Chemical Oxygen Demand | Total Organic Carbon | Activated Sludge Process | Wastewater Treatment Plant

 Chemical Oxygen Demand

Concept

A wastewater's Chemical Oxygen Demand (COD) is the total amount of oxygen required to chemically oxidize all organics in the wastewater to carbon dioxide and water. It is expressed in milligrams per liter. A reflux (vaporizing and then condensing) reaction involving potassium dichromate, sulfuric acid, and silver sulphate is used to determine the COD. This is a quick test procedure that produces reasonably consistent results.

Relevance

The COD test is used to determine the organic strength of wastewater because it is much faster (hours) than the standard Biological Oxygen Demand (BOD) test, which takes five days. Good correlations between COD and BOD values for a given wastewater treatment plant can generally be developed over time, but these correlations are dependent on the “normal” constituents of the wastewater and will not be consistent across plants. Correlations may also change over time, and they should be verified on a regular basis at each plant.

Total Organic Carbon

Concept

Total Organic Carbon (TOC) is another measure of wastewater's organic strength. This term encompasses a broader range of compounds. The total carbon concentration consists of

• Total Inorganic Carbon—the sum of the carbonate, bicarbonate, and carbon dioxide fractions.
• TOC refers to all carbon atoms that are covalently bound to organic molecules.
• Dissolved Organic Carbon (DOC)—the fraction of TOC that passes through a 0.45 mm filter paper;
• Suspended Organic Carbon (SOC)—the fraction of TOC retained on 0.45 mm filter paper.

The TOC test entails converting all inorganic carbon to CO2 at a pH of 2.0 or less and then purging the CO2 from the system with an inert gas. The methods for accomplishing all of this differ. The incorporation of in-line TOC measurement has been discovered to be a valuable process management tool that should be considered.

Relevance

The TOC test is primarily used when water reclamation is planned or practiced. The amount of organic and inorganic carbon that can pass untreated through a wastewater treatment plant and enter receiving water raises a number of health concerns. If that discharge is then used to supplement the water supply, the health risks are exacerbated. A TOC test can determine the relative magnitude of the threat, as well as the need for additional carbon reduction in wastewater treatment plant effluent, as well as a decrease in the relative volumes used for water supply augmentation or other wastewater reuse options. A TOC test can help assess the threat's relative severity, as well as the requirement for greater carbon reduction in wastewater treatment plant effluent and a reduction in the relative amounts used for water supply augmentation or other wastewater reuse possibilities. When used for these purposes, this is a surrogate, or indirect, test method that indicates the general water quality in relation to residual carbon concentrations.