Introduction

Many American cities are paying increasing attention to reducing the operating costs of urban water treatment plants while striving to comply with various regulations in the Environmental Protection Agency's Safe Drinking Water Act. Nowadays, many water treatment plants, such as the Engelwood Water Treatment Plant in Colorado, use Total Organic Carbon (TOC) monitoring technology to optimize their water treatment processes, significantly reducing the cost of water treatment while producing high-quality drinking water.

problem

The US Environmental Protection Agency's Disinfectants and Disinfection By Products Rule (D/DBPR) aims to limit the content of harmful by-products in drinking water, which mainly come from the interaction between naturally occurring organic matter (TOC) in water sources and the disinfection process materials used in water treatment plants. This rule not only specifies the concentration limits for disinfection by-products such as DBP, trihalomethanes (THM), and haloacetic acid (HAA5), but also specifies the removal rate of TOC. Many water treatment plants adopt traditional or direct filtration treatment processes, which use coagulants to remove TOC. The US Environmental Protection Agency uses the term "enhanced coagulation" to define processes that use traditional water treatment methods to limit DBP generation and improve TOC (also known as DBP precursor) removal efficiency.

Water treatment plants often believe that "strengthening coagulation" and improving TOC removal rate are equivalent to increasing operating costs. But if the water treatment plant can truly understand the TOC situation in its own water and the impact of changes in various process parameters on TOC, the operating costs will not increase but decrease. It has been proven that optimizing water treatment processes through monitoring TOC can significantly reduce the cost of water treatment. Although there is no foolproof solution to optimize the "enhanced coagulation" process, daily monitoring of process parameters that affect TOC removal rate can provide valuable information. Even in situations where water quality fluctuates frequently, water treatment plants can meet regulatory requirements for TOC removal rates without worrying about excessive or insufficient coagulant dosage.

In the coagulation process, the removal rate of TOC depends on multiple process parameters, including the type of coagulant, the amount of coagulant added, and the pH value of the water. Overall, the removal efficiency of TOC increases with the increase of coagulant dosage. But different coagulants have different TOC removal effects, especially under different pH values. The dosage of most coagulants has a threshold. After exceeding the threshold, continuing to add coagulants will not further improve the TOC removal rate, but will instead generate more waste. This will increase the consumption of coagulants and the cost of waste treatment, without further reducing the TOC content in the water. Therefore, it is crucial for water treatment plants to fully understand the impact of changes in various process parameters on TOC removal efficiency.

Solution

Although it is difficult to determine the specific correlation between all parameters in the water treatment process, TOC analyzers can help water treatment plants uncover the mystery of optimizing processes. One step in process optimization research is usually beaker testing, which involves extracting a small amount of water sample from a water treatment plant, placing it in multiple laboratory beakers, and then adding different chemical reagents separately. The beaker test combined with TOC instrument suction detection and turbidity detection can effectively compare various coagulants and their application effects. In the past, people observed the adhesion between particles through visual beaker tests to determine the turbidity removal effect. However, due to the fact that the optimal coagulation conditions for removing turbidity are not always consistent with those for removing TOC, water treatment plants must monitor both TOC and turbidity simultaneously.

After selecting a coagulant or a mixture of several coagulants through a beaker test, continuous TOC monitoring is required. Due to the frequent changes in the source water conditions of water treatment plants, including pH value, alkalinity, and organic matter composition, online TOC monitoring has become an effective method for frequently observing the process flow. The online TOC analyzer can continuously measure the TOC of inlet and outlet water, enabling water treatment plants to calculate the actual TOC removal rate based on the residence time of water in the plant. Water treatment plants can adjust the dosage of coagulants, pH value, and other parameters as necessary to further improve TOC removal efficiency. If water treatment plants rely solely on third-party laboratories to test TOC, the test results will always come too late and cannot be used to optimize current process parameters.

The water treatment plant equipped with TOC analyzer on site can make favorable decisions based on the water quality requirements and economic criteria of the plant, including selecting the most optimized coagulant dosage and other operating parameters for the process. If the TOC removal rate meets the drinking water standard, the water treatment plant can try to reduce the amount of coagulant used in order to save coagulant costs, while reducing the amount of waste generated and processing costs. Engelwood City Water Treatment Plant does exactly that.

For more details, please refer to the attached materials~