Source Trace Analysis – Modeling to Minimize Treatment Improvement Costs and Reduce DBPs
- Jeremy Hise, Wayne Zhang, Meric Selbes, Aron Griffin - Hazen and Sawyer
- Craig Lambert, Dan Seal - City of Frederick
Disinfection by-product (DBP) levels including total trihalomethanes (TTHMs) and five haloacetic acids (HAA5s) can vary due to seasonal changes in source water organic content, water treatment plant (WTP) and distribution system operations, seasonal temperature changes, and many other factors. These conditions can lead to moderate to high DBP concentrations in the distribution system. The purpose of this study was to perform an assessment of the current and near-future conditions within the City of Frederick’s (City) water distribution system in relation to the Stage 2 DBP Rule and evaluate potential improvements that could be made to better position the City for continued compliance. An evaluation of both treatment process and distribution system operational modifications was performed to improve DBP levels within the City’s water distribution system.
The City’s distribution system hydraulic model (H2OMAP/InfoWater) was used in this study to assess the distribution system. The system demand in the model was adjusted to represent water demands during summer when elevated DBP levels are observed. Flow, pressure, and tank elevation SCADA records from the City’s facilities (tanks and booster pumping stations) were used for model calibration. The predicted tank levels in the hydraulic model showed a good match with City’s SCADA records. The water age and source influences in the City’s distribution system was estimated using the hydraulic model.
The source tracing output from the model revealed that one of the sampling site’s associated with moderate to high HAA5 levels is influenced by one of the City’s WTPs. To mitigate the HAA5 levels at this sampling site, several water treatment process and distribution system management alternatives have been evaluated. Switching the pre-oxidant from chlorine to either chlorine dioxide or potassium permanganate was recommended at this WTP to reduce the formation of HAA5.
Moderate to high TTHM levels at select sampling sites were associated with another WTP operated by the City and some water supplied by the County. While switching to alternate coagulants or implementing granular activated carbon treatment at the WTP are effective for removing DBP precursors, they are associated with considerable capital costs. Moreover, these strategies may not yield intended results due to blending with the County’s water. Thus, cost effective alternatives were investigated for reducing the DBP levels in the distribution system. TTHM concentrations versus water age were investigated in the distribution system. Strong correlations (R2≥0.89) were found for the THM formation versus water age. Flushing, pipe looping, removal of tanks from operation, and optimization of tank operations were considered as options to reduce water age in the system. Hydraulic modeling determined that these strategies would have minimal impact on decreasing water age. Hydraulic modeling of the distribution system discovered that sample locations with high TTHM concentrations could be traced to the two storage tanks in the distribution system. Aeration could reduce TTHM concentrations in these tanks by over 40%, and thus, aeration was recommended for the City to implement. Lowering TTHM levels further in the distribution system would provide the City flexibility to compensate for seasonal variation and extreme weather events, while still maintaining regulatory compliance.
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