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 Rule in the United States. Systems that employ raw water with high levels of organics, utilize free-chlorine as a network residual and endure warm water temperatures will typically experience difficulties with THMs. 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 difficult to bear. Consecutive systems, however, have far fewer options.
The relatively low capital and low operating costs of in-tank aeration systems have been proven over the last 10 years as a cost effective tool-set for utilities to mitigate THM formation in the distribution network. Access to a set of network-oriented solutions allows both consecutive systems and larger water wholesalers to actively manage THM formation in transmission and distribution.
Active tank mixing, in-tank aeration, and head-space ventilation systems are three tools that through thoughtful combination, can yield meaningful reductions in distribution system THM levels. These technologies make water storage tanks a smart and active agent in the management and improvement of water quality instead of a passive vessel holding water of uncertain quality.
PSI Water Technologies Employs Time-Tested and Effective Analysis to Custom Engineer THM Removal Solution
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 contemplates the factors that contribute to THM formation in systems and drives an engineered THM removal solution that meets client needs for removal. Importantly, energy consumption is often the most significant factor in a system’s NPV (net present value) over a period of years. The PSI set of algorithms pays close attention to the energy consumption tradeoff versus initial equipment cost and can provide a series of equipment options based on client circumstances.
Basic Equipment Components in PAX TRS™ (THM Removal System)
Tank mixers such as the PAX Mixer line and the Tank Shark® mixer provide the fundamental energy required to ensure that THM concentrations are homogeneous throughout the tank’s stored water. Eliminating THM stratification in tanks can set up the optimal driving force for THMs to leave the water and enter the headspace of the tank. The PSI Neptune-Toolbox™ model determines the optimal amount of mixing energy required to ensure the tank is fully mixed for the purposes of THM removal. The 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 and power combinations for a mixer solution.
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 headspace) is accomplished through the addition of PAX Surface Aerators. These 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.