Wastewater effluent is the final product of previous treatment processes, and effluent that meets certain quality standards can be discharged into bodies of water, such as streams, rivers, or wetlands. Since the 1970s with the Clean Water Act in the United States, and with similar regulatory bodies throughout the world, wastewater treatment processes – both municipal and industrial – must continuously meet increasingly strict requirements for the quality of the effluent stream. This is an ongoing challenge, as increasing populations and growing industries use more water and produce more wastewater, making it necessary to improve the efficiency of removing byproducts and pollutants to meet established environmental regulatory limits.
Both the volume of flow and the amount and type of contaminants can vary in the effluent stream of a wastewater treatment process. Wastewater quality is categorized by the contaminant loading. Industrial wastewater typically contains a high organic fraction which can be toxic and demands rapid detection for early intervention.
The effluent stream in an industrial or municipal wastewater treatment process can contain numerous organic and inorganic contaminants. Because the effluent stream is typically released into the environment, the quality of the effluent must comply with regulations to avoid costly fines and a potentially hazardous environmental situation.
There are several commonly used measurement methods for identifying the amount of organic matter in wastewater, including biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC).
BOD testing is one of the most common methods. It involves introducing a small seed of bacteria to the effluent sample, and monitoring for a corresponding decrease in dissolved oxygen over a five-day period. This method has severe drawbacks. First, many wastewater treatment processes, especially industrial wastewater plants, cannot wait five days for water quality analysis when they need to make real-time decisions. Also, the presence of certain chemicals like solvents, chlorine, or sanitisers in the effluent stream can prevent the growth and activity of the bacteria, resulting in falsely low BOD test results. Because of this, the U.S. EPA Method 405.1 states, “There is no acceptable procedure for determining the accuracy of the BOD test.”
A COD test is a common alternative to BOD testing. COD uses chemical oxidation to measure oxidisable contaminants in the effluent. The result can be achieved within 30-120 minutes. COD is often used in conjunction with BOD testing.
TOC analyser oxidises the organic carbon to carbon dioxide (CO2) for the TOC measurement. OrganicMcarbon present in the sample is oxidised to carbon dioxide by a chemical oxidation process using hydroxyl radicals and catalysed ozone. The carbon dioxide generated after the oxidation process is then measured by a non-dispersive infrared (NDIR) CO2 detector. Measuring total organic carbon is an excellent method to measure wastewater effluent. This method can identify some organic pollutants directly, more accurately than BOD or COD tests, and can do so faster and in real-time.