How to Rapidly Reduce COD in Domestic Wastewater?

Wastewater treatment plants serving apartment complexes, buildings, and residential areas sometimes experience effluent COD levels approaching regulatory limits, particularly following sudden load surges or shortly before scheduled compliance sampling. QCVN 14:2025/BTNMT, applicable to domestic wastewater from September 1, 2025, sets COD limits based on discharge volume and the receiving water body, while allowing dischargers to choose between TOC and COD parameters

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.

Domestic wastewater treatment plants at apartment complexes, office buildings, and residential areas sometimes experience effluent COD spikes near legal limits, especially after shock loads or right before sampling inspections. QCVN 14:2025/BTNMT (effective from Sept 1, 2025) specifies COD limit values based on discharge flow rate and receiving water bodies, allowing dischargers to choose TOC or COD. Since domestic wastewater contains mostly easily biodegradable organic matter, COD can be reduced rapidly if the right method is applied at the correct treatment tank.

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:

01. Suspended Solids Check if the effluent is turbid or carrying carryover sludge flocs (secondary clarifier malfunction).

02. DO Level Measure DO level in the aerobic tank; inspect air diffusers and blowers.

03. SV30 Index Sludge settling volume after 30 minutes; check sludge color and odor.

04. Biological Tank pH Ensure influent pH remains within the optimal range for microorganisms.

05. Influent Load Investigate recent events such as septic tank desludging, kitchen grease surges, or heavy detergent discharges.

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.

High-Speed Portable & Benchtop COD Testing Kits

Includes thermoreactors & photometers for on-site COD/BOD/TOC analysis within 2 hours.

Explore EMIN COD Meters →

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.

Online COD Monitoring Sensors for PLC Systems

24/7 continuous monitoring, sending signals to trigger automated chemical dosing systems.

Discover Online COD Sensors →

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