• Webinar: "Chloramine Residual Management: Two Case Studies of Reservoir Control" AWWA Partnership for Safe Water

Distribution Network THM Mitigation in Consecutive and Wholesale Water Systems

Elevated trihalomethane (THM) levels are among the most common violations of the Stage 2 DBP (Disinfection Byproduct) Rule in the United States. Systems that employ raw water with high levels of organics and utilize free-chlorine as a primary or secondary disinfectant as well as endure warm water temperatures will typically experience difficulties with THM formation. For water wholesalers, treatment-plant based solutions such as activated carbon, enhanced coagulation and membranes can have significant capital costs as well as obligate utilities to recurring service and consumable costs that are significant. Consecutive systems (which purchase water from wholesalers) have fewer options and often have to rely on expensive system flushing and tank cycling to stay in compliance.

Over the past ten years, water reservoir or water storage tank aeration to volatilize THMs has become the preferred process option for utilities to mitigate THM formation in distribution networks. The relatively low capital, low operating costs and optional phased implementation of in-tank aeration systems have been proven to reduce THM formation in the distribution network. Furthermore, in combination with plant based solutions, tank oriented aeration systems can allow utilities to actively manage THM formation in transmission and distribution on a system-wide basis.

Active tank mixing, in-tank aeration, and head-space ventilation systems are three elements that through thoughtful combination, can yield meaningful reductions in distribution system THM levels. This integrated technology approach with performance warranties convert water storage tanks into smart and active agents in the management and improvement of water quality instead of a passive vessel holding water of uncertain quality.

Custom Engineered THM Removal Solutions

Using water storage assets as a starting place to effect the removal of THMs from water distribution networks makes sense from both a hydraulic and chemical engineering perspective. Each water distribution network will experience unique THM formation characteristics which dictate the level of THM formation through the network over time. Water characteristics such as water age, temperature, disinfectant concentration and disinfectant residual type all influence the formation of THMs. PSI Water Technologies has developed a robust model (Neptune-Toolbox™) that considers the factors that contribute to THM formation in water systems and helps derive an engineered THM removal solution that meets client needs for THM removal. Importantly, energy consumption is often the most significant factor in a system’s total cost over a period of years. The PSI model pays close attention to the energy consumption tradeoff versus initial equipment cost and can provide a best-in-class series of equipment options based on client circumstances and requirements.

Basic Equipment Components in PAX TRS^™ (THM Removal System)

Successful tank based THM aeration requires three elements; a well-mixed tank, a continual supply of fresh air circulating through the tank headspace and enough water surface area to achieve the desired level of removal. Tank mixers such as the PAX Mixer line and the Tank Shark® educator mixer provide the energy required to ensure that THM concentrations are homogeneous throughout the tank’s stored water. Eliminating THM stratification in tanks allows higher concentrations of THMs to enter the mass-transfer zone at the tank’s water surface allowing for optimal volatilization into the headspace. The PSI Neptune-Toolbox™ model determines the optimal amount of mixing energy required to ensure the tank is fully mixed as well as the need for additional aeration in the form of surface aerators for the purposes of enhanced THM removal. The PSI engineering model considers tank volume, fill/drain cycle, geometry and THM speciation to derive an optimal mixer energy input which allows our engineers select from available mixer form factors, aeration devices and ventilation combinations for optimal THM removal.

Headspace ventilation devices are critical to ensure that THMs which volatilize into the tank’s headspace are efficiently removed from the tank. This continuous process ensures that the partial pressure of THM species in the headspace remains low and continues to encourage additional THMs to leave the tank’s liquid volume and transfer into the gas phase (tank’s headspace). The PSI Powervent™ tank ventilation device has a number of configurations that accommodate a variety of tank geometries, power requirements and site specific issues.

In situations that require higher THM removal rates, additional mass transfer (the movement of THMs in the water into the air or tank headspace) is accomplished through the addition of PAX Surface Aerators. These electrically powered devices float on the water surface inside the tank and create additional surface area for THMs to volatilize into the tank’s headspace. The cost of additional horsepower is often necessitated by the regulatory need to remove large quantities of THM species.