What is BOD in Water Testing? BOD₅ Analyzer Operation Guide

In water quality testing, BOD (Biochemical Oxygen Demand) is a fundamental parameter used to evaluate the level of organic pollution in water. For operators new to water analysis, understanding what BOD means and how to measure it accurately is an essential part of daily laboratory work. This guide explains the basic concepts of BOD and introduces the measurement method using the Lianhua BOD₅ Analyzer with its advanced mercury-free manometric technology.

What is BOD?

BOD refers to the amount of dissolved oxygen consumed by microorganisms as they biochemically oxidize organic matter in a water sample. Simply put, the more organic matter present in the water, the more oxygen microorganisms will use to break it down.

A helpful analogy: imagine BOD as measuring "how much food a group of people can consume within a given time." The more active the microorganisms, the greater the oxygen consumption—indicating a relatively higher organic content in the water.

Understanding BOD₅

BOD₅ specifically refers to the dissolved oxygen consumed by microorganisms over a 5-day incubation period at a constant temperature of 20 °C. The 5-day standard was chosen because this period effectively reflects the degree of organic pollution while keeping the testing process efficient for routine laboratory work.

International standards such as HJ 505-2009 (China) and ISO 9408-1999 both recognize BOD₅ as a conventional indicator for water quality monitoring.

BOD vs. COD: What's the Relationship?

COD (Chemical Oxygen Demand) measures the total oxygen required to chemically oxidize all oxidizable substances—including both organic matter and certain inorganic compounds—using a strong oxidizing agent. The relationship between BOD and COD can be summarized as follows:

• COD is always greater than or equal to BOD (COD ≥ BOD), since chemical oxidation captures a broader range of substances than biological oxidation alone.

• The difference between COD and BOD helps estimate the relative amount of non-biodegradable organic matter in a sample.

Practical insight: When a water sample shows a high COD value but a low BOD value, it often indicates the presence of organic substances that microorganisms find difficult to degrade, such as certain chemical industrial residues.

How to Use the Lianhua BOD₅ Analyzer: Mercury-Free Manometric Method

Lianhua Meter Technology, founded in 1982, is an environmental enterprise dedicated to the research, development, production, sales, and service of water quality testing instruments. With a technical R&D team comprising over 30% of its workforce and branches or offices in 30 regions across China, Lianhua delivers reliable solutions for water analysis worldwide.

Below is the operating principle and procedure for the Lianhua BOD₅ Analyzer, which uses a mercury-free manometric sensing method.

Working Principle

In a sealed incubation bottle, microorganisms decompose organic matter by consuming dissolved oxygen and releasing carbon dioxide. A built-in sodium hydroxide absorbent captures the CO₂ produced, causing the internal pressure to drop. The instrument's pressure sensor monitors this pressure change in real time and automatically converts it into a BOD concentration reading.

Operating Procedure

Follow these simple steps to perform a BOD₅ test:

1. Sample Preparation: Transfer the appropriate amount of water sample into the incubation bottle.

2. Seal the Bottle: Attach the test cap and tighten it securely onto the bottle to ensure an airtight seal.

3. Start Measurement: Set the incubation period (adjustable from 1 to 30 days). The instrument will then run automatically.

Throughout the measurement period, the analyzer automatically stirs the sample, records pressure changes, and displays the BOD concentration directly once the incubation is complete—no manual calculations required.

Key Technical Features

Product Highlights from Lianhua

• Independent Test Caps: Each test cap works independently, allowing you to start a single sample measurement anytime without waiting for a full batch.

• Color LCD Display: Every test cap features its own color LCD screen for independent data display.

• Remote Monitoring: A dedicated mini-program allows you to check experimental status and results remotely from your smartphone.

• Multi-Site Management: Capable of managing multiple instruments and data across different monitoring sites simultaneously.

Frequently Asked Questions and Practical Tips

Q: Do I need to dilute high-concentration samples?
A: No, the Lianhua BOD₅ Analyzer has a direct measurement range of 0–4,000 mg/L. Water samples falling within this range can be tested directly without manual dilution.

Q: Is the incubation time fixed at 5 days?
A: No, the incubation period is adjustable from 1 to 30 days. Operators can select the appropriate testing cycle based on actual experimental needs.

Q: Will a power outage affect an ongoing test?
A: No, each test cap contains a built-in battery. A short-term power interruption will not disrupt an active test, and the instrument retains all stored historical data.

Conclusion

BOD₅ is a widely used indicator for evaluating the degree of organic pollution in water bodies. The Lianhua BOD₅ Analyzer uses mercury-free manometric sensing technology, which helps eliminate the tedious dilution, dissolved oxygen measurement, and manual calculation steps associated with traditional methods. It supports direct measurement across a full range of 0–4,000 mg/L, features adjustable incubation times, and equips each test cap with independent operation and battery backup for data security. With branches and offices in 30 regions nationwide, Lianhua Meter Technology stands ready to support your water testing needs with professional technical service.

Note: The experimental procedures described in this article are based on general measurement principles and the product specifications of the Lianhua BOD₅ Analyzer. Different water sample matrices may yield varying results. It is recommended to conduct method verification before routine testing.