PROBLEM: Light brown to brown colored pop-ups are rising in the secondary clarifiers of the conventional activated sludge plant. No odors are noticed as the sludge is broken up and the solids resettle. The MCRT is steady at 15 days, the F/M ratio is 0.02 and the SVI is 68 ml/g. The effluent ammonia is low, but the nitrate values are elevated. What is causing the sludge pop-ups? O Nitrification occurring in secondary clarifier sludge blanket O Nitrite-lock is occurring in the secondary clarifier sludge blanket O Denitrifycation occurring in secondary clarifier sludge blanket O Nitrification occurring in primary clarifier sludge blanket