How to Quickly Reduce COD in Domestic Wastewater? Effective Methods & Side Effects to Avoid
Emergency response solutions for plant operators at residential buildings, apartment complexes, and urban wastewater treatment plants.
Sudden COD Spike? Quickly Check 5 Points Before Dosing Chemicals
COD consists of particulate/colloidal forms (removable via physical settling or coagulation) and soluble forms (treated via biological or oxidative processes). Checking the following 5 parameters helps select the right solution:
Emergency COD Reduction Methods & Their Limitations
1. Coagulation & Flocculation (Particulate COD Treatment)
PAC or alum combined with polymers aggregates colloidal solids into larger flocs for settling. Ideal dosing point: After the biological tank, before the final clarifier, or in a dedicated coagulation tank.
- Effectiveness: Rapid reduction if COD is primarily composed of suspended particulate matter.
- Limitations: Ineffective against soluble COD. Overdosing increases residual aluminum concentrations and sludge volume.
2. Using H2O2 & Javel (Strong Oxidants for Rapid COD Breakdown)
Hydrogen peroxide (H2O2), sodium hypochlorite (Javel), or calcium hypochlorite instantly break down recalcitrant organic compounds. Dosing location: Disinfection tank or final storage tank. Never dose directly into biological tanks.
| Oxidizing Chemical | Rapid Advantages | Side Effects & Risks |
|---|---|---|
| H2O2 / Javel | Reduces soluble COD within minutes. | High chemical cost; recommended only for emergency situations. |
| Residual Chlorine | Provides disinfection while lowering COD. | Easily causes residual chlorine to exceed QCVN discharge limits; generates organochlorine byproducts. |
| Measurement Errors | Complete chemical reaction. | Residual H2O2 and chlorine cause false readings in COD testing (must wait for full decomposition). |
3. Increasing Aeration & Adding Bio-Augmentation
Increasing DO to 2–4 mg/L enhances microbial degradation (takes effect over hours). However, excessive aeration can break sludge flocs, increasing turbidity, TSS, and effluent COD. Microbial additives or molasses require days to acclimate and cannot provide immediate COD reduction within hours.
Warning: Common Mistakes That Cause COD Rebound or Violations
- Over-wasting sludge: Wasting sludge due to turbid water depletes active biomass, causing COD to surge after 1–2 days.
- Dosing oxidants into biological tanks: Kills the entire microbial population in the system.
- Diluting effluent with clean water: A severe discharge regulation violation that does not reduce total pollutant load.
- Dosing chemicals without testing: Leads to chemical overdosing and counterproductive side effects.
On-Site COD Testing with Specialized EMIN Equipment
To adjust chemical dosages accurately, operators need immediate data rather than waiting days for laboratory test results.
Includes thermoreactors & photometers for on-site COD/BOD/TOC analysis within 2 hours.
For automated plants, Online COD Sensors (UV254 principle) continuously monitor fluctuations and transmit signals to PLCs to automatically trigger dosing pumps when concentrations approach discharge limits.
24/7 continuous monitoring, sending signals to trigger automated chemical dosing systems.
