Understanding the denitrification rate in an anoxic tank is critical for optimizing wastewater treatment, ensuring effluent quality, and calculating the correct carbon source dosage. 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.
What Is Denitrification Rate?
The denitrification rate quantifies the amount of nitrate nitrogen (NO₃⁻-N) consumed by one gram of sludge per hour. Factors affecting this rate include:
Water temperature
Sludge age
Type of carbon source
Sludge microbial activity
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 carbon source optimization.
Materials and Equipment Required
Laboratory Equipment:
Conical flasks (250–500 mL) with rubber stoppers
Magnetic or hand stirrer
Timer
50 mL syringes and sample bottles
Filtration setup (filter paper and funnel, or vacuum device)
Spectrophotometer for nitrate nitrogen measurement
Portable dissolved oxygen (DO) meter
Reagents:
Potassium nitrate (analytical grade)
Carbon source: sodium acetate or glucose
Step 1: Collect Sludge from the Anoxic Tank
Sample 3–4 liters of activated sludge from the end of the anoxic tank. Do not use sludge from aerobic or anaerobic tanks.
Transport sludge to the laboratory within 1 hour.
Collect influent and effluent water samples from the anoxic tank for baseline nitrate measurements.
Tip: Sludge at the end of the anoxic tank contains stabilized denitrifying bacteria, ensuring reliable denitrification rate data.
Step 2: Prepare Sludge for Measurement
Measure MLSS (or MLVSS 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
Prepare a high-concentration nitrate solution (e.g., 10 mg NO₃⁻-N/mL).
Add sufficient nitrate solution to reach 30–40 mg/L nitrate nitrogen in the flask.
Stir immediately and take a zero-time sample (0 min). Filter and record.
Take additional samples every 30 minutes: 30, 60, 90 minutes (optional 120 min). Filter immediately.
Maintain low DO (<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.
Example:
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)
Reference: Normal rates range from 2–5 g NO₃⁻-N/(kg MLVSS·h).
Step 5: Determine Carbon Source Dosage
Calculate total active microorganisms in the anoxic tank:
Total MLVSS (kg)=MLVSS (g/L)×Tank Volume (m³)
Multiply by the measured denitrification rate to get daily nitrate removal capacity (kg/day).
Compare with required nitrate removal (inlet concentration – target effluent).
Add carbon source to fill the gap:
Sodium acetate: 3.5–5 g/g NO₃⁻-N
Glucose: 2.5–3.5 g/g NO₃⁻-N
Example:
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.
Conduct experiments in summer (max rate) and winter (min rate).
Interpolate intermediate rates to adjust carbon source dosing throughout the year.