Dear Ms Tomar, I have been ...

Dear Ms Tomar, I have been working with a variety of waste water treatment plants for for over 25 years.  I would start with the importance of aeration in the transfer of oxygen into the water and the removal of Carbon dioxide.  Pure oxygen resulting in pH levels of 0.5 to 1 units lower changing the population dynamics.  How the use of jet mix/ venturi systems cause the break up of bacterial floc interfering with settlement.  How surface aeration systems in a round tank does transfer air into the water but does not fully mix the tank resulting in anaerobic conditions developing in parts of the tank.  The difference between fine-bubble aeration and coarse air in oxygen transfer into the water and its importance in phosphate removal. 

The importance of anoxic selectors in the absorption of nutrients especially phosphate, prior to aeration.  The importance of ammonia oxidising bacteria in the control of ammonia and how they do not grow on organic materials but fix their carbon from carbonate in the water.

The importance of protozoa and metazoa in controlling the size of the floc and allow the transfer of oxygen and enhance bacterial settlement.

You will need to describe the different types of system called activated sludge as well as the supposed advantages and disadvantages of fixed and mobile media. You will also need to differentiate between plants using membrane separation rather than settlement tanks. This is before you even start on the micro-organisms themselves.

Once you have started this then you can describe the range of organisms involved under normal conditions.  Start up of an effluent plant.  How and when bacteria will be required to be added to an activated sludge plant.  The presence of filamentous forms of bacteria.  Conditions under which bacterial flocs weaken and form zoogleal growth.  Bacterial foam formation.  I could go on.