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Why are solids are floating on my secondary clarifier?

1/14/2015

 
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Secondary clarifiers can be running smoothly one day and then suddenly solids begin to float and carry over the weir into the effluent.  What are the conditions that cause floating sludge? And more importantly, what can be done to control it.




First floating sludge is most often caused by:
  • Denitrification – small nitrogen gas bubbles float the sludge in the clarifier creating floating sludge chunks with small bubbles entrapped

  •  Fats, Oils & Grease – simply put, FOG floats on water. When entrapped in floc, excessive grease or oil can cause floating biomass. This appears as a scum blanket that can cover the entire clarifier.

  • Viscous bulking or billowing sludge – viscous bulking can sometimes create floating sludge (more often it is just billowing over the weir versus floating). This is often caused by nutrient deficiencies (normally low phosphate) in industrial waters.

Solutions to floating sludge:

  • Denitrification problems can be often controlled by increasing the recycle pump to reduce sludge blanket depth/sludge retention time in the clarifier. The problem is often related to an increase in influent ammonia/nitrogen that is converted by beneficial nitrifiers into NO2 or NO3 via the autotrophic nitrification process. Without an anaerobic/anoxic step that removes NO2 or NO3 in the treatment system, this process occurs in the clarifier. Long run solutions include evaluating influent TKN/ammonia, anoxic denitrification zone residence time, availability of “food” or easily available BOD in the anoxic zone for denitficiation, and an overall system survey on sludge age, residence times, and influent makeup.

  • Fats, Oils & Grease – FOG created floating sludge involves a messy control process. First the scum/floating sludge needs to be removed or allowed to carry over the weir to polishing/tertiary treatment. Upstream, operators need to evaluate where the FOG increase originated. This can be a one-time slug or increased loadings of grease over time. It is best to prevent oils and grease from entering the biological treatment system. In cases where we have high levels of FOG in a system we encourage operators to increase wasting rates (remove entrapped FOG this way) and add cultures associated with FOG degradation/biosurfactant production. By using wasting and seeding steps together, the potential for significant biomass reduction is prevented while removing entrapped FOG that causes high effluent solids.

  • Viscous Bulking – solution here is to evaluate changes in influent makeup and changes in the environmental conditions in the biological treatment unit. While researching the exact cause of the bulking, operators need to begin wasting the bulking, viscous sludge. If nutrient residuals are low (<1.0 mg/L ammonia nitrogen or <0.5 mg/L ortho-phosphate) then begin adding nutrients to achieve residuals above the targets above. If heavy wasting is involved, we recommend adding cultures to promote the shift to a desirable biomass.


Sathyapriyan
8/26/2015 01:30:58 am

What are the other causes in STP sludge carry over on clarifier

Erik Rumbaugh
9/1/2015 05:02:02 am

Usually solids on the secondary clarifier are from denitrification, oil/grease, filamentous organisms especially Nocardia, or viscous bulking caused by influential makeup or nutrient deficiency.

yasser
4/19/2016 08:49:14 am

I would like to inquire about the layer from sludge on the clarifier surface in pre-treatment system.

Erik Rumbaugh
4/19/2016 11:50:43 am

Usually in pretreatment systems we find grease and fatty acids as a big culprit in creating foam on clarifiers. It is usually related to the influent makeup - what is your influent characteristics? How much pre-treatment is done? Also you could send a picture of the foam to my email (erumbaugh@asterbio.com)

Phi son
8/3/2018 02:06:10 am

Maybe solution is reducing fats and grease, or you have any others

Erik Rumbaugh
8/3/2018 05:25:52 am

Removing Fats, Oils, & Grease (FOG) not only helps with floating solids, it also helps lower COD/BOD, foaming - and even helps improve sludge dewatering characteristics (less water entrained). Ways to prevent FOG from entering a biological treatment unit:

Physical separation - Dissolved Air Floatation (DAF), grease skimmers grease interceptors upstream

Biological Pretreatment - collection systems and lift stations can be converted into part of the biological treatment system using biofilms. Can also help with odor control & septicity.

Riyaz Ahmed
8/10/2016 12:13:23 am

How to resolve the problem for floating sludge.

Erik Rumbaugh
8/11/2016 10:32:33 am

First steps to solving floating sludge is to look for what is causing the problem. In the interim period, I would immediately check clarifier bed depth and increase recycle if bed depth is too high. You may also want to put water spray on the clarifier to "knock" down the floating sludge. These two steps can help while you locate the true cause of the problem.

