I mainly agree with Waymon ...

I mainly agree with Waymon Hofheins with certain provisos.  It depends entirely on what you are trying to digest.  The theoretical ratio is 100 ppm Carbon: 5 Nitrogen: 1 of phosphate.  Different sources provide different ratios.  Sewage will provide a higher proportion of nitrogen due to the presence of urine.  Certain others will have a higher proportion of phosphate.  I am an advocate of using ammonia oxidising bacteria instead of adding urea.  These fix their carbon from an inorganic  source of bicarbonate and may need carbonate or bicarbonate added to assist them.  They are strictly aerobic but they are effective and once present is a less expensive way of adding nitrogen than urea.  The nitrite eventually produced is used by bacteria removing the carbon.  The pH range can be critical.  The growth rate at pH 7.5 is far higher than at pH 6.5 although a plant may become adapted to that pH.  If the plant is phosphate limiting, then phosphoric acid may be used.  In other cases nitric acid may be used.  It is important in an aerated activated sludge plant to ensure that the plant is well mixed as well as aerated.  The best solution is fine-bubble aeration directed to the bottom of a tank which is angled so as to stir the tank.  Others have aeration at the base of the tank.  Oxygen is inferior to the use of air.  This is for the simple reason that air is 70% Nitrogen which helps drive off the Carbon dioxide, which helps balance the pH to between 7 and 8.  Oxygen does not have this benefit so the pH drops to 6-7 and this impacts the bacteria.  Although we have referred to bacteria, it is desirable to have protozoa and rotifers feeding on the bacteria and this aids oxygen transfer into the floc, which forms and helps reduce sludge levels.