Why is the pH rising in my nitrifying bioreactor?

[Lesley Robertson]

bfinger1 at my-dejanews.com wrote in message
<7bh5ob$35k$1 at nnrp1.dejanews.com>...
>We have an 80 L, suspended growth nitrifying bioreactor.  The
>feed stream has approximately 800 ppm ammonium and flows in at
>15.3 mL/min.  The reactor is sparged with air at a rate of 50
>L/min.  From experience and the literature we know the pH should
>be dropping in this reactor.  The pH in other fixed-film reactors
>we have operated with this feed stream has always gone to 6.5 or
>lower.  We are getting approximately 40% nitrification, but the
>darn pH keeps rising towards 8.  We have to periodically add acid
>(HCl) to keep the free ammonia concentration below a toxic level.
> Were thinking some sort of CO2 related buffering is going on.
>Any ideas on why this is occurring would be greatly appreciated


What else is in your influent? Organics? Bicarb?
Does your N-balance close? That is, are you recovering all the NH4+ as
itself of NO2-/NO3-?
Lesley Robertson
Delft