From my point of view, I don't see any possible solution for your problem. Always prepare back up. Otherwise, you need to by pass the wastewater flow in order to keep the microbes safe and sound.

Best regards

physical mixer also can enhance DO .

you may use mobile air compressor ( in car workshop you can find samples) and use them for aeration .Also you should prepare a tube with a lot of small thin hollow foe attaching to end of air pomp and put it in the reactor or treatment channel.

I used O2 capsule for one case and it had suitable effects.But it is quite expensive .

adding kmno4 will increase the Do in the wastewater treatment plant without harming the microorganism

Dear Bhanu Prakash sir...

KMnO4 kills the bacteria  to the best of my knowledge..  

It Oxidizes organic compounds... by donating atomic Oxygen and not O2 gas...   

Hopefully, Agitator or peddle aerator will help in this regard as temporary basis; reduce the inflow of raw effluent which might be increase the BOD level, try to avoid the consequence of anaerobic degradation.

Nimantha, Contact your liquid oxygen supplier.  BOC used to have an emergency oxygen dissolving system for activated sludge called VITOX. 

In the meantime add alum to primary settling tanks and reduce MLSS