PH increase in wastewater treatment?

Question

in wastewater treatment system, bacteries are being used. normally I would expect that pH would decrease at the end of a reaction as a result of CO2 production by bacteria. how it is possible that pH increases??

Answer 1

add sodium bi carbonate

Answer 2

There could be hundreds of reasons for this including lack of DO, high aeration, and you don't even give us the chemical composition of the wastewater to work with.  Without complete details, I would have to go with a chemical change or the other two mentioned. There is also the formation of ammonia to consider.