WWEMA Window: Managing A High-Risk Treatment Change With Monitoring And Control
By Katy Guthrie, Hach Company
As utilities face compliance deadlines under the Stage 2 Disinfectants and Disinfection Byproducts Rule (DBPR), some are forced to consider alternative treatments. There are many options, not limited to ozone disinfection, enhanced coagulation, post-GAC filtration, and chloramination. Choosing the right alternative treatment depends on its effectiveness in reducing disinfection byproducts (DBPs), the cost and complexity of implementation and operation, and the potential for the chosen treatment to cause additional problems in the system.
Chloramination is often one of the most effective and least costly options to reduce DBPs, but is sometimes approached with wariness, due to the relative complexity of the process and the potential for nitrification in the distribution system. When nitrification occurs, not only is it costly to fix, it can also result in taste and odor complaints, and in severe cases, risk to public safety. These issues are very real, but can be controlled through proper monitoring.
When early testing confirmed that the City of Tulsa would be over the limit for trihalomethanes (TTHM) dictated by the Stage 2 DBPR, they were faced with the task of deciding which alternative treatment to implement. Although chloramination was originally their last choice, seven years of trials and cost/benefit analyses of 14 different treatment methods showed that it was the only viable option for them.
As soon as the utility realized that the switch to chloramination was inevitable, they began extensive pilot studies to test all potential negative impacts resulting from the conversion to chloramines. One study involved seeding pilot scale distribution systems with nitrifying bacteria to enhance the conditions for nitrification. Throughout the study, the city monitored free and total chlorine, ammonia, free ammonia, and monochloramine to control the process. By maintaining this level of control, they were never able to trigger a nitrification event in the test system.
In early August 2012, the City of Tulsa switched to chloramination. In order to assure they stay within the ideal portion of the breakpoint curve to prevent taste and odor issues, maintain an adequate chlorine residual, and prevent nitrification, they target 2.5 ppm of total chlorine and monochloramine, zero free chlorine, and barely detectable levels of free ammonia. The city went even further to maintain a high level of control. They expanded their distribution system monitoring program by adding nitrification leading indicators, free ammonia, monochloramine, total chlorine, and nitrite, to the 50 weekly bacteriological samples.
To expand their diligence, they developed a detailed SOP for responding to nitrification indicators, but so far they haven’t had to use it. Other than one section where they knew they’d have some issues, they’ve never seen nitrification in their distribution system, and they haven’t had to flush the system with free chlorine — a testament to the exceptional results that can be achieved with proper monitoring and proactive control.
Choosing chloramination as a compliance strategy doesn’t have to be painful. Through proper monitoring, utilities can be confident that they’ll comply with the regulation in a cost-effective manner while maintaining control over the process.