. This guide explains the laboratory procedure for measuring denitrification rates using activated sludge, as well as how to apply the results in real plant operations.
NO₃⁻-N) consumed by one gram of sludge per hour. Factors affecting this rate include:
Design values are often theoretical (e.g., 0.05 mg NO₃⁻-N/g·h), but actual operating rates may be much lower (e.g., 0.02 mg NO₃⁻-N/g·h), making precise measurement essential for .
Conical flasks (250–500 mL) with rubber stoppers
50 mL syringes and sample bottles
Step 1: Collect Sludge from the Anoxic Tank
3–4 liters of activated sludge. Do not use sludge from aerobic or anaerobic tanks.
within 1 hour.
Collect influent and effluent water samples from the anoxic tank for baseline nitrate measurements.
Sludge at the end of the anoxic tank contains stabilized denitrifying bacteria, ensuring reliable denitrification rate data.
Step 2: Prepare Sludge for Measurement
if possible) using standard laboratory methods.
Pour sludge into a conical flask, leaving 1/3 of the volume for stirring.
Pre-stir for 30 minutes to remove residual nitrate nitrogen (pre-stirring).
Measure the supernatant nitrate nitrogen; continue stirring if levels remain high.
Step 3: Conduct the Denitrification Experiment
high-concentration nitrate solution (e.g., 10 mg NO₃⁻-N/mL).
30–40 mg/L nitrate nitrogen in the flask.
zero-time sample (0 min). Filter and record.
Take additional samples every 30 minutes: 30, 60, 90 minutes (optional 120 min). Filter immediately.
<0.1 mg/L) throughout the experiment. Adjust stirring speed to avoid introducing air.
Step 4: Calculate the Denitrification Rate
Plot nitrate nitrogen concentrations at the four sampling points in an Excel spreadsheet-time on the x-axis and nitrate nitrogen concentration on the y-axis. Normally, a downward-sloping straight line will appear. The slope of this line represents the rate at which nitrate nitrogen decreases over time, expressed in mg NO₃⁻-N/(L·h). Divide this slope by the sludge concentration (MLSS or MLVSS) in the flask to obtain the specific denitrification rate.
For example: If the MLSS of the mixed liquor in the flask is 3500 mg/L (3.5 g/L), and the slope of the nitrate nitrogen decrease is 5.0 mg/(L·h), then the specific denitrification rate is 5.0 ÷ 3.5=1.43 mg NO₃⁻-N/(g MLSS·h). Converting to MLVSS, if the MLVSS/MLSS ratio is 0.65, the denitrification rate based on MLVSS is 1.43 ÷ 0.65=2.2 mg/(g MLVSS·h). Expressed in standard units, this is 2.2 g NO₃⁻-N/(kg MLVSS·h).
In actual operation, the normal denitrification rate is 2–5 g NO₃⁻-N/(kg MLVSS·h). Values below 2 indicate weak sludge activity or low water temperature, while values above 5 indicate highly active sludge.
MLSS=3,500 mg/L (3.5 g/L)
Nitrate decrease=5 mg/(L·h)
Specific rate=5 ÷ 3.5=1.43 mg NO₃⁻-N/(g MLSS·h)
MLVSS/MLSS=0.65 → Rate=2.2 g NO₃⁻-N/(kg MLVSS·h)
Normal rates range from 2–5 g NO₃⁻-N/(kg MLVSS·h).
Step 5: Determine Carbon Source Dosage
Total MLVSS (kg)=MLVSS (g/L)×Tank Volume (m³)
(kg/day).
Compare with required nitrate removal (inlet concentration – target effluent).
Sodium acetate: 3.5–5 g/g NO₃⁻-N
Glucose: 2.5–3.5 g/g NO₃⁻-N
Flow: 10,000 m³/d, MLVSS: 2,500 mg/L, tank volume: 2,000 m³
Total MLVSS: 5,000 kg
Denitrification rate: 2.2 g NO₃⁻-N/(kg MLVSS·h) → 264 kg/day removal
If required removal < 264 kg/day → add carbon source to reach target.
Step 6: Seasonal Adjustment
Denitrification rates are lower in winter due to reduced microbial activity.
.
Interpolate intermediate rates to adjust carbon source dosing throughout the year.