Online Instrumentation for Low-Level THM Detection

The City of Paso Robles, Calif., operates a 4.9 MGD activated sludge wastewater treatment facility. In 2015, the facility was upgraded to a biological nutrient removal (BRN) process in order to address whole effluent toxicity in its discharge, primarily attributed to ammonia nitrogen.

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The City of Paso Robles, Calif., operates a 4.9 MGD activated sludge wastewater treatment facility. In 2015, the facility was upgraded to a biological nutrient removal (BRN) process in order to address whole effluent toxicity in its discharge, primarily attributed to ammonia nitrogen. The removal of ammonia from the secondary effluent led to the formation of trihalomethanes (THM) in the chlorinated final effluent. Chloramination is being used to control THM formation.

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Paso Robles Wastewater Treatment Facility, Calif.

Treated wastewater from the facility is used for groundwater recharge (GWR), facing stringent restrictions on effluent toxic pollutants bromodichloromethane (BDCM) and dibromochloromethane (DBCM). Standard laboratory methods struggle to provide accurate, reliable and timely results of the low levels of DBCM (0.40 ppb) and BDCM (0.56 ppb) required under the city’s NPDES permitting. However, accurate and reliable high-frequency water quality analysis is imperative to control the chloramination process and ensure regulatory compliance.

The efficacy of a commercially available online THM monitor capable of low-level, real-time detection of DBCM and BDCM was pilot tested at the facility in 2016. The low-level detection THM analyzer was installed in June 2016 at the sampling location post-disinfection and pre-dechlorination. The online monitor provided accurate and reliable low-level detection of DBCM and BDCM formation at or below NPDES permit limits. The high-frequency data helped the city to evaluate its disinfection byproduct prevention strategy and ensure the quality of treated wastewater used in the GWR program.


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