Abstract: 5310 A. Introduction

1. General Discussion

Organic carbon in environmental and other water matrices may exist in a variety of oxidation states. Some of these compounds can be oxidized further via biological or chemical processes. Biochemical oxygen demand (BOD), carbonaceous BOD (CBOD), biodegradable organic carbon (BOC), chemical oxygen demand (COD), and total organic carbon (TOC) analytical methods may be used to characterize these fractions. Total organic carbon provides a more convenient and direct expression of total organic content than BOD, CBOD, BOC, or COD. Analyses of TOC provide a measurement of the organic content of a sample that is independent of the organic matter’s oxidation state and does not measure other organically bound elements (e.g., nitrogen and hydrogen) or inorganics that can contribute to the oxygen demand measured by BOD and COD. The amount of TOC in water sources may be used to define the matrix and its quality and can range from ≤0.1 to 25 mg/L in drinking water to ≥300 mg/L in wastewater influent and industrial effluents. Although none of these techniques (BOD, BOC, COD, TOC) are direct correlates of each other, several studies have demonstrated that correlations can be developed between any pair of them and TOC can be used as a surrogate for some of these other parameters in certain cases. This has been acceptable under some regulatory frameworks including National Pollutant Discharge Elimination System (NPDES) permits. The regulatory authority to develop a correlation is found in the Clean Water Act 40 CFR § 133.104(b) (sampling and test procedures); however, no methods are provided for establishing a long-term correlation or other repeatable empirical relationship. A correlation provides a measure of the strength of association between two continuous variables. Regression quantifies the effect of one continuous variable on another dependent continuous variable. A long-term study consists of a minimum of one year of samples collected no less than once weekly to capture seasonal variations in operating conditions. For regions with little to no seasonal variation, studies shorter than one year may be sufficient. Although a correlation establishes a relationship, regression establishes an equation to quantify the relationship that can be used to report a calculated value for the dependent variable. Correlation coefficients provide a metric of the strength of association between two variables. Generally, those greater than 0.50 but less than 0.70 indicate a moderately weak to strong correlation and those greater than 0.70 are sufficiently strong and indicative of an empirical relationship. These relationships may need to be established independently for each set of matrix conditions (e.g., various points in a treatment process and water types).

Organic contaminants in water may degrade ion-exchange capacity, serve as a carbon or energy source for undesired biological growth, react with disinfectants to form byproducts that could harm human health or aquatic organisms, or be otherwise detrimental to the water’s purpose. Before organically bound carbon can be analyzed, the molecules must be converted into carbon dioxide (CO₂), a molecular form that can be measured quantitatively. Current TOC methods use one of the following means of oxidation to convert organic carbon into carbon dioxide:

The CO₂ may be purged from the sample, dried, and transferred via carrier gas to a nondispersive infrared analyzer or coulometric titrator. Alternatively, CO₂ may be separated from the sample via a CO₂-selective membrane into high-purity water, where the increase in conductivity corresponds to the amount of CO₂ passing through the membrane.