A comparison of common analytical tools - BOD, COD, & TOC

SUEZ Water Technologies & Solutions designs and manufactures Sievers Total Organic Carbon (TOC) Analysers that enable near real-time reporting of organic carbon levels. Offering a variety of laboratory, on-line and portable analysers that cover a dynamic analytical range, these analysers provide stable and accurate data for treatment optimisation, quality control and regulatory compliance.

Traditional analytical tools used to determine the pollution degree of water and wastewater are lab-based tools that measure the oxygen demand required to oxidize contaminants. Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are the traditional tools often utilised. BOD and COD measure the oxygen-depleting effects of waste contaminants. The exception to this standard is the measurement of Total Organic Carbon (TOC), which directly measures inorganic and organic carbon. Below is a summary of each parameter, what they measure, and general strengths and weaknesses.

Analytical tools

BOD, Biochemical Oxygen Demand

BOD is an indicator of the fraction of organic matter that can be degraded by microorganisms, typically expressed as the amount (mg/L) of oxygen consumed over five days at 20°C. BOD includes the readily biodegradable organic carbon (carbonaceous or cBOD) and occasionally ammonia (nitrogenous or nBOD). cBOD is the primary driver of BOD and includes soluble, particulate, and colloidal organic carbon compounds.

Weaknesses

1. 1. Time: Five days
2. 2. Precision: Wide precision ranges on the order of +/-10-20%, showing limited reproducibility
3. 3. Interferences: Wastes containing suppressors to microbiological activity can inhibit and limit BOD results
4. 4. Specificity: The application of BOD is specific to a single sample and is not linear

COD, Chemical Oxygen Demand

1. COD is the amount of oxygen required for the chemical oxidation of compounds in water. This demand is determined using a strong oxidant, with most standard methods using dichromate and to a lesser extent permanganate.

Weaknesses

1. 1. Time: Several hours
2. 2. Oxidation limitation: Some organic molecules are relatively resistant to dichromate oxidation and may give a falsely low COD
3. 3. Inorganic carbon: The oxidation step does not distinguish between organic and inorganic carbons
4. 4. Additional inorganics: Chlorides, nitrite, ferrous iron, and sulfide may interfere
5. 5. Hazardous chemicals: Dichromate is toxic due to its oxidizing power and its use is being restricted in Europe

TOC, Total Organic Carbon

1. 1. TOC is a direct measure of all organic carbon in a sample. It is determined by oxidation of the organic carbon compounds. TOC analysers can be configured to measure different fractions of organic carbon including: TOC, Purgeable Organic Carbon (POC), Non-purgeable Organic Carbon (NPOC) and Dissolved Organic Carbon (DOC).

Depending on the organic carbon technique employed, Inorganic Carbon (IC) is removed or measured. Measuring for both inorganic carbon and organic carbon will provide Total Carbon (TC).

Strengths

1. 1. Direct: TOC analysers directly measure the amount of organic carbon without interferences
2. 2. Versatile: TOC analysers come in a variety of configurations
3. 3. Time: Less than 10 minutes
4. 4. Accuracy & Precision: TOC analysis often results in accuracy and precision within +/-5% over a large range of concentrations from 1 ppb to 50,000 ppm

Weaknesses

1. TOC does not identify the overall oxidation state. Most regulations are still written for BOD and COD reporting.

Available TOC technology from SUEZ  

The Sievers* InnovOx TOC Analysers with state-of-the-art supercritical water oxidation (SCWO) provides accurate measurement of the carbonaceous component of oxygen demand and supports correlation to these methods.

The advantages of measuring organic carbon with the Sievers InnovOx TOC Analyser include:

1. 1. No interference from Chlorides when measuring TOC
2. 2. Less than 10-minute analysis time, supports laboratory and on-line applications
3. 3. Complete oxidation of organic molecules unlike dichromate oxidation (COD)
4. 4. Provides direct data on process performance-organic load, separation, and removal

Water and wastewater samples contain a diverse set of organic contaminants. While COD, BOD, and TOC each provide information about the pollution degree of water and wastewater, only TOC provides a direct measure in minutes, allowing for process control and quick decision making. With the Sievers InnovOx TOC Analysers and SCWO technology, you can obtain accurate data without interference on process performance.