Trihalomethanes (THM), the word alone is enough to make you stop and pay attention. If your water system has already exceeded the limits of disinfection byproducts trihalomethanes or haloacetic acids (HAA5s) you may be looking for answers to rid your water system of these unwelcomed potential carcinogens. Disinfection byproducts (DBPs) were once thought to be a problem only for surface water systems, but we have noticed several groundwater systems, particularly in the Sparta aquifer area, with elevated THM levels. No one likes having to send out the required notices to their customers telling them that their water may be tainted. Additionally, DHH has started pursuing enforcement actions against water systems with recurring exceedances of this primary contaminant. DHH’s recent increase in minimum chlorine residuals has also contributed to higher levels of disinfection byproducts.
Disinfection byproducts are created within the distribution system when chlorine for disinfection reacts with naturally occurring organic (NOM) matter in the water over time. Since DBPs are produced within the distribution system, it is generally not feasible to remove them directly at each customer’s tap. Therefore, control of DBPs requires control of their precursors. The primary precursors for disinfection byproduct production are organic matter, chlorine dose, and contact time. Reduction of any of these precursors will reduce disinfection byproducts. The presence of bromide and high pH levels can also increase the rates of THM formation.
Control of DBPs can range from operational adjustments such as better monitoring of chlorine levels and more frequent flushing, to relatively simple changes such as moving the fill line or changing the setpoints for a tank, to more complex solutions such as switching to chloramines, to even more complex (and costly) solutions such as ozone treatment or membrane filtration. Sometimes an effective control plan may require multiple solutions. The main thing to remember is to not attempt to control DBPs haphazardly. By doing so, you could spend a lot of time and money on solutions that don’t work. Understand your water system first, and find out what each of your precursors are. Although there are many, many variables in a water distribution system, the chemical reactions creating DBPs are pretty straightforward and can be predicted.
Things to understand include:
Know your source water. Sample each of your water sources for precursors. Is the problem from only one well or from all of your wells? There is no point in treating all of the wells if the problem is with only one.
Know the typical water age in each part of the system. This can be done by computer modeling. Many rural water systems have water that is days old, yet the reactions creating THMs can sometimes occur in hours. In those cases, flushing and looping of lines will help, but will not solve the problem alone.
Know what kind of DBPs you are producing. Not all disinfection byproducts are created equal. Trihalomethanes include four different compounds and there are five different compounds which make up haloacetic acids. The presence of elevated bromide in the water can limit your treatment options because bromide reacts with ozone to create a whole different class of disinfection byproducts.
Know any other problems to be addressed. On some occasions, control of DBPs will also remove iron and improve the taste and color of the water. On other occasions, control of DBPs can make them worse. Each change to a water system will have some effect. You should carefully consider the implications of each proposed solution.
Know your operational skills and budget. Some solutions which cost less up front may turn into O&M headaches in the future. A careful comparison of alternatives should consider both short term and long term costs.
Know your funding sources. The days of “free money” are long gone, but there are some partial grants and low interest loans available.
Know your regulatory agency. You should always be responsive to the regulatory agency. Disinfection byproducts are classified as a primary contaminant and cannot be ignored. On the other hand, getting an exceedance doesn’t necessarily mean that you need to do something just for the sake of doing something. Although we do not presume to speak for DHH, they are rational people and would likely prefer to see you spend your money fixing the problem than paying fines. As long as you have a real plan and keep them in the loop, regulatory agencies will usually give you a reasonable amount of time to fix the problem.
Time and effort spent up front understanding your precursors and developing a control plan will save money and headaches down the road. Meyer, Meyer, LaCroix & Hixson, Inc. (MML&H) has the expertise to help you understand your precursors and to develop a plan for DBP control that best fits your particular needs.
