Contributed by Annie Pinet, Business Development Manager, RealTech
Chlorine has long been used as a primary disinfection method in many water and wastewater treatment applications. However, there is growing concern about the harmful DBPs produced by the use of chlorine, which has been the basis for regulations limiting their concentrations in water.
DBPs, such as triahalomethanes (THMs) and haloacetic acids (HAAs), can form when the natural organic matter (NOM) that is present in the water combines with chlorine. More specifically, the portion of NOM in the water that is aromatic is the most reactive with chlorine and therefore possesses the most potential for DBP formation.
Water treatment plants with high levels of NOM in their source water often find that they must remove organics in order to reduce DBP formation to comply with regulations such as the USEPA Stage 2 Disinfectants and Disinfection By-Products Rule (D/DBPR). Enhanced coagulation is a common means of achieving organics removal. In the enhanced coagulation process, a coagulant dose higher than required for optimal turbidity is used in order to properly remove NOM, and it is often applied at a depressed pH. Under these conditions, plants must use caution not to compromise turbidity performance, particularly if striving to meet the Partnership turbidity goals.
Process control testing plays an important role in balancing the need for both turbidity reduction and organics removal. Jar testing is a critical and necessary component of the enhanced coagulation process, but completing a jar test takes time. Online monitoring can provide real-time water quality information, allowing operators to immediately react to changes in turbidity or organics. Online turbidimeters are widely used to measure turbidity throughout the treatment process. Organics can be monitored using an online total organic carbon (TOC) analyzer; however use of such an analyzer may be cost prohibitive for many small utilities.
Monitoring UV254 absorbance is the most cost effective option for monitoring organics and can provide valuable information about the aromaticity of the organic content in the water and its likelihood to react with chlorine to form DBPs such as TTHM and HAA5. As cited by the USEPA - Stage 2 D/DBPR Operational Evaluation Guidance Manual "...UV254, which is generally linked to the aromatic and unsaturated components of NOM, is considered a good predictor of the tendency of a source water to form TTHM & HAA5." The relationship between UV254 absorbance, dissolved organic carbon, and DBP formation can be system specific, so developing a relationship between UV254 absorbance and DBP formation that works for your plant is an important part of using UV254 absorbance as a process control measurement.
Monitoring UV254 absorbance in a water supply and after coagulant addition can be an invaluable tool for providing a surrogate measurement for organics removal and indicating the potential formation of DBPs. The real-time data provided, combined with real time information from online turbidimeters and historical trends can help plants to achieve the delicate balance of optimizing removal of both turbidity and organics when using an enhanced coagulation process. Data from Real Tech's Real UV254 series of continuous monitors and portable meters can be used to monitor raw water, settled water, and in many other applications. More information about RealTech’s analyzers for online monitoring can be found at Real Tech.