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ruben vdk

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Optimal dosing point for extra phosphorus
May 21^st, 2012, 3:31am

Hi,

We have a 7000 m³/day wwtp with nitrogen removal by nitrification/denitrification and we're dosing extra phosphorus (H3PO4) in the selector (RAS + effluent physico chemical pre-treatment with FeCl3, pH > 10). We operate the primary clarifier in such a way that most of the time we have excess FeCl3 in the primary effluent. I was wondering whether this dosing point is effective due to the fact that FeCl3 and PO4 can form insoluble FePO4 salts.

In active sludge systems (with separate nitrification/denitrification basins), what is the best location to dose extra P? Would it be better in my case if I'd add it to the aeration basin which has a pH near neutral?

Regards, Ruben

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Tom Keenan - nesa

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Re: Optimal dosing point for extra phosphorus
Reply #1 - May 21^st, 2012, 6:55am

Why are you dosing "P" into the wastewater?
If you are dosing Ferric Chloride, how come the pH is > 10?
Generally Ferric will be acidic and therefore a lower pH would be expected.
It also seems strange that you are dosing ferric chloride, (which removes "P" and then you are dosing "P". Maybe a little more information might clarify this for me.
Thanks.

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ruben vdk

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Re: Optimal dosing point for extra phosphorus
Reply #2 - May 21^st, 2012, 9:57am

We're dosing H3PO4 ('P') because our effluent soluble PO4 content is lower than 0,5 mg/L.
pH > 10 because we are dosing Ca(OH)2 to neutralise pH and we had bad experiences working with lower pH (rotten sludge). The effect of FeCl3 on pH seems to be neglectable. Our influent pH can range anywhere between 1 and 12 (due to different processes in the factory)
The choice to use FeCl3 has been made in the past because our sister company is producing it.

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The sludge judge

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Re: Optimal dosing point for extra phosphorus
Reply #3 - May 21^st, 2012, 10:21am

The ferric chloride is likley tying up your phoshate. Cut back on the ferric first and see if the residual increases. You may find you don't need to add any extra P. What is your incoming BOD, TKN, and Total Phoshorus? Why are you dosing the ferric chloride to the primary?

You also don't want to have your pH that high/ not good for the bugs. This may also be a contributing factor to precipitation of phoshorus (this happens at high pH too

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Tom Keenan - nesa

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Re: Optimal dosing point for extra phosphorus
Reply #4 - May 21^st, 2012, 10:30am

If the pH of the raw wastewater is low then the effect of the Ferric Sulphate may not be noticeable. At higher pH values it is likely that the effect of the Ferric Sulphate will simply be to react with the Ca(OH)2.
The addition of either of these chemicals will effectively reduce the Phosphorous concentration in the water.
If the Phosphorous content is only 0.5 mg/l, then you probably need to add some phosphorous, but it seems foolish to be adding Phosphoric Acid at the same time that you are adding either Ferric or Ca(Oh)2. Both of these chemicals will precipitate the Phosphorous from the added Phosphoric acid.
If you need pH correction you would be better to consider using say Sulphuric Acid to reduce pH, (if necessary, i.e. if the Phosphoric Acid does not achieve the required reduction), and Sodium Hydroxide to increase pH, (if necessary).

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Jeff Naumann God Member Offline I love Water and Wastewater.com Posts: 1133 Torrance, California Gender: Company or Organization: Jeff Naumann & Associates	Re: Optimal dosing point for extra phosphorus Reply #5 - May 21^st, 2012, 1:57pm What is the treatment system that you are using? Does the system have a surge tank capacity before the treatment system? What are the influent concentrations for TSS, BOD, COD, oil and grease, TKN, Total Phosphorus, etc? What limits are placed on the effluent? Where is the treated effluent discharged?
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Re: Optimal dosing point for extra phosphorus
Reply #6 - May 22^nd, 2012, 1:20am

The sludge judge wrote on May 21^st, 2012, 10:21am:

The ferric chloride is likley tying up your phoshate. Cut back on the ferric first and see if the residual increases. You may find you don't need to add any extra P. What is your incoming BOD, TKN, and Total Phoshorus? Why are you dosing the ferric chloride to the primary?

You also don't want to have your pH that high/ not good for the bugs. This may also be a contributing factor to precipitation of phoshorus (this happens at high pH too

Our influent TKN is on average 325 mg/L, we do not measure BOD but average total COD is 4000 mg/L and TSS is on average 2700 mg/L and total P is 100 mg/L. The primary effluent has TKN of about 225 mg/L, Total P 7 mg/L and COD 1300 mg/L.

Most of the time our influent to the primary clarifier is already > 10, we used to have pH neutralisation with HCl but for some reason I do not know, this has been taken out of service.

The pH in the aeration basins are around 7.

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ruben vdk

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Re: Optimal dosing point for extra phosphorus
Reply #7 - May 22^nd, 2012, 1:26am

Tom Keenan - nesa wrote on May 21^st, 2012, 10:30am:

If the pH of the raw wastewater is low then the effect of the Ferric Sulphate may not be noticeable. At higher pH values it is likely that the effect of the Ferric Sulphate will simply be to react with the Ca(OH)2.
The addition of either of these chemicals will effectively reduce the Phosphorous concentration in the water.
If the Phosphorous content is only 0.5 mg/l, then you probably need to add some phosphorous, but it seems foolish to be adding Phosphoric Acid at the same time that you are adding either Ferric or Ca(Oh)2. Both of these chemicals will precipitate the Phosphorous from the added Phosphoric acid.
If you need pH correction you would be better to consider using say Sulphuric Acid to reduce pH, (if necessary, i.e. if the Phosphoric Acid does not achieve the required reduction), and Sodium Hydroxide to increase pH, (if necessary).

