As a rule of thumb, microorganisms need, pound for pound, 100:5:1 for BOD:N:P.  That means for every 100 lb of BOD, they need 5 lb of nitrogen, and 1 lb of phosphorus.  “Normal” domestic wastewater typically has the right amount of BOD, N, P and so we often don’t need to do anything to manage this balance.  This is not always this case, though.  Industrial or other contributors in your collection system can shift the balance and impact what you need at the WWTP to treat the wastewater.  Nutrient balance often becomes a factor when your WWTP receives numerical TN and/or TP permit limits.  In many instances you will need more carbon than what is available, and have to provide it via chemical addition. 

Do you use external carbon at your WWTP?  If so, what type do you use?