Riyaz Ahmed
8/12/2016 11:14:30 pm

Suppose If problem was not resolve mean what should I do?

Erik Rumbaugh
8/13/2016 05:56:56 am

Are the floating solids a new problem?

I would look for the following things:
1. Oil & Grease entering the biological unit
2. Look under the microscope for filamentous and non-filamentous bulking. If the solids are a foam, it could very well be nocardia filaments.
3. If you are not feeded secondary clarifier polymer, you may want to jar test polymers as a settling aid. Also, too much polymer can also cause floating solids.

The key is to find out what is causing the floating sludge. In many cases if the floating sludge is encapsulated with grease or has EPS bulking (non-filamentous) - the solution is to waste heavily and ensure conditions (nutrients and adding bacteria) to the biological unit promote floc forming microorganisms.

Riyaz Ahmed
8/13/2016 09:01:54 pm

Thanks for your co-operation
Then please explain what is the role of BOD, COD, MLSS & MLVSS in the WWTP?

Riyaz Ahmed
9/21/2016 12:49:43 am

What is DO, BOD and COD can you tell me please role?

Erik Rumbaugh
9/22/2016 05:32:58 am

D.O. is important with respect to solids as proper DO is required to form floc and remove pollutants. Signs of low DO include filamentous bulking with respect to floating solids. A quick look under a microscope can reveal if filaments in the floating solids are the cause. Filaments can be caused by more than just low DO, but it is among the most common causes.

Now for COD/BOD - they both measure oxygen demand in the sample. One dose it with chemicals (COD) in 2 hours and the other relies on a microbial seed and is run over a 5 day period (BOD). Either can give you the influent oxygen demand, but they do not tell you information on what the influent contains. Floating solids are often a sign of high influent fats, oils & grease (FOG) - but FOG is just regular COD/BOD in tests. To find your FOG number a separate and critical test needs to be run.

Riyaz Ahmed
12/24/2016 11:03:03 pm

Thanks....

Riyaz Ahmed
1/4/2017 09:49:28 pm

How to control the floating sludge in the secondary clarifier?
What is the main reason to floating sludge in secondary clarifier?

Jeseriel Moabelo
8/25/2018 01:40:42 am

How can I solve floating sludge caused by foamy effluent in pulp effluent industry?

Erik Rumbaugh link
8/25/2018 06:35:43 am

Foam in pulp & paper is usually a combination of wood soaps, starches, and other components of digesting lingo-cellulose wood. This foam usually causes problems on aeration basins where it can be carried by winds. Usually water sprays collapse the foam.

For your secondary clarifier's floating sludge try:
(1) water spray
(2) check skimmer efficiency & collection
(3) finally - you may want to add antifoam during very high foaming events.

Christopher Bagshaw
11/6/2018 12:03:47 pm

Could treatment of sewer lines (root control) cause a snowy white foam to form? As well as a micro kill? Thanks!

Erik Rumbaugh
11/6/2018 12:28:21 pm

It all depends on what is being used for root control & how much is entering the collection system. This document from Canada DPR may help:

https://www.cdpr.ca.gov/docs/license/pubs/sewrline.pdf

Afif
1/8/2019 11:57:37 pm

I am currently operating a wastewater treatment plant of which the influent consist of a mixture of casutic soda solution and wastewater from hot washing PET plastic bottles for the process of making PET plastic flakes.

My problem is, whenever the influent goes to the chemical treatment tank where coagulants and flocculants are added (as well as acid because the influent pH is 11 to 12), the sludge does not settle at the clarifier. Only after the influent is mixed with another influent from cold washing bottle process was the sludge able to settle. Fyi, the cold washing bottle process does not involve any chemical usage as compared to the hot washing. I read some journals, and it could be due to the increase in influent average temperature. Since the increase is depending on how frequent the the hot washing throws away the wastewater into the influent drain, I can only assume that it is due to the temperature change. But I am also contemplating the pH being the main cause for the inability for sludge to not settle. Because after mixing the cold wash influent and hot wash influent only were they able to settle. What do you think?

Also, may I request a good reference for a good read? I really need additional information.

