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Pnasr YaBB Newbies Offline I love Water and Wastewater.com Posts: 16 Company or Organization: Private	Activated lsudge SRT May 14^th, 2012, 8:05pm Hi, I am running a pilot scale Activated sludge. So I would like to know what would be the consequence of increasing the Sludge Retention time? Would it be sludge anaerobic degradation or nitification in final sedimentation tank and sludge floating, or less carbon removal or phosphorus taking place? Thank you
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The sludge judge

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Re: Activated lsudge SRT
Reply #1 - May 14^th, 2012, 8:15pm

increasing the SRT will generally promote endogeny (bugs eating themselves) and a lower F/M ratio. Usually at an SRT around 8 or more days nitrification is promoted. The benefits of a longer SRT are some less sludge production and less wasting. If the SRT is too long low F/M filaments and pin floc can cause problems.

I'm not sure what you mean by the second part of your question. As long as everything is aerobic there is no anaerobic activity. Nitrification happens in the aeration tank- if there's nitrate, low or no DO, and residual soluble BOD denitrification in the final clarifier may happen....

best regards,

Ryan

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Pnasr YaBB Newbies Offline I love Water and Wastewater.com Posts: 16 Company or Organization: Private	Re: Activated lsudge SRT Reply #2 - May 14^th, 2012, 8:20pm Thank you Ryan for your answer. Also, what is the main cause of floating sludge in final settling tank?
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grrun God Member Offline WaterandWastewa ter.Com is the best! Posts: 3525 Pekin, IL Gender: Company or Organization: Freelance Environmental Engine	Re: Activated lsudge SRT Reply #3 - May 14^th, 2012, 11:58pm Probably denitrification. RAS should be great enough to prevent the DO from depleting in the final clarifier. grrun
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Pnasr YaBB Newbies Offline I love Water and Wastewater.com Posts: 16 Company or Organization: Private	Re: Activated lsudge SRT Reply #4 - May 15^th, 2012, 12:02am Thank you for your reply. Could be the reason: an increase in sludge removal rate is final settlement tank?
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grrun God Member Offline WaterandWastewa ter.Com is the best! Posts: 3525 Pekin, IL Gender: Company or Organization: Freelance Environmental Engine	Re: Activated lsudge SRT Reply #5 - May 15^th, 2012, 11:17am Not as likely. Denitrification requires anoxic conditions and a source of BOD to occur. Sludge wasting may help reduce detention time for sludge in the final clarifier, but if the DO in the clarifier remains positive, then denitrification should not occur. grrun
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Pnasr YaBB Newbies Offline I love Water and Wastewater.com Posts: 16 Company or Organization: Private	Re: Activated lsudge SRT Reply #6 - May 15^th, 2012, 12:10pm I am a bit confused. So, if i am asked: Floating sludge is observed in FST in case of: (a) increase HRT in FST (b) increase DO in aertion tank (c) increase sludge removal in FST (d) increase sludge scrapper in FST What would be the best cause?
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grrun God Member Offline WaterandWastewa ter.Com is the best! Posts: 3525 Pekin, IL Gender: Company or Organization: Freelance Environmental Engine	Re: Activated lsudge SRT Reply #7 - May 15^th, 2012, 12:24pm (e) Anoxic conditions in FST! (f) A sludge age >7 days to promote nitrification in the aeration tank; sludge age of 5 days or less would also prevent nitrification in the aeration tank; which in turn would prevent denitrification in the FST. grrun
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Tom Keenan - nesa

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Re: Activated lsudge SRT
Reply #8 - May 17^th, 2012, 3:57am

Hi Pnasr,
You ask:

Floating sludge is observed in FST in case of:
(a) increase HRT in FST - Probably if anaerobic conditions develop, or if denitrification takes place. In both cases the gases generated can cause the sludge to float.

(b) increase DO in aertion tank Unlikely. Increased DO in aeration tank would be generally expected to help prevent anaerobic conditions developing, or denitrification taking place in the clarifier, so this would generally be expected to prevent, rather than cause, floating sludge.
Some people suggest that too much aeration in the aeration tank can cause sludge to float in the clarifier. Personally I see this as extremely unlikely. For this to happen the clarifier must have enough air to enable it to operate similar to a DAF, and this is most unlikely with conventional aeration systems and in the absence of some positive pressure to allow the air to dissolve.

(c) increase sludge removal in FST Unlikely. Increased sludge removal should have the opposite effect as it would reduce the hydraulic residence time in the clarifier. However increased sludge removal will reduce the overall sludge age in the treatment system and can therefore adversely affect the treatment process and the sludge settleability.

(d) increase sludge scrapper in FST Unlikely. Increasing the speed of the sludge scraper would be expected to release any gases that might form from anaerobic or denitrification processes and therefore would be expected to help prevent sludge from floating. However increased sludge scraper speed can cause turbulence and therefore could affect sludge settlement or sludge thickening in the clarifier.

What would be the best cause? When you say "best cause" do you mean the "most likely cause"? If so the most likely cause is denitrification, followed by anaerobic conditions.

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tomkeenan@nesa.ie
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