Caustic is added to the aeration tank-2 (series) to maintain neutral pH. But after 2 hrs, pH is dropped down to 5 to 5.5. What is the reason for this?? And what could be the best solution for the same ???? After aeration tank, Alum is added in the clarifier. The recirculation is 95%. But now, alum dosage is stopped completely. But even then, the same problem of pH dropping is happening.

Hi organica,

look at the following points:

1) buffer capacity of the influent: make a titration curve of the influent with HCl of a well know titer (strength, normality) an titrate from pH influent to pH e.g. 4 or 3. Note pH after each mL (or 0.5 or 0.1 mL, depends upon the slope...) of acid added. From this you have a good idea of the (pH)buffer capacity.
2) is there ammonium or other Kjeldahl nitrogen present in the influent, and how much?, Is you plant well nitrifying (nitrates appear in the aeration tank)? if so, is dentrification taking place?

alumn and other Al(III) (also Fe(III)) have a pretty strong tendency to lower the pH.

Best regards,

DS

When you say caustic is added; is this a manual addition or does the pH control caustic delivery? If the addition is manual, then the high RAS rate will continue to lower the pH from the alum addition. Because of your previous inquiry, I believe that you may need to provide more information about the inputs, outputs, and operation of the treatment system.

grrun

what is the source of your inlet water? I had encountered similar problem in a cola company ETP.