Total Organic Carbon (TOC) is an analytical measurement representing the total amount of carbon found within organic compounds dissolved or suspended in a water sample. These organic compounds are molecules derived from living or once-living matter, containing carbon atoms bonded to elements like hydrogen, oxygen, and nitrogen. Water naturally contains a variety of these carbon-based substances, ranging from large molecules to microscopic particles. As a unified measurement, TOC serves as a fundamental and non-specific indicator of the overall quality and organic load of a water source.
Where Organic Carbon Originates
Organic carbon enters water systems from sources broadly categorized as natural or human-made. Natural organic matter (NOM) is the most common contributor, originating primarily from the decomposition of plant and animal material within watersheds. This decay releases complex carbon compounds, such as humic and fulvic acids, which are large molecules that persist in the water.
Other natural sources include microbial activity, which excretes organic byproducts, and runoff from soil erosion that carries organic debris. Human (anthropogenic) sources introduce a different spectrum of organic compounds, often synthetic. These sources include industrial discharge containing solvents and detergents, agricultural runoff carrying pesticides and fertilizers, and effluent from treated or untreated sewage.
Why Monitoring TOC is Essential
Health and Safety Concerns
The presence of organic material, measured as TOC, is a significant concern for public health due to its role as a precursor to harmful chemical byproducts. When water is treated with chlorine for disinfection, the chlorine reacts with the organic compounds. This chemical reaction generates regulated substances known as disinfection byproducts (DBPs). The most common DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs), which are linked to potential long-term health risks. The higher the concentration of TOC in the source water, the greater the potential for DBP formation. Water treatment facilities are required to monitor and reduce TOC levels before adding disinfectants to minimize DBP generation.
Operational and Aesthetic Impacts
High TOC also disrupts the efficiency and economics of the water purification process itself. The organic load increases the demand for disinfectants because the chlorine is consumed reacting with the organic carbon instead of targeting harmful pathogens. Moreover, organic compounds can foul the membranes used in advanced filtration systems like reverse osmosis, requiring more frequent cleaning and replacement, which increases operational costs.
Beyond these operational and health concerns, elevated organic content can negatively affect the aesthetic quality of drinking water. High levels of TOC are often associated with objectionable taste, color, and odor, making the water unappealing to consumers. This aesthetic impact is another reason utilities work diligently to remove organic matter, ensuring the final product is both safe and palatable.
Determining the Level of TOC in Water
Measuring TOC concentration involves a specialized analytical process that isolates the organic carbon content. Water samples contain both Total Organic Carbon (TOC) and Total Inorganic Carbon (TIC), which includes dissolved carbon dioxide, carbonate, and bicarbonate. The first step is removing the inorganic carbon by acidifying the sample and purging it with an inert gas. This converts the inorganic species into gaseous \(text{CO}_2\) that is vented away.
Next, the remaining organic compounds are subjected to oxidation to convert the organic carbon into measurable carbon dioxide gas. Oxidation methods include high-temperature catalytic combustion or chemical methods like ultraviolet (UV) light combined with a chemical oxidant. The final step is measuring the amount of \(text{CO}_2\) produced using a non-dispersive infrared (NDIR) detector. This provides a direct reading of the organic carbon concentration, typically reported in milligrams of carbon per liter of water.