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Sludge thickening not happening

Sludge thickening not happening

I am a young Process/Environmental Engineer, recently started working for a company that designs, builds and commissions WWTPs. Here is the info :
On one of the WWTPs we built, there is a problem with sludge. WWTP is processing wastewater from pharmaceutical industry and it consists of: equalisation/neutralisation tank, coagulation addition, pipe flocculator and lamella plate clarification. Sludge from the clarification is pumped to a sludge thickening tank. After enough sludge of about 2%DS is collected, the sludge is being sent to a treatment with sludge filter bags (with addition of K-PE).
The issue is that sludge thickening is not doing well, with only 0.2%DS of the sample from the bottom of the sludge thickener. I advised the operator to recirculate the sludge from Lamella plate separator, back to the pipe flocculator, and to monitor the sludge consistence, assuming that this should improve the settleability of the sludge. Am I right?
I am also open to a suggestions on what could cause this situation.
P.S. On several pictures of the lamella separator in process there can be seen bubbles. I am not sure if this is due to a denitrification, or there is another explanation, but I guess this can be one of the reasons why solids tend to float, instead of settling.

Sorry for the long writing. Thank you in advance for your suggestions or advises.

RE: Sludge thickening not happening

In order to settle the suspended solids with a gravity plate settler, you have to carefully flocculate the solids. If the solids are not flocculated, the gravity plate settler will not work well.

Recirculating the sludge back to the gravity plate settler will most likely decrease the efficiency of the settler rather than improve the sludge settling.

Gravity plate settlers are more suited for inorganic solids. If you have organic materials, the organic materials may stick to the plates and decompose.

One would doubt that you would have much success with a bag filter as 10-20% of the solids will pass through the filter. A filter press would be a better solution for solids dewatering. You can recirculate the sludge through the filter press.

RE: Sludge thickening not happening

Thank you for your answer.
Yes, I agree that filter press would be a better solution, but filter bags were the chosen technology, and now I have to work with that.
Regarding the recirculation, I am not sure if You understood it correctly. The sludge is being recirculated to the step before the flocculation. Therefore, the sludge gets another extra dose of A-PE, and the "old" sludge itself should serve as a mean to "capture" smaller particles. I even consulted Metcalf & Eddy and the book also suggests that the recirculation should be done in order to improve the settleability of sludge.

Regarding the flocculation with A-PE, the doses were carefuly determined by JAR testing. Would you perheps recommend altering the doses to see what would happen?
Sometimes is the water heavy on organic loading, so I assume that sticking and decomposing on the plates may occur.

RE: Sludge thickening not happening

I assumed that you are referring to a gravity plate settler, when you say "lamella plate clarification". Lamella plates are sometimes installed in an existing clarifier to obtain 5-10% more capacity.

Solids contact clarifiers use (internal) sludge recycle. Contact with previously formed floc enables its adsorbent powers to be used; effects savings in the amounts of coagulant and other materials, which may be employed, such as coagulant aids, lime slurry, or activated carbon; requires a shorter detention period and consequently smaller ground space; and furnishes an excellent quality of effluent. Sludge recycle works particularly well and is used more frequently with lime softening and activated carbon.

M & E shows the sludge recirculation with a lamella plate installed in a clarifier. This is a different device than a gravity plate settler. A clarifier with lamella plates has a detention time of about 60 minutes. A gravity plate settler has a detention time of about 15 minutes. M & E (on page 414) is not showing sludge recycle with a gravity plate settler.

IMHO, lamella plates or tube settlers installed in a clarifier are useless. The reason that gravity plate settlers work is that there is a defined hydraulic flow through the plates that is maintained with orifices and weirs. Lamella plates or tube settlers installed in a clarifier do not greatly improve the flow pattern of the traditional clarifier.

Based on the short detention time in a gravity plate settler, the sludge recycle probably will not have much of an effect, nor improve the sludge thickening.

Gravity plate settlers are designed for Type 1 & 2 dilute sedimentation. Sludge thickening is hindered settling (Type 3 sedimentation). You can't achieve both in the same unit process.

By the way, what is K-PE?

RE: Sludge thickening not happening

It looks like it should be A-PE which i assume is anionic polyelectrolyte.
Depending on the nature of the sludge and the conditions under which it is being treated (none of which we know) anionic may not be the correct coagulant for use with the bags.

What is the coagulant being injected into the pipe flocculator?
What type of sludge are you processing?

