- Apr 4, 2022
- 3 min read

What Is Chemical Oxygen Demand (COD) And How To Reduce It? | YASA ET Tips

In water and wastewater treatment, Chemical Oxygen Demand (COD) is the most common measure to assay water quality. But what is COD exactly? How can it be reduced? A simple explanation by YASA ET.

Chemical Lab Test

What Is COD?

COD is an indicative measurement of the amount of oxygen that can be consumed by reactions in a measured solution. It is an indicator of the contents of reducing substances in the water, which are organic, nitrite, sulphide, ferrous salts, etc., and the organic is dominant.

COD is often used as a measurement of pollutants in water, wastewater, and aqueous hazardous wastes. One application of the COD test is to measure soluble COD in wastewater, since characterization of total COD in wastewater is critical for accurate modelling of biotransformation in wastewater treatment processes.

How To Reduce COD In Water And Wastewater?

1. COD Reduction by Precipitation Process (Coagulation & Flocculation)

The most important way to get rid of the sludge is to use coagulants and flocculants. The principle is to bind the sludge to one another so that a larger clumps of sludge and then be deposited in a sedimentation tank.

Some chemicals commonly used for coagulation and flocculation are PAC, Ferric Chloride, and PAM. This coagulation process, will greatly affect the value of COD. Especially in wastewater with a high amount of Total Suspended Solids (TSS).

2. Microbiology Process for COD Reduction

COD reduction process can be done using bacteria or microorganisms, derived from organic matter with a high content of biodegradable particles. This process is done following aeration and anaerobic processes.

In the aeration process, COD is reduced by making the bacteria to break down organic compounds in water. These bacteria are called heterotrophic bacteria and they break down organic compounds by consuming oxygen. This process is usually used in waste water with a COD of less than 3000 mg / L.

Instead, the anaerobic bacteria work in a room with minimal oxygen content. This process is also called fermentation, in which bacteria break down organic compounds from wastewater with three stages and one of them is by taking oxygen from organic compounds. The anaerobic process is suitable for waste water with BOD levels over 2000 mg / L.

3. Reverse Osmosis For COD Reduction

In the reverse osmosis process, cellophane-like membranes separate purified water from contaminated water. RO is when a pressure is applied to the concentrated side of the membrane forcing purified water into the dilute side, the rejected impurities from the concentrated side being washed away in the reject water.

RO can also act as an ultra-filter removing particles such as some micro-organisms that may be too large to pass through the pores of the membrane. The performance of RO membrane systems, water quality and plant performance can be enhanced with the careful selection and application of advanced treatment chemicals designed specifically for such water purification facilities and their sensitive membranes to effectively reduce COD.

4. Vacuum Evaporators

Vacuum evaporators have been used successfully in many industrial wastewater treatment applications, e.g., power and chemical plant wastewaters, metal finishing wastes, spent pulp liquors, emulsified oil streams, high soluble BOD (sugar) streams, and non-volatile aqueous organic or inorganic streams containing dyes, acids and bases.

Vacuum evaporators can reduce COD levels in the wastewater by evaporating the water and depositing the pollutants at the bottom. Often evaporators require pre-treatment or post-treatment equipment in order to reach the desired COD discharge standards.