Canadian Water and Wastewater Association (CWWA)

Aeration decreases THMs


WITH HIGH LEVELS of trihalomethanes (THMs), the township of North Shore in Northern Ontario, Canada, explored treatment options at its Serpent River'Water Treatment Plant. The Serpent River treatment process includes slow-sand filtration. limestone contact tanks, and sodium hypochlorite disinfection. After filtration, sodium hypochlorite is injected into the treatment process before water enters a clearwell. Chlorine contact time is achieved in a baffled clearwell before the water makes its way to rrvo distribution pumps.

Serpent River water contains dissolved organic compounds and total organic compounds, which contribute to water discoloration. Slow-sand lilters alone can't remove as much organics as can otherwise be removed through conventional filtration. Consequently, the slow-sand filtered water contains residual organic constituents.

When sodium hypochlorite comes into contact with organic compounds in filtered water, disinfection by-products (DBPs) are formed, including THMs. The utility's THM levels were ranging from 80 pgll to 150 pgll. The Ontario Ministry of Environment set the upper limit for THMs at 100 pglt and is considering reducing the limit to 80 pgll,. The idea to use aeration came from a study conducted by the Suisun-Solano (Calif.) Water Authoriff, which used a small recirculation pump and spray nozzles to considerably reduce THM levels (see Storage Tank Aeration Eliminates Trihalomethanes, Opflow, May 2008, page 28).

Laboratories usually report total THMs, which consist of bromodichloromethane, bromoform, chloroform, and dibromochloromethane. The utility is required to measure total THMs, which sums the above components. In this study, chloroform accounted for abowt 99 percent of total THMs.

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