Aeration difusser cleaning problems

We have our ceramic aeration diffusers cleaned annually with HCl gas and then purged with nitrogen. We usually did this in the spring of the year and shortly after this we would experience dinitrification with floating sludge on our final clarifiers and on our gravity thickener. We thought this was caused by the change in seasons here in Wisconsin and the nitrifiers increasing with the warmer temps. This year we had our cleaning done about two weeks ago and had the same type of increase in floating sludge. Our question is has anybody else seen this and is there something else we could use as a purge gas. Thanks, Terry