A wastewater sample has a high TOC value. What is the MOST significant concern regarding this finding in relation to drinking water treatment?

The most significant concern regarding a high Total Organic Carbon (TOC) value in a wastewater sample, in relation to drinking water treatment, is the potential for the formation of disinfection byproducts (DBPs). TOC refers to the total amount of carbon bound in organic molecules within the water. These organic molecules can be natural (like decaying leaves in a river) or synthetic (like industrial chemicals). Wastewater, by definition, contains a significant amount of organic matter from human and industrial sources, so a high TOC reading is not unusual, but it presents a challenge for drinking water treatment.

Drinking water treatment plants typically use disinfectants, such as chlorine, chloramine, or ozone, to kill harmful pathogens (disease-causing microorganisms) like bacteria and viruses, ensuring the water is safe to drink. However, when disinfectants react with the organic matter present in the water (the TOC), they can create DBPs. Many DBPs, such as trihalomethanes (THMs) and haloacetic acids (HAAs), are regulated because they are potentially carcinogenic (cancer-causing) and/or have other adverse health effects. The higher the TOC, the more organic material is available to react with the disinfectant, and therefore, the greater the potential for DBP formation. For example, if a wastewater sample has a TOC of 10 mg/L and is chlorinated, significantly more THMs and HAAs will likely form compared to a sample with a TOC of 1 mg/L treated with the same chlorine dose.

While other aspects of high TOC, such as taste and odor issues, or increased biological oxygen demand (BOD – the amount of oxygen needed by microorganisms to decompose the organic matter), are also concerns, the DBP formation potential poses the most serious risk to public health. Therefore, drinking water treatment plants often employ pretreatment processes, such as coagulation, flocculation, and filtration, or advanced oxidation processes (AOPs) to reduce TOC levels *before* disinfection, minimizing DBP formation and ensuring the production of safe drinking water. These pretreatment steps remove or break down the organic carbon, lessening the raw material available for DBP creation.