Sorry for the misunderstanding but we're not adding H3PO4 and FeCl3 at the same place. The FeCl3, Ca(OH)2 and polymer are added in front of the primary clarifier (to coagulate, flocculate and then settle the SS in the incoming wastewater). The effluent of this primary clarifier goes to the selector where it is mixed with return aerated sludge and then flows to a denitrification basin (and cycles between aeration and anoxic/denitrification basins).

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Re: Optimal dosing point for extra phosphorus
Reply #8 - May 22^nd, 2012, 2:16am

Jeff Naumann wrote on May 21^st, 2012, 1:57pm:

What is the treatment system that you are using?

Does the system have a surge tank capacity before the treatment system?

What are the influent concentrations for TSS, BOD, COD, oil and grease, TKN, Total Phosphorus, etc?

What limits are placed on the effluent?

Where is the treated effluent discharged?

We have a buffer basin at the inlet of the wwtp but it is limited in size (1000 m³, taking into account that is has a lot of settled solids, the usefull volume might be only 500 m³).

Our effluent is discharged in a local river and limits are < 250 mg/L COD, < 100 mg/L TSS, < 2 mg/L total P (with peaks to max 4 mg/L), < 50 mg/L TKN.

Our wwtp consists of a grid, sand and grease trap, buffer basin, neutralisation basin (adding FeCl3, Ca(OH)2 and polymer), primary clarifier (to remove SS), selector and two biological basins with separate nitrification and denitrification basins and a final clarifier.

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The sludge judge

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Re: Optimal dosing point for extra phosphorus
Reply #9 - May 22^nd, 2012, 8:51am

What is your COD and Total P leaving the primary clarifier? Before adding extra phos I recommend getting the pH neutralization fixed. Having the pH that high may be stripping out the phoshorus before the bacteria can get to it.

Also it's worth sending our a sample of your MLSS for a microscopic analysis and running the anthrone test. This will tell you if you truly don't have enough P at this time. 0.5 mg/L is still a residual and it's possible they may be getting enough. (note- you'll also see an increase in the SVI and in severe cases loss of BOD/ COD removal often accumptied by a grayish foam)

As for your origional question on the optimal dosing point, if you are really worried and want to dose right now I would dose at the head of the aeration tank where the pH is neutral and you don't have to worry about phoshorus getting tied up by high pH. Personally I would make fixing the possible culprit (improper pH balance) a top priority.

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The sludge judge God Member Offline Posts: 649 Wisconsin Gender: Company or Organization: private	Re: Optimal dosing point for extra phosphorus Reply #10 - May 22^nd, 2012, 8:55am I just re-read your post and see I skipped over reading the answer to my first question. 7 mg/L phos seems borderline (we don't know the BOD). Lowering the pH and/or the ferric chloride rate will increase the P. best regards, Ryan
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ruben vdk

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Re: Optimal dosing point for extra phosphorus
Reply #11 - May 22^nd, 2012, 9:11am

The sludge judge wrote on May 22^nd, 2012, 8:55am:

I just re-read your post and see I skipped over reading the answer to my first question. 7 mg/L phos seems borderline (we don't know the BOD). Lowering the pH and/or the ferric chloride rate will increase the P.

best regards,

Ryan

Isn't a pH around 10 necessary for the FeCl3 to work as a coagulant? I could try to lower the FeCl3 dosing rate but our influent is changing a lot and to prevent SS to overflow to the biological basin, my predecessor implemented that rule. And also, to prevent the sludge from starting to go bad/rot (at the bottom of the primary clarifier) this high pH was always thought as being beneficial.

The anthrone test is a test for carbohydrates? How can it be used to check for P-deficiency?

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Re: Optimal dosing point for extra phosphorus
Reply #12 - May 22^nd, 2012, 11:18am

I understand the need to optimize COD and TSS removal in primary treatment. I've seen ferric chloride work at much lower pH, but with that said each individual stream is different.

I would look into feeding magnesium hydroxide which even when overdosed typically only brings the pH up to 9. Have your chemical rep try this with ferric chloride addition or an alternative coagulant. You may be lucky and find a specific polymer works too by itself.

As for the anthrone test it measures the polysaccharide level in the sludge. >25% indicates lack of N or P.

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The sludge judge God Member Offline Posts: 649 Wisconsin Gender: Company or Organization: private	Re: Optimal dosing point for extra phosphorus Reply #13 - May 22^nd, 2012, 11:20am As for the sludge "rotting" in the bottom of the clarifier perhaps more frequent sludge removal may help.
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ruben vdk YaBB Newbies Offline I love Water and Wastewater.com Posts: 8 Gender: Company or Organization: PB Gelatins	Re: Optimal dosing point for extra phosphorus Reply #14 - May 23^rd, 2012, 10:47am The sludge judge wrote on May 22^nd, 2012, 11:20am: As for the sludge "rotting" in the bottom of the clarifier perhaps more frequent sludge removal may help. Those pumps are working 24/7 so can't change that. I will try adding the H3PO4 to the aeration basin.
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