# Phosphate Buffer - how much buffer to add wastewater (May/22/2006 )

I have an industrial wastewater of COD 30,000mg/l, and 1/10 dilution of the same wastewater (COD 3000 mg/l). I will make 500 ml batch reactors and treat these samples.

how much phosphate buffer (KH2PO4 & K2HPO4) I need to add them to keep their pH at 7 during batch treatability study. (X gr of KH2PO4 and Y gr of K2HPO4 to 500 ml......?? )

can you please explain in detail...or at least give me some source on this matter.

-paulanka-

QUOTE (paulanka @ May 23 2006, 06:23 AM)
how much phosphate buffer (KH2PO4 & K2HPO4) I need to add them to keep their pH at 7 during batch treatability study. (X gr of KH2PO4 and Y gr of K2HPO4 to 500 ml......?? )

Not sure what you are asking about - do you mean - how to prepare buffer of given pH? Or how much buffer should be added to be sure it will work?

First question is generally about buffer pH and Henderson-Hasselbalch equation, second is about buffer capacity.

Check also this buffer calculation example.

-Borek-

Then my question is related to buffer capacity...I already prepared a buffer stock solution with about 13,6 gr/L K2HPO4 and 6,8 gr/L KH2PO4 and its pH is around 6,8. But when added to my system pH becomes constant at around pH 8,5.

-paulanka-

QUOTE (paulanka @ May 24 2006, 02:34 AM)
Then my question is related to buffer capacity...I already prepared a buffer stock solution with about 13,6 gr/L K2HPO4 and 6,8 gr/L KH2PO4 and its pH is around 6,8. But when added to my system pH becomes constant at around pH 8,5.

What is starting pH of your samples?

Do you have any idea about waste composition? Could it contain any other buffer?

In what volume of water have you dissolved phosphates when creating buffer (or what is your buffer concentration)?

What amount of buffer do you add to the 500 mL samples?

Looks like your samples are basic and amount of buffer added is too small. Have you tried to double amount of buffer added?

-Borek-

(in preparing buffer I added previously mentioned amounts into 1L distille water)

Strangely my original sample pH was 4,8.

25ml buffer (from stock) /500ml to 3000mg/l COD
25ml/500ml to 5000mg/l COD
25ml/500ml to 10,000mg/l COD (current pH's = 8,5 )
(when I add 25ml to 10,000COD it gaves C:N:P = 100:5:2. may be it has not a buffer capacity for that concentration. its only nutrient here..)

I also added NH4Cl to supply CNP=100:5:2

ALso, Throughout reaction pH increases when I dont add any buffer (i mean when I only added them as nutrients)

-paulanka-

QUOTE (paulanka @ May 25 2006, 10:12 AM)
Strangely my original sample pH was 4,8.

I also added NH4Cl to supply CNP=100:5:2

My take is that there is some general error. You have acidic solution of pH=4.8. You add NH4Cl - which is slightly acidic on its own. Then you add pH 6.8 buffer - and suddenly pH rises to over 8? No idea what is going on.

-Borek-

voila;

In the first day of the experiment (before adding any buffer) i tried to use 15N NaOH to keep pH at 7. Maybe I added too much of NaOH which I saw its effect on second day... okay, here is a new important question related to that:

In biological systems C:N:P ratios of 100:5:1 or 100:5:2 are generally accepted optimal. I am supplying P as KH2PO4 and KH2PO4. So, if I keep this ratio at 100:5:5, then bacteria will take their nutrient need and the rest will act as buffer. Is that right??

Then the question springs to my mind is whether this ratio applicable to 30,000mg/l COD wastewater. As there must surely be a limit to solubility of chemicals.

-paulanka-