How to Detect COD in Wastewater: A Complete Guide

Chemical Oxygen Demand (COD) is one of the most critical parameters in water quality analysis. Whether you manage a wastewater treatment plant, run an environmental laboratory, or oversee industrial discharge compliance, understanding how to detect COD accurately is essential. In this guide, we’ll walk you through what COD is, why it matters, the standard methods used, and how to choose the right instrument for your needs.

What is COD?

COD stands for Chemical Oxygen Demand. It represents the amount of oxygen required to chemically oxidize organic and inorganic substances in water under specific conditions. The result is expressed as milligrams of oxygen consumed per liter of water (mg/L).

Simply put, COD measures the pollution load caused by reducing substances—primarily organic matter—in a water sample. A higher COD value indicates more severe organic pollution.

Why Measure COD in Wastewater?

Monitoring COD plays a vital role in environmental protection and process control. Here are the main reasons laboratories and industries measure COD regularly:

• Assessing Organic Pollution: COD directly reflects how much oxidizable organic material is present, helping you evaluate the degree of contamination.

• Environmental Compliance & Discharge Control: COD is a core indicator in wastewater discharge permits and surface water quality standards. Exceeding regulated COD limits can lead to heavy penalties.

• Treatment Efficiency Evaluation: By comparing COD values before and after treatment, you can calculate removal rates and fine-tune treatment processes for maximum efficiency.

• Operational Cost Management: Excessively high COD often means higher energy and chemical consumption during treatment. Keeping COD in check helps control operating costs.

Standards for COD Testing

In most markets, Chinese national standards or their international equivalents are widely used. The two most common standards for COD determination are:

• HJ 828-2017 – Water Quality – Determination of Chemical Oxygen Demand – Dichromate Method
  This is the arbitration method, offering high accuracy. The process takes about 2 to 3 hours and uses larger amounts of reagents.

• HJ/T 399-2007 – Water Quality – Determination of Chemical Oxygen Demand – Rapid Digestion Spectrophotometric Method
  This is the go-to method for routine testing. It’s fast (around 20 minutes), easy to operate, and highly suitable for batch sample analysis.

In European and North American contexts, you may also encounter methods such as ISO 6060 or EPA 410.4, but the principles remain similar to the dichromate approach.

COD Measurement Methods

Let’s take a closer look at the most common analytical methods.

Dichromate Method (HJ 828)

Principle: Under strong acidic conditions and heat, a known excess amount of potassium dichromate is used to oxidize the organic substances in the water sample. After the reaction, the remaining potassium dichromate is titrated with ferrous ammonium sulfate. The COD value is then calculated based on the oxidant consumed.

This method requires reflux equipment and is best suited for applications where the highest precision is needed, such as regulatory compliance verification or arbitration testing.

Rapid Digestion Spectrophotometric Method (HJ/T 399)

Principle: A water sample is mixed with a known quantity of potassium dichromate in a sealed digestion tube and heated at 165 °C for 15–20 minutes. After digestion, a spectrophotometer measures the absorbance of Cr⁶⁺ or Cr³⁺, and the COD value is read directly.

This method is the mainstream choice for daily laboratory testing because it reduces processing time, minimizes reagent usage, and is ideal for high-throughput environments.

Other Methods

Additional COD testing methods include:

• Chlorine Correction Method – Designed for high-chloride wastewater samples to avoid interference.

• Coulometric Titration – An electrochemical approach, less common in routine analysis.

How to Choose the Right COD Instrument

Selecting appropriate instruments can significantly improve your testing accuracy and workflow efficiency. Here are the three main categories, along with practical selection tips.

Benchtop Laboratory Analyzers

For wastewater treatment plants, environmental monitoring stations, and third-party laboratories, a benchtop setup that combines a digester with a spectrophotometer is recommended. Examples include the Lianhua LH-T600 and the 5B-3B series all-in-one COD analyzers.

When choosing a benchtop model, pay attention to:

• Measurement range: Commonly 20–1,500 mg/L for general wastewater; verify low-range accuracy if you test cleaner water.

• Chloride interference resistance: Look for integrated masking capability that can handle chloride concentrations up to 1,000–2,000 mg/L without manual dilution or additional pretreatment.

Portable Meters

For field inspections, drainage network surveys, and spot-checking, portable spectrophotometers offer great flexibility. The Lianhua LH-P700 series handheld meters allow you to run COD tests on-site with reliable results, eliminating the need to transport samples back to the lab.

Online COD Monitors

Key discharge points at industrial facilities often require continuous monitoring. Online COD analyzers (such as the CODcr-type models) perform automatic sampling, digestion, cleaning, and data transmission. When choosing an online monitor, make sure the device holds relevant environmental certifications in your region and supports the communication protocols required by local regulators.

Why Choose Lianhua Meter Technology for COD Testing?

With decades of experience in water quality instrumentation, Shanghai Lianhua Meter Instrument Co., Ltd. (Lianhua Meter Technology) provides a complete portfolio of COD testing solutions, from laboratory analyzers to portable meters and online monitors. Our instruments are designed to simplify operation, improve throughput, and deliver dependable accuracy—helping you stay compliant and optimize your treatment process.

Explore our product range at https://www.lianhuameter.com/ to find the right COD testing solution for your application, or contact our team for expert technical support.