Disinfection (with regard to wastewater) refers to a chemical process where pathogenic microorganisms are killed by a disinfecting agent. Chlorine is effective in destroying organic compounds and is relatively inexpensive when compared to other methods of disinfection; that's why chlorination has been the principle form of wastewater disinfection in North America for decades. In 1910, the city of Philadelphia was the first to chlorinate its wastewater before discharging into waterways (American Chemistry, 2007). Many other cities around the country soon adopted this disinfection technique and wastewater chlorination was widely used throughout the United States by the early 1900's. The chlorination of sewage remains the most common practice of wastewater disinfection today.
The Arcata Wastewater Treatment Plant uses chlorination to reduce pathogen loads in the wastewater before discharging it into the environment. After wastewater spends time in the enhancement marshes and treatment wetlands it is sent to the chlorine contact basin for disinfection. By the time the wastewater exits the chlorine contact basin it meets permitted standards for wastewater discharge. The National Discharge Pollution Elimination System (NDPES) permit program authorized by the Clean Water Act limits BOD and TSS levels in wastewater discharge to a monthly average of 30 milligrams per liter of effluent (EPA, 2008).
However, the primary reason is to remove pathogens from the wastewater.