There are several process variations in nitrification systems. In refinery wastewater plants two-stage biological treatment configurations are not uncommon. But those two-stage systems typically do not include a clarifier between the 1st and 2nd stage bioreactors as shown in Figure 1. This arrangement is more likely to be found in a municipal wastewater system designed to nitrify.
Nitrification consumes a lot of oxygen and oxygen shortages are not infrequent in petrochemical wastewater systems. Therefore, keeping track of oxygen demand is especially import for the industrial wastewater operator. A simple method for estimating oxygen consumption is to combine the oxygen requirements from the following two equations:
1.5 × CBOD5 removed (lb/d) = oxygen for carbonaceous BOD5 removal (lb/d)
4.6 × NH3 nitrified (lb/d) = oxygen for nitrification (lb/d)
When treating industrial wastewater nitrification is frequently inhibited or prevented by the presence of toxic organic compounds or heavy metals. This is because nitrifying organisms are much more sensitive to the biological environment than the typical microorganisms (bacteria) found in activated sludge. If your system is nitrifying, you have a good indication that the wastewater is low in toxicity.
Figure 1: Two stage nitrification system with two stages of clarifiers
The Modified Ludzack-Ettinger (MLE) process is a commonly used biological nitrogen removal method. Key to its operation is the internal recycle to feed more nitrate from the aerobic zone to the anoxic zone. The recycle flow rate is 2 to 4 times the influent flow rate. The result is both an increase in the denitrification rate and the overall nitrogen removal rate. The MLE process is portrayed in Figure 2.
Anoxic is a condition in which oxygen is absent but nitrate is present. (Anaerobic conditions exist when there is no oxygen or nitrate.) When the mixed liquor is subjected to anoxic conditions, facultative microorganisms that normally use dissolved oxygen instead consume the oxygen that is chemically bound to nitrate. The resulting reaction releases nitrogen gas to the atmosphere.
Denitrification is the reduction of nitrate to nitrogen gas. Denitrification uses the BOD of the influent wastewater as a substrate for carbon and energy and nitrate as an oxygen source. Be aware that denitrification can cause rising sludge in the secondary clarifier if the detention time in the clarifier is too long. Denitrification in the clarifier is most common when the sludge age is high such as in extended aeration plants.
Figure 2: Modified Ludzack-Ettinger Process
In Figure 3 there is yet another variation of the nitrification process. In this diagram you can see that methanol is being added as an inexpensive source of carbon for denitrification. The methanol is required when carbonaceous BOD removal is very complete leaving little substrate for the conversion of nitrate to nitrogen gas.
Figure 3: Methanol Addition for Denitrification