Adjust pH of the mixed liquor to a desired value; typically values between 7.5 and 8.4
The aeration system should provide sufficient oxygen to avoid limiting condition (DO> 2.0 mg/L)
Wait for approximate 30 min to reach and ensure stable initial condition
Execution of the test
Verify that the temperature, pH and DO readings are at the desired set values or at least within the selected intervals. Otherwise adjust the operating conditions accordingly and wait until the system stabilizes.
Once stable conditions are reached, add the ammonium solution (having previously adjusted its temperature to the target temperature of the test) to achieve a neither limiting nor inhibiting ammonium concentration in the mixed liquor. Typical and adequate values are between 20 and 40 mg N L-1.
Start the stopwatch to keep precise track of the sampling times, since formally the test starts with the addition of the ammonium solution.
Collect the activated sludge samples every 20-30 min throughout the execution of the test. Note that all samples need to be filtered through 0.45 μm pore size filters, except those used for the determination of TSS and VSS concentrations.
Check that the pH reading value remains close to the target pH set point every 20-30 min. Using NaOH 0,1 N solution to adjust pH. Record the volume of the NaOH titration solution added over time (VNaOH versus time) (20-40 min are usually adequate).
Conclude the test after 3 to 4 h or, if the sampling and analytical determination of ammonium in the collected samples allows it, when ammonium is depleted.
After the conclusion of the test, take a sample for the final VSS assessment.
A typical output of this test is outlined in Figure 1.
On the y-axis, the ammonium, nitrite and nitrate concentrations (in mg N L-1) are reported, while time (in hours) is reported on the X-axis. The linear regression over these data allows for the assessment of the ammonium removal and nitrate production rates (in mg N L-1 h-1).
Please note that the collected data should be sufficient to reliably estimate the corresponding removal/production rate (e.g. rNH4 and rNO3) by linear regression with a satisfactory coefficient of determination (e.g. R2 > 0.98). Thus, preferably at least 4 to 5 data points are needed to carry out the linear regression, implying that a larger number of samples will need to be collected in the beginning of the test.
Nitrite may accumulate up to few mg N L-1 when conventional activated sludge is used. However, if the activated sludge is used to perform (partial) nitritation tests, then a higher nitrite accumulation will be expected and its concentration should be monitored in time, similar to ammonium and nitrate. When using conventional activated sludge to perform the full oxidation of ammonium to nitrate, the ammonium removal rate should equal the nitrate production rate with a negligible accumulation of nitrite during the test. The maximum specific ammonium oxidation rate (as mg N g VSS-1 h-1) can be therefore computed as follows:
Figure 1. Typical profiles of ammonium, nitrite and nitrate in the test
The NaOH titration rates (QNaOH in mL min-1) are used to assess the alkalinity consumption of AOB and NOB.