Physical Methods for COD Reduction

Chemical oxygen demand (COD) is a key indicator of organic pollution in water. While biological and chemical treatments are common, physical methods offer distinct advantages—particularly for pretreatment, high-strength wastewater, or situations where biological processes are inhibited. Physical treatments primarily separate or concentrate pollutants without altering their chemical structure.

Key Physical Treatment Methods

1. Sedimentation and Flotation
   Gravity sedimentation removes suspended organic solids that contribute to particulate COD. Dissolved air flotation (DAF) effectively separates oils, greases, and fine suspended matter, significantly reducing COD loads before downstream treatment.

2. Filtration and Membrane Separation

• Sand or multimedia filtration captures larger organic particles.

• Microfiltration (MF) and ultrafiltration (UF) remove colloids and macromolecular organics, achieving moderate COD reduction.

• Nanofiltration (NF) and reverse osmosis (RO) can achieve high COD removal by rejecting dissolved organic compounds, though they produce a concentrated reject stream requiring further management.

3. Adsorption
   Activated carbon (granular or powdered) is the most common adsorbent. It effectively removes dissolved organic molecules responsible for soluble COD. Adsorption is especially useful as a polishing step after biological or chemical treatment.

4. Air Stripping
   For volatile organic compounds that contribute to COD, air stripping can provide efficient removal. It is typically applied when the target compounds have high volatility and low solubility.

5. Electrostatic Coagulation / Electrocoagulation (Physical Basis)
   Although often considered a physicochemical process, electrocoagulation uses electrical current to destabilize and aggregate suspended and colloidal organics, which are then removed by sedimentation or flotation.

Advantages and Limitations

Physical methods are generally robust, unaffected by toxicity, and produce rapid results. However, they do not degrade organic compounds; they merely separate or concentrate them. The resulting sludge or concentrate requires proper disposal or further treatment. In practice, physical methods are often integrated with biological or chemical processes to achieve complete COD removal.