The testing that should be done to determine if nitrification is taking place is very straight-forward and involves four separate tests on two different sets of samples as shown in Figure 1. Your first sample is typically collected from the primary clarifier overflow or the EQ tank effluent. In either case, the sample you need is the influent to the biological reactor. You need to measure the wastewater alkalinity, ammonia concentration, nitrate concentration, and pH. Your second sample is typically collected at the secondary clarifier overflow. If you have a two-stage nitrification system your sample locations are the same: inlet to the first-stage aeration basin and outlet of the clarifier.
Figure 1: Nitrification Test Locations
If you are in fact nitrifying, you should expect to see the clarifier effluent sample having a reduction in alkalinity, a reduction in ammonia, an increase in nitrate, and a reduction in the pH. Just because this is what we expect to see for results it doesn’t mean we will always get them. In Figure 2 you can see the nitrification test results for a chemical plant’s wastewater system.
Figure 2: Chemical Plant’s Nitrification Results
The alkalinity and pH in the secondary clarifier effluent are actually higher than the aeration influent values. Yet there was a major reduction in the ammonia concentration and an increase in the nitrate concentration. Clearly, this plant is nitrifying. So why the increase in alkalinity and pH? The two secondary clarifiers at this plant provide an excessive detention time and really only one clarifier needs to be in service at any time. But they just don’t want to take a clarifier out of service. So denitrification was taking place in the secondary clarifiers. Dentrification releases alkalinity back into the wastewater which increases the pH. This is shown in Figure 3.
Figure 3: Impact of Denitrification on pH
There is one other interesting point to observe about the nitrification test results shown in Figure 2. The question is often asked about why there is not an increase in the nitrate concentration proportional to the decrease in the ammonia concentration. After all, nitrification is the conversion of ammonia to nitrate. In Figure 2 we can see that our ammonia decreased by 36.92 mg/L (38.3 - 1.38) but the nitrate increased only 13 mg/L. Where is the missing 23.92 mg/L of ammonia (36.92 - 13)?
As shown in Figure 4 the missing ammonia can be accounted for by understanding that the quantity of ammonia not converted to nitrate is the quantity that was used by the bacteria to satisfy their nutrient need for nitrogen. This is shown, approximately, by the “N to biosynthesis” (nitrogen used in cell metabolism) line in Figure 4. You will never see the nitrate concentration increase by the exact amount of the reduction in the ammonia concentration.
Figure 4: Nitrogen Utilization by Bacteria