Erik Rumbaugh
1/9/2019 04:09:34 am

I am not sure of which coagulants and flocculants that you are using, but pH can often be a factor with the chemistry. Temperature is less of a factor. You can use standard reference books or polymer supplier training information to get an idea of which polymers are a best fit. Then you need to jar test to see which is the best coagulant & polymer combination and proper dose. For reference information, I like the Nalco Water Handbook which has recently been updated. Of course much of the information is available online too.

https://www.amazon.com/NALCO-Water-Handbook-Fourth/dp/1259860973/ref=sr_1_1?ie=UTF8&qid=1547035514&sr=8-1&keywords=nalco+water+handbook

Afif
1/9/2019 04:43:29 pm

Currently our pH is maintained within 7 to 8. We are using polyaluminium chloride as coagulant and poly acrylamide-sodium acrylate as flocculant. I am contemplating about reducing the pH within 6 to 7 range since both chemicals used was not able to settle the sludge.

Afif
1/9/2019 04:43:36 pm

Currently our pH is maintained within 7 to 8. We are using polyaluminium chloride as coagulant and poly acrylamide-sodium acrylate as flocculant. I am contemplating about reducing the pH within 6 to 7 range since both chemicals used was not able to settle the sludge.

Peter
12/10/2019 07:21:39 pm

Hi Afif.
I had a similar situation at a customer that treated PET plastics for recycling. The solution was applying a Ferrichloride product, which reduced the high pH levels (11-12) caused by the caustic washing process down to about 7.5 and water/solids separation of particles produced a supernatant turbidity of 15 NTU.

Afif
1/9/2019 12:07:05 am

*But I am also contemplating the pH being the main cause for the sludge's inability to settle

Erik Rumbaugh
1/10/2019 04:53:00 am

PAC is should be effective at your current pH. Lowering pH should not do too much. You may want to try a different coagulant - perhaps a DADMAC or PAC/DADMAC blend. With respect to your flocculant, you may want to try different charge densities. Again, all of this can be evaluated in Jar Testing inside the lab.

Farhan
9/18/2019 12:27:00 pm

My system is mbbr (roughning, ASP, Hybas)

I’m getting too problem with floating scum on clarifier surface, how I can reduce it?

In above system please let me know which below values are optimum for smooth controls

MLSS
SVI
RAS
DO IN DIFFERENT ZONES (ROUGHNING, ASP , HYBAS)

Erik Rumbaugh
9/18/2019 12:36:19 pm

Scum/foam on the clarifier can be from several sounces. If it is reduced with a water spray, it is probably denitrification (N2 gas) or excess EPS or high Fats/Oils/Grease in the floc trapping gas (CO2, N2, or O2).

If the spray does not break apart and settle the scum, it may be a Nocardia foam.

Once we know what type of foam/scum you have, we can find ways to control.

Mokajoka
9/26/2019 01:53:33 am

Ammonia parameter is high in the effluent. Why is that?

Erik Rumbaugh
9/26/2019 05:49:57 am

Effluent ammonia my not be related to floating solids. What you want to do is - (1) influent Total Nitrogen (TKN) - it is better than ammonia as most organic N is converted to ammonia in the biological system. (2) look for nitrate/nitrite in the effluent - if you have nitrate/nitrite you have AOB/NOB (ammonia removing bacteria). (3) Under low oxygen environents you convert NO3/NO2 into N2 gas which can float sludge but is degassed with a water spray. This blog has much more on ammonia oxidation and denitrification if you use the search function.

sandip k kalikate
1/27/2020 08:25:14 pm

what is the reason for clarrifier outlet turbidity is higher

n letner
10/11/2020 03:12:07 pm

so out of these options?
Denitrification is occuring in the secondary clarifiers.

Increase wasting, Increase RAS?
Decrease wasting, Increase RAS?
decrease RAS, Increase Wasting?
Decrease RAS, Decrease Wasting?

Erik Rumbaugh
10/11/2020 03:34:28 pm

First try increasing RAS ---- less time in the secondary clarifier gives less anoxic time for denitrification in the clarifier blanket.

MOHD SYIMIR SALIM
12/30/2020 12:52:28 am

Why the secondary clarifier cloudy.? But the sludge depth is more than one meter

Chris Pederson link
4/2/2021 03:13:13 pm

Thanks for clarifying how a nutrient deficit in wastewater could be the result for floating sludge. There is no way I could solve a problem like that on my own. I'd need a professional to help me out through the whole process.

Taylor Hicken link
5/10/2021 10:38:21 pm

I appreciated it when you shared that floating sludge can be caused by fats, oil, and grease from households. It is important to ensure that they are removed otherwise it might cause trouble to the whole system. I would like to think if someone is having issues with his tank, he should consider working with a reliable service that can help to lower the sludge level in the tank.


Comments are closed.

    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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