Phophate buffer. wastewater treatment?

Question

In a wastewater to keep pH at 7 one can use KH2PO4 and K2HPO4 as phosphate buffer. but is there an upper limit for their supplied concentration?

e.g. a wastewater sample with 2500 mg/l SCOD (pH = 5.5)(characteristics close to pulp and paper). I supply 1200 mg/l KH2PO4 & 2400 mg/l K2HPO4. at the end of reaction pH goes up to 8.6. I want to keep it 7 - 7.5 and for this purpose I am thinking of increasing concentrations of buffer.. but i dont know if there is a risk of phosphate overdosing???

Answer 1

I agree with the previous answer.  Usually, you try to increase the pH initially to make metals less soluble, and then precipitate them out.  I recommend adjusting the pH only prior to final release of the effluent, not with a buffer, with with either NaOH or H2SO4

Answer 2

In the real world, a buffer is never added to a wastewater system or pilot plant.  Also, you never try to maintain an exact pH.  You try to maintain it either on the acid side 5-7 or more usually on the basic side 7-9.  Other ranges can be used and which is used depends on the waste stream.  I usually use 7.5-8.5 on a twice per day check.  When it varies outside the range you use sulfuric acid, sodium hydroxide,  or carbonate to adjust the pH.  If you have it available you can use an auto feed and pH check, but this is not needed for wastewater studies.

Note that the addition of any of these will set up buffer systems in the wastewater.  In rare cases, soda ash can be added to aid this.  It is the carbonate and sulfate buffering systems that do the job.

In your system, once or twice per day, sulfuric acid would be added to bring the pH back down to around 7 with the system eventually buffering itself.  It sounds like you have a wastewater that is very low in buffering properties to start with, so you will need to run your system several cycles to get it to settle down, but it will still be a range.

Your way is not transferrable into the real world for two reasons.  One, the cost would be prohibative, and two, you are adding phosphate that you are going to have to removed somehow at the end.  That is very difficult and again would be cost prohibitive.  If your system can not be used in the real world, why study it.