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

RE: Sludge thickening not happening

Yes, I was referring to a gravity plate settler. M & E didn't show it on a figure, but it's written that "portion of the clarifier underflow can be recycled to the influent of the process to enhance settling..."
Anyway, I will let you know of the results of recirculation next week, because the plant operator is on a vacation. :(
K-PE is C-PE (cationic polyelectrolyte). My bad for the misspelling.

cationic PE is being used for sludge treatment. Anionic PE (polyacrylamyide) is being injected into the pipe flocculator.
They haven't yet collected any of the sludge. That is the main problem. All of the sludge goes back to the start of the process with the supernatant from sludge thickener, because no thickening is taking place.

I have seen couple of photos of the raw wastewater and there are huge differences in quality every day, even throughout the day (different color, quantity of visible particles, foam/no foam,etc), approx. 4 different types of wastewater coming to the WWTP. This of course depends on the manufacturing process.
My question is: Should the operator define the ideal FeCl3 and A-PE doses for every type of wastewater? I guess he should have good communication with the manufacturing department and they should notify him in advance on the quality of wastewater coming to the WWTP.

Thank you once again for your time.

RE: Sludge thickening not happening

Have you done any jar testing to determine if the coagulants you are using are correct and that the dose rates are close to what is required. I would suggest that is no1 thing to test if you have not done that already.

The differences in quality could be very significant and ideally during the design process for the plant these should have been sampled and assessed. Maybe they are similar chemically/physically so everything may be okay but it sounds like you might have very different waste streams to deal with. This may explain why there is a equalisation/neutralisation tank at the front of the plant as this will tend to blend the various streams together and flatten the peaks. If they are very different then this may totally wreck your jar testing results. You probably need to jar test each major stream as well as the various combinations to see if the required dose is about the same regardless of the waste being received. If its not and the quality/quantity of the stream varies a lot over time that might be a big part of your problem.

Do you add acid or base to neutralise? Maybe you need to as the pH may be too low/high.
In your last post you mention FeCl3. I assume this is your cationic coagulant. FeCL3 requires certain pH ranges (about 4-7 is ideal) and alkalinity to work well.

Denitrification is generally related to biological processes converting nitrates into nitrogen gas. What makes you think that this is occurring?

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

RE: Sludge thickening not happening

What are the details on the sludge thickening tank?

It is not possible to constantly revise the operation of the gravity plate settler over the course of a day. You should have an equalization tank.

RE: Sludge thickening not happening

jar testing has been done prior to designing, but it was done on just one sample. So, I assume that flocculation works 25% of the time, because there are roughly 4 types of wastewater, as I already said (conclusion based on pictures).
I will definitely suggest to the operator to sample each of the type of incoming wastewater, do the jar testing and define ideal doses for every one of them.
They are adding either acid or base, depending on the pH of the wastewater, both cases are possible.
I am avare that denitrification shouldn't be the problem, but when I saw couple of pictures, it looked like the bubbles are consequence of denitrification. When I took a second look, I realised that it could be foaming.

sludge thickening tank is just a rather small 3.5 m3 tank, with an overflow to the equalisation tank. Equalisation tank is very big, overdesigned I would say.

RE: Sludge thickening not happening

That answers the question.

The problem that you need to fix is to increase the size of the sludge thickening tank. Suggest you consider the use of a conical tank.

As I mentioned above, gravity plate settlers are designed for Type 1 & 2 dilute sedimentation. Sludge thickening is hindered settling (Type 3 sedimentation) and requires a much longer detention time.

RE: Sludge thickening not happening

I think I know what is the problem. The thing is, the operator discharges the water from thickener 1-2 times per day. Consider the fact that the sludge from the settler is being pumped to the thickener every 6 hours, at rate 50 l/min, for one minute. So the daily volume of sludge being transported to the thickener is 200 l. I guess 3.5 m3 is large enough, or am I wrong? BTW it is conical tank.
I told him to discharge the water from the thickener until it is half full, and than not to discharge any water from the thickener, until the tank is almost full. I hope this will give the sludge enough time to settle.

P.S. Can you give me some details on Type 1 & 2 dilute sedimentation? I am not sure if you are talking about discrete particle settling?

RE: Sludge thickening not happening

Type 1 & 2 dilute sedimentation is discrete particle settling.

Suggest you do a settling test on the solids. The test will verify if the size of the sludge tank is adequate.

RE: Sludge thickening not happening

Thank you for the answer on sedimentation types.
I will let you know when the results come.

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