Dissolved Organic Carbon (DOC)

What is DOC?

Dissolved Organic Carbon or DOC is a measurement of the amount of organic matter in water that can be passed through a filter, commonly 0.45 µm.

How is DOC measured?

A common method for Dissolved Organic Carbon analysis is Method 415.1. The procedure for analyzing DOC requires that the samples first be passed through a 0.45 µm filter. The test involves converting organic carbon to carbon dioxide (CO2) through oxidation (Combustion, UV promoted or Persulfate oxidation). The concentration of CO2 generated is measured using an infrared analyzer and reported as organic carbon (mg/L).

Why is DOC important?

For drinking water, Dissolved Organic Carbon is an important water quality parameter measured for several purposes. Elevated levels of DOC may interfere with the effectiveness of disinfection processes such as UV, ozone and chlorination thus should be monitored for removal prior to disinfection. In plants that disinfect with chlorine, DOC concentrations are a primary concern due to the harmful by-products that form when chlorine reacts with organic matter. DOC in finished water can lead to aesthetic problems and increase the potential for bacterial regrowth in the distribution system. Regulations for DOC are specific to each country, with aesthetic objective in drinking water being approximately 5 mg/L. Additionally, DOC is used in the calculation of SUVA which determines the aromatic portion of DOC, a major precursor for THM formation.

Measuring DOC with UV-VIS absorbance technology

Measurement of Dissolved Organic Carbon with UV-VIS technology has many advantages over traditional methods. A number of organic compounds in water strongly absorb UV radiation between 200-400 nm. Using absorbance data in this range and laboratory DOC data, excellent correlations can be made. For many applications, simply measuring absorbance at 254 nm and 550 nm will suffice for excellent correlations.

Where is DOC measured?

  • Influent to drinking water plant for source water quality fluctuations
  • Pre and post coagulation to optimize chemical dose for turbidity and organics removal
  • Pre and post carbon adsorption to monitor organics removal efficiency and optimize activated carbon dosing
  • Pre chlorination for DBP formation potential
  • Distribution system for general water quality and contamination events