What is the difference between BOD and COD?

Biochemical oxygen demand (BOD) measures the oxygen that microorganisms use to break down biodegradable organic matter over a set period, while chemical oxygen demand (COD) measures the oxygen equivalent of all chemically oxidizable material, so COD is usually higher and faster to determine.

Why are coliform bacteria used to test water?

Coliforms are easy to detect and are common in the intestines of warm-blooded animals, so their presence indicates possible fecal contamination and the potential for disease-causing organisms, serving as a practical indicator rather than a direct pathogen test.

Water Quality Parameters

Water quality parameters are the measurable characteristics used to describe the condition and suitability of water.

Definition

The set of physical, chemical, and biological properties measured to assess the condition of a water body or supply and its suitability for designated uses.

Scope

This topic covers the physical, chemical, and biological indicators used to characterize water quality. It addresses parameters such as dissolved oxygen, biochemical and chemical oxygen demand, pH, turbidity, suspended and dissolved solids, nutrients, and microbial indicators, and how these are measured by standardized methods. The interpretation of these parameters connects to the assessment of water pollution and treatment performance.

Core questions

What parameters indicate the oxygen status of water?
How do biochemical and chemical oxygen demand differ?
Which parameters indicate microbial contamination?
How are water quality parameters measured consistently?

Key theories

Oxygen demand as a pollution measure: Biochemical oxygen demand estimates the oxygen microbes need to degrade organic matter, while chemical oxygen demand measures total oxidizable material; both quantify the oxygen-depleting potential of a water sample.
Indicator organisms for contamination: Because direct testing for every pathogen is impractical, indicator organisms such as coliform bacteria are used to signal the likely presence of fecal contamination and associated pathogens.

Clinical relevance

Water quality parameters underpin the assessment of pollution, the design and monitoring of treatment, and the protection of drinking-water, recreational, and ecological uses; consistent measurement makes results comparable across sites and time.

Evidence & guidelines

Parameter measurement commonly follows standardized protocols such as Standard Methods for the Examination of Water and Wastewater; these are described here to explain how parameters are determined rather than as regulatory thresholds.

History

Standardized water-quality measurement developed alongside public-health and sanitary engineering in the nineteenth and twentieth centuries, with compendia of standard methods establishing reproducible procedures for key parameters.

Seminal works

davis2008
chapman1996
apha2017

Frequently asked questions

What is the difference between BOD and COD?: Biochemical oxygen demand (BOD) measures the oxygen that microorganisms use to break down biodegradable organic matter over a set period, while chemical oxygen demand (COD) measures the oxygen equivalent of all chemically oxidizable material, so COD is usually higher and faster to determine.
Why are coliform bacteria used to test water?: Coliforms are easy to detect and are common in the intestines of warm-blooded animals, so their presence indicates possible fecal contamination and the potential for disease-causing organisms, serving as a practical indicator rather than a direct pathogen test.