Organic Matter
Living things, both plants and animals, are composed of organic substances that contain carbon. Organic carbon can occur in water in naturally occurring organic matter (NOM) including plant and animal detritus. Organic carbon can also occur as traces of lubricants, liquid fuels, fertilizers, and pesticides. The 5-day test for carbonaceous biochemical oxygen demand (CBOD5) and tests for total organic carbon (TOC) measure the level of organic carbon in wastewater and in natural waters.

Carbonaceous Biochemical Oxygen Demand (CBOD)
Results of the CBOD5 are used to describe the potential of domestic and industry wastewaters to pollute surface waters. CBOD5 measures the amount of oxygen consumed by living organisms (bacteria) in decomposing the organic portion of a waste. In the Fargo-Moorhead (FM) area, during 2002 and early 2003, CBOD5 in untreated sewage ranged from 219 to 337 mg/L, but CBOD5 for treated sewage was much less, ranging from 4.1 to 15.8 mg/L. In the case of treated wastewater from the sewage treatment plants in the FM area, the CBOD5 of the treated wastewater is used to determine the optimal rate of discharge that will ensure the health of the receiving body of water (Red River). The rate of discharge is particularly important to ensure adequate levels of dissolved oxygen (DO).

Graph of results of the CBOD5 for untreated wastewater entering the water treatment plants in the FM area. The treated wastewater released to the Red River has CBOD5 levels less than 1/10th of the levels of the untreated wastewater.

Total Organic Carbon
TOC is a measure of the organic matter in water in terms of the organic carbon content. NOM from decaying plants and animals contributes to the TOC levels in rivers. Human sources of organic carbon can include petroleum products, pesticides, and herbicides. The following graph shows TOC levels at the intake for the water treatment plants in the FM metropolitan area from October 2001 to March 2003. The average TOC values illustrate the uniform and relatively high Red River TOC levels that are dominated by NOM.

TOC itself is not regulated but is important for the operators of water treatment plants when they consider disinfection practices. When chlorine is used to disinfect water supplies, it reacts with TOC to form chlorinated organic compounds that are collectively referred to as disinfection byproducts (DBP). DBPs are cancer-causing compounds. The levels of these compounds in drinking water are regulated under the U.S. Environmental Protection Agency's National Primary Drinking Water Standards. These standards are presently under review and may become more stringent. As such, local water treatment plant operators continually evaluate alternative disinfection practices to ensure healthful drinking water for the FM area.

Graph of TOC in water from the Red River entering the water treatment plants in the FM area. TOC in the water treatment intake water, dominated by NOM, is reduced by half in the local water treatment plants. TOC itself is not regulated but is important in controlling the levels of cancer-causing DBPs in drinking water.