the side and draw off them sequentially top to bottom to limit the discharge velocity or else make them relatively large compared to the flow rate so entry velocity is quite low. This will limit the risk of picking up sludge.

In this application, the flow rate must be 20 gpm leaving the tank (per the specs of a continuous neutralization system. For this reason, a single pipe may make the most sense so that a single batch can be continuously neutralized.

- Have you considered using a coagulant to increase the rate of settling and the density of the sludge in the bottom of the clarifier.

Yes, there will be an identical secondary clarifier after the neutralization step. Hydrated lime will be used for neutralization in this case, which also coagulates solids to some extent.

 

[ashtree]

The fine mesh may be problematic if you have much solids carry over. That mesh may block and then will limit your flow.
Better to understand the settleability issue and have multiple draw off levels so that you can select one that is above the sludge blanket.

As i said earlier you draw off from them sequentially as the level goes down making sure that you are always above the sludge layer.

In answer to your original question the level to which the sludge will settle and the amount of clean water to be drawn off the top is something that will be site unique and can only be worked out on site by jar testing and then actual operation.

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

 

[bimr]

Have you considered using a DAF for this application?

Regarding: 45 degree slanted cone bottoms. 45[sup]o[/sup] is less than the angle of repose and solids will build up unless a scraper is included. For that reason, devices such as tube settlers have 55[sup]o[/sup] angles