Use of an Online THM Analyzer for a 35 MGD WTP: Optimization Strategies for THM Compliance
- Allison Reinert, EIT, Ben Stanford, PhD – Hazen and Sawyer
- Tiffanie Hawley – Cape Fear Public Utility Authority
Disinfection Byproduct (DBP) compliance issues with the Stage 2 Disinfection/Disinfectant Byproducts (D/DBP) Rule are affecting utilities across North Carolina and the nation. Effectively maximizing existing treatment infrastructure as well as optimizing distribution systems (DSs) and investigating advanced technologies can be effective Stage 2 D/DBP Rule compliance strategies. As such, Cape Fear Public Utility Association’s staff evaluated both short-term and long-term solutions to ensure compliance with the Stage 2 Rule at the Sweeney Water Treatment Plant (WTP). The Sweeney WTP uses an advanced treatment process to treat water withdrawn from the Cape Fear River – raw water ozone, rapid mixing, superpulsators, intermediate ozone, biologically active carbon filtration, and UV disinfection with free chlorine added prior to the clearwells and point of entry to the DS. One area of concern for the Sweeney WTP and the entire Cape Fear River is the presence of elevated bromide levels in the river. Elevated bromide levels can increase brominated DBPs such as trihalomethanes (THMs), leading to higher total THMs in the DS. Additionally, the Sweeney WTP experiences large fluxes of organic matter in their source water throughout the year. High levels of organic matter increase the total organic carbon (TOC) concentration within the water which also impacts DBP formation in the plant and throughout the DS.
To address these concerns for Stage 2 compliance, research was completed to elucidate the relationship between THMs, bromide, and TOC by using an online THM analyzer that automatically collects samples 24/7 and provides speciated THM data along with the ability to process grab samples. Real time THM data were correlated with optimization efforts in the plant and DS.
Clearwell management (chlorine dose and contact time), preoxodiant dose (ozone), and DS operation highlight some of the operational parameters evaluated throughout this study and were correlated to hourly samples from the online THM analyzer.
The overall outcome of this research was a THM monitoring and response plan for the WTP, both at the plant and in the DS. The plan enables the WTP to monitor THMs holistically, while having the tools to optimize and adjust the treatment process to minimize DBP production. Additionally, bromide levels correlated to THMs helped to further the understanding of threshold concentrations for increased THM formation. Lastly, results from this study have be used in conjunction with the ongoing post treatment aeration design project to optimize the aeration system for more efficient THM and overall DBP control.
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