Why Test the COD Residual Rate of a Composite Carbon Source?
In modern biological nitrogen removal processes, the influent carbon-to-nitrogen ratio (C/N ratio) is often too low to support efficient denitrification. As a result, wastewater treatment plants must add an external carbon source to provide sufficient organic carbon for denitrifying bacteria.
Commonly used carbon sources for wastewater treatment include:
- Methanol
- Sodium Acetate
- Acetic Acid
- Glucose
- Composite Carbon Sources
Among these, composite carbon sources have become increasingly popular because of their high carbon efficiency and convenient application. However, product quality varies significantly among manufacturers. Low-quality products may contain organic substances that microorganisms cannot effectively biodegrade.
These non-biodegradable compounds may fail to improve total nitrogen (TN) removal, while simultaneously causing excessive COD residuals in the effluent, resulting in discharge standard violations.
According to industry practice, a COD residual rate of ≤10% is generally considered an indicator of a high-quality, biodegradable composite carbon source that supports stable wastewater treatment system operation.
Standard Test Method for Determining the COD Residual Rate of a Composite Carbon Source
Step 1. Prepare the Test Solution
Mix the composite carbon source thoroughly.
Based on its COD equivalent value (provided by the manufacturer or measured in the laboratory), dilute the sample with purified water to prepare Solution A with an approximate COD concentration of 10,000 mg/L.
Measure the actual COD of Solution A and record it as:
Y (mg/L)
Step 2. Prepare Activated Sludge Samples
Collect mixed liquor from the outlet section of the aeration tank in the wastewater treatment plant.
After stirring thoroughly, divide the mixed liquor equally into three 1-liter beakers.
Step 3. Add the Carbon Source
Randomly select two beakers and add:
- 10 mL of Solution A
Add 10 mL of purified water to the remaining beaker as the blank control.
Conduct aeration for all three beakers simultaneously under identical conditions.
Step 4. Measure Residual COD
After 24 hours of continuous aeration, collect the supernatant from each beaker and measure the COD.
Calculate the residual COD (M) using the following equation:
Residual COD (M)
= Average COD of the two dosed samples − COD of the blank control
Record the result as:
M (mg/L)
Step 5. Calculate the COD Residual Rate
The COD Residual Rate is calculated using the following formula:
COD Residual Rate (%) = (M × 1.01) ÷ (Y × 0.01) × 100%
A lower residual rate indicates better biodegradability and higher utilization efficiency of the external carbon source.
Example Calculation
Known Conditions
The original composite carbon source has a COD concentration of:
500,000 mg/L
Take 20 mL of the original solution and dilute it to 1 L with purified water.
The dilution factor is 50, giving a theoretical COD of approximately 10,000 mg/L.
Laboratory analysis shows:
Y = 9,800 mg/L
Aeration Test
Collect activated sludge from the end of the aeration tank.
Divide it into three beakers.
- Two beakers receive Solution A.
- One beaker receives purified water as the blank.
Aerate all samples continuously for 24 hours.
COD Results
Measured supernatant COD:
- Blank: 30 mg/L
- Sample 1: 38 mg/L
- Sample 2: 36 mg/L
Residual COD:
M = ((38 + 36) ÷ 2) − 30 = 7 mg/L
COD Residual Rate Calculation
COD Residual Rate
= (7 × 1.01) ÷ (9800 × 0.01) × 100%
= 7.21%
Test Result Interpretation
The measured COD residual rate is 7.21%, which is below the industry reference value of 10%.
This indicates that:
- The composite carbon source has excellent biodegradability.
- Microorganisms can efficiently utilize the organic carbon.
- The product effectively supports denitrification and total nitrogen removal.
- The risk of excessive effluent COD is very low.
- The carbon source is suitable for practical wastewater treatment applications.
Precautions During Testing
To ensure accurate and reliable results, pay attention to the following points:
- Thoroughly mix the composite carbon source before dilution.
- Use fresh activated sludge from a normally operating aeration tank.
- Keep all aeration conditions identical for every sample.
- Perform COD analysis using calibrated laboratory equipment.
- Conduct parallel tests whenever possible.
If abnormal results occur—for example:
- The blank control has a higher COD than the dosed samples.
- Significant differences exist between parallel samples.
Investigate potential causes such as sampling errors, mixing inconsistencies, laboratory operation, or instrument calibration before repeating the test.
Conclusion
Testing the COD residual rate of a composite carbon source is an effective method for evaluating its biodegradability and treatment performance. A COD residual rate of ≤10% generally indicates that the external carbon source can be efficiently utilized by microorganisms without causing excessive effluent COD.
When selecting a composite carbon source for wastewater treatment, plant operators should evaluate not only the purchase price but also biodegradability, denitrification efficiency, COD residual performance, and long-term operating costs to achieve stable and cost-effective biological nitrogen removal.
As a professional water treatment chemical manufacturer, we supply high-quality composite carbon sources, external carbon sources, and customized nutrient solutions for municipal and industrial wastewater treatment plants worldwide.
