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Survey Study of Trihalomethane Generation and Fate at 25 Municipal Wastewater Treatment Plants in North Carolina

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The North Carolina Division of Water Quality (NCDWQ) conducted an investigation in 2006 of trihalomethane (THM) generation and fate at 25 municipal wastewater treatment plants (WWTP) that disinfect using chlorination, in order to ensure that its' THM monitoring and enforcement policy was effective, particularly related to protecting ground waters of the state. The initial part of the study investigated six municipal wastewater treatment facilities permitted as lagoon treatment and spray irrigation facilities in order to ascertain whether or not ground waters surrounding these land irrigation facilities were being protected with regards to THM contamination emanating from the chlorination process.

The study scope was expanded to include 21 NPDES permitted (discharging) municipal mechanical activated-sludge type wastewater treatment facilities across North Carolina. The purpose of this portion of the study was to: (1) compare the THM generation and fate at mechanical activated-sludge type WWTPs with the lagoon-spray WWTPs in the first part of the study, (2) investigate the THM generation variability between mechanical activated sludge facilities that chlorinate, and (3) investigate the effects of the dechlorination process on THM generation, as almost all discharging WWTPs are required to dechlorinate in order to meet a maximum chlorine residual in the effluent outfall.

The results of the lagoon-spray WWTP study indicate that, despite earlier groundwater compliance data that indicated groundwater THM concentrations above the groundwater standards, no THM compounds were detected at the six spray irrigation facilities in any of the observation wells installed for this study, even wells located immediately within the wetted spray boundaries (far inside any review and compliance boundary). The total THM (TTHM) measured post-chlorination varied from non-detect to 5.4 μg/l. Those facilities that exhibited higher ammonia in their effluent generally had smaller amounts of THM generated, indicating that the ammonia was being oxidized to chloramines (combined chlorine residual), thereby reducing the amount of THMs generated. The spray irrigation process was found to remove on average 87% of the TTHM via volatilization.

The results of the 21 mechanical activated sludge WWTPs study indicate that the TTHM concentration varied from less than 1 μg/l to 66 μg/l immediately post-chlorination, and varied from less than 1 μg/l to 42 μg/l following the dechlorination process. The dechlorination process was found to reduce the TTHM concentration by an average of about 2 μg/l, with a maximum reduction of about 7 μg/l (ignoring one particular WWTP). Similar to the lagoon-spray WWTP study, the amount of THMs generated was generally correlated with higher chlorine residual. Those facilities that exhibited higher ammonia in their effluent generally had smaller amounts of THM generated, although these correlations were weaker.

Document Type: Research Article


Publication date: January 1, 2008

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