Lab scaled biofilm reactor are operated for treatment of wastewater with initial pH = 7.44, the pH of effluent after treatment from reactor increased to 8.15. The percentage removal of COD is greater than 70% with nitrification rate being higher than denitrification rate. What might be the possible reasons for rise in pH contradictory to the decrease in pH that must be observed during reduction of COD and nitrification occurring in the system?
The rise in pH in the effluent of a lab-scaled biofilm reactor, contrary to the expected decrease during reduction of COD (Chemical Oxygen Demand) and nitrification processes, can be attributed to specific factors within the reactor system.
Here are a few possible reasons for this contradictory pH behavior:
It's important to consider the specific conditions, wastewater composition, and reactor design when trying to explain the contradictory pH behavior. Monitoring and understanding other parameters such as alkalinity, bicarbonate concentration, and ammonium concentrations can help provide a more comprehensive explanation for the observed pH trend.
On a root there is a principle of electrical neutrality so when a cation is taken in a cation need be released.
So, when ammonium is up taken hydrogen ions are released resulting in acidification of the rhizosphere.
In a root when an anion is up taken it must release an anion for the uptake of nitrate the balance is releasing hydroxyl which means the rhizosphere is limed from the rhizosphere release.
In a controlled situation the use of mono and dibasic phosphate can result in a 7.4 buffering and many natural substances also. A carbonate bicarbonate buffer would also be effective.
Hope some of this is useful for your purposes.
Pratap Bikram Shahi , The processes in wastewater treatment that raise alkalinity and thus pH are:
I think in your case it is the mineralization of organic acids(2), because you measured more nitrification than denitrification(3) and you would properly have mentioned if your effluent had a lot of ammonia/ammonium(1).
Gaurav H Tandon
"Alkalinity Generation:" Check your processes
NH4+ +2O2 -> NO3- + 2H+ (thus consuming alkalinity)
2NO3- + 2H+ + {CH2O} -> N2 (producing alkalinity)
Pratap Bikram Shahi
With reference to the excellent clarifications provided by Henrik Rasmus Andersen it seems that the decay of organic nitrogen (urea, proteins) to ammonia is the main factor resulting in the pH increase in spite of the overall decrease in alkalinity. We observed this is many aerobic wastewater treatment units. As to verify this, check both TKN (Total Kjeldahl Nitrogen) and NH4+ (part of TKN) at influent and effluent.
Pratap Bikram Shahi
In addition, in systems with limited oxygen supply and/or mass transfer such as biofilm units, ammonia gets partly oxididized to nitrite NO2- which anammox bacteria use to convert ammonia to nitrogen gas. This most efficient ammonia removal process, which we apply at full scale, consumes 46% less alkalinty and 57.5% less aeration (energy). If interested, I would be happy to send you more details via email.
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