Hello,  In most of cases ...

Hello, 

In most of cases the manufacturers are able to solve your issue each using their own product, or one may use some thumb laws and make assumptions (very risky). This is the first iteration when dealing with your questions:

1.  The calculation shall be done in clean water. One needs to know ones wastewater quality, its alpha factor and do a conversion from your AOTR to SOTR. Assuming an alpha of 0.6, you will have a higher SOTR. (thumb low, you could divide to alpha, but the formula is more complex).
2. Aeration system design shall be performed at hourly rate, as this would deal with the instant process. SOTR / 14h will give the SOTR for design, in kgO2/h
3. having the water depth is OK, but one will need the immersion depth (iD) for the diffusers (subtract the height from bottom to diffuser upper surface). It may be that 0.3-0.4 m will be taken out of the 6m TWL. This iD will be always used further. 
4. From now on the SOTR is a function of: diffuser type, diffuser density and therefore coverage of bottom, immersion depth, and one needs to know the product
5. Having already an average OTE, which one must assume it was in wastewater, one have already a criterion to fulfill. The density will be increased until the SOTE (again converted for clean water) or actual (in wastewater)  OTE will go above your given average. This is not a function of one single diffuser disk. The shape of tank, the coverage ratio, the spaces between diffuser... all play a role. A huge role.
6. As output one has: amount of discs, SOTR figure, total airflow as figure, over-pressure demanded from blower. 
7. The last parameter 3.7 kgO2/h -  One might assume you were looking at SAE (in kgO2/kWh) or SSOTE (specific standard oxygen transfer efficiency - in %/m)
8. Usually a calculus is giving a total amount of aerated surface, that shall be quantified in real disk amount summing this surface. 
9. Some safety margins will be added. Engineering works this way. 

If located in India, I'd strongly suggest to inquire your plant to SSI Aeration, one of the companies I work with. Or try any other manufacturer.
I#d dare a question: why ceramic diffusers? What speaks against elastic membranes? What effluent do you treat?

I'd add that I am not a process engineer, but I have plenty of experience in dealing with fine bubble aeration at the most efficient standards, on behalf of established manufacturers, and quoted thousands of aeration projects worldwide. 

This File might help a little.