Hi All,

I am currently working at a municipal WWTP as a commissioning-process engineer. I have recently commissioned the dewatering equipments and some issues appeared lately with specific polymer consumption.

I have belt thickeners for thickening, the raw sludge concentration is around %1,2-1,3 and thickened sluge is %4-5. Specific consumption I obtaine 1,5 kg poly/ ton dry solid. It is okay here.

I have high speed centrifuges for dewatering, up to 3500 rpm.  Inlet sludge is the thickened sludge an dewatered sluge has good dryness, %24-25. Specific poly consumption is 10-11 kg poly/ ton dry solid.

I have guranteed values from the contract 8 kg poly/ ton dry solid for whole dewatering process. I fail.
If I decrease polymer dosing rate for centrifuges,  the supernetant water will contain high MLSS , it is also specified %98 solid capture in the contract.
I have used 3 types of cationic polymer for centrifuges, I have almost the same results.
I have %60-65 MLVSS/MLSS in the raw sludge
Do you have any tip to improve the specific poly consumption for centrifuges?
Best regards,
Serkan.

I agree with the above post- jar test to make sure you get the best polymer and also make sure there's no mechanical problem with your centrifuge.

Also having a microscopic evaluation of your sludge is a good idea because often times dewatering problems are the result of filament problems etc.

Dear Srknk,

maybe problem will be solved only after approaching sales people/application experts from different suppliers after some field tests.
Sounds like a problem we came across in one incident. After sludge flocs experience high shear at the decanter inlet the polymer has to ensure that not only high solids content is obtained but separation is fast enough. If the latter is not fulfilled part of the sludge will leave the centrifuge in the centrate. Increasing the polymer concentration is the wrong way. You need a polymer which can deliver both high separation velocity and high residual solids content. Unfortunately, current testing protocols cannot measure these properties directly and rely heavily on experience.
You can see more details about an alternative approach at an old Ask Tom article
http://www.waterandwastewater.com/www_services/ask_tom_archive/preselecting_flocculants_using_separation_analyzer.htm

Kind regards
T. Sobisch

The sludge judge wrote on May 29^th, 2012, 12:03pm:


for each polymer type I am doinga jar test and trying to obtain the best.  sludge settlent in the final sedimentation is okay, flametous not dominant

All good sound advice from the above contributors.  One question I would be asking is who guaranteed the amounts of poly you are trying to meet.  I am guessing it is a designer who has little in the way of real experience and at the design stage had to put something in to allow someone to determine whole life costs.  They had to write something and you are now stuck with it, and you are being measured against it which is the problem.

You are getting good separation, returning few solids back to the process stream, good %ds on the cake.  Dewatering is a bit of a black art and I would suggest chasing good dry cake is more important than trying to chase a possible unrealistic poly usage target, due to the high costs of moving cake about over the additional costs of some more poly.  The only way anyone can guarantee the amount of poly used on a system would be to fully test the system prior to installation, but then again I am guessing you have a new plant, and at the design stage there was no real sludge to work with.

Anyhow to try and guarantee the amount of poly used would be a folly as no one manufacturer builds poly plant, dewatering systems along with producing poly.  

I appreciate this does not solve your problem, but I would start talking to the client about the issue.  You have already started a chain of events which might solve your conundrum, but then again maybe not.  Let the client know but emphasis you are getting good capture (and minimising the negative effects of additional loading going back into the process stream - blowing more air into the system costs $$$).  Also you have a good cake so minimising the additional costs of haulage.

One last wee nugget for you to try out......dewatering fresh sludge over older sludge may reduce your poly consumption.

srknk wrote on May 30^th, 2012, 3:39am:


I have the dif at 4.5.
RPM at 3090
During operation torque is at 63%. Ger about 23%DS in cake from ilet of about 4.5% and use about 6/7 kg/tonDS poly. But for me to be able to get so low poly dosing is the addition of primary i feel.