Background | The Town of Wallkill has experienced high levels of trihalomethanes (THM) in its water system. THM occur within water systems as a result of the disinfection process. All drinking water sources could contain microorganisms such as bacteria and viruses, so the water is disinfected to destroy those microorganisms. Chlorine (by way of liquid sodium hypochlorite) is the most common disinfectant for treating drinking water. When chlorine is added to water that has natural organic matter, disinfection byproducts develop. THM are the most common type of byproduct. The level of THM in the water is therefore directly related to the organic content of the source water, the amount of chlorine that is added to the water as a disinfectant and the age of the water in the system.
Challenge | The Town is required to maintain a free chlorine residual of 0.2 mg/l within the distribution system. To maintain this level throughout the system, the initial chlorine residual for water leaving the treatment plants must be higher, and in some instances additional chlorine must be added later within the system. In the Town of Wallkill in particular, this addition occurs beyond the Cottage Street Pump Station in areas fed from the Washington Heights Storage Tanks, which are some of the farthest areas from the Town’s water treatment plants. The use of additional chlorine and the age of the water there have resulted in the production of more disinfection byproducts, mainly THM. The Town has recently exceeded the EPA limit of 80 parts per billion for THM on multiple occasions.
Solution | Our team was tasked with evaluating the treatment process, storage system and distribution network to discover the deficiencies contributing to high levels of THM. To improve water quality and lower THM levels in the short term, we have recommended the Town rehabilitate the Washington Heights Storage Tanks. The tank rehabilitation will include the removal of organic material from the tank floor and walls. Mixers will also be installed to circulate the water to lower its age and prevent thermal stratification. The most imperative long-term solution is to continue to rehabilitate the greensand filters at each of the Town’s water treatment plants, which the Town has already begun at the Braeside WTP. Another long-term solution is to construct a chlorine booster station near the Washington Heights Storage Tanks.
Result | Following the completion of this study, the Town will move forward with projects that will maintain its infrastructure and yield better water quality for the water district’s users.