How to Fix a Problem of Sludge Bulking in a Waste Water Treatment Plant?

We are running a small industrial wastewater treatment plant with capacity of 3000 m3/day for a food processing unit. We are using the activated sludge wastewater treatment process and facing the problem of sludge bulking for the last month. We find out that the reason is low level of dissolved oxygen. Can someone explain to me how we can fix this problem and how to avoid these issue?